U.S. Department of the Interior

U.S. Geological Survey

Prepared in cooperation with the California State Water Resources Control Board

A product of the California Groundwater Ambient Monitoring and Assessment (GAMA) Program

Status and Understanding of Groundwater Quality in the Tahoe-Martis, 

Central Sierra, and Southern Sierra Study Units, 2006–2007: California 

GAMA Priority Basin Project

Photo placement

Scientific Investigations Report 2011–5216
Front Cover Map: Groundwater basins categorized by sampling priority. Location of groundwater basin 

boundaries from California Department of Water Resources (CDWR, 2003).

Cover photographs:

Front cover: Lake Tahoe shoreline, El Dorado County, California. (Photograph taken by Cathy Munday,

U.S. Geological Survey.)

Back cover: Supply well in the Tehachapi-Cummings Valley, Kern County, California. (Photograph taken by 

Justin Kulongoski, U.S. Geological Survey.) 

Sampling priority

Study areas

Low-use basins

Priority basins

Areas that are outside CDWR-defined

 groundwater basins
Status and Understanding of Groundwater 

Quality in the Tahoe-Martis, Central Sierra, 

and Southern Sierra Study Units, 2006–2007: 

California GAMA Priority Basin Project

By Miranda S. Fram and Kenneth Belitz

A product of the California Groundwater Ambient Monitoring and Assessment 

(GAMA) Program

Prepared in cooperation with the California State Water Resources Control Board

Scientific Investigations Report 2011–5216

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Fram, M.S., and Belitz, Kenneth, 2012, Status and Understanding of Groundwater Quality in the Tahoe-Martis, Central 

Sierra, and Southern Sierra Study Units, 2006–2007—California GAMA Priority Basin Project: U.S. Geological Survey 

Scientific Investigations Report 2011-5216, 222 p.
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................2

Purpose and Scope ..............................................................................................................................3

Definitions and Locations of Study Units..........................................................................................6

Hydrogeologic Setting.................................................................................................................................10

Tahoe-Martis Study Unit....................................................................................................................12

Central Sierra Study Unit...................................................................................................................16

Southern Sierra Study Unit ...............................................................................................................22

Methods.........................................................................................................................................................24

Status Assessment.............................................................................................................................24

Relative-Concentrations and Water-Quality Benchmarks .................................................24

Datasets for Status Assessment.............................................................................................27

USGS-Grid Wells...............................................................................................................27

CDPH-Grid Wells...............................................................................................................27

Additional Data Used for Spatially Weighted Calculation .........................................33

Selection of Constituents for Additional Evaluation in the Status Assessment..............33

Calculation of Aquifer-Scale Proportions..............................................................................40

Understanding Assessment..............................................................................................................41

Evaluation of Potential Explanatory Factors ...........................................................................................42

Land Use...............................................................................................................................................42

Aquifer Lithology and Hydrologic Conditions.................................................................................43

Well Depth and Groundwater Age Distribution.............................................................................46

Geochemical Conditions....................................................................................................................50

Correlations Between Explanatory Factors ...................................................................................50

Status and Understanding of Water Quality............................................................................................55

Inorganic Constituents.......................................................................................................................55

Trace Elements...........................................................................................................................64

Factors Affecting Arsenic ...............................................................................................75

Factors Affecting Fluoride...............................................................................................81

Factors Affecting Molybdenum...............................................................................................86

Factors Affecting Boron ..................................................................................................94

Nutrients......................................................................................................................................98

Factors Affecting Nitrate...............................................................................................102

Radioactive Constituents........................................................................................................103

Factors Affecting Uranium, Gross Alpha Particle Activity, and Radon..................107

Major Ions and Trace Elements with SMCL Benchmarks ................................................111

Factors Affecting Total Dissolved Solids ....................................................................111

Factors Affecting Manganese and Iron......................................................................115
iv

Status and Understanding of Water Quality—Continued

Organic and Special-Interest Constituents..................................................................................122

Herbicides.................................................................................................................................127

Factors Affecting Herbicides........................................................................................127

Trihalomethanes.......................................................................................................................135

Factors Affecting Trihalomethanes..............................................................................135

Solvents.....................................................................................................................................140

Factors Affecting Solvents............................................................................................140

Other Volatile Organic Compounds.......................................................................................146

Special-Interest Constituents................................................................................................146

Factors Affecting Perchlorate ......................................................................................146

Summary......................................................................................................................................................152

Acknowledgments.....................................................................................................................................153

References Cited........................................................................................................................................154

Appendix A: Selection of CDPH-Grid Wells..........................................................................................163

Appendix B. Calculation of Aquifer-Scale Proportions ......................................................................170

Appendix C: Attribution of Potential Explanatory Factors..................................................................182

Appendix D: Comparison of CDPH and USGS-GAMA Data...............................................................210

Contents—Continued

Figures

Figure 1. Maps of California showing locations of the Tahoe-Martis, Central Sierra, and

Southern Sierra study units, California GAMA Priority Basin Project, and the

California hydrogeologic provinces, and selected major geologic features in

California .................................................................................... 4

Figure 2. Maps showing geographic features of theTahoe-Martis, Central Sierra, and

Southern Sierra study units, California GAMA Priority Basin Project ............... 7

Figure 3. Diagram showing conceptual model of groundwater flow in a

fractured-bedrock aquifer, and recharge of a small alluvial basin adjacent to

mountains consisting of fractured rock................................................... 11

Figure 4. Maps showing geologic features and areal distribution of USGS-grid, CDPH-grid, 

and USGS-understanding wells sampled in the Tahoe-Martis, Central Sierra, 

and Southern Sierra study units, California GAMA Priority Basin Project............ 13

Figure 5. Maps showing land use and areal distribution of USGS-grid, CDPH-grid, and

USGS-understanding wells sampled in the Tahoe-Martis, Central Sierra, and

Southern Sierra study units, California GAMA Priority Basin Project ............... 17

Figure 6. Ternary diagrams showing percentage of urban, agricultural, and undeveloped

land use in the Tahoe-Martis, Central Sierra, and Southern Sierra study units,

California GAMA Priority Basin Project in the study units, and the areas

surrounding each USGS-grid and USGS-understanding well ........................ 20

Figure 7. Box plots comparing method-detection levels used for analysis of constituents

by USGS-GAMA in the time period during which samples were collected for

the Tahoe-Martis study unit, and the most common reporting levels for the

same constituents used in the CDPH database for wells in the area

corresponding to the Tahoe-Martis study during the 3-year interval used in the

status assessment........................................................................... 26
v

Figure 8. Maps showing locations of grid cells and USGS-grid wells, CDPH-grid wells,

USGS-understanding wells, and other CDPH wells in the Tahoe-Martis, Central

Sierra, and Southern Sierra study units, California GAMA Priority Basin Project... 28

Figure 9. Box plot showing the density of septic tanks in the areas surrounding wells,

Tahoe-Martis, Central Sierra, and Southern Sierra study units, California

GAMA Priority Basin Project............................................................... 43

Figure 10. Bar chart showing aquifer lithology classification for grid wells and

USGS-understanding wells, Tahoe-Martis, Central Sierra, and Southern Sierra

study units, California GAMA Priority Basin Project .................................... 44

Figure 11. Graph showing relation between elevation and aridity index at well sites,

Tahoe-Martis, Central Sierra, and Southern Sierra study units, California

GAMA Priority Basin Project............................................................... 45

Figure 12. Box plots showing well depths and depths to top of screened or open interval

for grid and understanding wells, Tahoe-Martis, Central Sierra, and Southern

Sierra study units, California GAMA Priority Basin Project ........................... 47

Figure 13. Box plots showing depths to top of screened or open interval and well depths

for wells with groundwater classified as modern, mixed, or pre-modern in age,

Tahoe-Martis, Central Sierra, and Southern Sierra study units, California

GAMA Priority Basin Project............................................................... 48

Figure 14. Bar charts showing the relation between groundwater age classification and

well depth classification for the Tahoe-Martis, Central Sierra, and Southern

Sierra study units, California GAMA Priority Basin Project ........................... 49

Figure 15. Graphs showing relations between well depth and dissolved oxygen

concentration and depth to top of screened or open interval and pH for wells

with various aquifer lithologies, Tahoe-Martis, Central Sierra, and Southern

Sierra study units, California GAMA Priority Basin Project ........................... 51

Figure 16. Graphs showing maximum relative-concentrations of constituents detected in

grid wells by constituent class for the Tahoe-Martis, Central Sierra, and

Southern Sierra study units, California GAMA Priority Basin Project ............... 58

Figure 17. Graphs showing relative-concentrations of selected trace elements with

regulatory and non-regulatory health-based benchmarks, nutrients and

radioactive constituents with regulatory health-based benchmarks, and

salinity indicators and trace elements with non-regulatory SMCL benchmarks,

Tahoe-Martis, Central Sierra, and Southern Sierra study units, California

GAMA Priority Basin Project............................................................... 61

Figure 18. Maps showing relative-concentrations of arsenic for USGS- and CDPH-grid

wells, USGS-understanding wells, and all wells in the CDPH database with

data for arsenic during the 3-year intervals used in the status assessment for

the Tahoe-Martis, Central Sierra, and Southern Sierra study units, California

GAMA Priority Basin Project............................................................... 72

Figure 19. Graph showing relation between arsenic concentration and dissolved oxygen

concentration, pH, and aquifer lithology, Tahoe-Martis, Central Sierra, and

Southern Sierra study units, California GAMA Priority Basin Project ............... 75

Figure 20. Box plot showing arsenic concentrations in groundwater samples grouped by

oxidation-reduction class and pH, Tahoe-Martis, Central Sierra, and Southern

Sierra study units, California GAMA Priority Basin Project ........................... 78

Figures—Continued
vi

Figures—Continued

Figure 21. Graph showing relations among arsenic concentration, groundwater age

classification, aquifer lithology, and pH and total dissolved solids for oxic

groundwater samples from the Tahoe-Martis study unit, California GAMA

Priority Basin Project........................................................................ 80

Figure 22. Box plot showing relation between arsenic concentration and groundwater

age classification, Tahoe-Martis, Central Sierra, and Southern Sierra study

units, California GAMA Priority Basin Project .......................................... 81

Figure 23. Maps showing relative-concentrations of fluoride for USGS- and CDPH-grid

wells, USGS-understanding wells, and all wells in the CDPH database with

data for fluoride during the 3-year intervals used in the status assessment for

the Tahoe-Martis, Central Sierra, and Southern Sierra study units, California

GAMA Priority Basin Project............................................................... 82

Figure 24. Box plot showing relation between fluoride concentration and geographic

location within the Tahoe-Martis and Southern Sierra study units and between

fluoride concentration and pH for the Central Sierra study unit, California

GAMA Priority Basin Project............................................................... 85

Figure 25. Graphs showing relations between fluoride concentration and pH and calcium

concentration, and fluorite and calcite saturation indexes, Central Sierra study

unit, California GAMA Priority Basin Project............................................. 87

Figure 26. Maps showing relative-concentrations of molybdenum for USGS- and

CDPH-grid wells, and USGS-understanding wells for the Tahoe-Martis, Central

Sierra, and Southern Sierra study units, California GAMA Priority Basin Project... 88

Figure 27. Graph showing relations among molybdenum concentration, pH, aquifer

lithology, and oxidation-reduction class, Tahoe-Martis, Central Sierra, and

Southern Sierra study units, California GAMA Priority Basin Project ............... 91

Figure 28. Box plot showing relations among molybdenum, oxidation-reduction class,

and pH for wells with granitic aquifer geology, Tahoe-Martis, Central Sierra,

and Southern Sierra study units, California GAMA Priority Basin Project............ 92

Figure 29. Box plot showing relation between molybdenum concentration and well

location for wells with oxic groundwater and sedimentary aquifer lithology,

Tahoe-Martis and Southern Sierra study units, California GAMA Priority Basin

Project ....................................................................................... 93

Figure 30. Maps showing relative-concentrations of boron for USGS- and CDPH-grid

wells, USGS-understanding wells, and all wells in the CDPH database with

data for boron during the 3-year intervals used in the status assessment for

the Tahoe-Martis, Central Sierra, and Southern Sierra study units, California

GAMA Priority Basin Project............................................................... 95

Figure 31. Graph showing relation between boron and total dissolved solids

concentrations, Tahoe-Martis, Central Sierra, and Southern Sierra study units,

California GAMA Priority Basin Project................................................... 98

Figure 32. Maps showing relative-concentrations of nitrate for USGS- and CDPH-grid

wells, USGS-understanding wells, and all wells in the CDPH database with

data for nitrate during the 3-year intervals used in the status assessment for

the Tahoe-Martis, Central Sierra, and Southern Sierra study units, California

GAMA Priority Basin Project............................................................... 99

Figure 33. Box plot showing relations among nitrate concentration and agricultural land

use and detection of herbicides and solvents, Tahoe-Martis, Central Sierra,

and Southern Sierra study units, California GAMA Priority Basin Project............102
vii

Figures—Continued

Figure 34. Maps showing relative-concentrations of uranium for USGS- and CDPH-grid

wells, USGS-understanding wells, and all wells in the CDPH database with

data for uranium during the 3-year intervals used in the status assessment for

the Tahoe-Martis, Central Sierra, and Southern Sierra study units, California

GAMA Priority Basin Project...............................................................104

Figure 35. Graph showing relations among uranium, radon-222, and gross alpha particle

activities, Tahoe-Martis, Central Sierra, and Southern Sierra study units,

California GAMA Priority Basin Project...................................................107

Figure 36. Graph showing relations among uranium activity, dissolved oxygen

concentration, and aquifer lithology, Tahoe-Martis, Central Sierra, and

Southern Sierra study units, California GAMA Priority Basin Project ...............108

Figure 37. Box plot showing relation between uranium activity and location for

groundwater samples from wells with sedimentary aquifer lithology,

Tahoe-Martis and Southern Sierra study units, California GAMA Priority Basin

Project .......................................................................................110

Figure 38. Maps showing relative-concentrations of total dissolved solids for USGS- and

CDPH-grid wells, USGS-understanding wells, and all wells in the CDPH

database with data for total dissolved solids during the 3-year intervals used in

the status assessment for the Tahoe-Martis, Central Sierra, and Southern

Sierra study units, California GAMA Priority Basin Project ...........................112

Figure 39. Piper diagram showing major-ion composition and total dissolved solids

concentration for groundwater samples analyzed by USGS-GAMA or with data

in the 3-year interval used in the status assessment in the CDPH database,

Central Sierra study unit, California GAMA Priority Basin Project ..................116

Figure 40. Piper diagram showing major-ion composition and total dissolved solids

concentration for samples from the State Water Project California Aqueduct

and from groundwater samples analyzed by USGS-GAMA or data in the 3-year

interval used in the status assessment in the CDPH database, Southern Sierra

study unit, California GAMA Priority Basin Project ....................................117

Figure 41. Maps showing relative-concentrations of manganese for USGS- and

CDPH-grid wells, USGS-understanding wells, and all wells in the CDPH

database with data for manganese during the 3-year intervals used in the

status assessment for the Tahoe-Martis, Central Sierra, and Southern Sierra

study units, California GAMA Priority Basin Projec ....................................118

Figure 42. Bar chart showing relations among manganese relative-concentration, aquifer

lithology, and groundwater age, Tahoe-Martis, Central Sierra, and Southern

Sierra study units, California GAMA Priority Basin Project ...........................121

Figure 43. Graph showing area-weighted detection frequency and maximum

relative-concentration of organic and special-interest constituents detected in

USGS-grid wells in the Tahoe-Martis, Central Sierra, and Southern Sierra

study units, California GAMA Priority Basin Project ....................................123

Figure 44. Bar chart of area-weighted detection frequency and graph of

relative-concentrations of organic and special-interest constituents detected

in USGS-grid wells, Tahoe-Martis, Central Sierra, and Southern Sierra study

units, California GAMA Priority Basin Project ..........................................126

Figure 45. Maps showing relative-concentrations of herbicides for USGS- and CDPH-grid

wells, USGS-understanding wells, and all wells in the CDPH database with

data for herbicides during the 3-year intervals used in the status assessment

for the Tahoe-Martis, Central Sierra, and Southern Sierra study units,

California GAMA Priority Basin Project...................................................128
viii

Figures—Continued

Figure 46. Bar chart showing relations among detection frequencies of herbicides,

solvents, and trihalomethanes, and groundwater age class, Tahoe-Martis,

Central Sierra, and Southern Sierra study units, California GAMA Priority Basin

Project .......................................................................................131

Figure 47. Ternary diagram showing average percentages of agricultural, undeveloped,

and urban land use in the areas surrounding wells with and without detections

of herbicides, solvents, or trihalomethanes, Tahoe-Martis, Central Sierra, and

Southern Sierra study units, California GAMA Priority Basin Project ...............132

Figure 48. Box plots showing percentages of agricultural, undeveloped, and urban land

use in the areas surrounding wells with and without detections of herbicides,

solvents, or trihalomethanes (THMs), Tahoe-Martis, Central Sierra, and

Southern Sierra study units, California GAMA Priority Basin Project ...............133

Figure 49. Bar chart showing relation between detection frequency of herbicides and

groundwater age and percentage of agricultural land use in the areas

surrounding wells, Tahoe-Martis, Central Sierra, and Southern Sierra study

units, California GAMA Priority Basin Project ..........................................134

Figure 50. Maps showing relative-concentrations of trihalomethanes for USGS- and

CDPH-grid wells, USGS-understanding wells, and all wells in the CDPH

database with data for trihalomethanes during the 3-year intervals used in the

status assessment for the Tahoe-Martis, Central Sierra, and Southern Sierra

study units, California GAMA Priority Basin Project ....................................136

Figure 51. Bar chart showing relation between detection frequency of trihalomethanes

and groundwater age, percentage of urban land use, and septic tank density,

Tahoe-Martis, Central Sierra, and Southern Sierra study units, California

GAMA Priority Basin Project...............................................................139

Figure 52. Maps showing relative-concentrations of solvents for USGS- and CDPH-grid

wells, USGS-understanding wells, and all wells in the CDPH database with

data for solvents during the 3-year intervals used in the status assessment for

the Tahoe-Martis, Central Sierra, and Southern Sierra study units, California

GAMA Priority Basin Project...............................................................141

Figure 53. Graphs showing relation among total herbicide, total solvent, and total

trihalomethane concentrations and depth to top of screened or open interval

in wells and groundwater age classification, Tahoe-Martis, Central Sierra, and

Southern Sierra study units, California GAMA Priority Basin Project ...............144

Figure 54. Graph showing relation between total concentration of organic constituents

and depth to top of screened or open interval in wells, Tahoe-Martis, Central

Sierra, and Southern Sierra study units, California GAMA Priority Basin Project...145

Figure 55. Map showing relative-concentrations of perchlorate for USGS-grid wells,

USGS-understanding wells, and all wells in the CDPH database with data for

perchlorate during the 3-year intervals used in the status assessment, for the

Tahoe-Martis, Central Sierra, and Southern Sierra study units, California

GAMA Priority Basin Project...............................................................147

Figure 56. Graphs showing predicted probability of detecting perchlorate in groundwater

as a function of aridity index and Anthropogenic Score and observed

detection frequency and average aridity index in groups of samples for

perchlorate concentrations greater than or equal to 0.5 microgram per liter

(μg/L) and 1.0 μg/L, Tahoe-Martis, Central Sierra, and Southern Sierra study

units, California GAMA Priority Basin Project ..........................................151
ix

Tables

Table 1. Summary of constituent groups and number of wells sampled by the U.S.

Geological Survey for different analytical schedules for the Tahoe-Martis,

Central Sierra, and Southern Sierra study units, 2006–2007, California GAMA

Priority Basin Project........................................................................ 31

Table 2. Number of USGS-grid and CDPH-grid wells used in the status assessments for

inorganic constituents in the Tahoe-Martis, Central Sierra, and Southern

Sierra study units, 2006–2007, California GAMA Priority Basin Project............... 32

Table 3. Benchmark type and value for constituents selected for additional evaluation

in the status assessments of groundwater quality in the Tahoe-Martis, Central

Sierra, and Southern Sierra study units, 2006–2007, California GAMA Priority

Basin Project................................................................................. 34

Table 4. Constituents detected in samples collected by USGS-GAMA, but not selected

for additional evaluation in the status assessments for the Tahoe-Martis,

Central Sierra, and Southern Sierra study units, 2006–2007, California GAMA

Priority Basin Project........................................................................ 36

Table 5. Constituents reported at concentrations greater than benchmarks historically

in the California Department of Public Health database, but not during the

3-year time periods used in status assessment, Tahoe-Martis, Central Sierra,

and Southern Sierra study units, 2006–2007, California GAMA Priority Basin

Project ....................................................................................... 38

Table 6A. Results of Kruskal-Wallis and Wilcoxon rank-sum tests for differences in

values of selected potential explanatory factors between samples classified

into groups by aquifer lithology, groundwater age, study unit, or well depth,

Tahoe-Martis, Central Sierra, and Southern Sierra study units, 2006–2007,

California GAMA Priority Basin Project................................................... 52

Table 6B. Results of Spearman’s tests for correlations between selected potential

explanatory factors, Tahoe-Martis, Central Sierra, and Southern Sierra study

units, 2006–2007, California GAMA Priority Basin Project .............................. 53

Table 6C. Results of contingency table tests for associations between aquifer lithology,

study unit, well depth, and groundwater age, Tahoe-Martis, Central Sierra, and

Southern Sierra study units, 2006–2007, California GAMA Priority Basin Project ... 54

Table 7A. Number of inorganic constituents analyzed and detected by U.S. Geological

Survey-Groundwater Ambient Monitoring and Assessment (USGS-GAMA) by

benchmark and constituent type, Tahoe-Martis, Central Sierra, and Southern

Sierra study units, 2006–2007, California GAMA Priority Basin Project............... 56

Table 7B. Number of organic constituents analyzed and detected by U.S. Geological

Survey-Groundwater Ambient Monitoring and Assessment (USGS-GAMA) by

benchmark type and constituent type, Tahoe-Martis, Central Sierra, and

Southern Sierra study units, 2006–2007, California GAMA Priority Basin Project ... 57

Table 8A. Aquifer-scale proportions calculated using grid-based and spatially weighted

methods for those constituents that met criteria for additional evaluation in the

status assessment, Tahoe-Martis study unit, 2006–2007, California GAMA

Priority Basin Project........................................................................ 65

Table 8B. Aquifer-scale proportions calculated using grid-based and spatially weighted

methods for those constituents that met criteria for additional evaluation in the

status assessment, Central Sierra study unit, 2006–2007, California GAMA

Priority Basin Project........................................................................ 66
x

Tables

Table 8C. Aquifer-scale proportions calculated using grid-based and spatially weighted

methods for those constituents that met criteria for additional evaluation in the

status assessment, Southern Sierra study unit, 2006–2007, California GAMA

Priority Basin Project........................................................................ 68

Table 9A. Summary of aquifer-scale proportions for inorganic constituent classes with

health-based benchmarks, Tahoe-Martis, Central Sierra, and Southern Sierra

study units, 2006–2007, California GAMA Priority Basin Project ..................... 70

Table 9B. Summary of aquifer-scale proportions for inorganic constituent classes with

secondary maximum contaminant level (SMCL) benchmarks, Tahoe-Martis,

Central Sierra, and Southern Sierra study units, 2006–2007, California GAMA

Priority Basin Project........................................................................ 70

Table 9C. Summary of aquifer-scale proportions for organic constituent classes with

health-based benchmarks, Tahoe-Martis, Central Sierra, and Southern Sierra

study units, 2006–2007, California GAMA Priority Basin Project ..................... 71

Table 10A. Results of Kruskal-Wallis and Wilcoxon rank-sum tests for differences in

values of selected water-quality constituents between samples classified into

groups by aquifer lithology, groundwater age, study unit, or well depth,

Tahoe-Martis, Central Sierra, and Southern Sierra study units, 2006–2007,

California GAMA Priority Basin Project................................................... 76

Table 10B. Results of Spearman’s rho tests for correlations between selected potential

explanatory factors and selected water-quality constituents, Tahoe-Martis,

Central Sierra, and Southern Sierra study units, 2006–2007, California GAMA

Priority Basin Project........................................................................ 77
xi

Conversion Factors, Datums, and Abbreviations and Acronyms

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square foot (ft2) 0.09290 square meter (m2)

square mile (mi2) 2.590 square kilometer (km2) 

Flow rate

inch per year (in/yr) 25.4 millimeter per year (mm/yr)

Radioactivity

picocurie per liter (pCi/L) 0.037 Becquerel per liter (Bq/L)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

 °F=(1.8×°C)+32.

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

 °C=(°F–32)/1.8.

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm 

at 25 °C).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L). One milligram per liter is equivalent to 1 part per million (ppm); 

1 microgram per liter is equivalent to 1 part per billion (ppb).

Datums

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Conversion Factors, Datums, and Abbreviations and Acronyms
xii

Abbreviations and Acronyms

AB Assembly Bill (through the California State Assembly)

AL-US U.S. Environmental Protection Agency action level 

BQ benchmark quotient

GAMA Groundwater Ambient Monitoring and Assessment Program

HAL-US U.S. Environmental Protection Agency lifetime health advisory level 

HBSL health-based screening level

LRL laboratory reporting level

LSD land-surface datum

LT-MDL long-term method detection level

MCL maximum contaminant level

MCL-CA California Department of Public Health maximum contaminant level 

MCL-US U.S. Environmental Protection Agency maximum contaminant level 

MDL method detection limit

MRL minimum reporting level

NL-CA California Department of Public Health notification level 

pe electron activity

RC relative-concentration

RSD5-US U.S. Environmental Protection Agency risk-specific dose at a risk factor of 10–5

SI saturation index

SMCL secondary maximum contaminant level

SMCL-CA California Department of Public Health secondary maximum contaminant level 

SMCL-US U.S. Environmental Protection Agency secondary maximum contaminant level 

SWP State Water Project

TEAP terminal electron-acceptor processes

Organizations

BLM U.S. Bureau of Land Management

CDPH California Department of Public Health (Department of Health Services prior to July 1, 

2007)

CDPR California Department of Pesticide Regulation

CDWR California Department of Water Resources

LLNL Lawrence Livermore National Laboratory

NAWQA National Water-Quality Assessment Program (USGS) 

SWRCB State Water Resources Control Board (California)

TCCWD Tehachapi-Cummings County Water District

US United States

USEPA U.S. Environmental Protection Agency

USFS U.S. Forest Service

USGS U.S. Geological Survey

Conversion Factors, Datums, and Abbreviations and 

Acronyms—Continued
xiii

Selected Chemical Names

Ammonia-N ammonia as nitrogen

BDCM bromodichloromethane

CaWO4 scheelite

CCl4 carbon tetrachloride (tetrachloromethane)

CFC-11 trichlorofluoromethane

CFC-113 1,1,2-trichloro-1,2,2-trifluoroethane

CS2 carbon disulfide

DBCP 1,2-dibromo-3-chloropropane

EDB 1,2-dibromoethane (ethylene dibromide)

H2

CO3 carbonic acid

HCO3

– bicarbonate

MTBE methyl tert-butyl ether

NDMA N-nitrosodimethylamine

Nitrate-N nitrate as nitrogen

Nitrite-N nitrite as nitrogen

PCE perchloroethene (tetrachloroethene)

1,1,1-TCA 1,1,1-trichloroethane

TCE trichloroethene

1,2,3-TCP 1,2,3-trichloropropane

TDS total dissolved solids

THM trihalomethane

VOC volatile organic compound

Units of Measure

cm3 STP g–1 cubic centimeters at standard temperature and pressure per gram

δd delta notation; the ratio of a heavier isotope to the more common lighter isotope of an 

element, relative to a standard reference material, expressed as per mil

ft foot (feet)

ka kilo annum (thousand years ago)

Ma mega annum (million years ago)

meq/L milliequivalents per liter

mg/L milligram per liter (parts per million)

mi mile

μg/L microgram per liter (parts per billion)

μS/cm micro Siemen per centimeter

pCi/L picocurie per liter

per mil parts per thousand

pmc percent modern carbon

tanks/km2 septic tanks per square kilometer

TU tritium unit

Conversion Factors, Datums, and Abbreviations and 

Acronyms—Continued
xiv

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Status and Understanding of Groundwater Quality in the 

Tahoe-Martis, Central Sierra, and Southern Sierra Study 

Units, 2006–2007: California GAMA Priority Basin Project

 By Miranda S. Fram and Kenneth Belitz

Abstract

Groundwater quality in the Tahoe-Martis, Central Sierra, 

and Southern Sierra study units was investigated as part of 

the Priority Basin Project of the California Groundwater 

Ambient Monitoring and Assessment (GAMA) Program. 

The three study units are located in the Sierra Nevada region 

of California in parts of Nevada, Placer, El Dorado, Madera, 

Tulare, and Kern Counties. The GAMA Priority Basin Project 

is being conducted by the California State Water Resources 

Control Board, in collaboration with the U.S. Geological 

Survey (USGS) and the Lawrence Livermore National 

Laboratory. The project was designed to provide statistically 

robust assessments of untreated groundwater quality within the 

primary aquifer systems used for drinking water. The primary 

aquifer systems (hereinafter, primary aquifers) for each study 

unit are defined by the depth of the screened or open intervals 

of the wells listed in the California Department of Public 

Health (CDPH) database of wells used for municipal and 

community drinking-water supply. The quality of groundwater 

in shallower or deeper water-bearing zones may differ from 

that in the primary aquifers; shallower groundwater may be 

more vulnerable to contamination from the surface.

The assessments for the Tahoe-Martis, Central Sierra, 

and Southern Sierra study units were based on water-quality 

and ancillary data collected by the USGS from 132 wells in 

the three study units during 2006 and 2007 and water-quality 

data reported in the CDPH database. Two types of assessments 

were made: (1) status, assessment of the current quality of the 

groundwater resource, and (2) understanding, identification 

of the natural and human factors affecting groundwater 

quality. The assessments characterize untreated groundwater 

quality, not the quality of treated drinking water delivered to 

consumers by water purveyors.

Relative-concentrations (sample concentrations divided 

by benchmark concentrations) were used for evaluating 

groundwater quality for those constituents that have Federal 

or California regulatory or non-regulatory benchmarks for 

drinking-water quality. A relative-concentration (RC) greater 

than (>) 1.0 indicates a concentration above a benchmark. 

RCs for organic constituents (volatile organic compounds 

and pesticides) and special-interest constituents were 

classified as “high” (RC > 1.0), “moderate” (1.0 ≥ RC > 0.1), 

or “low” (RC ≤ 0.1). For inorganic constituents (major ions, 

trace elements, nutrients, and radioactive constituents), the 

boundary between low and moderate RCs was set at 0.5.

A new metric, aquifer-scale proportion, was used in 

the status assessment as the primary metric for evaluating 

regional-scale groundwater quality. High aquifer-scale 

proportion is defined as the percentage of the area of the 

primary aquifers with RC > 1.0 for a particular constituent 

or class of constituents; moderate and low aquifer-scale 

proportions are defined as the percentages of the area of the 

primary aquifer with moderate and low RCs, respectively. 

Percentages are based on an areal rather than a volumetric 

basis. Two statistical approaches—grid-based, which used 

one value per grid cell, and spatially weighted, which 

used multiple values per grid cell—were used to calculate 

aquifer-scale proportions for individual constituents and 

classes of constituents. The spatially weighted estimates of 

high aquifer-scale proportions were within the 90-percent (%) 

confidence intervals of the grid-based estimates in all cases.

The status assessment showed that inorganic constituents 

had greater high and moderate aquifer-scale proportions 

than did organic constituents in all three study units. In the 

Tahoe-Martis study unit, RCs for inorganic constituents with 

health-based benchmarks (primarily arsenic) were high in 20% 

of the primary aquifer, moderate in 13%, and low in 67%. 

In the Central Sierra study unit, aquifer-scale proportions 

for inorganic constituents with health-based benchmarks 

(primarily arsenic, uranium, fluoride, and molybdenum) were 

41% high, 36% moderate, and 23% low. In the Southern Sierra 

study unit, 32, 34, and 34% of the primary aquifer had high, 

moderate, and low RCs of inorganic constituents with healthbased benchmarks (primarily arsenic, uranium, fluoride, 

boron, and nitrate). The high aquifer-scale proportions for 

inorganic constituents with non-health-based benchmarks 

were 14, 34, and 24% for the Tahoe-Martis, Central Sierra, 

and Southern Sierra study units, respectively, and the primary 

constituent was manganese for all three study units.
2 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Organic constituents with health-based benchmarks were 

not present at high RCs in the primary aquifers of the Central 

Sierra and Southern Sierra study units, and were present at 

high RCs in only 1% of the Tahoe-Martis study unit. Moderate 

aquifer-scale proportions for organic constituents were < 5% in 

all three study units. Of the 173 organic constituents analyzed, 

22 were detected, and of those 22, 17 have health-based 

benchmarks. Organic constituents were detected in 20, 27, 

and 40% of the primary aquifers in the Tahoe-Martis, Central 

Sierra, and Southern Sierra study units, respectively. Four 

organic constituents had study-unit detection frequencies of 

>10%: the trihalomethane chloroform in the Tahoe-Martis 

study unit; chloroform and the herbicide simazine in the 

Central Sierra study unit; and chloroform, simazine, the 

herbicide atrazine, and the solvent perchloroethene in the 

Southern Sierra study unit. 

The second component of this study, the understanding 

assessment, identified the natural and human factors that may 

have affected groundwater quality in the three study units 

by evaluating statistical correlations between water-quality 

constituents and potential explanatory factors. The potential 

explanatory factors evaluated were land use, septic tank 

density, climate, relative position in the regional flow 

system, aquifer lithology, geographic location, well depth 

and depth to the top of the screened or open interval in the 

well, groundwater age distribution, pH, and dissolved oxygen 

concentration. Results of the statistical evaluations were used 

to explain the occurrence and distribution of constituents in 

the study units.

Aquifer lithology (granitic, metamorphic, sedimentary, 

or volcanic rocks), groundwater age distribution [modern 

(recharged since 1952), pre-modern (recharged before 1952), 

or mixed (containing both modern and pre-modern recharge)], 

geographic location, pH, and dissolved oxygen were the most 

significant factors explaining the occurrence patterns of most 

inorganic constituents. High and moderate RCs of arsenic 

were associated with pre-modern and mixed-age groundwater 

and two distinct sets of geochemical conditions: (1) oxic, 

high-pH conditions, particularly in volcanic rocks, and 

(2) low-oxygen to anoxic conditions and low- to neutral-pH 

conditions, particularly in granitic rocks. In granitic and 

metamorphic rocks, high and moderate RCs of uranium were 

associated with pre-modern and mixed-age groundwater, 

low-oxygen to anoxic conditions, and location within parts of 

the Central Sierra and Southern Sierra study units known to 

have rocks with anomalously high uranium content compared 

to other parts of the Sierra Nevada. High and moderate RCs 

of uranium in sedimentary rocks were associated with premodern-age groundwater, oxic and high-pH conditions, 

and location in the Tahoe Valley South subbasin within the 

Tahoe-Martis study unit. 

Land use within 500 meters of the well and groundwater 

age were the most significant factors explaining occurrence 

patterns of organic constituents. Herbicide detections were 

most strongly associated with modern- and mixed-age 

groundwater from wells with agricultural land use. 

Trihalomethane detections were most strongly associated 

with modern- and mixed-age groundwater from wells with 

>10% urban land use and (or) septic tank density >7 tanks 

per square kilometer. Solvent detections were not significantly 

related to groundwater age. Eighty-three percent of the wells 

with modern- or mixed-age groundwater, and 86% of wells 

with detections of herbicides and (or) THMs had depths to 

the top of the screened or open interval of < 170 feet. These 

observations suggest that modern groundwater has infiltrated 

to depths of approximately 170 feet below land surface.

Land use and occurrence of herbicides and solvents 

were the most significant factors explaining the occurrence 

of nitrate. Wells with > 5% agricultural land use and 

detection of a herbicide or solvent had the highest nitrate 

concentrations. Comparison between observed and predicted 

detection frequencies of perchlorate suggests that the 

perchlorate detected at concentrations < 1 microgram per 

liter likely reflects the distribution of perchlorate under 

natural conditions, and that the perchlorate detected at higher 

concentrations may reflect redistribution of originally natural 

perchlorate salts by irrigation in the agricultural areas of the 

Southern Sierra study unit. 

Introduction

Groundwater composes approximately half of the 

water used for public and domestic drinking-water supply in 

California (Kenny and others, 2009). To assess the quality 

of ambient groundwater in aquifers used for drinking-water 

supply and to establish a baseline groundwater-quality 

monitoring program, the California State Water Resources 

Control Board (SWRCB), in collaboration with the U.S. 

Geological Survey (USGS) and Lawrence Livermore National 

Laboratory (LLNL), implemented the Groundwater Ambient 

Monitoring and Assessment (GAMA) Program (California 

State Water Resources Control Board, 2010, website at 

http://www.waterboards.ca.gov/gama/). The statewide 

GAMA Program currently consists of three projects: (1) the 

GAMA Priority Basin Project, conducted by the USGS (U.S. 

Geological Survey, 2010, website at http://ca.water.usgs.gov/

gama/); (2) the GAMA Domestic Well Project, conducted by 

the SWRCB; and (3) the GAMA Special Studies, conducted 

by LLNL. On a statewide basis, the GAMA Priority 

Basin Project focused primarily on the deep portion of the 

groundwater resource, and the SWRCB Domestic Well Project 

generally focused on the shallow aquifer systems. 

The SWRCB initiated the GAMA Program in 2000 

in response to a legislative mandate (State of California, 

1999, 2001a, Supplemental Report of the 1999 Budget Act 

1999–00 Fiscal Year). The GAMA Priority Basin Project was 

initiated in response to the Groundwater Quality Monitoring 

Act of 2001 to assess and monitor the quality of groundwater 

in California (State of California, 2001b, Sections 10780–

10782.3 of the California Water Code, Assembly Bill 599). 
Introduction 3

The GAMA Priority Basin Project is a comprehensive 

assessment of statewide groundwater quality designed to 

help better understand and identify risks to groundwater 

resources and to increase the availability of information 

about groundwater quality to the public. For the GAMA 

Priority Basin Project, the USGS, in collaboration with the 

SWRCB, developed a monitoring plan to assess groundwater 

basins through direct sampling of groundwater and other 

statistically reliable sampling approaches (Belitz and others, 

2003; California State Water Resources Control Board, 2003). 

Additional partners in the GAMA Priority Basin Project 

include the California Department of Public Health (CDPH), 

California Department of Pesticide Regulation (CDPR), 

California Department of Water Resources (CDWR), and 

local water agencies and well owners (Kulongoski and Belitz, 

2004).

The range of hydrologic, geologic, and climatic 

conditions that exists in California must be considered in a 

statewide assessment of groundwater quality. Belitz and others 

(2003) partitioned the state into 10 hydrogeologic provinces, 

each with distinctive hydrologic, geologic, and climatic 

characteristics (fig. 1A). All of these hydrogeologic provinces 

include groundwater basins and subbasins designated by 

the CDWR (California Department of Water Resources, 

2003). Groundwater basins generally consist of relatively 

permeable, unconsolidated deposits of alluvial or volcanic 

origin. Eighty percent of the approximately 16,000 active and 

standby drinking-water wells listed in the statewide database 

maintained by the CDPH (hereinafter referred to as CDPH 

wells) are located in CDWR-designated groundwater basins 

within these hydrologic provinces. Groundwater basins 

and subbasins were prioritized for sampling on the basis 

of the number of CDPH wells in the basin, with secondary 

consideration given to municipal groundwater use, agricultural 

pumping, the number of historically leaking underground 

fuel tanks, and registered pesticide applications (Belitz and 

others, 2003). Of the 472 basins and subbasins designated by 

the CDWR, 116 priority basins, representing approximately 

95 percent (%) of the CDPH wells located in basins, were 

selected for the project.

In addition, some areas outside of the designated 

groundwater basins were included to represent the 20% 

of CDPH wells not located in groundwater basins. Of the 

10 hydrogeologic provinces, the Sierra Nevada contains 

the largest number of CDPH wells outside of designated 

groundwater basins. About 97% of the total area and 

approximately 85% of the CDPH wells in the Sierra 

Nevada are outside of designated groundwater basins. 

The Tahoe-Martis, Central Sierra, and Southern Sierra 

study units are located in the Sierra Nevada hydrogeologic 

province (fig. 1A). All three study units include areas outside 

of the designated groundwater basins; the Tahoe-Martis 

and Southern Sierra study units also include several small 

CDWR-designated groundwater basins. 

The goal of the GAMA Priority Basin Project is to 

produce three types of water-quality assessments for each 

study unit: (1) Status: assessment of the current quality of the 

groundwater resource, (2) Understanding: identification of the 

natural and human factors affecting groundwater quality and 

explanation of the relations between water quality and selected 

explanatory factors, and (3) Trends: detection of changes 

in groundwater quality (Kulongoski and Belitz, 2004). 

The assessments are intended to characterize the quality of 

groundwater within the primary aquifer systems of the study 

unit, not the treated drinking water delivered to consumers by 

water purveyors. The primary aquifer systems (hereinafter, 

primary aquifers) for the study units are defined by the depths 

of the screened or open intervals of the wells listed in the 

CDPH database for the study units. The CDPH database lists 

wells used for municipal and community drinking-water 

supplies, and includes wells from systems classified as nontransient (such as cities, towns, and mobile-home parks) and 

transient (such as schools, campgrounds, and restaurants). 

Groundwater quality in shallower or deeper parts of the 

aquifer systems may differ from that in the primary aquifers. 

In particular, shallower groundwater may be more vulnerable 

to surface contamination. As a result, samples from shallow 

wells (such as many private domestic wells and environmental 

monitoring wells) typically have higher concentrations of 

constituents from anthropogenic sources [such as volatile 

organic compounds (VOCs) and nitrate] than samples from 

wells screened in the deeper primary aquifer.

Purpose and Scope

The purposes of this report are to provide (1) study unit 

description: descriptions of the hydrogeologic settings of the 

Tahoe-Martis, Central Sierra, and Southern Sierra study units, 

(2) status assessment: assessments of the status of the current 

quality of groundwater in the primary aquifers in the three 

study units, and (3) understanding assessment: identification 

of the natural and human factors affecting groundwater quality 

and explanation of the relations between water quality and 

selected explanatory factors. Water-quality data for samples 

collected by the USGS for the GAMA Program in the three 

study units and details of sample collection, analysis, and 

quality-assurance procedures are reported by Fram and 

Belitz (2007), Ferrari and others (2008), and Fram and others 

(2009). Untreated groundwater samples were collected from 

the three study units between May 2006 and October 2007. 

Utilizing those same data, this report describes methods used 

in designing the sampling network, identifying CDPH data 

for use in the status assessment, estimating aquifer-scale 

proportions for constituents, analyzing ancillary datasets, and 

assessing the status and understanding of groundwater quality 

by statistical and graphical approaches.
4 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac11-0340_fig 01a

PACIFIC OCEAN

Valley

Klamath

Mountains

Cascades and

Modoc Plateau

Transverse Ranges and

selected Peninsular Ranges

Southern Coast

Ranges

Sierra

Nevada

Northern

Coast

Ranges

Desert

Central

Basin and Range

San Diego

Drainages

TAHOE-MARTIS

GAMA

STUDY UNIT

(figure 2A)

TAHOE-MARTIS

GAMA

STUDY UNIT

(figure 2A)

CENTRAL SIERRA

GAMA

STUDY UNIT

(figure 2B)

CENTRAL SIERRA

GAMA

STUDY UNIT

(figure 2B)

SOUTHERN SIERRA

GAMA

STUDY UNIT

(figure 2C)

SOUTHERN SIERRA

GAMA

STUDY UNIT

(figure 2C)

A

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006. 

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

Provinces from Belitz and others (2003)

NEVADA

0 200 MILES

0 200 KILOMETERS

42°

40°

38°

36°

124° 122° 120° 118° 116° 114°

34°

OREGON

MEXICO

ARIZONA UTAH

BakersfieldBakersfield

Los AngelesLos Angeles

San DiegoSan Diego

Redding

SacramentoSacramento

San

Francisco

Figure 1. (A) Locations of the Tahoe-Martis, Central Sierra, and Southern Sierra study units, California GAMA 

Priority Basin Project, and the California hydrogeologic provinces, and (B) selected major geologic features in 

California.
Introduction 5

sac11-0340_fig 01b

Melo

n

e s Fault Zone

Western

Metamorphic

Belt

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006. 

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

Geology simplified from Saucedo and others (2000).

EXPLANATION

Geologic units

Mesozoic granitic rocks

Paleozoic and Mesozoic metamorphic and

 sedimentary rocks and non-granitic

 plutonic rocks

Cenozoic sedimentary deposits

Cenozoic volcanic and volcaniclastic rocks

PACIFIC OCEAN

OREGON

NEVADA

MEXICO

ARIZONA UTAH

Owens

Valley

Central

Valley

S a n

A n d r e a s

F a u l t

Garlock Fault

B

0 200 MILES

0 200 KILOMETERS

Bakersfield

Los Angeles

San Diego

Redding

Sacramento

San

Francisco

Figure 1.—Continued
6 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

The status assessment uses two methods for calculating 

the areal proportion of the primary aquifer with groundwater 

of defined quality (aquifer-scale proportion). Both methods 

are based on equal-area grid cells covering the study unit: 

one uses one well to represent each cell, and the other uses 

multiple wells to represent each cell. The first method is based 

on water-quality data from 103 wells selected by the USGS for 

spatial coverage of one well per grid cell across the three study 

units (USGS-grid wells). Samples were collected by the USGS 

for analysis of anthropogenic organic constituents, naturally 

occurring inorganic constituents, and geochemical and agedating tracers (Fram and Belitz, 2007; Ferrari and others, 

2008; Fram and others, 2009). Water-quality data from the 

CDPH database also were used to supplement data collected 

by the USGS for the GAMA Program. The resulting set of 

water-quality data from USGS-grid wells and selected CDPH 

wells was considered to be representative of the primary 

aquifer systems in the three study units. The second method 

uses the USGS-grid wells, data from 29 additional wells 

sampled by the USGS for the GAMA Program, and data from 

many wells in the CDPH database. GAMA status assessments 

are designed to provide a statistically robust characterization 

of groundwater quality in the primary aquifers at the 

study-unit scale (Belitz and others, 2003). The statistically 

robust design also allows study units to be compared and 

results to be synthesized at regional and statewide scales.

To provide context, the water-quality data discussed 

in this report are compared to California and Federal 

drinking-water regulatory and non-regulatory benchmarks 

for treated drinking water. Groundwater quality is defined in 

terms of relative-concentrations, the ratio of the concentration 

of a constituent in groundwater to the concentration of the 

benchmark for that constituent. The assessments in this report 

characterize the quality of untreated groundwater resources 

in the primary aquifers in the study units, not the treated 

drinking water delivered to consumers by water purveyors. 

After withdrawal from the ground, water typically is treated, 

disinfected, and (or) blended with other waters to maintain 

acceptable water quality. Regulatory benchmarks apply to 

treated water that is served to the consumer, not to untreated 

groundwater.

The understanding assessment is based on water-quality 

data from the 103 USGS-grid wells and the 29 additional wells 

sampled by the USGS for the GAMA Program. The potential 

explanatory factors evaluated are land use, septic-tank density, 

climate, relative position in the regional flow system, aquifer 

lithology, well depth and depth to the top of the screened or 

open interval in the well, groundwater age distribution, pH, 

and dissolved oxygen concentration. Connections between 

potential explanatory factors and water quality are evaluated 

using statistical tests for correlations and by analysis of 

graphical relations.

Definitions and Locations of Study Units

The Tahoe-Martis study unit covers an area of 

approximately 460 square miles (mi2) [1,200 square 

kilometers (km2)] in El Dorado, Placer, Nevada, and Alpine 

Counties, California, in the northeast part of the Sierra Nevada 

hydrogeologic province (figs. 1A, 2A). The Tahoe-Martis 

study unit was divided into three study areas: the Martis study 

area, the Tahoe study area, and the Hard Rock study area. 

The boundary of the Martis study area corresponds to the 

CDWR Martis Valley basin (California Department of Water 

Resources, 2006a), and the boundaries of the Tahoe study area 

correspond to the South, West, and North subbasins of the 

CDWR Tahoe Valley basin (California Department of Water 

Resources, 2004a,b,c). The Hard Rock study area corresponds 

to the surface-water watersheds surrounding the Martis and 

Tahoe Valley basins and Lake Tahoe.

The Central Sierra study unit covers an area of 

approximately 370 mi2 (960 km2) in Madera and Mariposa 

Counties, California, in the central part of the Sierra Nevada 

hydrogeologic province (figs. 1A, 2B). The study unit consists 

of two study areas defined by surface-water watersheds, 

and it contains no CDWR-designated groundwater basins. 

The Coarse Gold study area corresponds to the watershed of 

the Upper Fresno River upstream of Hensley Lake, and the 

Wishon study area corresponds to the watershed of the North 

Fork Willow Creek, a tributary of the San Joaquin River.

The Southern Sierra study unit covers an area of 

approximately 1,800 mi2 (4,700 km2) in Kern and Tulare 

Counties, California, at the southern end of the Sierra Nevada 

hydrogeologic province (figs. 1A, 2C). The study unit area is 

defined by the watersheds of Tehachapi and Caliente Creeks, 

the small creeks at the southern end of the Sierra Nevada that 

drain towards the Central Valley, the Middle and Upper Kern 

Rivers, and the East Tehachapi closed drainage basin. The 

study unit includes six small groundwater basins, as defined by 

the CDWR (California Department of Water Resources, 2003). 

Wells sampled in the southern part of the study unit were in 

or near the Cummings Valley, Brite Valley, Tehachapi Valley 

West, and Tehachapi Valley East groundwater basins, and 

wells sampled in the northern part of the study unit were in 

or nearby the Kern River Valley groundwater basin (fig. 2C). 

No wells were sampled in the Walker Creek Valley basin. The 

Southern Sierra study unit was not divided into study areas.

As part of the GAMA Priority Basin Project, 

untreated-groundwater samples were collected from 

52 sites in the Tahoe-Martis study unit during the period 

June–September 2007 (Fram and others, 2009), 30 sites in the 

Central Sierra study unit during May 2006 (Ferrari and others, 

2008), and 50 sites in the Southern Sierra study unit during 

June 2006 (Fram and Belitz, 2007). Of the 132 sites, 7 were 

springs, and 125 were wells. For ease of discussion, all sites 

are referred to as wells unless the difference between well and 

spring is important to the discussion.
Introduction 7

sac11-0349_fig 02a

A

Martis Valley

basin

Tahoe Valley

North subbasin

Tahoe Valley

South subbasin

Tahoe Valley

West subbasin

South Fork American River

Caples Creek

Upper Truckee RiverUpper Truckee River

Truckee River

Rubicon River

LAKE TAHOE

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006.

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

120° 15’ 120°

39° 30’

39° 15’

39° 45’

39°

Truckee

Tahoe City

South Lake Tahoe

Meyers

SIERRA CO

NEVADA CO

PLACER CO

EL DORADO CO

ALPINE CO

AMADOR

CO

TAHOE-MARTIS GAMA STUDY UNIT

Box outlines the part of fig. 2A that is

shown in fig. 4A and subsequent figures

showing the Tahoe-Martis study unit

0 5 10 MILES

0 105 KILOMETERS

Hard Rock study area (part of watershed

 area that is within 3 km of a CDPH well)

Study areas

EXPLANATION

NEVADA

CALIFORNIA

Study unit watershed

California Department of Water 

 Resources groundwater basins

Figure 2. Geographic features of the (A) Tahoe-Martis, (B) Central Sierra, and (C) Southern Sierra study units, 

California GAMA Priority Basin Project.
8 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac11-0349_fig 02b

Coarse Gold study area

Wishon study area

Area of study unit within 3 km of CDPH well

Watershed boundary

EXPLANATION

MARIPOSA CO

MADERA CO

B

37° 30’

37° 15’

119° 45’ 119° 30’

Ahwahnee

Oakhurst

Wawona

Chiquito CreekChiquito Creek

North Fork Willow CreekNorth Fork Willow Creek

East Fork Chowchilla RiverEast Fork Chowchilla River

Upper Fresno RiverUpper Fresno River

San JoaquinSan Joaquin

RiverRiver

Chow c hilC

la

how c hilla

River

North Fork Willow

Creek

North Fork Willow

Creek

Hensley

Lake

Bass Lake

FRESNO CO

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006,

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

0 5 10 MILES

0 5 10 KILOMETERS

Figure 2.—Continued
Introduction 9

sac11-0349_fig 02c

California Department of Water

 Resources groundwater basins 

Sampled study unit (the part of watershed

 area that is within 3 km of a CDPH well)

Study unit watershed area

EXPLANATION

C

0 5 10 20 MILES

0 5 10 20 KILOMETERS

Kern River

Valley

Walker Creek

Valley

Tehachapi

Valley East Tehachapi

Brite Valley West

Valley

Bear

Valley

Cummings

Valley

TULARE CO

INYO CO

KERN CO

LOS ANGELES CO

Tehachapi

Bakersfield

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006,

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

LOS ANGELES AQUEDUCTLOS ANGELES AQUEDUCT

CALIFORNIA AQUEDUCTCALIFORNIA AQUEDUCT

Tehachapi CreekTehachapi Creek

Caliente Creek

Kern River Kern River

Poso Creek so Creek

Deer CreekDeer Creek

South Fork Kern RiverSouth Fork Kern River

Middle Fork Kern

River

Middle Fork Kern

River

Lake

Isabella

118° 30’ 118°

35° 30’

35°

36°

119°

Box outlines the part of fig. 2C that is

shown in fig. 5C and subsequent figures

showing the Southern Sierra study unit

SOUTHERN SIERRA

GAMA STUDY UNIT

Figure 2.—Continued
10 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Hydrogeologic Setting

The Sierra Nevada hydrogeologic province encompasses 

a broad range of geologic, hydrologic, and land-use settings. 

The Sierra Nevada is an asymmetric mountain range that 

extends for 400 miles (mi) approximately parallel to the long 

axis of the State of California (fig. 1A). The western side 

slopes gradually from the crest towards the Central Valley, and 

the eastern side is a steep escarpment that marks the western 

edge of active extension in the Basin and Range province. The 

elevation of the Sierra Nevada crest is highest in the south, 

with several peaks over 14,000 feet (ft) [4,270 meters (m)], 

and decreases northward, with the highest peaks north of 

Lake Tahoe only about 8,000 ft high (2,440 m). The western 

boundary of the Sierra Nevada province is defined by the limit 

of Quaternary sediments of the Central Valley. The eastern 

boundary is defined by the limits of Quaternary sediments in 

the Owens Valley, the Nevada State line, and the watersheds 

of the basins between the Owens Valley and the Nevada State 

line. The province is terminated by the Garlock Fault at the 

southern end (fig. 1B) and by the Cascades and Modoc Plateau 

in the north (fig. 1A).

The dominant geologic feature of the province is the 

Sierra Nevada batholith, a complex of Mesozoic tonalite, 

granodiorite, quartz diorite, and granite plutons that intruded 

the North American Plate above the subducting Farallon Plate, 

mostly between 80 and 150 mega annum (Ma; million years 

ago) (Evernden and Kistler, 1970; Saleeby and others, 2008). 

Roof pendants of older Mesozoic and Paleozoic metamorphic 

rocks, remnants of the terrain into which the plutons 

intruded, are scattered throughout the batholith, particularly 

in the southern part of the province (fig. 1B). The Western 

Metamorphic Belt occupies the foothills in the northern half of 

the province and consists of a deformed package of imbricate 

thrust slices of Mesozoic and Paleozoic ophiolites and oceanic 

sedimentary rocks that were accreted onto the western margin 

of the North American Plate as subduction proceeded (Day 

and others, 1985). The Mother Lode gold deposits are hosted 

by quartz veins injected along the Melones Fault zone, a major 

structural feature that likely marks the Mesozoic subduction 

plate boundary. Late Cenozoic (approximately 35 Ma to 1 Ma) 

volcanism blanketed areas of the Sierra Nevada, particularly in 

the northern part of the province. By the Late Miocene (16 Ma 

to 12 Ma), the Sierra Nevada existed as a prominent rain 

shadow, indicating that most of the uplift of the modern Sierra 

Nevada range had occurred by that time (Crowley and others, 

2008; Mulch and others, 2008). Continued uplift has occurred 

within the last 5 Ma (Wakabayashi and Sawyer, 2001). 

Portions of the Sierra Nevada were glaciated in the multiple 

major glacial advances during the Pleistocene (Birkeland, 

1964; Raub and others, 2006).

The Sierra Nevada is dominated by natural land use. 

Approximately 40% of the area is managed by the U.S. 

Forest Service (USFS), and approximately 20% is under 

the jurisdiction of the National Park Service or the Bureau 

of Land Management (BLM) (Sierra Nevada Ecosystem 

Project, 1996). Most of the remaining 40% is privately owned. 

Agricultural land use constitutes a small percentage of total 

land use and is concentrated in the valleys of the southern 

Sierra Nevada and areas of the foothills adjacent to the Central 

Valley. Free-range stock grazing on leased USFS and BLM 

lands as well as on private land is common in many parts of 

the Sierra Nevada province, but is not counted as agricultural 

land use by the land-use classification system used in this 

report (see appendix C). Urban land use also constitutes a 

small percentage of the total land use, but this category is 

growing rapidly. The permanent population of the province 

rose from 0.3 million in 1970 to 0.8 million in 2000 and 

is predicted to grow to 1.7–2.0 million by 2040 (Duane, 

1996a; Sierra Nevada Ecosystem Project, 1996). Much of the 

population growth has occurred along the Interstate 80 and 

Highway 50 corridors (highways that connect Sacramento 

to Reno, Nevada, and to the southern end of Lake Tahoe, 

respectively (fig. 1A). As of 1990, 80% of the permanent 

population lived on 3% of the land area. Recreation and 

tourism are the primary industries in the Sierra Nevada, and 

the resulting transient population is much larger than the 

permanent population. For example, recreational use of public 

lands was estimated at 50 to 60 million visitor days in 1993 

(Duane, 1996b), and was predicted to be 122 million visitor 

days in 2010 for USFS lands alone (U.S. Forest Service, 

2001).

Like much of California, the Sierra Nevada has a 

Mediterranean climate pattern: warm, dry summers and 

cool, wet winters. Annual precipitation ranges from 10 to 

80 inches per year (in/yr) [25 to 200 centimeters per year (cm/

yr)] and varies with elevation and latitude (PRISM Group, 

Oregon State University, 2007). Most of the precipitation 

falls in the winter season, between October and April. 

Precipitation is typically greater at higher elevations and 

more northerly locations. Above about 6,500 ft (1,980 m), 

most of the precipitation falls as snow. Runoff from Sierra 

Nevada watersheds, primarily in the form of snow melt, 

provides approximately 50% of California’s developed water 

(Carle, 2004).

Groundwater is used extensively for municipal, 

community, and domestic drinking-water supplies in the Sierra 

Nevada, and much of this groundwater comes from hard-rock 

aquifers rather than from groundwater basins. Approximately 

3% of the Sierra Nevada hydrogeologic province consists of 

CDWR-defined groundwater basins; the other 97% consists 

primarily of granitic and metamorphic rocks, with lower 

amounts of volcanic rocks and sediment deposits not in 

groundwater basins. Granitic and metamorphic rocks of the 

Sierra Nevada have low permeability except where fractured. 

Fractures and joints typically are more extensive in size and 

number in the upper few hundred feet of bedrock and typically 

decrease with depth (Page and others, 1984; Borchers, 1996) 
Hydrogeologic Setting 11

sac11-0349_fig 03

Joints, fractures,

and exfoliation

planes

Snow

Rain

Alluvial fan

Alluvial basin

aquifer

Direct infiltration

Fractured of precipitation

bedrock aquifer

Mountain-block

recharge

Mountain-front recharge

Recharge from

and discharge

to streams

Figure 3. Conceptual model of groundwater flow in a fractured-bedrock aquifer, and recharge of a small alluvial 

basin adjacent to mountains consisting of fractured rock.

(fig. 3). Fracture permeability tends to decrease with depth 

because of increased lithostatic pressure (Ingebritsen and 

Sanford, 1998); however, because crystalline rocks remain 

brittle to depths of several kilometers, some fracture 

permeability may persist to great depths (Freeze and Cherry, 

1979). The three-dimensional complexity and variability of 

fracture systems can cause well yields and water quality to 

vary widely on a local scale. 

Although groundwater basins compose a small part 

of the province area, they typically have a high density of 

groundwater use because they commonly contain population 

centers and have wells with much greater yields than those 

in the surrounding fractured-rock aquifers. Recharge to 

groundwater basins of the Sierra Nevada has two primary 

modes (fig. 3): (1) mountain-front recharge, which is runoff 

from precipitation on the mountain that percolates into the 

basin through the coarser alluvial fan deposits along the 

basin margin and through stream channels crossing the basin; 

and (2) mountain-block recharge, which is water that has 

percolated through the mountain bedrock aquifer and enters 

the basin from the subsurface. Precipitation falling directly 

on the basin percolates vertically downward through the 

sediments of the basin or enters stream channels.

The Tahoe-Martis, Central Sierra, and Southern Sierra 

study units represent three areas of the province with distinct 

geologic, hydrologic, and land-use characteristics. 
12 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Tahoe-Martis Study Unit

Lake Tahoe and the Tahoe Valley and Martis Valley 

groundwater basins lie in a structural valley between the 

Sierra Nevada crest to the west and the Carson Range to the 

east (fig. 4A). The average surface elevation of Lake Tahoe is 

6,225 ft (1,897 m), and the lowest area in the Martis Valley 

basin lies at 5,700 ft (1,740 m). The mountains bounding the 

valleys rise to over 9,500 ft (2,900 m). The valley was formed 

by extensional faulting that resulted in approximately 5,000 ft 

(1,525 m) of vertical displacement of the Sierra Nevada crest 

and Carson Range relative to the valley floor that occupies 

the graben between the two mountain ranges. Uplift of the 

Sierra Nevada crest occurred during the Miocene (Crowley 

and others, 2008; Mulch and others, 2008) and Pliocene (1.8 

to 5 Ma; Birkeland, 1963), although displacement along faults 

around Lake Tahoe has continued through the Pleistocene and 

Holocene to the present (Kent and others, 2005).

The bedrock in the Lake Tahoe area is primarily 

composed of Mesozoic (80 to 120 Ma) granitic rocks, mostly 

granodiorite plutons within the Sierra Nevada batholith 

(fig. 4A). Included within the granitic rocks are scattered 

remnants of the older metamorphic rocks into which the 

granitic plutons intruded. Uplift of the Sierra Nevada and 

Carson Range was accompanied by volcanic activity. Thick 

sequences of Miocene to Pleistocene (10 to 1 Ma) volcanic 

rocks cover the granitic bedrock in the Martis Valley basin 

and watershed area, and in the Lake Tahoe watershed 

approximately north of the Placer–El Dorado county line 

(Birkeland, 1963; Cousens and others, 2008). The older 

volcanic rocks are dominated by andesite tuffs, breccias, 

and lavas, and the younger rocks by basalt and latite lavas 

and cinder cones. Lake Tahoe is impounded by a dam of 

lava flows.

The Lake Tahoe and Truckee areas preserve evidence of 

four major glacial advances during the Pleistocene (fig. 2A; 

Birkeland, 1964). The topography of the Tahoe Valley 

subbasins and Martis Valley basin is dominated by glacial 

moraines and outwash plains, primarily from the Tahoe (70 to 

150 kilo annum (ka; thousand years ago) and Tioga (14 to 

26 ka) glaciations. The glaciers also formed ice dams that 

episodically raised the level of the lake as much as 600 ft 

above the current lake level, as evidenced by wave-cut terraces 

high on the mountains around the lake.

Average annual precipitation at stations in Truckee and 

on the California shores of Lake Tahoe is 30–35 in/yr 

(76–89 cm/yr). (Note that snowfall has been converted to 

equivalent depth of water.) Precipitation increases to an 

average of more than 80 in/yr (200 cm/yr) at the highest 

elevations on the western side of the Lake Tahoe watershed 

(Crippen and Pavelka, 1970; California Department of Water 

Resources, 2008; Western Regional Climate Center, 2008). 

At Lake Tahoe approximately half of the precipitation falls 

as snow, and at the highest elevations, nearly all of the 

precipitation falls as snow. Ninety percent of the precipitation 

falls between October and April (California Department 

of Water Resources, 2008; Western Regional Climate 

Center, 2008).

The groundwater-bearing units in the South, West, and 

North subbasins of the Tahoe Valley basin consist of Pliocene 

to Holocene glacial, lacustrine, and fluvial deposits, referred 

to as basin-fill deposits (Burnett, 1971; California Department 

of Water Resources, 2004a,b,c). The predominant sediments in 

the basins are glacial outwash sediments that were deposited 

on prograding deltas as the glaciers retreated. The outwash 

sediments are composed of sorted, stratified material; the 

grain size ranges from boulders to fine silt. Glacial tills also 

were deposited in lateral and terminal moraines. In the Tahoe 

Valley South subbasin, the sediments were derived primarily 

from the granitic rocks in the surrounding Sierra Nevada and 

Carson Ranges. Fluctuations in the level of Lake Tahoe during 

the glacial periods resulted in interbedding of lacustrine and 

glacial deposits. The lacustrine deposits range from coarse 

sand to silt and clay layers. The fine-grained lacustrine 

deposits constitute discontinuous confining layers within 

the basin fill. The relatively thin Holocene fluvial deposits 

in the modern stream channels do not contribute much to 

groundwater storage in the basins (California Department of 

Water Resources, 2004a).

The basin-fill deposits typically range from 50 to 150 ft 

(15 to 45 m) thick in the Tahoe Valley South subbasin, but 

thicknesses up to 1,600 ft (490 m) have been measured near 

the city of South Lake Tahoe (California Department of 

Water Resources, 2004a). Sediment thicknesses in the Tahoe 

Valley North and West subbasins are less well known, but 

are thought to range from 20 ft (6 m) to as much as 200 ft 

(60 m) (California Department of Water Resources, 2004b,c). 

Groundwater conditions range from unconfined to semiconfined to confined.

Groundwater is recharged primarily by infiltration 

of rain and snow melt. Recharge occurs through the 

surface of the basin-fill deposits, and from high elevations 

through the fractures and faults in the bedrock and surface 

soils (Thodal, 1997). Temperature profiles in wells in the 

South Tahoe subbasin and surrounding bedrock suggest 

that mountain-block and mountain-front recharge (fig. 3) 

contribute to recharge of the basin (Fogg and Trask, 2007). 

Streamflow characteristics of the Upper Truckee River and 

creeks in the Tahoe Valley South subbasin suggest that these 

streams are net “gaining” streams, receiving groundwater 

discharge as they traverse the subbasin (Rowe and 

Allander, 2000).
Hydrogeologic Setting 13

sac09-0340_fig 04a

Mesozoic granitic rocks

Cenozoic sedimentary deposits

USGS-grid

USGS-grid and CDPH-DG-grid

CDPH-DPH-grid

USGS-understanding

Gold

Uranium

Cenozoic volcanic and volcaniclastic rocks

Paleozoic and Mesozoic metamorphic rocks

and non-granitic plutonic rocks

Geologic units

Wells

Mines or mineral prospects

EXPLANATION

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006. 

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

Geologic units simplified from Saucedo 

and others (2000) and Crafford (2007).

Mines and mineral prospects from U.S. 

Geological Survey (2005).

0 5 10 MILES

0 5 10 KILOMETERS

39° 15’

120° 15’ 120°

39°

NEVADA CO

WASHOE CO

CARSON CO

DOUGLAS CO

PLACER CO

EL DORADO CO

ALPINE CO

NEVADA

CALIFORNIA

A

Martis and Tahoe study area boundary

Hard Rock study area boundary

LAKE TAHOE

Figure 4. Geologic features and areal distribution of USGS-grid, CDPH-grid, and USGS-understanding wells sampled 

in the (A) Tahoe-Martis, (B) Central Sierra, and (C) Southern Sierra study units, California GAMA Priority Basin Project.
14 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac09-0340_fig 04b

Mesozoic granitic rocks

Cenozoic sedimentary deposits

USGS-grid

USGS-grid and CDPH-DG-grid

CDPH-DPH-grid

USGS-understanding

Gold

Uranium

Cenozoic volcanic and volcaniclastic Tungsten

rocks

Paleozoic and Mesozoic metamorphic

rocks and non-granitic plutonic rocks

Geologic units

Study area

 boundary

Wells Mines or mineral

 prospects

EXPLANATION

Boundary of

 Bass Lake

 Tonalite

Not within

 Bass Lake

 Tonalite

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006,

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

Ahwahnee

Oakhurst

Wawona

37° 30’

37° 15’

119° 45’ 119° 30’

B

MADERA CO

MARIPOSA CO

FRESNO CO

0 5 10 MILES

0 5 10 KILOMETERS

Geologic units simplified from Saucedo

and others (2000) and Bateman (1992).

Mines and mineral prospects

from U.S. Geological Survey (2005).

Figure 4.—Continued
Hydrogeologic Setting 15

sac09-0340_fig 04c

CALIFORNIA AQUEDUCTCALIFORNIA AQUEDUCT

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006,

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD83)

C 118° 30’ 118°

35° 30’

35°

36°

119°

LOS ANGELES AQUEDUCTLOS ANGELES AQUEDUCT

Geologic units simplified from 

Saucedo and others (2000).

Mines and mineral prospects from 

U.S. Geological Survey (2005).

0 5 10 20 MILES

0 5 10 20 KILOMETERS

Tehachapi

Bakersfield

Gold

Uranium

Tungsten

Mines or mineral prospects

Mesozoic granitic rocks

Cenozoic sedimentary deposits

USGS-grid

USGS-grid and CDPH-DG-grid

CDPH-DPH-grid

USGS-understanding

Cenozoic volcanic and volcaniclastic rocks

Paleozoic and Mesozoic metamorphic rocks

and non-granitic plutonic rocks

Geologic units Watershed boundary

Wells

EXPLANATION

Study unit boundary

Figure 4.—Continued
16 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

The groundwater-bearing units in the Martis Valley 

basin consist of interlayered volcanic and sedimentary 

deposits of late Miocene to late Pleistocene age (7 to < 1 Ma) 

(Nimbus Engineers, 2001; California Department of Water 

Resources, 2006a). The sedimentary deposits are composed 

of glacial, lacustrine, and fluvial deposits with depositional 

characteristics similar to those of the basin-fill sedimentary 

deposits in the Tahoe Valley subbasins. The primary source of 

sediment deposited in the Martis Valley basin is the andesitic 

volcanic and volcaniclastic units located in and around the 

basin. The glacial outwash sediments that cover approximately 

half of the surface of the Martis Valley basin are up to 150 ft 

(45 m) thick (Birkeland, 1964). The volcanic units include 

andesitic lavas, tuffs, breccias, and volcaniclastic deposits.

The water-bearing sedimentary and volcanic units in 

the Martis Valley are up to 1,200 ft (365 m) thick and are 

underlain by low-permeability Miocene volcanic rocks. 

The stratigraphy of the basin is complex, but it is typically 

separated into upper and lower aquifer systems that appear 

to have limited interconnection (Nimbus Engineers, 2001). 

Groundwater recharge occurs primarily by infiltration of rain 

and snow melt in the basin and in the mountains surrounding 

the basin (fig. 3).

