U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2014–5238

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 Cascade Range and 

Modoc Plateau Study Unit, 2010: California GAMA Priority Basin Project

Photo placement
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: Well in Lassen County, California. (Photograph taken by Stephen Schmitt, 

U.S. Geological Survey.)

Back cover: View looking west toward Mount Shasta. (Photograph taken by George Bennett, 

U.S. Geological Survey.)

Sampling priority

Study area Low-use basins

Priority basins

Areas that are outside CDWR-defined

 groundwater basins
Status and Understanding of Groundwater Quality 

in the Cascade Range and Modoc Plateau Study 

Unit, 2010: California GAMA Priority Basin Project

By Miranda S. Fram and Jennifer L. Shelton

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 2014–5238

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2015

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Suggested citation:

Fram, M.S., and Shelton, J.L., 2015, Status and understanding of groundwater quality in the Cascade Range 

and Modoc Plateau study unit, 2010: California GAMA Priority Basin Project: U.S. Geological Survey Scientific 

Investigations Report 2014–5238, 131 p., http://dx.doi.org/10.3133/sir20145238.

ISSN 2328-0328 (online)

ISSN 2328-031X (print)
iii

Contents

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

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

Study-Unit Description..................................................................................................................................5

Eastside Sacramento Valley Study Area (ES)..................................................................................9

Honey Lake Valley Study Area (HL) ...................................................................................................9

Cascade Range and Modoc Plateau Low-Use Basins Study Area (LU) ...................................11

Shasta Valley and Mount Shasta Volcanic Area Study Area (SH).............................................13

Quaternary Volcanic Areas (QV) and Tertiary Volcanic Areas (TV) Study Areas ....................14

Methods.........................................................................................................................................................14

Status Assessment.............................................................................................................................15

Groundwater Quality Defined as Relative-Concentrations ................................................15

Data Used for Status Assessment ..........................................................................................16

Data for Grid-Based Calculations of Aquifer-Scale Proportions..............................17

Additional Data Used for Spatially Weighted Calculations of Aquifer-Scale 

Proportions...........................................................................................................18

Selection of Constituents for Evaluation................................................................................18

Calculation of Aquifer-Scale Proportions..............................................................................19

Understanding Assessment..............................................................................................................25

Selection of Constituents for Understanding Assessment.................................................25

Statistical Analysis ....................................................................................................................26

Characteristics of the Primary Aquifer System ......................................................................................26

Geology.................................................................................................................................................30

Land Use...............................................................................................................................................31

Hydrology .............................................................................................................................................34

Well Depth and Groundwater Age...................................................................................................34

Geochemical Conditions....................................................................................................................36

Status and Understanding of Groundwater Quality...............................................................................39

Inorganic Constituents.......................................................................................................................39

Trace Elements...........................................................................................................................58

Arsenic................................................................................................................................58

Boron ................................................................................................................................62

Molybdenum......................................................................................................................63

Vanadium............................................................................................................................63

Nutrients......................................................................................................................................64

Uranium and Radioactive Constituents..................................................................................64

Constituents with Secondary Maximum Contaminant Level (SMCL) Benchmarks........65

Total Dissolved Solids (TDS) ...........................................................................................65

Manganese and Iron........................................................................................................66

Organic and Special-Interest Constituents....................................................................................66

Herbicides...................................................................................................................................69

Trihalomethanes.........................................................................................................................74

Perchlorate .................................................................................................................................75
iv

Contents—Continued

Summary........................................................................................................................................................77

References Cited..........................................................................................................................................79

Appendix A: Attribution of Potential Explanatory Factors ....................................................................88

Aquifer Lithology and Study Area ....................................................................................................88

Land Use...............................................................................................................................................99

Septic Systems and Underground Storage Tanks ........................................................................99

Well Construction Information and Hydrologic Conditions .........................................................99

Groundwater Age Classification ....................................................................................................103

Geochemical Conditions..................................................................................................................106

References Cited ..............................................................................................................................110

Appendix B: Comparison of California Department of Public Health and U.S. Geological 

Survey-Groundwater Ambient Monitoring and Assessment Program Data......................113

References Cited ..............................................................................................................................119

Appendix C: Aquifer-Scale Proportions in Study Areas......................................................................120

References Cited ..............................................................................................................................120

Appendix D. Radioactivity.........................................................................................................................127

References Cited ..............................................................................................................................128

Appendix E: Additional Water-Quality Data...........................................................................................129

References Cited...............................................................................................................................129

Figures

1. Map showing location of the Cascade Range and Modoc Plateau study unit, 

California Groundwater Ambient Monitoring and Assessment Program Priority 

Basin Project, and the California hydrogeologic provinces..................................................4

2. Map showing geologic features of the Cascade Range and Modoc Plateau study 

unit, 2010, California Groundwater Ambient Monitoring and Assessment Program 

Priority Basin Project...................................................................................................................6

3. Map showing California Department of Water Resources defined groundwater 

basins and major hydrologic features in the Cascade Range and Modoc Plateau study 

unit, 2010, California Groundwater Ambient Monitoring and Assessment Program 

Priority Basin Project...................................................................................................................8

4. Map showing study areas and well locations in the Cascade Range and Modoc 

Plateau study unit, 2010, California Groundwater Ambient Monitoring and 

Assessment Program Priority Basin Project..........................................................................10

5. Map showing land use and major hydrologic features for the Cascade Range and 

Modoc Plateau study unit, 2010, California Groundwater Ambient Monitoring and 

Assessment Program Priority Basin Project..........................................................................32

6. Ternary diagrams showing percentage of urban, agricultural, and natural land use 

in the Cascade Range and Modoc Plateau study unit, 2010, California Groundwater 

Ambient Monitoring and Assessment Program Priority Basin Project. A, Average land 

use in the gridded area, average land use within 500-meters of USGS-grid wells, and 

average land use within 500-meters of California Department of Public Health wells for 

the study unit as a whole and for each of the six study areas; and B, land use within 

500-meters of each U.S. Geological Survey-grid well by study area.................................33
v

Figures—Continued

7. Plots showing depths to screened or open interval for U.S. Geological Surveygrid wells by study area, Cascade Range and Modoc Plateau study unit, 2010, 

California Groundwater Ambient Monitoring and Assessment Program Priority Basin 

Project. A, to top of screened or open interval; and B, to bottom of screened or 

open interval................................................................................................................................35

8. Bar charts showing the relations for wells in the Cascade Range and Modoc Plateau 

study unit, 2010, California Groundwater Ambient Monitoring and Assessment Program 

Priority Basin Project between groundwater A, age class and depth class; B, depth 

class and study area; and C, age class and study area.......................................................37

9. Boxplots showing values for groundwater samples from the six study areas, 

Cascade Range and Modoc Plateau study unit, 2010, California Groundwater Ambient 

Monitoring and Assessment Program Priority Basin Project. A, dissolved oxygen 

concentration and B, pH............................................................................................................38

10. Graph showing maximum relative-concentrations (RC) of constituents detected 

in U.S. Geological Survey-grid wells by constituent class, Cascade Range and 

Modoc Plateau study unit, 2010, California Groundwater Ambient Monitoring and 

Assessment Program Priority Basin Project..........................................................................40

11. Graphs showing relative-concentrations for water samples from wells in the 

Cascade Range and Modoc Plateau study unit, 2010, California Groundwater Ambient 

Monitoring and Assessment Program Priority Basin Project of selected A, trace 

elements with health-based benchmarks; B, nutrients and radioactive constituents 

with health-based benchmarks; and C, salinity indicators and trace metals with 

secondary maximum contaminant level benchmarks..........................................................41

12. Maps showing concentrations in water samples from U.S. Geological Survey-grid 

wells and all wells in the California Department of Public Health database with data 

during the 3-year period used in the status assessment from September 16, 2007, 

through September 16, 2010, Cascade Range and Modoc Plateau study unit, 2010, 

California Groundwater Ambient Monitoring and Assessment Program Priority Basin 

Project of A, arsenic; B, boron; C, molybdenum; D, vanadium; E, uranium; F, total 

dissolved solids; G, manganese; and H, iron..........................................................................43

13. Map showing locations of U.S. Geological Survey-grid wells, total dissolved solids 

concentrations, selected trace element concentrations, and selected geologic 

and hydrologic features of the Honey Lake Valley study area, Cascade Range and 

Modoc Plateau study unit, 2010, California Groundwater Ambient Monitoring and 

Assessment Program Priority Basin Project..........................................................................60

14. Piper diagrams showing samples from the Honey Lake Valley study area, Cascade 

Range and Modoc Plateau study unit, 2010, California Groundwater Ambient 

Monitoring and Assessment Program Priority Basin Project.............................................61

15. Graph showing detection frequency and maximum relative-concentration 

of organic and special-interest constituents detected in U.S. Geological 

Survey-grid wells in the Cascade Range and Modoc Plateau study unit, 2010, 

California Groundwater Ambient Monitoring and Assessment Program Priority 

Basin Project...............................................................................................................................67

16. Bar charts showing area-weighted detection frequency and graphs of relativeconcentrations of selected organic and special-interest constituents detected 

in U.S. Geological Survey-grid wells, Cascade Range and Modoc Plateau study 

unit, 2010, California Groundwater Ambient Monitoring and Assessment Program 

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

Figures—Continued

17. Maps showing relative-concentrations in groundwater samples from U.S. Geological 

Survey-grid wells, Cascade Range and Modoc Plateau study unit, 2010, California 

Groundwater Ambient Monitoring and Assessment Program Priority Basin Project 

for A, herbicides; B, trihalomethanes; and C, perchlorate ..................................................71

18. Graph showing detection frequencies of chloroform in groundwater samples 

grouped by percentage of urban land use and density of septic tanks in the 

500-meter buffer around the well, Cascade Range and Modoc Plateau study 

unit, 2010, California Groundwater Ambient Monitoring and Assessment Program 

Priority Basin Project.................................................................................................................75

19. Graph 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 by aridity index, Cascade Range and 

Modoc Plateau study unit, 2010, California Groundwater Ambient Monitoring and 

Assessment Program Priority Basin Project..........................................................................76

A1. Maps showing boundaries of the six study areas and study area buffers and 

locations of U.S. Geological Survey-grid wells and California Department of Public 

Health wells for A, the entire study unit; B, the northwestern; C, north–central; 

D, northeastern; E, west–central; F, southwestern; and G, southeastern parts of 

the Cascades Range and Modoc Plateau study unit, 2010, California Groundwater 

Ambient Monitoring and Assessment Program Priority Basin Project.............................89

B1. Piper diagrams showing A, samples collected by the U.S. Geological Survey for 

the study unit; and B, wells in the California Department of Public Health database 

having major-ion chemical analyses with acceptable cation-anion balance during 

the time period September 16, 2007, through September 17, 2010, Cascade Range 

and Modoc Plateau study unit, 2010, California Groundwater Ambient Monitoring 

and Assessment Program Priority Basin Project................................................................114

B2. Graphs showing comparison of data collected by the U.S. Geological Survey 

and data reported in the California Department of Public Health database during 

the time period September 16, 2007, through September 17, 2010, for A, nitrate; 

B, manganese; C, arsenic; D, vanadium; E, flouride, F, chloride, and; G, total 

dissolved solids, Cascade Range and Modoc Plateau study unit, 2010, California 

Groundwater Ambient Monitoring and Assessment Program Priority Basin 

Project ........................................................................................................................................117

B3. Graphs showing comparison of data collected by the U.S. Geological Survey and 

data reported in the California Department of Public Health database during the 

time period September 16, 2007, through September 17, 2010, for A, lead; B, grossalpha particle activity; and C, iron, Cascade Range and Modoc Plateau study unit, 

2010, California Groundwater Ambient Monitoring and Assessment Program Priority 

Basin Project.............................................................................................................................119

Tables

1. Relative-concentration categories used for assessing groundwater quality..................16

2. Study-area names, study-area and grid-cell sizes, and numbers of California 

Department of Public Health wells and U.S. Geological Survey-grid wells in each study 

area, Cascade Range and Modoc Plateau study unit, 2010, California Groundwater 

Ambient Monitoring and Assessment Program Priority Basin Project.............................17
vii

Tables—Continued

3. Summary of constituent groups analyzed in the 90 samples collected by the 

U.S. Geological Survey for the Cascade Range and Modoc Plateau study unit, 2010, 

California Groundwater Ambient Monitoring and Assessment Program Priority 

Basin Project...............................................................................................................................18

4A. Benchmark type and value and reporting limits for inorganic constituents detected 

at moderate or high relative-concentrations in samples collected for the Cascade 

Range and Modoc Plateau study unit, 2010, California Groundwater Ambient 

Monitoring and Assessment Program Priority Basin Project.............................................20

4B. Benchmark type and value and reporting limits for detected inorganic constituents 

having no benchmarks or present only at low relative-concentrations in the Cascade 

Range and Modoc Plateau study unit, 2010, California Groundwater Ambient 

Monitoring and Assessment Program Priority Basin Project.............................................21

5. Benchmark type and value and reporting limits for organic and special-interest 

constituents detected in samples collected for the Cascade Range and Modoc 

Plateau study unit, 2010, California Groundwater Ambient Monitoring and 

Assessment Program Priority Basin Project..........................................................................22

6. Constituents reported at concentrations greater than benchmarks in the California 

Department of Public Health database historically (November 16, 1982, to 

September 15, 2007), but not during the 3-year time period used in status assessment 

(September 16, 2007, to September 16, 2010), Cascade Range and Modoc Plateau 

study unit, 2010, California Groundwater Ambient Monitoring and Assessment 

Program Priority Basin Project.................................................................................................24

7A. Results of multi-stage Kruskal-Wallis tests for differences in values of selected 

potential explanatory factors between samples classified into groups by study 

area, aquifer lithology, age class, oxidation-reduction (redox) class, and depth 

class, Cascade Range and Modoc Plateau study unit, 2010, California Groundwater 

Ambient Monitoring and Assessment Program Priority Basin Project.............................27

7B. Results of Spearman’s tests for correlations between selected potential 

explanatory factors, Cascade Range and Modoc Plateau study unit, 2010, 

California Groundwater Ambient Monitoring and Assessment Program Priority 

Basin Project...............................................................................................................................29

7C. Results of contingency table tests for associations between selected potential 

explanatory factors, Cascade Range and Modoc Plateau study unit, 2010, 

California Groundwater Ambient Monitoring and Assessment Program Priority 

Basin Project...............................................................................................................................30

8. Aquifer-scale proportions for inorganic constituents detected at high or moderate 

relative concentrations in the Cascade Range and Modoc Plateau study unit, 2010, 

California Groundwater Ambient Monitoring and Assessment Program Priority 

Basin Project...............................................................................................................................51

9A. Summary of spatially weighted aquifer-scale proportions for inorganic constituent 

classes with health-based benchmarks, Cascade Range and Modoc Plateau study 

unit, 2010, California Groundwater Ambient Monitoring and Assessment Program 

Priority Basin Project.................................................................................................................52

9B. Summary of spatially weighted aquifer-scale proportions for inorganic constituent 

classes with secondary maximum contaminant level benchmarks, Cascade Range 

and Modoc Plateau study unit, 2010, California Groundwater Ambient Monitoring 

and Assessment Program Priority Basin Project..................................................................53
viii

Tables—Continued

10. Results of contingency table tests for differences in aquifer-scale proportions of 

selected inorganic constituents and constituent classes between study areas, 

Cascade Range and Modoc Plateau study unit, 2010, California Groundwater 

Ambient Monitoring and Assessment Program Priority Basin Project.............................54

11A. Results of multi-stage Kruskal-Wallis tests for differences in concentrations of 

selected water-quality constituents between U.S. Geological Survey-grid wells 

classified into groups by study area, aquifer lithology class, age class, oxidationreduction (redox) class, and depth class, Cascade Range and Modoc Plateau study 

unit, 2010, California Groundwater Ambient Monitoring and Assessment Program 

Priority Basin Project.................................................................................................................55

11B. Results of Spearman’s rho tests for correlations between values of selected 

potential explanatory factors and concentrations of selected water-quality 

constituents for the U.S. Geological Survey-grid wells, Cascade Range and 

Modoc Plateau study unit, 2010, California Groundwater Ambient Monitoring and 

Assessment Program Priority Basin Project..........................................................................57

12. Results of Spearman’s tests for correlations between concentrations of selected 

inorganic constituents in the U.S. Geological Survey-grid wells, Cascade Range and 

Modoc Plateau study unit, 2010, California Groundwater Ambient Monitoring and 

Assessment Program Priority Basin Project..........................................................................58

13. Summary of aquifer-scale proportions calculated using the spatially weighted 

method for organic constituent classes with health-based benchmarks and 

perchlorate for study areas, Cascade Range and Modoc Plateau study unit, 2010, 

California Groundwater Ambient Monitoring and Assessment Program Priority 

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

A1. Land use percentages, septic-tank density, underground storage tank density and 

geology classification for U.S. Geological Survey-grid wells, Cascade Range and 

Modoc Plateau study unit, 2010, California Groundwater Ambient Monitoring and 

Assessment Program Priority Basin Project..........................................................................96

A2. Hydrologic conditions and well construction information for U.S. Geological 

Survey-grid wells, Cascade Range and Modoc Plateau study unit, 2010, 

California Groundwater Ambient Monitoring and Assessment Program Priority 

Basin Project.............................................................................................................................100

A3. Tritium and carbon-14 threshold values used for groundwater age 

classification for the Cascade Range and Modoc Plateau study unit, 2010, 

California Groundwater Ambient Monitoring and Assessment Program Priority 

Basin Project.............................................................................................................................104

A4. Tritium, carbon-14 data, and groundwater age classes, Cascade Range and 

Modoc Plateau study unit, 2010, California Groundwater Ambient Monitoring and 

Assessment Program Priority Basin Project........................................................................104

A5. Oxidation-reduction classification system applied to U.S. Geological Surveygrid wells from the Cascade Range and Modoc Plateau study unit, 2010, 

California Groundwater Ambient Monitoring and Assessment Program Priority 

Basin Project.............................................................................................................................106
ix

Tables—Continued

A6. Oxidation-reduction classification, dissolved oxygen concentration, pH, and oxidationreduction species ratios for arsenic and iron, Cascade Range and Modoc Plateau 

study unit, 2010, California Groundwater Ambient Monitoring and Assessment 

Program Priority Basin Project...............................................................................................107

B1. Results of comparisons between concentrations of constituents in samples from 

wells with U.S. Geological Survey data and data in the California Department of Public 

Health database during the time period September 16, 2007, through September 17, 

2010, Cascade Range and Modoc Plateau study unit, 2010, California Groundwater 

Ambient Monitoring and Assessment Program Priority Basin Project...........................116

C1A. Aquifer-scale proportions for the Sacramento Valley Eastside study area for 

inorganic constituents detected at high or moderate relative-concentrations in 

the Cascade Range and Modoc Plateau study unit, 2010, California Groundwater 

Ambient Monitoring and Assessment Program Priority Basin Project...........................121

C1B. Aquifer-scale proportions for the Honey Lake Valley study area for inorganic 

constituents detected at high or moderate relative concentrations in the Cascade 

Range and Modoc Plateau study unit, 2010, California Groundwater Ambient 

Monitoring and Assessment Program Priority Basin Project...........................................122

C1C. Aquifer-scale proportions for the Cascade Range and Modoc Plateau Low 

Use Basins study area for inorganic constituents detected at high or moderate 

relative concentrations in the Cascade Range and Modoc Plateau study unit, 2010, 

California Groundwater Ambient Monitoring and Assessment Program Priority 

Basin Project.............................................................................................................................123

C1D. Aquifer-scale proportions for the Quaternary Volcanic Areas study area for 

inorganic constituents detected at high or moderate relative concentrations in 

the Cascade Range and Modoc Plateau study unit, 2010, California Groundwater 

Ambient Monitoring and Assessment Program Priority Basin Project...........................124

C1E. Aquifer-scale proportions for the Shasta Valley and Mount Shasta Volcanic 

Area study area for inorganic constituents detected at high or moderate relative 

concentrations in the Cascade Range and Modoc Plateau study unit, 2010, 

California Groundwater Ambient Monitoring and Assessment Program Priority 

Basin Project.............................................................................................................................125

C1F. Aquifer-scale proportions for the Tertiary Volcanic Areas study area for inorganic 

constituents detected at high or moderate relative concentrations in the Cascade 

Range and Modoc Plateau study unit, 2010, California Groundwater Ambient 

Monitoring and Assessment Program Priority Basin Project...........................................126

E1. Results for analyses of noble gases by Lawrence Livermore National Laboratory 

for samples collected for the Cascade Range and Modoc Plateau study unit, 2010, 

California Groundwater Ambient Monitoring and Assessment Program Priority 

Basin Project.............................................................................................................................129
x

Conversion Factors, Datums, 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

cubic foot per second (ft3

/s) 0.02832 cubic meter per second (m3/s)

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

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).

Elevation, as used in this report, refers to distance above the vertical datum.

*Transmissivity: The standard unit for transmissivity is cubic foot per day per square foot times 

foot of aquifer thickness [(ft3

/d)/ft2]ft. In this report, the mathematically reduced form, foot 

squared per day (ft2/d), is used for convenience.

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). Activities for radioactive 

constituents are given in picocuries per liter (pCi/L) or tritium units (TU). 
xi

Abbreviations and Acronyms

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

BQ benchmark quotient

CAMP Cascade Range and Modoc Plateau

CDPH California Department of Public Health (California Department of Health Services 

prior to July 1, 2007)

CDPR California Department of Pesticide Regulation

CDWR California Department of Water Resources

DO dissolved oxygen

ES Eastside Sacramento Valley Eastside

GAMA Groundwater Ambient Monitoring and Assessment Program

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

HBSL health-based screening level

HL Honey Lake Valley

ka kilo annum (thousand years ago)

LLNL Lawrence Livermore National Laboratory

LU Cascade Range and Modoc Plateau Low Use Basins

Ma mega annum (million years ago)

MCL-CA California Department of Public Health maximum contaminant level 

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

MTBE methyl tert-butyl ether

NAWQA National Water-Quality Assessment Program (USGS) 

NL-CA California Department of Public Health notification level 

per mil parts per thousand

pmc percent modern carbon 

QV Quaternary volcanic areas

RC relative-concentration

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

SH Shasta Valley and Mount Shasta volcanic area

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 

TEAP terminal electron-acceptor processes

SWRCB State Water Resources Control Board (California)

TDS total dissolved solids

THM trihalomethane

TV Tertiary volcanic areas

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

UST underground storage tank

VOC volatile organic compound
xii

Acknowledgments

The authors thank the following cooperators for their support: the State Water Resources 

Control Board, the Lawrence Livermore National Laboratory, the California Department of Public 

Health, and the California Department of Water Resources. We especially thank the cooperating 

well owners and water purveyors for their generosity in allowing the U.S. Geological Survey 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 State Water Resources Control Board.
Status and Understanding of Groundwater Quality in the 

Cascade Range and Modoc Plateau Study Unit, 2010: 

California GAMA Priority Basin Project

By Miranda S. Fram and Jennifer L. Shelton

Abstract 

Groundwater quality in the Cascade Range and Modoc 

Plateau study unit was investigated as part of the California 

State Water Resources Control Board’s Groundwater Ambient 

Monitoring and Assessment (GAMA) Program Priority Basin 

Project. The study was designed to provide a statistically 

unbiased assessment of untreated groundwater quality in the 

primary aquifer system. The depth of the primary aquifer 

system for the Cascade Range and Modoc Plateau study 

unit was delineated by the depths of the screened or open 

intervals of wells in the State of California’s database of 

public-supply wells. Two types of assessments were made: 

a status assessment that described the current quality of the 

groundwater resource, and an understanding assessment that 

made evaluations of relations between groundwater quality 

and potential explanatory factors representing characteristics 

of the primary aquifer system. The assessments characterize 

the quality of untreated groundwater, not the quality of treated 

drinking water delivered to consumers by water distributors. 

The status assessment was based on water-quality 

data collected in 2010 by the U.S. Geological Survey from 

90 wells and springs (USGS-grid wells) and on water-quality 

data compiled from the State of California’s regulatory 

compliance database for samples collected from 240 publicsupply wells between September 2007 and September 2010. 

To provide context, the water-quality data discussed in this 

report were compared to California and Federal drinkingwater regulatory and non-regulatory benchmarks for treated 

drinking water. Groundwater quality is defined in terms 

of relative concentrations (RCs), which are calculated by 

dividing the concentration of a constituent in groundwater by 

the concentration of the benchmark for that constituent. The 

RCs for inorganic constituents (major ions, trace elements, 

nutrients, and radioactive constituents) were classified 

as “high” (the RC is greater than 1.0, indicating that the 

concentration is above the benchmark), “moderate” (the RC 

is from 1.0 to greater than 0.5), or “low” (the RC is less than 

or equal to 0.5). For organic constituents (volatile organic 

compounds and pesticides) and special-interest constituents 

(perchlorate), the boundary between moderate and low RCs 

was set at 0.1. All benchmarks used for organic constituents 

were health-based. For inorganic constituents, health-based 

and aesthetic-based benchmarks were used. Constituents 

without benchmarks were not considered in the status 

assessment.

The primary metric used for quantifying regional-scale 

groundwater quality was the aquifer-scale proportion—the 

areal percentages of the primary aquifer system with high, 

moderate, and low RCs for a given constituent or class of 

constituents. The study unit was divided into six study areas 

on the basis of geologic differences (Eastside Sacramento 

Valley, Honey Lake Valley groundwater basin, Cascade Range 

and Modoc Plateau Low Use Basins, Quaternary Volcanic 

Areas, Shasta Valley and Mount Shasta Volcanic Area, and 

Tertiary Volcanic Areas), and each study area was divided 

into equal-area grid cells. Aquifer-scale proportions were 

calculated for individual constituents and constituent classes 

for each of the six study areas and for the study unit as a whole 

by using grid-based (one well per cell) and spatially weighted 

(many wells per cell) statistical methods. 

The status assessment showed that inorganic constituents 

were present at high and moderate RCs in greater proportions 

of the Cascade Range and Modoc Plateau study unit than 

were organic constituents. One or more inorganic constituents 

with health-based benchmarks were present at high RCs 

in 9.4 percent, and at moderate RCs in 14.7 percent of the 

primary aquifer system. Arsenic was present at high RCs in 

approximately 3 percent of the primary aquifer system; boron, 

molybdenum, uranium, and vanadium each were present at 

high RCs in approximately 2 percent of the primary aquifer 

system. One or more inorganic constituents with aestheticbased benchmarks were present at high RCs in 15.1 percent 

of the primary aquifer system and at moderate RCs in 

4.9 percent. Manganese, iron, and total dissolved solids were 

present at high RCs in approximately 12 percent, 5 percent, 

and 2 percent, respectively, of the primary aquifer system.

Organic constituents were not detected at high or 

moderate RCs in the primary aquifer system, and one or 

more organic constituents were detected at low RCs in 

approximately 40 percent of the primary aquifer system. 
2 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Two classes of organic constituents were detected 

in more than 10 percent of the primary aquifer system: 

trihalomethanes (chloroform only) and herbicides. The special 

interest constituent perchlorate was not detected at high RCs, 

but was detected at moderate RCs in approximately 2 percent 

of the primary aquifer system.

The understanding assessment relied on statistical tests 

to evaluate relations between concentrations of constituents 

and values of potential explanatory factors representing 

geology, land use, well construction, hydrologic conditions, 

groundwater age, and geochemical conditions. 

The majority of the high and moderate RCs of arsenic, 

boron, molybdenum, uranium, and total dissolved solids 

were in samples from the Honey Lake Valley groundwater 

basin study area. Groundwater mixing with hydrothermal 

fluids present in the study area, evaporative concentration of 

groundwater in the Honey Lake playa, presence of uraniumbearing sediment derived from the adjacent Sierra Nevada, 

and release of arsenic and other trace elements from sediments 

under high pH and low dissolved oxygen conditions all 

appeared to contribute to these elevated concentrations. 

Thermal springs are in many parts of the Cascade Range 

and Modoc Plateau study unit and could account for locally 

elevated concentrations of arsenic, boron, molybdenum, and 

total dissolved solids in samples from the other study areas. 

Vanadium concentrations were greater in oxic samples than in 

anoxic samples, but were not correlated with pH, contrary to 

expectations from previous studies.

Organic constituents were not detected at high or 

moderate RCs, and the occurrence of low organic constituents 

at low RCs ranged from 27 percent to 73 percent of the 

primary aquifers system in the six study areas. The Shasta 

Valley and Mount Shasta Volcanic study area had significantly 

greater occurrence of low RCs of herbicides compared to 

all of the other study areas, which could reflect the greater 

prevalence of modern groundwater in the Shasta Valley 

and Mount Shasta Volcanic study area and the presence 

of potential sources of herbicides, including applications 

to timberlands and roadside rights-of-way. The Eastside 

Sacramento Valley study area had the greatest occurrence of 

low concentrations of chloroform, and chloroform occurrence 

was most strongly associated with the combination of septictank density greater than two tanks per square kilometer and 

urban land use greater than 10 percent within a radius of 

500 meters of the well. These conditions were most prevalent 

in the Eastside Sacramento Valley study area. The detection 

frequency of low concentrations of perchlorate was consistent 

with the probability of occurrence expected under natural 

conditions, except in the Eastside Sacramento Valley study 

area, where detection frequencies were much higher than 

expected and could not be explained by known anthropogenic 

sources of perchlorate.

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 drinkingwater supply and to establish a baseline groundwaterquality monitoring program, the California State Water 

Resources Control Board (SWRCB), in cooperation with the 

U.S. Geological Survey (USGS) and Lawrence Livermore 

National Laboratory (LLNL), implemented the Groundwater 

Ambient Monitoring and Assessment (GAMA) Program 

(website at http://www.waterboards.ca.gov/gama/). The 

SWRCB initiated the GAMA Program in 2000 in response 

to a Legislative mandate (State of California, 1999, 2001a). 

The statewide GAMA Program currently consists of four 

projects: (1) the GAMA Priority Basin Project, carried out by 

the USGS (website at http://ca.water.usgs.gov/gama/); (2) the 

GAMA Domestic Well Project, carried out by the SWRCB; 

(3) the GAMA Special Studies, carried out by LLNL, and 

(4) the GeoTracker GAMA on-line groundwater information 

system, managed by the SWRCB. The SWRCB’s GAMA 

Domestic Well Project sampled private domestic wells on a 

voluntary, first-come-first-serve basis in six counties between 

2002 and 2011. From 2004 through 2012, the GAMA Priority 

Basin Project did water-quality assessments for groundwater 

resources used for public drinking-water supplies. The 

groundwater resources used for public-drinking water supplies 

typically are deeper than the groundwater resources used for 

domestic drinking-water supplies. In 2012, the GAMA Priority 

Basin Project began water-quality assessments of shallow 

aquifers, the groundwater resources typically used for private 

domestic and small system drinking-water supplies.

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). It is a comprehensive assessment 

of statewide groundwater quality designed to help understand 

and identify risks to groundwater resources better and to 

increase the availability of information about groundwater 

quality to the public. For the GAMA Priority Basin Project, 

the USGS, in cooperation 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 ranges of hydrologic, geologic, and climatic 

conditions in California were considered in the design of 

the statewide assessment of groundwater quality. Belitz and 

others (2003) partitioned the state into 10 hydrogeologic 

provinces, each with distinctive hydrologic, geologic, and 
Introduction 3

climatic characteristics (fig. 1). All these hydrogeologic 

provinces include groundwater basins designated by the 

CDWR (California Department of Water Resources, 1980; 

2003). Groundwater basins generally consist of relatively 

permeable, unconsolidated deposits of alluvial origin. 

Eighty percent of the approximately 16,000 active and 

standby public-supply wells listed in the statewide database 

maintained by the CDPH (hereinafter referred to as CDPH 

wells) are in CDWR-designated groundwater basins. [The 

CDPH Drinking Water Program which regulated water 

quality in public-supply wells was transferred to the SWRCB 

Division of Drinking Water on July 1, 2014, however the 

label “CDPH” is retained in this report for consistency with 

other GAMA Priority Basin Project publications and because 

the CDPH had jurisdiction over public-supply wells at the 

time that samples were collected for this study.] Twenty 

percent of the CDPH wells are in areas composed of igneous, 

metamorphic, or volcanic rocks, rather than in alluvial basins. 

Groundwater basins 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 the number of square-mile (mi2

) sections having 

registered pesticide applications (Belitz and others, 2003). 

Of the 472 basins designated by the CDWR, 116 contained 

approximately 95 percent of the CDPH wells in groundwater 

basins. These 116 basins were defined as “priority basins,” and 

the remaining 356 basins were defined as “low-use basins” 

(Belitz and others, 2003). All of the priority basins, selected 

low-use basins, and selected areas outside of groundwater 

basins were grouped into 35 GAMA Priority Basin Project 

study units that together represent approximately 95 percent of 

all CDPH wells. The entire Cascade Range and Modoc Plateau 

hydrogeologic province was defined as the Cascade Range and 

Modoc Plateau (CAMP) study unit (fig. 1). The CAMP study 

unit includes 3 priority basins, 47 low-use basins, and areas 

outside of groundwater basins.

The GAMA Priority Basin Project was designed to 

produce three types of water-quality assessments for each 

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

the groundwater resource; (2) Understanding, the identification 

of the natural and human factors affecting groundwater quality 

and an explanation of the relations between water quality and 

selected explanatory factors; and (3) Trends, the detection of 

changes in groundwater quality over time (Kulongoski and 

Belitz, 2004). These three objectives were modeled after those 

of the USGS National Water Quality Assessment (NAWQA) 

Program (Hirsch and others, 1988). The assessments are 

intended to characterize the quality of groundwater in the 

primary aquifer system of the study unit, not the treated 

drinking water delivered to consumers by water purveyors. 

The primary aquifer system for a study unit is defined by the 

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

the CDPH database for the study unit. The CDPH database 

lists wells used for public drinking-water supplies and 

includes wells from systems classified as community (such as 

cities, towns, and mobile-home parks); non-transient, noncommunity (such as schools, workplaces, and restaurants); and 

transient, non-community (such as campgrounds, parks, and 

highway rest areas) (California Department of Public Health, 

2013a). The purpose of the CDPH database is to house waterquality data for samples collected from public-supply wells for 

regulatory compliance. Groundwater quality in shallower or 

deeper parts of the aquifer system can differ from that in the 

primary aquifer system. In particular, shallower groundwater 

may be more vulnerable to surface contamination.

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

unit description of the hydrogeologic setting of the CAMP 

study unit, (2) a status assessment of the current status of 

groundwater quality in the primary aquifer system of the 

CAMP study unit, and (3) an understanding assessment

that identifies the natural and human factors that could be 

affecting groundwater quality in the CAMP study unit and a 

discussion of statistical tests of relations between groundwater 

quality and potential explanatory factors. Temporal trends in 

groundwater quality are not discussed in this report. Noble 

gas data provided by the Lawrence Livermore National 

Laboratory that were not available at the time of publication 

of the Data-Series Report (Shelton and others, 2013) are 

presented in appendix E of this report.

 Features of the hydrogeologic setting are described 

for the six study areas. Geology, land-use patterns, and 

hydrology in the study areas are summarized. Characteristics 

of the primary aquifer system, including geology, land 

use, hydrologic conditions, depth, groundwater age, and 

geochemical conditions are described by using ancillary data 

compiled for the 90 wells and springs sampled by the USGS 

for the GAMA Priority Basin Project (USGS-GAMA) in the 

CAMP study unit.

The status assessment is designed to provide a 

statistically representative characterization of groundwater 

quality in the primary aquifer system at the study-unit 

scale (Belitz and others, 2003; 2010). This report describes 

methods used in designing the sampling networks for the 

status assessment and estimating aquifer-scale proportions 

for constituents (Belitz and others, 2010). Aquifer-scale 

proportion is defined as the areal proportion of the primary 

aquifer system with groundwater of defined quality (Belitz 

and others, 2010). Water-quality data from 262 wells were 

used in the status assessment: 90 wells sampled by USGSGAMA for the CAMP study unit (Shelton and others, 2013) 

and 172 other wells in the CDPH database within the study 

unit that had water-quality data for samples collected between 

September 16, 2007, through September 16, 2010. Two 

methods were used to calculate aquifer-scale proportions 

from these data, both of which were based on a 90-cell grid 

covering the CAMP study unit; the methods either used data 

from one well per cell (grid-based method) or from many 

wells per cell (spatially weighted method) (Belitz and others, 

2010). Aquifer-scale proportions for constituents and classes 

of constituents were computed for the CAMP study unit as a 

whole and for the six study areas within the study unit.
4 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Figure 1. Location of the Cascade Range and Modoc Plateau (CAMP) study unit, California Groundwater Ambient Monitoring and 

Assessment (GAMA) Program Priority Basin Project, and the California hydrogeologic provinces.

OREGON

NEVADA

MEXICO

ARIZONA

40 ̊

42 ̊ 124 ̊ 122 ̊ 120 ̊ 118 ̊ 116 ̊ 114 ̊

38 ̊

36 ̊

34 ̊

PA C I F I C O C E A N

Bakersfield

Los Angeles

San Diego

Redding

SacramentoSacramento

San

Francisco

Valley

Klamath

Mountains

Cascade Range and

Modoc Plateau

(CAMP) study unit

Transverse Ranges and

selected Peninsular Ranges

Southern Coast

Ranges

Sierra

Nevada

Northern

Coast

Ranges

Desert

Central

Basin and Range

San Diego

Drainages

0 200 MILES

0 200 KILOMETERS

100

100

Base modified from U.S. Geological Survey and other

Federal and State digital data, various scales 

Albers Equal Area Projection

North American Datum of 1983

sac15-0550_fig 01
Study-Unit Description 5

To provide context, the water-quality data discussed in 

this report were compared to California and Federal drinkingwater regulatory and non-regulatory benchmarks for treated 

drinking water. Groundwater quality is defined in terms 

of relative-concentrations (RCs), which are calculated by 

dividing the concentration of a constituent in groundwater 

by the concentration of the benchmark for that constituent. 

The assessments in this report characterize the quality of 

untreated groundwater resources in the primary aquifer system 

in the study unit, not the treated drinking water delivered to 

consumers by water purveyors. After withdrawal from the 

ground, water may be 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 evaluates relations 

between groundwater quality and potential explanatory 

factors by using statistical tests. Potential explanatory factors 

examined include aquifer lithology, study area, land use near 

the well, septic and underground storage-tank densities near 

the well, depths to the top and bottom of the open or screened 

interval in the well, aridity index, groundwater age, oxidationreduction conditions, and pH.

Study-Unit Description

The CAMP study unit covers an area of approximately 

15,000 mi2 (39,000 square kilometers [km2]) in Siskiyou, 

Modoc, Shasta, Lassen, Tehama, Plumas, and Butte Counties 

(fig. 2). The study unit corresponds to the Cascade Range and 

Modoc Plateau hydrogeologic province in the northeastern 

corner of the State (fig. 1; Belitz and others, 2003). The 

province is defined on the basis of geologic and State 

boundaries; it is bounded to the west by the Mesozoic and 

Paleozoic rocks of the Klamath Mountains province, to the 

south by Mesozoic and Paleozoic rocks of the Sierra Nevada 

province, to the southwest by the Cenozoic sediment deposits 

of the Central Valley province, to the north by the Oregon 

State line, and to the east by the Nevada State line (fig. 2).

Hydrologic features of the CAMP study unit belong to 

three major watersheds: the Sacramento River watershed 

(CDWR basin numbers beginning with 5-), the Klamath River 

watershed (CDWR basin numbers beginning with 1-), and 

closed basins of the North Lahontan region (CDWR basin 

numbers beginning with 6-) (fig. 3) (California Department 

of Water Resources, 2003). The largest river in the study unit, 

the Pit River, flows across the study unit from south of Goose 

Lake in the northeast to Lake Shasta on the Sacramento River 

in the southwest. The headwaters of the Sacramento River 

are on the southwestern slopes of Mount Shasta. Creeks in 

the southwestern part of the study unit flow into the main 

stem of the Sacramento River. Along the Oregon border, the 

Lost River drainage, including Clear Lake and Tule Lake, is 

connected to the Klamath River by a set of canals and tunnels 

(not shown). The Shasta River flows through Shasta Valley 

and enters the Klamath River below Iron Gate Dam. In the 

North Lahontan region along the Nevada border, the basins are 

closed basins; rivers drain into perennial or intermittent lakes 

within the basins.
6 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

GLENN CO

TEHAMA CO

PLUMAS CO

BUTTE CO SIERRA CO

SHASTA CO LASSEN CO

SISKIYOU CO MODOC CO

TRINITY CO

0 10 20 30 40 MILES

0 10 20 30 40 KILOMETERS

Base modified from U.S. Geological Survey

digital data, various scales 

Albers Equal Area Projection

North American Datum of 1983

122° 121° 120°

42°

41°

40°

Geology modified from California

Division of Mines and Geology, 

CD-Rom 2000-007 (2000), GIS data

for the Geologic Map of California

Central

Valley

Province

Klamath

Mountains

Province

Cascade Range

and 

Modoc Plateau

Province

Sierra

Nevada

Province

Northern

Coast

Ranges

Province

Chico

monocline

Lassen area

Honey Lake fault zone

Medicine Lake

highlands

Mount

Shasta

Surprise Valley fault

zone

Study

area

CALIFORNIA

sac15-0550_fig 02

Figure 2. Geologic features of the Cascade Range and Modoc Plateau (CAMP) study unit, 2010, California Groundwater Ambient 

Monitoring and Assessment (GAMA) Program Priority Basin Project.
Study-Unit Description 7

EXPLANATION

Faults

Hydrologic provinces

Franciscan complex (KJf, KJFm, KJFs, TK)

Plio-Pleistocene sediment (QPc)

Quaternary alluvium (Q)

Other quaternary sediments (Qg, Qls, Qs)

Quaternary, mostly mafic, volcanic rocks (Qv, Qrv)

Pyroclastic deposits (Qvp, Qvrp, Tvp)

Tertiary, mostly mafic, volcanic rocks (Tv, Ti)

Tertiary nonmarine sediment (Ec, Mc, Ogc, Tc)

Granitic rocks (gr, grCz, grMz, grpC, grPz)

Other metamorphic rocks (gr-m, m, mv, Mzv, pCc, Pzv, pC, sch)

Metasedimentary rocks (ls, C, D, J, K, Kl, Ku, Pm, Pz, SO, Tr, Ca)

Ultramafic/mafic rocks (um, gb)

Water

Volcanic

Sedimentary

Metamorphic

Plutonic

Other

Simplifed geologic units

Codes in parenthesis are the geologic types from Saucedo and others 

(2000) included in each unit. Wells sampled by U.S. Geological 

Survey-Groundwater Ambient Monitoring and Assessment Program 

for this study are located in the bold codes (table A1).

County boundary

Figure 2. —Continued

Climate in the Cascade Range and Modoc Plateau 

hydrogeologic province primarily varies with elevation and 

location relative to mountain ranges. Land-surface elevations 

in the study unit range from approximately 1,500 feet (ft) 

along the margin of the Sacramento Valley to 14,179 ft on 

Mount Shasta. Precipitation is greatest on the western side 

of the study unit, reaching up to 80 inches per year 

(in/yr) on Mount Shasta and up to 120 in/yr in high elevations 

in Lassen Volcanic National Park (PRISM Climate Group, 

Oregon State University, 2010). Elevation ranges from 

4,000 to 5,500 ft throughout most of the central and eastern 

parts of the study unit, and climate is classified as the Great 

Basin desert (defined as high, cold desert). The central and 

eastern parts of the study unit are in the rain shadow of the 

Cascade Range and the Shasta Valley is in the rain shadow 

of the Klamath Mountains. Average annual precipitation 

is 10–20 in/yr in most of the central and eastern area and 

20–40 in/yr at higher elevations (Western Regional Climate 

Center, 2010). Much of the precipitation in the entire study 

unit falls as snow, particularly at elevations above 4,500 ft, 

and nearly all of the precipitation falls in the winter (between 

November and May). Summers are hot and dry.

The CAMP study unit consists entirely of Cenozoicage volcanic and sedimentary rocks and deposits (fig. 2). 

It was divided into six study areas on the basis of geologic 

features (fig. 4). The objective of dividing the study unit into 

these study areas was to investigate potential differences in 

groundwater quality among these geologically distinct aquifer 

systems. Four of the study areas correspond to CDWRdefined groundwater basins (California Department of Water 

Resources, 1980; 2003), and two correspond to mapped 

geologic units (Jennings, 1977; Saucedo and others, 2000) 

(appendix A).

CAMP-ES study area: Eastside Sacramento Valley,

CAMP-HL study area: Honey Lake Valley groundwater 

basin, 

CAMP-LU study area: Cascade Range and Modoc 

Plateau Low Use Basins, 

CAMP-QV study area: Quaternary Volcanic Areas 

mapped as Qv on the State geologic map,

CAMP-SH study area: Shasta Valley and Mount Shasta 

Volcanic Area, 

CAMP-TV study area: Tertiary Volcanic Areas mapped 

as Tv on the State geologic map.
8 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

1-4*

1-4.02*

1-17

1-3* 1-2.01*

1-18

Klamath

River

watershed

Sacramento River

watershed

North

Lahontan

watersheds

(5-55)*

5-50*

5-49

5-47

5-48

5-46

5-5

6-91 5-1.02

5-1.01

5-86

5-2.02* 6-1*

5-36 5-43

5-41

5-40

5-2.01*

5-3

5-44

6-98

1-2.02

5-35

5-34*

5-37

5-38

5-45

5-4*

5-54

5-53

5-51 5-52

5-7*

6-4*

6-3

6-100

6-101 6-92

6-93

6-96

6-97

6-94

6-95

6-2

LASSEN VOLCANIC

NATIONAL PARK

LAVA BEDS NATIONAL

MONUMENT

WHISKEYTOWN

NATIONAL

RECREATION

AREA

GLENN CO

TEHAMA CO

PLUMAS CO

SIERRA CO

BUTTE CO

LASSEN CO SHASTA CO

MODOC CO

SISKIYOU CO

TRINITY CO

Lake

Shasta

Goose

Lake Tule

Lake

Upper

Alkali

Lake

Eagle

Lake

Honey

Lake

Lake

Almanor

Lake

Oroville

Susan River

Sacramento River

North Fork Feather River

Klamath River

Pit River

Lost River

Shasta River

Clear

Lake

122° 121° 120°

42°

41°

40°

CALIFORNIA

Study

area

0 10 20 30

Base modified from U.S. Geological Survey and other

Federal and State digital data, various scales 

Albers Equal Area Projection

North American Datum of 1983 40 KILOMETERS

0 10 20 30 40 MILES

sac15-0550_fig 03

Figure 3. California Department of Water Resources (CDWR) defined groundwater basins and major hydrologic features in the 

Cascade Range and Modoc Plateau (CAMP) study unit, 2010, California Groundwater Ambient Monitoring and Assessment (GAMA)

Program Priority Basin Project.
Study-Unit Description 9

EXPLANATION

County boundary

Major watershed boundaries

Cascade Range and Modoc Plateau Province

CDWR defined groundwater basins that contain CDPH public-supply wells 

 (basin number marked with askerisk on map)

Modoc Plateau Pleistocene

 Volcanic Areas

Volcanic areas that contain CDPH public-supply wells and were never defined

 as groundwater basins by CDWR (mostly Tertiary Volcanic study area)

Mount Shasta Volcanic Area

Shasta Valley Volcanics

Modoc Plateau Recent

 Volcanic Areas

Sacramento Valley Eastside

 Tuscan Formation Highlands

Basin number

National parks, recreationa areas, or monuments

Volcanic areas that contain California Department of Public Health (CDPH) 

public-suppy wells, and were defined as groundwater basins by California 

Department of Water Resources [(CDWR) (1980)] but discontinued by CDWR 

(2003)

Basin name Study area

5-34

1-4.02

5-55

1-23 and 5-32

1-24, 5-33, and 6-103

SH

SH

ES

none

QV

Honey Lake Valley

Shasta Valley

Upper Klamath basin, Tule Lake 

 subbaasin

Butte Valley

Alturas basin, South Fork Pit River

 subbasin

Alturas basin, Warm Springs Valley 

 subbasin

Big Valley

Fall River Valley

Lake Almanor Valley

McCloud Area

Lake Britton Area

North Battle Creek

Surprise Valley 

CDWR, 2004a

CDWR, 2004b

CDWR, 2004c

CDWR, 2004d

CDWR, 2004e

CDWR, 2004f

CDWR, 2004g

CDWR, 2004h

CDWR, 2004i

CDWR, 2004j

CDWR, 2004k

CDWR, 2004l

CDWR, 2004m

HL

SH

LU

LU

LU

LU

LU

LU

LU

LU

LU

LU

LU

6-4

1-4

1-2.01

1-3

5-2.01

5-2.02

5-4

5-5

5-7

5-35

5-46

5-50

6-1

Basin

number Basin name

Study

area Reference

CDWR defined groundwater basins that do not contain CDPH public-supply wells

Basin

number Basin name

1-2.02

1-17

1-18

1-22

5-1

5-3

5-8

5-36

5-37

5-38

5-40

5-41

5-43

5-44

5-45

5-47

5-48

5-49

Upper Klamath basin, Lower

 Klamath Lake subbasin

Bray Town Area

Red Rock Valley

Fairchild Swamp Area

Goose Lake

Jess Valley

Mountain Meadows Valley

Round Valley

Toad Well Area

Pondosa Town Area

Hot Springs Valley

Egg Lake Valley

Rock Prairie Valley

Long Valley

Clayton Valley

Goose Valley

Burney Creek Valley

Dry Burney Creek Valley

Basin

number Basin name

5-49

5-51

5-52

5-53

5-54

5-86

6-2

6-3

6-91

6-92

6-93

6-94

6-95

6-96

6-97

6-98

6-99

6-100

6-101

Dry Burney Creek Valley

Butte Creek Valley

Grays Valley

Dixie Valley

Ash Valley

Joseph Creek

Madeline Plains

Willow Creek Valley

Cow Head Lake Valley

Pine Creek Valley

Harvey Valley

Grasshopper Valley

Dry Valley

Eagle Lake Area

Horse Lake Valley

Tuledad Canyon Valley

Painters Flat

Secret Valley

Bull Flat

sac15-0550_fig 03 legend

Figure 3. —Continued

Eastside Sacramento Valley Study Area (ES)

The ES study area (fig. 4) corresponds to the former 

CDWR-defined volcanic groundwater basin 5-55, the Eastside 

Sacramento Valley Tuscan Formation Highlands (fig. 3); 

California Department of Water Resources, 1980). It is no 

longer designated as a CDWR basin because volcanic areas 

statewide were reclassified as groundwater source areas 

rather than basins to restrict the definition of basins to alluvial 

basins (California Department of Water Resources, 2003). 