The groundwater-bearing units in the Hard Rock study 

area are the Mesozoic granitic rocks in the southern part and 

the Miocene to Pleistocene volcanic rocks in the northern part 

(fig. 4A). Extensive faulting on the margins of the structural 

valley containing Lake Tahoe and the Tahoe Valley and 

Martis Valley groundwater basins likely contributes to the 

permeability of the granitic and volcanic rocks. In addition, 

most of the volcanic rocks are stratified, and some have zones 

of high permeability between lava flow units and within some 

of the volcaniclastic deposits.

Land use in 88% of the Tahoe-Martis study unit 

is classified as undeveloped (figs. 5A, 6A). Most of the 

undeveloped areas are forested. At the highest elevations, 

which occur on the western side of the Hard Rock study 

area, the forests give way to alpine scrub vegetation and bare 

rock. The center of the Martis Valley has open grassland 

and shrubland. No cultivated agriculture occurs within the 

study unit, although some undeveloped lands are used for 

open-range cattle grazing (appendix C). Urban land use 

composes 12% of the study unit area and is concentrated in 

the Tahoe Valley South subbasin, along the shoreline of Lake 

Tahoe, and in the western half of the Martis Valley basin. 

Central Sierra Study Unit

The Central Sierra study unit area is defined by the 

surface-water watersheds of the Upper Fresno River and 

North Fork Willow Creek, a tributary of the San Joaquin River 

(fig. 2B). The Upper Fresno River watershed (Coarse Gold 

study area) ranges in elevation from about 6,000 ft (1,800 m) 

at the headwaters to about 500 ft (150 m) near Hensley Lake. 

The Willow Creek watershed (Wishon study area) ranges in 

elevation from about 8,500 ft (2,600 m) at the headwaters to 

about 1,500 ft (460 m) where the creek joins the San Joaquin 

River. Willow Creek flows through Bass Lake, a reservoir 

created for hydroelectric power generation.

Most of the Central Sierra study unit is underlain by 

granitic rocks, with a few areas underlain by metamorphic 

rocks (fig. 4B). Most of the granitic rocks are part of the 

Bass Lake Tonalite, a 114-Ma pluton that outcrops across 

over 800 mi2 (2,000 km2) in the central part of the Sierra 

Nevada batholith (Bateman, 1988). The Bass Lake Tonalite 

is primarily composed of hornblende-biotite tonalite, and 

includes facies ranging from granite to quartz diorite in 

composition. The study unit also includes smaller trondhjemite 

and granodiorite plutons related to the Bass Lake Tonalite. 

The arcuate band of metamorphic rocks in the center of the 

study unit is the Coarsegold roof pendant, which consists of 

metasedimentary and metavolcanic rocks (Bateman, 1992). 

The metasedimentary rocks are primarily slates and schists 

with lesser amounts of chert, marble, and metagreywacke, 

and the metavolcanic rocks are primarily pillow lavas with 

lesser amounts of tuff and breccia. The slates and schists 

contain masses of metagabbro that are likely coeval with the 

metavolcanic rocks.

The Central Sierra study unit is located southeast of the 

southeastern end of the Sierra Nevada’s Mother Lode Gold 

Belt. Placer and lode gold mining in the study unit began in 

1849, with most prospects located in a belt near the center of 

the study unit (fig. 4B; U.S. Geological Survey, 2005). The 

mines in the study unit were small compared to mines in the 

Mother Lode; between 1848 and 1965, gold mines in Madera 

County produced < 2% of the amount of gold produced in 

Nevada County, the richest county in the Mother Lode (Clark, 

1970). Most of the gold was found in quartz veins located near 

the contacts between granodiorite plutons and pendants of 

schist.
Hydrogeologic Setting 17

sac09-0340_fig 05a

A

NEVADA CO

WASHOE CO

CARSON CO

DOUGLAS CO

PLACER CO

EL DORADO CO

ALPINE

CO

NEVADA

CALIFORNIA

Truckee

Martis and Tahoe study area boundary

Hard Rock study area boundary

Urban

Undeveloped forest

USGS-grid

USGS-grid and CDPH-DG-grid

CDPH-DPH-grid

USGS-understanding

Undeveloped rock, shrub and grass

Agricultural

Land use

Wells

EXPLANATION

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006. 

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

Land use simplified from

Nakagaki and others (2007).

39° 15’

120° 15’ 120°

39°

0 5 10 MILES

0 5 10 KILOMETERS

South Lake Tahoe

Meyers

Tahoe City

LAKE TAHOE

Figure 5. Land use and areal distribution of USGS-grid, CDPH-grid, and USGS-understanding wells sampled 

in the (A) Tahoe-Martis, (B) Central Sierra, and (C) Southern Sierra study units, California GAMA Priority Basin 

Project.
18 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac09-0340_fig 05b

USGS-grid

USGS-grid and CDPH-DG-grid

CDPH-DPH-grid

USGS-understanding

Wells

EXPLANATION

Urban

Undeveloped forest

Undeveloped rock, shrub and grass

Agricultural

Land use

Study unit boundary

MARIPOSA CO

MADERA CO

FRESNO CO

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006,

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

37° 30’

37° 15’

119° 45’ 119° 30’

B

Oakhurst

Ahwahnee

Wawona

0 5 10 MILES

0 5 10 KILOMETERS

Land use simplified from

Nakagaki and others (2007).

Figure 5.—Continued
Hydrogeologic Setting 19

sac09-0340_fig 05c

Urban

Undeveloped forest

USGS-grid

USGS-grid and CDPH-DG-grid

CDPH-DPH-grid

USGS-understanding

Undeveloped rock, shrub, and grass

Agricultural

Land use

Study unit boundary

Wells

EXPLANATION

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006,

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

Land use simplified from

Nakagaki and others, 2007.

C

10 MILES

50 10 KILOMETERS

0 5

35°30’

35°15’

35°

118°30’ 118°15’

36°

35°45’

Tehachapi

Figure 5.—Continued
20 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac09-0340_fig 06a

0 20 40 60 80 100

PERCENT URBAN

0

20

40

60

80

100 0

20

40

60

80

100

PERCENT AGRICULTURE

PERCENT UNDEVELOPED

EXPLANATION

Study unit Data points:

TahoeMartis

Central

Sierra

Southern

Sierra

Average for grid wells

Average for understanding wells

Average for all California

Department of Public Health wells

Land use in entire area

A

Figure 6. Percentage of urban, agricultural, and undeveloped land use in 

the Tahoe-Martis, Central Sierra, and Southern Sierra study units, California 

GAMA Priority Basin Project in (A) the study units, and (B) the areas 

surrounding each USGS-grid and USGS-understanding well.
Hydrogeologic Setting 21

sac09-0340_fig 06b

0

20

40

60

80

100

0

0

PERCENT UNDEVELOPED

20

40

60

80

100

20

0

40

60

80

100

0 20 40 60 80 100

PERCENT AGRICULTURE

PERCENT URBAN

20

40

60

80

100

EXPLANATION

Tahoe-Martis study unit wells Central Sierra study unit wells Southern Sierra study unit wells

grid

understanding

Hard Rock grid and understanding

Martis grid and understanding 

Tahoe grid and understanding

Coarse Gold grid and understanding

Wishon grid and understanding

B

Figure 6.—Continued
22 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

The other commodity mined in Madera County was 

tungsten. The largest tungsten mine was northeast of the 

study unit, but there were numerous tungsten prospects within 

the study unit, particularly around Bass Lake (fig. 4B; U.S. 

Geological Survey, 2005). Mining activity peaked during 

World Wars I and II because tungsten is a component of 

high-strength steel alloy (Partridge, 1941). The tungsten 

occurs in the mineral scheelite (CaWO4) and is found in tactite 

masses along and near the contacts between granodiorite 

plutons and calcareous schists and marbles in xenoliths and 

pendants of metamorphic rocks (Krauskopf, 1953). Tactite is a 

skarn deposit, a rock type composed primarily of calcium-rich, 

silica-poor silicate minerals that forms in contact metamorphic 

aureoles in calcareous rocks.

The dominant source of groundwater in the study unit is 

the granitic rock. The granitic rocks are mantled by 0–100 ft 

(0–30 m) of grus (decomposed granitic rocks) and thin, 

discontinuous soils; however, these units typically are less 

permeable than the granitic bedrock (California Department 

of Water Resources, 1966). Groundwater moves through open 

joints, fractures, and exfoliation planes in the granitic bedrock 

(fig. 3). These fracture systems may be interconnected or 

isolated, resulting in variability in water levels, well yields, 

and water quality on local and regional scales. Groundwater 

recharge occurs primarily by infiltration of precipitation and 

snowmelt runoff, and seepage from surface-water bodies. 

Average annual precipitation ranges from < 20 in/yr (50 cm/yr) 

near Hensley Lake in the foothills to > 70 in/yr (180 cm/yr) in 

the highest elevations within the study unit (Todd Engineers, 

2002). More than half of the average annual precipitation 

falls during the months of December, January, and February 

(California Department of Water Resources, 2006b).

Land use in 95% of the Central Sierra study unit is 

classified as undeveloped (figs. 5B, 6A). The area is mainly 

forested in the higher elevations and is shrubland or grassland 

in the lower elevations. No cultivated agriculture occurs 

within the study unit, although some undeveloped lands are 

used for open-range cattle grazing (appendix C). Residential 

and commercial development composes 5% of the land use 

and is focused along major transportation routes. Population 

in eastern Madera County has increased from 5,853 in 

1970 to 36,376 in 1997, which has increased demand on 

the groundwater resources (Oakhurst Area Chamber of 

Commerce, 2006).

Southern Sierra Study Unit

The Southern Sierra study unit can be divided into three 

general hydrogeologic settings: the southern groundwater 

basins, the northern groundwater basin, and areas outside of 

the basins. 

The southern part of the Southern Sierra study unit is a 

seismically active area near the intersection of the San Andreas 

and Garlock Faults, two of California’s most prominent 

structural features (fig. 1B). Strain caused by motion on the 

San Andreas, Garlock, and White Wolf Faults appears to be 

partially accommodated by motion along many small faults 

in the region, which may have contributed to the formation 

of fault-bounded basins. The Bear Valley, Cummings Valley, 

Brite Valley, Tehachapi Valley East and West (hereinafter 

called the Tehachapi Valley) basins (fig. 2C) are all at least 

partially bounded by mapped faults (fig. 4C; Dibblee and 

Louke, 1970; Dibblee and Warne, 1970; Jennings, 1977). 

The basins contain alluvial fan and fluvial deposits derived 

from the erosion of the surrounding mountains. The oldest 

sedimentary deposits are Miocene in age (approximately 

20 Ma) and roughly concurrent with the initiation of strike-slip 

motion on the San Andreas fault. Recent earthquakes in the 

area, such as the magnitude 7.3 Kern County earthquake 

on the White Wolf fault in 1952, indicate that the basins are 

continuing to develop.

The primary groundwater-bearing units in the basins 

are alluvial sediments of Pleistocene to Holocene age (2 Ma 

to present). Creeks draining the Tehachapi Mountains and 

Sierra Nevada have deposited alluvial fans around the margins 

of the basins and floodplain deposits in the centers of the 

basins (Michael and McCann, 1962; Dibblee and Louke, 

1970; Dibblee and Warne, 1970; California Department of 

Water Resources, 2006c,d,e,f). Lithologically, the sediments 

are arkosic cobbles, gravels, sands, silts, and clays, with the 

coarser materials in the alluvial fans and the finer sediments 

in the floodplains. Maximum thicknesses of these alluvial 

deposits are approximately 500 ft (150 m) in the Cummings 

Valley and Brite Valley basins (Fugro-West Inc., ETIC 

Engineering, 2004; California Department of Water Resources, 

2006c,d) and > 600 ft (180 m) in the Tehachapi Valley basin. 

The Tehachapi basins have longer depositional histories 

than the Cummings Valley and Brite Valley basins, and have 

Miocene-Pliocene alluvial, fluvial, and pyroclastic deposits 

that are also water-bearing (Michael and McCann, 1962; 

Dibblee and Louke, 1970; California Department of Water 

Resources, 2006e,f). The eastern portion of the Tehachapi 

Valley basin has extensive Miocene-age volcanic rocks, 

primarily andesite lavas and flow breccias (fig. 4C; Dibblee 

and Louke, 1970).
Hydrogeologic Setting 23

Natural recharge occurs by infiltration of precipitation in 

the basins and by infiltration from the perennial and ephemeral 

creeks that drain the surrounding mountains (California 

Department of Water Resources, 2006c,d,e,f); however, 

the relative importance of natural recharge is decreasing as 

artificial recharge from irrigation and intentional recharge 

programs increases. Agricultural lands and urban/suburban 

landscapes in the basins are extensively irrigated.

The basins are managed collectively by the 

Tehachapi-Cummings County Water District (TCCWD). 

Since 1973, TCCWD has imported water from the State 

Water Project (SWP), and the Cummings Valley, Brite Valley, 

Tehachapi Valley, and Bear Valley (hereinafter referred 

to collectively as the Tehachapi-Cummings Valley) are 

all now supplied with imported water. The availability of 

imported water has changed the water-use patterns in the 

TCCWD region. The SWP water typically has relatively high 

concentrations of organic carbon, and disinfection of this 

water by municipalities in the basin produces unacceptable 

levels of disinfection byproducts. To avoid this problem, 

TCCWD manages an extensive conjunctive-use program 

in which SWP water allocated to municipal users in the 

TCCWD service area is used for artificial recharge and the 

municipal users pump an equivalent amount of groundwater 

(Tehachapi-Cummings County Water District, 2004a,b). 

SWP water is primarily recharged in spreading basins on the 

alluvial fans in the northeast corner of the Cummings Valley 

and the southern side of the Tehachapi Valley and in Lake 

Jacobsen on the southern side of Brite Valley. Since the arrival 

of imported water in 1973, groundwater levels in the TCCWD 

area have increased from 55 to 100 ft (from 17 to 30 m) and 

typically have returned approximately to the levels measured 

in 1950, prior to extensive groundwater pumping in the area 

(Tehachapi-Cummings County Water District, 2004a,b).

The Kern River Valley groundwater basin in the northern 

part of the Southern Sierra study unit has an irregular shape 

that follows the drainage pattern of the main stem and south 

fork of the upper Kern River (figs. 2C, 4C). The main stem 

lies in the trace of the Kern Canyon Fault from the river’s 

headwaters on the western slopes of Mt. Whitney (the highest 

peak in the conterminous 48 states) approximately 80 miles 

north of the study unit to Lake Isabella. The Kern Canyon 

Fault originally formed concurrently with the surrounding 

plutons and is still a major topographic and geologic 

discontinuity in the southern Sierra Nevada (Saleeby and 

others, 2008). Lake Isabella is impounded by Isabella Dam, 

which was constructed in 1953 by the U.S. Army Corps of 

Engineers, and is operated as part of the Federal Central 

Valley Project.

The primary water-bearing units are the Pleistocene to 

Holocene fluvial deposits in the channel of the Kern River 

and around Lake Isabella (California Department of Water 

Resources, 2006g). Natural recharge occurs by infiltration of 

precipitation falling in the basin and surrounding watershed, 

and infiltration of water from the Kern River. Landscape and 

agricultural irrigation are additional sources of recharge. 

In areas of the Southern Sierra study unit outside of the 

defined groundwater basins, groundwater is pumped from 

local aquifers in fractured hard rock (fig. 3). The Mesozoic 

granitic rocks, Paleozoic to Mesozoic metamorphic rocks, and 

the Miocene volcanic units are all locally water bearing. Most 

of the metamorphic rocks are part of the Kings Sequence, 

which is primarily composed of high metamorphic grade 

quartzites, marbles, and pelitic schists (Ross, 1989; Saleeby 

and others, 2008). Metamorphic grade increases sharply at the 

southern end of the Sierra Nevada, and metamorphic rocks 

around Brite Valley are primarily hypersthene granulites. 

The Mesozoic granitic rocks in the Southern Sierra study 

unit consist of numerous plutons ranging in age from 94 to 

105 Ma (Saleeby and others, 2008). Most of the plutons are 

tonalites or granodiorites, but plutons ranging in composition 

from granite to gabbro are present locally. As in the TahoeMartis and Central Sierra study units, groundwater in the 

granitic and metamorphic rocks of the Southern Sierra study 

unit moves through joints and fractures. The Kern River 

Valley region of the Southern Sierra study unit has numerous 

sites of lode gold and lode tungsten mining and prospecting, 

primarily associated with quartz veins and fracture systems at 

the boundaries between the granitic rocks and metamorphic 

roof pendants and within nearby granitic rocks (Troxel and 

Morton, 1962).

The study unit has approximately 8,000 ft (2,400 m) 

of topographic relief. The Cummings Valley and Tehachapi 

Valley basins are relatively flat with an elevation of 

approximately 4,000 ft (1,200 m), and the Tehachapi 

Mountains to the south rise over 8,000 ft (2,400 m). Average 

water level in Lake Isabella is 2,600 ft (790 m), and the peaks 

of the Sierra Nevada rise up to 10,000 ft (3,000 m) 40 mi 

(64 km) north of the lake. Precipitation at Lake Isabella 

Dam (elevation 2,635 ft; 803 m) and in the city of Tehachapi 

(4,017 ft; 1,224 m) averages 11 in/yr (28 cm/yr), with 80 

to 90% falling between November and April (California 

Department of Water Resources, 2007). Winter precipitation 

falls mostly as snow at elevations above 5,000 ft (1,500 m).

Land use in the Southern Sierra study unit is 90% 

undeveloped and is mostly shrublands and grasslands 

(figs. 5C, 6A). Some undeveloped lands are used for 

open-range cattle grazing (appendix C). Cultivated agricultural 

land use composes 8% of the area and is concentrated in the 

centers of the Tehachapi Valley and Cummings Valley and at 

the eastern end of the Kern River Valley basin. The dominant 

agricultural crops are sod, oats and alfalfa for fodder, carrots, 

and apples (Kern County Department of Agriculture and 

Measurement Standards, 2000). Urban land use, mostly 

residential, composes 2% of the area and occurs primarily 

in the Tehachapi Valley and Cummings Valley and the Kern 

River Valley basins.
24 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Methods

This section describes the methods used in the status

and understanding assessments. Methods used to collect and 

analyze groundwater samples and results for quality-control 

assessment for the analytes listed in table 1 are described by 

Fram and Belitz (2007), Ferrari and others (2008), and Fram 

and others (2009). Methods used for compilation of data on 

potential explanatory factors are described in appendix C.

Status Assessment

The status assessment was designed to quantify 

groundwater quality in areal proportions of the primary 

aquifer. This section describes the methods used for: 

(1) defining groundwater quality, (2) assembling the datasets 

used for the assessment, (3) determining which constituents 

warrant additional evaluation, and (4) calculating aquifer-scale 

proportions. 

Groundwater quality was defined in terms of 

relative-concentration (RC), which references the 

concentrations of constituents in groundwater to the 

concentrations of regulatory and non-regulatory benchmarks 

used to evaluate drinking-water quality. Constituents were 

selected for additional evaluation in the status assessment

on the basis of objective criteria by using these RCs. 

Groundwater-quality data collected by the USGS for the 

GAMA Priority Basin Project (USGS-GAMA) and data 

compiled from the CDPH database were used in the status 

assessment. Two statistical methods were used to calculate 

the areal proportions of the primary aquifer occupied 

by groundwater with high, moderate, or low RCs for a 

constituent or constituent class (aquifer-scale proportions): 

(1) the “grid‐based” method, which uses one value per cell to 

represent groundwater quality (Belitz and others, 2010), and 

(2) the “spatially weighted” method, which uses many values 

per cell.

The CDPH database contains records from more than 

25,000 wells, necessitating targeted retrievals to access 

water-quality data effectively. For example, for the areas 

representing the Tahoe-Martis, Central Sierra, and Southern 

Sierra study units, the CDPH database contains more than 

260,000 analyses from 755 wells over a period of 25 years. 

The CDPH data were used in three ways in the status 

assessment: (1) to fill in gaps in USGS-GAMA data for 

the grid-based calculations of aquifer-scale proportions, 

(2) to help identify constituents for additional evaluation in 

the assessment, and (3) to provide the majority of the data 

used in the spatially weighted calculations of aquifer-scale 

proportions. 

Relative-Concentrations and Water-Quality 

Benchmarks

To provide context for water-quality data, measured 

concentrations of constituents may be compared to 

water‐quality benchmarks that are typically applied to finished 

drinking water. Toccalino and others (2004), Toccalino and 

Norman (2006), and Rowe and others (2007) converted 

measured concentration to a Benchmark Quotient (BQ), 

which is the ratio of measured concentration to a water-quality 

benchmark. BQs are relative-concentrations. BQs that are 

< 1.0 indicate sample concentrations less than the benchmark, 

and values > 1.0 indicate sample concentrations greater than 

the benchmark. The use of BQs also permits comparison of 

constituents present at a wide range of concentrations on a 

single scale.

The BQ concept is used in this study; however, the ratio 

of measured concentration to a water-quality benchmark is 

called a relative-concentration (RC) rather than a BQ because 

the benchmarks used to calculate RCs are not the same as 

the benchmarks used to calculate BQs for all constituents. 

BQs are calculated using U.S. Environmental Protection 

Agency (USEPA) maximum contaminant levels (MCL-US) 

or USGS-USEPA Health-Based Screening Levels (HBSLs). 

HBSLs are determined using USEPA methodologies for 

establishing drinking-water guidelines, and the most recent 

USEPA peer-reviewed, publically-available human-health 

toxicity information (Toccalino, 2007). RCs are calculated 

using benchmarks established by the USEPA and CDPH (U.S. 

Environmental Protection Agency, 1999, 2009a,b; California 

Department of Public Health, 2006, 2008, 2010). HBSLs 

were not used in this study because they are not recognized by 

California drinking-water regulatory agencies, and the GAMA 

Program was created to specifically focus on groundwater 

quality in California.

The benchmarks used for each constituent for calculating 

RCs were selected in the following order of priority:

1. Regulatory, health-based CDPH and USEPA maximum 

contaminant levels (MCL-CA and MCL-US, respectively) 

and action levels (AL-US).

2. Non-regulatory CDPH and USEPA secondary maximum 

contaminant levels (SMCL-CA and SMCL-US, 

respectively). For constituents with both recommended 

and upper SMCL-CA levels, the values for the upper 

levels were used. 

3. Non-regulatory, health-based CDPH notification levels 

(NL-CA), USEPA lifetime health advisory levels 

(HAL-US) and USEPA risk-specific doses. Risk-specific 

doses for risks of 1 in 105 (RSD5-US) were calculated by 

dividing by 10 the USEPA values for the concentration 

of a constituent in drinking water corresponding to an 

estimated excess lifetime cancer risk of 1 in 104.
Methods 25

Note that for constituents with multiple types of benchmarks, 

this hierarchy may not result in selection of the benchmark 

with the lowest concentration. For example, zinc has an 

SMCL-CA of 5,000 micrograms per liter (μg/L) and a 

HAL-US of 2,000 μg/L; the comparison benchmark selected 

using this hierarchy is the SMCL-CA. Additional information 

on the types of benchmarks and listings of the benchmarks 

for all constituents analyzed are provided by Fram and Belitz 

(2007), Ferrari and others (2008), and Fram and others (2009).

Fifty-eight constituents detected in the Tahoe-Martis, 

Central Sierra, or Southern Sierra study units have MCL-US, 

MCL-CA, SMCL-CA, NL-CA, HAL-US, or RSD5-US 

benchmarks, and therefore can have RCs calculated. For 30 

of the 58 constituents, the benchmarks used in this study 

have the same concentrations as the benchmarks used by 

Toccalino (2007), thus, RCs and BQs for these constituents 

would have the same values. For 13 constituents, the 

MCL-CA or NL-CA benchmark used in this study has a lower 

concentration than the HBSL used by Toccalino (2007), thus, 

RCs would have higher values than BQs. For two constituents, 

the benchmarks used in this study have higher concentrations 

than the HBSLs, thus, RCs would have lower values than 

BQs. Another 13 constituents have MCL-CA or NL-CA 

values, but no MCL-US or HBSL values, and therefore, would 

have RCs but no BQs. 

For ease of discussion, RCs of constituents were 

classified into low, moderate, and high categories:

Category Organic and specialinterest constituents

Inorganic

constituents

High RC > 1.0 RC > 1.0

Moderate 1.0 ≥ RC > 0.1 1.0 ≥ RC > 0.5

Low 0.1 ≥ RC 0.5 ≥ RC

The boundary between “moderate” and “low” RCs was 

set at 0.1 for organic and special-interest constituents for 

consistency with other studies and reporting requirements 

(U.S. Environmental Protection Agency, 1998; Toccalino, 

2007). For organic constituents, detection at concentrations 

greater than one-tenth of a health-based benchmark value 

(RC > 0.1) commonly is used to identify constituents that 

may warrant additional monitoring to evaluate trends in their 

occurrences. Organic constituents typically are human-made 

and are infrequently detected at RCs > 0.1. Of the three 

special-interest constituents, two are organic compounds 

(1,2,3-trichloropropane and N-nitrosodimethylamine). The 

third, perchlorate, is an inorganic compound and is in the 

special-interest group, rather than the inorganic constituent 

group, because at the inception of the GAMA Priority Basin 

Project, the State of California was assessing potential 

regulation of perchlorate concentrations in drinking water and 

therefore had a “special interest” in perchlorate occurrence. An 

MCL-CA was promulgated in October 2007.

For inorganic constituents, the boundary between 

“moderate” and “low” RCs was set at 0.5. The primary reason 

for using a higher boundary value was to focus attention on 

the inorganic constituents of most immediate concern. In a 

national survey of water quality in aquifers used for public 

drinking-water supply, Toccalino and others (2010) found 

that organic constituents (pesticides and VOCs) were present 

at BQs > 0.1 in approximately 10% of the samples and 

that inorganic constituents (trace elements and radioactive 

constituents) were present at BQs >0.1 in approximately 

80% of the samples. By setting the boundary between low 

and moderate BQs at 0.1, Toccalino and others (2010) 

produced a conservative assessment of water quality that is 

protective of human health and provides an early indication of 

potential groundwater contamination issues. Concentrations 

of the human-made organic constituents may change rapidly 

in groundwater; therefore, such early warning is vital for 

planning and implementing measures to protect aquifer 

systems from further contamination and to mitigate existing 

contamination. Resources may be focused on the 10% of wells 

that have BQs >0.1 of organic constituents; however, a similar 

focusing of resources would not be possible for the inorganic 

constituents because most of the wells (80%) have inorganic 

constituents present at BQs >0.1. Inorganic constituents 

typically are naturally occurring in groundwater, and their 

concentrations usually are stable or change slowly compared 

to those of organic constituents. Thus, early warning of 

potential groundwater contamination by inorganic constituents 

may be less critical for management of potential water-quality 

problems. Having a boundary between low and moderate 

RC (or BQ) that is 0.5 allows identification of inorganic 

constituents—from among the many that may be present at 

concentrations approaching benchmarks—that may warrant 

more immediate attention from water-resource managers.

Setting the boundary between “moderate” and “low” 

RCs at 0.5 for inorganic constituents, rather than 0.1, also 

permitted use of water-quality data from the CDPH database 

in the assessments. For example, in the Tahoe-Martis study 

unit, 23 inorganic constituents had health-based benchmarks 

for which there were data from both the CDPH database and 

USGS-GAMA. RCs of reporting limits used by USGS-GAMA 

were < 0.1 for all 23 constituents (fig. 7). Thus, the difference 

between “low” and “moderate” RCs in groundwater samples 

could be distinguished whether the boundary was set at 0.1 or 

0.5. In contrast, the median RC of the most prevalent reporting 

limits used in the CDPH database was 0.1 (fig. 7). The RCs 

of the most prevalent reporting limits for 11 constituents 

were > 0.1. These 11 are the trace elements antimony, arsenic, 

beryllium, cadmium, chromium, lead, mercury, and thallium; 

the nutrient nitrite; and the radioactive constituents gross alpha 

particle activity and radium. If the boundary were set at 0.1, 

then data from the CDPH database would yield only minimum 
26 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

estimates of the prevalence of groundwater with moderate 

RCs of these 11 constituents. Elevated concentrations of 

arsenic and activities of gross alpha particles, in particular, are 

commonly detected in groundwater from the Tahoe-Martis, 

Central Sierra, and Southern Sierra study units. Setting both 

upper and lower bounds on the proportion of the primary 

aquifers with high and moderate RCs of these constituents is 

an important component of the status assessment portion of 

this study; thus, the boundary between low and moderate had 

to be set at an RC >0.1.

The decision to set the boundary at 0.5 (rather than 0.2, 

0.25, 0.675, or some other value > 0.1) is somewhat arbitrary, 

and represents a balance between the desire to be most 

protective of human health (lower boundary value), the desire 

to focus attention on those constituents of greatest potential 

concern (higher boundary value), and the limitations of the 

data in the CDPH database. For the Tahoe-Martis study unit, a 

boundary of 0.5 permits use of data from the CDPH database 

to identify moderate RCs for all inorganic constituents except 

for antimony. 

In this study, nutrients are grouped with trace elements 

and radioactive constituents rather than with organic 

constituents because nitrate concentrations, like the 

concentrations of many trace elements, may be affected by 

both natural and anthropogenic sources, whereas the VOCs 

and pesticides have only anthropogenic sources. However, 

the concentrations of nutrients in groundwater may be more 

affected by human activities than the concentrations of other 

inorganic constituents, and therefore, may be more likely to 

change rapidly. On that basis, one could choose to group the 

nutrients with organic constituents and use the boundary of 

RC=0.1 between low and moderate categories. The MCL-US 

for nitrate, as nitrogen (nitrate-N) is 10 milligrams per liter 

(mg/L); if RC  =  0.1 were used as the boundary, concentrations 

above 1 mg/L would be classified as moderate; if RC=0.5 

were used as the boundary, concentrations above 5 mg/L 

would be classified as moderate. Nitrate has both natural and 

anthropogenic sources. Mueller and Helsel (1996) define 

a nitrate concentration of 2 mg/L as the threshold above 

which nitrate is likely to be anthropogenic in origin, based 

on a national study of nitrate concentrations in groundwater. 

If elevated nitrate concentrations (moderate or high) are 

considered anthropogenic in origin (Toccalino and others, 

2010), then a boundary of 0.1 may overestimate the prevalence 

of groundwater contaminated by anthropogenic sources of 

nitrate, and a boundary of 0.5 may underestimate it.

Complex classifications could be devised based upon the 

properties and sources of individual constituents; however, use 

of a single moderate/low boundary value for each of the two 

major groups of constituents, organic and inorganic, provides 

a consistent criteria for distinguishing constituents occurring at 

moderate rather than low concentrations. sac09-0340_fig 07

1

0.1

0.01

0.001

0.0001

0.00001

10

CONSTITUENT CLASS

RELATIVE-CONCENTRATION OF REPORTING LIMIT

Trace elements,

nutrients, and

radioactive

Volatile organic

compounds

Pesticides and

fumigants

22 48 16

16 Number of constituents

California Department of Public Health (CDPH)

 database most common reporting levels

USGS-GAMA laboratory method detection levels

EXPLANATION

Relative-Concentration

75th percentile

Median (50th)

25th percentile

10th percentile

90th percentile

Value greater than

90th percentile

Value less than

10th percentile

Low Moderate High

Figure 7. Method-detection levels used for analysis of constituents by USGS-GAMA in the time period during which 

samples were collected for the Tahoe-Martis study unit, and the most common reporting levels for the same constituents 

used in the CDPH database for wells in the area corresponding to the Tahoe-Martis study during the 3-year interval used 

in the status assessment.
Methods 27

Datasets for Status Assessment

Three datasets were used in the status assessment: 

USGS-grid wells, CDPH-grid wells, and additional, non-grid 

wells from USGS and CDPH. This section describes how each 

dataset was constructed. Comparisons of USGS-GAMA and 

CDPH data are presented in appendix D.

USGS-Grid Wells

The primary data used for the grid-based calculations 

of aquifer-scale proportions are from wells sampled by 

USGS-GAMA. Detailed descriptions of the methods used 

to identify wells for sampling are given in Fram and Belitz 

(2007), Ferrari and others (2008), and Fram and others 

(2009). Briefly, each study unit or study area was divided into 

equal-area grid cells, and in each cell, one well was randomly 

selected to represent the cell (Scott, 1990). Wells primarily 

were selected from the population of wells in the statewide 

database maintained by CDPH. The three study units 

contained a total of 115 grid cells, and USGS-GAMA sampled 

wells in 103 of those cells (USGS-grid wells) (figs. 8A–C). 

Of the 103 USGS-grid wells, 92 were listed in the CDPH 

database, 3 were unlisted public-supply wells, and the other 

8 were irrigation, domestic, industrial, or monitoring wells 

screened or open at depths similar to the depths of CDPH 

wells in the cell or neighboring cells. USGS-grid wells were 

named with an alphanumeric GAMA_ID consisting of a prefix 

identifying the study unit or study area (TTAHO, TROCK, 

and TMART for the Tahoe, Hard Rock, and Martis study 

areas, respectively, in the Tahoe-Martis study unit; CGOLD 

and CWISH for the Coarse Gold and Wishon study areas, 

respectively, in the Central Sierra study unit; and SOSA for the 

Southern Sierra study unit) and a number indicating the order 

of sample collection (appendix A; fig. A1; table A1).

Samples were collected in accordance with protocols 

established by the USGS National Water-Quality Assessment 

(NAWQA) Program (Koterba and others, 1995) and the USGS 

National Field Manual (U.S. Geological Survey, variously 

dated). Detailed descriptions of the methods used to collect 

and analyze samples, and quality-control results are given in 

Fram and Belitz (2007), Ferrari and others (2008), and Fram 

and others (2009).

Samples collected from USGS-grid wells were analyzed 

for 166 to 243 constituents (table 1). Samples for analysis of 

field water-quality parameters, VOCs, pesticides, perchlorate, 

noble gases, tritium, and stable isotopes of water were 

collected from all wells. Samples for analysis of major and 

minor ions, trace elements, nutrients, carbon isotopes, and 

redox species were collected from most wells, and samples for 

analysis of radioactive constituents were collected from some 

wells.

CDPH-Grid Wells

Of the 115 grid cells in the three study units, 12 cells did 

not have a USGS-grid well, 28 cells had a USGS-grid well 

but no USGS data for major ions, trace elements, nutrients, 

and radioactive constituents, and 49 cells had a USGS-grid 

well but incomplete USGS data for radioactive constituents. 

The CDPH database was queried to identify wells to provide 

these missing inorganic data. CDPH wells with data during 

the most recent 3 years available at the time of sampling 

were considered. If the well had more than one analysis for a 

constituent in the 3-year interval, the most recent data were 

selected. The 3-year intervals were as follows:

Study unit 3-year interval

Number of 

wells with 

data

Tahoe-Martis May 1, 2004–April 30, 2007 157

Central Sierra June 1, 2003–May 31, 2006 169

Southern Sierra February 1, 2003–January 31, 2006 205

The decision tree used to identify suitable CDPH wells 

is described in appendix A. Briefly, the first choice was to 

use CDPH data from the same well as the USGS-grid well 

(“DG” CDPH-grid wells, where DG refers to CDPH and 

USGS; fig. A1; table A1). If the DG well did not have all of 

the needed data, a second well was randomly selected from the 

subset of CDPH wells with data (“DPH” CDPH-grid wells, 

where DPH refers to CDPH; fig. A1; table A1). No more 

than one DPH CDPH-grid well was selected in a cell. The 

combination of the USGS-grid wells and the DG and DPH 

CDPH-grid wells produced a grid-well network covering 108 

of the 115 grid cells in the three study units (figs. 8A,B,C). The 

remaining seven cells had no wells accessible for sampling 

and no wells with water-quality data in the CDPH database.

The CDPH database generally did not contain data for 

all of the missing inorganic constituents at every CDPH-grid 

well; therefore, the number of wells used for the grid-based 

assessment differed for various inorganic constituents 

(table 2). Although other organizations also collect 

water-quality data, the CDPH data are the only data available 

from a statewide database of groundwater chemistry that is 

suitable for comprehensive analysis.

CDPH data were not used to supplement USGS-grid 

well data for VOCs, pesticides, or perchlorate for the 

status assessment. A larger number of VOCs and pesticide 

compounds are analyzed for the USGS-GAMA Program 

than are available from the CDPH database. USGS-GAMA 

collected data for 88 VOCs and 63 pesticides and pesticide 

degradates at every well (table 1); the CDPH database for 

the three study units contains data for up to 61 VOCs and 

27 pesticides at a subset of the wells in the database. In 

addition, method detection limits for USGS-GAMA analyses 

of organic constituents were typically one to two orders 

of magnitude lower than the reporting limits for analyses 

compiled by the CDPH (fig. 7). 
28 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac09-0340_fig 08a

NEVADA CO

WASHOE CO

CARSON CO

DOUGLAS CO

PLACER CO

EL DORADO CO

ALPINE

CO

NEVADA

CALIFORNIA

Martis

Hard Rock 

Tahoe

Study areas

120° 15’ 120°

39° 15’

39°

0 5 10 MILES

0 5 10 KILOMETERS

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006,

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

A

Grid cell boundary

USGS-grid

USGS-grid and CDPH-DG-grid

CDPH-DPH-grid

USGS-understanding

CDPH well without data

CDPH well with water-quality data

Wells

EXPLANATION

LAKE TAHOE

Figure 8. Locations of grid cells and USGS-grid wells, CDPH-grid wells, USGS-understanding wells, and other 

CDPH wells in the (A) Tahoe-Martis, (B) Central Sierra, and (C) Southern Sierra study units, California GAMA 

Priority Basin Project.
Methods 29

sac09-0340_fig 08b

"

Coarse Gold study area

Wishon study area

Study areas

Grid cells

USGS-grid

USGS-grid and CDPH-DG-grid

CDPH-DPH-grid

USGS-understanding

CDPH well without data

CDPH well with water-quality data

Wells

EXPLANATION

0 5 10 MILES

0 5 10 KILOMETERS

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006,

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

37° 30’

37° 15’

119° 45’ 119° 30’

B

MARIPOSA CO

MADERA CO

FRESNO CO

Figure 8.—Continued
30 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac09-0340_fig 08c

TULARE CO

KERN CO

Study unit

Study areas

10 MILES

50 10 KILOMETERS

0 5

USGS-grid

USGS-grid and CDPH-DG-grid

CDPH-DPH-grid

USGS-understanding

CDPH well without data

CDPH well with water-quality data

Wells

Grid cell boundary

EXPLANATION

118°30’ 118°15’

36°

35°45’

35°30’

35°15’

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006,

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

C

Figure 8.—Continued
Methods 31

Table 1. Summary of constituent groups and number of wells sampled by the U.S. Geological Survey for different analytical schedules 

for the Tahoe-Martis, Central Sierra, and Southern Sierra study units, 2006–2007, California GAMA Priority Basin Project.

[Constituent names: NDMA, N-nitrosodimethylamine; TDS, total dissolved solids; 1,2,3-TCP, 1,2,3-trichloropropane. Other abbreviations: na, not analyzed]

Tahoe-Martis 

study unit schedule1

Central Sierra

study unit schedule1

Southern Sierra

study unit schedule1

Intermediate Slow Intermediate Slow Fast Intermediate Slow 

Total number of wells 42 10 16 14 27 16 7

Number of grid wells sampled 33 8 13 14 27 4 4

Number of understanding wells sampled 9 2 3 0 0 12 3

Number of constituents analyzed

Inorganic constituents

Specific conductance 1 1 1 1 1 1 1

Nutrients 5 5 5 5 na 5 5

Dissolved organic carbon na na 1 1 na 1 1

Major ions, alkalinity, and TDS 12 12 12 12 na 12 12

Trace elements 25 25 26 26 na 26 26

Radioactive constituents

Uranium isotopes6 1 1 1 1 na na na

Radon-222 1 1 na 1 na na 1

Radium isotopes7 na 1 na 1 na na 1

Gross alpha and beta particle activity8 na 2 na 2 na na 2

Organic constituents2

Volatile organic compounds (VOCs)3 85 85 85 85 85 85 85

Gasoline oxygenates4 3 3 3 3 3 3 3

Pesticides and degradates 63 63 83 83 63 63 63

Special-interest constituents

Perchlorate 1 1 1 1 1 1 1

NDMA and low-level 1,2,3-TCP5 na 1 2 2 na 2 2

Geochemical and age-dating tracers

Dissolved oxygen, pH, and temperature 3 3 3 3 3 3 3

Arsenic and iron redox species ratios 2 2 2 2 na 2 2

Chromium redox species ratio na na 1 1 1 1 1

Carbon-14 and δ13C of dissolved carbonates 2 2 2 2 na 2 2

Tritium9 1 1 1 1 na 1 1

Noble gases (helium, neon, argon, krypton, 

xenon), 3He/4He of helium, and tritium10

7 7 7 7 7 7 7

δ2H and δ18O stable isotopes of water 2 2 2 2 2 2 2 87Sr/86Sr of dissolved strontium 1 1 1 1 na na na

Sum: 215 219 239 243 166 217 221

1

 “Fast,” “intermediate,” and “slow” schedules refer to the amount of time required for a field crew to complete all work at a well.

2

 All schedules except the Intermediate schedule in the Tahoe-Martis study unit also had 14 pharmaceutical compounds analyzed. These results are not 

discussed in this report; thus, the compounds are not included in the count of constituents analyzed.

3

 Includes 10 constituents classified as fumigants or fumigant synthesis byproducts.

4

 In the Tahoe-Martis study unit, gasoline oxygenates were analyzed only in the Tahoe study area.

5

 Includes one analyte, 1,2,3-TCP, in common with VOC analyses. However, the laboratory reporting limit for the low-level analysis is 0.005 microgram per 

liter (μg/L) compared to 0.18 μg/L for the VOC analysis. Therefore, the low-level analysis is counted as a separate analysis. 1,2,3-TCP was not analyzed in the 

Tahoe-Martis study unit.

6

 Uranium activity equals the sum of the three isotopes measured: uranium-234, uranium-235, and uranium-238.

7

 Radium activity equals the sum of the two isotopes measured: radium-226 and radium-228.

8

 Both gross alpha particle and gross beta particle radiation were measured after 72-hour and 30-day holding times; data from the 72-hour measurement are 

used in this report.

9

 Analyzed at U.S. Geological Survey Stable Isotope and Tritium Laboratory, Menlo Park, California.

10 Analyzed at Lawrence Livermore National Laboratory, Livermore, California.
32 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Table 2. Number of USGS-grid and CDPH-grid wells used in the status assessments for inorganic constituents in the Tahoe-Martis, 

Central Sierra, and Southern Sierra study units, 2006–2007, California GAMA Priority Basin Project.

[CDPH, California Department of Public Health; GAMA, Groundwater Ambient Monitoring and Assessment Program; USGS, U.S. Geological Survey]

Constituent

Tahoe-Martis study unit 

(45 cells)

Central Sierra study unit 

(30 cells)

Southern Sierra study unit 

(40 cells)

Number of grid 

wells sampled 

by USGS-GAMA

Number of grid 

wells selected 

from CDPH

database

Number of grid 

wells sampled 

by USGS-GAMA

Number of grid 

wells selected 

from CDPH

database

Number of grid 

wells sampled 

by USGS-GAMA

Number of grid 

wells selected 

from CDPH

database

Inorganic constituents with health-based benchmarks

Trace elements

Aluminum 41 0 27 2 7 24

Antimony 41 0 27 2 7 24

Arsenic 41 0 27 2 7 25

Barium 41 0 27 2 7 24

Beryllium 41 0 27 2 7 24

Boron 41 0 27 0 7 8

Cadmium 41 0 27 2 7 24

Chromium 41 0 27 2 7 23

Copper 41 0 27 2 7 23

Fluoride 41 0 27 2 7 24

Lead 41 0 27 2 7 24

Mercury 41 0 27 2 7 24

Molybdenum 41 0 27 0 7 0

Nickel 41 0 27 2 7 24

Selenium 41 0 27 2 7 24

Strontium 41 0 27 0 7 0

Thallium 41 0 27 2 7 24

Uranium 41 0 27 0 7 1

Vanadium 41 0 27 0 7 7

Nutrients

Ammonia 41 0 27 0 7 0

Nitrate1 41 0 27 2 7 30

Nitrite 41 0 27 2 7 26

Radioactive constituents

Gross alpha particle activity 8 15 14 10 4 24

Gross beta particle activity 8 9 14 0 4 0

Radium activity 8 9 14 1 4 8

Radon-222 activity 40 0 14 0 4 2

Uranium activity 41 0 27 1 0 19

Inorganic constituents with secondary maximum contaminant level benchmarks

Iron 41 0 27 2 7 23

Manganese 41 0 27 2 7 23

Silver 41 0 27 1 7 23

Zinc 41 0 27 2 7 23

Chloride 41 0 27 2 7 23

Specific conductance 41 0 27 2 33 1

Sulfate 41 0 27 2 7 23

Total dissolved solids 41 0 27 2 7 23

1

 USGS-GAMA analyses were for nitrate plus nitrite; however, nitrite concentrations were negligible compared to nitrate in all cases.
Methods 33

Additional Data Used for Spatially Weighted Calculation

The spatially weighted calculations of aquifer-scale 

proportions used data from the USGS-grid wells, additional 

wells sampled by USGS-GAMA, and all wells in the CDPH 

database having water-quality data during the 3-year intervals 

prior to the USGS-GAMA sampling periods. For wells with 

both USGS-GAMA and CDPH data, only the USGS-GAMA 

data were used.

In addition to the 103 USGS-grid wells, a total of 

29 other wells were sampled in the three study units by 

USGS-GAMA. These additional wells were selected to 

increase sampling density in certain areas to help understand 

specific water-quality issues in those areas (figs. 8A,B,C; 

Fram and Belitz, 2007; Ferrari and others, 2008; Fram 

and others, 2009). These 29 wells are referred to as 

“USGS‐understanding” wells and were numbered in the 

order of collection with prefixes modified from those used 

for the USGS-grid wells (TMARTU, TROCKU, TTAHOU, 

CGOLDU, CWISHU, and SOSAFP,) (fig. A1; table C1). With 

the exception of four TTAHOU monitoring wells, all of the 

USGS-understanding wells were screened or open at similar 

depths to the grid wells and were included in the dataset for 

the spatially weighted calculations.

Selection of Constituents for Additional 

Evaluation in the Status Assessment

As many as 243 constituents were analyzed in samples 

from Tahoe-Martis, Central Sierra, and Southern Sierra study 

unit wells as part of the status assessment (table 1); however, 

only a subset of these constituents was selected for additional 

evaluation in this report. Three criteria were used to select 

constituents for additional evaluation in the status assessment:

1. Constituents present at high or moderate RCs in the 

USGS-grid wells or USGS-understanding wells used in 

the status assessment, or

2. Organic constituents having study-unit detection 

frequencies > 10% in the USGS-grid well dataset for a 

particular study unit, or

3. Constituents present at high or moderate RCs in the 

CDPH database within the 3-year intervals prior to the 

USGS-GAMA sampling periods.

These criteria identified 23 inorganic constituents, 12 organic 

constituents, and 1 constituent of special interest for additional 

evaluation for at least one of the three study units (table 3). 

An additional 27 inorganic constituents and 15 organic 

constituents were detected by USGS-GAMA, but were not 

selected for additional evaluation because they either have 

no established benchmarks or were only detected at low RCs 

(table 4). All 19 geochemical and age-dating tracers also 

were detected. The remaining 146 constituents that were not 

detected by USGS-GAMA in any of the three study units are 

listed in Fram and Belitz (2007), Ferrari and others (2008), 

and Fram and others (2009).

The CDPH database was used to identify constituents that 

have been reported at high RCs historically but not currently 

(table 5). The historical period was defined as the period 

starting with the earliest record maintained in the CDPH 

electronic database and ending just prior to the 3-year intervals 

used for the status assessment.

Study unit Historical period

Number of 

wells with 

data

Tahoe-Martis June 26, 1984–April 30, 2004 180

Central Sierra January 19, 1984–May 31, 2003 184

Southern Sierra January 12, 1977–January 31, 2003 252

Constituents may be historically high but not currently 

high because of improvement of groundwater quality with 

time or abandonment of wells with high concentrations 

of constituents. Historically high constituents that do not 

otherwise meet the criteria for additional evaluation in the 

status assessment were not considered representative of 

potential groundwater-quality concerns in the three study units 

during the current periods.

The Tahoe-Martis, Central Sierra, and Southern Sierra 

study units had 9, 6, and 17 historically high constituents, 

respectively (table 5). Eight of these constituents also 

were detected at moderate RCs in the 3-year intervals used 

in the status assessment, and were therefore included in 

the constituents selected for additional evaluation in the 

status assessment (table 3). Of the 32 total historically high 

constituents for the three study units, 27 were high in only 

1 well each.

Forty-five constituents with regulatory, health-based 

benchmarks that are reported in the CDPH database were not 

analyzed by USGS-GAMA. Most of these constituents are 

semi-volatile organic compounds. None of these constituents 

were reported at high concentrations in the 3-year intervals 

used for the status assessment in the three study units. 

Two constituents, heptachlor epoxide and di(2-ethylhexyl)

phthalate, were each reported at a high concentration once 

historically in the Southern Sierra study unit (table 5).
34 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Table 3. Benchmark type and value for constituents selected for additional evaluation in the status assessments of groundwater 

quality in the Tahoe-Martis, Central Sierra, and Southern Sierra study units, 2006–2007, California GAMA Priority Basin Project.

[Inorganic constituents selected if maximum concentration measured in USGS-GAMA samples or reported in CDPH database during the 3-year time period 

used for the status assessment was greater than 0.5 times benchmark concentration. Organic constituents selected if maximum concentration was greater than 

0.1 times benchmark concentration, or if study unit detection frequency at any concentration was greater than 10 percent. Benchmark type: Regulatory, healthbased benchmarks: MCL-US, USEPA maximum contaminant level; Prop MCL-US, proposed USEPA maximum contaminant level; AL-US, USEPA action 

level; MCL-CA, CDPH maximum contaminant level. Non-regulatory, health-based benchmarks: HAL-US, USEPA lifetime health advisory level; NL-CA, 

CDPH notification level. Non-regulatory, aesthetic-based benchmarks: SMCL-CA, CDPH secondary maximum contaminant level. Benchmark units: mg/L, 

milligram per liter; μg/L, microgram per liter; μS/cm, microsiemen per centimeter; pCi/L, picocurie per liter. Other Abbreviations: USGS, U.S. Geological 

Survey; GAMA, Groundwater Ambient Monitoring and Assessment Program; USEPA, U.S. Environmental Protection Agency; CDPH, California Department 

of Public Health; D-eval, detected by USGS-GAMA and selected for additional evaluation in status assessment for study unit; D, detected by USGS-GAMA 

but not selected for additional evaluation in status assessment for study unit; eval, not detected by USGS-GAMA in the study unit, but selected for additional 

evaluation in status assessment on the basis of concentrations reported in the CDPH database; –, not detected in study unit]

Constituent Typical use or source

Benchmarks Study unit

Type1 Value Units TahoeMartis

Central 

Sierra

Southern 

Sierra

Inorganic constituents with health-based benchmarks

Trace elements

Aluminum Naturally occurring MCL-CA 1,000 μg/L D D D-eval2

Antimony Naturally occurring MCL-US 6 μg/L D D D-eval2

Arsenic Naturally occurring MCL-US 10 μg/L D-eval D-eval D-eval

Boron Naturally occurring NL-CA 1,000 μg/L D-eval D-eval D-eval

Chromium Naturally occurring MCL-CA 50 μg/L D D-eval2 D-eval2

Fluoride Naturally occurring MCL-CA 2 mg/L D D-eval D-eval

Lead Naturally occurring AL-US 15 μg/L D D-eval D-eval

Molybdenum Naturally occurring HAL-US 40 μg/L D-eval D-eval D-eval

Selenium Naturally occurring MCL-US 50 μg/L D D-eval2 D

Strontium Naturally occurring HAL-US 4,000 μg/L D-eval D D

Uranium Naturally occurring MCL-US 30 μg/L D-eval D-eval D-eval

Nutrients

Nitrate, as nitrogen3 Natural, fertilizer, sewage MCL-US 10 mg/L D D-eval D-eval

Radioactive constituents

Gross alpha particle activity Naturally occurring MCL-US 15 pCi/L D-eval D-eval D-eval

Radium activity Naturally occurring MCL-US 5 pCi/L D-eval D-eval D-eval

Radon-222 activity Naturally occurring Prop MCL-US 4,000 pCi/L D-eval D-eval D-eval

Uranium activity Naturally occurring MCL-CA 20 pCi/L D-eval D-eval D-eval

Inorganic constituents with secondary maximum contaminant level benchmarks

Chloride Naturally occurring SMCL-CA 500 mg/L D-eval D-eval D

Iron Naturally occurring SMCL-CA 300 μg/L D-eval D-eval D-eval

Manganese Naturally occurring SMCL-CA 50 μg/L D-eval D-eval D-eval

Specific conductance Naturally occurring SMCL-CA 1,600 μS/cm D-eval D-eval D-eval

Sulfate Naturally occurring SMCL-CA 500 mg/L D-eval D D-eval

Total dissolved solids (TDS) Naturally occurring SMCL-CA 1,000 mg/L D-eval D-eval D-eval

Zinc Naturally occurring SMCL-CA 5,000 μg/L D D-eval D-eval

Organic and special-interest constituents with health-based benchmarks

Pesticides

Atrazine Herbicide MCL-CA 1 μg/L D D D-eval

Simazine Herbicide MCL-US 4 μg/L – D-eval D-eval

Volatile Organic Compounds

1,2-Dichloropropane Fumigant MCL-US 5 μg/L Eval – D

1,4-Dichlorobenzene Fumigant MCL-CA 5 μg/L – Eval –

DBCP Fumigant MCL-US 0.2 μg/L Eval – –
Methods 35

Table 3. Benchmark type and value for constituents selected for additional evaluation in the status assessments of groundwater 

quality in the Tahoe-Martis, Central Sierra, and Southern Sierra study units, 2006–2007, California Groundwater Ambient Monitoring and 

Assessment (GAMA) Priority Basin Project.—Continued

[Inorganic constituents selected if maximum concentration measured in USGS-GAMA samples or reported in CDPH database during the 3-year time period used 

for the status assessment was greater than 0.5 times benchmark concentration. Organic constituents selected if maximum concentration was greater than 0.1 

times benchmark concentration, or if study unit detection frequency at any concentration was greater than 10 percent. Benchmark type: Regulatory, healthbased benchmarks: MCL-US, USEPA maximum contaminant level; Prop MCL-US, proposed USEPA maximum contaminant level; AL-US, USEPA action 

level; MCL-CA, CDPH maximum contaminant level. Non-regulatory, health-based benchmarks: HAL-US, USEPA lifetime health advisory level; NL-CA, 

CDPH notification level. Non-regulatory, aesthetic-based benchmarks: SMCL-CA, CDPH secondary maximum contaminant level. Benchmark units: mg/L, 

milligram per liter; μg/L, microgram per liter; μS/cm, microsiemen per centimeter; pCi/L, picocurie per liter. Other Abbreviations: USGS, U.S. Geological 

Survey; GAMA, Groundwater Ambient Monitoring and Assessment Program; USEPA, U.S. Environmental Protection Agency; CDPH, California Department of 

Public Health; D-eval, detected by USGS-GAMA and selected for additional evaluation in status assessment for study unit; D, detected by USGS-GAMA but not 

selected for additional evaluation in status assessment for study unit; eval, not detected by USGS-GAMA in the study unit, but selected for additional evaluation 

in status assessment on the basis of concentrations reported in the CDPH database; –, not detected in study unit]

Constituent Typical use or source

Benchmarks Study unit

Type1 Value Units TahoeMartis

Central 

Sierra

Southern 

Sierra

Organic and special-interest constituents with health-based benchmarks—Continued

Volatile Organic Compounds—Continued

1,2-Dichloroethane Solvent, fumigant, plastics MCL-CA 0.5 μg/L Eval – –

Chloromethane Refrigerant, industrial HAL-US 30 μg/L Eval – –

Perchloroethene (PCE) Dry-cleaning, metal 

degreasing

MCL-US 5 μg/L D-eval D-eval D-eval

Carbon tetrachloride Solvent, industrial, drycleaningMCL-CA 0.5 μg/L D-eval – D-eval

Chloroform Disinfection byproduct MCL-US4 80 μg/L D-eval D-eval D-eval

Methyl tert-butyl ether 

(MTBE)

Gasoline oxygenate MCL-CA 13 μg/L D D-eval2 D

Benzene Gasoline hydrocarbon MCL-CA 1 μg/L – D-eval –

Constituents of special interest

Perchlorate Natural, rocket fuel, flares MCL-CA 6 μg/L – – D-eval

1

 Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CA are identical, and as MCL-CA when the MCL-CA is 

lower than the MCL-US or no MCL-US exists. Sources of benchmarks: 

MCL-CA: California Department of Public Health (2008),

MCL-US: U.S. Environmental Protection Agency (2009a),

SMCL-CA: California Department of Public Health (2006),

NL-CA: California Department of Public Health (2010),

HAL-US: U.S. Environmental Protection Agency (2009b),

Prop MCL-US: U.S. Environmental Protection Agency (1999).

2

 Selected on the basis of concentrations reported in CDPH database. Constituent also detected by USGS-GAMA at low relative-concentrations.

3

 Concentrations of nitrate, as nitrate, reported in the CDPH data are converted to concentrations of nitrate, as nitrogen, for comparison with USGS-GAMA 

data. 

4

 MCL-US benchmark for trihalomethanes is the sum of chloroform, bromodichloromethane, dibromochloromethane, and bromoform. 
36 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Table 4. Constituents detected in samples collected by USGS-GAMA, but not selected for additional evaluation in the status 

assessments for the Tahoe-Martis, Central Sierra, and Southern Sierra study units, 2006–2007, California GAMA Priority Basin Project.

[Detected constituents were not selected for additional evaluation if they had no benchmarks, or for inorganic constituents, if the maximum concentration 

measured in USGS-GAMA samples or reported in CDPH database during the 3-year time period used for the status assessment was less than or equal to 0.5 

times benchmark concentration, or for organic constituents, the maximum concentration was less than or equal to 0.1 times benchmark concentration and 

study unit detection frequency at any concentration was less than 10 percent. Benchmark type: Regulatory, health-based benchmarks: MCL-US, USEPA 

maximum contaminant level; AL-US, USEPA action level; MCL-CA, CDPH maximum contaminant level. Non-regulatory, health-based benchmarks: HAL-US, 

USEPA lifetime health advisory level; NL-CA, CDPH notification level. Non-regulatory, aesthetic-based benchmarks: SMCL-CA, CDPH secondary maximum 

contaminant level. None, no benchmark. Benchmark units: mg/L, milligram per liter; μg/L, microgram per liter; μS/cm, microsiemen per centimeter; pCi/L, 

picocurie per liter. Other Abbreviations: USGS, U.S. Geological Survey; GAMA, Groundwater Ambient Monitoring and Assessment Program; USEPA, U.S. 

Environmental Protection Agency; CDPH, California Department of Public Health; D, detection at low relative-concentration; std units, standard units; <, less 

than; >, greater than; –, not detected; na, not analyzed]

Constituent Typical use or source

Benchmarks Study unit

Type1 Value Units TahoeMartis

Central 

Sierra

Southern 

Sierra

Inorganic constituents with benchmarks

Ammonia, as nitrogen Naturally occurring HAL-US2 24.7 mg/L D D D

Barium Naturally occurring MCL-CA 1,000 μg/L D D D

Beryllium Naturally occurring MCL-US 4 μg/L D D D

Cadmium Naturally occurring MCL-US 5 μg/L D D D

Copper Naturally occurring AL-US 1,300 μg/L D D D

Gross beta particle activity Naturally occurring MCL-US 50 pCi/L D D D

Mercury Naturally occurring MCL-US 2 μg/L na D –

Nickel Naturally occurring MCL-CA 100 μg/L D D D

Nitrite, as nitrogen Naturally occurring MCL-US 1 mg/L D D D

Silver Naturally occurring SMCL-CA 100 μg/L – – D

Thallium Naturally occurring MCL-US 2 μg/L D – D

Vanadium Naturally occurring NL-CA 50 μg/L D D D

Inorganic constituents with no benchmarks

Bicarbonate Naturally occurring None None None D D D

Bromide Naturally occurring None None None D D D

Calcium Naturally occurring None None None D D D

Carbonate Naturally occurring None None None D D D

Cobalt Naturally occurring None None None D D D

Dissolved organic carbon Naturally occurring None None None na D D

Iodide Naturally occurring None None None D D D

Lithium Naturally occurring None None None D D D

Magnesium Naturally occurring None None None D D D

Nitrogen, total Naturally occurring None None None D D D

Orthophosphate Naturally occurring None None None D D D

Potassium Naturally occurring None None None D D D

Silica Naturally occurring None None None D D D

Sodium Naturally occurring None None None D D D

Tungsten Naturally occurring None None None D D D

Organic constituents with benchmarks

Bromodichloromethane Disinfection byproduct MCL-US3 80 μg/L D – –

Carbon disulfide Natural, industrial NL-CA 160 μg/L D D –

1,2-Dichlorobenzene Solvent, insecticide MCL-US 600 μg/L – – D

cis-1,2-Dichloroethene Solvent, PCE breakdown MCL-CA 6 μg/L – – D

Prometon Herbicide HAL-US 100 μg/L – – D

1,1,1-Trichloroethane Solvent, industrial, fumigant MCL-US 200 μg/L D – –

Trichloroethene (TCE) Solvent, PCE breakdown MCL-US 5 μg/L D – D

Trichlorofluoromethane (CFC-11) Refrigerant MCL-CA 150 μg/L – – D

1,2,3-Trichloropropane Fumigant, solvent HAL-US 40 μg/L na – D

1,1,2-Trichloro-1,2,2-trifluoroethane 

(CFC-113)

Refrigerant MCL-CA 1,200 μg/L – – D
Methods 37

Table 4. Constituents detected in samples collected by USGS-GAMA, but not selected for additional evaluation in the status 

assessments for the Tahoe-Martis, Central Sierra, and Southern Sierra study units, 2006–2007, California GAMA Priority Basin Project.—

Continued

[Detected constituents were not selected for additional evaluation if they had no benchmarks, or for inorganic constituents, if the maximum concentration 

measured in USGS-GAMA samples or reported in CDPH database during the 3-year time period used for the status assessment was less than or equal to 0.5 

times benchmark concentration, or for organic constituents, the maximum concentration was less than or equal to 0.1 times benchmark concentration and 

study unit detection frequency at any concentration was less than 10 percent. Benchmark type: Regulatory, health-based benchmarks: MCL-US, USEPA 

maximum contaminant level; AL-US, USEPA action level; MCL-CA, CDPH maximum contaminant level. Non-regulatory, health-based benchmarks: HAL-US, 

USEPA lifetime health advisory level; NL-CA, CDPH notification level. Non-regulatory, aesthetic-based benchmarks: SMCL-CA, CDPH secondary maximum 

contaminant level. None, no benchmark. Benchmark units: mg/L, milligram per liter; μg/L, microgram per liter; μS/cm, microsiemen per centimeter; pCi/L, 

picocurie per liter. Other Abbreviations: USGS, U.S. Geological Survey; GAMA, Groundwater Ambient Monitoring and Assessment Program; USEPA, U.S. 

Environmental Protection Agency; CDPH, California Department of Public Health; D, detection at low relative-concentration; std units, standard units; <, less 

than; >, greater than; –, not detected; na, not analyzed]

Constituent Typical use or source

Benchmarks Study unit

Type1 Value Units TahoeMartis

Central 

Sierra

Southern 

Sierra

Organic constituents with no benchmarks

Deethylatrazine Herbicide degradate (atrazine) None None None D D D

3,4-Dichloroaniline Herbicide degradate (diuron) None None None – D –

Dicrotophos Insecticide None None None D – –

Diisopropyl ether Gasoline oxygenate None None None D – –

Fipronil sulfide Insecticide degradate (fipronil) None None None – – D

Geochemical and age-dating tracers

Tritium Naturally occurring MCL-CA 20,000 pCi/L D D D

pH Naturally occurring SMCL-US <6.5 or >8.5 std units D D D

Dissolved oxygen and temperature Naturally occurring None None None D D D

Arsenic and iron redox species ratios Naturally occurring None None None D D D

Chromium redox species ratio Naturally occurring None None None na D D

Carbon-14 and δ13C of dissolved 

carbonates

Naturally occurring None None None D D D

Noble gases (helium, neon, argon, 

krypton, xenon) and 3He/4He of 

helium

Naturally occurring None None None D D D

δ2H and δ18O stable isotopes of water Naturally occurring None None None D D D 87Sr/86Sr of dissolved strontium Naturally occurring None None None D D na

1

 Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CA are identical, and as MCL-CA when the MCL-CA is lower 

than the MCL-US or no MCL-US exists. Sources of benchmarks: 

MCL-CA: California Department of Public Health (2008),

MCL-US, AL-US, and SMCL-US: U.S. Environmental Protection Agency (2009a),

SMCL-CA: California Department of Public Health (2006),

NL-CA: California Department of Public Health (2010),

HAL-US: U.S. Environmental Protection Agency (2009b),

Prop MCL-US: U.S. Environmental Protection Agency (1999).

2

 HAL-US benchmark is 30 mg/L for ammonia, as ammonia.

3

 MCL-US benchmark for trihalomethanes is the sum of chloroform, bromodichloromethane, dibromochloromethane, and bromoform.
38 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Table 5. Constituents reported at concentrations greater than benchmarks historically in the California Department of Public Health 

database, but not during the 3-year time periods used in status assessment, Tahoe-Martis, Central Sierra, and Southern Sierra study 

units, 2006–2007, California GAMA Priority Basin Project.

[Benchmark type: Regulatory, health-based benchmarks: MCL-US, USEPA maximum contaminant level; AL-US, USEPA action level; MCL-CA, CDPH 

maximum contaminant level. Non-regulatory, health-based benchmarks: HAL-US, USEPA lifetime health advisory level; NL-CA, CDPH notification level. 

Non-regulatory, aesthetic-based benchmarks: SMCL-CA, CDPH secondary maximum contaminant level. None, no benchmark. Benchmark units: mg/L, 

milligram per liter; μg/L, microgram per liter; μS/cm, microsiemen per centimeter; pCi/L, picocurie per liter. Other Abbreviations: USGS, U.S. Geological 

Survey; GAMA, Groundwater Ambient Monitoring and Assessment Program; USEPA, U.S. Environmental Protection Agency; CDPH, California Department 

of Public Health]

Constituent Typical use or source

Benchmark Date of most 

recent high 

value

Number of 

wells with 

historical data

Number of 

wells with a 

high value Type1 Value Units

Tahoe-Martis study unit

Methy tert-butyl ether (MTBE) Gasoline oxygenate MCL-CA 13 μg/L 07-27-99 85 1

Perchlorate Natural, rocket fuel, flares MCL-CA 6 μg/L 10-21-02 37 1

Nitrate, as nitrogen Naturally occurring MCL-US 10 mg/L 09-24-87 162 1

Aluminum Naturally occurring MCL-CA 1,000 μg/L 01-26-93 90 1

Cadmium Naturally occurring MCL-US 5 μg/L 08-05-03 90 1

Lead Naturally occurring AL-US 15 μg/L 09-19-96 79 6

Mercury Naturally occurring MCL-US 2 μg/L 08-27-96 90 3

Radium activity2 Naturally occurring MCL-US 5 pCi/L 08-08-01 6 1

Uranium activity2 Naturally occurring MCL-CA 20 pCi/L 08-28-02 36 2

Central Sierra study unit

Methy tert-butyl ether (MTBE)2 Gasoline oxygenate MCL-CA 13 μg/L 05-27-03 135 1

Nitrate, as nitrogen2,3 Naturally occurring MCL-US 10 mg/L 03-06-03 110 1

Antimony Naturally occurring MCL-US 6 μg/L 06-27-95 58 1

Copper Naturally occurring AL-US 1,300 μg/L 07-31-02 69 1

Lead2 Naturally occurring AL-US 15 μg/L 05-09-95 65 1

Thallium Naturally occurring MCL-US 2 μg/L 06-27-95 58 1

Southern Sierra study unit

Chloroform4 Disinfection byproduct MCL-US5 80 μg/L 07-16-96 205 1

1,1-Dichloroethane Solvent MCL-CA 5 μg/L 03-17-93 206 1

1,2-Dichloroethane Solvent, fumigant, plastics MCL-CA 0.5 μg/L 05-27-99 206 1

Dichloromethane Paint stripper, solvent MCL-US 5 μg/L 03-16-01 205 4

Heptachlor epoxide6 Insecticide degradate MCL-CA 0.01 μg/L 08-22-97 80 1

Di(2-ethylhexyl)phthalate6 Plasticizer MCL-CA 4 μg/L 04-08-91 104 1

Barium Naturally occurring MCL-CA 1,000 μg/L 11-20-96 218 1

Beryllium Naturally occurring MCL-US 4 μg/L 06-03-02 180 1

Cadmium Naturally occurring MCL-US 5 μg/L 03-10-92 218 1

Lead2 Naturally occurring AL-US 15 μg/L 08-13-01 219 13

Mercury Naturally occurring MCL-US 2 μg/L 09-16-84 218 1

Nickel Naturally occurring MCL-CA 100 μg/L 06-03-02 181 1

Selenium Naturally occurring MCL-US 50 μg/L 06-09-99 217 1

Thallium Naturally occurring MCL-US 2 μg/L 07-23-02 181 1

Zinc2 Naturally occurring SMCL-CA 5,000 μg/L 03-18-97 222 1

Sulfate2 Naturally occurring SMCL-CA 500 mg/L 07-21-99 225 1

Chloride Naturally occurring SMCL-CA 500 mg/L 07-22-99 225 1
Methods 39

Footnotes:

1

 Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CA are identical, and as MCL-CA when the MCL-CA is 

lower than the MCL-US or no MCL-US exists. Sources of benchmarks: 

MCL-CA: California Department of Public Health (2008),

MCL-US and AL-US: U.S. Environmental Protection Agency (2009a),

SMCL-CA: California Department of Public Health (2006),

NL-CA: California Department of Public Health (2010),

HAL-US: U.S. Environmental Protection Agency (2009b),

Prop MCL-US: U.S. Environmental Protection Agency (1999).

2

 Constituent detected at moderate relative concentrations within 3-year interval in CDPH database or in grid-well dataset, and therefore it was selected for 

additional evaluation in the status assessment for the study unit.

3

 Concentrations of nitrate, as nitrate, reported in the CDPH data are converted to concentrations of nitrate, as nitrogen, for comparison with USGS-GAMA 

data. 

4

 Study unit detection frequency for constituent was greater than 10 percent; therefore, it was selected for additional evaluation in the status assessment for the 

study unit.

5

 MCL-US benchmark for trihalomethanes is the sum of chloroform, bromodichloromethane, dibromochloromethane, and bromoform. 

6

 Constituent not analyzed by USGS-GAMA.