Groundwater source areas are non-basin areas that may serve 

as sources of recharge to adjacent basins by subsurface flow. 

The boundary between the ES study area and the Sacramento 

Valley is defined by the Chico monocline, a normal fault 

active within the last 1 mega-annum (Ma) moving the Sierra 

Nevada upward relative to the Central Valley (figs. 2; A1F). 

East of the Chico monocline, the Tuscan Formation dips less 

than 5 degrees; west of the monocline, the Tuscan Formation 

dips at least 20 degrees, disappearing beneath the younger 

valley sediments (Harwood and Helley, 1987). The other 

boundaries of the ES study area are defined by the extent of 

surface outcrops of the Tuscan Formation. 

The Tuscan Formation in the ES study area consists 

of many volcanic mudflows, or lahars. The lahars erupted 

approximately 3 Ma from vents on the flanks of now-extinct 

volcanoes west and south of Lassen Volcanic National Park, 

and form a unit up to 1,700 ft thick (Lydon, 1968). About three 

quarters of the exposed area is tuff breccia, a massive chaotic 

assemblage of boulder to pebble size fragments of andesitic 

lava in a clastic matrix of volcanic ash and fine volcanic debris 

(Lydon, 1968). The Formation grades westward from lahar 

deposits to volcanic sediments, and in the Central Valley, the 

Tuscan Formation sediments are an important aquifer. The 

tuff breccia is less permeable than the volcanic sediments, 

resulting in variable well yields. Several communities in 

the southern part of the ES study area have had to rely on 

drinking water brought in by tanker trucks because of low 

well yields (California Department of Water Resources, 

2003). The Tuscan Formation may be described in CDWR 

well completion reports as volcanic or sedimentary materials 

depending on the facies and on the interpretation of the person 

preparing the report. 

Honey Lake Valley Study Area (HL)

The HL study area (fig. 4) corresponds to the CDWRdefined Honey Lake Valley groundwater basin (fig. 3; CDWR 

basin number 6-4; California Department of Water Resources, 

2004a). The Honey Lake Valley is a topographically closed 

basin that is geologically part of the Basin and Range geologic 

province (California Department of Conservation, California 

Geological Survey, 2002). The basin is bounded on the south 

by the Honey Lake fault, which separates it from the Mesozoic 

granitic rocks of the Sierra Nevada, and on the other sides by 

Quaternary and Tertiary lava flows (figs. 2; A1G).
10 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Lake

Shasta

Goose

Lake

Upper

Alkali

Lake

Eagle

Lake

Honey

Lake

Lake

Almanor

Lake

Oroville

Susan River

Sacramento River

Klamath River

Pit River

Shasta River

122° 121° 120°

42°

41°

40°

CALIFORNIA

Study

area

0 10 20 30 40 KILOMETERS

Base modified from U.S. Geological Survey and other

Federal and State digital data, various scales 

Albers Equal Area Projection

North American Datum of 1983

0 10 20 30 40 MILES

Central

Valley

Province

Klamath

Mountains

Province

Cascade Range

and 

Modoc Plateau

Province

Sierra

Nevada

Province

Northern

Coast

Ranges

Province

sac15-0550_fig 04

Figure 4. Study areas and well locations in the Cascade Range and Modoc Plateau (CAMP) study unit, 2010, California Groundwater 

Ambient Monitoring and Assessment (GAMA) Program Priority Basin Project.
Study-Unit Description 11

EXPLANATION

County boundary

Hydrologic provinces

California Department of Public Health public-supply wells

U.S. Geological Survey-grid wells

Tertiary Volcanic Areas (TV)

Quaternary Volcanic Areas (QV)

Shasta Valley and Mount Shasta Volcanic Area (SH)

Eastside Sacramento Valley (ES)

Cascade Range and Modoc Plateau Low-Use Basins (LU)

Honey Lake Valley (HL)

Study areas

Buffer around California Department of Public Health 

 public-supply well

Other areas in the Cascade Range and Modoc Plateau Province

sac15-0550_fig 04_legend

Figure 4. —Continued

The Honey Lake Valley is a down-faulted basin 

containing up to 6,000 ft of Pliocene to Recent (5 Ma to 

present) sediments with interbedded lava flows and pyroclastic 

deposits (California Department of Water Resources, 1963; 

Handman and others, 1990). The primary water-bearing units 

are the Pleistocene to Recent lacustrine and alluvial sediments 

(California Department of Water Resources, 2004a). Honey 

Lake was on the western edge of Pleistocene Lake Lahontan, 

a large lake that covered most of northwestern Nevada 

from about 45,000 to about 10,000 years ago (Benson and 

Thompson, 1987). Near-shore deposits from this large lake 

are relatively coarse-grained and highly permeable and are an 

important source of groundwater to wells. Pleistocene volcanic 

rocks interbedded with the lake sediments on the north side of 

the basin act as conduits for groundwater recharge to the basin.

More than 40 intermittent and perennial streams flow 

into the valley and terminate at Honey Lake. The amount 

of groundwater recharge is low, however, because about 

90 percent of the total precipitation and stream inflow 

to the basin is lost by evapotranspiration (Handman and 

others, 1990). The sources of groundwater recharge are 

direct infiltration of precipitation at higher elevations in 

the watershed, infiltration of streamflow on alluvial fans 

on the valley margins, and infiltration of irrigation return 

water. In addition, upwelling of thermal waters along the 

faults bounding the basin could contribute up to 40 percent 

of recharge locally (Mayo and others, 2010). Faults within 

the basin limit lateral groundwater flow (Rose and others, 

1997). Groundwater discharges by evapotranspiration and 

by pumping. Most of the groundwater extraction in the basin 

is for irrigation. In addition, geothermal waters associated 

with the faults bounding the basin are extracted for power 

generation (Brown and Caldwell, 2007).

Cascade Range and Modoc Plateau Low-Use 

Basins Study Area (LU)

The LU study area includes 47 CDWR-defined 

groundwater basins (figs. 3, 4). Of these 47 basins, 11 contain 

wells listed in the CDPH database of wells used for public 

drinking-water supply. Hydrologic features of those 11 basins 

are discussed briefly here.

The Tule Lake subbasin of the Upper Klamath River 

groundwater basin is bounded on the east and west by northsouth trending normal faults, on the south by Pleistocene and 

Holocene lava flows of the Medicine Lake volcano, and on 

the north by the Oregon state line (figs. 2, 3; CDWR basin 

number 1-2.01; California Department of Water Resources, 

2004c). The subbasin is composed of Pliocene to Holocene 

age lacustrine deposits that have relatively low permeability 

and interbedded Miocene, Pliocene and Pleistocene basalt lava 

flows. The principal aquifer is the highly permeable Miocene/

Pliocene basalt lava flows beneath the lake sediments. In 2001, 

10 deep irrigation wells were drilled into this aquifer, and they 

have yields ranging from 4,000 to 12,000 gallons per minute 

(gpm) (Gannett and others, 2007). The subbasin primarily is 

recharged by subsurface flow through permeable basalt lava 

flows that are exposed in the highlands north and east of the 

subbasin and by subsurface flow from the adjacent Lower 

Klamath Lake subbasin (Gannett and others, 2007).

The surface-water hydrology of the Tule Lake subbasin 

has been extensively modified as part of the U.S. Bureau of 

Reclamation Klamath Project (Gannett and others, 2007). 

Prior to the project, Tule Lake was much larger, surrounded 

by vast wetlands, and fed by the Lost River. Hydraulic head 

gradients indicate the lake drains in the subsurface southward 

through the Medicine Lake Highlands toward the Pit River 

(fig. 3; Gannett and others, 2007). The Lost River is now 

connected to the Klamath River by a man-made canal, and 

much of the subbasin has been drained for agricultural use. 

The existing Tule Lake Sump collects irrigation return water 

that is then pumped out of the basin into Lower Klamath Lake. 

The subbasin is extensively irrigated with surface water from 

Upper Klamath Lake (in Oregon). Pumpage of groundwater 

has increased dramatically since 2001, largely because 

groundwater is used to augment surface-water supplies so that 

more surface water can be left in streams to help maintain fish 

populations (National Research Council, 2008).
12 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Butte Valley is a closed basin in the Klamath River 

watershed. The basin was formed by faulting and is defined 

by the surface extent of alluvial fill (figs. 2, 3; CDWR basin 

number 1-3; California Department of Water Resources, 

2004d). The main aquifer units are coarse-grained lake 

deposits and interbedded volcanic units, particularly the Butte 

Valley Basalt in the southeast part of the basin (Planert and 

Williams, 1995). Major sources of groundwater recharge 

include subsurface flow through volcanic units, infiltration 

of precipitation and stream flow, and irrigation return water. 

Major sources of groundwater discharge include subsurface 

flow through volcanic units; evapotranspiration; pumping, 

primarily for agricultural use (Planert and Williams, 1995).

The South Fork Pit River and Warm Springs Valley 

subbasins of the Alturas area are defined by surface exposure 

of Holocene alluvial deposits and the Plio-Pleistocene 

Alturas Formation (figs. 2, 3; CDWR basin numbers 5-2.01 

and 5-2.02; California Department of Water Resources, 

2004e, f). The basins are surrounded by Quaternary and 

Tertiary lava flows and separated by surface exposure of the 

Plio-Pleistocene Warm Springs Tuff member of the Alturas 

Formation. The basins are dissected by many northwestsouthwest trending faults, and the Alturas Formation is folded 

into three synclines with northwest-southwest trending axes 

(California Department of Water Resources, 1963). These 

structures affect groundwater flow in the basin. The Warm 

Springs subbasin has hot springs on faults that permit deep 

circulation of groundwater. The main water-bearing units in 

the basins are permeable beds of tuff, ashy sandstone, and 

diatomite in the lacustrine Alturas Formation. Basalt lava 

flows, which are exposed around the basins and also are 

interbedded with the Alturas Formation, serve as primary 

recharge areas. 

The boundaries of the Big Valley groundwater basin are 

defined by surface extent of Holocene alluvial deposits and 

the Pleistocene Bieber Formation, and the basin is surrounded 

by Pliocene (TV study area) and Pleistocene (QV study area) 

lava flows (figs. 2, 3; CDWR basin number 5-4; California 

Department of Water Resources, 2004g). The Pit River flows 

through the western side of the valley. The principal waterbearing units are pumiceous sand and volcanic sand layers in 

the lacustrine Bieber Formation.

The Fall River Valley is defined by surface exposure of 

Pleistocene and Holocene alluvial and lacustrine deposits, 

and is surrounded by Pliocene, Pleistocene, and Holocene 

volcanic rocks (figs. 2, 3; CDWR basin number 5-5; California 

Department of Water Resources, 2004h). The Fall River 

Springs, which historically have been among the largest 

springs in the United States at 1,400–2,000 cubic feet per 

second (ft3

/s) of discharge (Meinzer, 1927), discharge into the 

valley from the north and provide most of the base flow for the 

surface-water features in the valley. The source of the water 

for the Fall River Springs appears to be precipitation on the 

vast Medicine Lake Highlands 60 kilometers (km) to the north 

(Rose and others, 1996).

The Lake Almanor Valley groundwater basin is along 

the northwest shore of Lake Almanor (figs. 2, 3; CDWR basin 

number 5-7; California Department of Water Resources, 

2004i). The basin consists of Quaternary alluvial and 

lacustrine deposits and is bounded by Pliocene volcanic 

rocks. Lake Almanor is a reservoir operated by the Pacific 

Gas and Electric Company for power generation, recreation, 

and irrigation. The primary sources of inflow to the lake 

are submerged springs in the Pliocene volcanic rocks; the 

Upper North Fork Feather River, which drains the southern 

slope of the highlands in the Lassen Volcanic National Park 

area; and the Hamilton Branch, which drains Mountain 

Meadows reservoir (fig. 3; Plumas County Flood Control and 

Conservation District, 2007).

The McCloud Area groundwater basin is on the southeast 

slope of Mount Shasta between approximately 3,000 and 

6,000 ft elevation (figs. 2, 3; CDWR basin number 5-35; 

California Department of Water Resources, 2004j). The basin 

is defined by the surface extent of a thin veneer of mostly 

Holocene alluvial and glacial deposits. The basin is underlain 

and bounded on the north and east by Pliocene and Pleistocene 

volcanic rocks and on the south by Paleozoic metasedimentary 

deposits of the Klamath Mountains (figs. 2, 3). The area has 

many freshwater springs (for example, Poeschel and others, 

1986).

The Lake Britton groundwater basin is where Hat 

Creek joins the Pit River (fig. 3; CDWR basin number 5–46; 

California Department of Water Resources, 2004k). The basin 

is defined by the surface extent of Recent alluvial deposits and 

is surrounded and underlain by Pliocene (TV study area) and 

Pleistocene (QV study area) lava flows. 

The North Fork Battle Creek Valley groundwater 

basin is just west of Lassen Volcanic National park (fig. 3; 

CDWR basin number 5-50; California Department of Water 

Resources, 2004l). The basin is defined by the surface extent 

of Recent alluvial deposits and is surrounded and underlain by 

Pliocene (TV study area) and Pleistocene (QV study area) lava 

flows. These alluvial deposits are approximately 32 ft thick 

and overlie a succession of lava flows. The main water-bearing 

unit is the interbedded layer of sand, gravel, ash, and cinder 

between the lava flows.

The Surprise Valley groundwater basin is on the 

northeastern edge of the study unit (fig. 3; CDWR basin 

number 6-1; California Department of Water Resources, 

2004m). The long, narrow basin is a graben bounded on all 

sides by normal faults (fig. 2). The Surprise Valley fault is 

the westernmost large-offset normal fault in the northwestern 

Basin and Range, and there has been more than 15,000 ft 

(4.5 km) of vertical offset between the Warner Mountains 
Study-Unit Description 13

and the Surprise Valley in the last 14 Ma (Egger and Miller, 

2011). The valley is filled with over 5,000 ft of alluvial and 

lacustrine deposits, most of which are lacustrine sediments 

from Pleistocene Lake Surprise.

The Surprise Valley is a closed basin. Streams entering 

the valley, primarily from the Warner Mountains to the west, 

terminate in the three seasonal, shallow, saline lakes along 

the central axis of the valley (fig. 3). The primary source of 

groundwater recharge is infiltration of surface water through 

alluvial fans along the base of the Warner Mountains and 

through coarse stream deposits at the northern end of the 

valley (California Department of Water Resources, 1963). The 

principal aquifers are the Holocene alluvial fans and nearshore deposits from Pleistocene Lake Surprise. Groundwater 

discharges by evapotranspiration and pumping, primarily 

for agricultural uses. The Surprise Valley has numerous hot 

springs associated with the Surprise Valley fault and subsurface faults within the basin, and heated groundwater is 

extracted for use in geothermal power generation (Glen and 

others, 2013).

Shasta Valley and Mount Shasta Volcanic Area 

Study Area (SH)

The SH study area corresponds to the CDWR-defined 

Shasta Valley groundwater basin (CDWR basin number 

1-4; California Department of Water Resources, 2004b) 

and the former CDWR-defined volcanic groundwater basin 

5-34, Mount Shasta Volcanic Area (California Department 

of Water Resources, 1980) (figs. 3, 4). The CDWR Shasta 

Valley groundwater basin is defined by Quaternary alluvial 

deposits up to 140 ft thick along the western and northern 

sides of Shasta Valley. The groundwater basin appears to 

be hydrologically connected with the volcanic rocks of the 

Mount Shasta Volcanic Area (Mack, 1960). Mesozoic marine 

sedimentary and metasedimentary rocks, and Paleozoic 

metamorphic rocks likely underlie the SH study area 

(Chesterman and Saucedo, 1984), and are exposed in the 

Klamath Mountains on the western margin of the study area 

(fig. 2). 

Mount Shasta is part of the Cascade Range, which 

extends from northern California to southern British 

Columbia. The Cascade Range has been an active volcanic arc 

for the last approximately 36 Ma as a result of subduction of 

the Juan de Fuca plate under the North American plate (Bally 

and Palmer, 1989). The Mount Shasta magmatic system has 

been active for approximately 600,000 years, and the current 

mountain is a composite of four major cones that erupted 

over the last 250,000 years (Wood and Kienle, 1990). The 

most recent eruption in the youngest cone was in 1786, and 

there are still active fumaroles and hot springs on the summit. 

The mountain is formed primarily of andesitic lava flows and 

pyroclastic deposits. Mount Shasta also hosts seven glaciers 

(Howat and others, 2007).

Most of the SH study area consists of the Mount Shasta 

Volcanic Area (5-34; fig. 3). Approximately two-thirds of 

the Mount Shasta Volcanic Area is covered by a debris 

avalanche that fell from Mount Shasta between 300,000 and 

380,000 years ago (Crandell, 1989). The avalanche deposits 

are up to 300 ft thick and consist of two facies: a block facies 

containing blocks of andesite lava flows and volcanoclastic 

deposits up to several hundred feet across and a matrix 

facies consisting of an unsorted, unstratified mix of boulders 

through clay size material, primarily from andesite lava flows 

and material scoured from the valley floor. The principal 

aquifer in the valley is the Holocene Pluto’s Cave basalt lava 

flow from Mount Shasta (Mack, 1960; Blodgett and others, 

1988; California Department of Water Resources, 2004b). 

It is exposed on the surface in the southeastern part of the 

study unit. Many of the groundwater sources used for public 

drinking-water supplies in the SH study area are springs 

(Blodgett and others, 1988).

The Shasta River is listed as impaired under the Clean 

Water Act, because of elevated stream temperatures and 

low dissolved oxygen (DO) levels that have a detrimental 

effect on fish populations, and has a Total Maximum Daily 

Load (TMDL) Action Plan (North Coast Regional Water 

Quality Control Board, 2006). The Action Plan lists several 

anthropogenic factors that could be causing the increased 

temperatures and decreased DO levels, including irrigation 

return flows, impoundments, flow modifications and 

diversions, agricultural practices that decrease shade and 

increase inputs of organic matter, and wastewater discharge. 

Groundwater and spring discharges are the primary sources of 

cold water to the river. Groundwater pumping, primarily for 

agricultural uses, has become a contentious issue in the valley 

because pumping may decrease spring discharge and increase 

the depth to the water table, thereby decreasing inflow of cold 

water to the river (National Research Council, 2008).

Groundwater is recharged in the SH study area by 

infiltration of stream flow and snow melt into permeable 

lava flows, mostly in the southern part of the valley on the 

slopes of Mount Shasta. Much of the valley floor receives 

less than 15 in/yr of precipitation; thus, direct infiltration 

from precipitation is minimal. Percolation of irrigation water, 

water from behind small impoundments, and water from 

Lake Shastina (fig. A1B) also contribute to groundwater 

recharge. Groundwater is discharged by seepage to streams; 

evapotranspiration; and pumping for agricultural, municipal, 

and domestic supplies. Most of the groundwater is pumped 

from Pluto’s Cave basalt lava flow or from the alluvial fans 

on the west side of the valley (California Department of Water 

Resources, 2004b).
14 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Quaternary Volcanic Areas (QV) and Tertiary 

Volcanic Areas (TV) Study Areas

The QV and TV study areas consist of areas mapped as 

Quaternary volcanic rocks (Qv) and Tertiary volcanic rocks 

(Tv), respectively, on the State geologic map (Saucedo and 

others, 2000) (fig. 2). The QV study area includes the former 

CDWR-defined volcanic groundwater basins 5-33, 1-24, 

and 6-103, which are collectively called the Modoc Plateau 

Pleistocene Volcanic Areas (fig. 3; California Department 

of Water Resources, 1980). The former CDWR-defined 

volcanic groundwater basins 5-32, 1-23, and 6-102, which are 

collectively called the Modoc Plateau Recent Volcanic Areas 

(not shown on fig. 3), are mapped as Holocene volcanic rocks 

(Qrv) on the State geologic map and are not included in the 

QV study area. 

The QV and TV study areas both include Cascade Range 

and Modoc Plateau volcanic rocks. The oldest Cascade Range 

rocks are mid-Tertiary (35 Ma to 26 Ma) volcanic rocks of 

the Western Cascade series, an old, deeply eroded ancestor 

of the modern Cascade Range volcanoes, and are exposed 

north of the Shasta Valley (du Bray and John, 2011). The 

modern Cascade Range includes the Mount Shasta, Lassen, 

and Medicine Lake volcanic systems. The Lassen volcanic 

area, much of which is in Lassen Volcanic National Park, has 

a complex eruptive history, with 537 volcanic vents younger 

than 7 Ma (Guffanti and others, 1990). On a regional scale, it 

consists of hundreds of coalescing small volcanoes primarily 

formed of basalt and basaltic andesite lavas and, superimposed 

on this regional volcanism, is a series of five large volcanic 

centers composed of more silicic lavas (Clynne, 1990; 

Guffanti and others, 1990). Lassen is the most recent volcanic 

center and has been active for approximately 600,000 years. 

The youngest feature is a dacite dome field on the flank of the 

now-eroded main andesitic cone. One of these domes, Lassen 

Peak, last erupted in 1915–21. Medicine Lake Volcano is east 

of the main line of Cascade Range volcanic arc volcanoes. It is 

a large shield volcano covering approximately 770 mi2

 (2,000 

km2

) and formed primarily of basalt and basaltic andesite lavas 

erupted during the late Pleistocene (less than 1 Ma) (DonnellyNolan, 1988). The most recent eruptions were small rhyolite 

flows about 900 years ago (Donnelly-Nolan and others, 1990). 

Mount Shasta and Medicine Lake Volcano are composed 

of Quaternary and Holocene volcanic rocks and, thus, are 

partially included in the QV study area. Most of Lassen 

Volcanic National Park is in the QV study area, some is in the 

TV study area, and a small sliver is Holocene volcanic rocks.

The Modoc Plateau is the northern extension of the 

Walker Lane belt, a 60–190 miles (mi) (100–300 km) wide 

zone of distributed late Cenozoic dextral strike-slip and 

normal faulting between the Sierra Nevada and the Basin and 

Range provinces (Page, 1995; Oldow and Cashman, 2009). 

The Modoc Plateau is covered with volcanic rocks, primarily 

basalt and basaltic andesite lava flows, ranging in age from 

late Miocene to Recent (approximately 15 Ma to present). 

Locally, there are also pyroclastic deposits and more silicic 

volcanic features.

The QV and TV study areas were defined as separate 

areas in order to examine potential relations between 

groundwater water quality and age of the aquifer materials. 

The basalt and basaltic andesite lava flows that compose 

most of the QV and TV study areas may undergo changes 

in mineralogy with time. Primary igneous minerals and 

volcanic glass can become oxidized and hydrated, and new 

minerals can be deposited in voids. These differences in 

aquifer mineralogy could result in differences in groundwater 

composition and water quality. 

Groundwater in volcanic rocks primarily is contained in 

rubble zones at the tops and bottoms of lava flows, cavities 

between lava flows, cracks and fissures resulting from thermal 

and tectonic stresses, volcanic pipes and lava tubes, and open 

gas vesicles (Planert and Williams, 1995; Todd and Mays, 

2012).The distribution of permeable zones is unpredictable, 

although the probability of large groundwater yields generally 

is greater in areas near fault zones (Planert and Williams, 

1995). Hydraulic conductivity values from wells in Tertiary 

volcanic rocks in the Klamath Basin in Oregon and California 

range from less than 1 meter per day (m/d) to over 600 m/d 

(estimated from data in Gannett and others, 2007). For 

comparison, the hydraulic conductivity of sand generally 

ranges from 2 to 45 m/d (Todd and Mays, 2012). Within 

the TV study area, the Western Cascades series rocks have 

much lower permeability than Modoc Plateau lavas because 

secondary mineralization from hydrothermal alteration has 

filled many of the void spaces in the Western Cascades series 

rocks (Newcomb and Hart, 1958; Gannett and others, 2007). 

The volcanic units are highly permeable and generally 

have little soil or sediment on top of them, thus groundwater 

is readily recharged by infiltration of precipitation and 

snow melt and by capture of stream flow. The QV, TV, and 

SH study areas contain many springs, located where high 

permeability zones intersect the land surface, and these springs 

can be a large component of inflow to streams. These springs 

include several first magnitude springs, defined as springs 

with discharge greater than 100 cubic feet per second (ft3

/s; 

Meinzer, 1927). For example, five springs discharging in 

the QV study area near where Hat Creek joins the Pit River 

have a combined discharge of over 315,000 gpm (700 cubic 

feet per second), and isotopic data indicate the recharge area 

was approximately 30 mi (50 km) to the south in the high 

elevations in Lassen Volcanic National Park (Rose and others, 

1996).

Methods

This section describes the methods used for the status

and understanding assessments. Methods used to collect and 

analyze groundwater samples and results for the evaluation 

of quality-control data are described by Shelton and others 
Methods 15

(2013). Methods used for compiling data for potential 

explanatory factors are described in appendix A.

Status Assessment

The status assessment was designed to quantify 

groundwater quality in areal proportions of the primary 

aquifer system. The primary aquifer system is defined by 

the depth intervals over which wells listed in the CDPH 

database are screened or open. The use of the term “primary 

aquifer system” does not imply a discrete aquifer unit exists. 

In most groundwater basins, public drinking-water supply 

wells typically are screened or open at greater depths than are 

domestic wells (for example, Burow and others, 2008; Burton 

and others, 2012). Thus, the primary aquifer system generally 

corresponds to the deeper portion of the aquifer system that is 

tapped by public drinking-water supply wells. However, this 

segregation between the depths of public-supply and domestic 

wells may 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; Ingebritsen and Sanford, 1998). Nearly 

all of the wells used in status assessments for the CAMP 

study unit are listed in the CDPH database and are, therefore, 

classified as public drinking-water supply wells. To the extent 

that domestic wells in the study unit are screened or open over 

the same depth intervals as the CDPH wells, the assessments 

presented in this report also are applicable to the parts of the 

aquifer system used for domestic drinking-water supplies. 

This section describes the methods used for (1) defining 

groundwater quality, (2) assembling the data used for 

the assessment, (3) selecting constituents for evaluation, 

and (4) and calculating aquifer-scale proportions. Two 

statistical approaches were used for calculating aquifer-scale 

proportions: a “grid-based” approach that used one well to 

represent each grid cell, and a “spatially weighted” approach 

that used many wells to represent each grid cell (Belitz and 

others, 2010).

The CDPH database contains historical records from 

more than 25,000 wells, requiring the use of targeted retrievals 

to effectively access relevant water-quality data. For example, 

for the area representing the CAMP study unit, the CDPH 

database contains about 75,000 records for samples collected 

between 1982 and 2010 from 388 wells. The CDPH data were 

used in the spatially weighted calculations of aquifer-scale 

proportions.

Groundwater Quality Defined as RelativeConcentrations

In this study, groundwater-quality data are presented 

as relative concentrations (RCs), which is the ratio of a 

constituent’s concentration measured in a groundwater 

sample to the concentration of a constituent’s regulatory or 

non-regulatory benchmark used to evaluate drinking-water 

quality. The use of RC is similar to the approaches employed 

by other studies to place the concentrations of constituents 

in groundwater in a toxicological context (for example, U.S. 

Environmental Protection Agency, 1986; Toccalino and others, 

2004; Toccalino and Norman, 2006; Rowe and others, 2007). 

The RC is defined as follows:

Relativeconcentration RC Sampleconcentration

Benchmar 

( )

= k concentration

An RC value less than 1 indicates that the sample 

concentration was less than the benchmark concentration, 

and an RC value greater than 1 indicates that the sample 

concentration was greater than the benchmark concentration. 

The use of RCs permits comparison on a single scale 

for constituents that can be present at a wide range of 

concentrations. RCs can only be computed for constituents 

with water-quality benchmarks; therefore, constituents without 

water-quality benchmarks were not included in the status 

assessment.

Regulatory and non-regulatory benchmarks apply to 

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

groundwater. To place the results in context, however, 

concentrations of constituents measured in the untreated 

groundwater were compared to benchmarks established 

by the U.S. Environment Protection Agency (USEPA) 

and the California Department of Public Health (CDPH; 

U.S. Environmental Protection Agency, 1999a, 2009a; 2012; 

California Department of Public Health, 2010, 2013b). The 

benchmarks used for each constituent were selected in the 

following order of priority:

1. Regulatory, health-based levels established by the CDPH 

and the USEPA: maximum contaminant levels ( MCL-CA 

and MCL-US) and USEPA action levels (AL-US), 

respectively.

2. Non-regulatory, non-health based, aesthetic-based 

levels established by the CDPH: secondary maximum 

contaminant levels (SMCL-CA). The salinity indicators 

chloride, sulfate, and TDS have recommended and upper 

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

used. 

3. Non-regulatory, health-based levels established by the 

CDPH and the USEPA: CDPH notification levels (NLCA), USEPA lifetime health advisory levels (HAL-US), 

and USEPA risk-specific doses for 1:100,000 (10-5; 

RSD5-US).

For constituents with multiple types of benchmarks, 

this hierarchy sometimes did not result in selection of 

the benchmark with the lowest concentration. Additional 

information on the types of benchmarks used and lists of the 

benchmark values for all constituents analyzed are provided 

by Shelton and others (2013).
16 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

The two microbial indicators analyzed in samples 

from the CAMP study unit, total coliforms and E.coli, have 

drinking-water quality benchmarks, but are not included in 

the status assessments for the individual study units because 

the results are to be presented in one report for all 35 GAMA 

Priority Basin Project public-supply aquifer study units 

(Carmen Burton, USGS, written commun., 2014).

Toccalino and others (2004), Toccalino and Norman 

(2006), and Rowe and others (2007) used the ratio 

of measured sample concentration to the benchmark 

concentration, either MCL-US or Health-Based Screening 

Level (HBSL), and defined this ratio as the benchmark 

quotient (BQ). HBSLs are not used in this report because 

HBSLs are not currently used as benchmarks by California 

drinking-water regulatory agencies. Because different waterquality benchmarks were used to calculate the RCs and BQs, 

the terms are comparable but not interchangeable. 

For ease of discussion, the RCs of constituents were 

classified into low, moderate, and high categories (table 1). 

RC values greater than 1.0 were defined as “high” for all 

constituents. For inorganic constituents (trace elements, 

nutrients, radioactive constituents, and inorganic constituents 

having SMCL benchmarks) RC values greater than 0.5 and 

less than or equal to 1.0 were defined as “moderate,” and 

RC values less than or equal to 0.5 were defined as “low.” 

For organic and special-interest constituents, RC values 

greater than 0.1 and less than or equal to 1.0 were defined 

as “moderate,” and RC values less than or equal to 0.1 were 

defined as “low.” Although more complex classifications could 

be devised based on the properties and sources of individual 

constituents, use of a single moderate/low threshold value 

for each of the two major groups of constituents provided 

consistent objective criteria for distinguishing constituents 

present at moderate, rather than low, concentrations.

Other studies have used the same boundary value 

between low and moderate RCs for inorganic and organic 

constituents—either 0.5 (for example, U.S. Environmental 

Protection Agency, 1999b) or 0.1 (for example, Toccalino 

and others, 2010). The primary reason for using a higher 

boundary value for inorganic constituents in this study was to 

focus attention on the inorganic constituents most prevalent 

at concentrations closest to benchmark concentrations. 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 BQ 

greater than 0.1 in approximately 10 percent of the samples 

and that inorganic constituents (nutrients, trace elements and 

radioactive constituents) were present at BQ greater than 0.1 

in approximately 80 percent 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. Organic constituents generally are anthropogenic 

and enter groundwater as a result of human activities (both 

intentional, such as pesticide applications, and unintentional, 

such as leaks and spills) at the land surface. Concentrations of 

the organic constituents can change rapidly in groundwater; 

therefore, early warning (as given by using an RC of 0.1) 

could be vital for planning and implementing measures to 

protect aquifer systems from further contamination and to 

mitigate existing contamination. Inorganic constituents, on the 

other hand, typically occur naturally in groundwater, and their 

concentrations usually are stable or change slowly compared 

to those of organic constituents. Having a boundary between 

low and moderate RCs (or BQ) at 0.5 (rather than 0.1) allows 

identification of those inorganic constituents—from among 

the many that could be present—that are most prevalent at 

concentrations close to benchmarks and may therefore warrant 

more immediate attention from water-resource managers.

The boundary between low and moderate RC is not 

intended as a demarcation of the presence of contamination 

from anthropogenic sources. Unlike the other classes 

of inorganic constituents, concentrations of nutrients 

in groundwater commonly can be strongly affected by 

contamination from anthropogenic sources. Concentrations 

of nitrate in groundwater greater than 1 milligram per liter 

(mg/L) generally are considered to indicate contamination 

from anthropogenic sources (Nolan and others, 2002; 

Dubrovsky and others, 2010). Setting the boundary between 

low and moderate RCs at 0.5 for nitrate (which corresponds 

to 5 mg/L for nitrate), therefore, results in some groundwater 

samples with contamination from anthropogenic sources to be 

categorized as a low RC for nitrate. For this study, nitrate and 

the other nutrient constituents were categorized as inorganic 

constituents, and the boundary between low and moderate 

RCs was set at 0.5. Similarly, groundwater containing 

anthropogenic organic constituents with RCs less than 0.1 was 

classified as a low RC for organic constituents, even though 

contamination from anthropogenic sources was present.

Data Used for Status Assessment

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. The grid-based calculations of aquifer-scale 

Table 1. Relative-concentration categories used for assessing 

groundwater quality.

[Relative-concentration (RC) is defined as the concentration measured 

in the sample divided by the concentration of the selected benchmark. 

Abbreviations: >, greater than; ≤, less than or equal to]

RC 

category

RCs for organic and 

special-interest 

constituents

RCs for 

inorganic 

constituents

High >1 >1

Moderate >0.1 and ≤1 >0.5 and ≤1

Low ≤0.1 ≤0.5
Methods 17

proportions used data collected by USGS-GAMA for the 

CAMP study unit (appendix A), and the spatially weighted 

calculations of aquifer-scale proportions used the data from 

the grid-based calculations, plus data compiled for many wells 

in the CDPH database. Comparisons of USGS-GAMA and 

CDPH data are presented in appendix B.

Data for Grid-Based Calculations of Aquifer-Scale 

Proportions

The data used for the grid-based calculations of aquiferscale proportions were from the 90 wells and springs (USGSgrid wells) sampled by USGS-GAMA. Only a subset of 

the entire area of the CAMP study unit was included in the 

gridded area. The CAMP study unit had relatively few CDPH 

wells, and these wells were not evenly distributed; thus, if 

the entire study unit had been included in the gridded area, 

there would have been many grid cells that did not contain 

any CDPH wells (fig. 4). To minimize the number of cells 

without any wells, only the areas of the study unit near CDPH 

wells were included in the gridded area. For each study area, 

a 1.86-mi (3-kilometers; km) radius circle was drawn around 

each CDPH well in the study area. If the area within 3-km 

of the well included more than one study area, the circle 

was truncated at the boundaries of the study area containing 

the well (figs. A1A–G). For example, the area within 3-km 

of well LU-11 includes parts of the LU and TV study areas 

and areas not in any study area (fig. A1D). The 3-km buffer 

around LU-11 only includes the parts in the LU study area. 

The aggregate areas encompassed by these circles in each of 

the six study areas were divided into 15 equal-area grid cells 

(Scott, 1990). The sizes of the grid cells ranged from 19 square 

kilometers (km2) in the ES study area to 47 km2 in the QV 

study area (table 2).

All CDPH wells were assigned random rankings, and 

the highest ranked well in each cell that met basic sampling 

requirements, and for which permission could be obtained, 

was selected as the USGS-grid well and sampled. For cells 

without accessible CDPH wells, a CDPH well located close 

to the the cell boundary in an adjacent grid cell could be 

selected if the well was closer to the cell it was being selected 

to represent than it was to the USGS-grid well in the cell in 

which it was located, and the CDWR well completion report 

for the well indicated that the screened or open intervals were 

in the appropriate aquifer lithology. For cells not represented 

by a CDPH well, appropriate USGS-grid wells were selected 

by door-to-door canvassing. 

Of the 90 USGS-grid wells selected, 82 were listed in 

the CDPH database (74 wells and 8 springs), and the other 

8 (6 domestic wells, 1 irrigation well, and 1 spring used for 

irrigation) had screened or open intervals at depths similar 

to those of wells listed in the CDPH database. Seven USGSgrid wells were located in cells adjacent to the cells they 

were selected to represent. In two of these seven cases, a well 

in the Central Valley was selected to represent an Eastside 

Sacramento Valley study area cell (fig. A1F). The screened 

intervals in these two wells were deep enough to intersect the 

Tuscan Formation. The 90 USGS-grid wells were named with 

an alphanumeric GAMA_ID consisting of an initial prefix 

identifying the study unit (CAMP), a second prefix indicating 

the study area (ES, HL, LU, QV, SH, or TV), and followed 

by a number indicating the order of sample collection in each 

study area (appendix A). For ease of use, the prefix “CAMP” 

is dropped from the GAMA_ID in this report.

Table 2. Study-area names, study-area and grid-cell sizes, and numbers of California Department of Public Health (CDPH) wells 

and U.S. Geological Survey (USGS)-grid wells in each study area, Cascade Range and Modoc Plateau study unit, 2010, California 

Groundwater Ambient Monitoring and Assessment Program (GAMA) Priority Basin Project.

[Abbreviations: km2, square kilometers; na, not applicable; TDS, total dissolved solids]

Study area

Studyarea 

code

Area 

(km2

)

Gridded 

area 

(km2

)

Fraction of 

total 

gridded area

Grid-cell 

area 

(km2)

Number of 

CDPH wells 

with any data1

Number of CDPH wells 

with data for nitrate, 

arsenic, and TDS2

Number of 

USGS-grid 

wells

Sacramento Valley Eastside ES 2,004 287 0.104 19 26 15 15

Honey Lake Valley HL 1,261 371 0.134 25 34 15 15

Cascade Range and Modoc 

Plateau Low-Use Basins

LU 4,826 510 0.184 34 42 12 15

Quaternary Volcanic Areas QV 10,399 711 0.257 47 63 14 15

Shasta Valley and Mount 

Shasta Volcanic Area

SH 878 369 0.133 25 39 16 15

Tertiary Volcanic Areas TV 10,985 517 0.187 34 36 3 15

Other areas na 8,647 na na na na na na

Total 39,000 2,763 1 31 240 75 90

1

Of these, 63 also were USGS-grid wells.

2

Of these, 21 also were USGS-grid wells.
18 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Approximately 8 percent of the wells listed in the CDPH 

database for the CAMP study unit were springs; however, this 

could be a minimum estimate of the percentage of springs. 

Springs are sites where groundwater naturally flows from 

below ground to above land surface. In some cases, the 

connection between the spring and the distribution system 

had been improved with a horizontal well bore. In the CDPH 

database, a spring with a horizontal well bore may be given 

a name that identifies it as a well. Of the 90 USGS-grid 

wells sampled, 9 were springs and 81 were wells. For ease 

of discussion, all sites are referred to as wells, unless the 

difference between a well and a spring is important to the 

discussion.

Samples collected from USGS-grid wells were analyzed 

for 214 constituents (table 3). Water-quality data collected by 

USGS-GAMA are tabulated in Shelton and others (2013) and 

also are available from the SWRCB’s publically accessible 

internet database GeoTracker GAMA (website at http://www.

waterboards.ca.gov/gama/geotracker_gama.shtml) and the 

USGS’s publically accessible internet database NWISWeb 

(website at http://waterdata.usgs.gov/ca/nwis/).

Additional Data Used for Spatially Weighted Calculations 

of Aquifer-Scale Proportions

The spatially weighted calculations of aquifer-scale 

proportions used data from the 90 USGS-grid wells and 

from the 240 wells in the CDPH database with water-quality 

data for samples collected between September 16, 2007, 

and September 16, 2010. Of these 240 wells, 63 also were 

USGS-grid wells, and only the USGS-GAMA data were used. 

Many of the 177 wells having only CDPH data had data for 

a limited number of constituents, commonly only nitrate. For 

example, only 75 of the 177 wells had CDPH data for all three 

of the constituents nitrate, arsenic, and total dissolved solids 

(TDS) (table 2). Water-quality data collected by the CDPH are 

available from the SWRCB’s GeoTracker GAMA (website 

at http://www.waterboards.ca.gov/gama/geotracker_gama.

shtml).

Selection of Constituents for Evaluation

Aquifer-scale proportions are presented for a subset of 

214 constituents analyzed in samples collected by USGSGAMA for the CAMP study unit. This subset was selected by 

using the following criteria:

• Constituents present at high or moderate RCs (table 1) 

in the USGS-grid well dataset or in the CDPH database 

for any sample collected between September 16, 2007, 

and September 16, 2010. 

• Organic constituent classes having at least one 

constituent with an area-weighted detection frequency 

of greater than 10 percent in the USGS-grid well 

dataset.

Table 3. Summary of constituent groups analyzed in the 

90 samples collected by the U.S. Geological Survey (USGS) for the 

Cascade Range and Modoc Plateau study unit, 2010, California 

Groundwater Ambient Monitoring and Assessment Program

(GAMA) Priority Basin Project.

[Constituent names: E. coli, Escherichia coli. Other abbreviations and 

symbols: B, Boron; C, carbon; H, hydrogen; He, helium; O, oxygen; 

pCi/L, picocuries per liter; Sr, strontium; TDS, total dissolved solids; δ, delta; 

μg/L, microgram per liter]

Constituents Number of 

constituents analyzed

Inorganic constituents

Specific conductance 1

Gross alpha and gross beta particle 

activity1

2

Major ions and trace elements (including 

alkalinity and TDS)

35

Nutrients 5

Radon-222 1

Uranium and uranium isotopes2 1

Organic constituents

Pesticides and pesticide degradates 63

Volatile organic compounds (VOCs)3 85

Constituent of special interest

Perchlorate 1

Geochemical and age-dating tracers

Arsenic and iron redox species ratios 2

δ11B of dissolved boron 1

Carbon-14 and δ13C of dissolved carbonates

2

Dissolved oxygen, pH, and temperature 3

δ2

H and δ18O stable isotopes of water 2

Noble gases (helium, neon, argon, krypton, 

xenon), and 3

He/4He of dissolved helium

6

87Sr/86Sr of dissolved strontium 1

Tritium 1

Microbial constituents

Total coliform and E. coli 2

Sum 214

1

Both gross alpha and gross beta particle activities were measured after 

72-hour and 30-day holding times; data from the 30-day measurement are 

used in this report. 

2

Uranium concentration was measured in all samples; the activities of 

the uranium isotopes uranium-234, uranium-235, and uranium-238 were 

measured in a subset of samples. The two samples having total uranium 

activity greater than the California Department of Public Health maximum 

contaminant level (MCL-CA) of 20 pCi/L also had uranium concentration 

greater than the U.S. Environmental Protection Agency maximum 

contaminant level (MCL-US) of 30 μg/L.

3

Includes 10 constituents classified as fumigants or fumigant synthesis 

byproducts.
Methods 19

These criteria identified 17 inorganic constituents 

(table 4A), and 2 organic constituent classes (herbicides and 

trihalomethanes) and perchlorate (table 5). An additional 

28 inorganic constituents were detected by USGS-GAMA 

in samples from the CAMP study unit that either have no 

drinking-water quality benchmarks or were only detected 

at low RCs (table 4B). Aquifer-scale proportions are not 

presented for constituents only detected at low RCs because 

the proportion of the primary aquifer system having low RCs 

for those constituents is 100 percent. All of the 18 geochemical 

and age-dating tracers examined were detected in samples 

(table 4B). A total of 19 organic constituents with drinkingwater quality benchmarks were detected at low RCs, and 8 

organic constituents with no benchmarks also were detected 

(table 5). Because no organic constituents were detected at 

high or moderate RCs, the organic constituents were evaluated 

as classes rather than as individual constituents. The remaining 

121 constituents were not detected by USGS-GAMA in the 

CAMP study unit. A complete list of the constituents analyzed 

by USGS-GAMA in the CAMP study unit can be found in the 

CAMP Data Series Report (Shelton and others, 2013).

The CDPH database also was used to identify 

constituents that have been reported at high RCs historically 

but not currently (table 6). 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 

interval used for the status assessment: November 16, 1982, 

through September 15, 2007. Constituent concentrations 

could have been 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 concentrations of constituents 

that do not otherwise meet the criteria listed previously were 

not considered representative of potential groundwater-quality 

concerns in the study unit during the period of study.

The CAMP study unit had 12 historically high 

constituents (table 6). Of the nine inorganic constituents, one 

was also found at moderate RCs in the 3-year period used in 

the status assessment and was detected in moderate RCs in 

the USGS-grid wells (fluoride) (table 4A). Of the eight other 

inorganic constituents, two were not analyzed by USGSGAMA (mercury and radium-228) and the remaining five 

were either not detected or were detected only at low RCs in 

USGS-grid wells. Of the three volatile organic constituents, 

none were detected at moderate or high RCs in the 3-year 

period used in the status assessment; however, two were 

detected at low RCs (methyl tert-butyl ether [MTBE] and 

tetrachloroethene [PCE]) in USGS-grid wells in the study 

unit (table 5). Of the 12 historically high constituents, 9 were 

detected at high concentrations in only 1 CDPH well each 

(table 6).

Calculation of Aquifer-Scale Proportions

Seven primary aquifer systems were defined for the 

CAMP study unit: six to represent each of the six study areas 

and an aggregated system to represent the study unit as a 

whole. The proportions of these primary aquifer systems 

in the six study areas with high, moderate, and low RCs of 

constituents were calculated by using the grid-based and 

spatially weighted approaches of Belitz and others (2010). 

For ease of discussion, these proportions are referred to as 

“high-RC,” “moderate-RC,” and “low-RC” aquifer-scale 

proportions. Aquifer-scale proportions for the primary aquifer 

system in the study unit as a whole were calculated as an 

area-weighted combination of the proportions for the six study 

areas. Calculations of aquifer-scale proportions were made for 

individual constituents and for classes of constituents. Aquiferscale proportions for constituent classes were calculated 

by using the maximum RC for any constituent in the class 

to represent the class. For example, a well having a high 

RC for arsenic, moderate RC for fluoride, and low RCs for 

molybdenum, boron, selenium, and other trace elements would 

be counted as having a high RC for the class of trace elements 

with health-based benchmarks. 

The grid-based calculations used the USGS-grid well 

dataset. Aquifer-scale proportions were calculated for each of 

the study areas separately because cell sizes differed among 

the study areas (table 2). High-RC aquifer-scale proportion 

was calculated as the fraction of the USGS-grid wells in the 

study area having high RCs for a constituent (equation 1). 

The moderate-RC aquifer-scale proportion was calculated 

similarly. Confidence intervals for grid-based high-RC 

aquifer-scale proportions were computed by using the Jeffrey’s 

interval for the binomial distribution (Brown and others, 2001; 

Belitz and others, 2010).

P N

N SA g

high SA

high

SA

, = (1)

where

PSA g

high

,

is the grid-based high-RC aquifer-scale 

proportion for the study area SA,

NSA

high is the number of cells in the study area 

represented by a well having high RC for 

the constituent; and

NSA

is the number of cells in the study area having 

a well with data for the constituent (the 

value of this parameter is 15 for all 6 of the 

study areas, because the USGS-grid wells 

had data for all constituents evaluated).
20 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Table 4A. Benchmark type and value and reporting limits for inorganic constituents detected at moderate or high relativeconcentrations in samples collected for the Cascade Range and Modoc Plateau study unit, 2010, California Groundwater Ambient 

Monitoring and Assessment Program (GAMA) Priority Basin Project.

[Relative-concentration (RC) is defined as the concentration measured in the sample divided by the concentration of the selected benchmark. For inorganic 

constituents, RC greater than 1.0 is defined as high and RC less than or equal to 1 and greater than 0.5 is defined as moderate. Benchmark type: AL-US, USEPA 

action level. Regulatory, health-based benchmarks: MCL-CA, CDPH maximum contaminant level; MCL-US, USEPA maximum contaminant level; Prop 

MCL-US, proposed USEPA maximum contaminant level. Non-regulatory, health-based benchmarks: HAL-US, USEPA lifetime health advisory level; NL-CA, 

CDPH notification level. Non‐regulatory, aesthetic/technical-based benchmarks: SMCL-CA, CDPH secondary maximum contaminant level. Benchmark units: 

mg/L, milligrams per liter; pCi/L, picocuries per liter; μg/L, micrograms per liter; μS/cm, microsiemens per centimeter. Other abbreviations: CDPH, California 

Department of Public Health; na, not available; ssLc, sample-specific critical level; USEPA, U.S. Environmental Protection Agency; USGS, U.S. Geological 

Survey]

Constituent

Benchmarks

Units

Reporting limits Understanding 

assessment 

Type presented? 1 Value USGS CDPH2

Inorganic constituents with health-based benchmarks

Nutrients

Ammonia, as nitrogen HAL-US3 24.7 mg/L 0.01 na No

Nitrate, as nitrogen4 MCL-US 10 mg/L 0.02 0.1 No

Trace elements

Arsenic MCL-US 10 μg/L 0.02 2 Yes

Boron NL-CA 1,000 μg/L 3 100 Yes

Fluoride MCL-CA 2 mg/L 0.04 0.1 No

Lead5 AL-US 15 μg/L 1 0.2 No

Molybdenum HAL-US 40 μg/L 0.01 na Yes

Strontium HAL-US 4,000 μg/L 0.2 na No

Vanadium NL-CA 50 μg/L 0.08 2 Yes

Radioactive constituents

Gross alpha particle activity MCL-US 15 pCi/L ssLc 3 No

Radon-222 activity Prop MCL-US 4,000 pCi/L ssLc na No

Uranium MCL-US 30 μg/L 0.004 1 Yes

Inorganic constituents with secondary maximum contaminant level benchmarks

Chloride SMCL-CA 500 mg/L 0.06 1 No

Iron SMCL-CA 300 μg/L 6 50 Yes

Manganese SMCL-CA 50 μg/L 0.7 10 Yes

Specific conductance SMCL-CA 1,600 μS/cm 5 na No

Total dissolved solids (TDS) SMCL-CA 1,000 mg/L 12 na Yes

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 and SMCL-CA, California Department of Public Health (2013b); MCL-US and 

AL-US, U.S. Environmental Protection Agency (2009a); NL-CA, California Department of Public Health (2010); HAL-US, U.S. Environmental Protection 

Agency (2012); Prop MCL-US, U.S. Environmental Protection Agency (1999a).