Table 5. Constituents reported at concentrations greater than benchmarks historically in the California Department of Public Health 

database, but not during the 3-year time periods used in status assessment, Tahoe-Martis, Central Sierra, and Southern Sierra study 

units, 2006–2007, California GAMA Priority Basin Project.—Continued

[Benchmark type: Regulatory, health-based benchmarks: MCL-US, USEPA maximum contaminant level; AL-US, USEPA action level; MCL-CA, CDPH 

maximum contaminant level. Non-regulatory, health-based benchmarks: HAL-US, USEPA lifetime health advisory level; NL-CA, CDPH notification level. 

Non-regulatory, aesthetic-based benchmarks: SMCL-CA, CDPH secondary maximum contaminant level. None, no benchmark. Benchmark units: mg/L, 

milligram per liter; μg/L, microgram per liter; μS/cm, microsiemen per centimeter; pCi/L, picocurie per liter. Other Abbreviations: USGS, U.S. Geological 

Survey; GAMA, Groundwater Ambient Monitoring and Assessment Program; USEPA, U.S. Environmental Protection Agency; CDPH, California Department 

of Public Health]
40 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Calculation of Aquifer-Scale Proportions

The status assessment is intended to characterize the 

current quality of groundwater resources within the primary 

aquifers of the Tahoe-Martis, Central Sierra, and Southern 

Sierra study units. The primary aquifers are defined by the 

depth intervals over which wells listed in the CDPH database 

are screened, or open. The use of the term “primary aquifers” 

does not imply that there exists a discrete aquifer unit. In 

most groundwater basins, municipal and community supply 

wells typically are screened or open at greater depths than 

are domestic wells (Burow and others, 2008). Thus, because 

domestic wells are not listed in the CDPH database, the 

primary aquifer generally corresponds to the deeper portion 

of the aquifer system tapped by municipal and community 

supply wells; however, this segregation between the depths 

of municipal and domestic wells commonly does not apply in 

areas outside of groundwater basins. Wells in fractured-rock 

aquifers are most productive at depths where fractures in the 

local rock are saturated with water, and the density of fractures 

typically decreases with depth (Freeze and Cherry, 1979; Page 

and others, 1984; Borchers, 1996; Ingebritsen and Sanford, 

1998). Most of the wells used in the status assessment for the 

Tahoe-Martis, Central Sierra, and Southern Sierra study units 

are listed in the CDPH databases, and are therefore classified 

as municipal or community drinking-water supply wells. 

However, to the extent that domestic wells in the three study 

units are screened or open over the same depth intervals as the 

CDPH wells, the assessments presented in this report may also 

be applicable to the portions of the aquifer systems used for 

domestic drinking-water supplies. 

Two statistical approaches, grid-based and spatially 

weighted, were used to evaluate the proportions of the primary 

aquifers in the three study units with high, moderate, and low 

RCs of constituents. For ease of discussion, these proportions 

are referred to as “high,” “moderate,” and “low” aquifer‐scale 

proportions. Calculations of aquifer-scale proportions were 

made for individual constituents meeting the criteria for 

additional evaluation in the status assessment, and for classes 

of constituents. Detailed descriptions of the calculations are 

presented in appendix B, and brief summaries are given here.

The grid-based calculation uses the grid-well dataset 

assembled from the USGS-grid and CDPH-grid wells. For 

each constituent, the high aquifer-scale proportion was 

calculated by dividing the number of cells represented by a 

high RC for that constituent by the total number of grid cells 

with data for that constituent (Belitz and others, 2010). The 

moderate aquifer-scale proportion was calculated similarly. 

Confidence intervals for the high aquifer-scale proportions 

were computed using the Jeffreys interval for the binomial 

distribution (Brown and others, 2001). The grid-based 

approach is spatially unbiased; however, it may not detect 

constituents that are present at high RCs in small proportions 

of the aquifer system. For calculation of high aquifer-scale 

proportion for a class of constituents, cells were considered 

high if any of the constituents had a high RC. Cells were 

considered moderate if any of the constituents had a moderate 

RC, but none had a high RC.

The spatially weighted calculation uses the dataset 

assembled from all CDPH and USGS GAMA wells. For each 

constituent, the high aquifer-scale proportion was calculated 

by computing the proportion of wells with high values in each 

cell and then averaging the proportions for all cells (Isaaks 

and Srivastava, 1989; Belitz and others, 2010). The moderate 

aquifer-scale proportion was calculated similarly. Confidence 

intervals for spatially weighted detection frequencies of high 

concentrations are not described in this report. For calculation 

of high aquifer-scale proportion for a class of constituents, 

values for wells were considered high if the values for any of 

the constituents in that class were high. Values for wells were 

considered moderate if the values for any of the constituents 

were moderate, but no values for wells were high.

In addition, for each constituent, the detection 

frequencies of high and moderate RCs for individual 

constituents were calculated using the same dataset as used for 

the spatially weighted calculations. However, these detection 

frequencies are not spatially unbiased because the wells in the 

CDPH database are not uniformly distributed (figs. 8A,B,C). 

For example, if a constituent was present at high RCs in a 

small region of the aquifer that had a high density of wells, 

the detection frequency of high RCs would be greater than 

the high aquifer-scale proportion. Detection frequencies 

are provided for reference but were not used to assess 

aquifer-scale proportions. 

The Tahoe-Martis and Central Sierra study units 

had study areas whose cell sizes were different from one 

another. To obtain spatially unbiased results for these study 

units, aquifer-scale proportions were calculated for each 

study area in the study unit separately, and then combined, 

weighted by the relative areas of the study units (appendix B). 

Aquifer-scale proportions for the study areas are presented in 

appendix B; aquifer-scale proportions for the study units are 

discussed in the main body of the report.

The grid-based high aquifer-scale proportions were used 

to represent proportions in the primary aquifer unless the 

spatially weighted proportions were significantly different 

than the grid-based values. Significantly different results were 

defined as follows:

• If the grid-based high aquifer-scale proportion was zero 

and the spatially weighted proportion was non-zero, 

then the spatially weighted result was used. This 

situation can arise when a constituent is present at high 

concentrations in a small proportion of the primary 

aquifers.

• If the grid-based high aquifer-scale proportion was 

non-zero and the spatially weighted proportion was 

outside the 90% confidence interval around the 

grid-based proportion, then the spatially weighted 

proportion was used.
Methods 41

The grid-based moderate and low proportions were used in 

most cases because the reporting limits relative to benchmarks 

for many organic constituents and some inorganic constituents 

in the CDPH database were higher than the boundary between 

the moderate and low RC categories (fig. 7). However, 

if the grid-based moderate proportion was zero and the 

spatially weighted proportion non-zero, then the spatially 

weighted value was used as a minimum estimate for the 

moderate proportion.

Understanding Assessment

The purpose of the understanding assessment is to place 

groundwater quality within a physical and chemical context. A 

finite set of potential explanatory factors was considered: land 

use, aquifer lithology, well depth and depth to top of screened 

or open interval in well, relative position in the regional 

groundwater flow gradient, aridity index, density of septic 

systems, groundwater age, oxidation-reduction condition, 

and pH. Statistical tests were used to identify significant 

correlations between the constituents of interest and potential 

explanatory factors.

• Constituents with high aquifer-scale proportions of 

> 2%. These constituents were selected to focus the 

assessment for understanding on those constituents that 

have the greatest effect on groundwater quality.

• Classes of organic constituents that included 

constituents with study-unit detection frequencies of 

> 10%, regardless of concentration. 

The understanding assessment was based on the 

132 wells sampled by USGS-GAMA. Other CDPH wells 

were not used because data for many of the potential 

explanatory factors were not available. Samples from almost 

all USGS-GAMA wells were analyzed for dissolved oxygen, 

noble gases, and stable isotopes of water; samples from 

most wells were analyzed for tritium, carbon isotopes, and 

redox species of iron and arsenic (table 1). Data for these 

geochemical and age-dating tracers are not maintained in 

the CDPH database. Well construction data were also not 

available for most CDPH wells. The three study units were 

aggregated into one dataset for the understanding assessment.

Nonparametric statistical methods were used to test the 

significance of correlations between water-quality parameters 

and potential explanatory factors. Nonparametric statistics 

are robust techniques that generally are not affected by 

outliers and do not require that the data follow any particular 

distribution (Helsel and Hirsch, 2002). The significance level 

(p) used for hypothesis testing for this report was compared to 

a threshold value (α) of 5% (α=0.05) to evaluate whether the 

relation was statistically significant (p < α). 

Three different statistical tests were used because the 

set of potential explanatory factors included categorical and 

continuous variables. Groundwater age, aquifer lithology, 

well depth class, and study unit were treated as categorical 

variables because there were a finite number of values a 

well could be assigned: for example, groundwater ages were 

classified as modern, pre-modern, or mixed. Land use, septic 

tank density, aridity index, elevation, well depth, depth to top 

of screened or open interval, dissolved oxygen, and pH were 

treated as continuous variables because there were an infinite 

number of values a well could be assigned: for example, 

land use was represented by percentages of land-use types. 

Well depth was treated as both a continuous (depth in feet 

below land surface) and categorical (shallow, deep, or mixed) 

variable. Concentrations of water-quality constituents were 

treated as continuous variables. 

Correlations between potential explanatory factors, 

and between potential explanatory factors and water-quality 

constituents were tested for significance.

• Correlations between continuous variables were 

evaluated using the Spearman’s rho test to calculate 

the rank-order coefficient (ρ, rho) and the significance 

level of the correlation (p).

• Relations between categorical variables and continuous 

variables were evaluated using the Wilcoxon rank-sum 

and Kruskal-Wallis tests. The Kruskal-Wallis test is 

equivalent to the Wilcoxon rank-sum test when the 

categorical variable has only two possible values. The 

null hypothesis for both tests is that median values 

of the continuous variable in each of the independent 

groups defined by the categorical variable are not 

significantly different from one another. If the KruskalWallis test yielded a result of significance, then the 

Wilcoxon rank-sum test was applied pair-wise to the 

groups to determine which pairs had significantly 

different median values of the continuous variable.

• Relations between categorical variables were evaluated 

using contingency tables. For a contingency table 

analysis, the data are recorded as a matrix of counts. 

One variable is assigned to the columns and the 

other to the rows, and the entries in the cells of the 

matrix are the number of observations, Oij, which 

fall into the ith row and j

th column of the matrix. A 

test statistic is computed by comparing the observed 

counts (Oij) to the counts expected if the two variables 

are independent, and significance is determined by 

comparing the test statistic to the (1–α) quantile of a 

chi-squared distribution. If the contingency table test 

yielded a result of significance, then the location of the 

most important pairs was determined by comparing 

magnitudes of the components of the test statistic.
42 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Evaluation of Potential 

Explanatory Factors

The values assigned to wells for potential explanatory 

factors (land use, aquifer lithology and hydrologic conditions, 

well depth and groundwater age distribution, and geochemical 

conditions) are described in this section. Correlations among 

these potential explanatory factors that could affect apparent 

relations between potential explanatory factors and water 

quality are also described. Methods used for assigning the 

values are described in appendix C. 

Land Use

Briefly, land use was quantified as the percentages of 

three land-use types: agricultural, urban, and undeveloped. 

Land-use percentages were calculated for study units, study 

areas, and within a radius of 500 m (500-m buffers) around 

wells (Johnson and Belitz, 2009).

Land use based on all of the land within the study unit 

boundaries (figs. 5A,B,C) is > 88% undeveloped for all three 

study units (fig. 6A). In the Tahoe-Martis and Central Sierra 

study units, the remainder is urban land use. Only the Southern 

Sierra study unit contains areas of agricultural land use (8%). 

Compared to the land use in the entire study unit, the average 

land use around the CDPH wells in all three study units is 20 

to 30% more urban, and in the case of the Southern Sierra 

study unit, also almost 10% more agricultural. In contrast, the 

average land use around the grid wells in all three study units 

is only 10% more urban or agricultural than the overall land 

use in the study unit (fig. 6A). The difference between overall 

land use and land use around wells reflects the fact that wells 

are preferentially located where there are people living and 

working. The difference between the average land use around 

the CDPH wells and around the grid wells reflects the spatially 

distributed nature of the grid wells. The CDPH wells are more 

biased towards urban land use because more urban areas 

typically have a higher density of CDPH wells.

For the Tahoe-Martis study unit, overall land use is 12% 

urban, average land use around the grid wells is 23% urban, 

and average land use around the CDPH wells is 40% urban 

(fig. 6A). Land use around individual grid wells ranges from 

0 to 91% urban (table C1; fig. 6B). The TTAHO study area 

has a higher proportion of urban land use than do the TMART 

and TROCK study areas (fig. 5A), and accordingly, 10 of 

the 19 USGS-grid wells with ≥12% urban land use are from 

the TTAHO study area (table C1; fig. 6B). Average land use 

around the understanding wells is 49% urban in the TTAHO 

study area (fig. 6A) because 8 of the 11 USGS-understanding 

wells are in that study area (fig. 8A). The majority of the 

CDPH wells in the Tahoe-Martis study unit also are in the 

TTAHO study area (fig. 8A); thus, the average land use around 

the CDPH wells is strongly biased towards urban.

For the Central Sierra study unit, overall land use is 5% 

urban, average land use around the grid wells is 14% urban, 

and average land use around the CDPH wells is 27% urban 

(fig. 6A). Land use around individual grid wells ranges from 0 

to 71% urban (table C1; fig. 6B). Average land use around grid 

wells in the CGOLD and CWISH study areas is similar (15 

and 12% urban, respectively), although the areas of densest 

urban land use (the towns of Oakhurst and Ahwanee) are both 

in the CGOLD study area (fig. 5B).

For the Southern Sierra study unit, overall land use is 

2% urban and 8% agricultural, average land use around the 

grid wells is 5% urban and 16% agricultural, and average 

land use around the CDPH wells is 21% urban and 17% 

agricultural. Land use around individual grid wells typically 

is either a mixture of urban and undeveloped (0 to 49% 

urban) or a mixture of agricultural and undeveloped (0 to 89% 

agricultural) (table C1; fig. 6B). Average land use around the 

USGS-understanding wells is 6% urban and 52% agricultural 

(fig. 6A) because 8 of the 15 understanding wells are located 

in agricultural fields in the Cummings Valley and Tehachapi 

Valley basins (fig. 8C). Average land use around the CDPH 

wells is more urban and more agricultural than overall land 

use in the study unit because the majority of the wells are 

located in the CDWR groundwater basins, where all of the 

agricultural land use and most of the urban land use are 

concentrated.

The density of septic tanks in the 500-m buffers around 

the USGS wells in the three study units ranges from 0 to 

138 tanks per square kilometer (tanks/km2) (table C1). Wells 

in the Tahoe-Martis study unit have significantly lower septic 

tank densities (median = 0.5 tank/km2) than do wells in the 

Central Sierra (median = 4.4 tanks/km2) or Southern Sierra 

(median = 2.7 tanks/km2) study units (fig. 9; p <  0.001). 

For comparison, the median septic tank density around the 

1,810 wells sampled by USGS for the GAMA Priority Basin 

Project during 2004–2008 was 2.8 tanks/km2 (Tyler Johnson, 

U.S. Geological Survey, California Water Science Center, 

written commun., 2009). The low density in the Tahoe-Martis 

study unit reflects regulations that have required development 

of sewer systems and forbidden installation of septic systems 

in the Lake Tahoe watershed since 1972 to protect lake water 

quality (State of California, 1971). Septic-tank density likely 

is elevated in the Central Sierra study unit because local 

communities have only recently begun building sewer systems 

in response to recent population growth.
Evaluation of Potential Explanatory Factors 43

sac09-0340_fig 09

0

10

20

30

40

50

60

80

160

Tahoe-Martis Central Sierra Southern Sierra 

STUDY UNIT

SEPTIC TANK DENSITY, IN TANKS PER SQUARE KILOMETER

EXPLANATION

90th percentile

10th percentile

median

(50th percentile)

lower detached

25th percentile

75th percentile

upper detached

Tahoe-Martis significantly different 

from Central Sierra (p<0.001) and 

Southern Sierra (p<0.001)

Figure 9. Density of septic tanks in the areas surrounding wells, Tahoe-Martis, Central 

Sierra, and Southern Sierra study units, California GAMA Priority Basin Project.

Aquifer Lithology and Hydrologic Conditions

As discussed in the section “Hydrogeologic Setting,” 

the geologic settings of the Tahoe-Martis, Central Sierra, and 

Southern Sierra study units are complex, and wells are located 

in a range of rock types. For the purpose of examining broad 

relations between aquifer lithology and water quality, the 

geology of the study units was simplified into four rock types 

(appendix C):

• Granitic rocks: Mesozoic granitic rocks of the Sierra 

Nevada batholith;

• Metamorphic rocks: Mesozoic and Paleozoic 

metavolcanic, metasedimentary, mafic, and 

ultramafic rocks;

• Volcanic rocks: Cenozoic volcanic and volcaniclastic 

rocks; and

• Sedimentary deposits: Cenozoic alluvial, glacial, 

fluvial, and lacustrine sediments.

The Cenozoic volcanic rocks and sedimentary deposits include 

units that are Tertiary (primarily Miocene and Pliocene in age) 

and Quaternary (Pleistocene and Holocene epochs) in age. 

Analysis of groundwater chemistry as a function of geologic 

variations within each of the four classes is beyond the scope 

of this report. Chemical and mineralogical data for aquifer 

materials are not available for the wells used in this study.
44 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

have sedimentary surficial geology, but are screened or open 

in volcanic rocks at depth. Grid cells in the TROCK study 

area were evenly split between representation by wells with 

volcanic and granitic aquifer lithologies, with one well with 

sedimentary lithology.

Of the 29 cells with grid wells in the Central Sierra study 

unit (table A1), 27 are represented by wells with granitic 

aquifer lithology (fig. 10; table C1), in accordance with the 

dominance of granitic rocks over metamorphic rocks in the 

study unit (fig. 4B). 

0

5

10

15

20

25

30

35

40

45

NUMBER OF WELLS

TahoeMartisCentral

Sierra

Southern

Sierra

TahoeMartisCentral

Sierra

Southern

Sierra

GRID WELLS UNDERSTANDING WELLS

Granitic rocks (G)

G in well-log, G on geologic map

G in well-log, M on geologic map

G in well-log, S on geologic map

Aquifer lithology

EXPLANATION

Metamorphic rocks (M)

M in well-log, M on geologic map

M in well-log, V on geologic map

M in well-log, lake on geologic map

Sedimentary deposits (S)

S in well-log, S on geologic map

Volcanic rocks (V)

V in well-log, S on geologic map

V in well-log, V on geologic map

Figure 10. Aquifer lithology classification for grid wells and USGS-understanding wells, Tahoe-Martis, Central 

Sierra, and Southern Sierra study units, California GAMA Priority Basin Project.

Of the 41 cells with grid wells in the Tahoe-Martis study 

unit, 16 are represented by wells with sedimentary aquifer 

lithology, 18 by wells with volcanic aquifer lithology, and 7 

by wells with granitic aquifer lithology (fig. 10). Wells from 

the three study areas within the Tahoe-Martis study unit have 

markedly different aquifer lithologies (table C1). Thirteen 

of 14 grid cells in the TTAHO study area are represented 

by wells with sedimentary aquifer lithology. Twelve of 

14 grid cells in the TMART study area are represented by 

wells with volcanic aquifer lithology; 8 of these 12 wells 
Evaluation of Potential Explanatory Factors 45

wet

humid

dry sub-humid

semi-arid

arid

hyper-arid

Tahoe-Martis

Central Sierra

Southern Sierra

Study unit

EXPLANATION

0 2,000 4,000 6,000 8,000

0.00

0.25

0.50

0.75

1.00

1.25

1.50

1.75

ELEVATION OF WELL SITE, IN FEET NAVD 88

ARIDITY INDEX, DIMENSIONLESS

Climate

Classification

Figure 11. Relation between elevation and aridity index at well sites, Tahoe-Martis, Central Sierra, and 

Southern Sierra study units, California GAMA Priority Basin Project.

Of the 38 cells with grid wells in the Southern Sierra 

study unit (table A1), 21 are represented by wells with 

granitic or metamorphic aquifer lithology, 16 by wells with 

sedimentary aquifer lithology, and 1 by a well with volcanic 

aquifer lithology (fig. 10; table C1). Five wells with granitic 

aquifer lithology have sedimentary surficial geology but 

are primarily or solely screened or open in granitic rocks 

beneath the thin veneer of alluvial or fluvial sediments at the 

margins of the Tehachapi Valley, Cummings Valley, and Kern 

River Valley basins (table C1). Fourteen of the 15 USGS 

understanding wells have sedimentary aquifer lithology, and 

are located in the central parts of the Tehachapi Valley and 

Cummings Valley groundwater basins where the thickness of 

alluvial sediments is much greater.

Hydrologic conditions are represented by elevation 

and aridity index at the well site (table C2). Elevation was 

considered a proxy for relative position in a generalized 

regional groundwater flow system. The aridity index was 

used as an indicator of climate. The aridity index is defined 

as average annual precipitation divided by average annual 

evapotranspiration, and is equal to the UNESCO Aridity 

Index (United Nations Educational, Scientific, and Cultural 

Organization, 1979; United Nations Environment Programme, 

1997). The relation between aridity index and elevation 

is different for the three study units (fig. 11). Climate in 

the Tahoe-Martis study unit is dry sub-humid to humid at 

elevations below 6,500 ft, and climate at elevations above 

6,500 ft is typically characterized as wet. Climate in the 

Central Sierra study unit ranges from semi-arid at low 

elevations to wet at high elevations. Most of the Southern 

Sierra study unit is characterized by a semi-arid climate.
46 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Well Depth and Groundwater Age Distribution

Well construction characteristics differ among the three 

study units (table C2; figs. 12A,B). Grid wells from the 

Tahoe-Martis study unit have the widest ranges of both well 

depth and depth to top of screened or open interval, largely 

because of the large differences in geologic setting among 

the three study areas (fig. 4A). The median depth of TMART 

grid wells is 606 feet below land surface (ft bls), which is 

significantly greater than the median depth of TROCK grid 

wells (median = 73 ft bls; p=0.003) and greater than the 

depths of most of the TTAHO wells (median = 217 ft bls). 

Five of the 12 TROCK grid wells are springs emerging at 

ground surface rather than wells, and are defined as having 

depth and depth to top screened or open interval of 0 ft bls for 

the purposes of this report (table C2). TMART grid wells also 

had the greatest depth to top of the screened or open interval 

(median = 175 ft bls) (fig. 12B). The Truckee River Operating 

Agreement places restrictions on well locations and depths 

to the top of the screened or open interval for wells in the 

Martis Valley to minimize pumping from the shallow aquifer 

that may reduce flows in the Truckee River (U.S. Bureau of 

Reclamation, 2008). 

The median depth of grid wells from the Central Sierra 

study unit is 620 ft bls, with half of the wells having depths 

between 440 and 885 ft bls. Median depth to the top of 

the screened or open interval is 60 ft bls, with half of the 

wells having depth to top of screened or open interval in 

a narrow range of 52 to 92 ft bls. All of the Central Sierra 

study unit grid wells are drilled in hard rock, and many 

are open holes (no casing) below 50 ft bls. There were no 

significant differences in well construction characteristics 

between CGOLD and CWISH wells nor between grid and 

understanding wells. 

Compared to wells from the Central Sierra study unit, 

wells from the Southern Sierra study unit have shallower 

depths (median = 285 ft bls; p < 0.001), and greater depths 

to the top of the screened or open interval (figs. 12A,B). 

Within the Southern Sierra study unit, well depths varied 

with geologic setting (not shown). Grid wells from the Kern 

River Valley area have a median depth of 150 ft bls and a 

median depth to the top of the screened interval of 60 ft 

bls. In contrast, grid wells from the Tehachapi-Cummings 

Valley area are significantly deeper, with a median depth of 

375 ft bls (p=0.007) and a median depth to the top of screened 

interval of 140 ft bls (p=0.012). Understanding wells in the 

Tehachapi-Cummings Valley are typically deeper than the grid 

wells, but the differences were not significant (p > 0.3). 

Groundwater samples, particularly samples from 

wells with long screened or open intervals, typically 

represent mixtures of groundwaters that have different ages. 

Groundwater “age” refers to residence time in the aquifer 

system, which is the amount of time elapsed since the water 

was last in contact with the atmosphere. Data for the agedating tracers tritium, carbon-14, and helium-4 were used to 

classify groundwater age distributions into three categories: 

modern, mixed, and pre-modern (table C3). Samples with 

tritium activities indicating presence of a substantial amount 

of water recharged since approximately 1950 were classified 

as containing “modern” groundwater. Samples with carbon-14 

activities and helium-4 abundances indicating presence 

of a substantial amount of water recharged more than 

approximately 100 years ago were classified as containing 

“pre-modern” groundwater. Samples with characteristics of 

both modern and pre-modern groundwaters were classified 

as “mixed”. Groundwater age was treated as both a 2-factor 

classification and a 3-factor classification for tests of statistical 

relations between potential explanatory factors and water 

quality. For the 2-factor classification, the modern and 

mixed categories were combined into one category, “young” 

groundwater.

Of the 132 samples collected by USGS-GAMA, 

26 were classified as having “modern” groundwater age 

distributions, 53 as having “mixed” age distributions, and 39 

as having “pre‐modern” age distributions (table C4). Fourteen 

samples were classified as “modern or mixed” because they 

had insufficient data to confirm the presence or absence of 

pre-modern groundwater. (See appendix C for discussion.) 

Classified groundwater ages typically increase with 

well depth and with depth to the top of the screened or 

open interval. Groundwater with modern age distributions 

came from wells with significantly shallower depths than 

groundwater with mixed (p < 0.001) or pre-modern age 

distributions (p < 0.001) (fig. 13B). Groundwater with premodern age distributions came from wells with significantly 

greater depths to the top of the screened or open interval 

than groundwater with modern (p=0.001) or mixed age 

distributions (p=0.002) (fig. 13A). 
Evaluation of Potential Explanatory Factors 47

sac09-0340_fig 12a

0

200

400

600

800

1,000

1,200

1,400

90th percentile

10th percentile

median

(50th percentile)

lower detached

25th percentile

75th percentile

upper detached

12 number of wells

12 14 14 24 30 11 2 15

UNDERSTANDING WELLS

Central

Sierra

Southern

Sierra

TahoeMartis

TROCK

TahoeMartis

TTAHO

TahoeMartis

TMART

Central

Sierra

Southern

Sierra

TahoeMartis

GRID WELLS

WELL DEPTH, IN FEET BELOW LAND SURFACE

EXPLANATION

A

A

Central Sierra

Southern Sierra

Study units

Tahoe-Martis (separated into study areas 

for grid wells: TROCK, Hard Rock; 

TMART, Martis; TTAHO, Tahoe)

Figure 12. (A) Well depths and (B) depths to top of screened or open interval for grid and understanding wells, 

Tahoe-Martis, Central Sierra, and Southern Sierra study units, California GAMA Priority Basin Project.

100

0

200

300

400

500

DEPTH TO TOP OF SCREENED OR OPEN INTERVAL, 

IN FEET BELOW LAND SURFACE

9 13 13 23 29 11 2 11

UNDERSTANDING WELLS

Central

Sierra

Southern

Sierra

TahoeMartis

TROCK

TahoeMartis

TTAHO

TahoeMartis

TMART

Central

Sierra

Southern

Sierra

TahoeMartis

GRID WELLS

B
48 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac09-0340_fig 13

EXPLANATION

90th percentile

10th percentile

median

(50th percentile)

lower detached

25th percentile

75th percentile

upper detached

37 number of wells

Modern significantly 

different from mixed (p<0.001) 

and pre-modern (p<0.001)

Pre-modern significantly 

different from modern (p<0.001) 

and mixed (p=0.002)

200

400

600

800

1,000

1,200

1,400

100

0

0

200

300

400

500

WELL DEPTH, IN FEET DEPTH TO TOP OF SCREENED OR OPEN INTERVAL, IN FEET

GROUNDWATER AGE CLASSIFICATION

Modern Mixed Pre-modern

28 51 37

23 48 34

A

B

Figure 13. (A) Depths to top of screened or open interval and (B) well depths 

for wells with groundwater classified as modern, mixed, or pre-modern in age, 

Tahoe-Martis, Central Sierra, and Southern Sierra study units, California GAMA 

Priority Basin Project.
Evaluation of Potential Explanatory Factors 49

Classified groundwater ages were used to create a 

3-factor classification system for well depth. Wells of depth 

less than a critical depth were defined as shallow; wells with 

screened or open intervals beginning above the critical depth 

and ending below the critical depth were defined as mixed; 

and wells with screened or open intervals entirely below 

the critical depth were defined as deep. The critical depth 

of 170 ft bls was selected by optimizing the segregation of 

modern-age groundwater into shallow wells, mixed-age 

groundwater into mixed-depth wells, and pre-modern-age 

groundwater into deep wells.

Classified

well depth

Well

depth

Depth to top of 

the screened or 

open interval

Shallow < 170 ft bls < 170 ft bls

Mixed ≥ 170 ft bls < 170 ft bls

Deep ≥ 170 ft bls ≥ 170 ft bls

In the Tahoe-Martis study unit, most shallow wells 

had modern-age groundwater, most mixed-depth wells 

had mixed-age groundwater, and most deep wells had 

pre-modern-age groundwater (fig. 14A). Most of the wells in 

the Central Sierra study unit were classified as mixed depth, 

and the majority had mixed-age groundwater (fig. 14B). The 

segregation of groundwater ages into well depth classes was 

least strong in the Southern Sierra study unit (fig. 14C). Three 

depth categories in the Tahoe-Martis and Southern Sierra 

study units and the mixed-depth category in the Central Sierra 

study unit had some wells with each of the classified ages. 

These results indicate that there are local variations in the 

general age-depth relation.

NUMBER OF WELLS

TAHOE-MARTIS

STUDY UNIT

0 5 10 0 5 1510 0 5 10

DEPTH OF SCREENED OR OPEN INTERVAL

BELOW LAND SURFACE

Entire interval

< 170 ft

Entire interval

> 170 ft

Top of interval

< 170 ft; 

Bottom of 

interval > 170 ft

NUMBER OF WELLS

CENTRAL SIERRA

STUDY UNIT

NUMBER OF WELLS

SOUTHERN SIERRA

STUDY UNIT

EXPLANATION

Modern

Mixed

Pre-modern

Groundwater age

classification

A B C

< is less than;

> is greater than or

 equal to

Figure 14. Relation between groundwater age classification and well depth classification for the 

(A) Tahoe-Martis, (B) Central Sierra, and (C) Southern Sierra study units, California GAMA Priority Basin 

Project.
50 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Geochemical Conditions

Sufficient chemical data, including dissolved oxygen 

(DO) concentration, were available for 125 wells to classify 

their groundwater oxidation-reduction (redox) conditions 

(appendix C). Groundwater conditions were primarily oxic 

(76%), with 20% of the wells anoxic and 4% of the wells with 

mixed oxic/anoxic conditions (table C5). Wells with volcanic 

or sedimentary aquifer lithology typically had oxic conditions, 

regardless of well depth (fig. 15A). Anoxic conditions 

(DO <  0.5 mg/L) were more prevalent in wells with granitic 

or metamorphic aquifer lithology, particularly in deeper 

wells (fig. 15A). Anoxic conditions were further subdivided 

into suboxic, nitrate-reducing, manganese-reducing, and 

manganese-and-iron-reducing conditions (table C5).

Values for pH ranged from 5.3 to 9.4 (table C5; fig. 15B). 

Wells with granitic aquifer lithology included groundwater 

spanning the full range of pH values observed in the three 

study units. The range of pH values observed in groundwater 

from wells with sedimentary aquifer lithology was slightly 

narrower (6.0 to 9.2). All of the wells with sedimentary 

aquifer lithology and pH values > 8.1 were from the Tahoe 

Valley South subbasin. Wells with volcanic aquifer lithology 

had groundwater with the most restricted range of pH values 

(6.9 to 8.2).

Correlations Between Explanatory Factors

Apparent correlations between potential explanatory 

factors and water-quality constituents could result from 

correlations among potential explanatory factors; therefore, 

identification of statistically significant correlations between 

potential explanatory factors is important (tables 6A,B,C). 

The potential explanatory factors examined for the three study 

units are extensively correlated with one another. 

Nearly all of the potential explanatory factors are 

significantly correlated with aquifer lithology (tables 6A,C). 

Sedimentary aquifer lithology is positively correlated with 

urban and agricultural land use and negatively correlated with 

undeveloped land use. These relations reflect the fact that 

urban and agricultural development preferentially occur on 

sedimentary basin deposits because the land is flatter and the 

soil is typically more suitable for agriculture. Higher aridity 

index is significantly correlated with volcanic and granitic 

lithologies because it also is significantly greater in the two 

more northern study units (fig. 11; table 6A): nearly all of the 

wells with volcanic aquifer lithology are in the Tahoe-Martis 

study unit and three-quarters of the wells with granitic aquifer 

lithology are in the Central Sierra or Tahoe-Martis study units 

(fig. 10; table 6C). Aridity index typically increases with 

elevation and with latitude because of increasing precipitation.

Correlations between aquifer lithology and the 

geochemical potential explanatory factors, DO and pH, likely 

reflect causative relationships, but these may be partially 

obscured by the non-causative relations between aquifer 

lithology and other explanatory factors. DO concentrations 

and pH values are significantly greater in wells with 

sedimentary or volcanic aquifer lithology than in wells with 

granitic or metamorphic aquifer lithology (table 6A). DO has 

a significant inverse correlation with well depth (table 6B); 

however, wells with volcanic aquifer lithology and depths 

> 800 ft bls have DO concentrations of 4–10 mg/L (fig. 15A). 

Higher DO concentrations are significantly associated with 

groundwater with modern age distributions and well sites at 

higher elevations. Relatively recently recharged groundwater 

in shallow wells (and springs) near the proximal end of 

the regional groundwater system likely has not interacted 

extensively with organic matter or reduced inorganic aquifer 

materials, and thus would not have had its DO consumed. Well 

depth and depth to the top of screened or open interval have 

significant positive correlations with pH (fig. 15B; table 6B), 

and higher pH values are significantly associated with 

groundwater with pre-modern age distributions (table 6A). 

Precipitation is dilute and generally has low pH values 

controlled by equilibration between atmospheric carbon 

dioxide and dissolved carbonic acid (H2CO3). As this water 

recharges an aquifer, pH values generally rise as the carbonic 

acid is converted to bicarbonate (HCO3

–) by hydrolysis 

reactions with silicate minerals and dissolution of carbonate 

minerals (if present) (Stumm and Morgan, 1996).

Agricultural land use is positively correlated with depth 

to top of screened or open interval and negatively correlated 

to elevation and aridity index, which reflects the fact that 

agricultural land use is primarily located in centers of the 

Tehachapi-Cummings Valley of the Southern Sierra study 

unit, where wells and tops of screened intervals are relatively 

deep. The positive correlations between urban land use and 

sedimentary aquifer lithology and location in the Tahoe-Martis 

study unit are driven by the high degree of urbanization in the 

TTAHO study area (fig. 6B). 

Many other complex patterns of correlations involving 

aquifer lithology are evident in the statistical results 

(tables 6A,B,C), and these are discussed later in the report as 

part of the analysis of factors affecting individual constituents.
Evaluation of Potential Explanatory Factors 51

sac09-0340_fig 15ab

200

0

400

600

800

1,000

1,200

1,400

0 2 4 6 8 10

5 6 7 8 9 10

12 14 16 18

DISSOLVED OXYGEN CONCENTRATION, 

IN MILLIGRAMS PER LITER

WELL DEPTH, IN FEET BELOW LAND SURFACE

0

100

200

300

400

500

pH, IN STANDARD UNITS

DEPTH TO TOP OF SCREENED OR OPEN INTERVAL,

IN FEET BELOW LAND SURFACE

B

A

Tahoe-Martis study unit

Granitic rock

Sedimentary deposit

Volcanic rock

Aquifer lithology

EXPLANATION

Central Sierra study unit

Granitic rock

Metamorphic rock

Southern Sierra study unit

Granitic rock

Metamorphic rock

Sedimentary deposit

Volcanic rock

Figure 15. Relations between (A) well depth and dissolved oxygen concentration and (B) depth 

to top of screened or open interval and pH for wells with various aquifer lithologies, Tahoe-Martis, 

Central Sierra, and Southern Sierra study units, California GAMA Priority Basin Project.
52 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Table 6A. Results of Kruskal-Wallis and Wilcoxon rank-sum tests for differences in values of selected potential explanatory factors 

between samples classified into groups by aquifer lithology, groundwater age, study unit, or well depth, Tahoe-Martis, Central Sierra, 

and Southern Sierra study units, 2006–2007, California GAMA Priority Basin Project.

[Relation of median values in sample groups tested shown for Kruskal-Wallis and Wilcoxon rank-sum tests in which the populations were determined to be 

significantly different (two-sided test) on the basis of p-values (not shown) less than threshold value (α) of 0.05; ns, test indicated no significant differences 

between the sample groups. p-values were calculated using the Kruskal-Wallis test; if significant, then Wilcoxon rank-sum tests used to determine which 

differences were significant. Aquifer lithology class: G, granitic rocks; M, metamorphic rocks; S, sedimentary deposits; V, volcanic rocks. Study unit: T, 

Tahoe-Martis; C, Central Sierra; S, Southern Sierra. 3-factor depth class: shallow (Shal), well depth < 170 ft; Mixed (Mix), top of perforations < 170 ft and 

well depth > 170 ft; Deep (Deep) well depth > 170 ft and top of perforations > 170 ft. 2-factor age class: young, tritium > 0.5 TU; old, tritium < 0.5 TU. 

3-factor age class: Mod, modern; Mix, mixed modern and pre-modern; Pre, pre-modern (see appendix C for explanation). Other abbreviations: ns, no 

significant differences; TU, tritium units; ft, feet below land surface; >, greater than; <, less than]

Potential explanatory factor

Significant differences

Aquifer lithology 

class

(G, M, S, V)

Study unit 

(T, C, S)

3-Factor depth class 

(Shal, Mix, Deep)

2-Factor age class 

(young, old)

3-Factor age class 

(Mod, Mix, Pre)

Percent agricultural land use S > GV

M > V

S > CT Deep > ShalMix ns ns

Percent undeveloped land use GMV > S C > S ns ns ns

Percent urban land use S > GMV T > S ns ns ns

Density of septic tanks GM > S C > ST Mix > ShalDeep ns MixPre > Mod

Aridity index V > G > MS T > C > S ns ns Mod > MixPre

Elevation V > S > GM T > S > C ShalDeep > Mix ns Mod > Pre > Mix

Well depth GV > MS C > ST MixDeep > Shal old > young MixPre > Mod

Depth to top of perforation SV > GM ST > C Deep > Mix > Shal old > young Pre > ModMix

pH V > GM

S > G

ns Deep > Mix > Shal old > young Pre > Mix > Mod

Dissolved oxygen concentration SV > GM T > S > C ns ns Mod > MixPre

Explanation:

How to read results for significant differences. “GM > S” for density of septic tanks means the following:

G has significantly greater density than S.

M has significantly greater density than S.

Density in G is not significantly different than density in M.

Density in V is not significantly different than density in G, M, or S.
Evaluation of Potential Explanatory Factors 53

Table 6B. Results of Spearman’s tests for correlations between selected potential explanatory factors, Tahoe-Martis, Central Sierra, 

and Southern Sierra study units, 2006–2007, California GAMA Priority Basin Project.

[ρ (rho), Spearman’s correlation statistic; ρ values are shown for tests in which the variables were determined to be significantly correlated on the basis of p 

values (significance level of the Spearman’s test) less than threshold value (α) of 0.05 (not shown); ns, Spearman’s test indicates no significant correlation 

between factors; black text, significant positive correlation; red text, significant negative correlation]

ρ

Percent land use Density of 

septic

tanks

Aridity

index Elevation Welldepth

Depth 

to top of 

perforation

pH

Dissolved 

oxygen 

concentration

AgriculturalUndeveloped Urban

Percent

agricultural 

land use

–0.40 –0.29 ns –0.65 –0.21 ns ns ns ns

Percent

undeveloped

land use –0.66 ns 0.29 ns ns –0.29 –0.20 ns

Percent urban

land use ns ns 0.19 ns ns ns ns

Density of 

septic tanks –0.35 –0.53 0.33 ns ns –0.19

Aridity index

0.63 ns ns ns ns

Elevation

–0.26 ns ns 0.44

Well depth

0.39 0.39 –0.23

Depth to top 

of perforation 0.46 ns

pH

ns

Dissolved 

oxygen

concentration
54 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Table 6C. Results of contingency table tests for associations between aquifer lithology, study unit, well depth, and groundwater age, 

Tahoe-Martis, Central Sierra, and Southern Sierra study units, 2006–2007, California GAMA Priority Basin Project.

[Factors determined to be significantly associated on the basis of p-values (not shown) less than threshold value (α) of 0.05 for contingency table test; ns, test 

indicated no significant association between factors. For factors with significant association, combinations of classes contributing more than 10 percent to test 

statistic are listed; black text, classes occur together; red text, classes do not occur together. Aquifer lithology class: G, granitic rocks; M, metamorphic rocks; 

S, sedimentary deposits; V, volcanic rocks. Study Unit: T, Tahoe-Martis; C, Central Sierra, S, Southern Sierra. 3-factor depth class: shallow (Shal), well depth 

< 170 ft; Mixed (Mix), top of perforations < 170 ft and well depth > 170 ft; Deep (Deep) well depth > 170 ft and top of perforations > 170 ft. 2-factor age class:

young, tritium > 0.5 TU; old, tritium < 0.5 TU. 3-factor age class: Mod, modern; Mix, mixed modern and pre-modern; Pre, pre-modern (see appendix C for 

explanation). Other abbreviations: ns, no significant differences; TU, tritium units; ft, feet below land surface; >, greater than; <, less than]

Potential 

explanatory factor

Most significant associations

Aquifer lithology 

class

(G, M, S, V)

Study unit 

(T, C, S)

3-Factor depth class 

(Shal, Mix, Deep)

2-Factor age class 

(young, old)

3-Factor age class 

(Mod, Mix, Pre)

Aquifer lithology class

(G, M, S, V) C + G

T + V

C – S

Shal + M

Deep + C

Deep – M

Mix + G

ns ns

Study unit 

(T, C, S)

Mix + C

Deep – C

Shal – C

Mix – T

ns

Mod + T

Mix – T

Mod – C

Mix + C

Mod – S

3-Factor depth class 

(Shal, Mix, Deep) old + Deep

young – Deep

old – Shal

Mod + Shal

Pre + Deep

Mix + Mix

2-Factor age class 

(young, old) Pre + old

Pre – young

Mix – old

Mod – old

3-Factor age class 

(Mod, Mix, Pre)

Explanation: How to read results for factors with significant associations:

Study unit and aquifer lithology are significantly associated. The most important combinations of classes contributing to the significance of the association 

are: (1) Central Sierra study unit has wells with granitic aquifer lithology, (2) Tahoe-Martis study unit has wells with volcanic aquifer geology, and 

(3) Central Sierra study unit does not have wells with sediment aquifer lithology.
Status and Understanding of Water Quality 55

Status and Understanding of 

Water Quality

The status assessment was designed to identify the 

constituents or classes of constituents most likely to be 

water-quality concerns on the basis of their high RCs or their 

prevalence. The assessment applies only to constituents having 

regulatory or non-regulatory health-based or aesthetic-based 

benchmarks established by the USEPA or the CDPH (U.S. 

Environmental Protection Agency, 1999, 2009a,b; California 

Department of Public Health, 2006, 2008, 2010). The spatially 

distributed, randomized approach to well selection and data 

analysis yields a view of groundwater quality in which all 

areas of the primary aquifers are weighted equally; regions 

with a high density of groundwater use or with high density of 

potential contaminants were not preferentially sampled (Belitz 

and others, 2010).

The understanding assessment was designed to help 

answer the question of why specific constituents are, or 

are not, observed in the groundwater, and may improve 

our understanding of how human and natural sources of 

contaminants affect groundwater quality in the three study 

units. The assessment addresses a subset of the constituents 

selected for additional evaluation in the status assessment, and 

is based on statistical correlations between water quality and 

a finite set of potential explanatory factors. The assessment 

was not designed to identify specific sources of constituents to 

specific wells. 

The following discussion of the status and understanding 

assessment results is divided into two parts, one for inorganic 

constituents and the other for organic constituents, and each 

part has a tiered structure. Each part begins with a survey of 

how many constituents were detected at any concentration 

in USGS-GAMA samples compared to the number analyzed 

and a graphical summary of the RCs of constituents detected 

in the grid wells. Aquifer-scale proportions are presented 

for the subset of constituents that met criteria for additional 

evaluation based on RC, or for organic constituents, 

prevalence. Understanding assessment results are presented 

for the subset of status assessment constituents that had 

statistically significant correlations to potential explanatory 

factors. For constituents that have understanding assessment

results, those results are presented immediately following the 

status assessment results for that constituent. 

Inorganic Constituents

Inorganic constituents generally occur naturally 

in groundwater, although their concentrations may be 

influenced by human activities as well as by natural factors. 

All 50 inorganic constituents analyzed by USGS-GAMA 

were detected in groundwater from at least one of the 

three study units, and most were detected in all three 

study units (table 7A). Of these 50 constituents, 27 had 

regulatory or non-regulatory health-based benchmarks, 8 had 

non-regulatory aesthetic-based benchmarks, and 15 had no 

established benchmarks. Of the 35 inorganic constituents with 

benchmarks, 23 were identified for additional evaluation in 

the status assessment (table 3). The 27 inorganic constituents 

not selected for additional evaluation either had no established 

benchmarks or were only detected at low concentrations 

relative to their benchmarks (table 4). Most of the constituents 

without benchmarks are major or minor ions that are present 

in nearly all groundwater.

Nineteen of the 23 inorganic constituents selected for 

additional evaluation in the status assessment were included 

because they were detected at moderate or high RCs in the 

grid wells for at least one of the three study units: nitrate, 

arsenic, boron, fluoride, lead, molybdenum, strontium, 

uranium, iron, manganese, zinc, chloride, sulfate, total 

dissolved solids (TDS), specific conductance, gross alpha 

particle activity, uranium activity (sum of uranium-234, 

uranium-235, and uranium-238), radium activity (sum of 

radium-226 and radium-228), and radon-222 activity (table 3; 

figs. 16A,B,C). The majority of these 19 constituents were 

detected at moderate or high RCs in more than 10% of the grid 

wells for at least one of the three study units (fgs. 17A,B,C).

Uranium is shown as both a radioactive constituent and 

a trace element in figures 16A,B,C and 17A,B to demonstrate 

that similar results were obtained for comparison of 

uranium in groundwater to the MCL-CA of 20 picocuries 

per liter (pCi/L) or to the MCL-US of 30 μg/L. Uranium 

is classified as a radioactive constituent for calculations of 

aquifer-scale proportions of constituent classes. Similarly, 

total dissolved solids and specific conductance are both shown 

on figures 16A,B,C and 17C to demonstrate that similar 

results were obtained with these two different measures of 

groundwater salinity, and with grid-well datasets dominated 

by USGS-GAMA and CDPH data. For the Southern Sierra 

study unit, most of the specific conductance data were from 

USGS-grid wells, and most of the total dissolved solids data 

were from CDPH-grid wells (table 2). 
56 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Table 7A. Number of inorganic constituents analyzed and detected by U.S. Geological 

Survey-Groundwater Ambient Monitoring and Assessment (USGS-GAMA) by 

benchmark and constituent type, Tahoe-Martis, Central Sierra, and Southern Sierra 

study units, 2006–2007, California GAMA Priority Basin Project.

[Regulatory health-based benchmarks include: MCL-US, USEPA maximum contaminant level; 

AL-US, USEPA action level; and MCL-CA, CDPH maximum contaminant level. Non-regulatory 

health-based benchmarks include: HAL-US, USEPA lifetime health advisory level; and NL-CA, 

CDPH notification level. Non-regulatory aesthetic benchmarks include SMCL-CA, CDPH secondary 

maximum contaminant levels. Abbreviations: USEPA, U.S. Environmental Protection Agency; 

CDPH, California Department of Public Health]

Benchmark type Number 

analyzed

Number detected at any concentration

TahoeMartisCentral 

Sierra

Southern 

Sierra All

Major, minor, and trace elements

Regulatory health-based 15 114 15 14 15

Non-regulatory health-based 4 4 4 4 4

Non-regulatory aesthetic-based 8 7 7 8 8

No benchmark 12 12 12 12 12

Total: 39 37 38 38 39

Nutrients

Regulatory health-based 2 2 2 2 2

Non-regulatory health-based 1 1 1 1 1

Non-regulatory aesthetic-based 0 0 0 0 0

No benchmark 3 22 3 3 3

Total: 6 5 6 6 6

Radioactive

Regulatory health-based 5 5 5 34 5

Non-regulatory health-based 0 0 0 0 0

Non-regulatory aesthetic-based 0 0 0 0 0

No benchmark 0 0 0 0 0

Total: 5 5 5 4 5

Sum inorganic constituents

Regulatory health-based 22 21 22 20 22

Non-regulatory health-based 5 5 5 5 5

Non-regulatory aesthetic-based 8 7 7 8 8

No benchmark 15 14 15 15 15

Total: 50 47 49 48 50

1

 Mercury not analyzed in the Tahoe-Martis study unit.

2

 Dissolved organic carbon not analyzed in the Tahoe-Martis study unit.

3

 Uranium isotopes not analyzed in the Southern Sierra study unit.
Status and Understanding of Water Quality 57

Table 7B. Number of organic constituents analyzed and detected by U.S. Geological 

Survey-Groundwater Ambient Monitoring and Assessment (USGS-GAMA) by 

benchmark type and constituent type, Tahoe-Martis, Central Sierra, and Southern 

Sierra study units, 2006–2007, California GAMA Priority Basin Project.

[Regulatory health-based benchmarks include: MCL-US, USEPA maximum contaminant level; and 

MCL-CA, CDPH maximum contaminant level. Non-regulatory health-based benchmarks include: 

HAL-US, USEPA lifetime health advisory level; RSD5-US, USEPA risk-specific dose; NL-CA, 

CDPH notification level. Abbreviations: USEPA, U.S. Environmental Protection Agency; CDPH, 

California Department of Public Health; VOC, volatile organic carbon]

Benchmark type Number 

analyzed

Number detected at any concentration

TahoeMartisCentral 

Sierra

Southern 

Sierra All

VOCs

Regulatory 33 7 4 9 12

Non-regulatory 26 1 1 1 2

No benchmark 29 1 0 0 1

Total: 88 9 5 10 15

Pesticides and degradates1

Regulatory 7 1 2 2 2

Non-regulatory 19 0 0 1 1

No benchmark 59 2 2 2 4

Total: 85 3 4 5 7

Special interest

Regulatory 1 0 0 1 1

Non-regulatory 2 0 0 1 1

No benchmark 0 0 0 0 0

Total: 3 0 0 2 2

Sum organic and special-interest constituents

Regulatory 41 8 6 12 15

Non-regulatory 47 1 1 3 4

No benchmark 88 3 2 2 5

Total: 176 12 9 17 24

1

 Number analyzed refers to the Central Sierra study unit (table 1). For the Tahoe-Martis and 

Southern Sierra study units, the total number of pesticides and pesticide degradates analyzed was 63 

(3 with regulatory benchmarks, 17 with non-regulatory benchmarks, and 43 with no benchmarks).
58 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac09-0340_fig 16a

0.001

0.01

0.1

1

10

100

Bromodichloromethane

Carbon disulfide

Chloroform

Methyl tert-butyl ether

Perchloroethene

Trichloroethene

Atrazine

Ammonia

Nitrate

Nitrite

Aluminum

Antimony

Arsenic

Barium

Beryllium

Boron

Cadmium

Chromium

Copper

Fluoride

Lead

Molybdenum

Nickel

Selenium

Strontium

Thallium

Uranium

Vanadium

Chloride

Iron

Manganese

Specific conductance

Sulfate

Total dissolved solids

Zinc

Gross alpha

Gross beta

Radium

Radon-222

Uranium

Volatile

organic

compound

Pesticide SpecialinterestNutrient Trace

element

Radioactive

constituent

Inorganic

constituent

having

SMCLs

MAXIMUM RELATIVE-CONCENTRATION, DIMENSIONLESS

Organic and special-interest constituents Inorganic constituents

High

Moderate

Low

A 

EXPLANATION

Gross beta

Atrazine Name and center of symbol is location of data unless indicated by following 

location line: Constituent with analyses in >40 grid wells 

Name and center of symbol is location of data - Constituent with analyses 

in <25 grid wells and wells are not spatially representative

One volatile organic compound was detected at relative-concentration <0.001. SMCL, secondary maximum contaminant level; upper water-quality 

benchmark used for calculating relative-concentrations for constituents with an upper and recommended SMCL; >, greater than; <, less than

Relative-concentration

Tahoe-Martis study unit

Figure 16. Maximum relative-concentrations of constituents detected in grid wells by constituent class for 

the (A) Tahoe-Martis, (B) Central Sierra, and (C) Southern Sierra study units, California GAMA Priority Basin 

Project.
Status and Understanding of Water Quality 59

sac09-0340_fig 16b

100

10

1

0.1

0.01

0.001

Volatile

organic

compound

Pesticide SpecialinterestNutrient Trace

element

Radioactive

constituent

Inorganic

constituent

having

SMCLs

MAXIMUM RELATIVE-CONCENTRATION, DIMENSIONLESS

Organic and special-interest constituents Inorganic constituents

Benzene

Chloroform

Methyl tert-butyl ether

Perchloroethene

Atrazine

Simazine

Nitrate

Nitrite

Aluminum

Antimony

Arsenic

Barium

Beryllium

Boron

Cadmium

Chromium

Copper

Fluoride

Lead

Mercury

Molybdenum

Nickel

Selenium

Silver

Strontium

Thallium

Uranium

Vanadium Chloride

Iron Manganese

Sulfate

Zinc

Gross alpha

Gross beta

Radium

Radon-222

Uranium

TDS + SpC

B Central Sierra study unit

Relative-concentration

Gross beta

Atrazine Name and center of symbol is location of data unless indicated by following 

location line: Constituent with analyses in > 26 grid wells 

Name and center of symbol is location of data - Constituent with analyses 

in < 15 grid wells and wells are not spatially representative

EXPLANATION

High

Moderate

Low

One volatile organic compound and one nutrient were detected at relative-concentration <0.001. SMCL, secondary maximum contaminant level; upper 

water-quality benchmark used for calculating relative-concentrations for constituents with an upper and recommended SMCL; >, greater than; <, less 

than

Figure 16.—Continued
60 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac09-0340_fig 16c

Volatile

organic

compound

Pesticide Special

interest

Nutrient Trace

element

Radioactive

constituent

Inorganic

constituent

having

SMCLs

MAXIMUM RELATIVE-CONCENTRATION, DIMENSIONLESS

Organic and special interest constituents Inorganic constituents

Carbon tetrachloride

Chloroform

Methyl tert-butyl ether

Perchloroethene

Trichloroethene

Atrazine

Simazine

Perchlorate

Ammonia

Nitrate

Nitrite

Aluminum

Antimony

Arsenic

Barium

Beryllium

Boron

Cadmium

Chromium

Copper

Fluoride

Lead

Mercury

Molybdenum

Nickel

Selenium

Silver

Strontium

Uranium

Vanadium

Chloride

Iron

Manganese

Sulfate

Zinc

Gross alpha

Gross beta

Radium

Radon-222

Uranium

TDS + SpC

100

10

1

0.1

0.01

0.001

Relative-concentration

Gross beta

Atrazine Name and center of symbol is location of data unless indicated by following 

location line: Constituent with analyses in >27 grid wells

Name and center of symbol is location of data - Constituent with analyses 

in <20 grid wells and wells are not spatially representative

Three volatile organic compounds were detected at relative-concentration <0.001. SMCL, secondary maximum contaminant level; upper water-quality 

benchmark used for calculating relative-concentrations for constituents with an upper and recommended SMCL; >, greater than; <, less than

EXPLANATION

High

Moderate

Low

C Southern Sierra study unit

Figure 16.—Continued
Status and Understanding of Water Quality 61

sac09-0340_fig 17a

A 

Arsenic

Fluoride

Lead

Uranium

Molybdenum

Strontium

Boron

RELATIVE-CONCENTRATION, DIMENSIONLESS

0 1 2 3 4 >4

Trace elements

Coarse Gold study area, Central Sierra study unit

Wishon study area, Central Sierra study unit

Southern Sierra study unit

Hard Rock study area, Tahoe-Martis study unit

Martis study area, Tahoe-Martis study unit

Tahoe study area, Tahoe-Martis study unit

EXPLANATION

Relative-concentration

Low Moderate High

> is greater than

Figure 17. Relative-concentrations of selected (A) trace elements with regulatory and 

non-regulatory health-based benchmarks, (B) nutrients and radioactive constituents with 

regulatory health-based benchmarks, and (C) salinity indicators and trace elements with nonregulatory SMCL benchmarks, Tahoe-Martis, Central Sierra, and Southern Sierra study units, 

California GAMA Priority Basin Project.
62 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac09-0340_fig 17b

Coarse Gold study area, Central Sierra study unit

Wishon study area, Central Sierra study unit

Southern Sierra study unit

Hard Rock study area, Tahoe-Martis study unit

Martis study area, Tahoe-Martis study unit

Tahoe study area, Tahoe-Martis study unit

EXPLANATION

Gross alpha

Uranium

Radon-222

Radium

Nitrate

Radioactive constituents

Nutrients

RELATIVE-CONCENTRATION, DIMENSIONLESS

0 1 2 3 4 >4

B 

Relative-concentration

Low Moderate High

> is greater than

Figure 17.—Continued
Status and Understanding of Water Quality 63

sac09-0340_fig 17c

Coarse Gold study area, Central Sierra study unit

Wishon study area, Central Sierra study unit

Southern Sierra study unit

Hard Rock study area, Tahoe-Martis study unit

Martis study area, Tahoe-Martis study unit

Tahoe study area, Tahoe-Martis study unit

EXPLANATION

0 1 2 3 4 >4

C

Chloride

Sulfate

Manganese

Zinc

Iron

Specific

conductance

Total dissolved

solids

Salinity indicators

Trace elements

RELATIVE-CONCENTRATION, DIMENSIONLESS

Relative-concentration

Low Moderate High

> is greater than

Figure 17.—Continued
64 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Four of the 23 inorganic constituents selected for 

additional evaluation in the status assessment were included 

because they were reported at high or moderate RCs in the 

CDPH databases for at least one of the study units during the 

3-year interval prior to USGS-GAMA sampling: chromium, 

selenium, aluminum, and antimony (table 3). 

Aquifer-scale proportions for the constituents listed in 

table 3 are summarized in tables 8A,B,C for the three study 

units. Spatially weighted high aquifer-scale proportions 

fell within the 90% confidence intervals for their respective 

grid-based aquifer high proportions, providing evidence that 

the grid-based approach yields statistically equivalent results 

to the spatially weighted approach. Aquifer-scale proportions 

for classes of inorganic constituents are summarized in 

table 9A.

The high aquifer-scale proportion for any inorganic 

constituent having health-based benchmarks (nutrients, 

trace elements, and radioactive constituents) was 20% in the 

Tahoe-Martis study unit, 41% in the Central Sierra study unit, 

and 32% in the Southern Sierra study unit (table 9A). High 

RCs of trace elements in all three study units, and high RCs 

of radioactive constituents in the Central Sierra and Southern 

Sierra study units, contributed most to these high aquifer-scale 

proportions. For any inorganic constituent having non-healthbased benchmarks (SMCL constituents), the high aquifer-scale 

proportion was 14% in the Tahoe-Martis study unit, 34% in 

the Central Sierra study unit, and 24% in the Southern Sierra 

study unit (table 9B). High RCs of manganese accounted for 

most of the high aquifer-scale proportion.

Trace Elements

The constituent class trace elements includes a variety 

of metallic and non-metallic constituents that typically are 

present in groundwater at concentrations < 1 mg/L. Trace 

elements with health-based benchmarks had high aquifer-scale 

proportions of 19, 25, and 28% in the Tahoe-Martis, 

Central Sierra, and Southern Sierra study units, respectively 

(table 9A). Arsenic, fluoride, boron, and molybdenum 

accounted for most of the high and moderate RCs of trace 

elements (fig. 17A; tables 8A,B,C).

Arsenic had high RCs in 18, 15, and 22% of the 

primary aquifers in the Tahoe-Martis, Central Sierra, and 

Southern Sierra study units, respectively (tables 8A,B,C; 

fig. 17A). High RCs occurred in all study areas within the 

study units (figs. 18A,B,C; tables B1A–E). An estimated 

8% of groundwater resources used for drinking water in 

the United States have high RCs of arsenic (> 10 μg/L) 

(Focazio and others, 1999), and high concentrations of 

arsenic in groundwater resources used for drinking water are 

a worldwide concern (Smedley and Kinniburgh, 2002; Welch 

and others, 2006).

High aquifer-scale proportions for fluoride were 6.6% in 

the Central Sierra study unit, and 6.5% in the Southern Sierra 

study unit. Fluoride was not detected at moderate or high RCs 

in the Tahoe-Martis study unit (fig. 17A). Boron had high RCs 

in 5.9% of the Tahoe-Martis study unit and in 6.7% of the 

Southern Sierra study unit, and was detected at moderate RCs 

in all three study units. Molybdenum had high RCs in 5.9% of 

the Tahoe-Martis study unit and in 11% of the Central Sierra 

study unit, and was detected at moderate RCs in the Central 

Sierra and Southern Sierra study units.

Six trace elements were infrequently detected at high RCs 

or were only detected at moderate RCs. Lead was not detected 

at high RCs in any of the three study units, but was detected 

at moderate RCs in the Central Sierra and Southern Sierra 

study units (fig. 17A, tables 8B,C). The high aquifer-scale 

proportion for strontium was 4.9% in the Tahoe-Martis study 

unit; however, strontium was not detected at moderate RCs in 

any of the study units. Chromium and selenium in the Central 

Sierra study unit (table 8B) and aluminum and antimony in 

the Southern Sierra study unit (table 8C) were each reported at 

high RC in one well in the CDPH database during the 3-year 

intervals used for the status assessment. The resulting spatially 

weighted high aquifer-scale proportions were < 1% for 

chromium, selenium, and antimony, and 1.6% for aluminum, 

compared to 0% for the grid-based approach. The spatially 

weighted approach includes data from a larger number of 

wells than the grid-based approach, and therefore is more 

likely to include constituents present at high concentrations 

in small proportions of the primary aquifers. (Uranium is 

discussed in the section on radioactive constituents.)

Aluminum, cadmium, lead, and mercury in the 

Tahoe-Martis study unit; antimony, copper, and thallium in the 

Central Sierra study unit; and barium, beryllium, cadmium, 

mercury, nickel, selenium, and thallium in the Southern Sierra 

study unit were reported at high RCs historically in the CDPH 

database, but were not reported at high or moderate RCs in 

the CDPH database during the 3-year intervals used for the 

status assessment (table 5). For most of these historically high 

trace elements, a high RC was reported in just one well. For 

75% of the cases, all later samples from the same well had low 

RCs for the historically high trace element. For the remaining 

cases, the high RC occurred in the only sample collected from 

the well and analyzed for trace elements. (See appendix D for 

additional discussion.)
Status and Understanding of Water Quality 65

Table 8A. Aquifer-scale proportions calculated using grid-based and spatially weighted methods for those constituents that met criteria for additional evaluation in the status assessment, Tahoe-Martis study unit, 2006–2007, California GAMA Priority Basin Project.[Relative-concentration categories: high, concentrations greater than water-quality benchmark; moderate, concentrations greater than or equal to 0.1 of benchmark but less than benchmark for organic constituents (threshold for inorganic constituents is 0.5 of benchmark); low, concentrations less than 0.1 of benchmark for organic constituents (threshold for inorganic constituents is 0.5 of benchmark). Benchmark types and values listed in table 3. Other abbreviations: SMCL, secondary maximum contaminant level; DBCP, 1,2-dibromo-3-chloropropane]

Constituent

Raw detection frequency1

(percent)

Spatially weighted 

aquifer-scale

proportion1

(percent)

Grid-based 

aquifer-scale 

proportion2

(percent)

90-percent

confidence interval 

for grid-based 

high proportion3

(percent)

Number

of wells

Percent 

moderate

Percent 

high

Number 

of cells

Proportion 

moderate

Proportion 

high

Number 

of cells

Proportion 

moderate

Proportion 

high Lower Upper

Trace elementsArsenic 95 8.4 15.8 41 2.9 17 41 3.6 18 9.9 29

Boron 56 1.8 3.6 41 1.6 5.9 41 1.6 5.9 2.0 14

Molybdenum 48 2.1 4.2 41 0.5 5.4 41 0 5.9 1.5 13

Strontium 48 0 2.1 41 0 4.9 41 0 4.9 1.5 13

Uranium 49 6.1 0 41 2.0 0 41 2.0 0 0 3.2

Inorganic constituents with SMCL benchmarksIron 95 2.1 7.4 41 0.2 5.7 41 0 6.9 2.4 16

Manganese 94 0 6.4 41 0 10 41 0 13 6.6 24

Chloride 94 1.1 0 41 0.3 0 41 1.0 0 0 3.2

Specific conductance 95 0 3.1 41 5 2.8 41 5 6 2.0 14

Sulfate 94 0 1.1 41 0 2.5 41 0 4.9 1.5 13

Total dissolved solids (TDS) 96 0 3.1 41 0 7.7 41 0 11 4.9 21

Radioactive constituentsAdjusted gross alpha activity 60 1.7 3.3 25 0.3 0.5 24 1.1 1.1 0.1 10

Gross alpha particle activity 60 13 10 25 3.5 1.9 24 2.1 2.1 0.2 12

Radium activity 39 2.6 0 19 0.4 0 17 1.3 0 0 7.5

Radon-222 activity 67 7.5 6.0 40 11 0.9 40 12 1.0 0.1 6.7

Uranium activity 75 13 0 41 2.5 0 41 3.0 0 0 3.2

Volatile organic compounds - solventsCarbon tetrachloride 96 0 1.0 41 0 0.4 41 0 0 0 3.2

1,2-Dichloroethane 96 0 1.0 41 0 0.2 41 0 0 0 3.2

Chloroform 96 0 0 41 0 0 41 0 0 0 3.2

Chloromethane 96 1.0 0 41 0.4 0 41 0 0 0 3.2

Perchloroethene (PCE) 96 2.1 1.0 41 0.3 0.2 41 0 1.0 0.1 6.6

Volatile organic compounds - fumigants1,2-Dichloropropane 96 0 1.0 41 0 0.4 41 0 0 0 3.2

DBCP 72 0 1.4 41 0 0.2 41 0 0 0 3.2

1 Based on most recent analyses for each California Department of Public Health (CDPH) well during May 1, 2004–April 30, 2007, combined with data from U.S. Geological Survey (USGS) grid wells, and 

selected USGS-understanding wells.

2 Grid-based aquifer-scale proportions for organic consituents are based on samples collected by the USGS from 41 grid wells during June–October 2007.3 Based on the Jeffreys interval for the binomial distribution (Brown and others, 2001).
66 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project Table 8B. Aquifer-scale proportions calculated using grid-based and spatially weighted methods for those constituents that met criteria for additional evaluation in the status assessment, Central Sierra study unit, 2006–2007, California GAMA Priority Basin Project.

[Relative-concentration categories: high, concentrations greater than water-quality benchmark; moderate, concentrations greater than or equal to 0.1 of benchmark but less than benchmark for organic constituents (threshold for inorganic constituents is 0.5 of benchmark); low, concentrations less than 0.1 of benchmark for organic constituents (threshold for inorganic constituents is 0.5 of benchmark). Benchmark types and values listed in table 3. Other abbreviations: SMCL, secondary maximum contaminant level]

Constituent

Raw detection frequency1

(percent)

Spatially weighted

aquifer-scale proportion1

(percent)

Grid-based 

aquifer-scale proportion2

(percent)

90-percent confidence 

interval for grid-based 

high proportion3

(percent)

Number 

of wells

Percent 

moderate

Percent 

high

Number 

of cells

Proportion 

moderate

Proportion 

high

Number 

of cells

Proportion 

moderate

Proportion 

high Lower Upper

NutrientsNitrate 165 3.6 0 29 3.1 0 29 0 0 0 4.5

Trace elementsArsenic 98 10 22 29 15 19 29 19 15 6.6 28.0

Boron 33 15 0 27 13 0 27 13 0 0.0 4.8

Chromium 90 0 1.1 29 0 0.4 29 0 0 0 4.5

Fluoride 100 6.0 3.0 29 5.5 7.3 29 9.3 6.6 1.8 17

Lead 95 3.2 0 29 5.0 0 29 2.7 0 0 4.5

Molybdenum 30 33 10 27 29 9.1 27 29 11 4.1 24

Selenium 90 0 1.1 29 0 0.4 29 0 0 0 4.5

Uranium 53 15 17 27 13 16 27 12 15 6.6 29

Inorganic constituents with SMCL benchmarksIron 101 4.0 24 29 3.9 19 29 2.7 11 4.4 24

Manganese 101 6.9 36 29 1.9 29 29 3.8 32 20 47

Zinc 98 3.1 0 29 4.5 0 29 6.6 0 0 4.5

Chloride 100 2.0 3.0 29 1.0 1.3 29 0 0 0 4.5

Specific conductance 100 5.0 3.0 29 2.9 1.3 29 0 0 0 4.5

Total dissolved solids (TDS) 95 4.2 3.2 29 2.2 1.4 29 0 0 0 4.5

Radioactive constituentsAdjusted gross alpha activity 111 5.5 1.8 24 3.0 1.3 24 8.2 3.9 0.6 14

Gross alpha particle activity 111 16 18 24 15 22 24 25 16 6.9 31

Radium activity 15 7.1 0 15 5.5 0 15 5.5 0 0 8.5

Radon-222 activity 14 21 50 14 24 44 14 24 44 24 65

Uranium activity 77 16 21 28 11 18 28 15 14 5.9 27

PesticidesSimazine 71 0 0 27 0 0 26 0 0 0 5.0
Status and Understanding of Water Quality 67

Constituent

Raw detection frequency1

(percent)

Spatially weighted

aquifer-scale proportion1

(percent)

Grid-based 

aquifer-scale proportion2

(percent)

90-percent confidence 

interval for grid-based 

high proportion3

(percent)

Number 

of wells

Percent 

moderate

Percent 

high

Number 

of cells

Proportion 

moderate

Proportion 

high

Number 

of cells

Proportion 

moderate

Proportion 

high Lower Upper

Volatile organic compoundsBenzene 84 1.2 0 27 2.0 0 27 4.0 0 0 4.8

Chloroform 87 2.3 0 27 1.1 0 27 0 0 0 4.8

Perchloroethene (PCE) 84 1.2 0 27 0.5 0 27 0 0 0 4.8

Methyl tert-butyl ether (MTBE) 84 2.4 0 27 1.6 0 27 0 0 0 4.8

1,4-Dichlorobenzene 84 1.2 0 27 0.4 0 27 0 0 0 4.8

1 Based on most recent analyses for each California Department of Public Health (CDPH) well during June 1, 2003–May 31, 2006, combined with data from U.S. Geological Survey (USGS) grid wells, and 

USGS-understanding wells.

2 Grid-based aquifer-scale proportions for organic constituents are based on samples collected by the USGS from 27 grid wells during May 2006. 3 Based on the Jeffreys interval for the binomial distribution (Brown and others, 2001).