2

Nondetections are reported in the CDPH database as a concentration of zero or as less than the reporting limit. The most prevalent reporting limit, as inferred 

from data for nondetections, is listed.

3

HAL-US benchmark is 30 mg/L for ammonia as ammonia. To facilitate comparison to the analytical results, this HAL-US has been converted and reported as 

24.7 mg/L as nitrogen. The benchmark applies to total dissolved ammoniacal nitrogen (ammonia gas plus ammonium ion).

4

Concentrations of nitrate, as nitrate, reported in the CDPH data are converted to concentrations of nitrate, as nitrogen, for comparison with USGS-GAMA

data. 

5

Although lead was not detected at high or moderate RCs in USGS-GAMA samples, it was reported at high and moderate RCs in the CDPH database during 

September 16, 2007, through September 16, 2010, and, therefore, is included on this table.
Methods 21

Table 4B. Benchmark type and value and reporting limits for detected inorganic constituents having no benchmarks or present only at 

low relative-concentrations in the Cascade Range and Modoc Plateau study unit, 2010, California Groundwater Ambient Monitoring and 

Assessment Program (GAMA) Priority Basin Project.

[Relative-concentration (RC) is defined as the concentration measured in the sample divided by the concentration of the selected benchmark. For inorganic 

constituents, RC less than or equal to 0.5 is defined as low. Benchmark type: Regulatory, health-based benchmarks: AL-US, USEPA action level; 

MCL-CA, CDPH maximum contaminant level; MCL-US, USEPA maximum contaminant level. Non-regulatory, aesthetic/technical-based benchmarks: 

SMCL-CA, CDPH secondary maximum contaminant level. Benchmark units: cm3STP/gH2O, cubic centimeters of gas at standard pressure and temperature 

per gram of water; mg/L, milligrams per liter; pCi/L, picocuries per liter; per mil, parts per thousand; pmC, percent modern Carbon; std units, standard pH units; 

°C, degrees Celsius; μg/L, micrograms per liter. Other abbreviations: CDPH, California Department of Public Health; na, not available; ssLc, sample-specific 

critical level; USEPA, U.S. Environmental Protection Agency; USGS, U.S. Geological Survey; >, greater than; <, less than]

Constituent

Benchmarks

Units Reporting limits2

Type1 Value USGS CDPH

Inorganic constituents with benchmarks

Aluminum MCL-CA 1,000 μg/L 1.7 50

Antimony MCL-US 6 μg/L 0.027 6

Barium MCL-CA 1,000 μg/L 0.4 na

Beryllium MCL-US 4 μg/L 0.006 1

Cadmium MCL-US 5 μg/L 0.016 1

Chromium MCL-CA 50 μg/L 0.42 1

Copper AL-US 1,300 μg/L 1.7 10

Gross beta particle activity MCL-US 50 pCi/L ssLc na

Nickel MCL-CA 100 μg/L 0.36 10

Nitrite, as nitrogen MCL-US 1 mg/L 0.001 0.1

Selenium MCL-US 50 μg/L 0.03 2

Silver SMCL-CA 100 μg/L 0.005 1

Sulfate SMCL-CA 500 mg/L 0.09 2

Thallium MCL-US 2 μg/L 0.01 0.2

Zinc SMCL-CA 5,000 μg/L 4.8 20

Inorganic constituents with no benchmarks

Alkalinity, as CaCO3 none none mg/L 4 na

Bromide none none mg/L 0.01 na

Calcium none none mg/L 0.022 na

Cobalt none none μg/L 0.38 na

Iodide none none mg/L 0.001 na

Lithium none none μg/L 0.22 na

Magnesium none none mg/L 0.008 na

Nitrogen, total none none mg/L 0.05 na

Orthophosphate none none mg/L 0.004 0.04

Potassium none none mg/L 0.032 1

Silica none none mg/L 0.029 na

Sodium none none mg/L 0.06 na

Tungsten none none μg/L 0.11 na
22 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Table 4B. Benchmark type and value and reporting limits for detected inorganic constituents having no benchmarks or present only at 

low relative-concentrations in the Cascade Range and Modoc Plateau study unit, 2010, California Groundwater Ambient Monitoring and 

Assessment Program (GAMA) Priority Basin Project.—Continued

[Relative-concentration (RC) is defined as the concentration measured in the sample divided by the concentration of the selected benchmark. For inorganic 

constituents, RC less than or equal to 0.5 is defined as low. Benchmark type: Regulatory, health-based benchmarks: AL-US, USEPA action level; 

MCL-CA, CDPH maximum contaminant level; MCL-US, USEPA maximum contaminant level. Non-regulatory, aesthetic/technical-based benchmarks: 

SMCL-CA, CDPH secondary maximum contaminant level. Benchmark units: cm3STP/gH2O, cubic centimeters of gas at standard pressure and temperature 

per gram of water; mg/L, milligrams per liter; pCi/L, picocuries per liter; per mil, parts per thousand; pmC, percent modern Carbon; std units, standard pH units; 

°C, degrees Celsius; μg/L, micrograms per liter. Other abbreviations: CDPH, California Department of Public Health; na, not available; ssLc, sample-specific 

critical level; USEPA, U.S. Environmental Protection Agency; USGS, U.S. Geological Survey; >, greater than; <, less than]

Constituent

Benchmarks

Units Reporting limits2

Type1 Value USGS CDPH

Geochemical and age-dating tracers

3

He/4He of helium none none dimensionless na na

87Sr/86Sr of dissolved strontium none none dimensionless na na

Arsenic and iron redox species ratios none none dimensionless na na

Carbon-14 none none pmC na na

Dissolved oxygen none none mg/L na na

Noble gases (helium, neon, argon, krypton, xenon) none none cm3

STP/gH2O na na

pH SMCL-US <6.5 or >8.5 std units na na

Temperature none none °C na na

Tritium MCL-CA 20,000 pCi/L na na

δ11B of dissolved boron none none per mil na na

δ13C of dissolved carbonates none none per mil na na

δ2

H and δ18O stable isotopes of water none none per mil na 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 and SMCL-CA, California Department of Public Health (2013b); MCL-US and 

AL-US, U.S. Environmental Protection Agency (2009a); NL-CA, California Department of Public Health (2010); HAL-US, U.S. Environmental Protection 

Agency (2012); Prop MCL-US, U.S. Environmental Protection Agency (1999a).

2

Nondetections are reported in the CDPH database as a concentration of zero or as less than the reporting limit. The most prevalent reporting limit as inferred 

from data for nondetections is listed.

Table 5. Benchmark type and value and reporting limits for organic and special-interest constituents detected in samples collected for 

the Cascade Range and Modoc Plateau (CAMP) study unit, 2010, California Groundwater Ambient Monitoring and Assessment Program

(GAMA) Priority Basin Project.

[Benchmark type: Regulatory, health-based benchmarks: AL-US, U.S. Environmental Protection Agency (USEPA) action level; HAL-US, USEPA lifetime 

health advisory level; MCL-CA, California Department of Public Health (CDPH) maximum contaminant level; MCL-US, USEPA maximum contaminant level; 

Prop MCL-US, proposed USEPA maximum contaminant level; RSD5-US, U.S. Environmental Protection Agency risk specific dose at a risk factor of 10E–5. 

Non-regulatory, health-based benchmarks: NL-CA, CDPH notification level. Non-regulatory, aesthetic/technical-based benchmarks: SMCL-CA, CDPH 

secondary maximum contaminant level. Benchmark unit: μg/L, micrograms per liter. Other abbreviations: USGS, U.S. Geological Survey]

Constituent

Benchmarks Reporting limits In constituent 

class that has 

assessment results 

tabulated?

Type1 Value Units USGS CDPH2

Pesticides

Insecticides and fungicides

Carbaryl RSD5-US 400 μg/L 0.06 na No

Metalaxyl none none none 0.014 na No
Methods 23

Table 5. Benchmark type and value and reporting limits for organic and special-interest constituents detected in samples collected for 

the Cascade Range and Modoc Plateau (CAMP) study unit, 2010, California Groundwater Ambient Monitoring and Assessment Program 

(GAMA) Priority Basin Project.—Continued

[Benchmark type: Regulatory, health-based benchmarks: AL-US, U.S. Environmental Protection Agency (USEPA) action level; HAL-US, USEPA lifetime 

health advisory level; MCL-CA, California Department of Public Health (CDPH) maximum contaminant level; MCL-US, USEPA maximum contaminant level; 

Prop MCL-US, proposed USEPA maximum contaminant level; RSD5-US, U.S. Environmental Protection Agency risk specific dose at a risk factor of 10E–5 

μg/L;. Non-regulatory, health-based benchmarks: NL-CA, CDPH notification level. Non-regulatory, aesthetic/technical-based benchmarks: SMCL-CA, CDPH 

secondary maximum contaminant level. Benchmark unit: μg/L, micrograms per liter. Other abbreviations: USGS, U.S. Geological Survey]

Constituent

Benchmarks Reporting limits In constituent 

class that has 

assessment results 

tabulated?

Type1 Value Units USGS CDPH2

Pesticides—Continued

Herbicides and herbicide degradates

Deethylatrazine none none μg/L 0.014 na No

3,4-Dichloroaniline none none μg/L 0.0042 na No

Atrazine MCL-CA 1 μg/L 0.008 na Yes3

Hexazinone HAL-US 400 μg/L 0.008 na Yes3

Prometon HAL-US 100 μg/L 0.012 na Yes3

Simazine MCL-US 4 μg/L 0.006 na Yes3

Tebuthiuron HAL-US 500 μg/L 0.028 na Yes3

Volatile organic compounds (VOCs)

Trihalomethanes

Chloroform MCL-US4 80 μg/L 0.03 0.5 Yes

Solvents

1,1-Dichloroethene (1,1-DCE) MCL-CA 6 μg/L 0.02 0.5 No

Dichloromethane MCL-US 5 μg/L 0.04 0.5 No

Tetrachloroethene (PCE) MCL-US 5 μg/L 0.03 0.5 No

1,1,1-Trichloroethane (1,1,1-TCA) MCL-US 200 μg/L 0.03 0.5 No

Gasoline hydrocarbons and oxygenates

Benzene MCL-CA 1 μg/L 0.03 0.5 No

2-Ethyltoluene none none μg/L 0.03 na No

Isopropylbenzene NL-CA 770 μg/L 0.04 0.5 No

4-Isopropyltoluene none none μg/L 0.06 na No

Methyl tert-butyl ether (MTBE) MCL-CA 13 μg/L 0.1 1 No

n-Propylbenzene NL-CA 260 μg/L 0.04 0.5 No

1,2,3,4-Tetramethylbenzene none none μg/L 0.1 na No

1,2,3,5-Tetramethylbenzene none none μg/L 0.1 na No

1,2,3-Trimethylbenzene none none μg/L 0.1 na No

1,3,5-Trimethylbenzene NL-CA 330 μg/L 0.03 0.5 No

Other VOCs

Carbon disulfide NL-CA 160 μg/L 0.04 0.5 No

1,4-Dichlorobenzene MCL-CA 5 μg/L 0.03 0.5 No

Trichlorotrifluoroethane (CFC-113) MCL-CA 1,200 μg/L 0.03 0.5 No

Special-interest constituents

Perchlorate MCL-CA 6 μg/L 0.1 2 Yes

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 and SMCL-CA, California Department of Public Health (2013b); MCL-US and 

AL-US, U.S. Environmental Protection Agency (2009a); NL-CA, California Department of Public Health (2010); HAL-US and RSD5-US, U.S. Environmental 

Protection Agency (2012); Prop MCL-US, U.S. Environmental Protection Agency (1999a).

2

Nondetections are reported in the CDPH database as a concentration of zero or as less than the reporting limit. The most prevalent reporting limit as inferred 

from data for nondetections is listed.

3

These five herbicides were evaluated as a class by using the sum of the relative concentrations of the individual constituents.

4

The MCL-US for chloroform applies to the sum of the four trihalomethanes chloroform, bromodichloromethane, dibromochloromethane, and bromoform. 

Only chloroform was detected in the CAMP study unit.
24 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Table 6. Constituents reported at concentrations greater than benchmarks in the California Department of Public Health (CDPH)

database historically (November 16, 1982, to September 15, 2007), but not during the 3-year time period used in status assessment 

(September 16, 2007, to September 16, 2010), Cascade Range and Modoc Plateau (CAMP) study unit, 2010, California Groundwater 

Ambient Monitoring and Assessment Program (GAMA) Priority Basin Project.

[Benchmark type: Regulatory, health-based benchmarks: AL-US, USEPA action level; MCL-CA, CDPH maximum contaminant level; MCL-US, USEPA 

maximum contaminant level. Non-regulatory, aesthetic-based benchmarks: SMCL-CA, CDPH secondary maximum contaminant level. Benchmark 

units: mg/L, milligrams per liter; pCi/L, picocuries per liter; μg/L, micrograms per liter. Other abbreviations: mm/dd/yyyy, month/day/year; 

USEPA, U.S. Environmental Protection Agency]

The spatially weighted calculations used the USGS-grid 

well dataset and data from the CDPH database. High-RC 

aquifer-scale proportion was calculated for each constituent 

by computing the proportion of high-RC wells in each cell and 

then calculating the average proportion for the cells in each 

study area (equation 2; Isaaks and Srivastava, 1989; Belitz and 

others, 2010). The moderate-RC aquifer-scale proportion was 

calculated similarly.

P

W

W

N SA s

high

c

SA c

high

SA c

SA

,

,

, =

∑

(2)

where

PSA s

high

,

is the spatially weighted high-RC aquiferscale proportion for the study area SA,

WSA c

high

, is the number of wells in a particular cell 

in the study area having high RC for the 

constituent,

WSA c,

is the number of wells in a particular cell 

in the study area having data for the 

constituent, and

Σc is summation over the number of cells in the 

study area (the number of cells is 15 for all 

6 study areas).

Constituent

Benchmark Date of most 

recent high value

(mm/dd/yyyy)

Number of 

wells with 

historical data

Number of 

wells with a 

Type high value 1 Value Units

Inorganic constituents

Antimony MCL-US 6 μg/L 02/20/1997 226 1

Copper AL-US 1,300 μg/L 08/05/1992 214 1

Fluoride2 MCL-CA 2 mg/L 12/11/1996 270 1

Mercury3 MCL-US 2 μg/L 12/18/2003 231 1

Nickel MCL-CA 100 μg/L 05/21/2007 225 1

Nitrite, as nitrogen MCL-US 1 mg/L 02/07/2006 323 3

Radium-228 activity3 MCL-US 5 pCi/L 03/05/2007 133 1

Sulfate SMCL-CA 500 mg/L 11/17/1989 231 1

Thallium MCL-US 2 μg/L 12/14/1994 220 3

Organic constituents

Methy tert-butyl ether (MTBE) MCL-CA 13 μg/L 11/05/2002 210 1

Tetrachloroethene (perchloroethene, PCE) MCL-US 5 μg/L 02/22/2005 252 3

Trichloroethylene (TCE) MCL-US 5 μg/L 03/09/2005 252 1

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 and SMCL-CA, California Department of Public Health (2013b); MCL-US and 

AL-US, U.S. Environmental Protection Agency (2009a); NL-CA, California Department of Public Health (2010); HAL-US, U.S. Environmental Protection 

Agency (2012); Prop MCL-US, U.S. Environmental Protection Agency (1999a).

2

Constituent detected at moderate relative concentrations within 3-year period in the CDPH database and in the grid-well dataset; therefore, it was selected for 

additional evaluation in the status assessment for the study unit.

3

Constituent not analyzed by U.S. Geological Survey-GAMA for the CAMP study unit.
Methods 25

The grid-based and spatially weighted approaches both 

are designed to be spatially unbiased; however, the grid-based 

approach sometimes does not detect constituents that are 

present at high RCs (or moderate RCs) in small proportions of 

the primary aquifer system. The spatially weighted approach 

uses a greater number of wells and, therefore, has a greater 

chance of detecting small targets. This situation rarely 

occurred in the CAMP study unit because the 90 USGS-grid 

wells provided the majority of the data for all constituents 

except for nitrate.

High-RC aquifer-scale proportions for the study unit as a 

whole were calculated as an area-weighted combination of the 

grid-based or spatially weighted aquifer-scale proportions for 

the six study areas (equation 3). Moderate-RC aquifer-scale 

proportions were calculated similarly.

PSU

high

SA SA SA

high = ∑ F P (3)

where

PSU

high is the area-weighted high-RC aquifer-scale 

proportion for the CAMP study unit,

PSA

high is the high-RC aquifer-scale proportion for 

study area SA,

FSA

is the fraction of the study unit gridded area 

occupied by study area SA (table 2), and

ΣSA is summation over the six study areas.

Study unit detection frequencies for organic constituents 

also were calculated as area-weighted detection frequencies. 

The grid-based detection frequency in each study area was 

calculated by using equation 1 with NSA

high replaced by the 

number of samples with detections, and then the detection 

frequency for the study unit as a whole was calculated by 

using equation 3. Because of the area weighting, the study unit 

detection frequencies for organic constituents in this report can 

differ from the unweighted detection frequencies reported by 

Shelton and others (2013). 

In addition, for each constituent, the raw frequencies 

of occurrence of high and moderate RCs for individual 

constituents were calculated by using the same dataset as was 

used for the spatially weighted calculations. However, these 

raw occurrence frequencies are not spatially unbiased because 

the wells in the CDPH database are not uniformly distributed 

(fig. 4). 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 raw occurrence frequency of high RCs would be 

greater than the high aquifer-scale proportion. Raw occurrence 

frequencies are provided for reference, but were not used to 

assess aquifer-scale proportions.

Understanding Assessment

The purposes of the understanding assessment were 

to place groundwater quality in a physical and chemical 

context and to better understand the natural and human factors 

affecting groundwater quality. The assessment was based on 

the statistical strength of relations between concentrations 

or occurrences of selected water-quality constituents and 

values of selected potential explanatory factors. The potential 

explanatory factors evaluated were land use near the well, 

septic and underground-storage tank densities near the well, 

study area, aquifer lithology, depths to the top and bottom 

of the open or screened interval in the well, aridity index, 

groundwater age, oxidation-reduction conditions, and pH. 

Correlations among these factors that could affect apparent 

relations between aquifer lithology and water quality are also 

described. Data were compiled for the 90 USGS-grid wells 

sampled by USGS-GAMA. Other CDPH wells were not used 

for the understanding assessment because ancillary data for 

most of the CDPH wells were not available. This section 

describes the methods used for (1) selecting constituents 

for evaluation and (2) testing the statistical significance of 

correlations.

Selection of Constituents for Understanding 

Assessment

Constituents present at high RCs in greater than 

approximately 2 percent of the primary aquifer system were 

selected for evaluating relations between potential explanatory 

factors and groundwater quality. Nine inorganic constituents 

met this criterion and, therefore, have an understanding 

assessment section in this report (table 4A). No organic or 

special-interest constituents were present at high RCs in 

greater than approximately 2 percent of the primary aquifer 

system. 

Organic constituent classes containing at least one 

individual constituent with an area-weighted detection 

frequency in the study unit as a whole of greater than 

10 percent and special-interest constituents with an areaweighted detection frequency greater than 10 percent also 

were selected for evaluating relations between potential 

explanatory factors and groundwater quality. Two organic 

constituent classes, trihalomethanes and herbicides, and the 

special-interest constituent perchlorate met this criterion and, 

therefore, have an understanding assessment section in this 

report (table 5).
26 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Statistical Analysis

Nonparametric statistical methods were used to test the 

significance of correlations among the factors and between the 

factors and water-quality constituents. 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 percent (α=0.05) to evaluate whether 

the relation was statistically significant (p less than α).

Three different statistical tests were used because the 

set of potential explanatory factors included categorical and 

continuous variables. Groundwater age, aquifer lithology, 

study area, oxidation-reduction class, and depth class were 

treated as categorical variables: for example, groundwater ages 

were classified as modern, pre-modern, or mixed. Land use, 

septic-tank density, leaking or formerly leaking undergroundstorage tank (UST) density, aridity index, elevation, depths 

to top and bottom of screened or open interval, pH, and 

dissolved oxygen were treated as continuous variables; for 

example, land use was represented by percentages of land-use 

types. Depth and oxidation-reduction status were treated both 

as continuous (depth to top and bottom of screened or open 

interval and DO concentration, respectively) and as categorical 

(depth class as spring, shallow well, overlapping well, or 

deep well, and oxidation-reduction class as oxic or anoxic) 

variables (appendix A).

Correlations between continuous variables were 

evaluated by using the Spearman’s rho (ρ) test to calculate the 

rank-order correlation coefficient (ρ) and the significance level 

of the correlation (p).

Relations between categorical variables and continuous 

variables were evaluated by using a multi-stage KruskalWallis test to determine whether one or more of the groups 

had a significantly different median. The Kruskal-Wallis test 

is equivalent to the Wilcoxon rank-sum test for a categorical 

variable with two values. Pairwise Wilcoxon rank-sum tests 

were not used because the overall significance level for six 

pairwise tests with α=0.05 for a categorical variable with 

4 groups is α=0.26 (Helsel and Hirsch, 2002). If the KruskalWallis test detected a significant difference among the 

medians, then Tukey’s multiple comparison test was applied to 

the ranks of the data to determine which pairs had significantly 

different mean ranks. (Helsel and Hirsch, 2002).

Relations between categorical variables were evaluated 

by using contingency tables. For the 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 that are in the categories corresponding to the i

th

row and j

th column of the matrix. A test statistic is computed 

by comparing the observed counts 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 (Helsel and Hirsch, 2002). 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 

to each other. 

Contingency table tests also were used to evaluate 

whether aquifer-scale proportions for a constituent were 

significantly different among the six study areas. For these sixby-two contingency tables, the entries in the cells of the matrix 

are determined from the number of wells in a study area for 

which there were data for the constituent and the aquiferscale proportions. Contingency tables were constructed to 

evaluate whether the high-RC aquifer-scale proportion was 

significantly different among the study areas and to evaluate 

whether the proportion having high-RC or moderate-RC 

significantly differed among the study areas. For example, if 

a study area had 24 wells with data for a constituent, and the 

high-RC and moderate-RC aquifer-scale proportions were 

1.2 percent and 8.7 percent, respectively, the entries for that 

study area would be [0.29 23.7] in the contingency table 

testing whether the high-RC aquifer-scale proportion was 

significantly different among the study areas and [2.38 21.62] 

in the contingency table testing whether the proportion having 

high-RC or moderate-RC significantly differed among the 

study areas.

Characteristics of the Primary Aquifer 

System

The CAMP study unit covers a broad range of geologic, 

hydrologic, and land-use settings. Data for a finite set of 

potential explanatory factors were compiled: geology, land 

use and densities of leaking (or formerly leaking) underground 

storage tanks and septic systems, hydrologic conditions, well 

depth and groundwater age, and geochemical conditions. 

Methods used for assigning values of potential explanatory 

factors to the CAMP study-unit wells are described in 

appendix A. 

Correlations among explanatory factors are important to 

identify because apparent correlations between an explanatory 

factor and a water-quality constituent could reflect correlations 

between that explanatory factor and other explanatory factors 

rather than a causative relation between that explanatory 

factor and the water-quality constituent. Results of statistical 

tests of correlations among potential explanatory factors are 

summarized in tables 7A–C.
Characteristics of the Primary Aquifer System 27

Table 7A. Results of multi-stage Kruskal-Wallis tests for differences in values of selected potential explanatory factors between samples classified into groups by study area, aquifer lithology, age class, oxidation-reduction (redox) class, and depth class, Cascade Range and Modoc Plateau study unit, 2010, California Groundwater Ambient Monitoring and Assessment Program (GAMA) Priority Basin Project.[Kruskal-Wallis test p-values are listed in table. If Kruskal-Wallis test indicated significant differences (p-value less than a threshold value, α, of 0.05) among the groups, the groups that were significantly different were determined using Tukey’s test on the ranks of the data. Groups with signficant differences are listed in table. Study area: ES, Sacramento Valley Eastside; HL, Honey Lake Valley; LU, Cascade Range and Modoc Plateau Low-Use Basins; QV, Quaternary Volcanic Areas; SH, Shasta Valley and Mount Shasta Volcanic Area; TV, Tertiary Volcanic Areas. Geology class: G, granitic and sedimentary; M, metamorphic or metamorphic and sedimentary or metamorphic and volcanic; S, sedimentary; V, volcanic; VS, sedimentary and volcanic (see appendix A for explanation). Depth class: Deep, top and bottom of perforations >200 ft; Overlapping, top of perforations <200 ft and bottom of perforations >200 ft; Shallow, top and bottom of perforations <200 ft; Spring, groundwater emerges at land surface without pumping. Age class: Mixed, tritium>0.5 TU and 14C<90 pmc; Modern, tritum>0.5 TU and 14C>90 pmc; pre-modern, tritium<0.5 TU (see appendix A for explanation). Redox class: oxic, DO>1.0 mg/L and 

Mn<50 μg/L and Fe<100 μg/L; anoxic, DO<1.0 mg/L; mixed, DO>1.0 mg/L and Mn>50 μg/L and/or Fe>100 μg/L. Samples classified as mixed were grouped into the anoxic class for analysis (appendix A). Other abbreviations: DO, dissolved oxygen; ft, feet below land surface; mg/L, milligrams per liter; ns, no significant differences; pmc, percent modern carbon; TU, tritium units; USTs, leaking or formerly 

leaking underground-storage tanks; >, greater than; <, less than; μg/L, micrograms per liter]

p-value 

significant relations1

Study area

(ES, HL, SH, LU, QV, TV)

Aquifer lithology class

(G, M, S, V, VS)

Age class

(mod, mix, pre)

Redox class

(oxic, anoxic)

Depth class

(spring, shallow, overlap, deep)

Land-use characteristics

Percentage of agricultural land use <0.001

HL and LU>ES, QV, and TV

SH>QV

0.022

S>V and VS

0.772

ns

<0.001

Anoxic>Oxic

0.222

ns

Percentage of undeveloped land use <0.001

ES, QV, and TV>LU

0.007

V>M and S

VS>S

0.785

ns

0.006

Oxic>Anoxic

0.156

ns

Percentage of urban land use 0.026

LU>QV and TV

0.250

ns

0.963

ns

0.344

ns

0.110

ns

Density of septic tanks <0.001

ES and SH>LU and QV

0.314

ns

0.166

ns

0.015

Oxic>Anoxic

0.715

ns

Density of USTs <0.001

SH>HL, LU, and QV

ES>HL

0.094

ns

0.309

ns

0.387

ns

0.179

ns

Climate and hydrology variables

Aridity index <0.001

ES, QV, and TV>HL and LU

SH>HL

<0.001

V>G and S

VS>S

0.001

Mod>Pre

<0.001

Oxic>Anoxic

0.097

ns

Elevation <0.001

HL, LU, QV, and TV>ES and SH

0.025

G, S, V, and VS>M

0.268

ns

0.087

ns

0.762

ns
28 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010 p-value significant relations1 Study area (ES, HL, SH, LU, QV, TV) Aquifer lithology class (G, M, S, V, VS) Age class (mod, mix, pre) Redox class (oxic, anoxic) Depth class (spring, shallow, overlap, deep) Characteristics of primary aquifers Depth to top of screened or open interval

20.139

ns

0.031

V>M and S

VS>M

0.077

ns

0.325

ns

NA

Depth to bottom of screened or open 

interval3

0.038

ES>SH

0.323

ns

<0.001

Pre>Mod and mix

0.101

ns

NA

pH 0.001

HL>ES and SH

LU>ES

0.125

ns

<0.001

Pre>Mix>Mod

0.007

Anoxic>Oxic

<0.001

Deep and overlap> Spring and shallow

Dissolved oxygen concentration 0.017

QV and TV>HL

0.017

V>M, S, and VS

<0.001

Mod and mix>Pre

<0.001

Oxic>Anoxic

<0.001

Spring>Shallow, overlap, and deep

1 Explanation for reporting of significant differences using the results for percentage of agricultural land use by study unit as an example. The Tukey’s test results showed that the following seven pairs had 

significant differences: LU>ES, LU>QV, LU>TV, HL>ES, HL>QV, HL>TV, and SH>QV. Because both LU and HL have significantly greater agricultural land use than ES, QV, and TV, the notation for the first six pairs is condensed to ‘HL and LU>ES, QV, and TV.’ Because SH had significantly greater agricultural land use than QV, but not significantly greater than ES and TV, the seventh pair cannot be condensed with the first six. Agricultural land use was not significantly different for all the pairs not listed, in other words, agricultural land use in HL and LU were not significantly different from each other, agricultural land use in ES, QV, and TV were not significantly different from each other, and agricultural land use in SH was not significantly different from that in ES, HL, LU, or TV.

2Although the Kruskal-Wallis test indicated no significant differences among the six study areas, if the statistical tests were performed using 15 pairwise Wilconxon rank-sum tests, then HL>ES with a 

p-value of 0.014.

3Eleven wells did not have data for depth to the bottom of the screened or open interval, but did have data for well depth (table A2). Well depth data were used to represent data for the depth of the bottom 

Table 7A.

of the screened or open interval for these wells.

Results of multi-stage Kruskal-Wallis tests for differences in values of selected potential explanatory factors between samples classified into groups by study area, 

aquifer lithology, age class, oxidation-reduction (redox) class, and depth class, Cascade Range and Modoc Plateau study unit, 2010, California Groundwater Ambient Monitoring and Assessment Program (GAMA) Priority Basin Project.—Continued[Kruskal-Wallis test p-values are listed in table. If Kruskal-Wallis test indicated significant differences (p-value less than a threshold value, α, of 0.05) among the groups, the groups that were significantly different were determined using Tukey’s test on the ranks of the data. Groups with signficant differences are listed in table. Study area: ES, Sacramento Valley Eastside; HL, Honey Lake Valley; LU, Cascade Range and Modoc Plateau Low-Use Basins; QV, Quaternary Volcanic Areas; SH, Shasta Valley and Mount Shasta Volcanic Area; TV, Tertiary Volcanic Areas. Geology class: G, granitic and sedimentary; M, metamorphic or metamorphic and sedimentary or metamorphic and volcanic; S, sedimentary; V, volcanic; VS, sedimentary and volcanic (see appendix A for explanation). Depth class: Deep, top and bottom of perforations >200 ft; Overlapping, top of perforations <200 ft and bottom of perforations >200 ft; Shallow, top and bottom of perforations <200 ft; Spring, groundwater emerges at land surface without pumping. Age class: Mixed, tritium>0.5 TU and 14C<90 pmc; Modern, tritum>0.5 TU and 14C>90 pmc; pre-modern, tritium<0.5 TU (see appendix A for explanation). Redox class: oxic, DO>1.0 mg/L and 

Mn<50 μg/L and Fe<100 μg/L; anoxic, DO<1.0 mg/L; mixed, DO>1.0 mg/L and Mn>50 μg/L and/or Fe>100 μg/L. Samples classified as mixed were grouped into the anoxic class for analysis (appendix A). Other abbreviations: DO, dissolved oxygen; ft, feet below land surface; mg/L, milligrams per liter; ns, no significant differences; pmc, percent modern carbon; TU, tritium units; USTs, leaking or formerly 

leaking underground-storage tanks; >, greater than; <, less than; μg/L, micrograms per liter]
Characteristics of the Primary Aquifer System 29

Table 7B. Results of Spearman’s tests for correlations between selected potential explanatory factors, Cascade Range and Modoc 

Plateau study unit, 2010, California Groundwater Ambient Monitoring and Assessment Program (GAMA) Priority Basin Project.

[Abbreviations: ρ (rho), Spearman’s correlation statistic; USTs, leaking (or formerly leaking) underground storage tanks; <, less than p-values (significance 

level of the Spearman’s test) less than threshold value (α) of 0.05 are considered significant: bold black text, significant positive correlation; bold red text, 

significant negative correlation]

p-value

(ρ)

Percentage 

of natural 

land use

Percentage 

of urban 

land use

Density 

of 

septic 

tanks

Density 

of USTs

Aridity 

index Elevation

Depth 

to top of 

screened 

or open 

interval

Depth to 

bottom of 

screened 

or open 

interval1

pH

Dissolved 

oxygen 

concentration

Percentage of 

agricultural 

land use

<0.001

–0.64

0.065

0.20

0.380

–0.09

0.535

–0.07

<0.001

–0.58

0.562

0.06

0.522

–0.08

0.264

–0.13

<0.001

0.38

<0.001

–0.55

Percentage of 

natural land 

use

<0.001

–0.81

0.093

–0.18

0.539

–0.07

<0.001

0.40

0.648

–0.05

0.566

–0.07

0.411

–0.10

0.082

–0.18

0.001

0.335

Percentage of 

urban land use

0.055

0.20

0.342

0.10

0.036

–0.22

0.206

0.13

0.286

0.13

0.022

0.27

0.789

0.03

0.253

–0.122

Density of septic 

tanks

<0.001

0.44

0.003

0.31

<0.001

–0.38

0.811

–0.03

0.756

0.04

0.007

–0.28

0.103

0.17

Density of USTs 0.001

0.33

0.003

–0.31

0.061

0.23

0.030

0.25

0.024

–0.24

0.455

0.08

Aridity index 0.043

–0.21

0.523

0.08

0.967

0.00

<0.001

–0.67

<0.001

0.60

Elevation 0.361

0.12

0.958

–0.01

0.013

0.26

0.937

0.01

Depth to top of 

screened or 

open interval

<0.001

0.63

0.093

0.21

0.900

0.02

Depth to bottom 

of screened or 

open interval1

0.057

0.22

0.335

–0.11

pH <0.001

–0.54

1

Eleven wells did not have data for depth to the bottom of the screened or open interval, but did have data for well depth (table A2). Well depth data were used 

to represent data for the depth of the bottom of the screened or open interval for these wells.
30 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Table 7C. Results of contingency table tests for associations between selected potential explanatory factors, Cascade Range and 

Modoc Plateau study unit, 2010, California Groundwater Ambient Monitoring and Assessment Program (GAMA) Priority Basin Project.

[If contingency table test indicated a significant relation between the factors (p<0.05), then the correlations contributing most to that significant relation were 

identified from the components of the contingency table test statistic. Study area: ES, Sacramento Valley Eastside; HL, Honey Lake Valley; LU, Cascade 

Range and Modoc Plateau Low-Use Basins; QV, Quaternary Volcanic Areas; SH, Shasta Valley and Mount Shasta Volcanic Area; TV, Tertiary Volcanic Areas. 

Geology class: G, granitic and sedimentary; M, metamorphic or metamorphic and sedimentary or metamorphic and volcanic; S, sedimentary; V, volcanic; 

VS, sedimentary and volcanic (see appendix A for explanation). Depth class: Deep, top and bottom of perforations >200 ft; Overlapping, top of perforations 

<200 ft and bottom of perforations >200 ft; Shallow, top and bottom of perforations <200 ft; Spring, groundwater emerges at land surface without pumping. 

Age class: Mixed, tritium>0.5 TU and 14C<90 pmc; Modern, tritum>0.5 TU and 14C>90 pmc; pre-modern, tritium<0.5 TU (see appendix A for explanation). 

Redox class: anoxic, DO<1.0 mg/L; mixed, DO>1.0 mg/L and Mn>50 μg/L and/or Fe>100 μg/L; oxic, DO>1.0 mg/L and Mn<50 μg/L and Fe<100 μg/L. 

Samples classified as mixed were grouped into the anoxic class for analysis (appendix A). Other abbreviations: DO, dissolved oxygen; ft, feet below land 

surface; LUFTs, leaking underground fuel tanks; mg/L, milligrams per liter; ns, no significant differences; pmc, percent modern carbon; TU, tritium units; 

>, greater than; <, less than; μg/L, micrograms per liter]

p-value

signficant 

relations

Geology 

class

Depth 

class

Age 

class

Redox 

class

Study area <0.001

G and S lithology wells are in 

the HL study area; M lithology 

wells are in the SH study area; VS 

lithology wells are in the LU study 

area.

0.174

ns

0.017

HL wells are more likely to have 

pre-modern groundwater, and SH 

wells are less likely to have premodern groundwater.

0.029

Geology

Geology was quantified by two potential explanatory 

factors: study area and aquifer lithology. As discussed in the 

“Description of Study Unit” section, the six study areas are 

geologically distinct. Aquifer lithology in the CAMP study 

unit was defined by classifying the lithologic descriptions in 

the CDWR well completions reports and the surficial geology 

on the State geologic map (Jennings, 1977; Saucedo and 

others, 2000) into four categories: Mesozoic granitic rocks, 

Mesozoic and Paleozoic metamorphic rocks, Quaternary 

sedimentary deposits, and Tertiary and Quaternary volcanic 

rocks (appendix A). 

As expected from the definitions of the study areas, 

aquifer lithology was correlated with study area. Most CAMP 

study-unit wells had screened or open intervals in volcanic 

rocks (47 wells) or in both volcanic rocks and sedimentary 

deposits (17 wells) (table A1). As expected, all of the wells 

in the QV and TV study areas had open intervals in volcanic 

rocks, and for 26 of the 30, volcanic rocks were the only 

aquifer lithology. The majority of the wells in the LU study 

area had screened or open intervals in both sediment and 

volcanic rocks and only two were screened solely in sediment, 

likely reflecting preferential siting of public-supply wells in 

zones with greater yields. Wells in volcanic rocks generally 

had greater yields than those in sediments (Gannett and others, 

2007). The majority of the wells in the ES and SH study areas 

had open intervals in volcanic rocks. The HL study area was 

the only study area in which no wells had open intervals in 

volcanic rocks; all were screened in sediments. Three wells in 

the HL study area also had screened intervals in the underlying 

granitic rocks, and 4 wells in the SH study area also had 

screened or open intervals in underlying metamorphic rocks. 

These seven wells were located near the margins of the HL 

and SH groundwater basins where the alluvial sediments were 

relatively thin. One well in the TV study area north of the SH 

study area had open intervals in both the volcanic rocks of the 

Western Cascades series and the underlying metasedimentary 

rocks.

SH wells are more likely to have 

oxic groundwater.

Geology 

class

0.387

ns

0.360

ns

0.021

Wells in sedimentary deposits are 

more often anoxic than wells in 

volcanic rocks.

Depth class 0.018

Pre-modern groundwater is more 

likely to be found in deep wells than 

in springs or shallow wells, and 

modern groundwater is more likely 

to be found in shallow wells than in 

deep wells.

0.339

ns

Age class 0.072

ns
Characteristics of the Primary Aquifer System 31

Land Use

Land use was classified using an enhanced version of 

the satellite-derived (30-meter 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. About two-thirds 

of the CAMP study-unit wells had groundwater classified as 

mixed or pre-modern age, indicating presence of groundwater 

recharged many decades to thousands of years ago, perhaps 

(see ‘Well Depth and Groundwater Age’ section); therefore, 

land-use patterns from several decades ago were thought more 

likely to be relevant to the groundwater samples than current 

land-use patterns. The imagery is classified into 25 land-cover 

classifications (Nakagaki and Wolock, 2005). These 25 landcover classifications were condensed into 3 principal land-use 

categories: urban, agricultural, and natural (see appendix A). 

Land use in the whole CAMP study unit was 80.1 percent 

natural (forests, shrub lands, grasslands, rock, bare ground, 

and ice), 15.8 percent agricultural, and 4.1 percent urban 

(figs. 5, 6A). Natural land consists mainly of forests in the 

western part of the study unit and at higher elevations, and 

of shrub lands and grasslands in the eastern part and at lower 

elevations (fig. 5). Much of the natural land in the study unit is 

overseen by the U.S. Forest Service (Lassen, Modoc, ShastaTrinity, and Klamath National Forests) and may be used for 

open-range livestock grazing. Natural land use accounts for 

75 percent to 95 percent of the ES, HL, QV, SH, and TV study 

areas (fig. 6A).

Agricultural land use was unevenly divided among the 

study areas. More than half of the LU study area was used for 

agriculture; whereas, less than 2 percent of the ES, QV, and 

TV study areas were used for agriculture (fig. 6A). Most of the 

agricultural land was used for pasture or alfalfa hay and other 

silage; although wheat, barley, potatoes, wild rice, mint, and 

other crops also were grown (U.S. Department of Agriculture, 

2007). Urban land use constituted less than 5 percent of 

the land use in the CAMP study unit. The population was 

dispersed, with an average population density of less than 

20 people per square mile (people/mi2), compared to an 

average density of 239 people/mi2 for the state as a whole, and 

the population density was greater than 100 people/mi2 in only 

about 1 percent of the study unit (California Department of 

Finance, 2010).

Average land use around wells differed from the overall 

land use in the study unit because wells are preferentially 

located where there are people living, working, or recreating. 

Average land use in the area within the 500-m buffers around 

the USGS-grid wells was 17.7 percent urban and 10.9 percent 

agricultural. Unlike many other GAMA Priority Basin Project 

study units, the average land use around the USGS-grid 

wells was similar to the average land use around the CDPH 

wells (fig. 6A). This could reflect the dispersed population in 

the study unit. Of the 230 public water systems listed in the 

CDPH database for the study unit, nearly 60 percent served 

less than 100 people, and only 10 percent served more than 

1,000 people. Many of the GAMA Priority Basin Project 

study units have areas with dense populations that are served 

by public-water systems that have numerous wells in a small 

area. In contrast, the CDPH wells in the CAMP study unit are 

not markedly clumped. 

Land use was correlated with study area and aquifer 

lithology. Wells in the LU and HL study areas were 

surrounded by significantly greater percentages of agricultural 

land use than wells in the ES, QV, and TV study areas, and 

wells in the LU study area were surrounded by more urban 

land use than wells in the QV and TV study areas (table 7A). 

These correlations resulted from the concentration of 

agricultural activity in areas with thicker soils and sediments, 

and the greater density of population in agricultural areas 

compared to natural areas. Wells with sedimentary aquifer 

lithology were surrounded by significantly greater percentages 

of agricultural land use than wells with volcanic or volcanic 

and sedimentary aquifer lithology (table 7A). The percentage 

of urban land use was not correlated with aquifer lithology 

because the majority of wells in the LU study area had open 

intervals in volcanic rocks beneath the surficial sediments of 

the groundwater basins.

Septic tanks and leaking (or formerly leaking) 

underground storage tanks are markers of land-use patterns. 

Densities of septic tanks and USTs in the 500-m buffers 

around the USGS-grid wells were calculated from U.S. Census 

data (U.S. Census, 1990) and locations of environmental 

cleanup sites (California State Water Resources Control Board, 

2007), respectively (appendix A). Septic tanks generally are 

associated with dispersed residences or small communities 

because larger urban areas generally have collective sewer 

systems. Therefore, septic tanks can be in areas classified as 

natural or agricultural land use in addition to areas classified 

as urban land use. The density of septic tanks in the 500-m 

buffers around the USGS-grid wells in the study unit ranged 

from 0 to 256 tanks per square kilometer (tanks/km2

), with 

a median density of 1.2 tanks/km2

 (table A1). Dispersed 

residential development outside of cities is more likely to 

have individual septic systems than a collective sewer system 

because of the costs associated with constructing sewage 

collection and treatment systems. Septic-tank densities were 

greater in the ES and SH study areas than in the LU and QV 

study areas (table 7A). 

The density of USTs in the 500-m buffers around 

the USGS-grid wells in the study unit ranged from 0 to 

1.82 tanks/km2, with a median density of 0.01 tanks/km2

(table A1). The UST densities were greater in the SH study 

area than in the HL, LU, and QV study areas (table 7A). The 

UST density showed positive correlation with septic-tank 

density (table 7B), consistent with the higher UST and septictank densities in the SH study area compared to other study 

areas (table 7A).
32 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

122° 121° 120° 42°

41°

40°

0 10 20 30 40 KILOMETERS

Base modified from U.S. Geological Survey and other

Federal and State digital data, various scales 

Albers Equal Area Projection

North American Datum of 1983

0 10 20 30 40 MILES

EXPLANATION

County boundary

3-km buffer around California 

Department of Public Health wells

Agriculture Hydrologic provinces

Urban

Natural—forested

Natural—grassland and shrub land

Natural—rock, bare ground, ice

Land-use categories

GLENN CO

TEHAMA CO

PLUMAS CO

BUTTE CO SIERRA CO

SHASTA CO LASSEN CO

SISKIYOU CO MODOC CO

TRINITY CO

CALIFORNIA

Study

area

Central

Valley

Province

Klamath

Mountains

Province

Cascade Range

and 

Modoc Plateau

Province

Sierra

Nevada

Province Northern

Coast

Ranges

Province

sac15-0550_fig 05

Figure 5. Land use and major hydrologic features for the Cascade Range and Modoc Plateau (CAMP) study unit, 2010, California 

Groundwater Ambient Monitoring and Assessment (GAMA) Program Priority Basin Project.
Characteristics of the Primary Aquifer System 33

100

0

80

60

40

20

0

20

40

60

80

100

0

20

40

60

80

100

0

20

40

60

80

100

0

20

40

60

80

100

100

80

60

40

20

0

ES - Eastside Sacramento

HL - Honey Lake

LU - Low-Use Basins

QV - Quaternary Volcanics

SH - Shasta Valley

TV - Tertiary Volcanics

Land use in the 500-meter buffers 

around USGS-grid wells

EXPLANATION

A

B

ES - Eastside Sacramento

HL - Honey Lake

LU - Low-Use Basin

QV - Quaternary Volcanics

SV - Shasta Valley

TV - Tertiary Volcanics

Study areas

Study unit

Data point:

Land use 

averaged for

entire study

unit or

entire individual

study area

Data point:

Land use 

based on an

average for

the 500-meter

radius area 

around each

USGS grid well

Study-unit land use based on an

average for the 500-meter radius

area around each CDPH well

EXPLANATION

Percentage urban

Percentage agriculture

Percentage natural

Percentage urban

Percentage agriculture

Percentage natural

sac14-0550_fig 06

Figure 6. Percentage of urban, agricultural, and natural land use in the Cascade Range and Modoc Plateau (CAMP) study unit, 2010, 

California Groundwater Ambient Monitoring and Assessment Program (GAMA) Priority Basin Project. A, Average land use in the 

gridded area, average land use within 500-meters of USGS-grid wells, and average land use within 500-meters of California Department 

of Public Health (CDPH) wells for the study unit as a whole and for each of the six study areas; and B, land use within 500-meters of 

each U.S. Geological Survey (USGS)-grid well by study area.
34 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Hydrology

Hydrologic conditions were represented by the UNESCO 

aridity index (United Nations Educational, Scientific, and 

Cultural Organization, 1979; United Nations Environment 

Programme, 1997), and elevation at the well site (table A2).

The aridity index is the average annual precipitation divided 

by the average annual evapotranspiration; greater values 

correspond to wetter conditions (appendix A). Climate in the 

CAMP study unit ranges from arid to wet, and the aridity 

index is related to the elevation and position relative to the 

rain shadows of the Klamath Mountains and the Cascade 

Range. 

Aridity index showed a negative correlation with 

elevation for the dataset as a whole (table 7B), but showed 

positive correlations with elevation for individual study areas 

(Spearman’s test: ES, p=0.043, ρ=0.53; HL, p=0.001, ρ=0.78; 

SH, p<0.001, ρ=0.86). A positive correlation was expected 

because of the orographic effect: Air masses moving over 

rising terrain are forced upwards, and adiabatic cooling results 

in precipitation; therefore, precipitation generally increases 

with elevation on the windward side of mountain ranges. 

The leeward side of the mountain range is a dry area, a rain 

shadow, because the air mass descending the leeward side has 

already been stripped of moisture. Precipitation on the leeward 

side generally is still positively correlated with elevation, 

but the amount of precipitation at a given elevation is much 

less than on the windward side. Storm systems in northern 

California generally move from west to east, and a large part 

of the study unit is in the rain shadow east of the Klamath 

Mountains or the Cascade Range. The negative correlation 

for the dataset as a whole was driven by the fact that the HL 

study area and most of the LU study area receive the least 

precipitation because they are in the rain shadows of multiple 

mountain ranges, but the USGS-grid wells in those areas are 

at higher elevations than most of the USGS-grid wells in the 

other study areas.

Well Depth and Groundwater Age

The primary aquifer system in the CAMP study unit 

was defined as the depth interval over which wells in the 

CDPH database are screened or open. The 90 USGS-grid 

wells sampled by USGS-GAMA for the CAMP study unit 

were considered representative of the primary aquifer system, 

thus, depth characteristics of these sites can be used to define 

the primary aquifer system. Of the 63 wells having data for 

well depth and depth to the bottom of the screened or open 

intervals, the two depths were equal for 48 of the wells 

(76 percent). Therefore, depth to the bottom of the screened 

or open interval was assumed to be equal to well depth for the 

11 wells lacking data for depth to the bottom of the screened 

or open interval (table A2).

Nine of the sites sampled by USGS-GAMA were springs 

(table A2). At least one spring site was sampled in each of 

the study areas with the exception of the HL study area, and 

two-thirds of the springs were in either the QV or the SH 

study area. For the sites that were wells, median depths to 

the tops of the screened or open intervals ranged from 60 feet 

below land surface (ft bls) in the HL study area to 276 ft bls 

in the ES study area (fig. 7A), and there were no significant 

differences among the study areas (table 7A). Median depths 

to the bottoms of the screened or open interval (or bottom 

of well if data for the depth to the bottom of the screened or 

open interval were not available) ranged from 188 ft bls in the 

SH study area to 475 ft bls in the ES study area (fig. 7B), and 

wells in the ES study area were significantly deeper than those 

in the SH study area (table 7A). Springs were not included in 

the statistical tests involving relations with depths to tops or 

bottoms of the screened or open intervals.

Brown and Caldwell (2007) catalogued the depths of 

3,193 domestic and 407 irrigation wells in Lassen County 

from a CDWR database of well completion reports. Lassen 

County covers approximately one-quarter of the CAMP 

study area and includes all of the HL study area and parts 

of the LU, QV, and TV study areas. The median depths of 

wells in the primary aquifer system in those four study areas 

(216 to 344 ft bls; fig. 7B) was greater than the median depth 

of domestic wells in Lassen County (150 ft bls), and less 

than the median depth of irrigation wells in Lassen County 

(425 ft bls). This suggests that public-supply wells in the 

four study areas generally are deeper than domestic wells. 

However, the division between domestic and municipal wells 

for the CDWR well completion reports is not the same as the 

division between private and public wells for the CDPH. At 

the time of this study, the CDPH database for Lassen County 

listed 76 public-supply wells, whereas Brown and Caldwell 

(2007) reported 17 municipal wells in the CDWR well 

completion report database. Johnson and Belitz (2015) found 

that approximately 20 percent of wells reported as domestic 

on CDWR well completion reports statewide were listed as 

owned by an entity other than a private individual, and thus 

may be considered small-system or public-supply wells by 

CDPH. 