Table 8B. Aquifer-scale proportions calculated using grid-based and spatially weighted methods for those constituents that met criteria for additional evaluation in the status assessment, Central Sierra study unit, 2006–2007, California GAMA Priority Basin Project.—Continued[Relative-concentration categories: high, concentrations greater than water-quality benchmark; moderate, concentrations greater than or equal to 0.1 of benchmark but less than benchmark for organic constituents (threshold for inorganic constituents is 0.5 of benchmark); low, concentrations less than 0.1 of benchmark for organic constituents (threshold for inorganic constituents is 0.5 of benchmark). Benchmark types and values listed in table 3. Other abbreviations: SMCL, secondary maximum contaminant level]
68 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project Table 8C. Aquifer-scale proportions calculated using grid-based and spatially weighted methods for those constituents that met criteria for additional evaluation in the status assessment, Southern Sierra study unit, 2006–2007, California GAMA Priority Basin Project.

[Relative-concentration categories: high, concentrations greater than water-quality benchmark; moderate, concentrations greater than or equal to 0.1 of benchmark but less than benchmark for organic constituents (threshold for inorganic constituents is 0.5 of benchmark); low, concentrations less than 0.1 of benchmark for organic constituents (threshold for inorganic constituents is 0.5 of benchmark). Benchmark types and values listed in table 3. Other abbreviations: SMCL, secondary maximum contaminant level]

Constituent

Raw detection frequency1

(percent)

Spatially weighted 

aquifer-scale proportion1

(percent)

Grid-based 

aquifer-scale proportion2

(percent)

90-percent confidence 

interval for grid-based 

high proportion3

(percent)

Number 

of wells

Percent 

moderate

Percent

high

Number 

of cells

Proportion 

moderate

Proportion 

high

Number 

of cells

Proportion 

moderate

Proportion 

high Lower Upper

NutrientsNitrate 199 17 5.5 37 18 4.1 37 24 2.7 0.5 10Trace elements4

Aluminum 158 1.3 0.6 31 0.4 1.6 31 0 0 0 4.2Antimony 158 0 0.6 31 0 0.3 31 0 0 0 4.2Arsenic 168 14 15 32 9.3 18 32 6.3 22 12 36Boron 75 9.3 4.0 19 11 6.4 15 13 6.7 1.2 23

Chromium 151 0.7 0 30 1.7 0 30 0 0 0 4.4Fluoride 163 8.6 6.7 33 17 6.7 31 16 6.5 1.9 17

Lead 158 1.9 0 31 1.1 0 31 3.2 0 0 4.2Molybdenum 17 5.9 0 11 9.1 0 7 14 0 0 23Uranium 22 4.5 4.5 14 7.1 7.1 8 13 0 0 15Inorganic constituents with SMCL benchmarksIron 154 12 18 31 14 13 30 17 13 5.7 26Manganese 154 9.1 23 31 11 17 30 10 23 13 38

Zinc 153 2.0 0 30 2.4 0 30 3.3 0 0 4.4Sulfate 155 1.3 0 30 3.3 0 30 3.3 0 0 4.4Specific conductance 167 11 1.2 34 13 2.9 34 12 2.9 0.5 11Total dissolved solids (TDS) 154 11 1.9 31 20 4.3 30 13 3.3 0.6 12

Radioactive constituentsAdjusted gross alpha activity 143 5.6 0.7 29 9.6 0.3 28 7.1 7.1 2.1 18Gross alpha particle activity 143 7.7 13 29 12 18 28 11 21 12 37

Radium activity 30 0.0 3.3 12 0 1.2 12 0 0 0 11

Radon-222 activity 10 10 40 9 11 44 6 17 33 10 66

Uranium activity 77 12 14 21 12 19 19 16 21 14 44PesticidesAtrazine 161 0 0 37 0 0 35 0 0 0 3.8Simazine 161 0 0 37 0 0 35 0 0 0 3.8
Status and Understanding of Water Quality 69

Constituent

Raw detection frequency1

(percent)

Spatially weighted 

aquifer-scale proportion1

(percent)

Grid-based 

aquifer-scale proportion2

(percent)

90-percent confidence 

interval for grid-based 

high proportion3

(percent)

Number 

of wells

Percent 

moderate

Percent

high

Number 

of cells

Proportion 

moderate

Proportion 

high

Number 

of cells

Proportion 

moderate

Proportion 

high Lower Upper

Volatile organic compoundsCarbon tetrachloride 161 0.6 0 37 0.3 0 35 2.9 0 0 3.8Chloroform 161 0 0 37 0 0 35 0 0 0 3.8Perchloroethene (PCE) 162 0.6 0 37 0.3 0 35 0 0 0 3.8Special-interest constituentPerchlorate 89 11 0 35 10 0 35 8.6 0 0 3.8

1 Based on most recent analyses for each California Department of Public Health (CDPH) well during February 1, 2003–January 31, 2006, combined with data from U.S. Geological Survey (USGS) grid wells, 

and USGS-understanding wells.

2 Grid-based aquifer-scale proportions for organic consituents are based on samples collected by the USGS from 35 grid wells during June 2006.3 Based on the Jeffreys interval for the binomial distribution (Brown and others, 2001).4 Mercury and nickel were each reported at moderate relative-concentration in one CDPH well during February 1, 2003–January 31, 2006; the most recent values were low.

Table 8C. Aquifer-scale proportions calculated using grid-based and spatially weighted methods for those constituents that met criteria for additional evaluation in the status assessment, Southern Sierra study unit, 2006–2007, California GAMA Priority Basin Project.—Continued[Relative-concentration categories: high, concentrations greater than water-quality benchmark; moderate, concentrations greater than or equal to 0.1 of benchmark but less than benchmark for organic constituents (threshold for inorganic constituents is 0.5 of benchmark); low, concentrations less than 0.1 of benchmark for organic constituents (threshold for inorganic constituents is 0.5 of benchmark). Benchmark types and values listed in table 3. Other abbreviations: SMCL, secondary maximum contaminant level]
70 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Table 9A. Summary of aquifer-scale proportions for inorganic 

constituent classes with health-based benchmarks, Tahoe-Martis, 

Central Sierra, and Southern Sierra study units, 2006–2007, 

California GAMA Priority Basin Project.

[Relative-concentration categories: high, concentration of at least one 

constituent in group greater than water-quality benchmark; moderate, 

concentration of at least one constituent in group greater than 0.5 of 

benchmark and no constituents in group with concentration greater than 

benchmark; low, concentrations of all constituents in group less than or equal 

to 0.5 of benchmark]

Study unit Number

of cells

Aquifer-scale proportion

(percent)

Low Moderate High

Radioactive constituents with health-based benchmarks1

Tahoe-Martis 41 85 12 3.0

Central Sierra 28 43 26 31

Southern Sierra 29 62 17 21

Nutrients with health-based benchmarks

Tahoe-Martis 41 100 0 0

Central Sierra2 29 97 3.0 0

Southern Sierra 37 73 24 2.7

Trace elements with health-based benchmarks

Tahoe-Martis 41 77 3.6 19

Central Sierra 29 43 32 25

Southern Sierra 32 56 16 28

All inorganic constituents with health-based benchmarks3

Tahoe-Martis 41 67 13 20

Central Sierra 29 23 36 41

Southern Sierra 38 34 34 32

1

 Aquifer-scale proportions for radioactive constituents were calculated 

using gross alpha activity. If adjusted gross alpha activity were used instead, 

the high and moderate aquifer-scale proportions would be as follows: 

Tahoe-Martis, 2.0 percent high, 13 percent moderate; Central Sierra, 

27 percent high, 30 percent moderate; and Southern Sierra, 18 percent high, 

18 percent moderate.

2

 Based on spatially weighted calculation.

3

 Aquifer-scale proportions for all inorganic constituents with health-based 

benchmarks were calculated using gross alpha activity. If adjusted gross alpha 

activity were used instead, the high and moderate aquifer-scale proportions 

would be as follows: Tahoe-Martis, 19 percent high, 14 percent moderate; 

Central Sierra, unchanged; and Southern Sierra, 29 percent high, 34 percent 

moderate.

Table 9B. Summary of aquifer-scale proportions for inorganic 

constituent classes with secondary maximum contaminant level 

(SMCL) benchmarks, Tahoe-Martis, Central Sierra, and Southern 

Sierra study units, 2006–2007, California GAMA Priority Basin Project.

[Relative-concentration categories: high, concentration of at least one 

constituent in group greater than water-quality benchmark; moderate, 

concentration of at least one constituent in group greater than 0.5 of benchmark 

and no constituents in group with concentration greater than benchmark; low, 

concentrations of all constituents in group less than or equal to 0.5 of benchmark. 

Abbreviations: SMCL, secondary maximum contaminant level]

Study unit Number

of cells

Aquifer-scale proportion

(percent)

Low Moderate High

Major ions with SMCL benchmarks

Tahoe-Martis1 41 92 0 7.7

Central Sierra1 29 95 3.4 1.8

Southern Sierra 34 82 15 2.9

Trace elements with SMCL benchmarks

Tahoe-Martis 41 86 0 14

Central Sierra 29 58 9.3 32

Southern Sierra 30 57 20 23

All constituents with SMCL benchmarks

Tahoe-Martis 41 86 0 14

Central Sierra 29 55 11 34

Southern Sierra 34 56 21 24

1

 Based on spatially weighted calculation.
Status and Understanding of Water Quality 71

Table 9C. Summary of aquifer-scale proportions for organic constituent classes with 

health-based benchmarks, Tahoe-Martis, Central Sierra, and Southern Sierra study units, 

2006–2007, California GAMA Priority Basin Project.

[Relative-concentration categories: high, concentration of at least one constituent in group greater than waterquality benchmark; moderate, concentration of at least one constituent in group greater than 0.1 of benchmark 

and no constituents in group with concentration greater than benchmark; low, concentrations of all constituents 

in group less than or equal to 0.1 of benchmark]

Study unit Number 

of cells

Aquifer-scale proportions

(percent)

Not

detected

Detected

at low 

concentration

Detected

at moderate 

concentration

Detected

at high 

concentration

Volatile organic compounds

Tahoe-Martis 41 81 17 10.7 1.0

Central Sierra 27 77 19 4.0 0

Southern Sierra 35 69 29 2.9 0

Pesticides

Tahoe-Martis 41 97 3.0 0 0

Central Sierra 26 83 17 0 0

Southern Sierra 35 80 20 0 0

Any organic constituent

Tahoe-Martis 41 80 19 10.7 1.0

Central Sierra 27 73 23 4.0 0

Southern Sierra 35 60 37 2.9 0

1

 Spatially weighted proportion.
72 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac09-0340_fig 18a

NEVADA CO

PLACER CO

EL DORADO CO

ALPINE

CO

Mesozoic granitic rocks

Cenozoic sedimentary deposits

Low (< 5)

Moderate (5–10)

High (> 10)

Grid wells

Low (< 5)

Moderate (5–10)

High (> 10)

Understanding wells

Low (< 5)

Moderate (5–10)

High (> 10)

CDPH wells

Cenozoic volcanic and volcaniclastic rocks

Paleozoic and Mesozoic metamorphic rocks

and non-granitic plutonic rocks

Geologic units

Martis and Tahoe study area boundary

Arsenic concentration, in micrograms per liter

EXPLANATION

Hard Rock study area boundary

39° 15’

120° 15’ 120°

39°

0 5 10 MILES

0 5 10 KILOMETERS

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006. 

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

Geologic units simplified from Saucedo and 

others (2000) and Crafford (2007).

A

< is less than;

> is greater than

LAKE TAHOE

CALIFORNIA

NEVADA

Figure 18. Relative-concentrations of arsenic for USGS- and CDPH-grid wells, USGS-understanding wells, and all 

wells in the CDPH database with data for arsenic during the 3-year intervals used in the status assessment for the 

(A) Tahoe-Martis, (B) Central Sierra, and (C) Southern Sierra study units, California GAMA Priority Basin Project.
Status and Understanding of Water Quality 73

sac09-0340_fig 18b

FRESNO CO

MARIPOSA CO

MADERA CO

EXPLANATION

Mesozoic granitic rocks

Cenozoic sedimentary deposits

Cenozoic volcanic and volcaniclastic rocks

Paleozoic and Mesozoic metamorphic rocks

and non-granitic plutonic rocks

Geologic units

Study unit boundary 

Low (< 5)

Moderate (5–10)

High (> 10)

Grid wells

Low (< 5)

Moderate (5–10)

High (> 10)

Understanding wells

Low (< 5)

Moderate (5–10)

High (> 10)

CDPH wells

Arsenic concentration, in micrograms per liter

< is less than;

> is greater than

0 5 10 MILES

0 5 10 KILOMETERS

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006,

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

37° 30’

37° 15’

119° 45’ 119° 30’

B

Geologic units simplified from

Saucedo and others (2000).

Figure 18.—Continued
74 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac09-0340_fig 18c

EXPLANATION

Mesozoic granitic rocks

Cenozoic sedimentary deposits

Cenozoic volcanic and volcaniclastic rocks

Paleozoic and Mesozoic metamorphic rocks

 and non-granitic plutonic rocks

Geologic units

Study unit boundary

Low (< 5)

Moderate (5–10)

High (> 10)

Grid wells

Low (< 5)

Moderate (5–10)

High (> 10)

Understanding wells

Low (< 5)

Moderate (5–10)

High (> 10)

CDPH wells

Arsenic concentration, in micrograms per liter 

< is less than;

> is greater than

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006,

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

Geologic units simplified from 

Saucedo and others (2000).

C

35°15'

118°30’ 118°15’

36°

35°45’

35°30’

10 MILES

50 10 KILOMETERS

0 5

TULARE CO

KERN CO

Tehachapi

Figure 18.—Continued
Status and Understanding of Water Quality 75

Factors Affecting Arsenic

Arsenic is a semi-metallic trace element. Natural 

sources of arsenic in groundwater include dissolution of 

arsenic-bearing minerals and desorption of arsenic from 

mineral surfaces. Pyrite, the most common sulfide mineral in 

aquifer materials, may contain up to several percent arsenic. 

Potential anthropogenic sources of arsenic include copper 

ore smelting, coal combustion, arsenical pesticides, arsenical 

veterinary pharmaceuticals, and wood preservatives. The 

MCL-US for arsenic was lowered from 50 μg/L to 10 μg/L 

in 2002, and chronic exposure to arsenic concentrations 

between 10 and 50 μg/L in drinking water has been linked to 

increased cancer risk and to non-cancerous effects including 

skin damage and circulatory problems (U.S. Environmental 

Protection Agency, 2010).

The presence of elevated arsenic concentrations 

in groundwater typically is not related to high arsenic 

concentrations in aquifer sediments or rock, but rather to 

geochemical conditions that enhance arsenic solubility 

and hydrologic conditions that favor arsenic accumulation 

in groundwater (Smedley and Kinniburgh, 2002). In the 

Tahoe-Martis, Central Sierra, and Southern Sierra study 

units, high and moderate arsenic RCs occurred under two 

different sets of geochemical conditions: in oxic groundwater 

with relatively high pH values, and in anoxic groundwater, 

typically with relatively low pH values (fig. 19). Arsenic 

concentration had a significant positive correlation with pH 

and a significant negative correlation with DO (table 10B). 

Previous investigations of the occurrence of elevated arsenic 

in groundwater have identified two mechanisms corresponding 

to these two sets of conditions: (1) desorption from, or 

inhibition of sorption to, aquifer materials with increasing 

pH, and (2) release of arsenic from dissolution of iron or 

manganese oxyhydroxides under iron- or manganese-reducing 

conditions (Smedley and Kinniburgh, 2002; Belitz and others, 

2003; Welch and others, 2006; and many references therein).

DISSOLVED OXYGEN CONCENTRATION,

IN MILLIGRAMS PER LITER

0 2 4 6 8 10 12 14 16

ARSENIC CONCENTRATION, IN MICROGRAMS PER LITER

150

50

40

30

20

10

0

140

High

Moderate

Low

Relative-concentration

High

Moderate

Low

Aquifer lithology and pH

EXPLANATION

pH greater than or equal to 7.8

Granitic rock

Metamorphic rock

Sedimentary deposit

Volcanic rock

pH less than 7.8

Granitic rock

Metamorphic rock

Sedimentary deposit

Volcanic rock

Figure 19. Relation between arsenic concentration and dissolved oxygen concentration, pH, and 

aquifer lithology, Tahoe-Martis, Central Sierra, and Southern Sierra study units, California GAMA 

Priority Basin Project.
76 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Table 10A. Results of Kruskal-Wallis and Wilcoxon rank-sum tests for differences in values of selected water-quality constituents 

between samples classified into groups by aquifer lithology, groundwater age, study unit, or well depth, Tahoe-Martis, Central Sierra, 

and Southern Sierra study units, 2006–2007, California GAMA Priority Basin Project.

[Relation of median values in sample groups tested shown for Kruskal-Wallis and Wilcoxon rank-sum tests in which the populations were determined to be 

significantly different (two-sided test) on the basis of p-values (not shown) less than threshold value (α) of 0.05; ns, test indicated no significant differences 

between the sample groups. p-values were calculated using the Kruskal-Wallis test; if significant, then Wilcoxon rank-sum tests were used to determine which 

differences were significant. Aquifer lithology class: G, granitic rocks; M, metamorphic rocks; S, sedimentary deposits; V, volcanic rocks. Study unit: T, 

Tahoe-Martis; C, Central Sierra; S, Southern Sierra. 3-factor depth class: shallow (Shal), well depth < 170 ft; Mixed (Mix), top of perforations < 170 ft and 

well depth > 170 ft; Deep (Deep) well depth > 170 ft and top of perforations > 170 ft. 2-factor age class: young, tritium > 0.5 TU; old, tritium < 0.5 TU. 

3-factor age class: Mod, modern; Mix, mixed modern and pre-modern; Pre, pre-modern (see appendix C for explanation). Other abbreviations: SMCL, 

secondary maximum contaminant level; ns, no significant differences; TU, tritium units; ft, feet below land surface; >, greater than; <, less than; TDS, total 

dissolved solids; THM, trihalomethanes]

Water-quality 

constituents

Significant differences

Aquifer

lithology class

(G, M, S, V)

Study unit

(T, C, S)

3-Factor depth class

(Shal, Mix, Deep)

2-Factor age class

(young, old)

3-Factor age class

(Mod, Mix, Pre)

Nutrients and trace elements with health-based benchmarks

Nitrate S > GV and M > G S > CT ns ns Mix > ModPre

Fluoride GM > S > V CS > T MixDeep > Shal old > young PreMix > Mod

Arsenic ns ns ns ns PreMix > Mod

Boron ns CS > T MixDeep > Shal old > young PreMix > Mod

Molybdenum GMS > V CS > T MixDeep > Shal old > young PreMix > Mod

Inorganic constituents with SMCL benchmarks

TDS M > GSV S > C > T MixDeep > Shal ns PreMix > Mod

Iron M > SV C > ST ns ns ns

Manganese G > SV C > ST ns ns ns

Radioactive constituents

Radon-222 activity GM > SV C > T Mix > Deep ns Mix > ModPre

Gross alpha particle

activity

GMS > V ns ns ns Mix > ModPre

Uranium activity GMS > V CS > T Mix > Deep ns Mix > ModPre

Organic and special-interest constituents

Herbicides ns S > T ns young > old ModMix > Pre

Solvents S > GMV ns ns ns ns

THM M > GSV ns ns young > old Mix > Pre

Perchlorate S > GMV S > CT ns ns ns

Explanation: How to read results for significant differences. “G > SV” for manganese means the following:

G has significantly greater manganese than S,

G has significantly greater manganese than V,

Manganese in S is not significantly different than manganese in V,

Manganese in M is not significantly different than manganese in G, S, or V.
Status and Understanding of Water Quality 77

Table 10B. Results of Spearman’s rho tests for correlations between selected potential explanatory factors and selected water-quality 

constituents, Tahoe-Martis, Central Sierra, and Southern Sierra study units, 2006–2007, California GAMA Priority Basin Project.

[ρ (rho), Spearman’s correlation statistic; ρ values are shown for tests in which the variables were determined to be significantly correlated on the basis of p 

values (significance level of the Spearman’s test) less than threshold value (α) of 0.05 (not shown); ns, Spearman’s test indicates no significant correlation 

between factors; black text, significant positive correlation; red text, significant negative correlation. SMCL, secondary maximum contaminant level; TDS, total 

dissolved solids; THM, trihalomethanes]

ρ

Percent land use

Density

of septic

 tanks

Aridity

index Elevation Welldepth

Depth to 

top of 

perforation

pH

Dissolved 

oxygen

concentration

Agricultural Natural Urban

Nutrients and trace elements with health-based benchmarks

Nitrate 0.55 –0.38 ns ns –0.59 –0.25 ns 0.22 ns 0.28

Fluoride 0.39 ns –0.21 0.45 –0.62 –0.74 0.29 ns ns –0.42

Arsenic –0.22 ns ns 0.28 ns –0.26 0.26 ns 0.44 –0.27

Boron 0.25 ns ns 0.41 –0.36 –0.53 0.44 ns 0.34 –0.34

Molybdenum 0.22 ns ns 0.35 –0.50 –0.57 0.28 ns 0.35 –0.40

Inorganic constituents with SMCL benchmarks

TDS 0.50 ns –0.21 0.39 –0.59 –0.55 ns ns ns –0.20

Iron ns ns ns ns ns –0.23 ns ns –0.31 –0.34

Manganese ns 0.22 ns ns ns –0.29 ns ns –0.33 –0.52

Radioactive constituents

Radon-222 activity ns ns ns 0.29 –0.32 –0.30 ns –0.26 ns –0.32

Gross alpha particle 

activity

ns ns ns ns ns ns ns ns ns ns

Uranium activity ns ns ns 0.22 –0.31 –0.39 ns ns ns –0.22

Organic and special-interest constituents

Herbicides 0.28 –0.23 ns 0.20 –0.29 –0.26 ns ns ns ns

Solvents 0.18 –0.32 0.23 ns –0.18 ns ns ns ns ns

THM ns ns 0.27 ns ns ns ns ns ns ns

Perchlorate 0.51 –0.32 ns ns –0.34 ns ns ns ns 0.22
78 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Half of the groundwater samples from the three study 

units that had pH ≥ 7.8 and oxic (DO > 1 mg/L) conditions had 

moderate or high RCs of arsenic, compared to just 2% of the 

oxic groundwater samples with pH <  7.8 (fig. 20). Under oxic 

conditions, sediment grains and fracture surfaces often have 

coatings of iron and (or) manganese oxyhydroxides, resulting 

from weathering of primary iron oxide, iron sulfide, and 

ferro-magnesian silicate minerals. At high pH, the surfaces of 

these oxyhydroxides may be negatively charged and therefore 

inhibit sorption of anions to the surfaces. The dominant form 

of arsenic in solution can be predicted from the ratio of arsenic 

redox species, As+5/As+3, where As+5 is the amount of arsenic 

present in the more oxidized +5 oxidation state (arsenate) and 

As+3 is the amount of arsenic present in the more reduced 

+3 oxidation state (arsenite). In samples with moderate or 

high RCs of arsenic, As+5/As+3 ratios ranged from 3.5 to > 10, 

with 11 of the 15 samples having As+5/As+3> 10, indicating 

that the more oxidized form of dissolved arsenic, arsenate, 

was the dominant form. At pH ≥ 7.8, the primary arsenate 

species is HAsO4

–2, a relatively soluble oxyanion under those 

conditions (Smedley and Kinniburgh, 2002). The groundwater 

samples with elevated arsenic under oxic, high-pH conditions 

included wells with volcanic, granitic, and sedimentary aquifer 

lithologies (fig. 19). Previous studies have reported elevated 

arsenic concentrations under these geochemical conditions 

in groundwater from a range of aquifer lithologies (Ayotte 

and others, 1999; Smedley and Kinniburgh, 2002; Senior and 

Sloto, 2006).

90th percentile

10th percentile

median

(50th percentile)

lower detached

25th percentile

75th percentile

upper detached

50 number of wells

Oxic, low pH significantly 

different from oxic, high pH (p<0.001) 

and anoxic or mixed (p<0.001)

50 30 25

0

10

20

30

40

50

140

150

ARSENIC CONCENTRATION, IN MICROGRAMS PER LITER

OXIDATION REDUCTION CLASS

pH

Oxic Oxic Anoxic or mixed

< 7.8 > 7.8 any

Moderate

High

Low

EXPLANATION

Relative-concentration

High

Moderate

Low

< is less than;

> is greater than or equal to

Figure 20. Arsenic concentrations in groundwater samples grouped by oxidationreduction class and pH, Tahoe-Martis, Central Sierra, and Southern Sierra study units, 

California GAMA Priority Basin Project.
Status and Understanding of Water Quality 79

The primary mechanisms for increasing the pH of 

groundwater under oxic conditions are hydrolysis of silicate 

minerals and dissolution of calcite (which may be driven 

by cation exchange between calcium and magnesium in 

groundwater and sodium in clay minerals). Both mechanisms 

also result in increasing TDS. Aquifers in volcanic rocks 

generally have alkaline groundwater because dissolution 

and hydrolysis of volcanic glass, forming clay and zeolite 

minerals, cause groundwater to evolve towards higher pH 

(Hay and Sheppard, 1977; Gysi and Stefánsson, 2008). 

For oxic groundwater from wells with volcanic aquifer 

lithology (present in the Tahoe-Martis study unit only), 

arsenic was significantly positively correlated with pH and 

TDS (figs. 21A,B; p=0.005 and 0.012, respectively). All of 

the samples with moderate or high arsenic RCs from wells 

with volcanic aquifer lithology had mixed or pre-modern 

age distributions (figs. 21A,B), suggesting that increased 

contact time resulted in more water–rock interaction and 

more desorption of arsenic from iron and manganese 

oxyhydroxide surfaces.

In oxic groundwater from wells with sedimentary 

aquifer lithology in the Tahoe-Martis study unit, however, 

arsenic concentrations were significantly positively correlated 

with pH (p=0.010), but not correlated with TDS (p=0.388) 

(figs. 21A,B), suggesting that different mechanisms may have 

been responsible for the elevated pH. In addition, the presence 

of groundwater with pre-modern age distributions and low 

TDS (fig. 21B) suggests that increased contact time does 

not necessarily result in more hydrolysis and dissolution of 

minerals in aquifer materials. The occurrence of groundwater 

with high arsenic RC, high pH, and low TDS with pre-modern 

age distributions in the Tahoe-Martis study unit cannot be fully 

explained without further investigation using geochemical 

modeling.

Nearly half of the groundwater samples from the three 

study units that had anoxic (DO < 1 mg/L) or mixed oxidation 

conditions had moderate or high RCs of arsenic (fig. 20). Of 

the 16 anoxic or mixed oxidation groundwater samples with 

moderate or high RCs of arsenic, 13 also had moderate or 

high RCs of iron and (or) manganese, suggesting reductive 

dissolution of iron or manganese oxides as the source of 

arsenic. Anoxic and mixed oxidation groundwater samples 

with moderate or high RCs of arsenic had significantly 

lower As+5/As+3 ratios (0.1 to > 10; median 2.5) than oxic 

groundwater samples with moderate or high RCs of arsenic 

(3.5 to > 10; median > 10) (Wilcoxon rank-sum test, p=0.015), 

indicating greater relative abundance of the more reduced 

form of dissolved arsenic, arsenite, in the anoxic and mixed 

oxidation groundwater samples. Arsenite species typically are 

more toxic than arsenate species (National Research Council, 

2001). At pH < 9, the primary arsenite species at equilibrium 

is H3AsO3

0, which is a relatively soluble species under those 

conditions (Smedley and Kinniburgh, 2002). The large range 

of As+5/As+3 may reflect the complexities introduced by the 

slow kinetics of arsenic redox transformations (Lindberg and 

Runnels, 1984) or by groundwater mixing. Moderate and 

high RCs of arsenic under reducing conditions were detected 

in wells with all four aquifer lithologies, although most had 

granitic aquifer lithology (fig. 19). 

Whether arsenic was released by desorption under 

oxic, high-pH conditions, or by dissolution under anoxic 

conditions, accumulation of elevated concentrations of arsenic 

in groundwater also requires favorable hydrologic conditions. 

Arsenic accumulation is favored by longer contact times 

between groundwater and aquifer materials, minimizing 

the amount of flushing of the system and maximizing the 

reaction times of minerals in the aquifer materials (Smedley 

and Kinniburgh, 2002). Arsenic concentrations were 

significantly higher in groundwater with pre-modern and 

mixed age distributions than in groundwater with modern age 

distributions (table 10A; fig. 22). 

The significant negative correlations between arsenic and 

agricultural land use and elevation and significant positive 

correlation between arsenic and septic tank density (table 10B) 

likely reflect the fact that many of the samples with high and 

moderate RCs of arsenic were located in the Central Sierra 

study unit and in the Kern River Valley part of the Southern 

Sierra study unit (figs. 18B,C). Lower elevations and greater 

density of septic tanks were significantly associated with 

location in the Central Sierra and Southern Sierra study 

units (table 6A), and although the Southern Sierra study unit 

was the only study unit with agricultural land use, most of 

the agriculture land use was in the Tehachapi-Cummings 

Valley (fig. 5C). Inorganic arsenic-based insecticides were 

used extensively in agriculture, particularly in fruit orchards, 

prior to introduction of organochlorine pesticides around 

1950, and use continued until they were banned in the 1980s 

(Welch and others, 2000). Neither the Central Sierra study 

unit nor the Kern River Valley part of the Southern Sierra 

study unit had extensive agricultural land use in the past. 

Arsenical compounds currently are used as pesticides and 

wood preservatives in non-agricultural settings in California 

(Kegley and others, 2010). However, arsenic concentrations 

were not significantly correlated with anthropogenic organic 

constituents (Spearman’s rho test, p = 0.69 to 0.95), nitrate 

(p = 0.84), urban land use (table 10B), or modern groundwater 

(table 10A), suggesting that human activities were not a 

noticeable source of arsenic to groundwater. 
80 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac09-0340_fig 21

100

10

0.1

5.5 6.0

0 50 100 150 200 250

6.5 7.0 7.5 8.0 8.5 9.0 9.5

1

100

10

0.1

1

TOTAL DISSOLVED SOLIDS CONCENTRATION,

IN MILLIGRAMS PER LITER

ARSENIC CONCENTRATION, IN MICROGRAMS PER LITER ARSENIC CONCENTRATION, IN MICROGRAMS PER LITER

pH, IN STANDARD UNITS

A 

B

Modern

Granitic rock

Metamorphic rock

Volcanic rock

Groundwater age classification

 and aquifer lithology

EXPLANATION

Mixed

Sedimentary deposit

Volcanic rock

Pre-modern

Sedimentary deposit

Volcanic rock

Relative-concentration

High

Moderate

Low

High

Low

Moderate

High

Low

Moderate

Figure 21. Relations among arsenic concentration, groundwater age classification, aquifer 

lithology, and (A) pH and (B) total dissolved solids for oxic groundwater samples from the 

Tahoe-Martis study unit, California GAMA Priority Basin Project.
Status and Understanding of Water Quality 81

sac09-0340_fig 22

90th percentile

10th percentile

median

(50th percentile)

lower detached

25th percentile

75th percentile

upper detached

37 number of wells

10

0

20

30

40

50

Modern Mixed Pre-modern

GROUNDWATER AGE

ARSENIC CONCENTRATION, IN MICROGRAM PER LITER

27 54 37

Moderate

High

Low

EXPLANATION

Modern significantly 

different from mixed (p<0.001) 

and pre-modern (p<0.001)

Relative-concentration

High

Moderate

Low

Figure 22. Relation between arsenic concentration and groundwater age 

classification, Tahoe-Martis, Central Sierra, and Southern Sierra study units, California 

GAMA Priority Basin Project.

Factors Affecting Fluoride

Fluoride is the anion form of the element fluorine. 

Natural sources of fluoride in groundwater include dissolution 

of fluoride-bearing minerals, such as fluorite (CaF2) and 

fluorapatite [Ca5(PO4)3(F,OH)]. The main anthropogenic 

source of fluoride to water is addition of sodium fluoride 

or hexafluorosilicic acid during drinking‐water treatment 

as a public health measure to reduce dental caries. 

Hexafluorosilicic acid is a byproduct of production of 

phosphate fertilizers and hydrofluoric acid and the processing 

of aluminum. The MCL-CA for fluoride, 2 mg/L, is lower 

than the MCL-US for fluoride, 4 mg/L. Chronic exposure to 

fluoride concentrations in drinking water above the MCL may 

result in bone disease and tooth discoloration. Fluoride was 

detected at low RCs in the Tahoe-Martis study unit, and at low, 

moderate, and high RCs in the Central Sierra and Southern 

Sierra study units (figs. 17A,B,C and 23A,B,C).

Fluoride, molybdenum, and boron had significant positive 

correlations with each other (p < 0.001) and had similar 

patterns of significant correlations with potential explanatory 

factors (tables 10A,B). All three had significant positive 

correlations with agricultural land use and septic tank density 

and significant negative correlations with aridity index and 

elevation (table 10B). However, because the three constituents 

also had significant positive associations with location in the 

Central Sierra and Southern Sierra study units (table 10A), 

these correlations may reflect the significant associations 

between these potential explanatory factors and location in 

the Central Sierra and (or) the Southern Sierra study units 

(tables 6A,C). All three constituents had significant positive 

correlations with well depth and pH, and significant negative 

correlations with DO (table 10B). All were significantly 

associated with groundwater with pre-modern or mixed age 

distributions (table 10A), and all also had significant positive 

correlations with TDS (Spearman’s rho test, p < 0.001), 

manganese (p = 0.001 to p = 0.004), uranium (p < 0.001), and 

arsenic (p < 0.001).

Elevated concentrations of fluoride in groundwater 

typically are associated with long residence times and 

granitic aquifer lithology (Nordstrom and others, 1989; 

Kim and Jeong, 2005). Granitic rocks vary in mineralogical 

composition; high fluoride concentrations in groundwater 

typically are associated with granitic rocks containing more 

alkali feldspar than plagioclase feldspar, such as syenites 

and alkali granites. For the Tahoe-Martis, Central Sierra, 

and Southern Sierra study unit dataset, greater fluoride 

concentrations were significantly associated with granitic and 

metamorphic aquifer lithologies, and with groundwater with 

pre-modern and mixed age distributions (table 10A). 
82 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac09-0340_fig 23a

NEVADA CO

PLACER CO

EL DORADO CO

ALPINE

CO

Mesozoic granitic rocks

Cenozoic sedimentary deposits

Low (< 1)

Grid wells

Low (< 1)

Understanding wells

Low (< 1)

CDPH wells

Cenozoic volcanic and volcaniclastic rocks

Paleozoic and Mesozoic metamorphic rocks

and non-granitic plutonic rocks

Geologic units

Martis and Tahoe study area boundary

Fluoride concentration, in milligrams per liter

EXPLANATION

Hard Rock study area boundary

39° 15’

120° 15’ 120°

39°

0 5 10 MILES

0 5 10 KILOMETERS

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006. 

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

Geologic units simplified from Saucedo and 

others (2000) and Crafford (2007)

A

< is less than

LAKE TAHOE

CALIFORNIA

NEVADA

#

Figure 23. Relative-concentrations of fluoride for USGS- and CDPH-grid wells, USGS-understanding wells, and all 

wells in the CDPH database with data for fluoride during the 3-year intervals used in the status assessment for the 

(A) Tahoe-Martis, (B) Central Sierra, and (C) Southern Sierra study units, California GAMA Priority Basin Project.
Status and Understanding of Water Quality 83

sac09-0340_fig 23b

FRESNO CO

MARIPOSA CO

MADERA CO

EXPLANATION

Mesozoic granitic rocks

Cenozoic sedimentary deposits

Cenozoic volcanic and volcaniclastic rocks

Paleozoic and Mesozoic metamorphic rocks

and non-granitic plutonic rocks

Geologic units

Study unit boundary 

Low (< 1)

Moderate (1–2)

High (> 2)

Grid wells

Low (< 1)

Moderate (1–2)

Understanding wells

Low (< 1)

Moderate (1–2)

High (> 2)

CDPH wells

Fluoride concentration, in milligrams per liter

< is less than;

> is greater than

0 5 10 MILES

0 5 10 KILOMETERS

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006,

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

37° 30’

37° 15’

119° 45’ 119° 30’

B

Geologic units simplified from

Saucedo and others (2000).

Figure 23.—Continued
84 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac09-0340_fig 23c

EXPLANATION

Mesozoic granitic rocks

Cenozoic sedimentary deposits

Cenozoic volcanic and volcaniclastic rocks

Paleozoic and Mesozoic metamorphic rocks

 and non-granitic plutonic rocks

Geologic units

Study unit boundary

Low (< 1)

Moderate (1–2)

High (> 2)

Grid wells

Low (< 1)

Understanding wells

Low (< 1)

Moderate (1–2)

High (> 2)

CDPH wells

Fluoride concentration, in micrograms per liter 

< is less than;

> is greater than

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006,

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

Geologic units simplified from 

Saucedo and others (2000)

C

35°15'

118°30’ 118°15’

36°

35°45’

35°30’

10 MILES

50 10 KILOMETERS

0 5

TULARE CO

KERN CO

Figure 23.—Continued
Status and Understanding of Water Quality 85

Granitic and metamorphic rocks in the three study units 

yielded groundwater with different fluoride concentrations. 

None of the groundwater samples from the Tahoe-Martis 

study unit had moderate or high concentrations of fluoride 

(fig. 23A), and there was no significant difference in fluoride 

concentrations between groundwater from the southern and 

northern portions of the study unit (fig. 24). The northern 

portion of the study unit consists primarily of volcanic rocks 

and sediments derived from volcanic rocks, whereas the 

southern portion is granitic rocks and sediments derived 

from granitic rocks (fig. 4A). Fluoride concentrations in 

groundwater from the Central Sierra study unit and the 

Kern River Valley part of the Southern Sierra study unit 

were significantly higher than those in groundwater from 

the southern parts of the Tahoe-Martis and Southern Sierra 

study units (fig. 24). The Central Sierra study unit primarily 

consists of granitic rocks (figs. 4B, 10), and a large portion 

of the Southern Sierra study unit is either granitic rocks 

or sediments largely derived from granitic rocks (fig. 4C). 

Within the Southern Sierra study unit, fluoride concentrations 

in groundwater from the Kern River Valley watershed were 

greater than fluoride concentrations in groundwater from 

the Tehachapi-Cummings Valley watershed, similar to the 

spatial distribution of arsenic concentrations in groundwater 

(figs.18C, 23C). 

Tahoe-Martis study unit: north includes the Martis, Tahoe West, and Tahoe North Valleys 

and surrounding watersheds; south includes the Tahoe South Valley and surrounding 

watershed.

Central Sierra study unit: samples divided into two groups by pH.

Southern Sierra study unit: north includes the Kern River Valley and surrounding 

watershed; south includes the Tehachapi-Cummings Valleys and surrounding watershed.

Moderate

High

Low

31 21 19 11 16 24

0

1

2

3

4

north south pH<7.3 pH >7.3 north south

FLUORIDE CONCENTRATION, IN MILLIGRAMS PER LITER

TAHOE-MARTIS

STUDY UNIT

CENTRAL SIERRA

STUDY UNIT

SOUTHERN SIERRA

STUDY UNIT

EXPLANATION

Boxes with different colors

have significantly different

median values (p<0.05)

90th percentile

10th percentile

median

(50th percentile)

lower detached

25th percentile

75th percentile

upper detached

31 number of wells

Relative-concentration

High

Moderate

Low

Figure 24. Relation between fluoride concentration and geographic location within the 

Tahoe-Martis and Southern Sierra study units and between fluoride concentration and pH 

for the Central Sierra study unit, California GAMA Priority Basin Project.
86 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

The differences among fluoride concentrations in 

groundwater in the southern part of the Tahoe-Martis study 

unit, the Central Sierra study unit, and the northern and 

southern parts of the Southern Sierra study unit likely reflect 

regional differences in the composition of the plutons of the 

Sierra Nevada batholith. Ague and Brimhall (1987, 1988) 

defined the regional variations in the batholith on the basis of 

compositional systematics of the most commonly occurring 

mafic mineral, biotite [K(Mg,Fe)3(AlSi3O10)(OH,F)2]. They 

infer that the batholith was formed from I-type granitic 

magmas (igneous protolith) with variable contamination 

from sedimentary protolith materials. On a regional scale, the 

amount of contamination increases from west to east as the 

batholith overlaps the continental margin of the Precambrian 

continental craton. The granitic rocks fall into four types on 

the basis of magnesium/iron and fluoride/hydroxide ratios in 

biotite. Three types have high magnesium/iron ratios reflecting 

oxidized conditions, and have progressively increasing 

fluoride/hydroxide ratios from west to east, reflecting 

increased contamination from fluorine-rich rocks of the craton 

(called I-WC, I-MC, and I-SC for I-type granitic magma with 

weak, moderate, and strong contamination, respectively). 

The fourth type has low magnesium/iron ratio, reflecting 

anoxic conditions, and moderate to high fluoride/hydroxide 

ratios, and consists of I-type granitic magmas contaminated 

with reduced phyllites and pelitic schists (called I-SCR for 

I-type granitic magma with strong contamination by reduced 

materials). Reduced phyllites and pelitic schists are common 

lithologies in the pendants of metamorphic rocks exposed 

in both the Central Sierra and Southern Sierra study units. 

The I-SCR rocks are confined to a narrow belt extending 

from approximately 37°30' to approximately 35°30' between 

the foothills and the High Sierra (Ague and Brimhall, 1987, 

1988). The northern end of the belt passes through the Central 

Sierra study unit, and the southern end passes through the 

Kern River Valley portion of the Southern Sierra study unit. 

Within the boundaries of the belt, I-SCR rocks are interspersed 

with the west to east gradation of I-WC and I-MC rocks. The 

observation that higher fluoride concentrations were detected 

in groundwater from the Central Sierra study unit and northern 

part of the Southern Sierra study unit compared to fluoride 

concentrations in groundwater from the southern parts of the 

Tahoe-Martis and Southern Sierra study units (fig. 24) may 

indicate a correlation between higher groundwater fluoride 

concentrations and presence of I-SCR rocks. 

Among the groundwater samples from the Central 

Sierra study unit, samples with pH > 7.3 had significantly 

greater fluoride concentrations than samples with pH <  7.3 

(fig. 24). Fluoride concentrations in groundwater typically 

are limited by the low solubility of fluorite (CaF2) in waters 

containing calcium (Nordstrom and others, 1989; Kim and 

Jeong, 2005). For the Central Sierra study unit, the highest 

fluoride concentrations occurred in samples with low calcium 

concentrations and high pH values (figs. 25A, B). 

The relations among groundwater age, pH, and calcium 

and fluoride concentrations suggest a possible mechanism for 

generating the elevated fluoride concentrations: long residence 

times lead to increased reaction between groundwater and the 

granitic aquifer materials; pH increases with mineral reactions, 

contributing to precipitation of calcite; and the lower calcium 

concentrations result in increased dissolution of fluorite (Kim 

and Jeong, 2005, and references therein). Fluorite (CaF2) is 

an accessory mineral typically found in late-crystallizing, 

hydrothermal facies of evolved granitic systems, and is a 

common mineral in pegmatite veins (Deer and others, 1995).

Results from geochemical modeling support this 

suggested mechanism. Saturation indices (SI) for calcite 

and fluorite in groundwater samples from the Central Sierra 

study unit were calculated using PHREEQC (version 2.17; 

Parkhurst and Appelo, 1999). Groundwater samples along the 

trend of decreasing calcium with increasing pH are all close to 

saturation with calcite (–0.40 < SI < +0.13) (fig. 25A), indicating 

that calcite precipitation is a plausible mechanism for the 

decreasing calcium concentrations. Among the samples close 

to saturation with calcite, fluoride concentration increases 

as the degree of undersaturation with fluorite decreases, 

suggesting dissolution of fluorite is occurring (fig. 25B).

Not all of the samples with high pH and low calcium 

have high fluoride concentrations (fig. 25A), despite being 

strongly undersaturated with fluorite. This suggests that 

both favorable geochemical conditions and rocks containing 

sufficient fluorite are needed to produce groundwater with 

high fluoride concentrations. 

Factors Affecting Molybdenum

Molybdenum is a metallic trace element used in 

high-strength steel alloys. The main natural source of 

molybdenum to groundwater is dissolution of molybdenumbearing minerals in aquifer materials, the most common 

of which is the sulfide molybdenite (MoS2). Molybdenite 

generally forms in high-temperature environments and 

therefore occurs as a primary mineral in many igneous 

and contact metamorphic rocks. It may also precipitate at 

low temperatures in sediments under anoxic conditions. 

Potential anthropogenic sources include manufacture and 

use of molybdenum steel alloys, dry lubricants, and other 

industrial products. Molybdenum has a HAL-US of 40 μg/L in 

drinking water, and is included on the USEPA’s Contaminant 

Candidate List 3 (U.S. Environmental Protection Agency, 

2009c). Molybdenum is an essential trace nutrient for plants, 

in particular, for nitrogen-fixing species such as legumes. High 

levels of molybdenum in animals (including humans) may 

interfere with uptake of copper.

High RCs of molybdenum were detected in wells 

sampled by USGS-GAMA in the Tahoe-Martis and Central 

Sierra study units, and moderate RCs of molybdenum were 

detected in wells sampled by USGS-GAMA in all three study 

units (figs. 26A,B,C). There were no data for molybdenum 

concentrations in the CDPH database during the 3-year 

intervals used for the status assessment in any of the three 

study units. 
Status and Understanding of Water Quality 87

sac09-0340_fig 25ab

A 

B

0

20

40

60

80

CALCIUM CONCENTRATION, IN MILLIGRAMS PER LITER -3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

CALCITE SATURATION INDEX, DIMENSIONLESS

-4 -3 -2 -1 0

FLUORITE SATURATION INDEX, DIMENSIONLESS

56789 10

pH, IN STANDARD UNITS

< is less than;

> is greater than

< is less than;

> is greater than

Near saturation

with calcite

Calcite undersaturated

> 2 

1–2

0.5–1

< 0.5

Fluoride concentration, in milligrams per liter

EXPLANATION

Near saturation

with calcite

Calcite undersaturated

> 2 

1–2

0.5–1

< 0.5

Fluoride concentration, in milligrams per liter

EXPLANATION

Figure 25. Relations between fluoride concentration and (A) pH and calcium concentration, and (B) fluorite 

and calcite saturation indexes, Central Sierra study unit, California GAMA Priority Basin Project.
88 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac09-0340_fig 26a

NEVADA CO

PLACER CO

EL DORADO CO

ALPINE

CO

Mesozoic granitic rocks

Cenozoic sedimentary deposits

Low (< 20)

High (> 40)

Grid wells

Low (< 20)

Moderate (20–40)

High (> 40)

Understanding wells

Cenozoic volcanic and volcaniclastic rocks

Paleozoic and Mesozoic metamorphic rocks

and non-granitic plutonic rocks

Geologic units

Martis and Tahoe study area boundary

Molybdenum concentration, in micrograms per liter

EXPLANATION

Hard Rock study area boundary

39° 15’

120° 15’ 120°

39°

0 5 10 MILES

0 5 10 KILOMETERS

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006. 

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

Geologic units simplified from Saucedo and 

others (2000) and Crafford (2007)

A

< is less than:

> is greater than

LAKE TAHOE

CALIFORNIA

NEVADA

Figure 26. Relative-concentrations of molybdenum for USGS- and CDPH-grid wells, and USGS-understanding wells 

for the (A) Tahoe-Martis, (B) Central Sierra, and (C) Southern Sierra study units, California GAMA Priority Basin Project.
Status and Understanding of Water Quality 89

sac09-0340_fig 26b

FRESNO CO

MARIPOSA CO

MADERA CO

EXPLANATION

Mesozoic granitic rocks

Cenozoic sedimentary deposits

Cenozoic volcanic and volcaniclastic rocks

Paleozoic and Mesozoic metamorphic rocks

and non-granitic plutonic rocks

Geologic units

Study unit boundary 

Low (< 20)

Moderate (20–40)

High (> 40)

Grid wells

Low (< 20)

Understanding wells

Molybdenum concentration, in micrograms per liter

Moderate (20–40)

< is less than;

> is greater than

0 5 10 MILES

0 5 10 KILOMETERS

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006,

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

37° 30’

37° 15’

119° 45’ 119° 30’

B

Geologic units simplified from

Saucedo and others (2000).

Figure 26.—Continued
90 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac09-0340_fig 26c

EXPLANATION

Mesozoic granitic rocks

Cenozoic sedimentary deposits

Cenozoic volcanic and volcaniclastic rocks

Paleozoic and Mesozoic metamorphic rocks

 and non-granitic plutonic rocks

Geologic units

Study unit boundary

Low (< 20)

Moderate (20–40)

Grid wells

Low (< 20)

Moderate (20–40)

Understanding wells

Molybdenum concentration, in micrograms per liter 

< is less than;

> is greater than

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006,

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

Geologic units simplified from 

Saucedo and others (2000)

C

35°15'

118°30’ 118°15’

36°

35°45’

35°30’

10 MILES

50 10 KILOMETERS

0 5

TULARE CO

KERN CO

Figure 26.—Continued
Status and Understanding of Water Quality 91

Molybdenum concentrations were significantly 

correlated with the same set of potential explanatory 

factors as fluoride concentrations (tables 10A,B); however, 

subdivision of the data by study unit, geography, and pH, 

as was done for fluoride, did not yield the same pattern of 

significant differences among groups. For molybdenum, 

the correlations with oxidation-reduction conditions, pH, 

and aquifer lithology had the greatest explanatory value. 

Groundwater with moderate or high RCs of molybdenum 

occurred in wells with granitic aquifer or sedimentary aquifer 

lithologies, and molybdenum concentrations were lowest 

in wells with volcanic aquifer lithology (table 10A). The 

positive correlations between molybdenum and septic tank 

density and well depth (table 10B) likely reflect that greater 

septic tank density and deeper wells (table 6A) and granitic 

aquifer lithology (table 6C) were significantly associated with 

location in the Central Sierra study unit. In wells with granitic 

aquifer lithology, moderate and high RCs of molybdenum 

typically were associated with groundwater with either oxic 

and high pH conditions, or anoxic conditions with a range of 

pH values (fig. 27). The major soluble species of molybdenum 

at pH > 5 is the molybdate oxyanion MoO4

–2 (Evans and 

Barabash, 2010). Among the groundwater samples with anoxic 

conditions, all of the moderate and high RCs of molybdenum 

occurred in groundwater that was suboxic, nitrate-reducing, or 

manganese-reducing, and groundwater that was iron-reducing 

had significantly lower molybdenum concentrations (fig. 28). 

High

Moderate

Low

pH, IN STANDARD UNITS

56789 10

0

20

40

60

80

MOLYBDENUM CONCENTRATION,

IN MICROGRAMS PER LITER

Oxic Anoxic or mixed

Granitic rock

Metamorphic rock

Sedimentary deposit

Volcanic rock

Granitic rock

Metamorphic rock

Sedimentary deposit

Volcanic rock

Oxidation-reduction class and aquifer lithology

EXPLANATION

Relative-concentration

High

Moderate

Low

Figure 27. Relations among molybdenum concentration, pH, aquifer lithology, and oxidation-reduction class, 

Tahoe-Martis, Central Sierra, and Southern Sierra study units, California GAMA Priority Basin Project.
92 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Molybdenum present as the molybdate oxyanion adsorbed to 

manganese oxide minerals may be released during reductive 

dissolution of these oxides, similar to the way adsorbed 

arsenic is released during reductive dissolution of the oxides 

to which it is adsorbed. The reduction of iron from the +3 

oxidation state (Fe+3, ferric iron) to the +2 oxidation state 

(Fe+2, ferrous iron), of molybdenum from the +6 oxidation 

state (Mo+6) to the +4 oxidation state (Mo+4), and of sulfur 

from the +6 oxidation state (S+6 in sulfate) to the –2 oxidation 

state (S–2 in sulfide) occur at similar oxidation-reduction 

potentials. Mo+4 is highly insoluble in waters containing 

sulfide, and precipitates as molybdenite (MoS2) or other 

sulfide minerals and complexes (Anbar, 2004). Thirteen of 

the 14 wells with moderate or high RCs of molybdenum were 

in the Central Sierra study unit (fig. 26B). This may largely 

reflect that few groundwater samples from wells with granitic 

aquifer lithology in the Tahoe-Martis and Southern Sierra 

study units had either anoxic oxidation-reduction conditions 

(fig. 15A) or high pH values (fig. 15B). 

sac09-0340_fig 28

Suboxic/NO3-red/Mn-red 

and oxic/pH > 7.3 significantly 

different from Fe-red and

oxic/pH < 7.3 (p < 0.001 to p = 0.008) 

0

20

40

60

80

MOLYBDENUM CONCENTRATION,

IN MICROGRAMS PER LITER

OXIDATION-REDUCTION CLASS

Fe-red suboxic

or NO3-red

or Mn-red

oxic oxic

any any

pH VALUE

> 7.3 < 7.3

Oxidation-reduction classes: Fe-red, iron-reduction; Mn-red, manganese-reduction;

NO3

-red, nitrate-reduction; suboxic, dissolved oxygen (DO) < 1 and not Fe-red, Mn-red,

or NO3

-red; oxic, DO > 1.

5 10 13 8

High

Moderate

Low

EXPLANATION

90th percentile

10th percentile

median

(50th percentile)

lower detached

25th percentile

75th percentile

upper detached

5 number of wells

Relative-concentration

High

Moderate

Low

Figure 28. Relations among molybdenum, oxidation-reduction class, and pH for wells 

with granitic aquifer geology, Tahoe-Martis, Central Sierra, and Southern Sierra study 

units, California GAMA Priority Basin Project.
Status and Understanding of Water Quality 93

pH, IN STANDARD UNITS

5.5 6 .0 6.5 7 .0 7.5 8 .0 8.5 9 .0 9.5

0

10

20

30

40

50

80

90

MOLYBDENUM CONCENTRATION, IN MICROGRAMS PER LITER

Low

Moderate

High

< is less than;

> is greater than

EXPLANATION

Martis, Tahoe West, and Tahoe North Valleys

Tahoe South Valley, depth to top of screened interval <135 ft

Tahoe South Valley, depth to top of screened interval >135 ft

Tahoe-Martis study unit

Tehachapi Valley

Cummings Valley

Southern Sierra study unit

Relative-concentration

High

Moderate

Low

Figure 29. Relation between molybdenum concentration and well location for wells with oxic groundwater and 

sedimentary aquifer lithology, Tahoe-Martis and Southern Sierra study units, California GAMA Priority Basin Project.

In wells with sedimentary aquifer lithology, moderate and 

high RCs of molybdenum were detected in oxic groundwater 

from the Cummings Valley basin, in the southern part of 

the Southern Sierra study unit, and in deeper wells in the 

Tahoe Valley South subbasin in the Tahoe-Martis study unit 

(figs. 26A,C and 29). The differentiation between deep and 

shallow wells in the Tahoe Valley South subbasin is 135 ft bls; 

this depth was selected based on molybdenum data and not on 

information about groundwater ages or stratigraphic data for 

the basin. Molybdenum in the Mo+6 oxidation state is highly 

soluble in oxic water and primarily exists as the molybdate 

oxyanion (Anbar, 2004). Similar to many other metal 

oxyanions, molybdate can sorb to manganese oxyhydroxides, 

and sorbtion is weaker at higher pH. The three samples 

with the highest molybdenum concentrations were from the 

Tahoe Valley South subbasin, and all three samples had pH 

values > 8.7 (fig. 27). Granitic rocks are the primary source of 

sediments to the valley (fig. 4A).
94 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Factors Affecting Boron

Boron is a naturally occurring metalloid element with 

high solubility in water. Natural sources of boron include 

evaporate minerals, such as borax, ulexite, and colemanite, 

and boron-bearing silicate minerals, such as tourmaline, 

that are primarily found in igneous rocks (Hem, 1989; 

Klein and Hurlbut, 1993). Seawater contains approximately 

4,500 μg/L of boron, and boron also is associated with thermal 

springs and volcanic activity (Hem, 1989). Boron occurs in 

wastewater because borax is a component of many detergents. 

Other anthropogenic uses of boron compounds include 

borosilicate glass, boric acid insecticide, chemical reagents, 

semi-conductors, and fertilizers. Boron is an essential nutrient 

for plants, but is toxic to plants at high concentrations. The 

NL-CA for boron is 1,000 μg/L, and at higher concentrations, 

boron may adversely affect fetal development (U.S. 

Environmental Protection Agency, 2008). High RCs of boron 

were detected in samples from the Tahoe-Martis and Southern 

Sierra study units, and moderate RCs were detected in samples 

from all three study units (figs. 30A,B,C).

In the Tahoe-Martis, Central Sierra, and Southern 

Sierra study units, boron concentrations in groundwater had 

significant positive correlations with septic tank density, well 

depth, pH, and percent agricultural land use and significant 

negative correlations with elevation, aridity index, and DO 

(table 10B). Higher boron concentrations were significantly 

associated with pre-modern and mixed-age groundwater, 

and with location in the Central and Southern Sierra study 

units (table 10A). Boron had the same pattern of significant 

relations to potential explanatory factors as did fluoride and 

molybdenum, except that boron was not significantly related 

to aquifer lithology. Many of the correlations between boron 

and potential explanatory factors likely reflect correlations 

among the explanatory factors rather than causative relations 

explanatory of boron distribution in groundwater.

High and moderate RCs of boron occurred in three 

settings in the Tahoe-Martis, Central Sierra, and Southern 

Sierra study units: groundwater with high RCs of TDS, 

groundwater from wells with volcanic aquifer lithology, and 

groundwater from areas of the Central and Southern Sierra 

study units that also have high and moderate RCs of fluoride, 

molybdenum, arsenic, and uranium in groundwater. The only 

three samples with high RCs of boron corresponded to three of 

the four samples with high RCs of TDS (TTAHO-11, TROCK13, SOSA-35). These three samples also had high RCs of 

arsenic, anoxic or mixed oxic/anoxic oxidation-reduction 

conditions, and were from wells with metamorphic or granitic 

aquifer lithology.

In the Tahoe-Martis study unit, boron concentrations were 

significantly greater in wells with volcanic aquifer lithology 

than in wells with any other aquifer lithology (figs. 30A, 31). 

Fossil hot springs and other features associated with volatile 

emanations during volcanism are the likely sources of boron in 

the volcanic rocks. 

In the Central Sierra study unit, boron concentrations 

were significantly correlated with major-ion chemistry. 

Anion compositions of Central Sierra study unit groundwater 

primarily range from bicarbonate-dominated to chloridedominated (see section “Factors Affecting Total Dissolved 

Solids” for more information). Boron concentrations in 

groundwater samples with ≥ 20% chloride in the anions were 

significantly greater than boron concentrations in groundwater 

samples with < 20% chloride in the anions (fig. 31). Fluoride, 

arsenic, and molybdenum concentrations and pH also were 

significantly higher in samples with ≥ 24% chloride in 

the anions (Wilcoxon p  =  0.007, 0.006, 0.005, and 0.009, 

respectively); however, there was no significant difference 

in TDS concentrations between the two groups (Wilcoxon 

p =  0.28). These relations suggest the presence of at least two 

different compositional types of groundwater in the Central 

Sierra study unit: one with bicarbonate as the dominant anion 

and relatively lower concentrations of trace elements, and 

another with chloride as the dominant anion and relatively 

higher concentrations of trace elements. 

The geographic distribution of high and moderate RCs 

of boron in the Southern Sierra study unit was similar to those 

of arsenic and fluoride (figs. 18C, 23C, and 30C). For all three 

constituents, most of the wells with high and moderate RCs 

were located in the northern part of the study unit, in the Kern 

River Valley and surrounding watershed.

For the set of samples from the Central Sierra study unit 

and the northern part of the Southern Sierra study unit, boron 

concentrations were significantly positively correlated with 

fluoride concentrations (Spearman’s rho test, p <  0.001), and 

boron and fluoride concentrations were significantly positively 

correlated with lithium concentrations (p = 0.006 and 0.002, 

respectively). Boron, fluoride, and lithium are concentrated in 

hydrothermal fluids associated with granitic magmatism, and 

boron- and lithium-bearing minerals such as tourmaline and 

lepidolite (a lithium- and fluoride-bearing mica) co-occur with 

fluorite in pegmatite veins and contact aureoles around granitic 

plutons (Leeman and Sisson, 1996; London and others, 

1996). The close relations among boron, fluoride, and lithium 

concentrations in the groundwater samples suggest that the 

primary source of boron is hydrothermal fluids or mineral 

assemblages deposited from hydrothermal fluids. 
Status and Understanding of Water Quality 95

sac09-0340_fig 30a

NEVADA CO

PLACER CO

EL DORADO CO

ALPINE

CO

Mesozoic granitic rocks

Cenozoic sedimentary deposits

Low (< 500)

Moderate (500–1,000)

High (> 1,000)

Grid wells

Low (< 500)

Understanding wells

Cenozoic volcanic and volcaniclastic rocks

Paleozoic and Mesozoic metamorphic rocks

and non-granitic plutonic rocks

Geologic units

Martis and Tahoe study area boundary

Boron concentration, in micrograms per liter

EXPLANATION

Hard Rock study area boundary

39° 15’

120° 15’ 120°

39°

0 5 10 MILES

0 5 10 KILOMETERS

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006. 

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

Geologic units simplified from Saucedo and 

others (2000) and Crafford (2007)

A

< is less than;

> is greater than LAKE TAHOE CALIFORNIA NEVADA

Low (< 500)

CDPH wells

Figure 30. Relative-concentrations of boron for USGS- and CDPH-grid wells, USGS-understanding wells, and all 

wells in the CDPH database with data for boron during the 3-year intervals used in the status assessment for the 

(A) Tahoe-Martis, (B) Central Sierra, and (C) Southern Sierra study units, California GAMA Priority Basin Project.
96 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac09-0340_fig 30b

FRESNO CO

MARIPOSA CO

MADERA CO

EXPLANATION

Mesozoic granitic rocks

Cenozoic sedimentary deposits

Cenozoic volcanic and volcaniclastic rocks

Paleozoic and Mesozoic metamorphic rocks

and non-granitic plutonic rocks

Geologic units

Study unit boundary 

Low (< 500)

Moderate (500–1,000)

Grid wells

Low (< 500)

Understanding wells

Boron concentration, in micrograms per liter

< is less than

0 5 10 MILES

0 5 10 KILOMETERS

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006,

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

37° 30’

37° 15’

119° 45’ 119° 30’

B

Geologic units simplified from

Saucedo and others (2000).

Figure 30.—Continued
Status and Understanding of Water Quality 97

sac09-0340_fig 30c

EXPLANATION

Mesozoic granitic rocks

Cenozoic sedimentary deposits

Cenozoic volcanic and volcaniclastic rocks

Paleozoic and Mesozoic metamorphic rocks

 and non-granitic plutonic rocks

Geologic units

Study unit boundary

Low (< 500)

Moderate (500–1,000)

High (> 1,000)

Grid wells

Low (< 500)

Understanding wells

Boron concentration, in micrograms per liter 

< is less than;

> is greater than

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006,

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

Geologic units simplified from 

Saucedo and others (2000)

C

35°15'

118°30’ 118°15’

36°

35°45’

35°30’

10 MILES

50 10 KILOMETERS

0 5

Low (< 500)

Moderate (500–1,000)

High (> 1,000)

CDPH wells

TULARE CO

KERN CO

Figure 30.—Continued
98 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Nutrients

Nitrate was not detected at moderate or high RCs in 

the USGS-GAMA dataset or reported in the 3-year interval 

of the CDPH database used in the status assessment for the 

Tahoe-Martis study unit (fig. 32A). The CDPH database 

reported a high RC for nitrate in one well in 1987 (table 5), 

and during a survey of 32 wells around Lake Tahoe in 

1990–1992, a high RC of nitrate was detected in one well in 

Stateline, Nevada, which is just over the State line from the 

city of South Lake Tahoe (Thodal, 1997). The CDPH database 

for the Central Sierra study unit reported a high RC of nitrate 

in one well, but the high RC occurred prior to the 3-year 

interval used in the status assessment (table 5). The moderate 

aquifer-scale proportion for nitrate was 3.1%, based on the 

spatially weighted calculation (table 8B). The wells with 

moderate RCs primarily were located near areas of urban land 

use (fig. 32B).

The aquifer-scale proportions for nitrate in the Southern 

Sierra study unit were 2.7% high and 24% moderate 

(table 8C). Most of the wells with high or moderate RCs 

of nitrate were located in the centers of the Tehachapi and 

Cummings Valleys in the southern part of the study unit or 

along the margin of Lake Isabella in the northern part of the 

study unit (fig. 32C). 

sac09-0340_fig 31

BORON CONCENTRATION, IN MICROGRAMS PER LITER

TOTAL DISSOLVED SOLIDS CONCENTRATION,

IN MILLIGRAMS PER LITER

0 100 200 300 400 500 1,000 2,000 3,000

0

250

500

750

1,000

2,000

3,000

4,000

5,000

6,000

Low Low

Moderate

Moderate

High

High

EXPLANATION

Samples from wells in volcanic rocks or sediment

 primarily derived from volcanic rocks

Samples from wells in Kern River Valley

 and surrounding watershed

Samples from Tehachapi-Cummings Valley

 and surrounding watershed

Samples from wells in granitic rocks or sediment

 primarily from granitic rocks

Tahoe-Martis study unit

Samples with chloride comprising < 20% of anions

Samples with chloride comprising > 20% of anions

Central Sierra study unit

Southern Sierra study unit

Relative-concentration

Both constituents high

One constituent high,

one constituent moderate

One constituent moderate,

one constituent low

One constituent high,

one constituent low

Both constituents moderate

Both constituents low

< is less than;

> is greater than or equal to

Figure 31. Relation between boron and total dissolved solids concentrations, Tahoe-Martis, Central Sierra, and 

Southern Sierra study units, California GAMA Priority Basin Project.
Status and Understanding of Water Quality 99

Figure 32. Relative-concentrations of nitrate for USGS- and CDPH-grid wells, USGS-understanding wells, and 

all wells in the CDPH database with data for nitrate during the 3-year intervals used in the status assessment 

for the (A) Tahoe-Martis, (B) Central Sierra, and (C) Southern Sierra study units, California GAMA Priority 

Basin Project.

sac09-0340_fig 32a

A

NEVADA CO

PLACER CO

EL DORADO CO

ALPINE

CO

NEVADA

CALIFORNIA

Martis and Tahoe study area boundary

Hard Rock study area boundary

Urban

Undeveloped forest

Undeveloped rock, shrub and grass

Agricultural

Land use

EXPLANATION

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006. 

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

Land use simplified from

Nakagaki and others (2007).

39° 15’

120° 15’ 120°

0 5 10 MILES

0 5 10 KILOMETERS

LAKE TAHOE

Low (< 5)

Grid wells

Low (< 5)

Understanding wells

Nitrate as nitrogen concentration,

 in milligrams per liter 

< is less than

Low (< 5)

CDPH wells
100 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac09-0340_fig 32b

EXPLANATION

Urban

Undeveloped forest

Undeveloped rock, shrub and grass

Agricultural

Land use

Study unit boundary

MARIPOSA CO

MADERA CO

FRESNO CO

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006,

Albers Equal Area Conic Projection

Land use simplified from

Nakagaki and others (2007).

37° 30’

37° 15’

119° 45’ 119° 30’

B

0 5 10 MILES

0 5 10 KILOMETERS

Low (< 5)

Grid wells

Low (< 5)

Moderate (5–10)

Understanding wells

Nitrate as nitrogen concentration, in milligrams per liter 

< is less than

Low (< 5)

Moderate (5–10)

CDPH wells

Figure 32.—Continued
Status and Understanding of Water Quality 101

sac09-0340_fig 32c

Urban

Undeveloped forest

Undeveloped rock, shrub, and grass

Agricultural

Land use

Study unit boundary

EXPLANATION

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006,

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

Land use simplified from

Nakagaki and others (2007).

C

10 MILES

50 10 KILOMETERS

0 5

35°30’

35°15’

35°

118°30’ 118°15’

36°

35°45’

Low (< 5)

Moderate (5–10)

High (> 10)

Grid wells

Low (< 5)

Moderate (5–10)

Understanding wells

Low (< 5)

Moderate (5–10)

CDPH wells

Nitrate as nitrogen concentration,

 in milligrams per liter 

< is less than;

> is greater than

Figure 32.—Continued
102 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Factors Affecting Nitrate

Nitrate concentration had a significant positive correlation 

with percentage of agricultural land use (table 10B). Because 

all of the agricultural land use in the three study units is in 

the Southern Sierra study unit, many of the other correlations 

between nitrate and potential explanatory factors may reflect 

correlations between explanatory factors and location in 

the Southern Sierra study unit. For example, higher nitrate 

concentrations were significantly associated with metamorphic 

and sedimentary aquifer lithologies (table 10A), consistent 

with the greater abundance of these lithologies among wells 

from the Southern Sierra study unit compared to lithologies 

of wells from the Tahoe-Martis or Central Sierra study units 

(fig. 10). Nitrate concentrations had significant negative 

correlations with elevation and aridity index (table 10B), also 

reflecting location in the Southern Sierra study unit (fig. 11).

Groundwater samples with detections of herbicides and 

(or) solvents had significantly higher nitrate concentrations 

than those without detections of those organic constituents 

(Wilcoxon p  <  0.001). More than half of the wells with 

detections of herbicides and (or) solvents and > 5% agricultural 

land use had moderate or high RCs of nitrate (fig. 33). 

Groundwater samples with no detections of herbicides or 

solvents and from sites with < 5% agricultural land use had 

significantly lower nitrate concentrations than samples from 

wells with either characteristic. Positive correlations between 

nitrate and agricultural land use and application of agricultural 

chemicals commonly are observed in groundwater nationally 

(Nolan and others, 2002).

0

2

4

6

8

10

12

14

16

18

NITRATE CONCENTRATION, IN MILLIGRAMS PER LITER

AGRICULTURAL LAND USE GREATER THAN 5 PERCENT?

HERBICIDES OR SOLVENTS DETECTED?

yes no

yes no

yes no

yes no

14 11 16 66

High

Moderate

Low

EXPLANATION

Boxes with different colors

have significantly different

median values (p < 0.05)

90th percentile

10th percentile

median

(50th percentile)

lower detached

25th percentile

75th percentile

upper detached

37 number of wells

Relative-concentration

High

Moderate

Low

Figure 33. Relations among nitrate concentration and agricultural land use and 

detection of herbicides and solvents, Tahoe-Martis, Central Sierra, and Southern Sierra 

study units, California GAMA Priority Basin Project.
Status and Understanding of Water Quality 103

For the entire dataset, nitrate concentration had a 

significant positive correlation with depth to the top of the 

screened or open interval (table 10B) because wells in the 

Southern Sierra study unit had significantly greater depths 

to the top of the screened or open interval than wells in 

the Central Sierra study unit and Hard Rock study area of 

the Tahoe-Martis study unit (fig. 12B). However, nitrate 

concentration was not significantly correlated with depth 

to top of screened or open interval in any of the study units 

(Spearman’s rho test: Tahoe-Martis, p=0.11; Central Sierra, 

p =  0.81; and Southern Sierra, p =0.16). 

Radioactive Constituents

The class of radioactive constituents includes constituents 

whose abundance is measured as activity rather than 

concentration. Activity is measured in units of picocuries per 

liter (pCi/L), and one picocurie equals approximately two 

atoms decaying per minute. The number of atoms decaying 

is equal to the number of alpha or beta particles emitted. 

Radioactive constituents with health-based benchmarks were 

present at high RCs in 3, 31, and 21% of the primary aquifers 

in the Tahoe-Martis, Central Sierra, and Southern Sierra study 

units, respectively (table 9A). Gross alpha particle, uranium, 

and radon-222 activities accounted for most of the moderate 

and high RCs in all three study units (tables 8A,B,C; fig. 17B).

The MCL-US (15 pCi/L) for gross alpha particle activity 

applies to adjusted gross alpha activity, which is equal to 

measured gross alpha activity minus uranium activity (U.S. 

Environmental Protection Agency, 2009b). Data collected 

by USGS-GAMA and data compiled in the CDPH database 

are reported as gross alpha activity without correction for 

uranium activity. Gross alpha is used a screening tool to 

determine whether other radioactive constituents must be 

analyzed. For regulatory purposes, analysis of uranium is 

only required if gross alpha activity is greater than 15 pCi/L; 

therefore, the CDPH database contains far more data for gross 

alpha activity than for uranium. As a result, it is not always 

possible to calculate adjusted gross alpha activity. The CDPH 

database contains even fewer data for radium and radon-222 

than it does for uranium. The primary contributors to gross 

alpha activity are uranium and radium, and samples with 

high or moderate RCs of gross alpha generally have high or 

moderate RCs of uranium. Because of the lack of data for 

other radioactive constituents in the CDPH database, gross 

alpha data without correction for uranium are the primary data 

used in this report to avoid underestimating the prevalence 

of groundwater with moderate and high RCs of radioactive 

constituents as a class.

 Aquifer-scale proportions for both gross alpha and 

adjusted gross alpha are listed in tables 8A,B,C for comparison 

purposes. Uranium data were not available for all CDPH 

wells with gross alpha data; in these cases, the adjusted gross 

alpha was estimated from the relation between gross alpha and 

adjusted gross alpha in the 31 samples with USGS-GAMA 

data for both gross alpha and uranium (Fram and Belitz, 2007; 

Ferrari and others, 2008; Fram and others, 2009). High and 

moderate proportions for gross alpha are larger than those for 

adjusted gross alpha. Aquifer-scale proportions for radioactive 

constituents as a class were calculated using gross alpha, 

uranium, radium, and radon data (table 9A). The resulting 

high and moderate proportions were slightly greater than 

the high and moderate proportions calculated using adjusted 

gross alpha, uranium, radium, and radon data, although the 

differences were not significant.

In the Tahoe-Martis study unit, the high aquifer-scale 

proportions for gross alpha particles and radon-222 were 

2.0 and 1.0%, respectively (table 8A). Neither uranium 

nor radium were present at high RCs during the period of 

study, although both were reported at high RCs in the CDPH 

database historically (table 5). A high RC of uranium was 

detected in one USGS-understanding well (TTAHOU-08), 

but this well was not included in the dataset for the spatially 

weighted calculation of aquifer-scale proportion because it is 

a shallow monitoring well not representative of the primary 

aquifer. Gross alpha particles, radium, and uranium each had 

moderate aquifer-scale proportions of less than or equal to 

3.0%, and the moderate aquifer-scale proportion for radon-222 

was 12% (table 8A, fig. 17B). In the Tahoe-Martis study unit, 

all of the wells with moderate or high RCs of radioactive 

constituents were located in the Tahoe Valley South subbasin 

and surrounding watershed (fig. 34A).

High RCs of gross alpha particle radiation and uranium 

were present in 16 and 14%, respectively, of the primary 

aquifer in the Central Sierra study unit (table 8B). The 

moderate aquifer-scale proportions for gross alpha particle 

radiation and uranium were 25 and 15%, respectively. Radium 

and radon-222 data were available for only half of the grid 

cells; thus, the estimates of aquifer-scale proportions have 

greater uncertainty. Radon-222 was detected at high RCs, and 

both constituents were detected at moderate RCs (table 8B, 

fig. 17B). Wells with moderate or high RCs of radioactive 

constituents were distributed across both study areas within 

the study unit (fig. 34B). The CWISH study area had greater 

moderate and high aquifer-scale proportions for all radioactive 

constituents than did the CGOLD study area (appendix B, 

tables B1D,E and B2A).

The high aquifer-scale proportions for gross alpha 

particle radiation and uranium in the Southern Sierra 

study unit were both 21%, and the moderate aquifer-scale 

proportions were 11 and 16%, respectively (table 8C; 

fig. 17B). Radium and radon-222 data were available for less 

than one-third and less than one-sixth, respectively, of the grid 

cells; thus, the estimates of aquifer-scale proportions may not 

be representative. Most of the wells with moderate or high 

RCs of radioactive constituents were located in the Kern River 

Valley and surrounding watershed (fig. 34C).
104 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac09-0340_fig 34a

NEVADA CO

PLACER CO

EL DORADO CO

ALPINE

CO

Mesozoic granitic rocks

Cenozoic sedimentary deposits

Low (< 10)

Moderate (10–20)

USGS-grid wells

Low (< 10)

Moderate (10–20)

High (> 20)

USGS-understanding wells

Low (< 10)

Moderate (10–20)

CDPH wells

Cenozoic volcanic and volcaniclastic rocks

Paleozoic and Mesozoic metamorphic rocks

and non-granitic plutonic rocks

Geologic units

Martis and Tahoe study area boundary

Uranium relative-concentration,

 in picocuries per liter

EXPLANATION

Hard Rock study area boundary

39° 15’

120° 15’ 120°

39°

0 5 10 MILES

0 5 10 KILOMETERS

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006. 

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

Geologic units simplified from Saucedo and 

others (2000) and Crafford (2007).

A

< is less than

LAKE TAHOE

CALIFORNIA

NEVADA

> is greater than

Figure 34. Relative-concentrations of uranium for USGS- and CDPH-grid wells, USGS-understanding wells, and all 

wells in the CDPH database with data for uranium during the 3-year intervals used in the status assessment for the 

(A) Tahoe-Martis, (B) Central Sierra, and (C) Southern Sierra study units, California GAMA Priority Basin Project.
Status and Understanding of Water Quality 105

sac09-0340_fig 34b

FRESNO CO

MARIPOSA CO

MADERA CO

EXPLANATION

Mesozoic granitic rocks

Cenozoic sedimentary deposits

Cenozoic volcanic and volcaniclastic rocks

Paleozoic and Mesozoic metamorphic rocks

and non-granitic plutonic rocks

Geologic units

Study unit boundary 

Low (< 10)

Moderate (10–20)

High (> 20)

USGS-grid wells

Low (< 10)

USGS-understanding wells

Low (< 10)

Moderate (10–20)

High (> 20)

CDPH wells

Uranium relative-concentration, in picocuries per liter

< is less than;

> is greater than

0 5 10 MILES

0 5 10 KILOMETERS

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006,

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

37° 30’

37° 15’

119° 45’ 119° 30’

B

Geologic units simplified from

Saucedo and others (2000).

Figure 34.—Continued
106 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac09-0340_fig 34c

EXPLANATION

Mesozoic granitic rocks

Cenozoic sedimentary deposits

Cenozoic volcanic and volcaniclastic rocks

Paleozoic and Mesozoic metamorphic rocks

 and non-granitic plutonic rocks

Geologic units

Study unit boundary

Low (< 10)

Moderate (10–20)

High (> 20)

USGS-grid wells

Low (< 10)

USGS-understanding wells

Low (< 10)

Moderate (10–20)

High (> 20)

CDPH wells

Uranium relative-concentration, in picocuries per liter

< is less than;

> is greater than

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006,

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

Geologic units simplified from 

Saucedo and others (2000).

C

35°15'

118°30’ 118°15’

36°

35°45’

35°30’

10 MILES

50 10 KILOMETERS

0 5

TULARE CO

KERN CO

Figure 34.—Continued
Status and Understanding of Water Quality 107

0 10 20 30 40 50 200 400 600

0

15

30

45

60

300

600

900

0

4,000

8,000

12,000

16,000

80,000

160,000

URANIUM ACTIVITY, IN PICOCURIES PER LITER

RADON-222 ACTIVITY, IN PICOCURIES PER LITER

Low

Low

Moderate

Moderate

High

High

GROSS ALPHA PARTICLE ACTIVITY, IN PICOCURIES PER LITER

EXPLANATION

Gross alpha particle

Radon-222

Constituent

Relative-concentration

Both constituents high

One constituent high,

one constituent moderate

One constituent moderate,

one constituent low

One constituent high,

one constituent low

Both constituents moderate

Both constituents low

Figure 35. Relations among uranium, radon-222, and gross alpha particle activities, Tahoe-Martis, Central Sierra, 

and Southern Sierra study units, California GAMA Priority Basin Project.

Factors Affecting Uranium, Gross Alpha Particle Activity, 

and Radon

Uranium, gross alpha particle, and radon-222 activities 

had significant positive correlations with one another, although 

the correlation between uranium and gross alpha particle 

activities (Spearman’s rho test, p  <  0.001, rho = 0.89) was 

considerably stronger than the correlation between uranium 

and radon-222 activities (p  <  0.001, rho=0.49) (fig. 35). 

The primary source of the alpha particles counted during 

laboratory analysis of gross alpha particle activity is the decay 

of dissolved uranium in the sample, thus a high degree of 

correlation was expected (Thomas and others, 1993). 

The source of radon-222 in groundwater is the decay of 

radium-226 (a member of the uranium-238 decay series) in 

aquifer materials. Groundwater in crystalline rocks typically 

has low radium activities because radium sorbs strongly to 

mineral surfaces, particularly to altered feldspars (Zapecza and 

Szabo, 1988; Thomas and others, 1993). Radon, however, is 

an inert gas that readily diffuses out of the aquifer materials 

and into the groundwater. Ayotte and others (2007) observed 

greater activities of radon-222 in groundwater from crystalline 

bedrock aquifers in the northern United States compared to 

aquifers composed of glacial sediments derived from the 

crystalline bedrock. They attributed the greater radon-222 

activities in the crystalline bedrock aquifers to concentrations 

of sorbed radium on fracture surfaces. Because of the 

complicating geological and sorptive factors between uranium 

and radon, a less strong correlation was expected compared 

to the correlation between gross alpha particle activity and 

uranium (fig. 35).
108 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

The following discussion of factors affecting the 

radioactive constituents primarily is focused on uranium. The 

Central Sierra study unit and the Kern River Valley area of 

the Southern Sierra study unit correspond to the northwestern 

and southeastern ends, respectively, of a band of granitic rocks 

in the western Sierra Nevada known to have groundwater 

with greater radioactivity than groundwater elsewhere in the 

Sierra Nevada (California Department of Water Resources, 

1990). Uranium activities had significant positive associations 

with location in the Central and Southern Sierra study units 

(table 10A). This relation between uranium activity and 

location accounts for the significant negative correlations 

between uranium activity and aridity index and elevation, and 

the significant positive correlation with septic tank density 

(table 10B).

Median uranium activities in samples in the three 

groundwater age classes were significantly different: uranium 

activities were lowest in groundwater with modern age 

distribution, higher in groundwater with pre-modern age 

distribution, and highest in groundwater with mixed age 

distribution (table 10A). The relations between uranium 

activity and groundwater age in part reflect the complex 

interrelations among groundwater age, aquifer lithology, study 

unit, and well depth (tables 6A,B). 

Uranium activity had a significant negative correlation 

with DO concentration (table 10B). However, the relations 

between uranium and DO in individual samples from the 

three study units indicated that elevated uranium activities 

occurred under oxic and anoxic conditions (fig. 36). Elevated 

uranium activities in groundwater from wells with granitic 

or metamorphic aquifer lithologies primarily occurred in 

samples with DO concentrations < 2.2 mg/L; however, 

elevated uranium activities in groundwater from wells with 

sedimentary aquifer lithology primarily occurred in samples 

with DO concentrations > 4 mg/L.

High

Low

Moderate

024 68 10 12 14 16 18

0

10

20

30

40

50

200

400

600

DISSOLVED OXYGEN CONCENTRATION, IN MILLIGRAMS PER LITER

URANIUM ACTIVITY, IN PICOCURIES PER LITER

Tahoe-Martis study unit

Granitic rock

Sedimentary deposit

Volcanic rock

Aquifer Lithology

EXPLANATION

Central Sierra study unit

Granitic rock

Metamorphic rock

Southern Sierra study unit

Granitic rock

Metamorphic rock

Sedimentary deposit

Relative-concentration

High

Moderate

Low

Figure 36. Relations among uranium activity, dissolved oxygen concentration, and aquifer 

lithology, Tahoe-Martis, Central Sierra, and Southern Sierra study units, California GAMA Priority 

Basin Project.
Status and Understanding of Water Quality 109

Uranium activities in the 29 groundwater samples with 

granitic or metamorphic aquifer lithologies and either anoxic 

conditions or DO concentrations <  2.2 mg/L ranged from 

0.05 to 519 pCi/L (fig. 36), and were significantly higher 

than uranium activities in the 18 groundwater samples with 

DO concentrations ≥  2.2 mg/L (Wilcoxon rank-sum test, 

p =0.024). The 29 low-oxygen samples also came from 

significantly deeper wells (p= 0.023). High uranium activities 

are typically associated with oxic groundwater (Hem, 1989). 

Geochemical modeling was done to evaluate the occurrence of 

high uranium activities in anoxic groundwater.. 

Speciation of dissolved uranium and saturation indices 

of uranium minerals in groundwater were calculated 

using PHREEQC, version 2.17 (Parkhurst and Appelo, 

1999) with updated thermodynamic constants for uranium 

complexes (Jurgens and others, 2010). For each sample, the 

saturation index for uraninite was calculated at a range of 

oxidation-reduction potentials (by adjusting the variable pe, 

the electron activity, in PHREEQC). Uraninite, UO2, occurs 

as an accessory mineral in granitic rocks and pegmatites. 

The pe value for saturation (SI =0) with uraninite ranged 

from approximately –1 to +2. At lower pe, uraninite is 

supersaturated (SI  > 0) and would be expected to precipitate 

from the groundwater, and at higher pe, uraninite is 

undersaturated (SI  <  0) and would be expected to dissolve, 

if it were present. Reduction of iron oxides typically occurs 

at pe values <  0, whereas reduction of manganese oxides, 

nitrate, and oxygen typically occurs at pe values above 0 

(Stumm and Morgan, 1996; Appelo and Postma, 2005). Thus, 

the calculations suggest that groundwater with oxic, suboxic, 

nitrate-reducing, and manganese-reducing oxidation-reduction 

conditions had sufficiently high oxidation potential to dissolve 

uraninite. All of the groundwater samples with granitic or 

metamorphic aquifer lithology and high or moderate relative 

activities of uranium (> 10 pCi/L) were classified as either 

oxic, suboxic, nitrate-reducing, or manganese-reducing. 

Calculated pe for manganese-reducing samples with data for 

iron oxidation-reduction species (table C6) ranged from 2.1 

to 3.7. No samples classified as iron-reducing had high or 

moderate relative activities of uranium. 

Note that approximately one-quarter of the groundwater 

samples with granitic or metamorphic aquifer lithologies 

and sufficiently oxidizing conditions had high or moderate 

RCs of uranium, indicating that sufficiently oxidizing 

conditions are a necessary, but not sufficient, condition for 

producing groundwater with elevated uranium activities 

(fig. 36). The PHREEQC speciation calculations indicate 

that dissolved uranium primarily occurs in complexes with 

calcium and carbonate Ca2UO2(CO3)3

0 and CaUO2(CO3)3–2. 

Uranium was significantly correlated with calcium 

(Spearman’s rho test, p < 0.001, rho=0.59) and bicarbonate 

(p=0.001, rho=0.53). The groundwater samples with granitic 

or metamorphic aquifer lithologies, sufficiently oxidizing 

conditions, and high or moderate RCs of uranium had 

bicarbonate concentrations of 89 to 251 mg/L; however, only 

approximately one-third of the groundwater samples with 

bicarbonate concentration > 89 mg/L had high or moderate 

RCs of uranium (Ferrari and others, 2008). This indicates that 

sufficiently high concentrations of bicarbonate and calcium 

are a necessary, but not sufficient, condition for producing 

groundwater with elevated uranium activities.

The distribution of uranium in groundwater in granitic 

rocks may be quite heterogeneous, even within a single pluton. 

For example, wells CWISH-08 and CWISH-09 are located 

less than half of a mile apart, are screened or open over a 

similar depth interval in the same granitic pluton, have similar 

oxidation-reduction conditions, and have groundwater with 

mixed age distributions. Yet, groundwater from CWISH-09 

has uranium activity of 1 pCi/L, and groundwater from 

CWISH-09 has uranium activity of 44 pCi/L. Twenty-three 

of the USGS-GAMA sites in the Central Sierra study unit 

were located in the Bass Lake Tonalite. In the 20 samples 

with oxidation-reduction conditions favoring uranyl as the 

dominant uranium species, uranium activities ranged from < 1 

to 519 pCi/L. Similarly large ranges of uranium concentrations 

in groundwater from a relatively small geographic area within 

a granitic pluton have been observed elsewhere (Asikainen 

and Kahlos, 1979; Gascoyne, 1989).

Mineralogical observations of granitic rocks of the 

Sierra Nevada suggest that weathering of uraniferous titanite 

may be an important source of uranium to groundwater 

(Wollenberg and Smith, 1968; Thomas and others, 1993). 

Titanite commonly is present as an accessory mineral in 

Sierra Nevada granitic rocks (Ague and Brimhall, 1988). 

Uranium also occurs in apatite, another common accessory 

mineral. Weathering of biotite may also be a source, based on 

observations of uranium concentrated in the rims of biotite 

grains and in zircon inclusions in biotite grains in Sierra 

Nevada granitic rocks with abundant biotite (Wollenberg and 

Smith, 1968). In the several hundred samples of Bass Lake 

Tonalite examined by Bateman (1992), the modal abundances 

of biotite and hornblende varied from 5 to 25% and from 

0 to 20%, respectively. These heterogeneities in relative 

abundances of the major silicate minerals suggest that the 

uranium-bearing accessory minerals, such as titanite and 

zircon, may also be heterogeneously distributed within the 

pluton. Uranium mobilized from primary minerals also may 

be redeposited; high concentrations of uranium adsorbed to 

iron oxyhydroxide coatings on fractures have been observed in 

granitic rocks (Gascoyne, 1989). Small-scale heterogeneities 

in groundwater flow patterns in fractured rock systems may 

therefore result in a wide range of groundwater uranium 

concentrations, even within a small geographic area.
110 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

TAHOE-MARTIS STUDY UNIT SOUTHERN SIERRA

STUDY UNIT

Tahoe-Martis study unit north: Martis, Tahoe North, and Tahoe South Valleys;

Tahoe-Martis study unit south: Tahoe South Valley (divided into two groups

 by depth to top of screened interval in well);

Southern Sierra study unit north: Kern River Valley;

Southern Sierra study unit south: Tehachapi-Cummings Valleys

Tahoe-Martis study unit south >135 ft and Southern Sierra north

significantly different than the three other categories (p<0.001 to p=0.013) 

High

Moderate

Low

north south

<135 ft depth

south

>135 ft depth

north south 0

10

20

30

40

URANIUM ACTIVITY, IN PICOCURIES PER LITER

7 8 8 5 16 EXPLANATION

90th percentile

10th percentile

median

(50th percentile)

lower detached

25th percentile

75th percentile

upper detached

7 number of wells

Relative-concentration

High

Moderate

Low

Figure 37. Relation between uranium activity and location for groundwater samples from 

wells with sedimentary aquifer lithology, Tahoe-Martis and Southern Sierra study units, 

California GAMA Priority Basin Project.

Uranium activity in groundwater samples from wells with 

sedimentary aquifer lithology ranged from below detection to 

39 pCi/L (figs. 36, 37). High and moderate RCs of uranium 

were detected in groundwater from the Tahoe Valley South 

subbasin and the Kern River Valley (figs. 34, 37). Within 

the Tahoe Valley South subbasin, uranium activity was 

significantly higher in wells with depth to top of screened or 

open interval > 135 ft bls. Sources of sediment to the Tahoe 

Valley South subbasin likely had some variation over time 

because variation in the extent of glaciation may have drawn 

sediment from different areas. Elevated activity of uranium in 

sediment of the Kern River Valley may reflect that the source 

of sediment is the surrounding granitic rocks, which include 

rocks with elevated uranium.
Status and Understanding of Water Quality 111

The mechanisms for mobilizing uranium from sediment 

in the Tahoe Valley South subbasin appear to be different than 

the mechanism inferred for mobilization of uranium from 

sediment of the Eastern San Joaquin Valley, which primarily 

is composed of material derived from Sierra Nevada granitic 

rocks. In the Eastern San Joaquin Valley, Jurgens and others 

(2010) reported higher uranium concentration groundwater 

with modern age distribution from the shallower parts of the 

aquifer system, and reported a strong positive correlation 

between uranium and bicarbonate. Jurgens and others (2010) 

concluded that the uranium is mobilized from aquifer sediment 

by downward moving irrigation return water that has high 

concentrations of bicarbonate. In contrast, in the Tahoe Valley 

South subbasin, uranium activity was positively correlated 

with depth to top of screened or open interval (p=0.005, 

rho=0.67), high and moderate RCs of uranium occurred in 

pre-modern and mixed-age groundwater, and uranium was not 

correlated with bicarbonate (p =  0.53). Further investigation 

with geochemical modeling would be needed to understand 

the mechanism for solubilization of uranium in this high-pH, 

low-bicarbonate groundwater.

Major Ions and Trace Elements with 

SMCL Benchmarks

Constituents with SMCL benchmarks were present at 

high RCs in 14, 34, and 24% of the primary aquifers in the 

Tahoe-Martis, Central Sierra, and Southern Sierra study units, 

respectively (table 9B). The constituent most commonly 

responsible for the high RCs was manganese, which was 

present at high RCs in 13, 32, 23% of the three study units, 

respectively (tables 8A,B,C; fig. 17C). High aquifer-scale 

proportions for iron were 6.9, 11, and 13% for the three study 

units, respectively. Manganese, iron, and zinc also were 

detected at moderate RCs in the Central Sierra and Southern 

Sierra study units, but not in the Tahoe-Martis study unit.

The high aquifer-scale proportion for TDS in the 

Tahoe-Martis study unit was 11% (table 8A). No moderate 

or high RCs of TDS or specific conductance were reported in 

the CDPH database for the Tahoe-Martis study unit (fig. 38A). 

However, two of the three wells with high RCs of TDS in 

the grid well dataset were CDPH wells, but had no TDS or 

specific conductance data reported in the CDPH database. Of 

the three wells with high RCs for TDS, one had moderate RC 

for chloride, and one had high RC for sulfate (fig. 17C).

The high aquifer-scale proportion for TDS in the Central 

Sierra study unit was 1.4% (spatially weighted; table 8B). 

The CDPH database reported high and moderate RCs of TDS 

or specific conductance in several wells in the vicinity of 

Oakhurst and Ahwanee (figs. 2B, 38B). The high and moderate 

RCs of TDS or specific conductance were all associated with 

high or moderate RCs of chloride. No high or moderate RCs 

of sulfate were reported in the CDPH database.

The high aquifer-scale proportion for TDS in the 

Southern Sierra study unit was 3.3% (table 8C), and 

groundwater samples with moderate and high RCs of TDS 

or specific conductance were collected from the northern and 

southern portions of the study unit (fig. 38C). A subset of 

the samples with moderate or high RCs of TDS or specific 

conductance also had moderate RCs of sulfate (fig. 17C). No 

high or moderate RCs of chloride were reported.

Factors Affecting Total Dissolved Solids

Higher TDS concentration was significantly associated 

with location in the Southern Sierra study unit, and lower TDS 

concentration was significantly associated with location in the 

Tahoe-Martis study unit (table 10A). TDS concentration had 

significant positive correlations with agricultural land use, and 

septic tank density, and significant negative correlations with 

urban land use, aridity index, elevation, and DO (table 10B). 

Many of these correlations reflect correlations between 

the explanatory factors and study unit location (table 6A), 

and therefore may not indicate causative relations. In the 

understanding assessment dataset, all of the groundwater 

samples with high RCs of TDS had anoxic or mixed oxic/

anoxic oxidation-reduction conditions (table C5, SOSA-35, 

TTAHO-11, TROCK-12, -13). Anoxic groundwater may 

be more chemically aggressive because redox reactions in 

which oxygen is consumed produce hydrogen ions that are 

then available to participate in hydrolysis or dissolution of 

minerals.
112 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac09-0340_fig 38a

NEVADA CO

PLACER CO

EL DORADO CO

ALPINE

CO

Mesozoic granitic rocks

Cenozoic sedimentary deposits

Low (< 500)

High (> 1,000)

Grid wells

Low (< 500)

Understanding wells

Low (< 500)

CDPH wells

Cenozoic volcanic and volcaniclastic rocks

Paleozoic and Mesozoic metamorphic rocks

and non-granitic plutonic rocks

Geologic units

Martis and Tahoe study area boundary

Total dissolved solids concentration,

 in milligrams per liter

EXPLANATION

Hard Rock study area boundary

39° 15’

120° 15’ 120°

39°

0 5 10 MILES

0 5 10 KILOMETERS

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006. 

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

Geologic units simplified from Saucedo and 

others (2000) and Crafford (2007).

A

< is less than;

> is greater than

LAKE TAHOE

CALIFORNIA

NEVADA

Figure 38. Relative-concentrations of total dissolved solids for USGS- and CDPH-grid wells, USGS-understanding 

wells, and all wells in the CDPH database with data for total dissolved solids during the 3-year intervals used in the 

status assessment for the (A) Tahoe-Martis, (B) Central Sierra, and (C) Southern Sierra study units, California GAMA 

Priority Basin Project.
Status and Understanding of Water Quality 113

sac09-0340_fig 38b

FRESNO CO

MARIPOSA CO

MADERA CO

EXPLANATION

Mesozoic granitic rocks

Cenozoic sedimentary deposits

Cenozoic volcanic and volcaniclastic rocks

Paleozoic and Mesozoic metamorphic rocks

and non-granitic plutonic rocks

Geologic units

Study unit boundary 

Low (< 500)

Grid wells

Low (< 500)

Understanding wells

Low (< 500)

Moderate (500–1,000)

High (> 1,000)

CDPH wells

Total dissolved solids concentration, in milligrams per liter

< is less than;

> is greater than

0 5 10 MILES

0 5 10 KILOMETERS

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006,

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

37° 30’

37° 15’

119° 45’ 119° 30’

B

Geologic units simplified from

Saucedo and others (2000).

Figure 38.—Continued
114 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac09-0340_fig 38c

EXPLANATION

Mesozoic granitic rocks

Cenozoic sedimentary deposits

Cenozoic volcanic and volcaniclastic rocks

Paleozoic and Mesozoic metamorphic rocks

 and non-granitic plutonic rocks

Geologic units

Study unit boundary

Low (< 500)

Moderate (500–1,000)

High (> 1,000)

Grid wells

Low (< 500)

Moderate (500–1,000)

Understanding wells

Low (< 500)

Moderate (500–1,000)

High (> 1,000)

CDPH wells

Total dissolved solids concentration,

 in milligrams per liter

< is less than;

> is greater than

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006,

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

Geologic units simplified from 

Saucedo and others (2000).

C

35°15'

118°30’ 118°15’

36°

35°45’

35°30’

10 MILES

50 10 KILOMETERS

0 5

TULARE CO

KERN CO

Figure 38.—Continued
Status and Understanding of Water Quality 115

In the Central Sierra study unit, high and moderate RCs 

of TDS were associated with groundwater anion compositions 

dominated by chloride. All of the samples with moderate or 

high RCs (> 500 mg/L) had anion compositions with > 60% 

chloride and <15% sulfate; however, most groundwater 

with low RCs of TDS had anion compositions with > 75% 

bicarbonate (fig. 39). The Central Sierra study unit wells 

with high and moderate RCs of TDS were located in granitic 

rocks in the vicinities of Ahwanee and Oakhurst (fig. 2B, 

38B). This area is near the northern end of a 60-mile long, 

northwest-trending band of granitic rocks in which some 

springs and wells yield groundwater with high chloride 

concentrations compared to most of the groundwater in 

the region (Mack and Ferrell, 1979; Mack and Schmidt, 

1981). All of the data for samples with moderate or high 

RCs of TDS were from the CDPH database, thus, no 

groundwater age distribution data are available to evaluate 

the potential hypothesis that the chloride-rich groundwater 

is a mixture of meteoric water and connate water derived 

from metamorphosed Paleozoic and early Mesozoic marine 

sedimentary rocks beneath the Sierra Nevada batholith (Mack 

and Ferrell, 1979; Mack and Schmidt, 1981).

In the Southern Sierra study unit, TDS concentration 

was significantly higher in wells with sedimentary aquifer 

lithology than in wells with granitic or metamorphic aquifer 

lithology (Wilcoxon, p = 0.001). Wells with granitic or 

metamorphic aquifer lithology typically were located in the 

hills surrounding the alluvial basins or were relatively deep 

wells at the margin of the basin where sediment thickness was 

small. Groundwater from these wells may be representative 

of water recharging the basins by mountain-front or 

mountain-block recharge (fig. 3). 

For the wells with sedimentary aquifer lithology in 

the Southern Sierra study unit, TDS concentration was 

significantly higher in wells with shallower depths to the 

top of the screened or open interval (Spearman’s rho test, 

p=  0.039, rho = –0.44), but not significantly associated 

with groundwater age (p =0.23). TDS concentration had 

a significant positive correlation with percent sulfate in 

the anions (p  <  0.001, rho=0.57) (fig. 40). The major ion 

composition of current State Water Project water (T. Kraus, 

U.S. Geological Survey, California Water Science Center, 

written commun., 2010) is not similar to groundwater sampled 

by CDPH and USGS-GAMA wells (fig. 40), suggesting that 

artificial recharge in the Tehachapi-Cummings Valley has 

not yet infiltrated to the depths tapped by the wells. This is in 

accord with the prevalence of groundwater with pre-modern 

age distributions (SWP water has modern age distributions).

Factors Affecting Manganese and Iron

Concentrations of iron and manganese in groundwater are 

strongly influenced by oxidation-reduction conditions in the 

aquifer, and as such, both had significant negative correlations 

with DO concentration (table 10B), and a significant positive 

correlation with each other (Spearman’s rho test, p  <  0.001). 

Groundwater with low DO was present in all three study 

units and in wells in all four aquifer lithologies, although 

the majority of the samples with low DO were from wells 

with granitic aquifer lithology (fig. 15A). Accordingly, high 

RCs of manganese were present in all three study units and 

most commonly present in the Central Sierra study unit 

(figs. 41A,B,C).

The dominant form of iron in solution can be predicted 

from the ratio of iron redox species, Fe+3/Fe+2, where Fe+3 is 

the amount of iron present in the more oxidized +3 oxidation 

state (ferric iron) and Fe+2 is the amount of arsenic present 

in the more reduced +2 oxidation state (ferrous iron). Of the 

52 samples with detection of iron and data for iron species, 

the 10 with moderate or high RCs of iron all had Fe+3/Fe+2

<  0.08, and the 42 with low RCs of iron all had Fe+3/Fe+2

>  0.25 (table C5). The greater relative abundance of Fe+2 

(lower Fe+3/Fe+2) in samples with moderate or high RCs of 

iron confirms that elevated iron concentration was indeed 

associated with reduced conditions. Two of the samples with 

high or moderate RCs of iron also had DO concentration 

> 1 mg/L, suggesting that they represented mixtures of oxic 

and iron-reducing anoxic groundwaters (table C5, TTAHO-12 

and CWISH-04). 

Of the groundwater samples classified as iron-reducing 

(iron > 100 μg/L), three-quarters were also classified as 

manganese-reducing (manganese > 50 μg/L); however, of the 

groundwater samples classified as manganese-reducing, only 

one-half were also classified as iron-reducing. The difference 

reflects the sequence of terminal electron-accepting processes 

(TEAP) in which manganese-reduction is energetically more 

favorable than iron-reduction (McMahon and Chappelle, 

2008), and may also reflect relative abundances of iron and 

manganese oxides available for reduction in specific aquifer 

types.

Low DO concentration was significantly associated with 

granitic and metamorphic aquifer lithologies, groundwater 

with mixed or pre-modern age distributions (table 6A), 

and higher manganese concentration (table 10B). Higher 

manganese concentration was significantly associated with 

granitic aquifer lithology, but manganese concentration was 

not significantly associated with groundwater age distribution 

(table 10A). These relations may suggest both residence time 

and aquifer lithology may be important causative factors for 

elevated manganese concentrations in groundwater.
116 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac09-0340_fig 39

SULFATE PLUS CHLORIDE

CALCIUM PLUS MAGNESIUM

80 20

100 0

60 40

MAGNESIUM 40 60 SODIUM PLUS POTASSIUM

80

0 100

20

0

100 0

100 80 60 40 20 0

CALCIUM

80

60

40

20

80

60

40

20

0

80 80

60 60

40 40

20 20

0

0

0

100 100

100 100

100

80 20

CHLORIDE

20 40 60 80

80

0 100

20

40 60

60 40SULFATE

BICARBONATE

PERCENT

PERCENT

PERCENT

EXPLANATION

Total dissolved solids concentration

High (>1,000 mg/L)

Moderate (500–1,000 mg/L)

Low (250–500 mg/L)

Lower (<250 mg/L)

Figure 39. Major-ion composition and total dissolved solids concentration for groundwater samples analyzed by USGS-GAMA 

or with data in the 3-year interval used in the status assessment in the CDPH database, Central Sierra study unit, California 

GAMA Priority Basin Project.
Status and Understanding of Water Quality 117

sac09-0340_fig 40

SULFATE PLUS CHLORIDE

CALCIUM PLUS MAGNESIUM

80 20

100 0

60 40 MAGNESIUM

40 60

SODIUM PLUS POTASSIUM

80

0 100

20

100 80 60 40 20 0

CALCIUM

80

60

40

20

80

100100

60

40

20

80

60

40

20

0

0

0

0

80

100 100

60

40

20

100

20

100 0

80

CHLORIDE

0 20 40 60 80

80

1000

20

40 60

60 40

SULFATE

BICARBONATE

PERCENT

PERCENT

PERCENT

EXPLANATION

Total dissolved solids concentration

Kern River Valley

Moderate (500–1,000 mg/L)

Low (<500 mg/L)

Tehachapi-Cummings Valley

Moderate (500–1,000 mg/L)

Low (<500 mg/L)

State Water Project

Low (<500 mg/L)

Figure 40. Major-ion composition and total dissolved solids concentration for samples from the State Water Project California 

Aqueduct and from groundwater samples analyzed by USGS-GAMA or data in the 3-year interval used in the status assessment in 

the CDPH database, Southern Sierra study unit, California GAMA Priority Basin Project.
118 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac09-0340_fig 41a

NEVADA CO

PLACER CO

EL DORADO CO

ALPINE

CO

Mesozoic granitic rocks

Cenozoic sedimentary deposits

Low (< 25)

High (> 50)

Grid wells

Low (< 25)

Understanding wells

Low (< 25)

Moderate (25–50)

High (> 50)

CDPH wells

Cenozoic volcanic and volcaniclastic rocks

Paleozoic and Mesozoic metamorphic rocks

and non-granitic plutonic rocks

Geologic units

Martis and Tahoe study area boundary

Manganese concentration, in micrograms per liter

EXPLANATION

Hard Rock study area boundary

39° 15’

120° 15’ 120°

39°

0 5 10 MILES

0 5 10 KILOMETERS

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006. 

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

Geologic units simplified from Saucedo and 

others (2000) and Crafford (2007).

A

< is less than;

> is greater than

LAKE TAHOE

CALIFORNIA

NEVADA

Figure 41. Relative-concentrations of manganese for USGS- and CDPH-grid wells, USGS-understanding wells, and all 

wells in the CDPH database with data for manganese during the 3-year intervals used in the status assessment for the 

(A) Tahoe-Martis, (B) Central Sierra, and (C) Southern Sierra study units, California GAMA Priority Basin Project.
Status and Understanding of Water Quality 119

sac09-0340_fig 41b

FRESNO CO

MARIPOSA CO

MADERA CO

EXPLANATION

Mesozoic granitic rocks

Cenozoic sedimentary deposits

Cenozoic volcanic and volcaniclastic rocks

Paleozoic and Mesozoic metamorphic rocks

and non-granitic plutonic rocks

Geologic units

Study unit boundary 

Low (< 25)

Moderate (25–50)

High (> 50)

Grid wells

Low (< 25)

High (> 50)

Understanding wells

Low (< 25)

Moderate (25–50)

High (> 50)

CDPH wells

Manganese concentration, in micrograms per liter

< is less than;

> is greater than

0 5 10 MILES

0 5 10 KILOMETERS

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006,

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

37° 30’

37° 15’

119° 45’ 119° 30’

B

Geologic units simplified from

Saucedo and others (2000).

Figure 41.—Continued
120 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac09-0340_fig 41c

EXPLANATION

Mesozoic granitic rocks

Cenozoic sedimentary deposits

Cenozoic volcanic and volcaniclastic rocks

Paleozoic and Mesozoic metamorphic rocks

 and non-granitic plutonic rocks

Geologic units

Study unit boundary

Low (< 25)

Moderate (25–50)

High (> 50)

Grid wells

Low (< 25)

High (> 50)

Understanding wells

Low (< 25)

Moderate (25–50)

High (> 50)

CDPH wells

Manganese concentration, in micrograms per liter 

< is less than;

> is greater than

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006,

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

Geologic units simplified from 

Saucedo and others (2000).

C

35°15'

118°30’ 118°15’

36°

35°45’

35°30’

10 MILES

50 10 KILOMETERS

0 5

TULARE CO

KERN CO

Figure 41.—Continued
Status and Understanding of Water Quality 121

Among the samples from wells with granitic aquifer 

lithology, 60% of samples with pre-modern age had a high 

RC of manganese, compared to 26% of samples with mixed 

age, and 10% with modern age (fig. 42). Given sufficiently 

long residence times, the oxidation state of groundwater in 

granitic aquifers typically is controlled by the Fe+3 – Fe+2

redox couple because minerals containing ferrous iron are 

prevalent in granitic rocks (Gascoyne, 1997; MacQuarrie and 

Mayer, 2005; Sidborn and Neretnicks, 2007). Biotite typically 

is most abundant; chlorite, magnetite, pyrite, hornblende, 

and other ferrous-iron bearing silicates and oxides may also 

be important. Non-oxidative dissolution of biotite, followed 

by essentially instantaneous oxidation of the released ferrous 

iron to ferric iron oxyhydroxides, consumes DO. Oxidation of 

the iron may be coupled with nitrate reduction, followed by 

manganese reduction once the DO has been consumed. 

Reducing conditions in aquifers with sedimentary 

lithology typically are produced by consumption of DO 

by oxidation of sedimentary organic matter (Appelo and 

Postma, 2005). In the Tahoe-Martis and Southern Sierra 

study units, approximately 10% of samples with mixed or 

modern groundwater ages from wells with sedimentary aquifer 

lithologies had a high RC of manganese, and none of the 

samples with pre-modern groundwater ages had a high RC 

of manganese (fig. 42). Absence of reducing conditions, even 

with long residence times, suggests sediment in the Tahoe 

Valley and Tehachapi-Cummings Valley basins has relatively 

low organic matter content.

Figure 42. Relations among manganese relative-concentration, aquifer lithology, and groundwater age, Tahoe-Martis, 

Central Sierra, and Southern Sierra study units, California GAMA Priority Basin Project.

0

10

20

30

40

50

60

70

PERCENTAGE OF SAMPLES WITH HIGH 

RELATIVE-CONCENTRATION OF MANGANESE

AQUIFER LITHOLOGY

Granitic rocks Metamorphic

rocks

Sedimentary

deposits

Volcanic rocks

10

22

10

11

13

14

6 6

9

17

EXPLANATION

Modern

Mixed

Pre-modern

Granitic, sedimentary, and volcanic aquifer lithologies

All ages

Number indicates the number of wells in each category

Metamorphic aquifer lithology
122 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Organic and Special-Interest Constituents

The organic and special-interest constituents are 

organized by constituent class: volatile organic compounds 

(VOCs), pesticides, and special interest. VOCs are present in 

paints, solvents, fuels, fuel additives, refrigerants, fumigants, 

and disinfected water, and are characterized by their tendency 

to evaporate. VOCs typically persist longer in groundwater 

than in surface water because groundwater is isolated from 

the atmosphere. Pesticides include herbicides, insecticides, 

and fungicides, and are used to control unwanted vegetation 

(weeds), insects, fungi, and other pests in agricultural, 

urban, and suburban settings. The special-interest group 

includes three chemically unrelated constituents (perchlorate, 

N-nitrosodimethylamine, and 1,2,3-trichloropropane) that 

are of interest in California because they have recently been 

found in groundwater because of recent advances in analytical 

methods.

USGS-GAMA included analysis of a large number of 

organic constituents, many of which are not subject to any 

regulation in drinking water, and used analytical methods with 

lower detection limits than required for regulatory sampling. 

In the Tahoe-Martis, Central Sierra, and Southern Sierra 

study units, however, the vast majority of organic constituents 

detected were ones already subject to regulation in drinking 

water. Of the 88 VOCs analyzed, 15 were detected at least 

once, and of these, 12 have MCL-US or MCL-CA benchmarks 

(table 7B). Of the 63 pesticides and pesticide degradates 

analyzed, 7 were detected at least once, and of these, 2 were 

parent compounds with MCL-US or MCL-CA benchmarks. 

In all, of the 72 organic constituents with no health-based 

benchmarks analyzed, 5 were detected in groundwater 

(table 7B, table 4).

Figures 43A,B,C summarize the study-unit detection 

frequencies and maximum RCs of organic constituents and 

constituents of special interest detected in the grid wells for 

the three study units. In the Tahoe-Martis study unit grid well 

dataset, two organic constituents met criteria for additional 

evaluation: perchloroethene (PCE), because it had a maximum 

RC > 1, and chloroform, because it had a study-unit detection 

frequency > 10% (figs. 43A, 44). Six other organic constituents 

were detected in grid wells at RCs  <  0.1 and study-unit 

detection frequencies <  10%: methyl tert-butyl ether (MTBE), 

atrazine, bromodichloromethane, trichloroethene (TCE), 

1,1,1-trichloroethane, and carbon disulfide (fig. 43A). Five 

additional organic constituents were selected for additional 

evaluation in the status assessment because they were 

reported at RCs  >  0.1 in the CDPH database during the 3-year 

interval used for the status assessment: carbon tetrachloride, 

1,2-dichloroethane, chloromethane, 1,2-dichloropropane, and 

DBCP (table 3). 

In the Central Sierra study unit grid well dataset, 

three organic constituents met criteria for additional 

evaluation: benzene, because it had a maximum RC  > 0.1, 

and chloroform and simazine, because they had study-unit 

detection frequencies > 10% (figs. 43B, 44). Four other organic 

constituents were detected in grid wells at RCs  <  0.1 and 

study-unit detection frequencies < 10%: MTBE, atrazine, 

PCE, and carbon disulfide (fig. 43B; table 3). MTBE and PCE, 

and one additional constituent, 1,4-dichlorobenzene, were 

selected for additional evaluation because they were reported 

at RCs > 0.1 in the CDPH database during the 3-year interval 

used for the status assessment (table 3).

In the Southern Sierra study unit grid well dataset, 

six organic and special-interest constituents met criteria 

for additional evaluation in the status assessment: carbon 

tetrachloride and perchlorate, because they had maximum 

RCs > 0.1, and chloroform, PCE, simazine, and atrazine, 

because they had study-unit detection frequencies > 10% 

(figs. 43C, 44). Five other organic constituents were detected 

in grid wells at RCs <  0.1 and study-unit detection frequencies 

< 10%: MTBE, TCE, trichlorofluoromethane (CFC-11), 

1,1,2-trichloro-1,2,2-trifluoroethane (CFC-113), and prometon 

(fig. 38C). Three other organic constituents were detected only 

in understanding wells at RCs <  0.1: 1,2-dichlorobenzene, 

cis-1,2-dichloroethene, and 1,2-dichloropropane (tables 3, 4).

Results of the status assessment for individual 

constituents are listed in tables 8A,B,C and for constituent 

classes in table 9C. Note that the organic constituent class 

does not include the constituents of special interest. Organic 

constituents with health-based benchmarks were not present 

at high RCs in the primary aquifers of the Central Sierra and 

Southern Sierra study units, and were present in 1.0% of 

the Tahoe-Martis study unit primary aquifers. Aquifer-scale 

proportions for moderate RCs of organic constituents were 

< 5% in all three study units. Organic constituents were 

detected at low RCs in 19, 23, and 37% of the primary 

aquifers in the Tahoe-Martis, Central Sierra, and Southern 

Sierra study units, respectively (table 9C).
Status and Understanding of Water Quality 123

sac09-0340_fig 43a

0 5 10 15 20

0.01

0.001

0.0001

0.00001

0.1

1

10

1,1,1-Trichloroethane

Bromodichloromethane

Carbon disulfide

Chloroform

Methyl tert-butyl ether

Perchloroethene

Trichloroethene

Atrazine

AREA-WEIGHTED DETECTION FREQUENCY AT ANY CONCENTRATION, IN PERCENT

MAXIMUM RELATIVE-CONCENTRATION, DIMENSIONLESS

Relative-concentration

Low Moderate High

EXPLANATION

Atrazine Name and center of symbol is location of data

Carbon tetrachloride, 1,2-dichloroethane, 1,2-dichloropropane, and 1,2-dibromo-3-chloropropane each were reported at 

relative-concentration greater than 1 in one well in the Tahoe-Martis study unit in the California Department of Public Health 

database for May 1, 2004, to April 30, 2007

High

Moderate

Low

A Tahoe-Martis study unit

Figure 43. Area-weighted detection frequency and maximum relative-concentration of organic and 

special-interest constituents detected in USGS-grid wells in the (A) Tahoe-Martis, (B) Central Sierra, and 

(C) Southern Sierra study units, California GAMA Priority Basin Project.
124 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac09-0340_fig 43b

15 20

AREA-WEIGHTED DETECTION FREQUENCY AT ANY CONCENTRATION, IN PERCENT

MAXIMUM RELATIVE-CONCENTRATION, DIMENSIONLESS

Atrazine

Benzene

Carbon disulfide

Chloroform

Methyl tert-butyl ether

Perchloroethene

Atrazine

Simazine

B

0.01

0.001

0.0001

0.00001

0.1

1

10

0 5 10

EXPLANATION

Name and center of symbol is location of data

Central Sierra study unit

High

Moderate

Low

Relative-concentration

Low Moderate High

Figure 43.—Continued
Status and Understanding of Water Quality 125

sac09-0340_fig 43c

0 5 10 15 20

AREA-WEIGHTED DETECTION FREQUENCY AT ANY CONCENTRATION, PERCENT

MAXIMUM RELATIVE-CONCENTRATION, DIMENSIONLESS

Carbon tetrachloride

Trichlorofluoromethane

1,1,2-Trichloro1,2,2-trifluoroethane

Chloroform

Methyl tert-butyl ether

Perchloroethene

Trichloroethene

Atrazine

Prometon

Simazine

Perchlorate

C 

0.01

0.001

0.0001

0.00001

0.1

1

10

Southern Sierra study unit

Relative-concentration

Low Moderate High

Atrazine

EXPLANATION

Name and center of symbol is location of data

Figure 43.—Continued
126 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac09-0340_fig 44

10% detection line

Volatile organic compounds

Pesticides

Special interest

Chloroform

Perchloroethene

Benzene

Carbon

tetrachloride

Simazine

Atrazine

Perchlorate

RELATIVE-CONCENTRATION,

DIMENSIONLESS

AREA-WEIGHTED

DETECTION FREQUENCY,

PERCENT

0 20 40 60 80 0.0001100 0.001 0.01 1010.1

EXPLANATION

Hard Rock (13 grid wells)

Martis (14 grid wells)

Tahoe (14 grid wells)

Tahoe-Martis study unit

Coarse Gold (27 grid wells)

Wishon (9 grid wells)

Central Sierra study unit

Southern Sierra (35 grid wells)

Southern Sierra study unit

Relative-concentration

High

Moderate

Low

Figure 44. Area-weighted detection frequency and graph of relative-concentrations of organic and special-interest 

constituents detected in USGS-grid wells, Tahoe-Martis, Central Sierra, and Southern Sierra study units, California 

GAMA Priority Basin Project.
Status and Understanding of Water Quality 127

Herbicides

Herbicides were not detected at high or moderate 

concentrations in any of the three study units (tables 8A,B,C; 

figs. 45A,B,C). The maximum RC detected was 0.012 

(atrazine in the Tahoe-Martis study unit; fig. 44). No 

detections of herbicides analyzed by USGS-GAMA were 

reported in the CDPH databases for the three study units; 

however, the detection limits for pesticide compounds in the 

CDPH database are greater than an RC of 0.1 (fig. 7). No 

herbicides had a study-unit detection frequency >10% in the 

Tahoe-Martis study unit. The study-unit detection frequency 

for simazine was 15% in the Central Sierra study unit, and 

the detection frequencies for simazine and atrazine in the 

Southern Sierra study unit were 11 and 15%, respectively 

(fig. 44). Ninety-two percent of the samples containing 

atrazine also contained low concentrations of deethylatrazine, 

a degradation product of atrazine that does not have a 

benchmark. Co-occurrence of atrazine and deethylatrazine 

may reflect the relatively high degree of persistence of 

atrazine in groundwater environments (Kolpin and others, 

1998). Atrazine, deethylatrazine, and simazine were the most 

frequently detected pesticide compounds in groundwater in 

major aquifers across the USA (Gilliom and others, 2006). 

Simazine is most commonly used on orchards and vineyards 

and on rights-of-way for weed control; atrazine is most 

commonly used on forage grasses, corn, and managed forests, 

also for weed control (Kegley and others, 2010).

Factors Affecting Herbicides

Rather than consider atrazine and simazine separately for 

the understanding assessment, concentrations of all herbicides 

with benchmarks (simazine, atrazine, and prometon) were 

summed and treated as a constituent class. The presence 

of herbicides was significantly related to groundwater age 

(table 10A). Herbicide detection frequency in samples 

classified as modern or mixed age was 18%, compared to 

0% in samples classified as pre-modern age (fig. 46). Despite 

the significant association between modern and mixed-age 

groundwaters and shallower well depths and depths to top 

of screened or open interval (table 6A), herbicides were 

not significantly related to classified well depth (table 10A) 

or significantly correlated to well depth or depth to top of 

screened or open interval (table 10B). The absence of the 

expected significant correlation between herbicides and any 

measures of well depth may be because of insufficient data 

for statistical mass: only 12 of the 17 samples with herbicide 

detections had sufficient well construction information to 

classify well depth.

Herbicides had a significant positive correlation with 

agricultural land use, and a significant negative correlation 

with undeveloped land use (table 10B; figs. 47, 48). The 

positive relation with agricultural land use reflects herbicide 

occurrence patterns in the Southern Sierra study unit, which 

is the only study unit with any agricultural land use (fig. 6A). 

Eleven of the 17 samples with herbicide detections are from 

the Southern Sierra study unit (figs. 45A,B,C), and of those 11, 

10 have > 5% agricultural land use within the 500-m buffer 

around the well. When land use and classified groundwater 

age are used together to stratify the dataset, the interrelations 

among herbicides, groundwater age, and land use are apparent. 

The detection frequency of herbicides in samples classified as 

modern or mixed age from wells with > 5% agricultural land 

use is 66%, which is significantly greater than the detection 

frequency in pre-modern-age groundwater samples or in 

modern/mixed-age samples from wells with < 5% agricultural 

land use or in the dataset as a whole (contingency table tests; 

p <  0.001; fig. 49). 

Herbicides were not significantly correlated to either 

undeveloped or urban land use when only samples with 

< 5% agricultural land use were considered (Spearman’s rho 

test, p= 0.19 and p=  0.16, respectively). Among samples 

with < 5% agricultural land use, the detection frequency of 

herbicides was greater in modern/mixed-age groundwater 

(9%) than in pre-modern-age groundwater (0%), although the 

difference was not statistically significant (contingency table 

test; p  =  0.057; fig. 49). Simazine and atrazine both have nonagricultural applications, thus, the presence of herbicides in 

young water from non-agricultural settings is not unexpected.

The significant negative correlations between herbicides 

and elevation and aridity index, and significant positive 

correlation with septic tank density (table 10B) reflect that 

herbicides were more prevalent in the Central Sierra and 

Southern Sierra study units than in the Tahoe-Martis study 

unit (table 6A). Neither elevation nor septic tank density were 

significantly correlated to herbicides when samples with > 5% 

agricultural land use were removed.
128 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac09-0340_fig 45a

NEVADA CO

PLACER CO

EL DORADO CO

ALPINE

CO

Not detected

Low (< 0.1 atrazine)

Grid wells

Not detected

Understanding wells

Not detected

CDPH wells

Martis and Tahoe study area boundary

Herbicide concentration, in micrograms per liter

EXPLANATION

Hard Rock study area boundary

39° 15’

120° 15’ 120°

39°

0 5 10 MILES

0 5 10 KILOMETERS

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006. 

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

Land use simplified from

Nakagaki and others (2007).

A

< is less than

LAKE TAHOE

CALIFORNIA

NEVADA

Urban

Undeveloped forest

Undeveloped rock, shrub, and grass

Agricultural

Land use

Figure 45. Relative-concentrations of herbicides for USGS- and CDPH-grid wells, USGS-understanding wells, and all 

wells in the CDPH database with data for herbicides during the 3-year intervals used in the status assessment for the 

(A) Tahoe-Martis, (B) Central Sierra, and (C) Southern Sierra study units, California GAMA Priority Basin Project.
Status and Understanding of Water Quality 129

sac09-0340_fig 45b

FRESNO CO

MARIPOSA CO

MADERA CO

EXPLANATION

Study unit boundary 

Not detected

 Low (< 0.4 simazine or

 < 0.1 atrazine)

Grid wells

Not detected

Understanding wells

Not detected

CDPH wells

Herbicide concentration, in micrograms per liter

< is less than

0 5 10 MILES

0 5 10 KILOMETERS

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006,

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

37° 30’

37° 15’

119° 45’ 119° 30’

B

Land use simplified from

Nakagaki and others (2007).

Urban

Undeveloped forest

Undeveloped rock, shrub, and grass

Agricultural

Land use

Figure 45.—Continued
130 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac09-0340_fig 45c

EXPLANATION

Study unit boundary

Not detected

 Low (< 0.4 simazine or

 0.1 atrazine)

Grid wells

Not detected

 Low (< 0.4 simazine or

 < 0.1 atrazine)

Understanding wells

Not detected

CDPH wells

Herbicide concentration, in micrograms per liter 

< is less than

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006,

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

Land use simplified from

Nakagaki and others (2007).

C

35°15'

118°30’ 118°15’

36°

35°45’

35°30’

10 MILES

50 10 KILOMETERS

0 5

TULARE CO

KERN CO

Urban

Undeveloped forest

Undeveloped rock, shrub, and grass

Agricultural

Land use

Figure 45.—Continued
Status and Understanding of Water Quality 131

sac09-0340_fig 46

10

15

0

5

20

25

30

DETECTION FREQUENCY, IN PERCENT

CONSTITUENT CLASS

Herbicides Solvents Trihalomethanes

EXPLANATION

Modern or Mixed (n=93)

Pre-modern (n=39)

Groundwater age classification

Modern/Mixed significantly different than pre-modern for 

herbicides (p=0.005) and trihalomethanes (p<0.001)

Figure 46. Relations among detection frequencies of herbicides, solvents, and trihalomethanes, and 

groundwater age class, Tahoe-Martis, Central Sierra, and Southern Sierra study units, California GAMA Priority 

Basin Project.
132 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac09-0340_fig 47

0

20

40

60

80

100 0

20

40

60

80

100

0 20 40 60 80 100

PERCENT UNDEVELOPED

PERCENT AGRICULTURE

PERCENT URBAN

Average land use for all USGS-grid 

and USGS-understanding wells

EXPLANATION

Herbicides

Trihalomethanes

Solvents

Data point:

Average land

use for wells

with detection

Data point:

Average land

use for wells

with no detection

Organic constituent

Figure 47. Average percentages of agricultural, undeveloped, and urban 

land use in the areas surrounding wells with and without detections of 

herbicides, solvents, or trihalomethanes, Tahoe-Martis, Central Sierra, and 

Southern Sierra study units, California GAMA Priority Basin Project.
Status and Understanding of Water Quality 133

sac09-0340_fig 48

0

20

40

60

80

100

LAND USE AROUND WELL, IN PERCENT

Herbicides Solvents THMs

Agric

p=0.002

Undev

p=0.007

Agric

p=0.046

Undev

p<0.001

Urban

p=0.011

Urban

p=0.002

EXPLANATION

90th percentile

10th percentile

median

(50th percentile)

lower detached

25th percentile

75th percentile

upper detached

37 number of wells

Detected

Not detected

17 114 17114 22 110 22 110 22 110 23 105

Figure 48. Percentages of agricultural, undeveloped, and urban land use in the areas 

surrounding wells with and without detections of herbicides, solvents, or trihalomethanes 

(THMs), Tahoe-Martis, Central Sierra, and Southern Sierra study units, California GAMA 

Priority Basin Project.
134 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac09-0340_fig 49

0

20

40

60

80

HERBICIDE DETECTION FREQUENCY, IN PERCENT

> 5 < 5 any any

15

78

38

131

(all data)

AGRICULTURAL LAND USE, IN PERCENT

GROUNDWATER AGE CLASSIFICATION

Modern or any

Mixed

Modern or Pre-modern

Mixed

Detection frequencies in the three sub-groups are all significantly different from one another 

(p<0.050). Number of samples in each group indicated above bar. <, less than; >, greater than

Figure 49. Relation between detection frequency of herbicides 

and groundwater age and percentage of agricultural land use in the 

areas surrounding wells, Tahoe-Martis, Central Sierra, and Southern 

Sierra study units, California GAMA Priority Basin Project.
Status and Understanding of Water Quality 135

of bromodichloromethane. The MCL-US benchmark for 

trihalomethanes applies to the sum of the four chlorinated 

and (or) brominated species. Therefore, the sum of both 

trihalomethane species was used for the understanding 

assessment rather than the chloroform concentration alone. 

THM concentration was significantly correlated with 

percentage of urban land use (table 10B; figs. 47, 48). 

Nationally, THMs have also been strongly correlated with 

percentage of urban land use (Zogorski and others, 2006). 

Potential urban sources of THMs include recharge from 

landscape irrigation with disinfected water, leakage from 

water distribution systems, and industrial and commercial 

usage of chlorinated disinfectants and reagents (Ivahnenko and 

Barbash, 2004). However, 9 of the 23 samples with detections 

of THMs had < 10% urban land use, suggesting that other 

explanatory factors were also important. 

The samples were divided into groups by groundwater 

age class (pre-modern or modern/mixed), percentage of 

urban land use (greater than or less than 10%), and septic 

tank density (greater than or less than 7 tanks/km2). The 

three groups with modern/mixed age groundwater and either 

urban land use > 10% or septic tank density > 7 tanks/km2 or 

both had THM detection frequencies of 33–50% (fig. 51). 

THM detection frequencies in samples with pre-modern 

groundwater (0%) or with modern/mixed groundwater, urban 

land use < 10%, and septic tank density < 7 tanks/km2 (4.6%) 

were significantly lower (contingency tests, p < 0.004). These 

results suggest septic systems may be a source of THMs to 

groundwater. However, the increased detection frequency of 

THMs in groundwater from non-urbanized areas with higher 

density of septic tanks may also be because of differences 

in well maintenance procedures. Shock chlorination is 

a recommended procedure for treatment of bacterial 

contamination and odor problems in domestic wells, and may 

result in a reservoir of chlorinated water in the well bore and 

surrounding aquifer material (Seiler, 2006). Small systems, 

such as schools, campgrounds, restaurants, small community 

associations, and domestic well owners, may be more likely to 

maintain their wells following guidelines for domestic wells 

than are large systems, such as municipalities.

THM concentration was significantly greater in wells 

with metamorphic aquifer lithology compared to any of the 

other aquifer lithology categories (table 10A). However, wells 

with metamorphic aquifer lithology did not have significantly 

higher percentages of urban land use or occurrence of younger 

water—two factors that are significantly correlated with 

THM concentration—suggesting that another factor was also 

important. Wells with metamorphic aquifer lithology had 

significantly higher densities of septic tanks in the 500-m 

buffers around the wells compared to wells with all other 

aquifer lithologies (table 6A). 

Trihalomethanes

Water used for drinking water and other household uses 

in domestic and public (municipal and community) systems 

commonly is disinfected with chlorine solutions (bleach). In 

addition to disinfecting the water, the chlorine reacts with 

organic matter to produce THMs and other chlorinated and 

(or) brominated disinfection byproducts.

The study detection frequencies of the THM chloroform 

were 15, 16, and 17% in the Tahoe-Martis, Central Sierra, and 

Southern Sierra study units, respectively (fig. 44). Note that 

these detection frequencies for the Tahoe-Martis and Central 

Sierra study units are lower than those reported by Fram and 

others (2009) and Ferrari and others (2008) for two reasons: 

(1) four wells with detections of chloroform (one from the 

TMART, two from the TROCK, and one from the CWISH 

study areas) were removed from the calculation because 

the sampling point was downstream from the chlorination 

point, and (2) the detection frequencies reported here are 

weighted by the areas of the study areas within the study units 

(appendix B). Chloroform is the most commonly detected 

VOC in groundwater nationally (Zogorski and others, 2006).

The maximum RC measured in the grid well networks 

for the three study units was 0.014 (fig. 39). Moderate RCs 

of chloroform were reported for two wells in the CDPH 

database for the Central Sierra study unit (table 8B). Both 

well sampling points were coded as “untreated” in the 

database. Because this coding is not necessarily reliable, the 

spatially weighted moderate aquifer-scale proportions may 

not be as reliable as the grid-based proportions. The high and 

moderate aquifer-scale proportions of chloroform in all three 

study units, calculated using the grid-based method, were 

zero (tables 8A,B,C). 

Factors Affecting Trihalomethanes

The disinfection byproduct chloroform was the only 

organic constituent with a detection frequency of > 10% 

in all three study units. Detections of chloroform were not 

distributed evenly within each of the three study units. In 

the Tahoe-Martis study unit, chloroform was detected most 

frequently in the Tahoe Valley South subbasin (fig. 50A). 

In the Central Sierra study unit, chloroform was detected 

more frequently in the CGOLD study area than in the 

CWISH study area (fig. 50B). In the Southern Sierra study 

unit, chloroform was detected more frequently in the Kern 

River Valley and surrounding watershed than it was in the 

Tehachapi-Cummings Valleys and surrounding watershed 

(fig. 50C). 

Two samples with chloroform detections analyzed 

by USGS-GAMA and a number of samples in the CDPH 

databases with chloroform detections also had detections 
136 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac09-0340_fig 50a

NEVADA CO

PLACER CO

EL DORADO CO

ALPINE

CO

Not detected

 Low (< 8 chloroform, bromodichloromethane)

Grid wells

Not detected

 Low (< 8 chloroform, bromodichloromethane)

Understanding wells

Not detected

 Low (< 8 chloroform, bromodichloromethane)

CDPH wells

Martis and Tahoe study area boundary

Trihalomethane concentration,

 in micrograms per liter

EXPLANATION

Hard Rock study area boundary

39° 15’

120° 15’ 120°

39°

0 5 10 MILES

0 5 10 KILOMETERS

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006. 

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

Land use simplified from

Nakagaki and others (2007).

A

< is less than

LAKE TAHOE

CALIFORNIA

NEVADA

Urban

Undeveloped forest

Undeveloped rock, shrub, and grass

Agricultural

Land use

Figure 50. Relative-concentrations of trihalomethanes for USGS- and CDPH-grid wells, USGS-understanding wells, 

and all wells in the CDPH database with data for trihalomethanes during the 3-year intervals used in the status 

assessment for the (A) Tahoe-Martis, (B) Central Sierra, and (C) Southern Sierra study units, California GAMA Priority 

Basin Project.
Status and Understanding of Water Quality 137

sac09-0340_fig 50b

FRESNO CO

MARIPOSA CO

MADERA CO

EXPLANATION

Study unit boundary 

Not detected

 Low (< 8 chloroform)

Grid wells

Not detected

Understanding wells

Not detected

 Low (< 8 chloroform or bromodichloromethane

 or dibromochloromethane or bromoform)

Moderate (8–80 chloroform)

 

CDPH wells

Trihalomethane concentration, in micrograms per liter

< is less than

0 5 10 MILES

0 5 10 KILOMETERS

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006,

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

37° 30’

37° 15’

119° 45’ 119° 30’

B

Land use simplified from

Nakagaki and others (2007).

Urban

Undeveloped forest

Undeveloped rock, shrub, and grass

Agricultural

Land use

Figure 50.—Continued
138 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac09-0340_fig 50c

EXPLANATION

Study unit boundary

Not detected

 Low (< 8 chloroform)

Grid wells

Not detected

 Low (< 8 chloroform)

Understanding wells

 Low (< 5)

 Low (< 8 chloroform or bromodichloromethane

 or dibromochloromethane or bromoform)

Moderate (8–80 total trihalomethanes)

 

CDPH wells

Trihalomethane concentration, in micrograms per liter 

< is less than

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006,

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

Land use simplified from

Nakagaki and others (2007).

C

35°15'

118°30’ 118°15’

36°

35°45’

35°30’

10 MILES

50 10 KILOMETERS

0 5

TULARE CO

KERN CO

Urban

Undeveloped forest

Undeveloped rock, shrub, and grass

Agricultural

Land use

Figure 50.—Continued
Status and Understanding of Water Quality 139

sac09-0340_fig 51

0

10

20

30

40

50

60

TRIHALOMETHANE DETECTION FREQUENCY, IN PERCENT

URBAN LAND USE GREATER THAN 10 PERCENT?

SEPTIC TANK DENSITY GREATER THAN 7 TANKS PER SQUARE KILOMETER?

GROUNDWATER AGE CLASSIFICATION

yes yes no no

yes no yes no any

any

Modern or Mixed Pre-modern

Bars of different colors have significantly different detection frequencies. 

Number of samples in each group indicated above bar.

12

21

14

44

37

Figure 51. Relation between detection frequency of trihalomethanes 

and groundwater age, percentage of urban land use, and septic tank 

density, Tahoe-Martis, Central Sierra, and Southern Sierra study units, 

California GAMA Priority Basin Project.
140 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Solvents

Solvents are used for a variety of industrial, commercial, 

and domestic purposes (Zogorski and others, 2006). The only 

solvent with a study-unit detection frequency > 10% was 

PCE, with a frequency of 14% in the Southern Sierra study 

unit (fig. 44). Note that Fram and others (2009) reported that 

17% of the Tahoe-Martis study-unit grid wells had detections 

of PCE, but because the majority of the detections were 

in wells from the TTAHO study area (the smallest study 

area), the area-weighted detection frequency of PCE in the 

Tahoe‐Martis study unit is < 10% (fig. 44). PCE is primarily 

used for dry-cleaning of fabrics and degreasing metal parts, 

and is an ingredient in a wide range of products including 

paint removers, polishes, printing inks, lubricants, and 

adhesives (Doherty, 2000).

The high aquifer-scale proportion for solvents in the 

Tahoe-Martis study unit was 1.0%, and the solvents present 

at high RCs in 1% or less of the primary aquifers were PCE, 

carbon tetrachloride, and 1,2-dichloroethane (table 8A). 

Wells with high RCs of solvents were located in the Tahoe 

Valley South subbasin (fig. 52A). PCE and chloroethane 

were present at moderate RCs in < 1% of the primary aquifer 

(spatially weighted). 

No solvents were present at high RCs in the Central 

Sierra study unit (fig. 52B). The moderate aquifer-scale 

proportion of PCE was < 1% (spatially weighted) (table 8B).

No solvents were present at high RCs in the Southern 

Sierra study unit. The moderate aquifer-scale proportion for 

solvents was 2.9%, composed of PCE and carbon tetrachloride 

(table 8C). Wells with moderate RCs of solvents were 

located in the Kern River and Tehachapi-Cummings Valleys 

(fig. 52C). Three additional solvents were reported at high 

RCs historically in the CDPH database (1,1-dichloroethane, 

1,2-dichloroethane, and dichloromethane; table 5).

Factors Affecting Solvents

Because many of the factors associated with the 

occurrences of individual chlorinated solvents were similar 

in the NAWQA groundwater studies nationally (Zogorski and 

others, 2006), solvents were considered as a class rather than 

as individual constituents for the understanding assessment. 

Total solvent concentration was the sum of the concentrations 

of all chlorinated solvents with benchmarks. Twenty of 

the 26 solvent detections were PCE, which was also the 

most frequently detected solvent in groundwater nationally 

(Zogorski and others, 2006). Three detections were of solvents 

that are also breakdown products of PCE (trichloroethene and 

cis-1,2-dichloroethene; table 4). 

Solvent concentration had significant positive 

correlations with percentages of both urban and agricultural 

land use and a significant negative correlation with percentage 

of undeveloped land use (table 10B; figs. 47, 48)—as would 

be expected given the myriad of uses for solvents in human 

activities. Solvent concentration was significantly greater in 

wells with sedimentary aquifer lithology (table 10A); wells 

with sedimentary aquifer lithology had significantly greater 

percentages of agricultural and urban land use (table 6A).

Although solvent concentration was significantly 

correlated to herbicide and THM concentrations (Spearman’s 

rho test, p= 0.002 and p = 0.004, respectively), the relations 

of solvents to groundwater age and measures of depth were 

different from those of herbicides and THMs. Unlike herbicide 

and THM concentrations, solvent concentration was not 

significantly related to classified groundwater age (table 10A; 

fig. 46). THM, herbicide, and solvent concentrations were 

not significantly related to classified well depth (table 10A), 

nor were they significantly correlated to well depth or depth 

to top of screened or open interval (table 10B). Of the 

11 wells with herbicide detections and well construction 

information, 9 (82%) had depth to top of screened or open 

interval < 170 ft bls, and all had groundwater with modern 

or mixed age (fig. 53A). Similarly, 89% of the wells with 

THM detections had depth to top of screened or open interval 

< 170 ft bls, and all had groundwater with modern or mixed 

age (fig. 53B). In contrast, of the 19 wells with solvent 

detections and well construction information, only 63% had 

depth to top of screened or open interval < 170 ft bls, and 68% 

had groundwater with modern or mixed age (fig. 53C). The 

median depth to top of screened or open interval for wells 

with detections of herbicides or THMs and no detections of 

solvents (89 ft) was significantly less than the median depth 

to top of screened or open interval for wells with detections 

of solvents and no detections of herbicides or THMs (180 ft) 

(Wilcoxon rank-sum test, p=0.023) (fig. 54).