Groundwater “age” refers to the amount of time elapsed 

since the water was last in contact with the atmosphere and 

is related to its residence time in the aquifer system. Data 

for the age-dating tracers tritium and carbon-14 were used 

to classify groundwater ages into three categories: modern, 

mixed, and pre-modern (appendix A). Tritium values greater 

than 0.5 tritium units (TU) were defined as indicating presence 

of some groundwater recharged since 1952. The 14C values 

greater than 90 percent modern carbon (pmc) were defined 

as indicating presence of some groundwater recharged since 

1952. Samples with tritium activities less than 0.5 TU were 

classified as “pre-modern” groundwater; samples with tritium 

activities greater than 0.5 TU and 14C values greater than 

90 pmc were classified as “modern” groundwater. Samples 

with tritium activities greater than 0.5 TU and 14C values less 

than 90 pmc were classified as “mixed” groundwater.
Characteristics of the Primary Aquifer System 35

sac15-0550_fig 07

0

200

400

600

800

1,000

2,000

3,000

13 14 14 11 10 12

Depth to bottom of screened or open interval or well depth, 

in feet below land surface

Study area

Eastside

Sacramento

Honey

Lake

Low-Use

Basins

Quaternary

Volcanics

Shasta

Valley

Tertiary

Volcanics

Eastside

Sacramento

Honey

Lake

Low-Use

Basins

Quaternary

Volcanics

Shasta

Valley

Tertiary

Volcanics

Depth to top of screened or open interval, in feet below land surface

0

200

400

600

800

1,000

2,000

3,000

12 11 13 12 8 9

EXPLANATION

A

B

50th (median) percentile

10th percentile

90th percentile 

25th percentile

75th percentile

Data under 10th percentile

11 Number of USGS-grid

wells with data

Data over 90th percentile

Figure 7. Depths to screened or open interval for U.S. Geological Survey (USGS)-grid wells by study area, Cascade Range and Modoc 

Plateau (CAMP) study unit, 2010, California Groundwater Ambient Monitoring and Assessment (GAMA) Program Priority Basin Project. 

A, to top of screened or open interval; and B, to bottom of screened or open interval.
36 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Of the 90 USGS-grid well samples collected by USGSGAMA, 34 were classified as having modern groundwater, 20 

as having mixed groundwater, and 33 as having pre-modern 

groundwater (table A4). In addition, three wells with tritium 

activities greater than 0.5 TU could not be categorized as 

modern or mixed because of a lack of 14C data; these wells 

were excluded from statistical tests and plots for which the 

three age classes were handled separately.

Wells with pre-modern groundwater had significantly 

greater depths to the bottoms of the screened or open 

intervals than did wells with modern or mixed groundwater 

(table 7A). Classified groundwater ages were used to create a 

classification system for depth class. The boundary between 

shallow and deep—the critical depth—was determined by 

optimizing the segregation of modern and pre-modern age 

samples into shallow and deep wells, respectively. Wells 

with screened or open intervals entirely above the critical 

depth were defined as shallow (22 wells); wells with screened 

or open intervals beginning above the critical depth and 

ending below the critical depth were defined as overlapping 

(28 wells); and wells with screened or open intervals entirely 

below the critical depth were defined as deep (23 wells) 

(table A2). For the CAMP study unit, the critical depth was 

200 ft bls (fig. 8A). Wells deeper than 200 ft bls and lacking 

data for depth to the top of the screened or open interval were 

defined as overlapping or deep (five wells); wells with the 

depth to the top of the screened or open interval less than 

200 ft bls and lacking data for well depth were defined as 

overlapping or shallow (one well); these wells were excluded 

from statistical tests and plots for which the four depth classes 

were handled separately. Two wells had insufficient data for 

classification. The nine springs were considered a separate 

class.

This classification system was based on the assumption 

that depth class and age class were related. Pre-modern 

groundwater was more common in deep wells and less 

common in springs and shallow wells, and modern 

groundwater was more common in shallow wells and 

less common in deep wells (table 7C); however, all three 

groundwater age classes were found in all four depth classes, 

and about 20–35 percent of the samples from all four depth 

classes yielded mixed-age groundwater (fig. 8A). Age class 

also was significantly related to study area. The HL study area 

had a greater proportion of pre-modern groundwater, and the 

SH study area had a greater proportion of mixed groundwater 

and a lower proportion of pre-modern groundwater compared 

to the other study areas (table 7C; fig. 8C). Age class and 

depth class were not correlated with measures of land use 

(table 7A). Sites with modern groundwater had significantly 

greater aridity indices (indicating wetter conditions) than 

sites with pre-modern groundwater, reflecting the greater 

abundance of pre-modern groundwater in the HL study area, 

the most arid (lowest aridity index value) study area.

Geochemical Conditions

Groundwater geochemical conditions were represented 

by oxidation-reduction conditions and pH. Oxidationreduction conditions were classified on the basis of dissolved 

oxygen (DO), nitrate, manganese, and iron concentrations 

by using a modified version of the classification scheme of 

McMahon and Chapelle (2008) and Jurgens and others (2009). 

The primary modification was that the DO threshold for 

separating oxic from anoxic groundwater was increased from 

0.5 mg/L to 1 mg/L (Fram and Belitz, 2012). For a majority 

of the sites in the CAMP study unit (72 of the 90 USGS-grid 

wells [80 percent]), the groundwater was classified as oxic 

(DO greater than or equal to 1 mg/L) (tables A5, A6). At 

least some portion of the water was anoxic in the remaining 

20 percent of the wells; 5 wells (6 percent) were classified as 

mixed, and 13 wells (14 percent) were classified as anoxic. 

Mixed and anoxic conditions were further subdivided into 

suboxic, nitrate-reducing, manganese-reducing, and ironreducing conditions, or a combination of two reducing 

conditions (tables A5, A6). Correlations between oxidationreduction conditions and other potential explanatory factors 

were tested with oxidation-reduction conditions represented by 

a continuous variable (DO concentration) and by a categorical 

variable (redox class). For statistical tests involving redox 

class, anoxic and mixed categories were combined.

Oxidation-reduction conditions and pH were significantly 

related to study area, aquifer lithology, groundwater age, and 

depth class (table 7A). DO concentrations were significantly 

greater in springs than in shallow, overlapping, or deep wells, 

greater in modern and mixed groundwater than in pre-modern 

groundwater, and greater in samples from volcanic aquifer 

lithology than in samples from sedimentary aquifer lithology 

(table 7A). These observations imply that the CAMP study 

unit springs discharge water that has had extensive, recent 

interaction with the atmosphere. Groundwater newly entering 

the aquifer system likely has not interacted extensively with 

organic matter or reduced inorganic aquifer materials, and thus 

DO would not have been consumed. The sedimentary deposits 

in the CAMP study unit include lacustrine sediments, which 

commonly contain organic matter. In contrast, volcanic rocks 

generally have a low abundance of organic matter; thus, DO in 

groundwater in volcanic rocks can be consumed less rapidly 

than DO in groundwater in sedimentary deposits of the CAMP 

study unit. These relations could account for the significantly 

higher DO concentrations in samples from the QV and TV 

study areas compared to the HL study area (table 7A; fig. 9A) 

and the significantly greater proportion of samples classified 

as oxic in the SH study area compared to the other study areas 

(table 7C).
Characteristics of the Primary Aquifer System 37

Modern

Modern or mixed

Mixed

Pre-modern

Age class

Number of U.S. Geological Survey-grid wells

0

2

4

6

8

10

12

Study areas

14

EXPLANATION

Eastside

Sacramento

Honey

Lake

Low-Use

Basins

Quaternary

Volcanics

Shasta

Valley

Tertiary

Volcanics

Spring

Shallow

Shallow or overlapping

Overlapping

Overlapping or deep

Deep

Unknown

Depth class

Number of U.S. Geological Survey-grid wells

0

2

4

6

8

10

12

14

EXPLANATION

Number of U.S. Geological Survey-grid wells

Modern

Mixed 

Pre-modern

Groundwater age class

Spring

Shallow

well

Overlapping

well

Deep

well

Depth class

Wells with age class ‘modern or 

mixed’ (table A4) or depth class 

’shallow or overlapping; or 

‘overlapping or deep’ (table A2) 

are not plotted.

0 2 4 6 8 10 12 14

EXPLANATION

A

B

C

sac15-0550_fig 08

Figure 8. Bar charts showing the relations for wells in the Cascade Range and Modoc Plateau (CAMP) study unit, 2010, California 

Groundwater Ambient Monitoring and Assessment (GAMA) Program Priority Basin Project between groundwater A, age class and 

depth class; B, depth class and study area; and C, age class and study area.
38 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010 pH

5.5

6.0

6.5

7.0

7.5

8.0

8.5

9.0

9.5

Dissolved oxygen, in milligrams per liter

0

2

4

6

8

10

12

14

Study areas

Eastside

Sacramento

Honey

Lake

Low-Use

Basins

Quaternary

Volcanics

Shasta

Valley

Tertiary

Volcanics

Eastside

Sacramento

Honey

Lake

Low-Use

Basins

Quaternary

Volcanics

Shasta

Valley

Tertiary

Volcanics

EXPLANATION

50th (median) percentile

10th percentile

90th percentile 

25th percentile

75th percentile

Data under 10th percentile

Data over 90th percentile

A

B

sac14-0550_fig 09

Figure 9. Values for groundwater samples from the six study areas, Cascade Range and Modoc Plateau study (CAMP) unit, 

2010, California Groundwater Ambient Monitoring and Assessment (GAMA) Program Priority Basin Project. A, dissolved oxygen 

concentration and B, pH.
Status and Understanding of Groundwater Quality 39

The pH values showed a strong negative correlation with 

DO concentration (table 7B), and, accordingly, pH values 

were significantly greater in deep and overlapping wells than 

in springs or shallow wells, and in pre-modern groundwater 

than in modern or mixed groundwater (table 7A). Precipitation 

in the CAMP study unit is dilute and acidic: median 

specific conductance values were less than 5 microsiemens 

per centimeter (μS/cm), and median pH values were 

approximately 5.4 for annual wet deposition at National 

Atmospheric Deposition Program sites within the boundaries 

of the CAMP study unit (National Atmospheric Deposition 

Program, 2012). These low pH values are primarily controlled 

by the equilibrium between atmospheric carbon dioxide and 

carbonic acid (H2CO3) in solution (Stumm and Morgan, 1996). 

As the length of contact time between groundwater and the 

aquifer materials increases, pH values generally rise as acid 

is consumed by weathering reactions of silicate minerals and 

dissolution of carbonate minerals (if present) (Stumm and 

Morgan, 1996). The pH values were greater in the HL study 

area than in the ES and SH study areas (fig. 9B), reflecting that 

samples from the HL study area were more commonly premodern age.

Status and Understanding of 

Groundwater Quality

The following discussion 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 the number of constituents that were 

detected at any concentration in the USGS-grid well samples 

compared to the number analyzed, and a graphical summary 

of the RCs of constituents detected in the USGS-grid wells. 

Aquifer-scale proportions are then presented for constituent 

classes and for the subset of individual constituents that were 

present at moderate or high RCs (constituents present only 

at low RCs have aquifer-scale proportions of 100 percent 

low-RC). Finally, results of statistical tests for relations 

between water quality and potential explanatory factors are 

presented for the smaller subset of individual constituents and 

constituent classes that met further criteria based on RCs or, 

for organic constituents, detection frequency.

Inorganic Constituents

Inorganic constituents generally occur naturally in 

groundwater, although their concentrations can be influenced 

by human activities as well as by natural factors (Hem, 1985). 

Of the 45 inorganic constituents analyzed by USGS-GAMA, 

24 had regulatory or non-regulatory health-based benchmarks, 

8 had non-regulatory aesthetic-based secondary maximum 

contaminant level (SMCL) benchmarks, and 13 had no 

established benchmarks (table 4A, 4B). Of the 32 inorganic 

constituents with benchmarks, 17 were detected at moderate 

or high RCs in samples collected by USGS-GAMA for the 

CAMP study unit or were reported in the CDPH database 

at moderate or high RCs in samples collected from any 

well between September 16, 2007, and September 16, 2010 

(table 4A). The other 28 inorganic constituents either had no 

established benchmarks or were only detected at low RCs 

(table 4B). Most of the constituents without benchmarks are 

major or minor ions that are present in nearly all groundwater.

Of the 17 inorganic constituents, 16 were detected at 

moderate or high RCs in the USGS-grid wells: the nutrients 

ammonia and nitrate; the trace elements arsenic, boron, 

fluoride, molybdenum, strontium, and vanadium; the 

radioactive constituents gross alpha particle activity, radon222 activity, and uranium; and the constituents with SMCL 

benchmarks chloride, iron, manganese, specific conductance, 

and total dissolved solids (TDS) (table 4A; figs. 10, 11A–C). 

The majority of these 16 constituents were detected at 

moderate or high RCs in 6 percent or less of the grid wells 

(figs. 11A–C). Lead was reported at moderate or high RCs in 

4 wells in the CDPH database between September 16, 2007, 

and September 16, 2010; thus the high-RC and moderate-RC 

aquifer-scale proportions for lead were not zero when 

calculated by using the spatially weighted approach (table 4A). 

However, discrepancies between lead concentrations measured 

by USGS-GAMA and those reported by CDPH for samples 

from the same wells indicate that the data in the CDPH 

database for samples from wells in the CAMP study unit could 

be unreliable (appendix B). Lead was not included in the 

calculation of aquifer-scale proportions for trace elements as a 

class or for inorganic constituents as a class.

Aquifer-scale proportions for individual inorganic 

constituents are summarized in table 8 for the CAMP study 

unit and in tables C1A–F for the six study areas. Aquifer-scale 

proportions for inorganic constituent classes are summarized 

in table 9A for inorganic constituents with health-based 

benchmarks and in table 9B for inorganic constituents 

with SMCL benchmarks, and results of statistical tests of 

differences in aquifer-scale proportions among the study 

areas are given in table 10. The geographic distributions of 

concentrations of the six inorganic constituents for which 

understanding assessment results are presented are shown in 

figures 12A–F. Results of statistical tests for relations between 

water quality and potential explanatory factors are presented 

in tables 11A–B and 12 for these six constituents.
40 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Constituent class

Maximum relative-concentration, dimensionless

0.001

0.01

0.1

1

10

1,1-DCE

Benzene

Carbon disulfide

Dichloromethane

MTBE

PCE

Chloroform

1,4-DCB

Atrazine

Simazine

Perchlorate

Ammonia

Nitrate

Nitrite

Aluminum

Antimony

Arsenic

Barium

Beryllium

Boron

Cadmium

Chromium, Lead

Copper

Fluoride

Molybdenum

Nickel

Selenium

Strontium

Thallium

Vanadium Gross alpha

Gross beta

Radon-222

Uranium Chloride, TDS

Iron

Manganese

Sulfate

Zinc

Organic and special-interest 

constituent classes Inorganic constituent classes

High-RC

Moderate-RC

Low-RC

Volatile

organic

compounds

Pesticides Specialinterest

constituents

Nutrients Trace elements

with healthbased benchmarks

Radioactive

constituents

Inorganic

consituents

with SMCL

benchmarks

sac14-0550_fig 10

Figure 10. Maximum relative-concentrations (RC) of constituents detected in U.S. Geological Survey (USGS)-grid wells by constituent 

class, Cascade Range and Modoc Plateau (CAMP) study unit, 2010, California Groundwater Ambient Monitoring and Assessment

(GAMA) Program Priority Basin Project. (TDS, total dissolved solids; MTBE, methyl tert-butyl ether; PCE, tetrachloroethene; 

1,1-DCE, 1,1-dichloroethene; 1,4-DCB, 1,4-dichlorobenzene; SMCL, secondary maximum contaminant level)
Status and Understanding of Groundwater Quality 41

Arsenic

Relative-concentration, dimensionless

0

0.5

1.0

1.5

2.0

Boron Fluoride Molybdenum Strontium Vanadium

>2

High-RC

Moderate-RC

Low-RC

20

9.4

11 5.5 2.9

Uranium

0

0.5

1.0

1.5

2.0

Radon-222 Gross alpha Nitrate Ammonia

Relative-concentration, dimensionless

>2

High-RC

Moderate-RC

Low-RC

2.5

A

B

ES - Eastside Sacramento Valley

HL - Honey Lake Valley

LU - Low-Use Basins

Study area

QV - Quaternary Volcanics

SH - Shasta Valley

TV - Tertiary Volcanics

EXPLANATION

sac15-0550_fig11ab

Figure 11. Relative-concentrations (RC) for water samples from wells in the Cascade Range and Modoc Plateau (CAMP) study unit, 

2010, California Groundwater Ambient Monitoring and Assessment (GAMA) Program Priority Basin Project of selected A, trace elements 

with health-based benchmarks; B, nutrients and radioactive constituents with health-based benchmarks; and C, salinity indicators and 

trace metals with secondary maximum contaminant level (SMCL) benchmarks.
42 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Iron

Relative-concentration, dimensionless

0

0.5

1.0

1.5

2.0

Manganese Total dissolved solids Chloride

>2

C

High-RC

Moderate-RC

Low-RC

2.2

3.9

3.6

2.1

2.9 3.7

2.9

3.3

ES - Eastside Sacramento Valley

HL - Honey Lake Valley

LU - Low-Use Basins

Study area

QV - Quaternary Volcanics

SH - Shasta Valley

TV - Tertiary Volcanics

EXPLANATION

sac15-0550_fig 11c

Figure 11. —Continued
Status and Understanding of Groundwater Quality 43

Lake

Shasta

Goose

Lake

Upper

Alkali

Lake

Eagle

Lake

Honey

Lake

Lake

Almanor

Lake

Oroville

Sacramento River

North Fork Feather River

Klamath River

Pit River

Shasta River

Central

Valley

Province

Klamath

Mountains

Province

Cascade Range

and 

Modoc Plateau

Province

Sierra

Nevada

Northern Province

Coast

Ranges

Province

122° 121° 120° 42°

41°

40°

CALIFORNIA

Study

area

0 10 20 30 40 KILOMETERS

Base modified from U.S. Geological Survey and other

Federal and State digital data, various scales 

Albers Equal Area Projection

North American Datum of 1983

0 10 20 30 40 MILES

A

EXPLANATION

County boundary

Hydrologic provinces

Tertiary Volcanic Areas (TV)

Quaternary Volcanic Areas (QV)

Shasta Valley and Mount Shasta Volcanic Area (SH)

Eastside Sacramento Valley (ES)

Cascade Range and Modoc Plateau Low-Use Basins (LU)

Honey Lake Valley (HL)

Study areas

USGS-grid

wells

CDPH publicsupply wells

≤5 (low)

>5 to ≤10 (medium)

>10 (high)

Concentration of arsenic,

Other areas in the Cascade Range in micrograms per liter

 and Modoc Plateau Province

sac15-0550_fig 12a

Figure 12. Concentrations in water samples from U.S. Geological Survey (USGS)-grid wells and all wells in the California Department 

of Public Health (CDPH) database with data during the 3-year period used in the status assessment from September 16, 2007, through 

September 16, 2010, Cascade Range and Modoc Plateau (CAMP) study unit, 2010, California Groundwater Ambient Monitoring and 

Assessment (GAMA) Program Priority Basin Project of A, arsenic; B, boron; C, molybdenum; D, vanadium; E, uranium; F, total dissolved 

solids; G, manganese; and H, iron.
44 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Lake

Shasta

Goose

Lake

Upper

Alkali

Lake

Eagle

Lake

Honey

Lake Lake

Almanor

Sacramento River

North Fork Feather River

Klamath River

Pit River

Shasta River

Central

Valley

Province

Klamath

Mountains

Province

Cascade Range

and 

Modoc Plateau

Province

Sierra

Nevada

Northern Province

Coast

Ranges

Province

122° 121° 120° 42°

41°

40°

CALIFORNIA

Study

area

0 10 20 30 40 KILOMETERS

Base modified from U.S. Geological Survey and other

Federal and State digital data, various scales 

Albers Equal Area Projection

North American Datum of 1983

0 10 20 30 40 MILES

B

EXPLANATION

USGS-grid

wells

CDPH publicsupply wells

≤500 (low)

>500 to ≤1,000 (medium)

>1,000 (high)

Concentration of boron,

in micrograms per liter

County boundary

Hydrologic provinces

Tertiary Volcanic Areas (TV)

Quaternary Volcanic Areas (QV)

Shasta Valley and Mount Shasta Volcanic Area (SH)

Eastside Sacramento Valley (ES)

Cascade Range and Modoc Plateau Low-Use Basins (LU)

Honey Lake Valley (HL)

Study areas

Other areas in the Cascade Range

 and Modoc Plateau Province

sac15-0550_fig 12b

Figure 12. —Continued
Status and Understanding of Groundwater Quality 45

Lake

Shasta

Goose

Lake

Upper

Alkali

Lake

Eagle

Lake

Honey

Lake Lake

Almanor

Sacramento River

North Fork Feather River

Klamath River

Pit River

Shasta River

Central

Valley

Province

Klamath

Mountains

Province

Cascade Range

and 

Modoc Plateau

Province

Sierra

Nevada

Northern Province

Coast

Ranges

Province

122° 121° 120° 42°

41°

40°

CALIFORNIA

Study

area

0 10 20 30 40 KILOMETERS

Base modified from U.S. Geological Survey and other

Federal and State digital data, various scales 

Albers Equal Area Projection

North American Datum of 1983

0 10 20 30 40 MILES

C

EXPLANATION

USGS-grid wells

≤20 (low)

>20 to ≤40 (medium)

>40 (high)

Concentration of molybdenum,

in micrograms per liter

County boundary

Hydrologic provinces

Tertiary Volcanic Areas (TV)

Quaternary Volcanic Areas (QV)

Shasta Valley and Mount Shasta Volcanic Area (SH)

Eastside Sacramento Valley (ES)

Cascade Range and Modoc Plateau Low-Use Basins (LU)

Honey Lake Valley (HL)

Study areas

Other areas in the Cascade Range

 and Modoc Plateau Province

sac15-0550_fig 12c

Figure 12. —Continued
46 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Lake

Shasta

Goose

Lake

Upper

Alkali

Lake

Eagle

Lake

Honey

Lake Lake

Almanor

Sacramento River

North Fork Feather River

Klamath River

Pit River

Shasta River

Central

Valley

Province

Klamath

Mountains

Province

Cascade Range

and 

Modoc Plateau

Province

Sierra

Nevada

Northern Province

Coast

Ranges

Province

122° 121° 120° 42°

41°

40°

CALIFORNIA

Study

area

0 10 20 30 40 KILOMETERS

Base modified from U.S. Geological Survey and other

Federal and State digital data, various scales 

Albers Equal Area Projection

North American Datum of 1983

0 10 20 30 40 MILES

D

EXPLANATION

USGS-grid

wells

CDPH publicsupply wells

≤25 (low)

>25 to ≤50 (medium)

>50 (high)

Concentration of vanadium,

in micrograms per liter

County boundary

Hydrologic provinces

Tertiary Volcanic Areas (TV)

Quaternary Volcanic Areas (QV)

Shasta Valley and Mount Shasta Volcanic Area (SH)

Eastside Sacramento Valley (ES)

Cascade Range and Modoc Plateau Low-Use Basins (LU)

Honey Lake Valley (HL)

Study areas

Other areas in the Cascade Range

 and Modoc Plateau Province

sac15-0550_fig 12d

Figure 12. —Continued
Status and Understanding of Groundwater Quality 47

Lake

Shasta

Goose

Lake

Upper

Alkali

Lake

Eagle

Lake

Honey

Lake Lake

Almanor

Sacramento River

North Fork Feather River

Klamath River

Pit River

Shasta River

Central

Valley

Province

Klamath

Mountains

Province

Cascade Range

and 

Modoc Plateau

Province

Sierra

Nevada

Northern Province

Coast

Ranges

Province

122° 121° 120° 42°

41°

40°

CALIFORNIA

Study

area

0 10 20 30 40 KILOMETERS

Base modified from U.S. Geological Survey and other

Federal and State digital data, various scales 

Albers Equal Area Projection

North American Datum of 1983

0 10 20 30 40 MILES

E

EXPLANATION

USGS-grid

wells

CDPH publicsupply wells

≤15 (low)

>15 to ≤30 (medium)

>30 (high)

Concentration of uranium,

in micrograms per liter

sac15-0550_fig 12e

County boundary

Hydrologic provinces

Tertiary Volcanic Areas (TV)

Quaternary Volcanic Areas (QV)

Shasta Valley and Mount Shasta Volcanic Area (SH)

Eastside Sacramento Valley (ES)

Cascade Range and Modoc Plateau Low-Use Basins (LU)

Honey Lake Valley (HL)

Study areas

Other areas in the Cascade Range

 and Modoc Plateau Province

Figure 12. —Continued
48 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Lake

Shasta

Goose

Lake

Upper

Alkali

Lake

Eagle

Lake

Honey

Lake Lake

Almanor

Sacramento River

North Fork Feather River

Klamath River

Pit River

Shasta River

Central

Valley

Province

Klamath

Mountains

Province

Cascade Range

and 

Modoc Plateau

Province

Sierra

Nevada

Northern Province

Coast

Ranges

Province

122° 121° 120° 42°

41°

40°

CALIFORNIA

Study

area

0 10 20 30 40 KILOMETERS

Base modified from U.S. Geological Survey and other

Federal and State digital data, various scales 

Albers Equal Area Projection

North American Datum of 1983

0 10 20 30 40 MILES

F

EXPLANATION

USGS-grid

wells

CDPH publicsupply wells

≤500 (low)