The greater depth of penetration of solvents into the 

aquifer systems, compared to herbicides and THMs, may 

reflect differences in either patterns of usage of these organic 

compounds or chemical behavior of these constituents in 

aquifers. Because solvents have such a wide variety of uses 

(Doherty, 2000), it is possible that their widespread uses 

preceded herbicide use and water disinfection in these parts of 

the Sierra Nevada. Also, solvents generally are more mobile 

in groundwater than are herbicides because solvents typically 

sorb less to mineral surfaces and sedimentary organic matter.
Status and Understanding of Water Quality 141

sac09-0340_fig 52a

NEVADA CO

PLACER CO

EL DORADO CO

ALPINE

CO

Not detected

 Low (< 0.5 perchloroethene or

 < 20 1,1,1-trichloroethane or < 0.5 trichloroethene)

High (> 5 perchloroethene)

Grid wells

Not detected

Low (< 0.5 perchloroethene)

Understanding wells

Not detected

 Moderate (3–30 chloromethane

 or 0.5–5 perchloroethene)

 High (> 0.5 carbon tetrachloride or

 > 0.5 1,2-dichloroethane)

CDPH wells

Martis and Tahoe study area boundary

Solvent concentration, in micrograms per liter

EXPLANATION

Hard Rock study area boundary

39° 15’

120° 15’ 120°

39°

0 5 10 MILES

0 5 10 KILOMETERS

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006. 

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

Land use simplified from

Nakagaki and others (2007).

A

< is less than;

> is greater than

LAKE TAHOE

CALIFORNIA

NEVADA

Urban

Undeveloped forest

Undeveloped rock, shrub, and grass

Agricultural

Land use

Figure 52. Relative-concentrations of solvents for USGS- and CDPH-grid wells, USGS-understanding wells, and all 

wells in the CDPH database with data for solvents during the 3-year intervals used in the status assessment for the 

(A) Tahoe-Martis, (B) Central Sierra, and (C) Southern Sierra study units, California GAMA Priority Basin Project.
142 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac09-0340_fig 52b

FRESNO CO

MARIPOSA CO

MADERA CO

EXPLANATION

Study unit boundary 

Not detected

Low (< 0.5 perchloroethene)

Grid wells

Not detected

Understanding wells

Not detected

Moderate (0.5–5 perchloroethene)

CDPH wells

Solvent concentration, in micrograms per liter

< is less than

0 5 10 MILES

0 5 10 KILOMETERS

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006,

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

37° 30’

37° 15’

119° 45’ 119° 30’

B

Land use simplified from

Nakagaki and others (2007).

Urban

Undeveloped forest

Undeveloped rock, shrub, and grass

Agricultural

Land use

Figure 52.—Continued
Status and Understanding of Water Quality 143

sac09-0340_fig 52c

EXPLANATION

Study unit boundary

Not detected

 Low (< 0.5 perchloroethene or

 < 0.5 trichloroethene)

Moderate (0.05–5 perchloroethene)

Grid wells

Not detected

 Low (< 0.5 perchloroethene or

 < 0.6 cis-1,2-dichloroethene)

Moderate (0.05–5 perchloroethene)

Understanding wells

Not detected

Low (< 3 chloromethane)

Moderate (3–30 chloromethane or 0.5–5 perchloroethene)

CDPH wells

Solvent concentration, in micrograms per liter 

< is less than

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006,

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

Land use simplified from

Nakagaki and others (2007).

C

35°15'

118°30’ 118°15’

36°

35°45’

35°30’

10 MILES

50 10 KILOMETERS

0 5

Urban

Undeveloped forest

Undeveloped rock, shrub, and grass

Agricultural

Land use

Figure 52.—Continued
144 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Figure 53. Relation among (A) total herbicide, (B) total solvent, and (C) total trihalomethane concentrations and depth to 

top of screened or open interval in wells and groundwater age classification, Tahoe-Martis, Central Sierra, and Southern 

Sierra study units, California GAMA Priority Basin Project.

sac09-0340_fig 53

TOTAL HERBICIDE CONCENTRATION,

IN MICROGRAMS PER LITER

0.000 0.003 0.006 0.009 0.012 0.015

0

100

200

300

400

500

DEPTH TO TOP OF SCREENED OR OPEN INTERVAL,

IN FEET BELOW LAND SURFACE

0.0 0.1 0.2 0.3 0.4 0.5

0

100

200

300

400

500

TOTAL TRIHALOMETHANE CONCENTRATION,

IN MICROGRAMS PER LITER

DEPTH TO TOP OF SCREENED OR OPEN INTERVAL,

IN FEET BELOW LAND SURFACE

0

100

200

300

400

500

TOTAL SOLVENT CONCENTRATION,

IN MICROGRAMS PER LITER

DEPTH TO TOP OF SCREENED OR OPEN INTERVAL,

IN FEET BELOW LAND SURFACE

0 0.1 0.2 0.3 2 25

EXPLANATION

Groundwater age classification

Modern

 Tahoe-Martis study unit

 Central Sierra study unit

 Southern Sierra study unit

Modern or Mixed

 Tahoe-Martis study unit

 Central Sierra study unit

 Southern Sierra study unit

Mixed

 Tahoe-Martis study unit

 Central Sierra study unit

 Southern Sierra study unit

Pre-modern

 Tahoe-Martis study unit

 Central Sierra study unit

 Southern Sierra study unit

A C

B
Status and Understanding of Water Quality 145

sac09-0340_fig 54

0.0 0.1 0.2 0.3 0.4 0.5 2 25

TOTAL CONCENTRATION OF SOLVENTS,

HERBICIDES, AND TRIHALOMETHANES,

IN MICROGRAMS PER LITER

0

100

200

300

400

500

DEPTH TO TOP OF SCREENED OR OPEN INTERVAL,

IN FEET BELOW LAND SURFACE

EXPLANATION

Type of organic constituents detected

None

Herbicides or THMs

Solvents and herbicides or THMs

Solvents

Tahoe-Martis study unit

Central sierra study unit

Southern Sierra study unit

Tahoe-Martis study unit

Southern Sierra study unit

Tahoe-Martis study unit

Central sierra study unit

Southern Sierra study unit

Tahoe-Martis study unit

Southern Sierra study unit

Figure 54. Relation between total concentration of organic constituents and depth to top of 

screened or open interval in wells, Tahoe-Martis, Central Sierra, and Southern Sierra study units, 

California GAMA Priority Basin Project.
146 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Other Volatile Organic Compounds

No fuel components had detection frequencies of > 10% 

in any of the three study units (figs. 43A,B,C). The gasoline 

hydrocarbon benzene was detected in one Central Sierra 

study-unit grid well at an RC of 0.17 (fig. 43B), which resulted 

in a moderate aquifer-scale proportion of 4.0% (table 8B). 

The gasoline oxygenate MTBE was reported at high RCs 

historically in the Tahoe-Martis and Central Sierra study units 

(table 5). Use of MTBE in gasoline was banned in the Lake 

Tahoe Basin in 2000 and in the State of California at the end 

of 2003 (State of California, 2003). Among the wells currently 

used in the Tahoe-Martis study unit, no moderate or high 

RCs of MTBE were reported in the CDPH database or were 

detected by USGS-GAMA (fig. 43A). MTBE was reported at 

moderate RCs in the Central Sierra study unit CDPH database; 

the moderate aquifer-scale proportion is 1.6% (table 8B). Note 

that this is a minimum estimate because the reporting limit 

for MTBE in the CDPH database for Central Sierra, 2 μg/L, 

is greater than one-tenth of the benchmark value for MTBE, 

which is the MCL-CA of 13 μg/L. 

The CDPH database for the Tahoe-Martis study unit 

reported one detection each of two fumigants—1,2-dibromo3-chloropropane (DBCP) and 1,2-dichloropropane—at high 

RCs in the 3-year interval (table 8A). Neither constituent had 

any other detections at any concentration in the USGS-GAMA 

dataset for the Tahoe-Martis study unit or in the entire period 

of record of the CDPH database available for this study.

Special-Interest Constituents

Perchlorate was analyzed at all USGS-GAMA wells 

in the three study units (table 1). N-nitrosodimethylamine 

(NDMA) was analyzed at all wells in the Central Sierra 

study unit and at a subset of the wells in the Tahoe-Martis 

and Southern Sierra study units. Low-level 1,2,3-TCP was 

analyzed at all wells in the Central Sierra study unit and at a 

subset of the wells in the Southern Sierra study unit. 

NDMA was not detected in samples from any of the three 

study units, and perchlorate was not detected in samples from 

the Tahoe-Martis and Central Sierra study units. 1,2,3-TCP 

was detected at low RC in one USGS-understanding well 

in the Southern Sierra study unit. The CDPH database 

for the Tahoe-Martis study unit reported one detection of 

perchlorate at a high RC historically (table 5); this was the 

only detection of perchlorate reported in the CDPH database 

for the Tahoe-Martis study unit for the entire period of record. 

The study-unit detection frequency for perchlorate in the 

Southern Sierra study unit was 12% (fig. 43C). The moderate 

aquifer-scale proportion for perchlorate in the Southern Sierra 

study unit was 8.6%, and no high RCs of perchlorate were 

detected (table 8C; fig. 44). 

Factors Affecting Perchlorate

Perchlorate concentration had significant positive 

correlations with agricultural land use, sedimentary aquifer 

lithology, and DO concentration, and significant negative 

correlations with aridity index and percentage of undeveloped 

land use (tables 10A,B). Perchlorate concentration also had 

significant positive correlation with nitrate, total herbicide, and 

TDS concentrations (Spearman’s rho test, p < 0.001, p=0.009, 

and p < 0.001, respectively). Many of these correlations reflect 

the fact that perchlorate was only detected in the Southern 

Sierra study unit (figs. 55A,B,C).

There are both natural and anthropogenic potential 

sources of perchlorate to groundwater. Perchlorate is formed 

naturally in the atmosphere, and very low concentrations are 

found in precipitation (Dasgupta and others, 2005; Parker and 

others, 2009; Rajagopalan and others, 2009). The distribution 

of perchlorate under natural conditions in groundwater 

is likely correlated with climate because the extent of 

evaporative concentration of precipitation in the hydrologic 

cycle is likely to increase with increasing aridity (Fram and 

Belitz, 2011). Perchlorate salts accumulate in unsaturated 

zones and soils (Rao and others, 2007).

Three main sources of anthropogenic perchlorate 

have potential to reach California groundwater: industrial, 

agricultural, and disinfection. Most known sites of perchlorate 

contamination from industrial sources are associated with 

facilities that manufactured or used solid rocket fuel; other 

sites include the following: military munitions storage, use, 

and disposal, and manufacturing and disposal of pyrotechnics, 

safety flares, and explosives (U.S. Environmental Protection 

Agency, 2005a; California Department of Toxic Substances 

Control, 2007; California State Water Resources Control 

Board, 2007). Colorado River water imported to southern 

California is contaminated with perchlorate downstream 

from perchlorate manufacturing facilities in southern 

Nevada (U.S. Environmental Protection Agency, 2005b). 

Agricultural sources include the Chilean nitrate fertilizer 

applied extensively in the early to mid-1900s (Aziz and 

others, 2006; Dasgupta and others, 2006; Böhlke and others, 

2009). Irrigation can also be considered an agricultural 

source if it redistributes perchlorate salts that were originally 

naturally present in the unsaturated zone (Fram and Belitz, 

2011). Redistribution likely results in higher concentrations 

and greater detection frequencies of perchlorate in deep 

groundwater than would be present under natural conditions. 

Finally, perchlorate is a degradation product of hypochlorite 

solutions that are widely used for drinking-water disinfection 

(Massachusetts Department of Environmental Protection, 

2006; Greiner and others, 2008). 
Status and Understanding of Water Quality 147

sac09-0340_fig 55a

A

NEVADA CO

PLACER CO

EL DORADO CO

ALPINE

CO

NEVADA

CALIFORNIA

Martis and Tahoe study area boundary

Hard Rock study area boundary

Urban

Undeveloped forest

Undeveloped rock, shrub, and grass

Agricultural

Land use

Perchlorate concentration, in micrograms per liter

EXPLANATION

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006. 

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

Land use simplified from

Nakagaki and others (2007).

39° 15’

120° 15’ 120°

0 5 10 MILES

0 5 10 KILOMETERS

LAKE TAHOE

Not detected (< 0.5)

USGS-grid wells

Not detected (< 0.5)

USGS-understanding wells

Figure 55. Relative-concentrations of perchlorate for USGS-grid wells, USGS-understanding wells, and all wells 

in the CDPH database with data for perchlorate during the 3-year intervals used in the status assessment, for the 

(A) Tahoe-Martis, (B) Central Sierra, and (C) Southern Sierra study units, California GAMA Priority Basin Project.
148 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac09-0340_fig 55b

Perchlorate concentration, in micrograms per liter

EXPLANATION

Urban

Undeveloped forest

Undeveloped rock, shrub, and grass

Agricultural

Land use

Study unit boundary

MARIPOSA CO

MADERA CO

FRESNO CO

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006. 

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

37° 30’

37° 15’

119° 45’ 119° 30’

B

0 5 10 MILES

0 5 10 KILOMETERS

Not detected (< 0.5)

USGS-grid wells

Not detected (< 0.5)

CDPH wells

Not detected (< 0.5)

USGS-understanding wells

Land use simplified from

Nakagaki and others (2007).

Figure 55.—Continued
Status and Understanding of Water Quality 149

sac09-0340_fig 55c

Urban

Undeveloped forest

Undeveloped rock, shrub, and grass

Agricultural

Land use

Study unit boundary

EXPLANATION

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006,

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

Land use simplified from

Nakagaki and others (2007)..

C

10 MILES

50 10 KILOMETERS

0 5

35°30’

35°15’

35°

118°30’ 118°15’

36°

35°45’

Not detected (< 0.5)

Low (0.5–0.6)

Moderate (0.6–6)

USGS-grid wells

Not detected (< 0.5) 

Low (0.5–0.6)

Moderate (0.6–6)

USGS-understanding wells

Not detected (< 4)

Moderate (4–6)

CDPH wells

Perchlorate concentration, in micrograms per liter

< is less than

Figure 55.—Continued
150 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Fram and Belitz (2011) used logistic regression to 

quantify the relation between the probability of perchlorate 

detection in groundwater resources sampled by the GAMA 

Priority Basin Project and water-quality indicators and other 

proxies of natural and anthropogenic perchlorate sources 

and processes. Their best-fit model was constructed with 

aridity index as the proxy for perchlorate under natural 

conditions, and an Anthropogenic Score as the proxy for 

anthropogenic sources and processes affecting perchlorate. 

The Anthropogenic Score summed four binary categorical 

variables: presence or absence of herbicides and fumigants, 

presence or absence of solvents and fuel components, 

nitrate-N concentration above or below 3 mg/L, and presence 

or absence of known sites of perchlorate contamination 

within a specific distance of the well. This model was applied 

to the data from the Tahoe-Martis, Central Sierra, and 

Southern Sierra study units to assess whether the observed 

concentrations and detection frequencies of perchlorate can 

be accounted for by natural conditions or require additional 

inputs of perchlorate from anthropogenic sources or processes.

Samples were assigned Anthropogenic Scores on the 

basis of nitrate concentrations and presence or absence 

of detections of herbicides, fumigants, solvents, and fuel 

components (Fram and Belitz, 2011). None of the well sites 

were near any of the known or suspected sites of groundwater 

contamination with industrial sources of perchlorate. Samples 

were then divided into groups by study unit, Anthropogenic 

Score, and aridity index for calculation of detection 

frequencies. 

The observed detection frequencies were compared to 

predicted probabilities of detecting perchlorate as a function 

of Anthropogenic Score and aridity index (fig. 56). Sample 

groups from the Tahoe-Martis and Central Sierra study units 

with Anthropogenic Scores of 0 or 1 had detection frequencies 

of 0% for perchlorate at concentrations ≥ 0.5 μg/L and 

average aridity index values ranging from 0.55 to 1.0. The 

absence of detections of perchlorate is consistent with the 

low detection frequencies predicted under natural conditions 

(Anthropogenic Score = 0) for that range of aridity index 

(5.3% to 1.8%; fig. 56A). Sample groups from the Southern 

Sierra study unit with Anthropogenic Scores of 0, 1, or 2 

and 3 had progressively increasing detection frequencies 

of perchlorate of 5.0%, 31%, and 38%, respectively, which 

are values consistent with predicted values (fig. 56A). The 

detection frequency of perchlorate at concentrations ≥ 1.0 μg/L 

was 0% in all groups except for the group with Anthropogenic 

Score = 2 and 3 from the Southern Sierra study unit, which 

had a detection frequency of 31% (fig. 56B). These results 

suggest that both natural and anthropogenic sources of 

perchlorate may be identified in the Southern Sierra study unit. 

The observed detection frequencies for concentrations 

≥ 0.5 μg/L for the groups with Anthropogenic Scores of 1 or 2 

are significantly higher than the study-unit detection frequency 

of 12%. Of the 10 Southern Sierra study-unit samples with 

detection of perchlorate, 6 are USGS-understanding wells 

located in the centers of the Tehachapi-Cummings Valley 

basins (fig. 55C). These six wells have 30% to 86% (average 

63%) agricultural land use, which is significantly higher than 

the average agricultural land use around the grid wells (20%; 

fig. 6A). Agricultural lands in the Tehachapi-Cummings Valley 

basins are heavily irrigated, suggesting that the source of the 

elevated perchlorate in groundwater may be redistribution 

of naturally deposited perchlorate salts in the unsaturated 

zone by recharge of irrigation water (Fram and Belitz, 2011). 

Three of the six USGS-understanding wells have perchlorate 

concentrations ≥ 1 μg/L, suggesting that these higher 

concentrations of perchlorate in groundwater are indicative of 

anthropogenic influence.
Status and Understanding of Water Quality 151

sac09-0340_fig 56ab

ARIDITY INDEX, DIMENSIONLESS 

A

B

Anthropogenic

Score (AS) =3

AS=1

AS=2

AS=0

Anthropogenic

Score (AS) =3

AS=1

AS=2

AS=0

0

10

20

30

40

50

0.0 0.2 0.4 0.6 0.8 1.0 1.2

0

10

20

30

40

50

PREDICTED PROBABILITY OF DETECTION OR

OBSERVED DETECTION FREQUENCY FOR

CONCENTRATION GREATER THAN OR EQUAL TO

0.5 MICROGRAM PER LITER, IN PERCENT

PREDICTED PROBABILITY OF DETECTION OR

OBSERVED DETECTION FREQUENCY FOR

CONCENTRATION GREATER THAN OR EQUAL TO

1 MICROGRAM PER LITER, IN PERCENT

EXPLANATION

AS=0

AS = 1

Tahoe-Martis study unit

Average Anthropogenic Score (AS)

AS=0

AS = 1

Central Sierra study unit

AS=0

AS = 1

AS = 2 and 3

Southern Sierra study unit

Figure 56. Predicted probability of detecting perchlorate in groundwater as a function of aridity 

index and Anthropogenic Score and observed detection frequency and average aridity index in 

groups of samples for perchlorate concentrations greater than or equal to (A) 0.5 microgram per 

liter (μg/L) and (B) 1.0 μg/L, Tahoe-Martis, Central Sierra, and Southern Sierra study units, California 

GAMA Priority Basin Project.
152 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Summary

Groundwater quality in the Tahoe-Martis, Central Sierra, 

and Southern Sierra study units was investigated as part of 

the Priority Basin Project of the California Groundwater 

Ambient Monitoring and Assessment (GAMA) Program. 

The three study units are located in the Sierra Nevada region 

of California in parts of Nevada, Placer, El Dorado, Madera, 

Tulare, and Kern Counties. The GAMA Priority Basin Project 

is designed to provide a statistically-robust characterization of 

untreated groundwater quality in the primary aquifers used for 

public and community drinking-water supplies.

The assessments are based on water-quality and ancillary 

data collected by the USGS from 132 wells in the three study 

units during 2006–2007, and water-quality data from the 

California Department of Public Health (CDPH) database. 

The primary aquifer systems are defined by the depths of the 

screened or open intervals of the wells listed in the CDPH 

database for each study unit. The quality of groundwater in 

shallower or deeper water-bearing zones may differ from 

that in the primary aquifers; shallower groundwater may 

be more vulnerable to contamination from the surface, and 

deeper groundwater may be more susceptible to water–rock 

interaction. Two types of assessments were made: (1) status, 

assessment of the current quality of the groundwater resource, 

and (2) understanding, identification of the natural and human 

factors affecting groundwater quality.

Relative-concentrations (sample concentrations divided 

by benchmark concentrations) were used for evaluating 

groundwater quality for those constituents that have Federal 

and (or) California regulatory or non-regulatory benchmarks 

for drinking-water quality. A relative-concentration (RC) 

greater than (>) 1.0 indicates a concentration above a 

benchmark. Organic (volatile organic compounds and 

pesticides) and special-interest (perchlorate) constituent 

RCs were classified as “high” (RC > 1.0), “moderate” 

(1.0 ≥ RC > 0.1), or “low” (RC ≤ 0.1). For inorganic (major 

ion, trace element, nutrient, and radioactive) constituents, the 

boundary between low and moderate RCs was set at 0.5.

Aquifer-scale proportion was used as the primary 

metric in the status assessment for evaluating regional-scale 

groundwater quality. High aquifer-scale proportion is defined 

as the percentage of the primary aquifer with RC > 1.0 for a 

particular constituent or class of constituents; moderate and 

low aquifer-scale proportions are defined as the percentage of 

the primary aquifer with moderate and low RCs, respectively. 

The methods used to calculate aquifer-scale proportions 

are based on an equal-area grid; thus, the proportions are 

areal rather than volumetric. Two statistical approaches—

grid-based, which used one value per grid cell, and spatially 

weighted, which used multiple values per cell—were used to 

calculate aquifer-scale proportions for individual constituents 

and classes of constituents. The spatially weighted estimates of 

high aquifer-scale proportions were within the 90-percent (%) 

confidence intervals of the grid-based estimates in all cases.

Inorganic constituents had significantly greater high 

and moderate aquifer-scale proportions than did organic 

constituents in all three study units. In the Tahoe-Martis 

study unit, RCs for inorganic constituents with healthbased benchmarks (primarily arsenic) were high in 20% 

of the primary aquifer, moderate in 13%, and low in 67%. 

In the Central Sierra study unit, aquifer-scale proportions 

for inorganic constituents with health-based benchmarks 

(primarily arsenic, uranium, fluoride, and molybdenum) were 

41% high, 36% moderate, and 23% low. In the Southern 

Sierra study unit, 32, 34, and 34% of the primary aquifer 

had high, moderate, and low RCs, respectively, of inorganic 

constituents with health-based benchmarks (primarily arsenic, 

uranium, fluoride, boron, and nitrate). The high aquifer-scale 

proportions for inorganic constituents with non-health-based 

benchmarks were 14, 34, and 24% for the Tahoe-Martis, 

Central Sierra, and Southern Sierra study units, respectively, 

and the primary constituent was manganese for all three study 

units.

Organic constituents with health-based benchmarks were 

not present at high RCs in the primary aquifers of the Central 

Sierra and Southern Sierra study units, and were present at 

high RCs in only 1% of the Tahoe-Martis study unit. Moderate 

aquifer-scale proportions for organic constituents were < 5% in 

all three study units. Of the 173 organic constituents analyzed 

for, 22 were detected, and of those 22, 17 have health-based 

benchmarks. Organic constituents were detected in 20, 27, 

and 40% of the primary aquifers in the Tahoe-Martis, Central 

Sierra, and Southern Sierra study units, respectively. Four 

organic constituents had study-unit detection frequencies of 

> 10%: the trihalomethane chloroform in the Tahoe-Martis 

study unit; chloroform and the herbicide simazine in the 

Central Sierra study unit; and chloroform, simazine, the 

herbicide atrazine, and the solvent perchloroethene in the 

Southern Sierra study unit. 

The understanding assessment used statistical 

correlations between concentrations of constituents and values 

of selected potential explanatory factors to identify the factors 

potentially affecting the concentrations and occurrences of 

constituents found at high RCs or, for organic constituents, 

with study-unit detection frequencies > 10%. The potential 

explanatory factors evaluated were geographic location, 

land use, septic tank density, climate, relative position in 

the regional flow system, aquifer lithology, well depth and 

depth to the top of the screened or open interval in the well, 

groundwater age distribution, pH, and dissolved oxygen 

concentration. Data from the three study units were aggregated 

into one dataset for the statistical tests.
Acknowledgments 153

Aquifer lithology (granitic, metamorphic, sedimentary, 

or volcanic), groundwater age distribution (modern, mixed, 

or pre-modern), pH, DO, and geographic location were the 

most significant explanatory factors with potential causative 

value for most of the inorganic constituents evaluated in 

the understanding assessment (arsenic, fluoride, boron, 

molybdenum, nitrate, uranium, total dissolved solids, iron, 

and manganese). High and moderate RCs of arsenic were 

associated with pre-modern and mixed-age groundwater and 

two distinct sets of geochemical conditions, reflecting two 

mechanisms for solubilization of arsenic. Correlations between 

elevated arsenic concentrations and oxic, high-pH conditions, 

particularly in volcanic rocks, suggest desorption of arsenic 

from iron and manganese oxyhydroxides. Correlations 

between elevated arsenic concentrations and low-oxygen to 

anoxic, low- to neutral-pH conditions, particularly in granitic 

rocks, suggest release of arsenic by reductive dissolution of 

iron and manganese oxyhydroxides.

In granitic and metamorphic rocks, high and moderate 

RCs of uranium were associated with pre-modern and 

mixed-age groundwater, low-oxygen to anoxic conditions, 

and location within parts of the Central Sierra and Southern 

Sierra study units known to have rocks with anomalously 

high uranium contents compared to other parts of the Sierra 

Nevada. High and moderate RCs of uranium in sedimentary 

rocks were associated with pre-modern-age groundwater, 

oxic and high-pH conditions, low total dissolved solids, and 

location in the Tahoe Valley South subbasin. 

Elevated manganese and iron concentrations were 

associated with anoxic conditions, older groundwater, and 

granitic and metamorphic aquifer lithologies. Of the samples 

with pre-modern ages from wells in granitic rocks, 60% 

had high RCs of manganese. Most wells in sedimentary 

and volcanic rocks had oxic groundwater, regardless of age, 

and approximately 5 and 10%, respectively, of the samples 

from wells in those rock types had moderate or high RCs of 

manganese. 

Land use within 500 meters of the well and groundwater 

age were the most significant explanatory factors with 

potential causative value for the organic constituents. 

Herbicide detections were most strongly associated with 

modern- and mixed-age groundwater from wells with any 

agricultural land use. Trihalomethane detections were most 

strongly associated with modern- and mixed-age groundwater 

from wells with > 10% urban land use and (or) septic tank 

density > 7 tanks per square kilometer. Eighty-four percent 

of the wells with modern- or mixed-age groundwater and 

86% of wells with detections of herbicides and (or) THMs 

had depths to the top of the screened or open interval of 

< 170 feet below land surface. These results suggest that 

modern (post-1952) groundwater has infiltrated to a depth of 

approximately 170 feet below land surface. Solvent detections 

were not significantly related to groundwater age, and only 

63% of wells with detections of solvents had depths to the 

top of the screened or open interval of < 170 feet below land 

surface, suggesting that solvents have different behavior than 

herbicides and THMs in these aquifer systems.

Moderate RCs of perchlorate and moderate and high 

RCs of nitrate were detected in the Southern Sierra study unit 

only. Elevated nitrate concentrations were associated with 

agricultural land use and detections of herbicides and (or) 

solvents, suggesting an anthropogenic source for the moderate 

and high RCs of nitrate. Comparison between observed and 

predicted detection frequencies of perchlorate suggests that 

the perchlorate detected at concentrations < 1 microgram 

per liter likely reflects the distribution of perchlorate under 

natural conditions and that the perchlorate detected at higher 

concentrations may reflect redistribution of original, natural 

perchlorate salts by irrigation in the agricultural areas of the 

Southern Sierra study unit.

Acknowledgments

The authors thank the following cooperators for their 

support: the State Water Resources Control Board (SWRCB), 

Lawrence Livermore National Laboratory, California 

Department of Public Health, and California Department of 

Water Resources. We especially thank the cooperating well 

owners and water purveyors for their generosity in allowing 

the USGS to collect samples from their wells. Funding for this 

work was provided by State of California bonds authorized by 

Proposition 50 and administered by the SWRCB. We thank 

Bryant Jurgens (USGS California Water Science Center) for 

assistance with the PHREEQC modeling and Tyler Johnson 

(USGS California Water Science Center) for GIS support. 

Approximately 30 members of the USGS-GAMA team 

collected the data presented in this report.
154 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

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Aeschbach-Hertig, W., Peeters, F., Beyerle, U., and Kipfer, 

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Appendix A 163

Of the 115 grid cells in the three study units, 11 cells did 

not have a USGS-grid well, 28 cells had a USGS-grid well 

but no USGS-GAMA data for major ions, trace elements, 

nutrients, and radioactive constituents, and 49 cells had a 

USGS-grid well but incomplete USGS-GAMA data for 

radioactive constituents. The CDPH database was queried to 

identify wells to provide these missing inorganic data. CDPH 

wells with data in the most recent 3 years available at the time 

of sampling were considered. If a well had more than one 

analysis for a constituent in the 3-year interval, the most recent 

data were selected. The 3-year intervals were as follows: 

Study unit 3-year interval

Number of 

wells with 

data

Tahoe-Martis May 1, 2004–April 30, 2007 157

Central Sierra June 1, 2003–May 31, 2006 169

Southern Sierra February 1, 2003–January 31, 2006 205

No quality-control data were available for data stored 

in the CDPH database. The only measure of data quality 

available was the cation/anion balance for the major-ion 

results. We made the broad assumption that if the major-ion 

data were of acceptable quality, then the data for trace 

elements, nutrients, and radioactive constituents would 

also be of acceptable quality (appendix D). Therefore, to 

minimize selection of potentially poor-quality data, the 

algorithm included calculation of the cation/anion balance as a 

quality-control check for the analyses (Hem, 1989). The total 

positive charge on dissolved cation species in a water sample 

must equal the total negative charge on dissolved anion 

species in the sample. Cation-anion balance was calculated as 

the absolute value of the difference between the total cations 

and total anions divided by the average of the total cations and 

total anions, expressed as a percentage, and acceptable cationanion balance was defined as > 90%:

cations anions

balance 1 *100, cations anions

2

where

cations is the sum of calcium, magnesium, 

sodium, and potassium in

milliequivalents per liter, and

anions is the sum of chloride, fluorid

    − = − +

 

∑ ∑

∑ ∑

∑

∑ e, nitrate,

 sulfate, bicarbonate, and carbonate

in milliequivalents per liter.

A multi-step algorithm was used to select data from 

the CDPH database to fill in the gaps in the USGS-grid well 

datasets for these inorganic constituents. The algorithm 

maintained the principles of spatially distributed, randomized 

well selection originally used to identify the USGS-grid wells 

for sampling.

1. If the USGS-grid well was a well with data in the CDPH 

database, then the most recent CDPH data for that well 

for each missing inorganic constituent was used to 

supplement the USGS-GAMA data for that cell. The 

well was given an alphanumeric code name consisting 

of the same prefix and number as the USGS-well and 

separated by a second prefix “DG” (table A1). The “DG” 

second prefix indicates that the CDPH-grid well and the 

USGS-grid well are the same well. 

2. If CDPH “DG” well did not have data for all of the 

missing inorganic constituents, a second CDPH well 

could be selected to provide the remainder of the data. 

a. The cation/anion balances for the most recent 

analyses of all the CDPH wells in the cell were 

calculated. The wells with acceptable cation/anion 

balances were then divided into priority groups 

based on the number of missing constituents for 

which they could provide data. The well with the 

highest random rank among the wells in the highest 

priority group was then selected, and the most 

recent CDPH data from that well for each missing 

inorganic constituent was used to supplement the 

USGS-GAMA data for the cell. The random ranks 

were the same random ranks originally assigned to 

the CDPH wells during selection of the USGS-grid 

wells.

b. If no wells in the cell had data in the CDPH database 

with acceptable cation/anion balances, the wells 

were divided into priority groups based on the 

number of missing constituents for which they could 

provide data. The well with the highest random rank 

among the wells in the highest priority group was 

then selected, and the most recent CDPH data from 

that well for each missing inorganic constituent was 

used to supplement the USGS-GAMA data for the 

cell.

c. The well was given a name consisting of the same 

prefix and number as the USGS-grid well, separated 

by a second prefix, “DPH” (table A1). The “DPH” 

second prefix indicates that the CDPH-grid well and 

the USGS-grid well in a cell are different wells.

Appendix A: Selection of CDPH-Grid Wells
164 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

3. If there was no USGS-grid well in the grid cell, a 

CDPH-grid well was selected as described in step 2. The 

well was given a name consisting of the same prefix used 

by the USGS wells in the study area or study unit, the 

second prefix “DPH”, and the next number in the series 

after the USGS-grid well numbers (table A1). 

The result of this process was selection of one or two 

CDPH-grid wells per cell. Forty-one “DG” CDPH-grid wells 

and 34 “DPH” CDPH-grid wells were selected (figs. A1A,B,C; 

table A1). Of the 115 cells in the three study units, 44 

cells had only USGS-GAMA inorganic data, 31 cells had 

USGS-GAMA data supplemented by CDPH data, 33 cells had 

only CDPH inorganic data, and 7 cells had no inorganic data. 

Reporting limits used in the CDPH database typically 

were higher than reporting limits for the same constituents 

analyzed by USGS-GAMA (fig. 7). However, for all inorganic 

constituents, except thallium and antimony, the predominant 

reporting limit in the CDPH database was lower than half of 

the benchmark value, indicating that the moderate and low 

RCs could be distinguished. If the reporting limit were greater 

than half of the benchmark value, then a calculated proportion 

of moderate RC would be only a minimum estimate of the 

proportion. Calculation of high and moderate aquifer-scale 

proportions therefore were not affected by differences in 

reporting limits between USGS-collected and CDPH data. 

Other comparisons between the USGS-GAMA data and the 

CDPH data are described in appendix D.

The CDPH database typically did not contain data for 

all of the missing inorganic constituents at every CDPH-grid 

well; therefore, the number of wells used for the grid-based 

assessment was different for different constituents (table 2). 

For example, in the Southern Sierra study unit, 37 wells 

(7 USGS-grid and 30 CDPH-grid) were used for the gridbased assessment of nitrate, but only 15 wells (7 USGS-grid 

and 8 CDPH-grid) were used for the grid-based assessment of 

boron.
Appendix A 165

Table A1. Nomenclature for USGS-grid and CDPH-grid wells used in the status assessments for the Tahoe-Martis, Central Sierra, and 

Southern Sierra study units, 2006–2007, California GAMA Priority Basin Project.

[CDPH-grid wells are labeled “DG” if the USGS-grid and CDPH-grid wells are the same well, and are labeled “DPH” if the CDPH-well and USGS-grid well 

are different wells or if no USGS-grid well exists. Recent data from the CDPH database from CDPH-grid wells provide data for inorganic constituents for grid 

cells lacking USGS data for inorganic constituents. Identification number prefixes: TMART, Tahoe-Martis study unit, Martis study area grid well; TROCK, 

Tahoe-Martis study unit, Hard Rock study area grid well; TTAHO, Tahoe-Martis study unit, Tahoe study area; CGOLD, Central Sierra study unit, Coarse Gold 

study area grid well; CWISH, Central Sierra study unit, Wishon study area grid well; SOSA, Southern Sierra grid well. Abbreviations: CDPH, California 

Department of Public Health; USGS, U.S. Geological Survey; na, not applicable; –, no well sampled or selected]

Well identification number

USGS-grid “DG” CDPH-grid “DPH” CDPH-grid

Tahoe-Martis study unit (45 grid cells)

TMART-01 – –

TMART-02 – –

TMART-03 TMART-DG-03 –

TMART-04 TMART-DG-04 –

TMART-05 TMART-DG-05 –

TMART-06 – –

TMART-07 – –

TMART-08 TMART-DG-08 –

TMART-09 – –

TMART-10 – –

TMART-11 – –

TMART-12 – –

TMART-13 – –

TMART-14 – –

– – –

TROCK-01 – –

TROCK-02 – –

TROCK-03 – TROCK-DPH-03

TROCK-04 – TROCK-DPH-04

TROCK-05 TROCK-DG-05 –

TROCK-06 TROCK-DG-06 TROCK-DPH-06

TROCK-07 TROCK-DG-07 –

TROCK-08 – –

TROCK-09 – –

TROCK-10 – –

TROCK-11 – –

TROCK-12 – TROCK-DPH-12

TROCK-13 – –

– – –

– – –

TTAHO-01 – TTAHO-DPH-01

TTAHO-02 – –

TTAHO-03 TTAHO-DG-03 –

TTAHO-04 TTAHO-DG-04 TTAHO-DPH-04

TTAHO-05 – –

TTAHO-06 – –

TTAHO-07 – TTAHO-DPH-07

TTAHO-08 TTAHO-DG-08 –

TTAHO-09 – TTAHO-DPH-09

TTAHO-10 – –

TTAHO-11 – –

TTAHO-12 TTAHO-DG-12 –

TTAHO-13 – –

TTAHO-14 TTAHO-DG-14 TTAHO-DPH-14

– – –

Well identification number

USGS-grid “DG” CDPH-grid “DPH” CDPH-grid

Central Sierra study unit (30 grid cells)

CGOLD-01 – –

CGOLD-02 – –

CGOLD-03 – –

CGOLD-04 – –

CGOLD-05 CGOLD-DG-05 –

CGOLD-06 – –

CGOLD-07 – –

CGOLD-08 – –

CGOLD-09 – –

CGOLD-10 – –

CGOLD-11 – CGOLD-DPH-11

CGOLD-12 CGOLD-DG-12 –

CGOLD-13 CGOLD-DG-13 –

CGOLD-14 – CGOLD-DPH-14

CGOLD-15 CGOLD-DG-15 –

CGOLD-16 – CGOLD-DPH-16

CGOLD-17 – –

CGOLD-18 – CGOLD-DPH-18

– – CGOLD-DPH-19

– – –

CWISH-01 – –

CWISH-02 – –

CWISH-03 – –

CWISH-04 – –

CWISH-05 – –

CWISH-06 – CWISH-DPH-06

CWISH-07 – –

CWISH-08 – –

CWISH-09 – –

– – CWISH-DPH-10

Southern Sierra study unit (40 grid cells)

SOSA-01 SOSA-DG-01 –

SOSA-02 SOSA-DG-02 –

SOSA-03 – –

SOSA-04 SOSA-DG-04 –

SOSA-05 SOSA-DG-05 –

SOSA-06 SOSA-DG-06 SOSA-DPH-06

SOSA-07 – SOSA-DPH-07

SOSA-08 SOSA-DG-08 SOSA-DPH-08

SOSA-09 SOSA-DG-09 –

SOSA-10 SOSA-DG-10 –

SOSA-11 SOSA-DG-11 SOSA-DPH-11

SOSA-12 – SOSA-DPH-12

SOSA-13 – SOSA-DPH-13
166 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Table A1. Nomenclature for USGS-grid and CDPH-grid wells used in the status assessments for the Tahoe-Martis, Central Sierra, and 

Southern Sierra study units, 2006–2007, California GAMA Priority Basin Project.—Continued

[CDPH-grid wells are labeled “DG” if the USGS-grid and CDPH-grid wells are the same well, and are labeled “DPH” if the CDPH-well and USGS-grid well 

are different wells or if no USGS-grid well exists. Recent data from the CDPH database from CDPH-grid wells provide data for inorganic constituents for grid 

cells lacking USGS data for inorganic constituents. Identification number prefixes: TMART, Tahoe-Martis study unit, Martis study area grid well; TROCK, 

Tahoe-Martis study unit, Hard Rock study area grid well; TTAHO, Tahoe-Martis study unit, Tahoe study area; CGOLD, Central Sierra study unit, Coarse Gold 

study area grid well; CWISH, Central Sierra study unit, Wishon study area grid well; SOSA, Southern Sierra grid well. Abbreviations: CDPH, California 

Department of Public Health; USGS, U.S. Geological Survey; na, not applicable; –, no well sampled or selected]

Well identification number

USGS-grid “DG” CDPH-grid “DPH” CDPH-grid

Southern Sierra study unit (40 grid cells)—Continued

SOSA-14 SOSA-DG-14 –

SOSA-15 – SOSA-DPH-15

SOSA-16 SOSA-DG-16 –

SOSA-17 SOSA-DG-17 –

SOSA-18 SOSA-DG-18 SOSA-DPH-18

SOSA-19 SOSA-DG-19 –

SOSA-20 SOSA-DG-20 –

SOSA-21 SOSA-DG-21 SOSA-DPH-21

SOSA-22 – –

SOSA-23 SOSA-DG-23 –

SOSA-24 SOSA-DG-24 –

SOSA-25 SOSA-DG-25 –

SOSA-26 SOSA-DG-26 –

SOSA-27 SOSA-DG-27 –

Well identification number

USGS-grid “DG” CDPH-grid “DPH” CDPH-grid

Southern Sierra study unit (40 grid cells)—Continued

SOSA-28 – SOSA-DPH-28

SOSA-29 SOSA-DG-29 –

SOSA-30 SOSA-DG-30 SOSA-DPH-30

SOSA-31 SOSA-DG-31 SOSA-DPH-31

SOSA-32 – SOSA-DPH-32

SOSA-33 – SOSA-DPH-33

SOSA-34 SOSA-DG-34 SOSA-DPH-34

SOSA-35 – –

– – SOSA-DPH-36

– – SOSA-DPH-37

– – SOSA-DPH-38

– – –

– – –
Appendix A 167

sac09-340_fig A01a

0 5 10 MILES

0 5 10 KILOMETERS

120° 15’

39° 15’

39°

120°

-03

-12

-09

-01

-01

-U-01

-DPH-03 -10

-06 -07

-13

-10

-13 TROCK

TTAHO

DPH-12

-12

-04

-11

-14

-02

-02

-05

-DPH-01

-DPH-04

-DG-04

-DG-03

-DG-14

-DPH-14

-DG-07

-02

-08

-09

-11

-01

-DG-05

-DG-03

-DG-08

-DG-04

-DG-05

-DPH-04

-DG-06

-DPH-06

-U-01

-U-02

TMART

Martis (cell size 9.8 km2; label prefix TMART)

Tahoe (cell size 6.2 km2; label prefix TTAHO)

USGS-grid

USGS-grid and CDPH-DG-grid

CDPH-DPH-grid

USGS-understanding

Hard Rock (cell size 30 km2; label prefix TROCK)

Study area grid cells

Wells

Grid cell boundary

A EXPLANATION

120°

-09 -U-02

-U-03

-U-01

-U-04

-U-08

-13

-U-07

-U-06

-U-05

-DG-08

-DG-12

-11

-10 -07

-06

-DPH-09

-DPH-07

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006.

Albers Equal Area Conic Projection

North American Datum of 1983 (NAD 83)

Figure A1. Maps showing locations of grid cells and USGS-grid, CDPH-grid, and USGS-understanding wells in the 

(A) Tahoe-Martis, (B) Central Sierra, and (C) Southern Sierra study units, California GAMA Priority Basin Project.
168 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac09-0340_fig A01b

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006,

Albers Equal Area Conic Projection

37° 30’

37° 15’

119° 45’ 119° 30’

B

"

0 5 10 MILES

0 5 10 KILOMETERS

USGS-grid

USGS-grid and CDPH-DG-grid

CDPH-DPH-grid

USGS-understanding

Wells

EXPLANATION

Coarse Gold study area (cell size 25 km2; label prefix CGOLD)

Wishon study area (cell size 18 km2; label prefix CWISH)

Study area grid cells

Study unit boundary

CGOLD CWISH

Grid cell boundary

-07

-16

-08

-04

-03

-05

-01

-07 -02

-06

-DPH-06

-U-01 -DPH-10

-09

-DPH-16

-DPH-18

-DPH-14

-DPH-11

-U-01

-DG-13

-DG-05

-18

-08

-10

-11

-17

-01

-09

-03

-02

-DPH-19

-DG-12

-04

-06

U-02 -14

-15

Figure A1.—Continued
Appendix A 169

sac09-0340_fig A01c

USGS-grid

USGS-grid and CDPH-DG-grid

CDPH-DPH-grid

USGS-understanding

Study unit boundary

Grid cell boundary

Wells

EXPLANATION

Cell size 26 km2; label prefix SOSA

Grid cells

10 MILES

50 10 KILOMETERS

0 5

35°30’

35°15’

35°

118°30’ 118°15’

36°

35°45’

C

-DG-29

-DPH-12

-DPH-07

-DPH-21

-DG-19

-DPH-18

-DPH-38

-DPH-30

-DG-31

-DG-30

-DPH-31

-DG-10

-DG-16

-DG-14

-DPH-36

-DG-20

-DG-17

-DPH-10

-DG-04

-DG-05

-DG-06

-33 -35

-28

-DPH-06

-DG-26

-DG-27

-DG-25

-DG-24

-DG-08

-DPH-08 -DPH-11

-DG-01

-DG-11

-DG-09

-DPH-28

-DPH-33

-DG-02

-DPH-15

-15

-18

-03

-DG-21

FP-02

-22

-12

-07

FP-04

-DPH-32

-DG-34

-DPH-34

-DPH-13 FP-03

FP-15

FP-14

FP-13

FP-05

FP-01

FP-06

-32

-13

-DG-23

FP-10

FP-12

FP-08

FP-09

FP-11

FP-07

Figure A1.—Continued
170 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

The methods used to calculate aquifer-scale proportions 

using the grid-based and spatially weighted approaches are 

described in this appendix. 

Grid-Based Calculation

In the grid-based approach, one value per grid cell is used 

to represent the primary aquifer. A hypothetical example is 

shown in figure B1A. The proportion of the primary aquifer 

with high RCs is calculated by dividing the number of cells 

represented by a high RC for that constituent by the total 

number of grid cells with data for that constituent. Proportions 

of moderate and low RCs are calculated similarly. In the 

example, 1 of the 16 cells with data for constituent A is 

represented by a high RC, thus the high proportion is 6.25%. 

Similarly, the moderate proportion for constituent A is 19% 

(3 of 16 cells are represented by moderate RCs). Only 12 of 

the cells have data for constituent B; the high proportion for 

B is 17% (2 of 12 cells), and the moderate proportion is 25% 

(3 of 12 cells).

The grid-based aquifer-scale proportions for constituent 

classes also are calculated on a one-value-per-grid-cell basis. A 

cell with a high RC for any constituent in the class is defined 

as a high cell, and the high proportion is the number of high 

cells divided by the number of cells with data for any of the 

constituents in the class. For the example shown in figure B1A, 

cells 4 and 11 are defined as high cells, and the high 

proportion is 12.5% (2 of 16). The moderate proportion for 

the constituent class is calculated similarly, except that a cell 

already defined as high cannot also be defined as moderate. A 

cell with a moderate RC for any constituent in the class that 

does not also have a high RC for any constituent in the class 

is defined as moderate. For the example shown in figure B1A, 

cells 3, 5, 7, and 12 are defined as moderate, and the moderate 

proportion is 25% (4 of 16). 

The Tahoe-Martis and Central Sierra study units were 

subdivided into study areas, each of which had different sized 

cells. A hypothetical example of this situation is shown in 

figure B1B. The study unit is divided into two study areas—

SA1, which has 12 equal-area cells, and SA2, which has 

9 equal-area cells. SA1 covers 75% of the study unit, and 

SA2 covers 25%. The cells in SA1 are larger than the cells 

in SA2. Now suppose that a constituent has high RCs in one 

cell of SA1 and in four cells of SA2. The detection frequency 

of high RCs of the constituent in the grid wells is 24% (5 of 

21 wells), but this detection frequency in the grid wells is not 

equal to the high aquifer-scale proportion for the constituent in 

the study unit. The proportion for the study unit is determined 

by calculating the aquifer-scale proportions in each study area 

separately, and then calculating the area-weighted sum:

,

where

is the aquifer-scale proportion for the study unit,

is the aquifer-scale proportion for a study area,

 and

is the fraction of the total study unit area occupied

by the study area

SU SA SA

SU

SA

SA

P PF

P

P

F

= ∑

.

For the example in figure B1B, the study unit aquifer-scale 

proportion is 17%. 

1 4 0.17 0.75 0.25 .

12 9

   = +      

Spatially Weighted Calculation

The spatially weighted calculations of aquifer-scale 

proportions included all wells in the CDPH database with 

data in the 3-year interval prior to USGS-GAMA sampling 

in the study unit, all the USGS-grid wells, and most of the 

USGS-understanding wells. All of the USGS-understanding 

wells in the Central Sierra and Southern Sierra study units 

were included because they were either wells listed in the 

CDPH database, or were wells with screened or open intervals 

similar to those of nearby CDPH wells. All of the USGSunderstanding wells in the Tahoe-Martis study unit were 

included, except for four monitoring wells with short screened 

or open intervals at depths not similar to nearby CDPH wells 

(TTAHOU-05, -06, -07, and -08). 

Appendix B. Calculation of Aquifer-Scale Proportions
Appendix B 171

sac11-0340_figB01ab

1 2 3 4

8 7 6 5

9 10 11 12

16 15 14 13

1 2 3 4

8 7 6 5

9 10

12 11

1 2 3

4

7 8

6 5

9

Constituent

A B

Relative-concentration

Grid cell

SA1 grid cell

SA2 grid cell

8

8

8

A

B

EXPLANATION

EXPLANATION

 High

 Moderate

 Low

 No data

Relative-concentration

 High

 Moderate

 Low

Figure B1. Diagrams demonstrating calculation of aquifer-scale proportions for constituents using the 

grid-based approach in a study unit with (A) one grid cell size and (B) study areas with different grid cell sizes.
172 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

For each constituent, the high aquifer-scale proportion 

was calculated by (1) computing the proportion of wells 

having high RCs in each grid cell, and then (2) averaging 

the grid cell values computed in step (1) (Isaaks and 

Srivastava, 1989):

,

where

is the number of cells with data for the constituen

is the aquifer-scale proportion for the study unit,

is the number of wells in a given cell, and

is the number 

,

o

t

high

total

SU

SU

total

high

W

W P

C

P

C

W

W

=

∑

f wells in a given cell with high RC

for the constituent.

The same procedures were used to calculate the proportions 

of moderate and low concentrations. The resulting proportions 

are spatially unbiased (Isaaks and Srivastava, 1989). 

For the example shown in figure B2, the high 

aquifer-scale proportion is 14%:

0010000 0 9

3 5 3 0 4 1 2 11 11 0.14 . 8

+++++++ +

=

The moderate aquifer-scale proportion is 39%.

Raw Detection Frequencies

The raw detection frequencies of wells with high RCs 

for constituents were calculated using the same data as used 

for the spatially weighted approach. However, raw detection 

frequencies are not spatially unbiased because the wells in the 

CDPH database are not uniformly distributed. Consequently, 

high RCs occurring in spatially clustered wells in a particular 

area representing a small part of the primary aquifer could 

be given a disproportionately high weight compared to 

spatially unbiased methods. Raw detection frequencies of high 

concentrations are provided for reference in this report but 

were not used to assess aquifer-scale proportions. 

For the example shown in figure B2, the raw detection 

frequencies of high and moderate RCs are both 25% (10 

of 40 wells). The raw detection frequency of high RCs is 

greater than the high aquifer-scale proportion because the 

wells with high RCs primarily are clustered into one cell. 

Cell 9 represents one-eighth of the total area of the study 

unit, and contains 9 of the 10 wells with high concentrations. 

The raw detection frequency for moderate RCs is less than 

the moderate aquifer-scale proportion because the wells with 

moderate RCs are dispersed among multiple cells, each with 

relatively few wells (cells 2, 3, 5, and 6). Half of the total area 

of the study unit therefore is characterized by moderate or 

mostly moderate RCs. 

1 2 3

4

7 8

6 5

9 Grid cell 8

EXPLANATION

Relative-concentration

 High

 Moderate

 Low

Figure B2. Diagram demonstrating calculation of aquifer-scale proportions for a constituent 

using the spatially weighted approach.
Appendix B 173

Study Area Aquifer-Scale Proportion Results

Raw detection frequencies, spatially weighted 

aquifer-scale proportions, and grid-based aquifer-scale 

proportions for constituents in the TTAHO, TMART, and 

TROCK study areas of the Tahoe-Martis study unit are 

listed in tables B1A,B,C. Each study area had 15 cells, but 

covered different areal percentages of the total study-unit 

area: TTAHO 14%, TMART 22%, and TROCK 64%. If the 

area-weighting calculation had not been done, then TTAHO, 

and to a lesser extent, TMART, cells would have contributed 

more to the overall study-unit aquifer-scale proportions than 

warranted for an equal-area result. Summaries of aquiferscale proportions for constituent classes by study area are 

given in tables B2A,B for inorganic constituent classes and in 

table B2C for organic constituent classes.

Groundwater quality differed significantly between the 

primary aquifers of the three Tahoe-Martis study unit study 

areas. The only inorganic constituent with a health-based 

benchmark detected at high RCs in the TMART study area 

was arsenic (table B1B). Arsenic also was detected at high 

RCs in the TROCK and TTAHO study areas (tables B1A,C). 

TTAHO was the only study area with radioactive constituents 

detected at high RCs (table B1A). 

Organic constituents were detected in 36, 8, and 50% of 

the primary aquifers in the TMART, TROCK, and TTAHO 

study areas, respectively (table B2C). Pesticides were only 

detected in the TMART study area. No organic constituents 

were detected at moderate or high RCs in the TMART study 

area (table B2C). Several solvent constituents were detected 

at moderate and high RCs in approximately 1% each of the 

TROCK and TTAHO study areas (tables B1A,C).

Raw detection frequencies, spatially weighted 

aquifer-scale proportions, and grid-based aquifer-scale 

proportions for constituents in the CGOLD and CWISH 

study areas of the Central Sierra study unit are listed in 

tables B1D,E. The CGOLD study area has 20 cells and 

composes 73% of the study unit area; the CWISH study area 

has 10 cells and composes 27% of the study unit area. If the 

area-weighting calculation had not been done, then CWISH 

cells would have contributed more to the overall study unit 

aquifer-scale proportions than warranted for an equal-area 

result. Summaries of aquifer-scale proportions for constituent 

classes by study area are given in table B2A for organic 

constituent classes and in table B2B for inorganic constituent 

classes.

Groundwater quality differed significantly between 

the primary aquifers of the two study areas in the Central 

Sierra study unit. Eighty percent of the primary aquifer in the 

CWISH study area had high RCs of at least one inorganic 

constituent with a health-based benchmark, compared to 26% 

of the primary aquifer in the CGOLD study area (table B2A). 

The high aquifer-scale proportions of arsenic, gross alpha 

particle radiation, and uranium each were ≥ 40% in the 

CWISH study area (table B1E). Fluoride, molybdenum, 

and radon-222 also were present at high RCs. All of the 

same constituents were present at high RCs in the CGOLD 

study area, but in lower proportions of the primary aquifer 

(table B1D). High RCs of manganese and iron were detected 

in both study areas; high RCs of TDS were only detected in 

the CGOLD study area. Organic constituents were detected 

in 11 and 28% of the primary aquifers in the CWISH and 

CGOLD study areas, respectively (table B2C), and both study 

areas had detections of organic constituents at moderate RCs 

(tables B1D,E).
174 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Table B1A. Aquifer-scale proportions calculated using grid-based and spatially weighted methods for those constituents that met 

criteria for additional evaluation in the status assessment, Tahoe study area, Tahoe-Martis study unit, 2006–2007, California GAMA 

Priority Basin Project.

[Tahoe study area covers 13.99 percent of the Tahoe-Martis study unit. Relative-concentration categories: high, concentrations greater than water-quality 

benchmark; moderate, concentrations greater than or equal to 0.1 of benchmark but less than benchmark for organic constituents (threshold for inorganic 

constituents is 0.5 of benchmark); low, concentrations less than 0.1 of benchmark for organic constituents (threshold for inorganic constituents is 0.5 of 

benchmark). Benchmark types and values listed in table 3. Other abbreviations: SMCL, secondary maximum contaminant level]

Constituent

Raw detection frequency1

(percent)

Spatially weighted

aquifer-scale 

proportion1

(percent)

Grid-based

aquifer-scale

proportion2

(percent)

Number

of wells

Percent

moderate

Percent

high

Number

of cells

Proportion 

moderate

Proportion 

high

Number 

of cells

Proportion 

moderate

Proportion 

high

Trace elements

Arsenic 45 13 16 14 9.2 15 14 14 14

Boron 22 0 4.5 14 0 7.1 14 0 7.1

Molybdenum 18 5.6 5.6 14 3.6 3.6 14 0 7.1

Strontium 18 0 0 14 0 0 14 0 0

Uranium 18 17 0 14 14 0 14 14 0

Inorganic constituents with SMCL benchmarks

Iron 43 2.3 9.3 14 1.2 12 14 0 14

Manganese 44 0 6.8 14 0 11 14 0 14

Chloride 44 2.3 0 14 2.4 0 14 7.1 0

Specific conductance 45 0 2.2 14 0 2.4 14 0 7.1

Sulfate 44 0 0 14 0 0 14 0 0

Total dissolved solids (TDS) 45 0 2.2 14 0 2.4 14 0 7.1

Radioactive constituents

Adjusted gross alpha activity 34 2.9 5.9 13 1.9 3.8 13 7.7 7.7

Gross alpha particle activity 34 21 18 13 14 13 13 15 15

Radium activity 28 3.6 0 11 3.0 0 11 9.1 0

Radon-222 activity 35 8.6 11 14 4.2 6.1 14 7.1 7.1

Uranium activity 42 24 0 14 18 0 14 21 0

Volatile organic compounds - solvents

Carbon tetrachloride 48 0 0 14 0 0 14 0 0

1,2-Dichloroethane 48 0 2.1 14 0 1.2 14 0 0

Chloroform 48 0 0 14 0 0 14 0 0

Chloromethane 48 0 0 14 0 0 14 0 0

Tetrachloroethylene (PCE) 48 4.2 2.1 14 2.4 1.2 14 0 7.1

Volatile organic compounds - fumigants

1,2-Dichloropropane 48 0 0 14 0 0 14 0 0

DBCP 36 0 2.8 14 0 1.2 14 0 0

1

 Based on most recent analyses for each California Department of Public Health (CDPH) well during May 1, 2004–April 30, 2007, combined with data from 

U.S. Geological Survey (USGS) grid wells, and selected USGS-understanding wells.

2

 Grid-based aquifer-scale proportions for organic constituents are based on samples collected by the USGS from 41 grid wells during June–October 2007.
Appendix B 175

Table B1B. Aquifer-scale proportions calculated using grid-based and spatially weighted methods for those constituents that met 

criteria for additional evaluation in the status assessment, Martis study area, Tahoe-Martis study unit, 2006–2007, California GAMA 

Priority Basin Project.

[Martis study area covers 22.11 percent of the Tahoe-Martis study unit. Relative-concentration categories: high, concentrations greater than water-quality 

benchmark; moderate, concentrations greater than or equal to 0.1 of benchmark but less than benchmark for organic constituents (threshold for inorganic 

constituents is 0.5 of benchmark); low, concentrations less than 0.1 of benchmark for organic constituents (threshold for inorganic constituents is 0.5 of 

benchmark). Benchmark types and values listed in table 3. Other abbreviations: SMCL, secondary maximum contaminant level]

Constituent

Raw detection frequency1

(percent)

Spatially weighted

aquifer-scale 

proportion1

(percent)

Grid-based

aquifer-scale

proportion2

(percent)

Number

of wells

Percent

moderate

Percent

high

Number

of cells

Proportion 

moderate

Proportion 

high

Number 

of cells

Proportion 

moderate

Proportion 

high

Trace elements

Arsenic 16 13 31 14 7.1 32 14 7.1 29

Boron 15 6.7 0 14 7.1 0 14 7.1 0

Molybdenum 15 0 0 14 0 0 14 0 0

Strontium 15 0 0 14 0 0 14 0 0

Uranium 16 0 0 14 0 0 14 0 0

Inorganic constituents with SMCL benchmarks

Iron 16 0 0 14 0 0 14 0 0

Manganese 16 0 6.3 14 0 7.1 14 0 7.1

Chloride 16 0 0 14 0 0 14 0 0

Specific conductance 16 0 0 14 0 0 14 0 0

Sulfate 16 0 0 14 0 0 14 0 0

Total dissolved solids (TDS) 16 0 0 14 0 0 14 0 0

Radioactive constituents

Adjusted gross alpha activity 10 0 0 7 0 0 6 0 0

Gross alpha particle activity 10 0 0 7 0 0 6 0 0

Radium activity 3 0 0 3 0 0 2 0 0

Radon-222 activity 15 0 0 14 0 0 14 0 0

Uranium activity 16 0 0 14 0 0 14 0 0

Volatile organic compounds - solvents

Carbon tetrachloride 16 0 0 14 0 0 14 0 0

1,2-Dichloroethane 16 0 0 14 0 0 14 0 0

Chloroform 16 0 0 14 0 0 14 0 0

Chloromethane 16 0 0 14 0 0 14 0 0

Tetrachloroethylene (PCE) 16 0 0 14 0 0 14 0 0

Volatile organic compounds - fumigants

1,2-Dichloropropane 16 0 0 14 0 0 14 0 0

DBCP 16 0 0 14 0 0 14 0 0

1

 Based on most recent analyses for each California Department of Public Health (CDPH) well during May 1, 2004–April 30, 2007, combined with data from 

U.S. Geological Survey (USGS) grid wells, and selected USGS-understanding wells.

2

 Grid-based aquifer-scale proportions for organic constituents are based on samples collected by the USGS from 41 grid wells during June–October 2007.
176 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Table B1C. Aquifer-scale proportions calculated using grid-based and spatially weighted methods for those constituents that met 

criteria for additional evaluation in the status assessment, Hard Rock study area, Tahoe-Martis study unit, 2006–2007, California GAMA 

Priority Basin Project.

[Hard Rock study area covers 63.90 percent of the Tahoe-Martis study unit. Relative-concentration categories: high, concentrations greater than water-quality 

benchmark; moderate, concentrations greater than or equal to 0.1 of benchmark but less than benchmark for organic constituents (threshold for inorganic 

constituents is 0.5 of benchmark); low, concentrations less than 0.1 of benchmark for organic constituents (threshold for inorganic constituents is 0.5 of 

benchmark). Benchmark types and values listed in table 3. Other abbreviations: SMCL, secondary maximum contaminant level]

Constituent

Raw detection frequency1

(percent)

Spatially weighted

aquifer-scale 

proportion1

(percent)

Grid-based

aquifer-scale

proportion2

(percent)

Number

of wells

Percent

moderate

Percent

high

Number

of cells

Proportion 

moderate

Proportion 

high

Number 

of cells

Proportion 

moderate

Proportion 

high

Trace elements

Arsenic 34 0 8.8 13 0 12 13 0 15

Boron 19 0 5.3 13 0 7.7 13 0 7.7

Molybdenum 15 0 6.7 13 0 7.7 13 0 7.7

Strontium 15 0 6.7 13 0 7.7 13 0 7.7

Uranium 15 0 0 13 0 0 13 0 0

Inorganic constituents with SMCL benchmarks

Iron 36 2.8 8.3 13 0 6.4 13 0 7.7

Manganese 34 0 5.9 13 0 12 13 0 15

Chloride 34 0 0 13 0 0 13 0 0

Specific conductance 34 0 5.7 13 7.7 3.8 13 7.7 7.7

Sulfate 34 0 2.9 13 0 3.8 13 0 7.7

Total dissolved solids (TDS) 35 0 5.7 13 0 12 13 0 15

Radioactive constituents

Adjusted gross alpha activity 16 0 0 5 0 0 5 0 0

Gross alpha particle activity 16 6.3 0 5 2.5 0 5 0 0

Radium activity 8 0 0 5 0 0 4 0 0

Radon-222 activity 17 12 0 12 17 0 12 17 0

Uranium activity 17 0 0 13 0 0 13 0 0

Volatile organic compounds - solvents

Carbon tetrachloride 32 0 3.1 13 0 0.7 13 0 0

1,2-Dichloroethane 32 0 0 13 0 0 13 0 0

Chloroform 32 0 0 13 0 0 13 0 0

Chloromethane 32 3.1 0 13 0.7 0 13 0 0

Perchloroethene (PCE) 32 0 0 13 0 0 13 0 0

Volatile organic compounds - fumigants

1,2-Dichloropropane 32 0 3.1 13 0 0.7 13 0 0

DBCP 20 0 0 13 0 0 13 0 0

1

 Based on most recent analyses for each California Department of Public Health (CDPH) well during May 1, 2004–April 30, 2007, combined with data from 

U.S. Geological Survey (USGS) grid wells, and selected USGS-understanding wells.

2

 Grid-based aquifer-scale proportions for organic constituents are based on samples collected by the USGS from 41 grid wells during June–October 2007.
Appendix B 177

Table B1D. Aquifer-scale proportions calculated using grid-based and spatially weighted methods for those constituents that met 

criteria for additional evaluation in the status assessment, Coarse Gold study area, Central Sierra study unit, 2006–2007, California 

Groundwater GAMA Priority Basin Project.

[Course Gold area covers 72.58 percent of the Central Sierra study unit. Relative-concentration categories: high, concentrations greater than water-quality 

benchmark; moderate, concentrations greater than or equal to 0.1 of benchmark but less than benchmark for organic constituents (threshold for inorganic 

constituents is 0.5 of benchmark); low, concentrations less than 0.1 of benchmark for organic constituents (threshold for inorganic constituents is 0.5 of 

benchmark). Benchmark types and values listed in table 3. Other abbreviations: SMCL, secondary maximum contaminant level]

Constituent

Raw detection frequency1

(percent)

Spatially weighted

aquifer-scale 

proportion1

(percent)

Grid-based

aquifer-scale

proportion2

(percent)

Number

of wells

Percent

moderate

Percent

high

Number

of cells

Proportion 

moderate

Proportion 

high

Number 

of cells

Proportion 

moderate

Proportion 

high

Nutrients

Nitrate 129 4.7 0 19 4.2 0 19 0 0

Trace elements

Arsenic 72 13 18 19 21 17 19 26 5.3

Boron 23 8.7 0 18 5.6 0 18 5.6 0

Chromium 67 0 1.5 19 0 0.6 19 0 0

Fluoride 76 2.6 2.6 19 3.2 6.3 19 5.3 5.3

Lead 71 2.8 0 19 3.2 0 19 0 0

Molybdenum 20 30 10 18 28 8.3 18 28 11

Selenium 67 0 1.5 19 0 0.6 19 0 0

Uranium 41 20 4.9 18 17 2.8 18 17 0

Inorganic constituents with SMCL benchmarks

Iron 77 2.6 27 19 3.2 23 19 0 16

Manganese 77 7.8 40 19 0 34 19 5.3 37

Zinc 75 2.7 0 19 3.4 0 19 5.3 0

Chloride 76 2.6 3.9 19 1.4 1.8 19 0 0

Specific conductance 76 6.6 3.9 19 4.1 1.8 19 0 0

Total dissolved solids (TDS) 71 5.6 4.2 19 3.0 1.9 19 0 0

Radioactive constituents

Adjusted gross alpha activity 87 5.7 0 17 3.2 0 17 5.9 0

Gross alpha particle activity 87 16 13 17 15 13 17 24 5.9

Radium activity 9 0 0 10 0 0 10 0 0

Radon-222 activity 9 33 22 9 33 22 9 33 22

Uranium activity 62 16 15 18 12 5.8 18 17 0

Pesticide

Simazine 75 0 0 19 0 0 19 0 0

Volatile organic compounds

Benzene 61 1.6 0 18 2.8 0 18 5.6 0

Chloroform 64 1.6 0 18 0.7 0 18 0 0

Perchloroethene (PCE) 61 1.6 0 18 0.7 0 18 0 0

Methyl tert-butyl ether (MTBE) 61 3.3 0 18 2.2 0 18 0 0

1,4-Dichlorobenzene 61 0 0 18 0 0 18 0 0

1

 Based on most recent analyses for each California Department of Public Health (CDPH) well during June 1, 2003–May 31, 2006, combined with data from 

U.S. Geological Survey (USGS) grid wells, and selected USGS-understanding wells.

2

 Grid-based aquifer-scale proportions for organic constituents are based on samples collected by the USGS from 27 grid wells during May 2006.
178 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Table B1E. Aquifer-scale proportions calculated using grid-based and spatially weighted methods for those constituents that met 

criteria for additional evaluation in the status assessment, Wishon study area, Central Sierra study unit, 2006–2007, California GAMA 

Priority Basin Project.

[Wishon study area covers 27.42 percent of the Central Sierra study unit. Relative-concentration categories: high, concentrations greater than water-quality 

benchmark; moderate, concentrations greater than or equal to 0.1 of benchmark but less than benchmark for organic constituents (threshold for inorganic 

constituents is 0.5 of benchmark); low, concentrations less than 0.1 of benchmark for organic constituents (threshold for inorganic constituents is 0.5 of 

benchmark). Benchmark types and values listed in table 3. Other abbreviations: SMCL, secondary maximum contaminant level]

Constituent

Raw detection frequency1

(percent)

Spatially weighted

aquifer-scale 

proportion1

(percent)

Grid-based

aquifer-scale

proportion2

(percent)

Number

of wells

Percent

moderate

Percent

high

Number

of cells

Proportion 

moderate

Proportion 

high

Number 

of cells

Proportion 

moderate

Proportion 

high

Nutrients

Nitrate 36 0 0 10 0 0 19 0 0

Trace elements

Arsenic 26 3.8 35 10 0 26 10 0 40

Boron 10 30 0 9 33 0 9 33 0

Chromium 23 0 0 10 0 0 10 0 0

Fluoride 24 17 4.2 10 12 10 10 20 10

Lead 24 4.2 0 10 10 0 10 10 0

Molybdenum 10 40 10 9 33 11 9 33 11

Selenium 23 0 0 10 0 0 10 0 0

Uranium 12 0 58 9 0 50 9 0 56

Inorganic constituents with SMCL benchmarks

Iron 24 8.3 13 10 5.8 7.9 10 10 0

Manganese 24 4.2 21 10 7.1 15 10 0 20

Zinc 23 4.3 0 10 7.5 0 10 10 0

Chloride 24 0 0 10 0 0 10 0 0

Specific conductance 24 0 0 10 0 0 10 0 0

Total dissolved solids (TDS) 24 0 0 10 0 0 10 0 0

Radioactive constituents

Adjusted gross alpha activity 24 4.3 8.7 7 2.4 4.8 7 14 14

Gross alpha particle activity 24 17 38 7 16 45 7 29 43

Radium activity 5 20 0 5 20 0 5 20 0

Radon-222 activity 5 0 100 5 0 100 5 0 100

Uranium activity 15 13 47 10 10 52 10 10 50

Pesticide

Simazine 13 0 0 9 0 0 9 0 0

Volatile organic compounds

Benzene 23 0 0 9 0 0 9 0 0

Chloroform 23 4.3 0 9 2.2 0 9 0 0

Perchloroethene (PCE) 23 0 0 9 0 0 9 0 0

Methyl tert-butyl ether (MTBE) 23 0 0 9 0 0 9 0 0

1,4-Dichlorobenzene 23 4.3 0 9 1.6 0 9 0 0

1

 Based on most recent analyses for each California Department of Public Health (CDPH) well during June 1, 2003–May 31, 2006, combined with data from 

U.S. Geological Survey (USGS) grid wells, and selected USGS-understanding wells.

2

 Grid-based aquifer-scale proportions for organic constituents are based on samples collected by the USGS from 27 grid wells during May 2006.
Appendix B 179

Table B2A. Summary of aquifer-scale proportions calculated using the grid-based method for study areas 

for inorganic constituent classes with health-based benchmarks, Tahoe-Martis and Central Sierra study units, 

2006–2007, California GAMA Priority Basin Project.

[Relative-concentration categories: high, concentration of at least one constituent in group greater than water-quality benchmark; 

moderate, concentration of at least one constituent in group greater than 0.5 of benchmark and no constituents in group with 

concentration greater than benchmark; low, concentrations of all constituents in group less than or equal to 0.5 of benchmark. 

TMART, Tahoe-Martis study unit, Martis study area; TROCK, Tahoe-Martis study unit, Hard Rock study area; TTAHO, TahoeMartis study unit, Tahoe study area; CGOLD, Central Sierra study unit, Coarse Gold study area; CWISH, Central Sierra study unit, 

Wishon study area]

Number

of cells

Aquifer-scale proportion

(percent)

Detected at low 

relative-concentration

Detected 

at moderate 

relativeconcentration

Detected at

high relativeconcentration

Radioactive constituents1

Tahoe-Martis

TMART 14 100 0 0

TROCK 13 85 15 0

TTAHO 14 64 14 21

Central Sierra

CGOLD 18 56 28 17

CWISH 10 10 20 70

Nutrients

Tahoe-Martis

TMART 14 100 0 0

TROCK 13 100 0 0

TTAHO 14 100 0 0

Central Sierra

CGOLD 19 96 2

4.2 0

CWISH 10 100 0 0

Trace elements

Tahoe-Martis

TMART 14 64 7.1 29

TROCK 13 85 0 15

TTAHO 14 64 14 21

Central Sierra

CGOLD 19 47 37 16

CWISH 10 30 20 50

All inorganic constituents with health-based benchmarks3

Tahoe-Martis

TMART 14 64 7.1 29

TROCK 13 69 15 15

TTAHO 14 57 14 29

Central Sierra

CGOLD 19 32 42 26

CWISH 10 0 20 80

1

 Aquifer-scale proportions for radioactive constituents were calculated using gross alpha activity. If adjusted gross alpha activity 

were used instead, the high and moderate aquifer-scale proportions would be as follows: TMART, unchanged; TROCK, unchanged; 

TTAHO, 14 percent high, 21 percent moderate; CGOLD, 11 percent high, 33 percent moderate; and CWISH, unchanged.

2

 Based on spatially weighted calculation.

3

 Aquifer-scale proportions for all inorganic constituents with health-based benchmarks were calculated using gross alpha activity. 

If adjusted gross alpha activity were used instead, the high and moderate aquifer-scale proportions would be as follows: TMART, 

unchanged; TROCK, unchanged; TTAHO, 21 percent high, 21 percent moderate; CGOLD, unchanged; and CWISH, unchanged.
180 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Table B2B. Summary of aquifer-scale proportions calculated using the grid-based method 

for study areas for inorganic constituent classes with SMCL benchmarks, Tahoe-Martis and 

Central Sierra study units, 2006–2007, California GAMA Priority Basin Project.

[Relative-concentration categories: high, concentration of at least one constituent in group greater than waterquality benchmark; moderate, concentration of at least one constituent in group greater than 0.5 of benchmark 

and no constituents in group with concentration greater than benchmark; low, concentrations of all constituents 

in group less than or equal to 0.5 of benchmark. TMART, Tahoe-Martis study unit, Martis study area; TROCK, 

Tahoe-Martis study unit, Hard Rock study area; TTAHO, Tahoe-Martis study unit, Tahoe study area; CGOLD, 

Central Sierra study unit, Coarse Gold study area; CWISH, Central Sierra study unit, Wishon study area; SMCL, 

secondary maximum contaminant level]

Number

of cells

Aquifer-scale proportion

(percent)

Detected at 

low relativeconcentration

Detected 

at moderate 

relativeconcentration

Detected at

high relativeconcentration

Major ions with SMCL benchmarks1

Tahoe-Martis

TMART 14 100 0 0

TROCK 13 88 0 12

TTAHO 14 98 0 2.4

Central Sierra

CGOLD 19 93 4.7 2.5

CWISH 10 100 0 0

Trace elements with SMCL benchmarks

Tahoe-Martis

TMART 14 93 0 7.1

TROCK 13 85 0 15

TTAHO 14 79 0 21

Central Sierra

CGOLD 19 58 5.3 37

CWISH 10 60 20 20

All inorganic constituents with SMCL benchmarks

Tahoe-Martis

TMART 14 93 0 7.1

TROCK 13 85 0 15

TTAHO 14 79 0 21

Central Sierra

CGOLD 19 58 5.3 37

CWISH 10 60 20 20

1

 Based on spatially weighted calculation.
Appendix B 181

Table B2C. Summary of aquifer-scale proportions calculated using the grid-based method for study areas for organic 

constituent classes with health-based benchmarks, Tahoe-Martis and Central Sierra study units, 2006–2007, California 

GAMA Priority Basin Project.

[Relative-concentration categories: high, concentration of at least one constituent in group greater than water-quality benchmark; 

moderate, concentration of at least one constituent in group greater than 0.1 of benchmark and no constituents in group with concentration 

greater than benchmark; low, concentrations of all constituents in group less than or equal to 0.1 of benchmark. TMART, Tahoe-Martis 

study unit, Martis study area; TROCK, Tahoe-Martis study unit, Hard Rock study area; TTAHO, Tahoe-Martis study unit, Tahoe study area; 

CGOLD, Central Sierra study unit, Coarse Gold study area; CWISH, Central Sierra study unit, Wishon study area]

Number

of cells

Aquifer-scale proportion

(percent)

Not

detected

Detected at 

low relativeconcentration

Detected 

at moderate 

relativeconcentration

Detected at

high relativeconcentration

Volatile organic compounds

Tahoe-Martis

TMART 14 71 29 0 0

TROCK 13 92 6.6 10.7 10.7

TTAHO 14 50 44 12.4 13.6

Central Sierra

CGOLD 18 72 22 15.6 0

CWISH 9 89 11 0 0

Pesticides

Tahoe-Martis

TMART 14 86 14 0 0

TROCK 13 100 0 0 0

TTAHO 14 100 0 0 0

Central Sierra

CGOLD 17 76 24 0 0

CWISH 9 100 0 0 0

Any organic constituent

Tahoe-Martis

TMART 14 64 36 0 0

TROCK 13 92 6.6 0.7 0.7

TTAHO 14 50 44 2.4 3.6

Central Sierra

CGOLD 18 67 28 5.6 0

CWISH 9 89 11 0 0

1

 Based on spatially weighted calculation.
182 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Land Use 

Land use was classified using an enhanced version of 

the satellite-derived (30-m pixel resolution), nationwide 

USGS National Land Cover Dataset (Nakagaki and others, 

2007). This dataset has been used in previous national and 

regional studies relating land use to water quality (Gilliom and 

others, 2006; Zogorski and others, 2006). The data represent 

land use during the early 1990s. The imagery is classified 

into 25 land‐cover classifications (Nakagaki and Wolock, 

2005). These 25 land-cover classifications were condensed 

into 3 principal land-use categories: urban, agricultural, and 

undeveloped. Land-use statistics for the study unit, study 

areas, and areas within a 500-m radius around each study 

well (500-m buffers) were calculated for classified datasets 

using ArcGIS. A 500-m radius centered on the well has been 

shown to be effective at correlating urban land use with VOC 

occurrence for the purposes of statistical characterization 

(Johnson and Belitz, 2009). Land-use data for USGS-grid, 

CDPH-grid, and USGS-understanding wells are listed in 

table C1.

Land-cover classes are based on features distinguishable 

in Level II remote sensing data (high-elevation aerial 

photography; Anderson and others, 1976). Urban land use 

includes high, moderate, and low intensity development 

and developed open space. Agricultural land use includes 

cultivated crops and land used for pasture or hay. Undeveloped 

land use includes everything else. In this classification, 

open-range grazing, such as that practiced on USFS 

and USBLM lands in the Sierra Nevada, is classified as 

undeveloped land use, not agricultural land use. In the 

seven national forests entirely within the Sierra Nevada 

hydrogeologic province (Plumas, Tahoe, Lake Tahoe Basin 

Management Unit, El Dorado, Stanislaus, Sierra, and 

Sequoia), there were approximately 3,640,000 acres of 

grazing allotments and approximately 165,000 billed animalunit months (AUM) in 2004 (U.S. Government Accounting 

Office, 2005). This corresponds to a density of approximately 

0.05 cattle per acre per month. For comparison, cattle density 

on irrigated pasture in the San Joaquin Valley averages 8 cattle 

per acre per month (Gildersleeve, 2006).

Septic Systems

Septic tank density was determined from housing 

characteristics data from the 1990 U.S. Census (U.S. Census 

Bureau, 1990). The density of septic tanks in each housing 

census block was calculated from the number of tanks and 

block area. The density of septic tanks around each well was 

then calculated from the area-weighted mean of the block 

densities for blocks within a 500-m buffer around the well 

location. (Tyler Johnson, USGS, written commun., 2009) 

(table C1).

Aquifer Lithology

Aquifer lithology was classified into four categories 

on the basis of lithologic information from driller’s logs 

and on the California State geologic map (Jennings, 1977; 

Saucedo and others, 2000). The State geologic map shows 

the lithologic unit exposed at the surface, which may not be 

the same as the lithologic unit at the depth range over which 

the well is screened or open. Therefore, if the lithologic 

category estimated from the geologic map disagreed with 

the lithology described in the driller’s log, the category 

from the driller’s log was used. In addition, several sites 

located near lakes plotted in the lake on the map; lithologic 

category for these sites was estimated from the geologic units 

mapped surrounding the lake. The 132 wells sampled by 

USGS‐GAMA and the 34 “DPH” CDPH-grid wells selected in 

the three study units were located on areas classified into 8 of 

the 66 geologic units defined on the California state geologic 

map (Jennings, 1977; Saucedo and others, 2000). These eight 

geologic units were grouped into four lithologic categories on 

the basis of rock type and age:

• Granitic rocks: Mesozoic granitic rocks (California 

State geologic map unit: grMz).

• Metamorphic rocks: Paleozoic and Mesozoic 

metavolcanic, metasedimentary, ultramafic, and mafic 

rocks (California state geologic map units: m).

• Sedimentary deposits: Cenozoic sedimentary deposits, 

including alluvial, fluvial, lacustrine, and glacial 

sediments (California State geologic map units: Q, 

Qg). These Quaternary-age sedimentary deposits are 

Pleistocene and Holocene in age.

• Volcanic rocks: Cenozoic volcanic rocks, including 

lava flows and pyroclastic deposits (California State 

geologic map units: Ti, Tv, Tvp, Qv). The Tertiary-age 

volcanic rocks (Ti, Tv, Tvp) are primarily Miocene and 

Pliocene in age, and the Quaternary-age volcanic rocks 

(Qv) are Pleistocene and Holocene in age.

The lithologic categories and geologic units assigned to each 

USGS-grid, USGS-understanding, and CDPH-grid well are 

listed in table C1.

Geologic time is divided into four eras: Cenozoic 

(65.5 Ma to present), Mesozoic (251 Ma to 65.5 Ma), 

Paleozoic (542 Ma to 251 Ma), and Precambrian 

(approximately 3,900 Ma to 542 Ma) (Walker and Geissman, 

2009). The Cenozoic Era consists of the Tertiary (65.5 Ma 

to 2.6 Ma) and Quaternary (2.6 Ma to present) periods. 

The Tertiary period consists of the Paleocene (65.5 Ma to 

55.8 Ma), Eocene (55.8 Ma to 33.9 Ma), Oligocene (33.9 Ma 

to 23.0 Ma), Miocene (23.0 to 5.3 Ma), and Pliocene (5.3 

Ma to 2.6 Ma) epochs. The Quaternary period consists of the 

Pleistocene (2.6 Ma to 10 ka) and Holocene (10 ka to present) 

epochs.

Appendix C: Attribution of Potential Explanatory Factors
Appendix C 183

Table C1. Land-use percentages, septic tank density, and aquifer lithology classification for USGS-grid, USGS-understanding, 

and CDPH-grid wells, Tahoe-Martis, Central Sierra, and Southern Sierra study units, 2006–2007, California GAMA Priority Basin 

Project.

[Well identification numbers: TMART, Tahoe-Martis study unit, Martis study area; TROCK, Tahoe-Martis study unit, Hard Rock study area; TTAHO, 

Tahoe-Martis study unit, Tahoe study area; CGOLD, Central Sierra study unit, Coarse Gold study area; CWISH, Central Sierra study unit, Wishon study 

area; SOSA, Southern Sierra study unit. Geologic units: grMz, Mesozoic granitic rocks; m, Paleozoic-Mesozoic metasedimentary and metavolcanic 

rocks; Ti, Tertiary shallow intrusive rocks; Tv, Tertiary volcanic rocks; Tvp, Tertiary pyroclastic rocks; Qv, Quaternary volcanic rocks; Q, Quaternary 

non-marine sediment; Qg, Quaternary glacial sediment. Aquifer lithology class: G, granitic rocks; M, metamorphic rocks; S, sedimentary deposits; V, 

volcanic rocks. Other abbreviations: USGS, U.S. Geological Survey; CDPH, California Department of Public Health; na, not available; tanks/km2, 

number of septic tanks per square kilometer]

Well 

identification

number

Land-use percentages1 Septic density2 Aquifer lithology classification

Agricultural Undeveloped1 Urban Tanks/km2 Geologic unit3 Aquifer

lithology class

Tahoe-Martis study unit USGS-grid wells

TMART-01 0 94 6 13 Qv8 V8

TMART-02 0 100 0 2 Qv V

TMART-03 0 100 0 2 Tv V

TMART-04 0 64 36 3 Qv8 V8

TMART-05 0 100 0 2 Qv8 V8

TMART-06 0 48 52 81 Qv8 V8

TMART-07 0 75 25 3 Qg S

TMART-08 0 95 5 24 Qv8 V8

TMART-09 0 100 0 0 Qv8 V8

TMART-10 0 13 87 1 Qv V

TMART-11 0 74 26 2 Qg S

TMART-12 0 91 9 1 Qv8 V8

TMART-13 0 100 0 7 Qv V

TMART-14 0 100 0 7 Tv8 V8

TROCK-01 0 64 36 1 grMz G

TROCK-02 0 100 0 0 grMz G

TROCK-03 0 100 0 1 Tvp V

TROCK-04 0 100 0 2 Tvp8 V8

TROCK-05 0 96 4 2 Tvp V

TROCK-06 0 50 50 0 Qv V

TROCK-07 0 33 67 0 grMz6 G6

TROCK-08 0 100 0 0 grMz G

TROCK-09 0 100 0 0 grMz6 G6

TROCK-10 0 88 12 7 Tvp V

TROCK-11 0 100 0 0 Qg S

TROCK-12 0 100 0 3 Tvp8 V8

TROCK-13 0 100 0 7 grMz6 G6

TTAHO-01 0 84 16 0 Q S

TTAHO-02 0 88 12 0 Q S

TTAHO-03 0 39 61 0 Qg S

TTAHO-04 0 63 37 0 Qg S

TTAHO-05 0 92 8 0 Q S

TTAHO-06 0 9 91 4 Q S

TTAHO-07 0 92 8 0 Q S

TTAHO-08 0 21 79 0 Qg S

TTAHO-09 0 97 3 0 Q S

TTAHO-10 0 79 21 0 Q S

TTAHO-11 0 96 4 0 grMz G

TTAHO-12 0 50 50 0 Q S

TTAHO-13 0 43 57 0 Qg S

TTAHO-14 0 45 55 105 Qg S
184 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Table C1. Land-use percentages, septic tank density, and aquifer lithology classification for USGS-grid, USGS-understanding, 

and CDPH-grid wells, Tahoe-Martis, Central Sierra, and Southern Sierra study units, 2006–2007, California GAMA Priority Basin 

Project.—Continued

[Well identification numbers: TMART, Tahoe-Martis study unit, Martis study area; TROCK, Tahoe-Martis study unit, Hard Rock study area; TTAHO, 

Tahoe-Martis study unit, Tahoe study area; CGOLD, Central Sierra study unit, Coarse Gold study area; CWISH, Central Sierra study unit, Wishon study 

area; SOSA, Southern Sierra study unit. Geologic units: grMz, Mesozoic granitic rocks; m, Paleozoic-Mesozoic metasedimentary and metavolcanic 

rocks; Ti, Tertiary shallow intrusive rocks; Tv, Tertiary volcanic rocks; Tvp, Tertiary pyroclastic rocks; Qv, Quaternary volcanic rocks; Q, Quaternary 

non-marine sediment; Qg, Quaternary glacial sediment. Aquifer lithology class: G, granitic rocks; M, metamorphic rocks; S, sedimentary deposits; V, 

volcanic rocks. Other abbreviations: USGS, U.S. Geological Survey; CDPH, California Department of Public Health; na, not available; tanks/km2, 

number of septic tanks per square kilometer]

Well 

identification

number

Land-use percentages1 Septic density2 Aquifer lithology classification

Agricultural Undeveloped1 Urban Tanks/km2 Geologic unit3 Aquifer

lithology class

Tahoe-Martis study unit USGS-understanding wells

TMARTU-01 0 100 0 7 Qv V

TROCKU-01 0 100 0 1 Tvp V

TROCKU-02 0 85 15 0 Qv V

TTAHOU-01 0 61 39 0 Q S

TTAHOU-02 0 9 91 57 Qg S

TTAHOU-03 0 26 74 4 Q S

TTAHOU-04 0 20 80 0 Qg S

TTAHOU-05 0 43 57 0 Qg S

TTAHOU-06 0 43 57 0 Qg S

TTAHOU-07 0 43 57 0 Qg S

TTAHOU-08 0 43 57 0 Qg S

Tahoe-Martis study unit CDPH-grid wells

TROCK-DPH-03 na na na na Tvp V

TROCK-DPH-04 na na na na Qg S

TROCK-DPH-06 na na na na Qv V

TROCK-DPH-12 na na na na Tv V

TTAHO-DPH-01 na na na na Q S

TTAHO-DPH-04 na na na na Q S

TTAHO-DPH-07 na na na na Qg S

TTAHO-DPH-09 na na na na Q S

TTAHO-DPH-14 na na na na Qg S

Central Sierra study unit USGS-grid wells

CGOLD-01 0 100 0 0 grMz G

CGOLD-02 0 64 36 6 grMz G

CGOLD-03 0 100 0 4 grMz G

CGOLD-04 0 100 0 11 grMz G

CGOLD-05 0 52 48 18 grMz G

CGOLD-06 0 100 0 10 grMz G

CGOLD-07 0 96 4 5 grMz G

CGOLD-08 0 36 64 34 grMz G

CGOLD-09 0 100 0 0 grMz G

CGOLD-10 0 100 0 4 m M

CGOLD-11 0 86 14 4 m M

CGOLD-12 0 77 23 14 grMz G

CGOLD-13 0 87 13 13 grMz G

CGOLD-14 0 29 71 50 grMz G

CGOLD-15 0 100 0 4 grMz G

CGOLD-16 0 100 0 5 grMz4 G4

CGOLD-17 0 100 0 1 grMz G

CGOLD-18 0 94 6 9 grMz G

CWISH-01 0 75 25 9 grMz G

CWISH-02 0 34 66 3 grMz G
Appendix C 185

Table C1. Land-use percentages, septic tank density, and aquifer lithology classification for USGS-grid, USGS-understanding, 

and CDPH-grid wells, Tahoe-Martis, Central Sierra, and Southern Sierra study units, 2006–2007, California GAMA Priority Basin 

Project.—Continued

[Well identification numbers: TMART, Tahoe-Martis study unit, Martis study area; TROCK, Tahoe-Martis study unit, Hard Rock study area; TTAHO, 

Tahoe-Martis study unit, Tahoe study area; CGOLD, Central Sierra study unit, Coarse Gold study area; CWISH, Central Sierra study unit, Wishon study 

area; SOSA, Southern Sierra study unit. Geologic units: grMz, Mesozoic granitic rocks; m, Paleozoic-Mesozoic metasedimentary and metavolcanic 

rocks; Ti, Tertiary shallow intrusive rocks; Tv, Tertiary volcanic rocks; Tvp, Tertiary pyroclastic rocks; Qv, Quaternary volcanic rocks; Q, Quaternary 

non-marine sediment; Qg, Quaternary glacial sediment. Aquifer lithology class: G, granitic rocks; M, metamorphic rocks; S, sedimentary deposits; V, 

volcanic rocks. Other abbreviations: USGS, U.S. Geological Survey; CDPH, California Department of Public Health; na, not available; tanks/km2, 

number of septic tanks per square kilometer]

Well 

identification

number

Land-use percentages1 Septic density2 Aquifer lithology classification

Agricultural Undeveloped1 Urban Tanks/km2 Geologic unit3 Aquifer

lithology class

Central Sierra study unit USGS-grid wells—Continued

CWISH-03 0 97 3 1 grMz G

CWISH-04 0 95 5 12 grMz G

CWISH-05 0 95 5 3 grMz G

CWISH-06 0 100 0 3 grMz G

CWISH-07 0 94 6 3 grMz G

CWISH-08 0 100 0 0 grMz G

CWISH-09 0 100 0 0 grMz G

Central Sierra study unit USGS-understanding wells

CGOLDU-01 0 66 34 14 grMz G

CGOLDU-02 0 100 0 3 m M

CWISHU-01 0 48 52 3 grMz G

Central Sierra study unit CDPH-grid wells

CGOLD-DPH-11 na na na na grMz G

CGOLD-DPH-14 na na na na grMz G

CGOLD-DPH-16 na na na na grMz G

CGOLD-DPH-18 na na na na grMz G

CGOLD-DPH-19 na na na na grMz G

CWISH-DPH-06 na na na na grMz G

CWISH-DPH-10 na na na na grMz G

Southern Sierra study unit USGS-grid wells

SOSA-01 40 60 0 0 m M

SOSA-02 0 100 0 0 Ti V

SOSA-03 45 55 0 17 m5 M5

SOSA-04 0 100 0 0 grMz G

SOSA-05 0 100 0 5 grMz G

SOSA-06 0 100 0 5 grMz G

SOSA-07 0 100 0 0 Q S

SOSA-08 21 79 0 3 grMz6 G6

SOSA-09 0 95 5 133 m M

SOSA-10 2 97 0 1 Q S

SOSA-11 57 43 0 3 Q S

SOSA-12 0 100 0 0 Q S

SOSA-13 60 22 18 2 Q S

SOSA-14 0 98 2 17 m5 M5

SOSA-15 0 100 0 1 grMz G

SOSA-16 7 44 49 39 Q S

SOSA-17 0 100 0 36 m M

SOSA-18 0 96 4 17 Q S

SOSA-19 0 92 8 23 m M

SOSA-20 0 87 13 36 m M

SOSA-21 0 54 46 34 Q S

SOSA-22 0 100 0 0 grMz G
186 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Table C1. Land-use percentages, septic tank density, and aquifer lithology classification for USGS-grid, USGS-understanding, 

and CDPH-grid wells, Tahoe-Martis, Central Sierra, and Southern Sierra study units, 2006–2007, California GAMA Priority Basin 

Project.—Continued

[Well identification numbers: TMART, Tahoe-Martis study unit, Martis study area; TROCK, Tahoe-Martis study unit, Hard Rock study area; TTAHO, 

Tahoe-Martis study unit, Tahoe study area; CGOLD, Central Sierra study unit, Coarse Gold study area; CWISH, Central Sierra study unit, Wishon study 

area; SOSA, Southern Sierra study unit. Geologic units: grMz, Mesozoic granitic rocks; m, Paleozoic-Mesozoic metasedimentary and metavolcanic 

rocks; Ti, Tertiary shallow intrusive rocks; Tv, Tertiary volcanic rocks; Tvp, Tertiary pyroclastic rocks; Qv, Quaternary volcanic rocks; Q, Quaternary 

non-marine sediment; Qg, Quaternary glacial sediment. Aquifer lithology class: G, granitic rocks; M, metamorphic rocks; S, sedimentary deposits; V, 

volcanic rocks. Other abbreviations: USGS, U.S. Geological Survey; CDPH, California Department of Public Health; na, not available; tanks/km2, 

number of septic tanks per square kilometer]

Well 

identification

number

Land-use percentages1 Septic density2 Aquifer lithology classification

Agricultural Undeveloped1 Urban Tanks/km2 Geologic unit3 Aquifer

lithology class

Southern Sierra study unit USGS-grid wells—Continued

SOSA-23 72 28 0 3 Q S

SOSA-24 16 84 0 5 grMz6 G6

SOSA-25 26 74 0 5 Q S

SOSA-26 0 100 0 5 grMz G

SOSA-27 34 66 0 4 grMz6 G6

SOSA-28 0 95 5 0 Q S

SOSA-29 0 100 0 0 m M

SOSA-30 25 75 0 1 grMz6 G6

SOSA-31 89 9 2 1 Q S

SOSA-32 28 71 1 3 Q S

SOSA-33 0 100 0 15 grMz G

SOSA-34 39 60 1 0 Q S

SOSA-35 0 99 1 0 m9 M9

Southern Sierra study unit USGS-understanding wells

SOSAFP-01 61 21 18 1 Q S

SOSAFP-02 1 93 6 33 m5 M5

SOSAFP-03 32 50 19 11 Q S

SOSAFP-04 7 91 3 138 Q S

SOSAFP-05 9 76 15 77 Q S

SOSAFP-06 81 14 5 1 Q S

SOSAFP-07 79 21 0 1 Q S

SOSAFP-08 71 29 0 2 Q S

SOSAFP-09 68 32 0 3 Q S

SOSAFP-10 86 14 0 3 Q S

SOSAFP-11 74 26 0 3 Q S

SOSAFP-12 70 30 0 3 Q S

SOSAFP-13 39 60 0 0 Q S

SOSAFP-14 65 34 1 0 Q S

SOSAFP-15 30 43 27 14 Q S

Southern Sierra study unit CDPH-grid wells

SOSA-DPH-06 na na na na grMz G

SOSA-DPH-07 na na na na Q S

SOSA-DPH-08 na na na na grMz6 G6

SOSA-DPH-11 na na na na grMz G

SOSA-DPH-12 na na na na m M

SOSA-DPH-13 na na na na Q S

SOSA-DPH-15 na na na na grMz G

SOSA-DPH-18 na na na na Q S

SOSA-DPH-21 na na na na grMz7 G7

SOSA-DPH-28 na na na na m M

SOSA-DPH-30 na na na na Q S
Appendix C 187

Table C1. Land-use percentages, septic tank density, and aquifer lithology classification for USGS-grid, USGS-understanding, 

and CDPH-grid wells, Tahoe-Martis, Central Sierra, and Southern Sierra study units, 2006–2007, California GAMA Priority Basin 

Project.—Continued

[Well identification numbers: TMART, Tahoe-Martis study unit, Martis study area; TROCK, Tahoe-Martis study unit, Hard Rock study area; TTAHO, 

Tahoe-Martis study unit, Tahoe study area; CGOLD, Central Sierra study unit, Coarse Gold study area; CWISH, Central Sierra study unit, Wishon study 

area; SOSA, Southern Sierra study unit. Geologic units: grMz, Mesozoic granitic rocks; m, Paleozoic-Mesozoic metasedimentary and metavolcanic 

rocks; Ti, Tertiary shallow intrusive rocks; Tv, Tertiary volcanic rocks; Tvp, Tertiary pyroclastic rocks; Qv, Quaternary volcanic rocks; Q, Quaternary 

non-marine sediment; Qg, Quaternary glacial sediment. Aquifer lithology class: G, granitic rocks; M, metamorphic rocks; S, sedimentary deposits; V, 

volcanic rocks. Other abbreviations: USGS, U.S. Geological Survey; CDPH, California Department of Public Health; na, not available; tanks/km2, 

number of septic tanks per square kilometer]

Well 

identification

number

Land-use percentages1 Septic density2 Aquifer lithology classification

Agricultural Undeveloped1 Urban Tanks/km2 Geologic unit3 Aquifer

lithology class

Southern Sierra study unit CDPH-grid wells—Continued

SOSA-DPH-31 na na na na Q S

SOSA-DPH-32 na na na na Q S

SOSA-DPH-33 na na na na grMz G

SOSA-DPH-34 na na na na Q S

SOSA-DPH-36 na na na na Q S

SOSA-DPH-37 na na na na Q S

SOSA-DPH-38 na na na na grMz6 G6

1

 Land-use percentages within 500-meter radius of well site (Johnson and Belitz, 2009).

2

 Septic tank density within 500-meter radius of well site, based on 1990 U.S. Census data.

3

 Geologic units from geologic map of California (Jennings, 1977, and Saucedo and others, 2000).

4

 Geologic unit based on surficial geology was metamorphic; classification changed to granitic based on driller’s log.

5

 Geologic unit based on surficial geology was water; classification changed to metamorphic based on nearest surficial geologic unit.

6

 Geologic unit based on surficial geology was sedimentary; classification changed to granitic based on driller’s log.

7

 Geologic unit based on surficial geology was water; classification changed to granitic based on nearest surficial geology unit.

8

 Geologic unit based on surficial geology was sedimentary; classification changed to volcanic based on driller’s log.

9

 Geologic unit based on surficial geology was volcanic; classification changed to metamorphic based on driller’s logs of adjacent wells.
188 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Well Construction Information and 

Hydrologic Conditions

Well construction data were obtained primarily from 

driller’s logs. On occasion, well construction data were 

obtained from ancillary records of well owners or the 

USGS National Water Information System database. Well 

identification verification procedures are described by Fram 

and Belitz (2007), Ferrari and others (2008), and Fram 

and others (2009). Well depths and depths to the top and 

bottom of the screened or open interval for USGS-grid, 

USGS-understanding, and CDPH-grid wells are listed in 

table C2. Wells drilled in hard rock commonly do not have 

casings; the borehole is left open. For these wells, the top of 

the screened or open interval was defined as the base of the 

sanitary seal, and the bottom was defined as the depth of the 

well. Springs were assigned a value of zero for the well depth 

and depth to top and bottom of the screened or open interval. 

Wells were classified as production wells, monitoring wells, 

or springs (table C2). Production wells have pumps that pump 

the groundwater from the aquifer to a distribution system. 

Monitoring wells include short-screened wells installed 

specifically as monitoring wells and wells that were once 

production wells, but no longer have pumps. Wells were 

classified as springs if water could flow from the aquifer into 

the distribution system without a pump, and if the well was 

either drilled horizontally or had no borehole.

The climate at each well site was represented by an 

aridity index (United Nations Environment Programme, 

1997; United Nations Educational, Scientific, and Cultural 

Organization, 1979):

 . 

averageannual precipitation aridityindex averageannual evapotranspiration

=

Higher values of the index correspond to wetter conditions. 

Average annual precipitation for each well site was extracted 

from the PRISM average annual precipitation for 1971–2000 

GIS coverage (PRISM Group, Oregon State University, 

2007). Average annual evapotranspiration for each well site 

was extracted from a GIS coverage modified from Flint and 

Flint (2007). The modification consisted of calibrating the 

evapotranspiration values to the measured California Irrigation 

Management Information System reference evapotranspiration 

values (California Irrigation Management Information System, 

2005; Alan Flint, USGS California Water Science Center, oral 

commun., 2009).

Elevation of the intersection of the well or spring and 

the land surface was used as a proxy for relative position in 

the groundwater flow system. All three study units have a 

large range in elevation of well sites—approximately 2,300 ft, 

5,500 ft, and 5,000 ft, for Tahoe-Martis, Central Sierra, and 

Southern Sierra, respectively. Elevations were obtained from 

U.S. Geological Survey digital elevation GIS coverage and are 

reported in feet relative to NAVD 88 (table C2). 

Groundwater Age Classification

Groundwater dating techniques provide a measure of 

the time since the groundwater was last in contact with the 

atmosphere. Techniques aimed at estimating groundwater 

residence times or ‘age’ include those based on tritium (3H) 

(for example, Tolstikhin and Kamensky, 1969; Torgersen and 

others, 1979), carbon-14 (14C) activity (for example, Vogel 

and Ehhalt, 1963; Plummer and others, 1993), dissolved 

noble gases, particularly helium-4 (4He) accumulation (for 

example, Davis and DeWiest, 1966; Andrews and Lee, 1979; 

Kulongoski and others, 2008), and tritium in combination with 

its decay product helium-3 (3He) (Poreda and others, 1988; 

Schlosser and others, 1989). 

Tritium (3H) is a short-lived radioactive isotope 

of hydrogen with a half-life of 12.32 years (Lucas and 

Unterweger, 2000). Tritium is produced naturally in the 

atmosphere from the interaction of cosmogenic radiation 

with nitrogen (Craig and Lal, 1961), by above-ground 

nuclear weapons testing (peak 1952 to 1963; Michel, 1989), 

and by the operation of nuclear reactors. Tritium enters the 

hydrological cycle following oxidation to tritiated water. 

Consequently, the presence of tritium in groundwater 

may be used to identify water that has exchanged with the 

atmosphere in the past 60 years. For the Tahoe-Martis, Central 

Sierra, and Southern Sierra study units, presence of tritium 

> 0.5 TU (tritium units) was defined as indicating presence of 

water recharged more recently than 1950. Water recharged 

since 1950 is defined as “modern” groundwater. Previous 

investigations have used a range of tritium values from 0.3 

to 1.0 TU as thresholds for distinguishing pre-1950 from 

post-1950 water (Michel, 1989; Plummer and others, 1993; 

Michel and Schroeder, 1994; Clark and Fritz, 1997; Manning 

and others, 2005).

Carbon-14 (14C) is a widely used chronometer based 

on the radiocarbon content of organic and inorganic carbon. 

Dissolved inorganic carbonate species typically are used 

for 14C dating of groundwater. Carbon-14 is formed in the 

atmosphere by the interaction of cosmic-ray neutrons with 

nitrogen, and to a lesser degree, with oxygen and carbon. 

Carbon-14 is incorporated into carbon dioxide and mixed 

throughout the atmosphere. The carbon dioxide dissolves 

in precipitation which eventually recharges the aquifer. 

Carbon-14 activity in groundwater, expressed as percent 

modern carbon (pmc), reflects exposure to the atmospheric 

14C source and is governed by the decay constant of 14C (with 

a half-life of 5,730 yrs). Carbon-14 can be used to estimate 

groundwater ages ranging from 1,000 to approximately 

30,000 years before present because of its half-life. Calculated 

14C ages in this study are referred to as “uncorrected” because 

they have not been adjusted to consider exchanges with 

sedimentary sources of carbon (Fontes and Garnier, 1979). 
Appendix C 189

Table C2. Hydrologic conditions and well construction information for USGS-grid, USGS-understanding, and CDPH-grid wells, TahoeMartis, Central Sierra, and Southern Sierra study units, 2006–2007, California GAMA Priority Basin Project.

[Well identification numbers: TMART, Tahoe-Martis study unit, Martis study area; TROCK, Tahoe-Martis study unit, Hard Rock study area; TTAHO, TahoeMartis study unit, Tahoe study area; CGOLD, Central Sierra study unit, Coarse Gold study area; CWISH, Central Sierra study unit, Wishon study area; SOSA, 

Southern Sierra study unit. Other abbreviations: na, not available; ft, feet; LSD, land-surface datum]

Well 

identification 

number

Hydrologic conditions Well construction information

(feet below LSD)

Aridity

index1

Elevation of LSD 

(ft above NAVD 88)2

Well

type3

Well depth 

(ft below LSD)

Top of screened

or open interval 

(ft below LSD)

Bottom of 

screened or 

open interval

(ft below LSD)

Tahoe-Martis study unit USGS-grid wells

TMART-01 0.65 5,847 Production 1,132 280 1,110

TMART-02 0.82 6,125 Production 690 65 680

TMART-03 0.59 5,596 Production 383 363 383

TMART-04 0.70 5,889 Production 1,026 100 1,026

TMART-05 0.66 5,968 Production 1,370 460 1,360

TMART-06 0.76 5,879 Production 900 40 900

TMART-07 0.80 5,884 Production 106 50 106

TMART-08 0.71 5,973 Production 612 125 612

TMART-09 0.78 5,940 Production 120 70 120

TMART-10 1.02 6,604 Production 600 500 600

TMART-11 0.68 5,914 Production 132 na na

TMART-12 0.74 5,871 Production 250 175 250

TMART-13 0.87 5,857 Production 175 175 175

TMART-14 0.78 5,840 Production 800 217 800

TROCK-01 0.95 6,044 Production 270 na na

TROCK-02 1.13 6,584 Spring 0 0 0

TROCK-03 0.73 6,064 Production 292 232 292

TROCK-04 1.58 7,209 Spring 0 0 0

TROCK-05 1.34 6,758 Spring 0 0 0

TROCK-06 0.82 6,314 Production 218 75 218

TROCK-07 0.82 6,319 Production 320 190 320

TROCK-08 1.29 8,107 Spring 0 0 0

TROCK-09 1.00 7,615 Spring 0 0 0

TROCK-10 0.89 6,316 Production 610 na 610

TROCK-11 0.77 6,354 Production 48 na 48

TROCK-12 0.99 6,056 Production na na na

TROCK-13 1.00 5,984 Production 102 93 98

TTAHO-01 0.82 6,232 Production 50 45 50

TTAHO-02 0.79 6,443 Production 880 240 880

TTAHO-03 0.78 6,293 Production 152 124 144

TTAHO-04 0.82 6,405 Production 240 60 210

TTAHO-05 0.79 6,400 Production 482 228 452

TTAHO-06 0.64 6,232 Production 495 138 480

TTAHO-07 0.80 6,309 Production 266 108 160

TTAHO-08 0.58 6,254 Production 156 86 156

TTAHO-09 0.72 6,214 Production 100 70 100

TTAHO-10 0.89 6,319 Production 250 95 245

TTAHO-11 1.05 6,472 Monitoring 200 na na

TTAHO-12 0.85 6,304 Monitoring 325 106 315

TTAHO-13 0.56 6,281 Monitoring 135 130 135

TTAHO-14 0.48 6,241 Production 250 135 245
190 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Table C2. Hydrologic conditions and well construction information for USGS-grid, USGS-understanding, and CDPH-grid wells, TahoeMartis, Central Sierra, and Southern Sierra study units, 2006–2007, California GAMA Priority Basin Project.—Continued

[Well identification numbers: TMART, Tahoe-Martis study unit, Martis study area; TROCK, Tahoe-Martis study unit, Hard Rock study area; TTAHO, TahoeMartis study unit, Tahoe study area; CGOLD, Central Sierra study unit, Coarse Gold study area; CWISH, Central Sierra study unit, Wishon study area; SOSA, 

Southern Sierra study unit. Other abbreviations: na, not available; ft, feet; LSD, land-surface datum]

Well 

identification 

number

Hydrologic conditions Well construction information

(feet below LSD)

Aridity

index1

Elevation of LSD 

(ft above NAVD 88)2

Well

type3

Well depth 

(ft below LSD)

Top of screened

or open interval 

(ft below LSD)

Bottom of 

screened or 

open interval

(ft below LSD)

Tahoe-Martis study unit USGS-understanding wells

TMARTU-01 0.81 5,996 Production 900 140 900

TROCKU-01 1.23 6,984 Production 317 285 317

TROCKU-02 0.88 6,645 Production 860 380 800

TTAHOU-01 0.89 6,304 Production 330 130 310

TTAHOU-02 0.55 6,259 Production 418 110 400

TTAHOU-03 0.67 6,284 Production 380 186 366

TTAHOU-04 0.54 6,263 Production 247 152 220

TTAHOU-05 0.56 6,281 Monitoring 335 325 335

TTAHOU-06 0.56 6,281 Monitoring 255 245 255

TTAHOU-07 0.56 6,281 Monitoring 163 158 163

TTAHOU-08 0.56 6,281 Monitoring 93 88 93

Tahoe-Martis study unit CDPH-grid wells

TROCK-DPH-03 na 7,092 Production 317 285 317

TROCK-DPH-04 na 7,405 Spring 0 0 0

TROCK-DPH-06 na 6,595 Spring 0 0 0

TROCK-DPH-12 na 6,307 Spring 0 0 0

TTAHO-DPH-01 na 6,336 Production na na na

TTAHO-DPH-04 na 6,372 Production 114 49 109

TTAHO-DPH-07 na 6,340 Production 225 55 220

TTAHO-DPH-09 na 6,357 Production 495 110 230

TTAHO-DPH-14 na 6,340 Production 240 136 232

Central Sierra study unit USGS-grid wells

CGOLD-01 0.29 760 Production 1,007 100 1,007

CGOLD-02 0.54 2,220 Production 750 100 750

CGOLD-03 0.48 1,900 Production 680 50 680

CGOLD-04 0.55 2,300 Production 500 55 500

CGOLD-05 0.51 2,200 Production na na na

CGOLD-06 0.67 3,240 Production 377 60 377

CGOLD-07 0.92 4,400 Production 462 60 462

CGOLD-08 0.55 2,310 Production 225 na 225

CGOLD-09 0.26 660 Production 1,027 70 1,027

CGOLD-10 0.77 3,680 Production 1,025 50 1,025

CGOLD-11 0.58 2,200 Production na na na

CGOLD-12 0.37 1,300 Production 880 100 880

CGOLD-13 0.38 1,090 Production 302 60 302

CGOLD-14 0.55 2,280 Production na na na

CGOLD-15 0.53 2,200 Production 355 50 355

CGOLD-16 0.87 4,800 Production 500 220 500

CGOLD-17 0.40 1,200 Production 300 200 300

CGOLD-18 0.72 3,030 Production 631 56 631

CWISH-01 0.73 3,360 Production 608 52 608

CWISH-02 0.64 2,660 Production 520 66 520

CWISH-03 0.78 3,500 Production 472 50 472
Appendix C 191

Table C2. Hydrologic conditions and well construction information for USGS-grid, USGS-understanding, and CDPH-grid wells, TahoeMartis, Central Sierra, and Southern Sierra study units, 2006–2007, California GAMA Priority Basin Project.—Continued

[Well identification numbers: TMART, Tahoe-Martis study unit, Martis study area; TROCK, Tahoe-Martis study unit, Hard Rock study area; TTAHO, TahoeMartis study unit, Tahoe study area; CGOLD, Central Sierra study unit, Coarse Gold study area; CWISH, Central Sierra study unit, Wishon study area; SOSA, 

Southern Sierra study unit. Other abbreviations: na, not available; ft, feet; LSD, land-surface datum]

Well 

identification 

number

Hydrologic conditions Well construction information

(feet below LSD)

Aridity

index1

Elevation of LSD 

(ft above NAVD 88)2

Well

type3

Well depth 

(ft below LSD)

Top of screened

or open interval 

(ft below LSD)

Bottom of 

screened or 

open interval

(ft below LSD)

Central Sierra study unit USGS-grid wells—Continued

CWISH-04 0.76 3,400 Production 700 52 700

CWISH-05 0.72 3,480 Production 202 85 202

CWISH-06 0.61 1,800 Production 900 20 200

CWISH-07 0.65 2,900 Production 900 100 900

CWISH-08 1.19 5,400 Production 925 81 925

CWISH-09 1.24 5,390 Production 700 70 700

Central Sierra study unit USGS-understanding wells

CGOLDU-01 0.37 1,300 Production 675 20 675

CGOLDU-02 0.53 1,950 Production 100 30 100

CWISHU-01 0.64 2,560 Production na na na

Central Sierra study unit CDPH-grid wells

CGOLD-DPH-11 na 2,269 Production na na na

CGOLD-DPH-14 na 2,327 Production 780 60 780

CGOLD-DPH-16 na 3,459 Spring 0 0 0

CGOLD-DPH-18 na 3,219 Production na na na

CGOLD-DPH-19 na 1,294 Production na na na

CWISH-DPH-06 na 2,606 Production 125 20 125

CWISH-DPH-10 na 3,803 Production na na na

Southern Sierra study unit USGS-grid wells

SOSA-01 0.32 5,049 Production na na na

SOSA-02 0.20 2,822 Production 102 54 102

SOSA-03 0.20 2,606 Production 270 130 270

SOSA-04 0.19 3,333 Production 540 300 540

SOSA-05 0.28 2,940 Production 470 370 470

SOSA-06 0.28 3,245 Production 580 300 580

SOSA-07 0.26 2,800 Production 51 21 51

SOSA-08 0.30 3,770 Production 700 300 700

SOSA-09 0.21 3,840 Production 300 60 300

SOSA-10 0.22 2,720 Production 174 90 174

SOSA-11 0.24 4,335 Production 332 50 332

SOSA-12 0.29 3,000 Production 70 25 70

SOSA-13 0.22 4,010 Production 520 280 510

SOSA-14 0.22 2,600 Production 305 150 305

SOSA-15 0.76 5,060 Spring 0 0 0

SOSA-16 0.23 2,549 Production na na na

SOSA-17 0.30 3,330 Production 150 20 150

SOSA-18 0.24 3,025 Production 615 120 615

SOSA-19 0.24 3,045 Production na na na

SOSA-20 0.24 2,964 Production 165 na 165

SOSA-21 0.21 2,671 Production na na na

SOSA-22 0.66 7,200 Spring 0 0 0

SOSA-23 0.26 3,845 Production 460 200 460

SOSA-24 0.26 4,226 Production 350 243 350
192 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Table C2. Hydrologic conditions and well construction information for USGS-grid, USGS-understanding, and CDPH-grid wells, TahoeMartis, Central Sierra, and Southern Sierra study units, 2006–2007, California GAMA Priority Basin Project.—Continued

[Well identification numbers: TMART, Tahoe-Martis study unit, Martis study area; TROCK, Tahoe-Martis study unit, Hard Rock study area; TTAHO, TahoeMartis study unit, Tahoe study area; CGOLD, Central Sierra study unit, Coarse Gold study area; CWISH, Central Sierra study unit, Wishon study area; SOSA, 

Southern Sierra study unit. Other abbreviations: na, not available; ft, feet; LSD, land-surface datum]

Well 

identification 

number

Hydrologic conditions Well construction information

(feet below LSD)

Aridity

index1

Elevation of LSD 

(ft above NAVD 88)2

Well

type3

Well depth 

(ft below LSD)

Top of screened

or open interval 

(ft below LSD)

Bottom of 

screened or 

open interval

(ft below LSD)

Southern Sierra study unit USGS-grid wells—Continued

SOSA-25 0.30 4,124 Production 180 84 180

SOSA-26 0.27 4,633 Production 490 140 490

SOSA-27 0.28 4,094 Production 600 140 400

SOSA-28 0.25 3,914 Production 200 80 200

SOSA-29 0.37 3,533 Production 42 21 42

SOSA-30 0.23 2,770 Production 375 220 375

SOSA-31 0.20 2,650 Production 120 78 120

SOSA-32 0.25 4,180 Production 400 200 400

SOSA-33 0.23 3,100 Production 195 115 195

SOSA-34 0.23 3,970 Production 180 84 174

SOSA-35 0.22 4,260 Production na na na

Southern Sierra study unit USGS-understanding wells

SOSAFP-01 0.22 4,029 Production 546 267 546

SOSAFP-02 0.19 2,640 Production 55 10 55

SOSAFP-03 0.23 4,072 Production 604 400 590

SOSAFP-04 0.21 3,940 Production 290 50 285

SOSAFP-05 0.22 4,040 Production 480 180 480

SOSAFP-06 0.22 4,045 Production 420 na na

SOSAFP-07 0.26 3,862 Production 472 202 472

SOSAFP-08 0.26 3,840 Production 402 54 396

SOSAFP-09 0.26 3,850 Production 300 na na

SOSAFP-10 0.27 3,820 Production 357 119 na

SOSAFP-11 0.26 3,850 Production 300 na na

SOSAFP-12 0.27 3,838 Production 550 na na

SOSAFP-13 0.23 4,060 Production 478 212 478

SOSAFP-14 0.23 4,055 Production 500 180 500

SOSAFP-15 0.22 3,980 Production 294 114 288

Southern Sierra study unit CDPH-grid wells

SOSA-DPH-06 na 3,324 Production na na na

SOSA-DPH-07 na 2,708 Production na na na

SOSA-DPH-08 na 3,999 Production 340 200 340

SOSA-DPH-11 na 5,378 Production na na na

SOSA-DPH-12 na 3,245 Production na na na

SOSA-DPH-13 na 4,188 Production 604 400 590

SOSA-DPH-15 na 6,044 Spring 0 0 0

SOSA-DPH-18 na 2,735 Production 585 150 270

SOSA-DPH-21 na 2,681 Production 450 220 450

SOSA-DPH-28 na 4,228 Production 130 70 130

SOSA-DPH-30 na 2,705 Production na na na

SOSA-DPH-31 na 2,724 Production 253 121 253
Appendix C 193

Table C2. Hydrologic conditions and well construction information for USGS-grid, USGS-understanding, and CDPH-grid wells, TahoeMartis, Central Sierra, and Southern Sierra study units, 2006–2007, California GAMA Priority Basin Project.—Continued

[Well identification numbers: TMART, Tahoe-Martis study unit, Martis study area; TROCK, Tahoe-Martis study unit, Hard Rock study area; TTAHO, TahoeMartis study unit, Tahoe study area; CGOLD, Central Sierra study unit, Coarse Gold study area; CWISH, Central Sierra study unit, Wishon study area; SOSA, 

Southern Sierra study unit. Other abbreviations: na, not available; ft, feet; LSD, land-surface datum]

Well 

identification 

number

Hydrologic conditions Well construction information

(feet below LSD)

Aridity

index1

Elevation of LSD 

(ft above NAVD 88)2

Well

type3

Well depth 

(ft below LSD)

Top of screened

or open interval 

(ft below LSD)

Bottom of 

screened or 

open interval

(ft below LSD)

Southern Sierra study unit CDPH-grid wells—Continued

SOSA-DPH-32 na 4,256 Production na na na

SOSA-DPH-33 na 2,802 Production na na na

SOSA-DPH-34 na 4,111 Production na na na

SOSA-DPH-36 na 2,553 Production na na na

SOSA-DPH-37 na 3,311 Production 107 47 107

SOSA-DPH-38 na 3,266 Production 665 73 665

1

Aridity index is average annual precipitation divided by average annual evapotranspiration.

2

 Land-surface datum (LSD) is a datum plane that is approximately at land surface at each well. The elevation of the LSD is described in feet above the North 

American Vertical Datum 1988 (NAVD 88).

3

 Production wells had installed pumps that brought groundwater to the surface; monitoring wells were sampled with monitoring well pumps; springs had 

groundwater reaching the surface without pumps. Well depth, and depths to top and bottom of screened interval defined as 0 feet for springs for this study.

The 14C age (residence time) is calculated on the basis of the 

decrease in 14C activity because of radioactive decay since 

groundwater recharge, relative to an assumed initial 14C 

concentration (Clark and Fritz, 1997). An average initial 14C 

activity of 100 percent modern carbon (pmc) is assumed for 

this study, with estimated errors on calculated groundwater 

ages up to ± 20%. Groundwater with a 14C activity of 

> 88 pmc is reported as having an age of <1,000 years; no 

attempt is made to refine 14C ages < 1,000 years. Measured 

values of percent modern carbon can be > 100 pmc because 

the definition of the 14C activity in “modern” carbon does 

not include the excess 14C produced in the atmosphere by 

above-ground nuclear weapons testing. For the Tahoe-Martis, 

Central Sierra, and Southern Sierra study units, 14C activity 

< 88 pmc was defined as indicative of presence of groundwater 

recharged before 1950. The threshold value of 88 pmc was 

selected because all groundwater samples with tritium < 0.5 

TU also had 14C < 88 pmc. 

Helium (He) is a naturally occurring inert gas initially 

included during accretion of the planet, and later produced 

by radioactive decay of lithium, uranium, and thorium in the 

Earth. Helium (3He plus 4He) concentrations in groundwater 

often exceed the expected solubility equilibrium values as 

a result of air-bubble entrainment, or subsurface production 

of both isotopes, and their subsequent release into the 

groundwater (for example, Morrison and Pine, 1955; Andrews 

and Lee, 1979; Torgersen, 1980; Torgersen and Clarke, 1985). 

There are four primary sources of He in groundwater:

,

where

is the total amount of helium in the groundwater

sample;

is the helium derived from equilibration with the

atmosphere at the time of recharge;

total equil exair trit terr

total

equil

ex

He He He He He

He

He

He

= + ++

is the helium derived from dissolved air bubbles

("excess" air);

is the helium produced by radioactive decay

of tritium in the sample, and

is the helium produced by radioactive decay of

uran

air

trit

terr

He

He

ium and thorium in aquifer materials or

emanating from deeper in the Earth's crust or

mantle.

Heequil, Heexair, and Heterr all consist of helium-3 (3He) and 

helium-4 (4He); however, Hetrit consists only of 3He.
194 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Heequil is a function of temperature at the time of 

recharge. Recharge temperatures were calculated from 

dissolved neon, argon, krypton, and xenon using methods 

described in Aeschbach-Hertig and others (1999, 2000) to 

model the Heexair component. The best model for the Heexair

component for each groundwater sample was selected 

by comparing the sums of the weighted squared standard 

deviations between the modeled and measured noble-gas 

concentrations (χ2). The model with the lowest χ2 value (least 

amount of deviation between the modeled and measured 

concentrations) was selected. The χ2 was compared to the 

value of a chi-squared distribution with the appropriate 

number of degrees of freedom for the model and a significance 

level (α) of 0.01 (χ2

α=0.01). Recharge temperatures were only 

calculated for groundwater samples for which χ2 was less than 

χ2

α=0.01 (Aeschbach-Hertig and others, 2000).

The presence of large concentrations of Heterr commonly 

is indicative of long groundwater residence times. For the 

purpose of estimating groundwater residence times, the 

amount of Heterr is converted to the parameter %Heterr–c, the 

percent of Heterr in Hetotal corrected for excess air:

% 100. terr terr c

total exair

He

He

He He − = × −

(Hetrit is neglected in calculation of %Heterr–c because it 

typically is very small.) For Tahoe-Martis, Central Sierra, 

and Southern Sierra study units, values of %Heterr–c >50% 

were defined as indicative of the presence of pre-modern 

groundwater. This threshold of 50% is greater than the 

thresholds used in other studies (for example, Kulongoski and 

others, 2010; Landon and others, 2010). A higher threshold 

was selected to reduce the number of samples for which 

groundwater age class inferred from %Heterr–c disagreed with 

that inferred on the basis of 14C and tritium data.

By determining the concentrations of tritium and its 

decay product helium-3 (Hetrit), the time that the water has 

resided in the aquifer can be calculated more precisely than 

using tritium alone for water (for example, Poreda and others, 

1988; Schlosser and others, 1989). Tritium-helium ages were 

computed as described by Schlosser and others (1989). The 

calculation requires determining how much of the total 3He 

was contributed from each of the components Hetrit, Heterr, 

Heequil, and Heexair. The amounts of 3He in Heequil and Heexair

are determined from equilibrium with atmospheric He and 

the model used for Heexair. The amount of 3He in Heterr is 

determined from the concentration of Heterr and the 3He/4He 

ratio of the Heterr. The remaining 3He is Hetrit. The uncertainty 

in this calculation of Hetrit, and therefore the uncertainty in the 

tritium-helium age, increases as Heterr increases.

The 3He/4He ratio of Heterr was determined by the linear 

regression of Heterr/Hetotal and δ3He [(δ3He = Rmeas/Ratm –1) × 

100 percent, where Rmeas/Ratm is the measured 3He/4He ratio in 

the sample divided by the 3He/4He ratio in the atmosphere] for 

groups of related groundwater samples containing < 1 tritium 

unit. Heterr has two primary sources: He from decay of 

radioactive isotopes of uranium, thorium, and lithium in the 

aquifer rocks or sediment (HeUTh), and He emanating from the 

lower crust and mantle below (Hemant). HeUTh was assumed 

to have a 3He/4He ratio of 2.0 × 10–8 (Rmeas/Ratm = 0.015) 

(Andrews, 1985), and Hemant was assumed to have a 3He/4He 

ratio of 8.3 × 10–6 (Rmeas/Ratm = 6), which is the median value 

measured in continental basalts and mantle xenoliths in the 

Western United States (Dodson and others, 1998). Samples 

were divided into four groups—(1) Tahoe-Martis study unit, 

Martis study area, (2) the rest of the Tahoe-Martis study unit, 

(3) Central Sierra study unit, and (4) Southern Sierra study 

unit. For each group, the fraction of Hemant in Heterr was 

calculated from the 3He/4He ratio of Heterr determined from 

the linear regression. The Martis study area was considered 

separately from the rest of the Tahoe-Martis study unit 

because the geologic setting (recent volcanic activity and 

active faulting) suggested that the volume of Hemant might 

be greater than in other regions. Hetrit was then calculated 

for groundwater samples containing tritium ≥ 1 TU by mass 

balance for 3He. 

Noble gas concentrations, modeled recharge 

temperatures, and measured 3He/4He ratios are reported in 

table C3, and tritium, modeled 3H/3He ages,14C ages, and 

%Heterr–c are reported in table C4. Because of uncertainties 

in age distributions, particularly the uncertainties caused by 

mixing of waters of different ages in wells with long screened 

or open intervals and high withdrawal rates, the uncorrected 

14C ages and the ages calculated from Heterr accumulation 

were not specifically used for quantifying the relation between 

age and water quality in this report. While more sophisticated 

lumped parameter models for analyzing age distributions that 

incorporate mixing are available (Cook and Böhlke, 2000), 

use of these alternative models to understand age mixtures was 

beyond the scope of this report. Using the data for tritium, 14C, 

and %Heterr–c, the age distribution of groundwater samples 

was classified as “Modern,” “Mixed,” or “Pre-modern” 

following the decision-tree shown in figure C1. Classification 

into modern, mixed, and pre-modern categories was sufficient 

to provide an appropriate and useful characterization for the 

purposes of examining groundwater quality.
Appendix C 195

Table C3. Results for analyses of noble gases in samples collected for the Tahoe-Martis, Central Sierra, and 

Southern Sierra study units, 2006–2007, California GAMA Priority Basin Project.

[The five-digit number in parentheses below the constituent name is the U.S. Geological Survey parameter code used to uniquely 

identify a specific constituent or property. GAMA identification numbers: TMART, Tahoe-Martis study unit, Martis study area; 

TROCK, Tahoe-Martis study unit, Hard Rock study area; TTAHO, Tahoe-Martis study unit, Tahoe study area; CGOLD, Central 

Sierra study unit, Coarse Gold study area; CWISH, Central Sierra study unit, Wishon study area; SOSA, Southern Sierra study 

unit. Measurement errors: Helium-3/helium-4 ratios, and helium-4, neon, and argon concentrations have 2-percent errors; 

krypton and xenon concentrations have 3-percent errors; errors for modeled recharged temperatures are ≤ 0.3 °C for 90 percent of 

samples. Other abbreviations: cm3 STP g–1 H20, cubic centimeters at standard temperature and pressure per gram of water; °C, 

degrees Celsius; ≤, less than or equal to; na, not available]

GAMA 

identification 

number

Helium-3/ 

Helium-4 

(atom ratio) 

(61040)

Helium-4 Neon Argon Krypton Xenon Modeled 

recharge 

temperature 

(°C)

(cm3 STP/gH2

O)

(85561) (61046) (85563) (85565) (85567)

× 10–7 × 10–7 × 10–7 × 10–4 × 10–8 × 10–8

Tahoe-Martis study unit USGS-grid wells

TMART-01 31.57 5.06 1.95 3.54 7.53 1.13 10.3

TMART-02 14.27 0.64 2.43 3.45 7.45 1.04 11.3

TMART-03 33.66 4.70 1.90 3.25 7.39 1.06 10.5

TMART-04 na na na na na na na

TMART-05 33.50 39.07 9.04 7.22 12.14 1.52 7.5

TMART-06 26.61 1.22 1.84 3.30 7.72 1.10 8.9

TMART-07 17.40 0.48 1.73 2.97 6.69 0.98 13.0

TMART-08 26.04 21.73 1.82 3.23 7.73 1.07 9.4

TMART-09 15.61 0.45 2.01 3.54 7.77 1.13 9.7

TMART-10 9.36 0.99 2.14 3.60 8.39 1.18 7.3

TMART-11 14.15 0.35 1.63 2.94 6.99 0.98 12.2

TMART-12 14.50 0.48 2.06 3.51 8.14 1.14 8.7

TMART-13 7.62 40.84 1.50 3.15 7.39 1.08 10.3

TMART-14 33.06 1.49 1.85 3.09 7.16 1.03 11.5

TROCK-01 17.64 0.95 3.38 4.68 9.93 1.30 7.5

TROCK-02 13.71 0.36 1.64 3.03 7.08 1.05 10.1

TROCK-03 12.69 0.46 1.82 3.24 7.48 1.06 10.4

TROCK-04 19.50 0.52 2.33 3.67 8.48 1.19 5.7

TROCK-05 17.67 0.42 1.93 3.44 8.13 1.19 6.2

TROCK-06 17.88 0.41 1.83 3.28 7.77 1.10 8.4

TROCK-07 15.96 0.57 1.88 3.30 7.71 1.13 8.2

TROCK-08 13.69 0.32 1.47 2.90 6.99 1.05 8.9

TROCK-09 13.62 0.35 1.58 3.27 7.70 1.19 5.4

TROCK-10 14.88 0.36 1.56 3.06 7.38 1.12 9.2

TROCK-11 16.58 0.53 2.22 3.64 8.13 1.14 8.7

TROCK-12 na na na na na na na

TROCK-13 20.89 21.90 8.42 7.90 14.90 1.79 4.3

TTAHO-01 25.07 0.47 2.14 3.72 8.69 1.22 6.5

TTAHO-02 15.81 0.61 2.22 3.47 8.09 1.12 8.2

TTAHO-03 15.11 0.45 1.90 3.23 7.85 1.12 8.6

TTAHO-04 15.84 1.08 4.43 5.40 10.66 1.42 5.8

TTAHO-05 15.35 0.43 1.75 3.27 7.79 1.14 7.6

TTAHO-06 5.50 2.27 2.19 3.59 8.24 1.14 8.6

TTAHO-07 10.99 0.83 1.95 3.39 7.78 1.15 7.8

TTAHO-08 8.25 1.04 1.81 3.28 7.41 1.09 8.9

TTAHO-09 13.86 0.74 2.98 4.11 8.96 1.22 8.4

TTAHO-10 4.07 3.19 2.39 3.73 8.48 1.18 8.2

TTAHO-11 na na na na na na na

TTAHO-12 14.89 0.53 2.15 3.77 8.08 1.23 7.4

TTAHO-13 28.59 0.46 1.92 3.57 7.90 1.19 6.5

TTAHO-14 3.23 2.91 3.04 4.03 8.68 1.19 8.6
196 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Table C3. Results for analyses of noble gases in samples collected for the Tahoe-Martis, Central Sierra, 

and Southern Sierra study units, 2006–2007, California GAMA Priority Basin Project.—Continued

[The five-digit number in parentheses below the constituent name is the U.S. Geological Survey parameter code used to 

uniquely identify a specific constituent or property. GAMA identification numbers: TMART, Tahoe-Martis study unit, Martis 

study area; TROCK, Tahoe-Martis study unit, Hard Rock study area; TTAHO, Tahoe-Martis study unit, Tahoe study area; 

CGOLD, Central Sierra study unit, Coarse Gold study area; CWISH, Central Sierra study unit, Wishon study area; SOSA, 

Southern Sierra study unit. Measurement errors: Helium-3/helium-4 ratios, and helium-4, neon, and argon concentrations 

have 2-percent errors; krypton and xenon concentrations have 3-percent errors; errors for modeled recharged temperatures are 

≤ 0.3 °C for 90 percent of samples. Other abbreviations: cm3 STP g–1 H20, cubic centimeters at standard temperature and 

pressure per gram of water; °C, degrees Celsius; ≤, less than or equal to; na, not available]

GAMA 

identification 

number

Helium-3/ 

Helium-4 

(atom ratio) 

(61040)

Helium-4 Neon Argon Krypton Xenon Modeled 

recharge 

temperature 

(°C)

(cm3 STP/gH2

O)

(85561) (61046) (85563) (85565) (85567)

× 10–7 × 10–7 × 10–7 × 10–4 × 10–8 × 10–8

Tahoe-Martis study unit USGS-understanding wells

TMARTU-01 31.94 1.59 1.80 3.24 7.91 1.08 8.9

TROCKU-01 na na na na na na na

TROCKU-02 15.14 0.68 1.84 3.16 7.33 1.05 9.8

TTAHOU-01 5.82 2.32 2.03 3.49 7.85 1.15 7.6

TTAHOU-02 6.32 1.28 2.29 3.53 7.78 1.15 8.4

TTAHOU-03 1.93 10.32 2.22 3.78 7.88 1.14 10.5

TTAHOU-04 11.50 1.00 2.57 3.84 8.31 1.13 10.0

TTAHOU-05 4.14 0.98 1.49 3.06 7.40 1.06 8.2

TTAHOU-06 5.76 1.77 2.41 3.65 8.05 1.16 7.9

TTAHOU-07 13.57 0.67 1.96 3.46 7.97 1.16 7.3

TTAHOU-08 14.93 0.35 1.90 4.39 10.49 1.42 8.2

Central Sierra study unit USGS-grid wells

CGOLD-01 4.97 4.85 2.03 3.19 7.10 0.97 18.9

CGOLD-02 5.50 1.97 3.35 4.39 8.79 1.11 17.0

CGOLD-03 2.91 4.55 2.42 3.49 7.26 0.97 19.3

CGOLD-04 7.08 1.12 2.27 3.55 7.76 1.05 15.4

CGOLD-05 8.42 38.06 2.51 3.75 8.21 1.12 14.1

CGOLD-06 5.15 1.70 2.32 3.60 8.01 1.08 13.6

CGOLD-07 2.59 23.66 2.30 3.83 8.65 1.22 8.8

CGOLD-08 6.69 2.75 4.26 5.03 10.21 1.30 13.2

CGOLD-09 0.88 23.42 2.52 3.66 7.83 1.05 17.5

CGOLD-10 13.68 0.37 1.71 3.17 7.56 1.08 12.1

CGOLD-11 na na na na na na na

CGOLD-12 4.09 2.84 2.54 3.58 7.75 1.00 18.9

CGOLD-13 6.56 14.17 10.35 8.85 16.20 1.65 19.1

CGOLD-14 4.45 61.71 2.40 3.66 8.00 1.05 15.8

CGOLD-15 3.32 3.72 2.44 3.58 7.66 1.02 16.9

CGOLD-16 15.53 0.39 1.87 3.56 7.92 1.10 11.4

CGOLD-17 na na na na na na na

CGOLD-18 2.63 23.83 1.93 3.33 7.46 1.07 13.1

CWISH-01 4.46 44.27 2.56 3.75 8.25 1.10 13.5

CWISH-02 0.99 65.01 2.39 3.74 8.33 1.16 11.7

CWISH-03 3.07 52.25 3.97 4.78 9.51 1.30 10.0

CWISH-04 14.22 0.42 1.63 3.21 7.56 1.04 13.1

CWISH-05 5.21 3.84 3.10 4.06 8.57 1.14 13.4

CWISH-06 0.20 69.44 2.28 3.69 8.02 1.15 12.5

CWISH-07 0.75 29.09 2.31 3.57 7.83 1.05 15.0

CWISH-08 2.00 3.45 2.12 3.78 8.66 1.26 6.7

CWISH-09 1.71 13.70 1.74 3.42 8.28 1.17 7.1
Appendix C 197

Table C3. Results for analyses of noble gases in samples collected for the Tahoe-Martis, Central Sierra, 

and Southern Sierra study units, 2006–2007, California GAMA Priority Basin Project.—Continued

[The five-digit number in parentheses below the constituent name is the U.S. Geological Survey parameter code used to 

uniquely identify a specific constituent or property. GAMA identification numbers: TMART, Tahoe-Martis study unit, Martis 

study area; TROCK, Tahoe-Martis study unit, Hard Rock study area; TTAHO, Tahoe-Martis study unit, Tahoe study area; 

CGOLD, Central Sierra study unit, Coarse Gold study area; CWISH, Central Sierra study unit, Wishon study area; SOSA, 

Southern Sierra study unit. Measurement errors: Helium-3/helium-4 ratios, and helium-4, neon, and argon concentrations 

have 2-percent errors; krypton and xenon concentrations have 3-percent errors; errors for modeled recharged temperatures are 

≤ 0.3 °C for 90 percent of samples. Other abbreviations: cm3 STP g–1 H20, cubic centimeters at standard temperature and 

pressure per gram of water; °C, degrees Celsius; ≤, less than or equal to; na, not available]

GAMA 

identification 

number

Helium-3/ 

Helium-4 

(atom ratio) 

(61040)

Helium-4 Neon Argon Krypton Xenon Modeled 

recharge 

temperature 

(°C)

(cm3 STP/gH2

O)

(85561) (61046) (85563) (85565) (85567)

× 10–7 × 10–7 × 10–7 × 10–4 × 10–8 × 10–8

Central Sierra study unit USGS-understanding wells

CGOLDU-01 3.00 3.99 3.28 4.21 8.58 1.06 19.0

CGOLDU-02 5.73 1.49 2.06 3.21 7.13 0.95 17.6

CWISHU-01 2.59 14.95 6.85 5.23 9.36 1.17 19.6

Southern Sierra study unit USGS-grid wells

SOSA-01 11.45 0.72 2.23 3.48 7.83 1.09 11.2

SOSA-02 10.26 0.85 2.51 3.35 6.95 0.90 20.3

SOSA-03 9.93 4.37 3.44 4.50 8.98 1.11 17.6

SOSA-04 1.07 46.74 4.73 5.12 9.87 1.22 13.7

SOSA-05 13.03 0.92 3.84 4.39 9.02 1.12 15.7

SOSA-06 12.72 1.02 3.90 4.74 9.69 1.22 14.0

SOSA-07 6.70 0.93 1.93 3.42 7.84 1.11 12.3

SOSA-08 5.66 1.71 3.07 4.14 8.65 1.11 14.0

SOSA-09 5.79 1.37 3.05 4.19 8.98 1.21 11.1

SOSA-10 7.17 13.54 2.19 3.45 7.80 1.03 15.0

SOSA-11 13.81 3.10 10.07 7.89 14.87 1.54 15.7

SOSA-12 13.37 0.50 2.14 3.36 7.78 1.09 13.1

SOSA-13 11.69 0.76 2.84 3.78 8.03 1.02 16.2

SOSA-14 8.96 1.91 3.51 3.95 7.78 0.95 22.4

SOSA-15 13.89 0.41 1.78 3.35 7.99 1.16 7.4

SOSA-16 3.70 8.61 2.38 3.31 6.97 0.91 20.1

SOSA-17 5.01 1.89 2.54 3.51 7.77 1.02 14.5

SOSA-18 6.78 5.39 3.21 3.63 7.61 1.02 17.2

SOSA-19 8.06 1.09 2.79 3.77 7.88 1.04 16.0

SOSA-20 15.52 0.63 2.67 3.67 7.92 1.09 13.6

SOSA-21 4.82 5.26 4.92 4.77 9.04 1.12 17.1

SOSA-22 na na na na na na na

SOSA-23 11.64 1.85 6.21 5.59 9.82 1.15 18.1

SOSA-24 10.77 1.15 3.86 4.40 8.60 1.07 16.2

SOSA-25 8.64 0.64 1.78 3.20 7.30 0.99 14.9

SOSA-26 10.62 1.16 3.70 4.42 8.97 1.15 13.6

SOSA-27 12.10 1.70 6.09 6.27 11.59 1.36 12.4

SOSA-28 6.48 3.07 3.99 4.15 8.46 1.06 15.1

SOSA-29 13.02 0.54 1.99 3.19 7.11 0.98 15.4

SOSA-30 6.54 8.70 2.67 3.54 7.47 0.96 19.3

SOSA-31 na na na na na na na

SOSA-32 13.41 0.64 2.57 3.51 7.55 1.02 14.6

SOSA-33 5.32 2.21 3.36 4.19 8.79 1.08 16.6

SOSA-34 12.90 1.54 5.58 5.72 10.36 1.18 18.0

SOSA-35 1.74 8.72 2.25 3.48 7.79 1.01 14.3
198 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Table C3. Results for analyses of noble gases in samples collected for the Tahoe-Martis, Central Sierra, 

and Southern Sierra study units, 2006–2007, California GAMA Priority Basin Project.—Continued

[The five-digit number in parentheses below the constituent name is the U.S. Geological Survey parameter code used to 

uniquely identify a specific constituent or property. GAMA identification numbers: TMART, Tahoe-Martis study unit, Martis 

study area; TROCK, Tahoe-Martis study unit, Hard Rock study area; TTAHO, Tahoe-Martis study unit, Tahoe study area; 

CGOLD, Central Sierra study unit, Coarse Gold study area; CWISH, Central Sierra study unit, Wishon study area; SOSA, 

Southern Sierra study unit. Measurement errors: Helium-3/helium-4 ratios, and helium-4, neon, and argon concentrations 

have 2-percent errors; krypton and xenon concentrations have 3-percent errors; errors for modeled recharged temperatures are 

≤ 0.3 °C for 90 percent of samples. Other abbreviations: cm3 STP g–1 H20, cubic centimeters at standard temperature and 

pressure per gram of water; °C, degrees Celsius; ≤, less than or equal to; na, not available]

GAMA 

identification 

number

Helium-3/ 

Helium-4 

(atom ratio) 

(61040)

Helium-4 Neon Argon Krypton Xenon Modeled 

recharge 

temperature 

(°C)

(cm3 STP/gH2

O)

(85561) (61046) (85563) (85565) (85567)

× 10–7 × 10–7 × 10–7 × 10–4 × 10–8 × 10–8

Southern Sierra study unit USGS-understanding wells

SOSAFP-01 3.04 3.48 2.90 3.84 8.15 1.02 16.1

SOSAFP-02 3.40 37.92 1.99 3.26 7.24 1.02 15.1

SOSAFP-03 4.64 2.42 3.43 3.89 8.05 1.04 14.8

SOSAFP-04 9.56 1.31 3.60 4.14 8.11 1.04 16.7

SOSAFP-05 3.34 2.12 2.26 3.40 7.36 1.00 15.0

SOSAFP-06 8.82 1.23 3.30 4.05 8.24 1.02 17.8

SOSAFP-07 12.70 0.50 2.14 3.70 7.97 1.08 15.4

SOSAFP-08 7.63 1.28 2.91 3.90 8.20 1.09 13.7

SOSAFP-09 13.35 0.74 3.55 4.96 9.61 1.14 22.5

SOSAFP-10 3.71 2.27 2.54 3.55 7.43 0.99 16.5

SOSAFP-11 8.52 1.16 2.57 3.74 7.92 1.02 16.2

SOSAFP-12 3.92 2.36 2.60 3.67 8.00 1.03 15.2

SOSAFP-13 4.71 2.39 3.24 4.07 8.48 1.07 15.9

SOSAFP-14 12.87 1.26 4.51 4.77 9.21 1.13 18.5

SOSAFP-15 13.80 0.79 3.25 4.05 8.19 1.04 16.3
Appendix C 199

Table C4. Tritium, tritium-helium, terrigenic helium, and carbon-14 data and groundwater age classifications, Tahoe-Martis, Central 

Sierra, and Southern Sierra study units, 2006–2007, California GAMA Priority Basin Project.

[Groundwater age classification based on decision tree shown in figure C1. Modern, groundwater recharged since 1952; Pre-modern, groundwater recharged 

before 1952; Mixed, groundwater containing components recharged before and after 1952. Errors: average 14C counting error was 0.36 percent with standard 

deviation of 0.11 percent; average tritium-helium age error was 6 percent with standard deviation of 3 percent; %Heterr–c errors less than 4 percent for values 

greater than 50 percent; average %Heterr–c error for values less than 50 percent was 5.2 percent with standard deviation of 3.4 percent. Abbreviations: 3Hetrit, 

helium-3 formed from decay of tritium; %Heterr–c, concentration of terrigenic helium divided by total helium concentration corrected for helium from excess air; 14C, carbon-14; TU, tritium units; nd, not datable; na, not available; <, less than; >, greater than]

GAMA

identification

number

Tritium

(TU)

3

Hetrit

(TU)

Tritium-Helium 

age (years)

%Heterr–c

(percent)

Percent 

modern 

carbon

Uncorrected 14C 

age (years)

Groundwater age 

classification1

Tahoe-Martis study unit USGS-grid wells

TMART-01 1.3 nd nd 92 72 2,580 Mixed

TMART-02 0.5 <1 >50 6 76 2,120 Mixed

TMART-03 0.0 <1 >50 92 30 9,670 Pre-modern

TMART-04 1.1 na na na 73 2,430 Mixed

TMART-05 0.3 <1 >50 99 41 7,170 Pre-modern

TMART-06 2.8 nd nd 68 99 <1,000 Mixed

TMART-07 3.0 nd nd 16 102 <1,000 Modern

TMART-08 1.0 nd nd 98 83 1,400 Mixed

TMART-09 4.8 3.4 9.4 0 102 <1,000 Modern

TMART-10 0.1 <1 >50 56 73 2,400 Pre-modern

TMART-11 4.0 0.7 2.7 0 83 1,380 Mixed

TMART-12 0.2 <1 >50 0 86 1,120 Pre-modern

TMART-13 0.0 <1 >50 99 43 6,630 Pre-modern

TMART-14 0.2 <1 >50 74 72 2,510 Pre-modern

TROCK-01 3.9 18.5 31 16 105 <1,000 Modern

TROCK-02 3.5 nd nd 0 108 <1,000 Modern

TROCK-03 0.1 <1 >50 7 82 1,500 Pre-modern

TROCK-04 2.8 12.2 29.7 0 99 <1,000 Modern

TROCK-05 2.2 6.7 24.9 0 99 <1,000 Modern

TROCK-06 1.6 6.9 29.8 0 99 <1,000 Modern

TROCK-07 3.6 11.4 25.4 25 99 <1,000 Modern

TROCK-08 2.9 0.1 0.3 0 na na Modern or Mixed

TROCK-09 2.8 nd nd 0 113 <1,000 Modern

TROCK-10 2.0 1.7 11 0 89 <1,000 Modern

TROCK-11 3.2 6.1 19.1 0 111 <1,000 Modern

TROCK-12 0.1 na na na 2 2 32,080 Pre-modern

TROCK-13 0.1 <1 >50 98 1 2 38,030 Pre-modern

TTAHO-01 3.2 21.5 36.3 0 104 <1,000 Modern

TTAHO-02 0.2 <1 >50 15 83 1,420 Pre-modern

TTAHO-03 5.9 2.5 6.2 0 101 <1,000 Modern

TTAHO-04 4.0 8.8 20.6 0 97 <1,000 Modern

TTAHO-05 2.1 4.3 19.7 7 89 <1,000 Modern

TTAHO-06 0.9 <1 >50 82 84 1,280 Mixed

TTAHO-07 1.1 9.6 40.5 50 92 <1,000 Mixed

TTAHO-08 2.4 8.8 27.3 63 92 <1,000 Mixed

TTAHO-09 0.0 <1 >50 0 77 1,980 Pre-modern

TTAHO-10 0.1 <1 >50 88 69 2,930 Pre-modern

TTAHO-11 1.4 na na na 2 2 32,980 Modern or Mixed

TTAHO-12 2.1 3.1 16.1 3 108 <1,000 Modern

TTAHO-13 1.5 <1 >50 2 90 <1,000 Modern

TTAHO-14 0.0 <1 >50 85 85 1,200 Pre-modern
200 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Table C4. Tritium, tritium-helium, terrigenic helium, and carbon-14 data and groundwater age classifications, Tahoe-Martis, Central 

Sierra, and Southern Sierra study units, 2006–2007, California GAMA Priority Basin Project.—Continued

[Groundwater age classification based on decision tree shown in figure C1. Modern, groundwater recharged since 1952; Pre-modern, groundwater recharged 

before 1952; Mixed, groundwater containing components recharged before and after 1952. Errors: average 14C counting error was 0.36 percent with standard 

deviation of 0.11 percent; average tritium-helium age error was 6 percent with standard deviation of 3 percent; %Heterr–c errors less than 4 percent for values 

greater than 50 percent; average %Heterr–c error for values less than 50 percent was 5.2 percent with standard deviation of 3.4 percent. Abbreviations: 3Hetrit, 

helium-3 formed from decay of tritium; %Heterr–c, concentration of terrigenic helium divided by total helium concentration corrected for helium from excess air; 14C, carbon-14; TU, tritium units; nd, not datable; na, not available; <, less than; >, greater than]

GAMA

identification

number

Tritium

(TU)

3

Hetrit

(TU)

Tritium-Helium 

age (years)

%Heterr–c

(percent)

Percent 

modern 

carbon

Uncorrected 14C 

age (years)

Groundwater age 

classification1

Tahoe-Martis study unit USGS-understanding wells

TMARTU-01 0.5 <1 >50 76 76 2,170 Mixed

TROCKU-01 1.6 na na na 93 <1,000 Modern or Mixed

TROCKU-02 0.2 <1 >50 40 73 2,480 Pre-modern

TTAHOU-01 2.8 20.1 37.2 83 102 <1,000 Mixed

TTAHOU-02 0.7 <1 >50 66 88 <1,000 Mixed

TTAHOU-03 0.0 <1 >50 96 58 4,360 Pre-modern

TTAHOU-04 3.1 9.1 24.3 49 107 <1,000 Mixed

TTAHOU-05 0.0 <1 >50 62 82 1,520 Pre-modern

TTAHOU-06 2.4 3.3 15.2 76 98 <1,000 Mixed

TTAHOU-07 4.1 9.9 21.8 35 88 <1,000 Modern

TTAHOU-08 5.9 1.7 4.6 0 na na Modern or Mixed

Central Sierra study unit USGS-grid wells

CGOLD-01 1.0 <1 >50 91 64 3,530 Mixed

CGOLD-02 1.0 nd nd 72 87 1,030 Mixed

CGOLD-03 0.1 <1 >50 90 79 1,820 Pre-modern

CGOLD-04 1.8 nd nd 57 110 <1,000 Mixed

CGOLD-05 0.5 <1 >50 99 79 1,850 Mixed

CGOLD-06 1.9 nd nd 73 98 <1,000 Mixed

CGOLD-07 -0.1 <1 >50 98 60 4,090 Pre-modern

CGOLD-08 3.0 6.2 19.9 79 109 <1,000 Mixed

CGOLD-09 0.7 <1 >50 98 54 4,840 Mixed

CGOLD-10 3.1 0.1 0.7 0 110 <1,000 Modern

CGOLD-11 2.3 na na na 99 <1,000 Modern or Mixed

CGOLD-12 0.2 <1 >50 84 72 2,580 Premodern

CGOLD-13 1.6 <1 >50 96 99 <1,000 Mixed

CGOLD-14 2.3 <1 >50 99 84 1,360 Mixed

CGOLD-15 1.8 5.7 25.3 88 94 <1,000 Mixed

CGOLD-16 5.6 2.7 6.9 0 na na Modern or Mixed

CGOLD-17 3.4 na na na 106 <1,000 Modern or Mixed

CGOLD-18 2.0 <1 >50 98 60 4,080 Mixed

CWISH-01 1.1 <1 >50 99 38 7,750 Mixed

CWISH-02 0.0 <1 >50 99 61 3,820 Pre-modern

CWISH-03 0.0 <1 >50 99 18 13,830 Pre-modern

CWISH-04 3.5 1.9 7.6 4 109 <1,000 Modern

CWISH-05 2.8 26.3 41.5 88 104 <1,000 Mixed

CWISH-06 0.0 <1 >50 99 19 13,350 Pre-modern

CWISH-07 1.1 nd nd 99 65 3,370 Mixed

CWISH-08 1.4 nd nd 88 91 <1,000 Mixed

CWISH-09 2.9 29.8 43 97 100 <1,000 Mixed

Central Sierra study unit USGS-understanding wells

CGOLDU-01 0.7 <1 >50 88 83 1,410 Mixed

CGOLDU-02 3.5 3.4 12 70 115 <1,000 Mixed

CWISHU-01 0.3 <1 >50 97 62 3,800 Premodern
Appendix C 201

Table C4. Tritium, tritium-helium, terrigenic helium, and carbon-14 data and groundwater age classifications, Tahoe-Martis, Central 

Sierra, and Southern Sierra study units, 2006–2007, California GAMA Priority Basin Project.—Continued

[Groundwater age classification based on decision tree shown in figure C1. Modern, groundwater recharged since 1952; Pre-modern, groundwater recharged 

before 1952; Mixed, groundwater containing components recharged before and after 1952. Errors: average 14C counting error was 0.36 percent with standard 

deviation of 0.11 percent; average tritium-helium age error was 6 percent with standard deviation of 3 percent; %Heterr–c errors less than 4 percent for values 

greater than 50 percent; average %Heterr–c error for values less than 50 percent was 5.2 percent with standard deviation of 3.4 percent. Abbreviations: 3Hetrit, 

helium-3 formed from decay of tritium; %Heterr–c, concentration of terrigenic helium divided by total helium concentration corrected for helium from excess air; 14C, carbon-14; TU, tritium units; nd, not datable; na, not available; <, less than; >, greater than]

GAMA

identification

number

Tritium

(TU)

3

Hetrit

(TU)

Tritium-Helium 

age (years)

%Heterr–c

(percent)

Percent 

modern 

carbon

Uncorrected 14C 

age (years)

Groundwater age 

classification1

Southern Sierra study unit USGS-grid wells

SOSA-01 0.7 <1 >50 31 na na Mixed

SOSA-02 3.4 nd nd 34 na na Modern or Mixed

SOSA-03 3.4 <1 >50 89 73 2,490 Mixed

SOSA-04 0.8 <1 >50 99 na na Mixed

SOSA-05 0.6 <1 >50 0 na na Mixed

SOSA-06 -0.3 <1 >50 0 60 na Pre-modern

SOSA-07 2.4 nd nd 53 92 <1,000 Mixed

SOSA-08 0.5 <1 >50 70 na na Mixed

SOSA-09 0.7 <1 >50 59 na na Mixed

SOSA-10 2.5 <1 >50 97 51 5,270 Mixed

SOSA-11 0.8 <1 >50 42 na na Mixed

SOSA-12 4.1 0.1 0.6 0 na na Modern or Mixed

SOSA-13 0.0 <1 >50 5 84 1,320 Pre-modern

SOSA-14 1.0 <1 >50 71 na na Mixed

SOSA-15 4.5 0.3 1.2 0 92 <1,000 Modern

SOSA-16 1.4 <1 >50 95 na na Mixed

SOSA-17 0.9 <1 >50 76 na na Mixed

SOSA-18 1.1 <1 >50 92 na na Mixed

SOSA-19 1.8 nd nd 48 na na Mixed

SOSA-20 2.9 5.1 18.1 0 110 na Modern

SOSA-21 2.2 25.3 45.2 91 na na Mixed

SOSA-22 2.8 na na na 100 <1,000 Modern or Mixed

SOSA-23 -0.1 <1 >50 27 na na Pre-modern

SOSA-24 -0.0 <1 >50 24 na na Pre-modern

SOSA-25 0.3 <1 >50 36 na na Pre-modern

SOSA-26 1.6 nd nd 32 na na Modern or Mixed

SOSA-27 1.0 nd nd 9 na na Modern or Mixed

SOSA-28 2.3 20.0 40.4 84 na na Mixed

SOSA-29 2.6 1.8 9.4 11 na na Modern or Mixed

SOSA-30 1.4 <1 >50 95 na na Mixed

SOSA-31 3.9 7.9 25.3 45 123 na Modern

SOSA-32 0.2 <1 >50 0 88 na Pre-modern

SOSA-33 0.2 <1 >50 77 na na Pre-modern

SOSA-34 1.6 nd nd 2 na na Modern or Mixed

SOSA-35 0.1 <1 >50 95 na na Pre-modern

Southern Sierra study unit USGS-understanding wells

SOSAFP-01 0.1 <1 >50 88 66 3,310 Pre-modern

SOSAFP-02 3.2 <1 >50 99 26 10,700 Mixed

SOSAFP-03 -0.1 <1 >50 80 67 3,140 Pre-modern

SOSAFP-04 0.5 <1 >50 47 87 1,080 Mixed

SOSAFP-05 0.2 <1 >50 80 65 3,400 Pre-modern

SOSAFP-06 0.4 <1 >50 48 78 1,870 Pre-modern

SOSAFP-07 1.8 nd nd 0 106 <1,000 Modern
202 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Table C4. Tritium, tritium-helium, terrigenic helium, and carbon-14 data and groundwater age classifications, Tahoe-Martis, Central 

Sierra, and Southern Sierra study units, 2006–2007, California GAMA Priority Basin Project.—Continued

[Groundwater age classification based on decision tree shown in figure C1. Modern, groundwater recharged since 1952; Pre-modern, groundwater recharged 

before 1952; Mixed, groundwater containing components recharged before and after 1952. Errors: average 14C counting error was 0.36 percent with standard 

deviation of 0.11 percent; average tritium-helium age error was 6 percent with standard deviation of 3 percent; %Heterr–c errors less than 4 percent for values 

greater than 50 percent; average %Heterr–c error for values less than 50 percent was 5.2 percent with standard deviation of 3.4 percent. Abbreviations: 3Hetrit, 

helium-3 formed from decay of tritium; %Heterr–c, concentration of terrigenic helium divided by total helium concentration corrected for helium from excess air; 14C, carbon-14; TU, tritium units; nd, not datable; na, not available; <, less than; >, greater than]

GAMA

identification

number

Tritium

(TU)

3

Hetrit

(TU)

Tritium-Helium 

age (years)

%Heterr–c

(percent)

Percent 

modern 

carbon

Uncorrected 14C 

age (years)

Groundwater age 

classification1

Southern Sierra study unit USGS-understanding wells—Continued

SOSAFP-08 0.3 <1 >50 58 85 1,200 Pre-modern

SOSAFP-09 1.0 nd nd 0 101 <1,000 Modern

SOSAFP-10 -0.1 <1 >50 81 79 1,770 Pre-modern

SOSAFP-11 1.0 3.1 25.2 56 87 1,010 Mixed

SOSAFP-12 0.2 <1 >50 81 na na Pre-modern

SOSAFP-13 0.4 <1 >50 80 na na Pre-modern

SOSAFP-14 0.6 <1 >50 0 71 2,670 Mixed

SOSAFP-15 1.4 0.2 2.4 0 92 <1,000 Modern

1

 Some samples intially classified as Modern or Mixed based on available groundwater age tracer data were subsequently classified as one or the other 

(underlined) on the basis of classified well depth.

2

 Sample oxidation-reduction state was anoxic (table C6) and stable carbon isotope ratio was between 0 and –10 per mil, suggesting that the carbon isotope 

composition of the sample may have been altered by methanogenesis. The uncorrected carbon-14 apparent age thus may be too old.

Are there 

14C data?

Is tritium > 

0.5 TU?

Is 14C

> 88 pmc?

Are there 

Heterr–c data?

Is Heterr–c

< 50%?

Modern or

Mixed

Pre-modern

Modern

Mixed

Mixed

Modern or Mixed

no yes

no yes

no yes

no yes

no yes

EXPLANATION

Decision question

Age classification

Three tracers are used for age classification:

Tritium activity, reported in tritium units (TU),

Carbon-14 activity (14C), reported as uncorrected

 percent modern carbon (pmc),

Percent of terrigenic helium in total helium corrected

 for excess air (Heterr–c)

Figure C1. Decision tree used for assigning groundwater age 

classifications on the basis of tritium, carbon-14, and terrigenic 

helium data, Tahoe-Martis, Central Sierra, and Southern Sierra 

study units, California GAMA Priority Basin Project.
Appendix C 203

Geochemical Conditions

Geochemical conditions investigated as potential 

explanatory factors in this report include oxidation-reduction 

characteristics and pH. Oxidation-reduction (redox) 

conditions influence the mobility of many organic and 

inorganic constituents (McMahon and Chapelle, 2008). 

Redox conditions along groundwater flow paths commonly 

proceed along a well-documented sequence of Terminal 

Electron Acceptor Processes (TEAP), in which a single 

TEAP typically dominates at a particular time and aquifer 

location (Chapelle and others, 1995; Chapelle, 2001). The 

typical TEAP sequence is oxygen reduction (oxic), nitrate 

reduction, manganese reduction, iron reduction, sulfate 

reduction, and methanogenesis. This sequence is the order 

predicted from equilibrium thermodynamics and corresponds 

to progressively decreasing oxidation-reduction potentials. 

However, the kinetics of many TEAPs are slow, and the 

reactions typically only proceed at consequential rates when 

mediated by biological catalysis (Stumm and Morgan, 1996; 

Appelo and Postma, 2005). Microbes present in groundwater 

and on aquifer sediment or rock produce enzymes that 

catalyze the reactions. The microbes couple reduction of 

these inorganic species with oxidation of organic matter to 

produce energy needed for growth. Groundwater samples may 

contain chemical species that suggest more than one TEAP 

is operating. Evidence for more than one TEAP may indicate 

mixing of waters from different redox zones upgradient of the 

well, a well that is screened across more than one redox zone, 

or spatial heterogeneity in microbial activity in the aquifer. 

In addition, different redox couples often are not consistent, 

indicating electrochemical disequilibrium in groundwater, 

complicating the assessments of redox conditions (Lindberg 

and Runnels, 1984; Appelo and Postma, 2005).

In this report, oxidation-reduction conditions were 

represented in three ways: by DO concentration, by classified 

oxidation-reduction condition, and by ratios of species of 

redox-sensitive trace elements. DO concentrations were 

measured at USGS-grid and USGS-understanding wells (Fram 

and Belitz, 2007; Ferrari and others, 2008; Fram and others, 

2009), but are not reported in the CDPH database (table C5). 

Oxidation-reduction conditions were classified on the basis 

of DO, nitrate, manganese, and iron concentrations using a 

modified version of the classification scheme of McMahon 

and Chapelle (2008) and Jurgens and others (2009) (table C6). 

For wells without USGS inorganic constituent data, the most 

recent data in the CDPH database within the 3-year interval 

used for the status assessment were used. 

Arsenic, iron, and chromium occur as different species 

depending on the redox state of the groundwater. The ratio 

of the amount of the more oxidized species to the amount of 

the more reduced species for each constituent may provide 

information about the progress of the TEAP involving the 

constituent. The following ratios are reported in table C5:

• As+5/As+3, where As+5 is the amount of arsenic present 

in the more oxidized +5 oxidation state (arsenate) 

and As+3 is the amount of arsenic present in the more 

reduced +3 oxidation state (arsenite);

• Fe+3/Fe+2, where Fe+3 is the amount of iron present in 

the more oxidized +3 oxidation state (ferric iron) and 

Fe+2 is the amount of iron present in the more reduced 

+2 oxidation state (ferrous iron);

• Cr+6/Cr+3, where Cr+6 is the amount of chromium 

present in the more oxidized +6 oxidation state 

(chromate) and Cr+3 is the amount of chromium 

present in the more reduced +3 oxidation state 

(chromite).

Total concentrations of As, Fe, and Cr, and concentrations 

of As+3, Fe+2, and Cr+6 were reported by Fram and Belitz 

(2007), Ferrari and others (2008), and Fram and others 

(2009). The concentrations of As+5, Fe+3, Cr+3 Fe(III), As(V), 

and Cr(III) were calculated by difference. As+5/As+3 was 

reported as > 10 if total arsenic was detected and As+3 was not 

detected, and as < 0.01 if total arsenic concentration equaled 

As+3 concentration. Similarly, Fe+3/Fe+2 was reported as > 10 

if total iron was detected and Fe+2 was not detected, and as 

< 0.01 if total iron concentration equaled Fe+2 concentration. 

Cr+6/ Cr+3 was reported as > 10 if total chromium equaled Cr+6, 

and as < 0.01 if total chromium was detected and Cr+6 was not 

detected.
204 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Table C5. Oxidation-reduction classification, dissolved oxygen concentration, pH, and oxidation-reduction species ratios for 

arsenic, iron, and chromium, Tahoe-Martis, Central Sierra, and Southern Sierra study units, 2006–2007, California GAMA Priority 

Basin Project.

[Well identification numbers: TMART, Tahoe-Martis study unit, Martis study area; TROCK, Tahoe-Martis study unit, Hard Rock study area; 

TTAHO, Tahoe-Martis study unit, Tahoe study area; CGOLD, Central Sierra study unit, Coarse Gold study area; CWISH, Central Sierra study unit, 

Wishon study area; SOSA, Southern Sierra study unit. Oxidation-reduction classification: NO3–red, nitrate-reducing; Mn-red, manganese-reducing; 

MnFe-red, manganese and iron reducing; noDO, no dissolved oxygen data available; noMn, no manganese data available. Ratios of oxidized to 

reduced species of metals: Fe+3/Fe+2, ratio of the amount of iron in the +3 oxidation state (ferric iron) to the amount in the +2 oxidation state (ferrous 

iron); As+5/As+3, ratio of the amount of arsenic in the +5 oxidation state (arsenate) to the amount in the +3 oxidation state (arsenite); Cr+6/Cr+3, ratio 

of the amount of chromium in the +6 oxidation state (chromate) to the amount in the +3 oxidation state (chromite). Other abbreviations: mg/L, 

milligrams per liter; <, less than; >, greater than; na, not available; –, concentration too low to measure ratio; CDPH, California Department of Public 

Health; USGS, U.S. Geological Survey]

Well identification 

number

Source of 

inorganic 

data

pH

(standard 

units)

Dissolved 

oxygen 

(mg/L)

Oxidation-reduction 

classification1

Ratios of oxidized and reduced 

species of metals

Fe+3/

Fe+2

As+5/

As+3

Cr+6/

Cr+3

Tahoe-Martis study unit USGS-grid wells

TMART-01 USGS 8.2 6.5 Oxic 1.0 >10 –

TMART-02 USGS 8.4 6.6 Oxic – >10 1.0

TMART-03 USGS 7.9 0.2 Anoxic Mn-red 1.0 3.0 –

TMART-04 USGS 8.0 10.2 Oxic – >10 <0.01

TMART-05 USGS 7.9 7.2 Oxic 0.29 >10 –

TMART-06 USGS 7.0 8.6 Oxic – >10 –

TMART-07 USGS 6.7 4.1 Oxic – – –

TMART-08 USGS 8.2 6.0 Oxic >10 >10 –

TMART-09 USGS 6.8 9.6 Oxic – – –

TMART-10 USGS 8.0 6.1 Oxic – >10 –

TMART-11 USGS 7.0 9.4 Oxic 0.33 >10 –

TMART-12 USGS 8.1 8.8 Oxic – – –

TMART-13 USGS 8.0 8.9 Oxic >10 >10 –

TMART-14 USGS 7.5 3.9 Oxic >10 >10 –

TROCK-01 USGS 7.3 6.2 Oxic >10 – –

TROCK-02 USGS 6.0 2.8 Oxic – – –

TROCK-03 USGS 7.6 8.1 Oxic – – –

TROCK-04 USGS 7.3 11.9 Oxic – – –

TROCK-05 USGS 7.2 10.7 Oxic >10 – –

TROCK-06 USGS 6.9 9.1 Oxic >10 – –

TROCK-07 USGS 6.7 6.8 Oxic – >10 –

TROCK-08 USGS 5.5 9.5 Oxic – – –

TROCK-09 USGS 5.9 8.1 Oxic – – –

TROCK-10 USGS 7.4 7.6 Oxic – – –

TROCK-11 USGS 6.5 6.0 Oxic 0.75 >10 –

TROCK-12 USGS 5.8 0.5 Anoxic MnFe-red <0.01 0.52 –

TROCK-13 USGS 7.3 12.6 Mixed anoxic Mn-red/oxic >10 >10 –

TTAHO-01 USGS 7.5 4.9 Oxic 3.0 – –

TTAHO-02 USGS 8.1 7.6 Oxic – – –

TTAHO-03 USGS 6.1 8.8 Oxic – – –

TTAHO-04 USGS 6.7 9.5 Oxic – >10 >10

TTAHO-05 USGS 7.6 6.9 Oxic 0.67 0.60 –

TTAHO-06 USGS 8.9 5.2 Oxic – >10 –

TTAHO-07 USGS 8.0 8.4 Oxic – >10 >10

TTAHO-08 USGS 7.6 4.2 Oxic – >10 –

TTAHO-09 USGS 8.1 5.7 Oxic >10 >10 1.0

TTAHO-10 USGS 8.6 5.3 Oxic – >10 –

TTAHO-11 USGS 6.3 0.2 Anoxic MnFe-red 0.01 0.36 –

TTAHO-12 USGS 6.5 4.8 Mixed oxic/Fe-red 0.03 – –

TTAHO-13 USGS 6.2 0.8 Anoxic Mn-red 0.80 – –

TTAHO-14 USGS 8.9 9.6 Oxic – <0.01 –
Appendix C 205

Table C5. Oxidation-reduction classification, dissolved oxygen concentration, pH, and oxidation-reduction species ratios 

for arsenic, iron, and chromium, Tahoe-Martis, Central Sierra, and Southern Sierra study units, 2006–2007, California GAMA 

Priority Basin Project.—Continued

[Well identification numbers: TMART, Tahoe-Martis study unit, Martis study area; TROCK, Tahoe-Martis study unit, Hard Rock study area; 

TTAHO, Tahoe-Martis study unit, Tahoe study area; CGOLD, Central Sierra study unit, Coarse Gold study area; CWISH, Central Sierra study unit, 

Wishon study area; SOSA, Southern Sierra study unit. Oxidation-reduction classification: NO3–red, nitrate-reducing; Mn-red, manganese-reducing; 

MnFe-red, manganese and iron reducing; noDO, no dissolved oxygen data available; noMn, no manganese data available. Ratios of oxidized 

to reduced species of metals: Fe+3/Fe+2, ratio of the amount of iron in the +3 oxidation state (ferric iron) to the amount in the +2 oxidation state 

(ferrous iron); As+5/As+3, ratio of the amount of arsenic in the +5 oxidation state (arsenate) to the amount in the +3 oxidation state (arsenite); Cr+6/

Cr+3, ratio of the amount of chromium in the +6 oxidation state (chromate) to the amount in the +3 oxidation state (chromite). Other abbreviations: 

mg/L, milligrams per liter; <, less than; >, greater than; na, not available; –, concentration too low to measure ratio; CDPH, California Department of 

Public Health; USGS, U.S. Geological Survey]

Well identification 

number

Source of 

inorganic 

data

pH

(standard 

units)

Dissolved 

oxygen 

(mg/L)

Oxidation-reduction 

classification1

Ratios of oxidized and reduced 

species of metals

Fe+3/

Fe+2

As+5/

As+3

Cr+6/

Cr+3

Tahoe-Martis study unit USGS-understanding wells

TMARTU-01 USGS 7.9 7.9 Oxic – >10 –

TROCKU-01 USGS 7.3 16.2 Oxic – – –

TROCKU-02 USGS 8.0 5.3 Oxic – >10 –

TTAHOU-01 USGS 8.8 4.5 Oxic – >10 –

TTAHOU-02 USGS 7.4 3.7 Oxic – – –

TTAHOU-03 USGS 9.0 1.2 Oxic 0.67 >10 –

TTAHOU-04 USGS 6.6 9.0 Oxic – – –

TTAHOU-05 USGS 9.2 5.8 Oxic – >10 –

TTAHOU-06 USGS 8.4 6.6 Oxic – – –

TTAHOU-07 USGS 7.3 1.6 Oxic – >10 –

TTAHOU-08 USGS 6.1 11.4 Oxic – – –

Tahoe-Martis study unit CDPH-grid wells

TROCK-DPH-03 CDPH na na Oxic (noDO) na na na

TROCK-DPH-04 CDPH na na Oxic (noDO) na na na

TROCK-DPH-06 CDPH na na Oxic (noDO) na na na

TROCK-DPH-12 CDPH na na Oxic (noDO) na na na

TTAHO-DPH-01 CDPH na na Oxic (noDO) na na na

TTAHO-DPH-04 CDPH na na Oxic (noDO) na na na

TTAHO-DPH-07 CDPH 6.3 na Oxic (noDO)2 na na na

TTAHO-DPH-09 CDPH 7.9 na Oxic (noDO) na na na

TTAHO-DPH-14 CDPH 9.0 na Oxic (noDO) na na na

Central Sierra study unit USGS-grid wells

CGOLD-01 USGS 7.4 0.8 Anoxic NO3-red >10 >10 –

CGOLD-02 USGS 6.9 0.2 Anoxic MnFe-red <0.01 >10 –

CGOLD-03 USGS 7.1 0.2 Anoxic MnFe-red 0.03 – –

CGOLD-04 USGS 6.2 5.4 Oxic 0.27 >10 –

CGOLD-05 USGS 8.0 1.4 Oxic – 3.8 –

CGOLD-06 USGS 6.4 0.8 Anoxic Mn-red >10 >10 –

CGOLD-07 USGS 9.4 0.2 Suboxic – >10 –

CGOLD-08 USGS 6.0 1.6 Oxic 1.0 – –

CGOLD-09 USGS 8.7 1.1 Oxic – 10.0 –

CGOLD-10 USGS 6.0 1.5 Mixed anoxic Mn-red/oxic 2.4 >10 –

CGOLD-11 USGS 6.7 0.2 Anoxic MnFe-red 0.07 >10 –

CGOLD-12 USGS 7.5 0.2 Anoxic Mn-red 0.42 >10 –

CGOLD-13 USGS 7.2 8.4 Mixed anoxic Mn-red/oxic 0.69 >10 –

CGOLD-14 USGS 6.9 1.1 Oxic 0.67 >10 –

CGOLD-15 USGS 7.6 0.2 Suboxic 1.0 >10 –

CGOLD-16 USGS 6.8 9.3 Oxic 0.50 – –

CGOLD-17 USGS 7.3 2.2 Oxic >10 >10 –

CGOLD-18 USGS 8.5 3.4 Oxic >10 3.5 –
206 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Table C5. Oxidation-reduction classification, dissolved oxygen concentration, pH, and oxidation-reduction species ratios 

for arsenic, iron, and chromium, Tahoe-Martis, Central Sierra, and Southern Sierra study units, 2006–2007, California GAMA 

Priority Basin Project.—Continued

[Well identification numbers: TMART, Tahoe-Martis study unit, Martis study area; TROCK, Tahoe-Martis study unit, Hard Rock study area; 

TTAHO, Tahoe-Martis study unit, Tahoe study area; CGOLD, Central Sierra study unit, Coarse Gold study area; CWISH, Central Sierra study unit, 

Wishon study area; SOSA, Southern Sierra study unit. Oxidation-reduction classification: NO3–red, nitrate-reducing; Mn-red, manganese-reducing; 

MnFe-red, manganese and iron reducing; noDO, no dissolved oxygen data available; noMn, no manganese data available. Ratios of oxidized 

to reduced species of metals: Fe+3/Fe+2, ratio of the amount of iron in the +3 oxidation state (ferric iron) to the amount in the +2 oxidation state 

(ferrous iron); As+5/As+3, ratio of the amount of arsenic in the +5 oxidation state (arsenate) to the amount in the +3 oxidation state (arsenite); Cr+6/

Cr+3, ratio of the amount of chromium in the +6 oxidation state (chromate) to the amount in the +3 oxidation state (chromite). Other abbreviations: 

mg/L, milligrams per liter; <, less than; >, greater than; na, not available; –, concentration too low to measure ratio; CDPH, California Department of 

Public Health; USGS, U.S. Geological Survey]

Well identification 

number

Source of 

inorganic 

data

pH

(standard 

units)

Dissolved 

oxygen 

(mg/L)

Oxidation-reduction 

classification1

Ratios of oxidized and reduced 

species of metals

Fe+3/

Fe+2

As+5/

As+3

Cr+6/

Cr+3

Central Sierra study unit USGS-grid wells—Continued

CWISH-01 USGS 6.9 0.4 Anoxic Mn-red >10 >10 –

CWISH-02 USGS 8.3 0.2 Suboxic – 0.10 –

CWISH-03 USGS 7.2 0.3 Anoxic Mn-red 2.2 1.6 –

CWISH-04 USGS 5.7 2.2 Mixed oxic/Fe-red 0.0 – –

CWISH-05 USGS 6.0 1.0 Oxic 1.2 >10 –

CWISH-06 USGS 9.3 na Oxic (noDO) – 0.9 –

CWISH-07 USGS 7.8 1.3 Oxic – 8.4 –

CWISH-08 USGS 6.5 2.1 Oxic – – –

CWISH-09 USGS 5.3 3.2 Oxic 3.5 – –

Central Sierra study unit USGS-understanding wells

CGOLDU-01 USGS 7.2 0.2 Anoxic Mn-red 0.30 >10 –

CGOLDU-02 USGS 6.8 6.3 Oxic – >10 –

CWISHU-01 USGS 8.3 12.2 Oxic >10 >10 –

Central Sierra study unit CDPH-grid wells

CGOLD-DPH-11 CDPH 7.0 na Anoxic MnFe-red na na na

CGOLD-DPH-14 CDPH 6.0 na Oxic (noDO)2 na na na

CGOLD-DPH-16 CDPH 6.8 na Oxic (noDO)2 na na na

CGOLD-DPH-18 CDPH 7.1 na Oxic (noDO) na na na

CGOLD-DPH-19 CDPH 6.4 na Oxic (noDO) na na na

CWISH-DPH-06 na na na Insufficient na na na

CWISH-DPH-10 CDPH 7.7 na Oxic (noDO) na na na

Southern Sierra study unit USGS-grid wells

SOSA-01 CDPH na 1.4 Oxic na na –

SOSA-02 CDPH na 4.3 Oxic (noMn) na na <0.01

SOSA-03 USGS 6.9 11.6 Oxic na >10 <0.01

SOSA-04 CDPH na 2.2 Oxic (noMn) na na –

SOSA-05 CDPH na 4.7 Oxic na na –

SOSA-06 CDPH na 0.1 Anoxic Mn-red na na –

SOSA-07 USGS 6.5 0.1 Anoxic Mnfe-red 0.02 1.1 –

SOSA-08 CDPH na 0.6 Anoxic NO3-red na na –

SOSA-09 CDPH na 0.6 Suboxic na na –

SOSA-10 USGS 6.0 0.8 Anoxic MnFe-red 0.04 na –

SOSA-11 CDPH na 10.5 Oxic (noMn) na na >10

SOSA-12 CDPH na 2.1 Oxic (noMn) na na 5.0

SOSA-13 USGS 7.6 7.0 Oxic – na –

SOSA-14 CDPH na 6.0 Oxic (noMn) na na –

SOSA-15 USGS 6.8 7.8 Oxic – – –

SOSA-16 CDPH na 7.0 Oxic na na –

SOSA-17 CDPH na 1.2 Oxic na na –

SOSA-18 CDPH na 5.4 Oxic (noMn) na na –
Appendix C 207

Table C5. Oxidation-reduction classification, dissolved oxygen concentration, pH, and oxidation-reduction species ratios 

for arsenic, iron, and chromium, Tahoe-Martis, Central Sierra, and Southern Sierra study units, 2006–2007, California GAMA 

Priority Basin Project.—Continued

[Well identification numbers: TMART, Tahoe-Martis study unit, Martis study area; TROCK, Tahoe-Martis study unit, Hard Rock study area; 

TTAHO, Tahoe-Martis study unit, Tahoe study area; CGOLD, Central Sierra study unit, Coarse Gold study area; CWISH, Central Sierra study unit, 

Wishon study area; SOSA, Southern Sierra study unit. Oxidation-reduction classification: NO3–red, nitrate-reducing; Mn-red, manganese-reducing; 

MnFe-red, manganese and iron reducing; noDO, no dissolved oxygen data available; noMn, no manganese data available. Ratios of oxidized 

to reduced species of metals: Fe+3/Fe+2, ratio of the amount of iron in the +3 oxidation state (ferric iron) to the amount in the +2 oxidation state 

(ferrous iron); As+5/As+3, ratio of the amount of arsenic in the +5 oxidation state (arsenate) to the amount in the +3 oxidation state (arsenite); Cr+6/

Cr+3, ratio of the amount of chromium in the +6 oxidation state (chromate) to the amount in the +3 oxidation state (chromite). Other abbreviations: 

mg/L, milligrams per liter; <, less than; >, greater than; na, not available; –, concentration too low to measure ratio; CDPH, California Department of 

Public Health; USGS, U.S. Geological Survey]

Well identification 

number

Source of 

inorganic 

data

pH

(standard 

units)

Dissolved 

oxygen 

(mg/L)

Oxidation-reduction 

classification1

Ratios of oxidized and reduced 

species of metals

Fe+3/

Fe+2

As+5/

As+3

Cr+6/

Cr+3

Southern Sierra study unit USGS-grid wells—Continued

SOSA-19 CDPH na 1.4 Oxic na na –

SOSA-20 CDPH na 5.1 Oxic na na –

SOSA-21 CDPH na 1.2 Oxic (noMn) na na –

SOSA-22 USGS 6.1 6.6 Oxic >10 – –

SOSA-23 CDPH na 8.0 Oxic na na 2.5

SOSA-24 CDPH na 0.2 Anoxic Mn-red na na –

SOSA-25 CDPH na 4.2 Oxic na na >10

SOSA-26 CDPH na 2.1 Oxic na na –

SOSA-27 CDPH na 2.0 Mixed anoxic MnFe-red/oxic na na –

SOSA-28 CDPH na 6.3 Oxic (noMn) na na 2.0

SOSA-29 CDPH na 1.4 Oxic (noMn) na na –

SOSA-30 CDPH na 6.6 Oxic (noMn) na na –

SOSA-31 CDPH na 7.8 Oxic (noMn) na na –

SOSA-32 CDPH 7.4 8.4 Oxic (noMn) na na <0.01

SOSA-33 CDPH 7.2 0.1 Suboxic (noMn) na na –

SOSA-34 CDPH 7.2 11.6 Oxic (noMn) na na 3.0

SOSA-35 USGS 8.6 0.2 Suboxic na na –

Southern Sierra study unit USGS-understanding wells

SOSAFP-01 USGS 7.8 5.2 Oxic – na 1.3

SOSAFP-02 USGS 6.2 0.1 Anoxic MnFe-red <0.01 2.0 –

SOSAFP-03 USGS 8.1 3.8 Oxic >10 >10 3.0

SOSAFP-04 USGS 7.9 8.3 Oxic – na 2.0

SOSAFP-05 USGS 7.7 3.5 Oxic – >10 1.0

SOSAFP-06 USGS 7.7 7.9 Oxic >10 >10 5.0

SOSAFP-07 USGS 7.6 8.5 Oxic na na <0.01

SOSAFP-08 USGS 7.4 3.1 Oxic – – 3.0

SOSAFP-09 USGS 7.0 12.3 Oxic – >10 >10

SOSAFP-10 USGS 7.4 4.0 Oxic >10 – 4.5

SOSAFP-11 USGS 7.7 5.2 Oxic >10 >10 >10

SOSAFP-12 USGS 7.3 3.3 Oxic – – >10

SOSAFP-13 USGS 7.8 7.0 Oxic >10 na <0.01

SOSAFP-14 USGS 7.7 9.7 Oxic na na <0.01

SOSAFP-15 USGS 7.7 9.4 Oxic >10 >10 >10

Southern Sierra study unit CDPH-grid wells

SOSA-DPH-06 CDPH 8.2 na Anoxic MnFe-red na na na

SOSA-DPH-07 CDPH 7.1 na Anoxic MnFe-red na na na

SOSA-DPH-08 CDPH 7.6 na Oxic (noDO) na na na

SOSA-DPH-11 CDPH 8.0 na Oxic (noDO) na na na

SOSA-DPH-12 CDPH na na Insufficient na na na

SOSA-DPH-13 CDPH 8.1 na Oxic (noDO) na na na
208 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Table C5. Oxidation-reduction classification, dissolved oxygen concentration, pH, and oxidation-reduction species ratios 

for arsenic, iron, and chromium, Tahoe-Martis, Central Sierra, and Southern Sierra study units, 2006–2007, California GAMA 

Priority Basin Project.—Continued

[Well identification numbers: TMART, Tahoe-Martis study unit, Martis study area; TROCK, Tahoe-Martis study unit, Hard Rock study area; 

TTAHO, Tahoe-Martis study unit, Tahoe study area; CGOLD, Central Sierra study unit, Coarse Gold study area; CWISH, Central Sierra study unit, 

Wishon study area; SOSA, Southern Sierra study unit. Oxidation-reduction classification: NO3–red, nitrate-reducing; Mn-red, manganese-reducing; 

MnFe-red, manganese and iron reducing; noDO, no dissolved oxygen data available; noMn, no manganese data available. Ratios of oxidized 

to reduced species of metals: Fe+3/Fe+2, ratio of the amount of iron in the +3 oxidation state (ferric iron) to the amount in the +2 oxidation state 

(ferrous iron); As+5/As+3, ratio of the amount of arsenic in the +5 oxidation state (arsenate) to the amount in the +3 oxidation state (arsenite); Cr+6/

Cr+3, ratio of the amount of chromium in the +6 oxidation state (chromate) to the amount in the +3 oxidation state (chromite). Other abbreviations: 

mg/L, milligrams per liter; <, less than; >, greater than; na, not available; –, concentration too low to measure ratio; CDPH, California Department of 

Public Health; USGS, U.S. Geological Survey]

Well identification 

number

Source of 

inorganic 

data

pH

(standard 

units)

Dissolved 

oxygen 

(mg/L)

Oxidation-reduction 

classification1

Ratios of oxidized and reduced 

species of metals

Fe+3/

Fe+2

As+5/

As+3

Cr+6/

Cr+3

Southern Sierra study unit CDPH-grid wells

SOSA-DPH-15 CDPH 7.0 na Oxic (noDO) na na na

SOSA-DPH-18 CDPH 7.2 na Oxic (noDO) na na na

SOSA-DPH-21 CDPH 7.3 na Oxic (noDO) na na na

SOSA-DPH-28 CDPH 8.1 na Oxic (noDO) na na na

SOSA-DPH-30 CDPH 8.2 na Anoxic MnFe-red na na na

SOSA-DPH-31 CDPH 7.0 na Oxic (noDO)2 na na na

SOSA-DPH-32 CDPH na na Insufficient na na na

SOSA-DPH-33 CDPH 7.6 na Anoxic MnFe-red na na na

SOSA-DPH-34 CDPH 8.4 na Oxic (noDO)2 na na na

SOSA-DPH-36 na na na Insufficient na na na

SOSA-DPH-37 CDPH na na Insufficient na na na

SOSA-DPH-38 CDPH 8.1 na Oxic (noDO) na na na

1

 Oxidation-reduction classification criteria are given in table C6.

2

 Iron concentrations from California Department of Public Health (CDPH) database were greater than criteria.
Appendix C 209

Table C6. Oxidation-reduction classification system applied to samples from the TahoeMartis, Central Sierra, and Southern Sierra study units, 2006–2007, California GAMA 

Priority Basin Project.

[Oxidation-reduction classes: noDO, no dissolved oxygen data available; noMn, no manganese data 

available; NO3-red, nitrate-reducing; Fe-red, iron-reducing; Mn-red, manganese-reducing; MnFe-red, 

manganese and iron reducing. Other abbreviations: mg/L, milligram per liter; μg/L, microgram per liter; 

≥, greater than or equal to; <, less than; –, data not available; any, any concentration]

Oxidationreduction 

class

Number

of wells

Dissolved 

oxygen1

(mg/L)

Nitrate, as 

nitrogen 

(mg/L)

Manganese 

(μg/L)

Iron2

(μgL)

Oxic classes

Oxic 87 ≥ 1 Any < 50 < 200

Oxic (noDO) 1 – Any < 50 < 200

Oxic (noMn) 13 ≥ 1 Any – –

Anoxic classes

Suboxic 5 < 1 < 0.5 < 50 < 200

Suboxic (noMn) 1 < 1 < 0.5 – –

Anoxic NO3-red 2 < 1 ≥ 0.5 < 50 < 200

Anoxic Mn-red 9 < 1 Any ≥50 < 200

Anoxic MnFe-red 8 < 1 Any ≥50 ≥ 200

Mixed classes

Mixed anoxic Mn-red/oxic 3 ≥ 1 Any ≥ 50 < 200

Mixed oxic/Fe-red2 2 ≥ 1 Any < 50 ≥ 200

Mixed anoxic MnFe-red/oxic 1 ≥ 1 Any ≥ 50 ≥ 200

1

 Criteria of 1 mg/L of dissolved oxygen used instead of the criteria of 0.5 mg/L used by McMahon and 

Chapelle (2008) and Jurgens and others (2009) because measurements with values less than 1 mg/L were 

less precise in this study.

2

 Criteria of 200 μg/L of iron used instead of the criteria of 100 μg/L used by McMahon and Chapelle 

(2008) and Jurgens and others (2009) because 200 μg/L sometimes was the reporting limit for iron in the 

CDPH database.
210 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Data acquired by USGS-GAMA and data from the CDPH 

database were compared to assess the validity of merging data 

from these two sources for the status assessment. Comparisons 

were made using wells that were analyzed by USGS-GAMA 

for inorganic and radioactive constituents and had data in the 

CDPH database within the 3-year intervals used for the status 

assessment. All data were combined because the number of 

wells in each study unit with data from both sources was 

small. Because the number of data pairs for many of the 

constituents was small, the reported concentrations were 

converted to RCs by dividing by the appropriate benchmark 

concentration which permitted combining constituents into 

constituent classes for the purpose of comparing data from 

the two sources. Five constituent classes were considered: 

nutrients, trace elements with health-based benchmarks, 

radioactive constituents, trace elements with SMCL 

benchmarks, and salinity indicators with SMCL benchmarks. 

Constituents with 10 or more data pairs also were examined 

individually. The data were evaluated in three ways:

• The Wilcoxon signed-rank test was used to determine 

whether the RCs for a constituent or constituent 

class were systematically greater in one data source 

compared to the other. The test evaluates whether the 

median difference between the two values in each data 

pair is significantly different from zero (Helsel and 

Hirsch, 2002). 

• The coefficient of determination (R2) of linear 

regression of the RCs for a constituent or constituent 

class from the two data sources was used to evaluate 

the variance in the data. 

• The potential effect on the results of the status 

assessment of using data from one source or the 

other was evaluated by comparing the proportions of 

high and moderate RCs for a constituent or class of 

constituents calculated by using data from the two 

sources. 

The data for nutrients, many trace elements, and 

radioactive constituents from both sources included many nondetections. This resulted in five types of data pairs, only two of 

which were included in the quantitative comparisons.

• Both sources reported a detection of the constituent. 

Pairs of this type were included in the quantitative 

comparisons.

• Both sources reported a non-detection of the 

constituent. Pairs of this type were not included in the 

quantitative comparisons.

• One source reported a non-detection of the constituent, 

and the other source reported a detection with 

concentration greater than the reporting limit for the 

non-detection. Pairs of this type were included in the 

quantitative comparisons, and the reporting limit was 

used as the concentration for the non-detection.

• One source reported a non-detection of the constituent, 

and the other source reported a detection with 

concentration less than the reporting limit for the 

non-detection. USGS-GAMA had lower reporting 

limits than CDPH for all of the inorganic constituents 

(fig. 7); thus, this type of data pair was common. 

Pairs of this type were not included in the quantitative 

comparisons.

• USGS-GAMA reports a detection and CDPH reports 

a non-detection with a value of zero rather than a 

reporting limit. Pairs of this type were only included 

in the quantitative comparisons if the concentration 

reported by USGS-GAMA was greater than one-half of 

the benchmark value for the constituent.

Among the nutrients, only nitrate had data pairs available 

for comparison (27 pairs) (table D1). Nitrate concentrations 

from the CDPH database were significantly higher than 

those measured by USGS-GAMA (Wilcoxon signed-rank, 

p=0.042), and data for RCs greater than approximately 

0.25 deviated both above and below the 1:1 line (fig. D1A). 

However, there were no differences in the proportions of high 

RCs (0%) or moderate RCs (15%) between data from the two 

sources (fig. D2).

Appendix D: Comparison of CDPH and USGS-GAMA Data
Appendix D 211

Table D1. Results of comparisons between relative-concentrations of constituents 

in samples from wells with U.S. Geological Survey-Groundwater Ambient Monitoring 

and Assessment (USGS-GAMA) data and data in the California Department of Public 

Health (CDPH) database within the 3-year intervals used in the status assessment, 

Tahoe-Martis, Central Sierra, and Southern Sierra study units, 2006–2007, California 

GAMA Priority Basin Project.

[Results shown for constituents and constituent classes containing 10 or more data pairs. Relation 

of median differences between data pairs shown for Wilcoxon signed-rank tests in which the 

populations were determined to be significantly different (two-sided test) on the basis of p-values 

(not shown) less than threshold value (α) of 0.05; ns, test indicated no significant differences 

between the sample groups. R2 for linear regression between relative-concentrations of USGSGAMA and CDPH data in data pairs. Abbreviations: USGS, U.S. Geological Survey; GAMA, 

Groundwater Ambient Monitoring and Assessment Program; CDPH, California Department of 

Public Health; >, greater than]

Constituent or 

constituent class

Number of

data pairs

Significant 

difference R2

Inorganic constituents with health-based benchmarks

Nitrate 27 CDPH > USGS 0.81

Trace elements1 62 ns 0.76

Arsenic 18 ns 0.84

Fluoride 14 ns 0.86

Radioactive consituents2 36 USGS > CDPH 0.93

Gross alpha particle activity 14 ns 0.98

Inorganic constituents with SMCL benchmarks

Trace metals3 22 ns 0.77

Salinity indicators 122 USGS > CDPH 0.91

Specific conductance 44 ns 0.80

Chloride 27 USGS > CDPH 0.83

Sulfate 22 ns 0.98

Total dissolved solids 29 ns 0.93

1

Trace element data pairs: aluminium (2), arsenic (18), barium (5), boron (2), chromium (4), 

fluoride (14), mercury (1), nickel (1), selenium (4), vanadium (4), uranium (6).

2

Radioactive constituent data pairs: gross alpha particle activity (14), gross beta particle activity 

(1), radium activity (5), radon-222 activity (7), uranium activity (9).

3

Trace metal data pairs: iron (6), manganese (7), silver (4), zinc (5).
212 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac09-0340_fig D1ab

0.00

0.25

0.50

0.75

1.00

RELATIVE-CONCENTRATION USING CDPH DATA, 

DIMENSIONLESS

A 

0.00 0.25 0.50 0.75 1.00

RELATIVE-CONCENTRATION USING USGS-GAMA DATA, 

DIMENSIONLESS

B

RELATIVE-CONCENTRATION USING USGS-GAMA DATA, 

DIMENSIONLESS

RELATIVE-CONCENTRATION USING CDPH DATA, 

DIMENSIONLESS

0.00 0.25 0.50 0.75 1.00 1.25 1.50

0.00

0.25

0.50

0.75

1.00

1.25

1.50

10

20

30

2.00

2.00 10 20 30

High

Moderate

Low

Moderate

Low

EXPLANATION

EXPLANATION

Nitrate

Constituent

Arsenic

Barium

Boron

Chromium

Copper

Fluoride

Mercury

Nickel

Selenium

Uranium

Vanadium

Figure D1. Graphs showing comparisons of relative-concentrations measured by USGS-GAMA and 

relative-concentrations reported in the CDPH during the 3-year intervals used for the status assessment for wells 

with data from both sources for (A) nitrate, (B) trace elements with health-based benchmarks, (C) radioactive 

constituents, (D) metals with SMCL benchmarks, and (E) salinity indicators with SMCL benchmarks, Tahoe-Martis, 

Central Sierra, and Southern Sierra study units, California GAMA Priority Basin Project.
Appendix D 213

sac09-0340_fig D1cd

0.

RELATIVE-CONCENTRATION USING USGS-GAMA DATA, 

DIMENSIONLESS

0.0 0.5 1.0 1.5 2.0 25 50 75

High

Moderate

Low

C

25

50

75

RELATIVE-CONCENTRATION USING CDPH DATA, 

DIMENSIONLESS

2.0

1.5

0.5

1.0

0.0

D 

RELATIVE-CONCENTRATION USING USGS-GAMA DATA, 

DIMENSIONLESS

RELATIVE-CONCENTRATION USING CDPH DATA, 

DIMENSIONLESS

3

3.0

2.5

1.5

0.5

0.0

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3 4 5

1.0

2.0

4

5

High

Moderate

Low

Uranium activity

Radium activity

Radon-222 activity

Gross alpha particle activity

Gross beta particle activity

EXPLANATION

Constituent

Iron

Manganese

Silver

Zinc

EXPLANATION

Constituent

Figure D1.—Continued
214 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac09-0340_fig D1e

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

E

RELATIVE-CONCENTRATION USING USGS-GAMA DATA, 

DIMENSIONLESS

RELATIVE-CONCENTRATION USING CDPH DATA, 

DIMENSIONLESS

Moderate

Low

Specific conductance

Total dissolved solids

Chloride

Sulfate

EXPLANATION

Constituent

Figure D1.—Continued
Appendix D 215

sac09-0340_fig D2

AQUIFER-SCALE PROPORTION USING USGS-GAMA DATA, PERCENT

30

25

20

15

10

5

0

0 5 10 15 20 25 30

AQUIFER-SCALE PROPORTION USING CDPH DATA, PERCENT

EXPLANATION

Constituent class

Data points:

High aquifer-scale

proportion

Data points:

Moderate aquifer-scale

proportion

Nutrients

Trace elements having HBBs

Trace elements having SMCLs

Salinity indicators having SMCLs

Radioactive constituents

HBB, health-based benchmark; SMCL, secondary maximum contaminant level; CDPH, 

California Department of Public Health; USGS-GAMA, U.S. Geological Survey 

Groundwater Ambient Monitoring and Assessment Program

Figure D2. Graphs showing comparisons of proportions of high 

and moderate relative-concentrations for constituent classes in 

the set of wells with data from the USGS-GAMA and the CDPH 

database within the 3-year intervals used in the status assessment, 

Tahoe-Martis, Central Sierra, and Southern Sierra study units, 

California GAMA Priority Basin Project.
216 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Trace elements with health-based benchmarks had 

62 data pairs available for comparison, 32 of which were 

either pairs of arsenic or fluoride data (table D1). There 

were no significant differences between USGS-GAMA and 

CDPH data for trace elements as a class or for arsenic or 

fluoride individually. Data for RCs greater than approximately 

0.25 deviated above and below the 1:1 line, particularly for 

arsenic (fig. D1B). The relatively high R2 value for arsenic 

is strongly controlled by the three data pairs with RC > 2; if 

those three data pairs are excluded, the R2 value decreases 

to 0.19. Similarly, the relatively high R2 value for trace 

elements as a class is strongly controlled by the five data 

pairs (three for arsenic and two for uranium) with RC > 2; if 

those five data pairs are excluded, the R2 value decreases to 

0.56. Despite the poor agreement between the USGS-GAMA 

and CDPH trace element data on a well-by-well basis, the 

agreement between the aggregate datasets is sufficient for 

the purposes of the status assessment. For trace elements as 

a class, USGS-GAMA data and CDPH data yielded similar 

proportions with high and moderate RCs: 15% high and 

13% moderate for USGS-GAMA data, and 16% high and 

11% moderate for CDPH data (fig. D2). The proportions for 

arsenic also were similar: 39% high and 28% moderate for 

USGS-GAMA data, and 39% high and 22% moderate for 

CDPH data. These results indicate that the agreement between 

the aggregate datasets is sufficient for calculating aquifer-scale 

proportions of high, moderate, and low RCs of trace elements, 

which is the central objective of the status assessment. 

Radioactive constituents had 36 data pairs available for 

comparison, 14 of which were gross alpha particle activity 

(table D1). RCs measured by USGS-GAMA for radioactive 

constituents were significantly greater than RCs reported 

in the CDPH database (Wilcoxon signed-rank, p=0.003). 

The relatively high R2 value for radioactive constituents 

is strongly controlled by the four data pairs with RC > 2 

(fig. D1C); if those four data pairs are excluded, the R2

value decreases to 0.76. Agreement between the aggregate 

datasets for radioactive constituents was poorer than 

agreements for nutrients and for trace elements with healthbased benchmarks. For radioactive constituents as a class, 

USGS-GAMA data yielded 14% with high RCs and 19% with 

moderate RCs; CDPH data yielded 11% with high RCs and 

5.6% with moderate RCs (fig. D2). The difference between 

the proportions with moderate RCs mainly was due to greater 

proportion of moderate RCs of uranium activity in the USGSGAMA data. The reason for the systematic discrepancy 

between USGS-GAMA and CDPH results for radioactive 

constituents is unknown. 

Trace elements with SMCL benchmarks had 22 data pairs 

available for comparison (table D1). There was no significant 

difference between RCs measured by USGS-GAMA and RCs 

reported by CDPH. The relatively high R2 value for the class 

is strongly controlled by the two data pairs with RCs > 3; if 

those two data pairs are excluded, the R2 value decreases to 

0.35 (fig. D1D). Agreement between the aggregate datasets 

for trace elements with SMCL benchmarks was poorer than 

agreement for salinity indicators with SMCL benchmarks. 

For trace elements with SMCL benchmarks as a class, 

USGS-GAMA data yielded 27% with high RCs and 9.1% with 

moderate RCs, whereas, CDPH data yielded 23% with high 

RCs and 18% with moderate RCs (fig. D2). 

Salinity indicators with SMCL benchmarks had 122 data 

pairs available for comparison, and specific conductance, 

total dissolved solids, chloride, and sulfate each had a 

sufficient number of data pairs for evaluation as individual 

constituents (table D1). RCs measured by USGS-GAMA for 

salinity indicators as a class and for chloride as an individual 

constituent were significantly greater than RCs reported in 

the CDPH database (Wilcoxon signed-rank, p=0.024 and 

p=0.041, respectively). However, agreement between the 

aggregate datasets was good: for salinity indicators as a class, 

USGS-GAMA data yielded 0% high and 2.5% moderate, and 

CDPH data yielded 0% high and 1.6% moderate (fig. D2).

Major ion data for grid wells in each study unit were 

compared with major ion data from all wells in the CDPH 

database for each study unit to evaluate whether the grid 

wells in each study unit were representative of the range of 

groundwater types pumped by wells used for municipal and 

community drinking water supply in that study unit. The 

CDPH well dataset consisted of all wells with a complete 

major ion analysis within the 3-year intervals used for the 

status assessment. If there were multiple analyses available, 

the most recent one with an acceptable cation-anion balance 

was selected. The datasets were compared using Piper 

diagrams (Piper, 1944; Hem, 1989). Groundwater types are 

defined on a Piper diagram according to the cations and anions 

present in the greatest proportions (fig. D3).

Because Piper diagrams show relative abundances of 

cations and anions, only major ion analyses with acceptable 

cation-anion balances were plotted (see appendix A for 

definition of acceptable cation-anion balance). Calculating 

cation-anion balance for major ion analyses from the CDPH 

database occasionally included adjusting the reported 

alkalinity value. The CDPH database has fields for bicarbonate 

alkalinity and total alkalinity, but the data in these fields were 

not always populated correctly. Total alkalinity is a measured 

value, and bicarbonate alkalinity is a calculated value that is 

typically 1.22 times the total alkalinity (in units of milligrams 

per liter as CaCO3) for water with pH values in the range 

of pH values for most groundwater. This relationship was 

not consistently observed in the CDPH databases for these 

study units .In particular, total alkalinity and bicarbonate 

alkalinity were commonly reported as the same value. 
Appendix D 217

sac09-0340_fig D3

CALCIUM CHLORIDE

PERCENT

100

80

60

40

20

0

100

80

60

40

20

0

100

80

60

40

20

0

100

80

60

40

20

0

CALCIUM PLUS MAGNESIUM

SULFATE PLUS CHLORIDE

MAGNESIUM

SODIUM PLUS POTASSIUM

SULFATE

BICARBONATE

PERCENT

PERCENT

80

100

60

40

0

20

100 80 60 40 20 0 0 20 40 60 80 100

0

20

40

60

80

100

20

0

80

100

80

100 0

20

60 40

40 60

Ca

Mg

NaK

Ca + Mg

NaK + Mg

mixed

Ca + NaK

mixed

HCO3 HCO3 + CI

HCO3 

+ SO4 SO4 

CI + SO4 CI

EXPLANATION

Cations

Calcium

Magnesium

Sodium plus Potassium

Ca

Mg

Nak

Anions

Bicarbonate

Chloride

Sulfate

HCO3

CI

SO4

Figure D3. Piper diagram showing definitions of groundwater types.
218 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

Other inconsistencies included total alkalinity greater than 

bicarbonate alkalinity, which, in the absence of other anions, 

would indicate significant amounts of non-carbonate alkalinity, 

and presence of only total alkalinity or bicarbonate alkalinity 

data, not both. For major ion analyses with cation-anion 

balance < 90%, cation-anion balance was recalculated by 

substituting total alkalinity values equaling 1.22 or 0.82 

times the reported total alkalinity or the reported bicarbonate 

alkalinity. If any substitution resulted in a cation-anion balance 

of >  90%, the major ion analysis was used for the Piper 

diagram with the substituted value. 

The range of groundwater types represented by the 

grid wells was similar to the range of groundwater types 

with major ion data in the CDPH database in each of the 

three study units (figs. D4A–C). The anion composition of 

the majority of CDPH and grid wells from all three study 

units was classified as bicarbonate-type (HCO3), and most 

of the remainder was classified as bicarbonate-chloride 

(HCO3-Cl) (Tahoe-Martis and Central Sierra study units) or as 

bicarbonate-sulfate (HCO3-SO4) (Southern Sierra study unit). 

The cation composition of the majority of CDPH and grid 

wells was classified as calcium-sodium/potassium (Ca-NaK), 

calcium-magnesium (Ca-Mg), or mixed in the Tahoe-Martis 

study unit; Ca-NaK, Ca, or mixed in the Central Sierra study 

unit; and Ca-NaK or mixed in the Southern Sierra study unit. 

The similarities in the ranges of anion and cation compositions 

in the CDPH wells and the grid wells in each of the three 

study units suggest that the grid wells do indeed represent the 

diversity of groundwater types used for public supply in the 

three study units.

Several differences in groundwater types between the 

CDPH wells and grid wells were noted.

Tahoe-Martis study unit: One of the grid wells in the 

Hard Rock study area had Ca as the major cation and SO4

as the major cation (fig. D4A). Ca- and SO4-dominated 

groundwater was not seen in any other wells from the 

Tahoe-Martis, Central Sierra, or Southern Sierra study units. 

The well is listed in the CDPH database; however, major ion 

data were not available for comparison with the USGS-GAMA 

data.

Central Sierra study unit: A minority of the CDPH 

wells had anion compositions classified as Cl or Cl-HCO3

groundwater (fig. D4B). The wells with Cl-type groundwater 

were all located in five adjacent cells around the city of 

Oakhurst (fig. A1B), and represented one-third of the total 

number of wells (CDPH plus USGS) in those five cells. The 

Cl-HCO3–type wells were located in the same five adjacent 

cells, plus in two other, non-adjacent cells. Together, Cl and 

Cl-HCO3 types accounted for nearly two-thirds of the total 

number of wells in the seven cells. The grid wells for these 

seven cells included four Cl-HCO3–type samples, but no 

Cl-type samples. The Cl and Cl-HCO3–type samples also have 

a wide range of cation composition, from 14 to 89% NaK; 

however, the four grid wells had 63 to 87% NaK. Thus, the 

grid well dataset may not adequately represent the Cl-type 

groundwater, particularly low-NaK varieties.

Southern Sierra study unit: One CDPH well has markedly 

higher chloride than all of the other CDPH wells, and this well 

was selected as a grid well.
Appendix D 219

sac09-0340_fig D4a

80

100

60

40

0

20

SULFATE PLUS CHLORIDE

MAGNESIUM

PERCENT

80 100

CALCIUM PLUS MAGNESIUM

SODIUM PLUS POTASSIUM

CALCIUM CHLORIDE

PERCENT

100 80 60 40 20 0 0 20 40 60

SULFATE

PERCENT

20

0

80

100

80

0 100

20

60 40

40 60

BICARBONATE

100 100

80

60

40

20

0

80

60

40

20

0

100

80

60

40

20

0

100

80

60

40

20

0

0

20

40

60

80

100

EXPLANATION

USGS-grid and CDPH-grid wells

USGS-understanding well

CDPH well (most recent analysis with charge

balance greater than 90 percent)

A

Figure D4. Piper diagram for grid wells, USGS-understanding wells, and all wells in the CDPH database that had major ion 

data with acceptable charge balances during the 3-year intervals used in the status assessment for the (A) Tahoe-Martis, 

(B) Central Sierra, and (C) Southern Sierra study units, California GAMA Priority Basin Project.
220 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

sac09-0340_fig D4b

80

100

60

40

0

20

SULFATE PLUS CHLORIDE

MAGNESIUM

PERCENT

80 100

CALCIUM PLUS MAGNESIUM

SODIUM PLUS POTASSIUM

CALCIUM CHLORIDE

PERCENT

100 80 60 40 20 0 0 20 40 60

SULFATE

PERCENT

20

0

80

100

80

0 100

20

60 40

40 60

BICARBONATE

100 100

80

60

40

20

0

80

60

40

20

0

100

80

60

40

20

0

100

80

60

40

20

0

0

20

40

60

80

100

EXPLANATION

USGS-grid and CDPH-grid wells

USGS-understanding well

CDPH well (most recent analysis with charge

balance greater than 90 percent)

B

Figure D4.—Continued
Appendix D 221

sac09-0340_fig D4c

80

100

60

40

0

20

SULFATE PLUS CHLORIDE

MAGNESIUM

PERCENT

80 100

CALCIUM PLUS MAGNESIUM

SODIUM PLUS POTASSIUM

CALCIUM CHLORIDE

PERCENT

100 80 60 40 20 0 0 20 40 60

SULFATE

PERCENT

20

0

80

100

80

0 100

20

60 40

40 60

BICARBONATE

100 100

80

60

40

20

0

80

60

40

20

0

100

80

60

40

20

0

100

80

60

40

20

0

0

20

40

60

80

100

EXPLANATION

USGS-grid and CDPH-grid wells

USGS-understanding well

CDPH well (most recent analysis with charge

balance greater than 90 percent)

C

Figure D4.—Continued
222 Status and Understanding of Groundwater Quality, Tahoe-Martis, Central and Southern Sierra Study Units, 2006–2007: California GAMA Priority Basin Project

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Fram and Belitz—Status and Understanding of Groundwater Quality in the Tahoe-Martis, Central Sierra, and Southern Sierra Study Units, 2006–2007: 

California GAMA Priority Basin Project —SIR 2011–5216