>500 to ≤1,000 (medium)

>1,000 (high)

Concentration of total dissolved solids,

in milligrams per liter

County boundary

Hydrologic provinces

Tertiary Volcanic Areas (TV)

Quaternary Volcanic Areas (QV)

Shasta Valley and Mount Shasta Volcanic Area (SH)

Eastside Sacramento Valley (ES)

Cascade Range and Modoc Plateau Low-Use Basins (LU)

Honey Lake Valley (HL)

Study areas

Other areas in the Cascade Range

 and Modoc Plateau Province

sac15-0550_fig 12f

Figure 12. —Continued
Status and Understanding of Groundwater Quality 49

Lake

Shasta

Goose

Lake

Upper

Alkali

Lake

Eagle

Lake

Honey

Lake Lake

Almanor

Sacramento River

North Fork Feather River

Klamath River

Pit River

Shasta River

Central

Valley

Province

Klamath

Mountains

Province

Cascade Range

and 

Modoc Plateau

Province

Sierra

Nevada

Northern Province

Coast

Ranges

Province

122° 121° 120° 42°

41°

40°

CALIFORNIA

Study

area

0 10 20 30 40 KILOMETERS

Base modified from U.S. Geological Survey and other

Federal and State digital data, various scales 

Albers Equal Area Projection

North American Datum of 1983

0 10 20 30 40 MILES

G

County boundary

Hydrologic provinces

Tertiary Volcanic Areas (TV)

Quaternary Volcanic Areas (QV)

Shasta Valley and Mount Shasta Volcanic Area (SH)

Eastside Sacramento Valley (ES)

Cascade Range and Modoc Plateau Low-Use Basins (LU)

Honey Lake Valley (HL)

Study areas

Other areas in the Cascade Range

 and Modoc Plateau Province

EXPLANATION

USGS-grid

wells

CDPH publicsupply wells

≤25 (low)

>25 to ≤50 (medium)

>50 (high)

Concentration of manganese,

in micrograms per liter

sac15-0550_fig 12g

Figure 12. —Continued
50 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Lake

Shasta

Goose

Lake

Upper

Alkali

Lake

Eagle

Lake

Honey

Lake Lake

Almanor

Sacramento River

North Fork Feather River

Klamath River

Pit River

Shasta River

Central

Valley

Province

Klamath

Mountains

Province

Cascade Range

and 

Modoc Plateau

Province

Sierra

Nevada

Northern Province

Coast

Ranges

Province

122° 121° 120° 42°

41°

40°

CALIFORNIA

Study

area

0 10 20 30 40 KILOMETERS

Base modified from U.S. Geological Survey and other

Federal and State digital data, various scales 

Albers Equal Area Projection

North American Datum of 1983

0 10 20 30 40 MILES

H

EXPLANATION

USGS-grid

wells

CDPH publicsupply wells

≤150 (low)

>150 to ≤300 (medium)

>300 (high)

Concentration of iron,

in micrograms per liter

County boundary

Hydrologic provinces

Tertiary Volcanic Areas (TV)

Quaternary Volcanic Areas (QV)

Shasta Valley and Mount Shasta Volcanic Area (SH)

Eastside Sacramento Valley (ES)

Cascade Range and Modoc Plateau Low-Use Basins (LU)

Honey Lake Valley (HL)

Study areas

Other areas in the Cascade Range

 and Modoc Plateau Province

sac15-0550_fig 12h

Figure 12. —Continued
Status and Understanding of Groundwater Quality 51

Table 8. Aquifer-scale proportions for inorganic constituents detected at high or moderate relative concentrations in the Cascade 

Range and Modoc Plateau study unit, 2010, California Groundwater Ambient Monitoring and Assessment Program (GAMA) Priority 

Basin Project.

[Relative-concentration (RC) is defined as the concentration measured in the sample divided by the concentration of the selected benchmark. RC categories for 

inorganic constituents: high, RC>1.0; moderate, 1.0≥RC>0.5; low, RC≤0.5. Inorganic constituents not listed in this table either do not have benchmarks or were 

detected only at low RCs. Benchmark types and values listed in table 4A. Other abbreviations: SMCL, secondary maximum contaminant level]

Constituent

Number

Raw occurrence 

frequency1

(percent)

Aquifer-scale proportions 

(percent)

90-percent 

confidence 

interval for 

grid-based 

high-RC 

proportion3

Wells Cells ModerateRC High-RC

Spatially weighted1 Grid-based2

ModerateRC High-RC ModerateRC High-RC

Nutrients

Ammonia 90 90 1.2 0 1.2 0 1.2 0 0.0–1.5

Nitrate 253 90 1.8 0.4 1.9 0.9 1.8 0.9 0.1–3.9

Trace elements and minor ions with health-based benchmarks

Arsenic 159 90 6.4 3.8 7.1 3.7 7.4 2.7 0.9–6.7

Boron 115 90 6.8 2.1 7.5 2.5 7.5 2.5 0.8–6.4

Fluoride 151 90 1.2 0 1.3 0 1.8 0 0.0–1.5

Lead 137 90 1.5 2.1 0.9 1.1 0 0 0.0–1.5

Molybdenum 90 90 0 2.2 0 2.1 0 2.1 0.5–5.8

Strontium 90 90 0.9 0 0.9 0.0 0.9 0 0.0–1.5

Vanadium 120 90 9.3 1.4 9.9 1.3 11.3 1.8 0.5–5.4

Radioactive constituents

Gross-alpha particle activity 150 90 1.8 1.8 0.6 2.1 0 1.8 0.5–5.4

Adjusted gross-alpha particle activity 150 90 0.7 0.7 0.3 0.9 0.9 0 0.0–1.5

Radon-222 90 90 0 0.9 0 0.9 0 0.9 0.1–3.9

Uranium 95 90 0 3.2 0 2.0 0 1.8 0.5–5.4

Inorganic constituents with SMCL benchmarks

Chloride 146 90 0 0.9 0 1.2 0 1.2 0.2–4.4

Iron 155 90 3.8 7.5 2.8 5.6 1.2 4.0 1.6–8.5

Manganese 154 90 3.2 10.3 2.5 12.1 1.2 10.4 6.0–16.6

Total dissolved solids (TDS) 159 90 6.9 1.3 9.3 1.9 10.1 1.2 0.2–4.4

1

Based on the most recent analyses for each California Department of Public Health well during September 16, 2007, through September 16, 2010, combined 

with data from U.S. Geological Survey (USGS)-grid wells.

2

Grid-based aquifer-scale proportions are based on samples collected by the USGS from 90 grid wells during July 12 through October 14, 2010. 

3

Based on the Jeffrey’s interval for the binomial distribution (Brown and others, 2001).
52 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Table 9A. Summary of spatially weighted aquifer-scale proportions for inorganic constituent classes with health-based benchmarks, 

Cascade Range and Modoc Plateau (CAMP) study unit, 2010, California Groundwater Ambient Monitoring and Assessment Program

(GAMA) Priority Basin Project.

[Relative-concentration (RC) is defined as the concentration measured in the sample divided by the concentration of the selected benchmark. Study area 

abbreviations: ES, Sacramento Valley Eastside study area; HL, Honey Lake Valley study area; LU, Cascade Range and Modoc Plateau Low-Use Basins 

study area; QV, Quaternary Volcanic Areas study area; SH, Shasta Valley and Mount Shasta Volcanic Area study area; TV, Tertiary Volcanic Areas study area. 

Study unit: aquifer-scale proportions are area weighted by the percent of the study unit covered by each study area: ES=10.4 percent, HL=13.4 percent, 

LU=18.4 percent, QV=25.7 percent, SH=13.3 percent, TV=18.7 percent. Relative-concentration categories: high, RC greater than 1.0 for at least one 

constituent in the class; moderate, RC is less than or equal to 1.0 and greater than 0.5 for at least one constituent in the class and no constituents with RC greater 

than 1; low, RC less than or equal to 0.5 for all constituents in the class]

Constituent classes 

and 

study areas

Number 

of 

wells

Aquifer-scale proportions 

(percent)

Low-RC or not detected Moderate-RC High-RC

Nutrients

ES 30 100 0 0

HL 35 92.2 1.1 6.7

LU 45 95.3 4.7 0

QV 64 100 0 0

SH 42 88.9 11.1 0

TV 40 100 0 0

CAMP study unit 256 96.6 2.5 0.9

Trace elements1

ES 28 91.1 8.9 0

HL 34 55.3 28.3 16.4

LU 27 77.8 18.9 3.3

QV 31 88.7 11.3 0

SH 29 74.4 16.7 8.9

TV 25 85.3 1.3 13.3

CAMP study unit 174 79.9 13.6 6.5

Radioactive constituents

ES 24 100 0 0

HL 23 78.3 0 21.7

LU 27 96.7 3.3 0

QV 29 100 0 0

SH 27 100 0 0

TV 22 100 0 0

CAMP study unit 152 96.5 0.6 2.9

Any inorganic constituent with a health-based benchmark1, 2

ES 29 91.1 8.9 0

HL 36 35.8 26.1 38.1

LU 30 74.4 22.2 3.3

QV 37 88.7 11.3 0

SH 32 68.9 22.2 8.9

TV 27 85.3 1.3 13.3

CAMP study unit 190 76.0 14.7 9.4

1

Data for lead are not included because of possible data quality problems for lead in the California Department of Public Health (CDPH) database for samples 

collected between September 2007 and September 2010 (see appendix B).

2

The 66 CDPH wells that had data for nitrate and no data for trace elements or radioactive constituents were not included. See text for discussion.
Status and Understanding of Groundwater Quality 53

Table 9B. Summary of spatially weighted aquifer-scale proportions for inorganic constituent classes with secondary maximum 

contaminant level (SMCL) benchmarks, Cascade Range and Modoc Plateau (CAMP) study unit, 2010, California Groundwater Ambient 

Monitoring and Assessment Program (GAMA) Priority Basin Project.

[Relative-concentration (RC) is defined as the concentration measured in the sample divided by the concentration of the selected benchmark. Study area 

abbreviations: ES, Sacramento Valley Eastside study area; HL, Honey Lake Valley study area; LU, Cascade Range and Modoc Plateau Low-Use Basins 

study area; QV, Quaternary Volcanic Areas study area; SH, Shasta Valley and Mount Shasta Volcanic Area study area; TV, Tertiary Volcanic Areas study area. 

Study unit: aquifer-scale proportions are area weighted by the percent of the study unit covered by each study area: ES=10.4 percent, HL=13.4 percent, 

LU=18.4 percent, QV=25.7 percent, SH=13.3 percent, TV=18.7 percent. Relative-concentration categories: high, RC greater than 1.0 for at least one 

constituent in the class; moderate, RC less than or equal to 1.0 and greater than 0.5 for at least one constituent in the class and no constituents with RC greater 

than 1; low, RC less than or equal to 0.5 for all constituents in the class]

Constituent classes 

and 

study areas

Number 

of 

wells

Aquifer-scale proportions 

(percent)

Low-RC or not detected Moderate-RC High-RC

SMCL salinity indicators1

ES 24 100.0 0 0

HL 32 65.6 34.4 0

LU 30 90.0 6.7 3.3

QV 29 93.3 6.7 0

SH 26 86.7 13.3 0

TV 23 93.3 0 6.7

CAMP study unit 164 88.8 9.3 1.9

SMCL metals2

ES 27 87.9 1.0 11.1

HL 31 71.1 0 28.9

LU 24 71.1 6.7 22.2

QV 30 86.7 0 13.3

SH 29 89.4 1.7 8.9

TV 24 94.4 0 5.6

CAMP study unit 165 83.7 1.6 14.8

Any inorganic SMCL constituent3

ES 23 87.6 1.3 11.1

HL 31 53.3 17.8 28.9

LU 24 71.1 6.7 22.2

QV 29 86.7 0 13.3

SH 26 85.0 8.3 6.7

TV 21 91.1 0 8.9

CAMP study unit 154 80.0 4.9 15.1

1

Salinity indicators with SMCL benchmarks: total dissolved solids, specific conductance, chloride, and sulfate.

2

Metals with SMCL benchmarks: iron, manganese, and zinc.

3

Aquifer-scale proportions for any inorganic SMCL constituent were calculated using only wells with data for at least one salinity indicator and at least one 

SMCL metal.
54 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Table 10. Results of contingency table tests for differences in aquifer-scale proportions of selected inorganic constituents and 

constituent classes between study areas, Cascade Range and Modoc Plateau study unit, 2010, California Groundwater Ambient 

Monitoring and Assessment Program (GAMA) Priority Basin Project.

[If contingency table test indicated a significant difference in aquifer-scale proportions among the study areas (p<0.05), then pairs with the greatest differences 

were identified from the components of the contingency table test statistic. Study area: ES, Sacramento Valley Eastside; HL, Honey Lake Valley; LU, Cascade 

Range and Modoc Plateau Low-Use Basins; QV, Quaternary Volcanic Areas; SH, Shasta Valley and Mount Shasta Volcanic Area; TV, Tertiary Volcanic Areas. 

Other abbreviations: <, less than; ns, no significant differences; RC, relative concentration; >, greater than]

Constituents Proportion high-RC or moderate-RC 

compared to proportion low-RC

Proportion high-RC compared to 

proportion moderate-RC or low-RC

Individual inorganic constituents

Arsenic <0.001

HL>ES, LU, QV, SH, and TV

0.025

HL>ES and QV

Boron 0.001

HL>ES, LU, QV, and SH

0.022

TV>ES and QV

Molybdenum 0.533

ns

0.533

ns

Vanadium 0.275

ns

0.508

ns

Uranium 0.038

HL>TV

0.038

HL>TV

Total dissolved solids 0.003

HL>ES, LU, QV, and TV

0.296

ns

Manganese <0.001

HL>ES, LU, QV, SH, and TV

0.025

HL>ES and TV

Iron 0.005

LU>HL, QV, and SH

0.187

ns

Classes of inorganic constituents with health-based benchmarks

Nutrients (nitrate) 0.020

SH>QV and TV

HL>QV

0.011

HL>QV

Trace elements 0.007

HL>ES, QV, and TV

0.049

HL>ES and QV

Radioactive constituents <0.001

HL>ES, QV, SH, and TV

<0.001

HL>ES, LU, QV, SH, and TV

Any inorganic constituent <0.001

HL>ES, LU, QV, SH, and TV

<0.001

HL>ES, LU, QV, SH, and TV

 TV>QV
Status and Understanding of Groundwater Quality 55

Table 11A. Results of multi-stage Kruskal-Wallis tests for differences in concentrations of selected water-quality constituents between U.S. Geological Survey (USGS)-grid 

wells classified into groups by study area, aquifer lithology class, age class, oxidation-reduction (redox) class, and depth class, Cascade Range and Modoc Plateau study unit, 2010, California Groundwater Ambient Monitoring and Assessment Program (GAMA) Priority Basin Project.

[Kruskal-Wallis test p-values are listed in table. If Kruskal-Wallis test indicated significant differences (p-value less than a threshold value, α, of 0.05) among the groups, the groups that were significantly different were determined using Tukey’s test on the ranks of the data. Groups with signficant differences are listed in table. Study area: ES, Sacramento Valley Eastside; HL, Honey Lake Valley; LU, Cascade Range and Modoc Plateau Low-Use Basins; QV, Quaternary Volcanic Areas; SH, Shasta Valley and Mount Shasta Volcanic Area; TV, Tertiary Volcanic Areas. Geology class: G, granitic and sedimentary; M, metamorphic or metamorphic and sedimentary or metamorphic and volcanic; S, sedimentary; V, volcanic; VS, sedimentary and volcanic (see appendix A for explanation). Depth class: Deep, top and bottom of perforations >200 ft; Overlapping, top of perforations <200 ft and bottom of perforations >200 ft; Shallow, top and bottom of perforations <200 ft; Spring, groundwater emerges at land surface without pumping. Age class: Mixed, tritium>0.5 TU and 14C<90 pmc; Modern, tritum>0.5 TU and 14C>90 pmc; Pre-modern, tritium<0.5 TU (see appendix A for explanation). Redox class: anoxic, DO<1.0 mg/L; mixed, DO>1.0 mg/L and Mn>50 μg/L and/or Fe>100 μg/L; oxic, DO>1.0 mg/L and Mn<50 μg/L and Fe<100 μg/L. Samples classified as mixed were grouped in to the anoxic class for analysis (appendix A). Other abbreviations: DO, dissolved oxygen; ft, feet below land surface; mg/L, milligrams per liter; ns, no significant differences; pmc, percent modern carbon; SMCL, secondary maximum contaminant level; THM, Trihalomethane; TU, tritium units; μg/L, micrograms per liter; <, less than; >, greater than]

p-value

significant 

relations1

Study area

(ES, HL, SH, LU, QV, TV)

Aquifer 

lithology class

(G, M, S, V, VS)

Age class 

(pre-modern, mixed, modern)

Redox class

(oxic, anoxic)

Depth class

(spring, shallow, overlap, deep)

Inorganic constituents with health-based benchmarks (HBB)

Arsenic 0.002

HL>ES, QV, and TV

0.001

S>M and V

<0.001

Pre-modern and mixed>Modern

0.712 0.497

Boron <0.001

HL>ES, QV, and TV

SH>ES and QV

0.001

S>V and VS

M>V

<0.001

Pre-modern and mixed>Modern

0.077 0.089

Molybdenum <0.001

HL>ES, LU, QV, SH, and TV

LU>ES

<0.001

G and S>V

S>VS

<0.001

Pre-modern and mixed>Modern

0.028

Anoxic>Oxic

0.209

Vanadium 0.035

ES>TV

0.048

S, V, and VS>M

0.06 0.006

Oxic>Anoxic

0.472

Uranium <0.001

HL>ES, LU, QV, and TV

0.001

G, M, and S>V

G>VS

0.113 0.458 0.373

Inorganic constituents with SMCL benchmarks

Total dissolved 

solids (TDS)

<0.001

HL>ES, QV and TV

<0.001

M, S, and VS>V

S>VS

0.007

Pre-modern>Modern

<0.001

Anoxic>Oxic

0.009

Overlap and shallow>Spring

Manganese 0.147 0.087

ns

0.071 0.004

Anoxic>Oxic

0.711

Iron 0.628 0.996

ns

0.738 0.010

Anoxic>Oxic

0.527
56 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010 p-value significant relations1 Study area (ES, HL, SH, LU, QV, TV) Aquifer lithology class (G, M, S, V, VS) Age class (pre-modern, mixed, modern) Redox class (oxic, anoxic) Depth class (spring, shallow, overlap, deep) Organic and special interest constituents Herbicides 0.028 SH>ES, HL, LU, QV, and TV 0.081 ns 0.014 Modern and mixed>Pre-modern 0.007 Oxic>Anoxic 0.094 THMs (chloroform) 0.012 ES>LU and QV 0.282 ns 0.436 0.157 0.086 Perchlorate 0.024 ES>QV and TV HL>QV 0.155 ns 0.296 0.027 Oxic>Anoxic 0.222

1Explanation for reporting of significant differences using the results for arsenic by study unit as an example. The Tukey’s test results showed that the following three pairs had significant differences: HL>ES, 

Table 11A.

HL>QV, and HL>TV. Because HL has greater arsenic concentrations than ES, QV, and TV, the notation is condensed to ‘HL>ES, QV, and TV.’ Arsenic concentrations in ES, QV, and TV were not significantly different from one another. Arsenic concentrations were not significantly different for all of the pairs not listed.

Results of multi-stage Kruskal-Wallis tests for differences in concentrations of selected water-quality constituents between U.S. Geological Survey (USGS)-grid 

wells classified into groups by study area, aquifer lithology class, age class, oxidation-reduction (redox) class, and depth class, Cascade Range and Modoc Plateau study unit, 2010, California Groundwater Ambient Monitoring and Assessment Program (GAMA) Priority Basin Project.—Continued[Kruskal-Wallis test p-values are listed in table. If Kruskal-Wallis test indicated significant differences (p-value less than a threshold value, α, of 0.05) among the groups, the groups that were significantly different were determined using Tukey’s test on the ranks of the data. Groups with signficant differences are listed in table. Study area: ES, Sacramento Valley Eastside; HL, Honey Lake Valley; LU, Cascade Range and Modoc Plateau Low-Use Basins; QV, Quaternary Volcanic Areas; SH, Shasta Valley and Mount Shasta Volcanic Area; TV, Tertiary Volcanic Areas. Geology class: G, granitic and sedimentary; M, metamorphic or metamorphic and sedimentary or metamorphic and volcanic; S, sedimentary; V, volcanic; VS, sedimentary and volcanic (see appendix A for explanation). Depth class: Deep, top and bottom of perforations >200 ft; Overlapping, top of perforations <200 ft and bottom of perforations >200 ft; Shallow, top and bottom of perforations <200 ft; Spring, groundwater emerges at land surface without pumping. Age class: Mixed, tritium>0.5 TU and 14C<90 pmc; Modern, tritum>0.5 TU and 14C>90 pmc; Pre-modern, tritium<0.5 TU (see appendix A for explanation). Redox class: anoxic, DO<1.0 mg/L; mixed, DO>1.0 mg/L and Mn>50 μg/L and/or Fe>100 μg/L; oxic, DO>1.0 mg/L and Mn<50 μg/L and Fe<100 μg/L. Samples classified as mixed were grouped in to the anoxic class for analysis (appendix A). Other abbreviations: DO, dissolved oxygen; ft, feet below land surface; mg/L, milligrams per liter; ns, no significant differences; pmc, percent modern carbon; SMCL, secondary maximum contaminant level; THM, Trihalomethane; TU, tritium units; μg/L, micrograms per liter; <, less than; >, greater than]
Status and Understanding of Groundwater Quality 57

Table 11B. Results of Spearman’s rho tests for correlations between values of selected potential explanatory factors and concentrations of selected water-quality constituents for the U.S. Geological Survey (USGS)-grid wells, Cascade Range and Modoc Plateau study unit, 2010, California Groundwater Ambient Monitoring and Assessment Program(GAMA) Priority Basin Project.[ρ (rho), Spearman’s correlation statistic. p values (significance level of the Spearman’s test) less than threshold value (α) of 0.05 are considered significant: bold black text, significant positive correlation; bold red text, significant negative correlation. Abbreviations: DO, dissolved oxygen; GAMA, Groundwater Ambient Monitoring and Assessment Program; SMCL, secondary maximum contaminant level; THM, Trihalomethane; USTs, leaking or formerly leaking underground-storage tanks; <, less than]

p-value

(ρ)

Percentage of 

agricultural 

land use

Percentage of 

natural land 

use

Percentage of 

urban land use

Density of 

septic tanks

Density of 

USTs

Aridity 

index Elevation

Depth to top 

of screened or 

open interval

Depth to bottom 

of screened or 

open interval1

pH DO concentration

Nutrients and trace elements with health-based benchmarks

Arsenic <0.001

0.35

0.338

–0.10

0.740

–0.04

0.139

–0.16

0.077

–0.19

<0.001–0.55

0.972

–0.00

0.9720.00

0.5770.07

<0.001

0.49

<0.001

–0.41

Boron <0.001

0.58

<0.001

–0.36

0.4380.08

0.086

–0.18

0.324

–0.10

<0.001–0.66

0.485

–0.07

0.598

–0.07

0.4610.09

<0.001

0.55

<0.001

–0.62

Molybdenum <0.001 0.49 0.008 –0.28 0.343 0.10 0.008 –0.28 0.019 –0.25 <0.001 –0.78 0.090 0.18 0.958 0.01 0.498 0.08 <0.001 0.66 <0.001 –0.60

Vanadium 0.549

0.06

0.2760.12

0.112

–0.17

0.3130.11

0.3880.09

0.980

–0.00

0.032

–0.23

0.3460.12

0.2550.13

0.5270.08

0.8700.02

Uranium <0.001

0.40

0.020

–0.24

0.5100.07

0.450

0.08

0.169

–0.15

<0.001–0.53

0.1580.15

0.296

–0.13

0.099

–0.19

<0.001

0.38

0.049

–0.21

Inorganic constituents with SMCL benchmarks

Total dissolved 

solids (TDS)

<0.001

0.59

<0.001

–0.37

0.3260.10

0.428

–0.08

0.448

–0.08

<0.001–0.68

0.038

–0.22

0.269

–0.14

0.9390.01

<0.001

0.42

<0.001

–0.73

Manganese 0.012 0.26 0.010 –0.27 0.046 0.21 0.028 –0.23 0.385 –0.09 0.002 –0.32 0.187 0.14 0.193 0.16 0.274 0.13 0.010 0.27 <0.001 –0.47

Iron 0.805

–0.03

0.796

–0.03

0.6330.05

0.099

–0.18

0.738

–0.04

0.516

–0.07

0.1260.16

0.9390.01

0.939

–0.01

0.831

–0.02

0.066

–0.20

Organic and special interest constituents

Herbicides 0.851

–0.02

0.688

–0.04

0.7800.03

0.0580.20

0.3670.10

0.2050.14

0.294

–0.11

0.090

–0.21

0.066

–0.22

0.047

–0.21

0.0510.21

THMs (chloroform)

0.358

–0.10

0.123

–0.16

0.0060.29

0.0370.22

0.0990.18

0.1900.14

0.005

–0.29

0.132

–0.19

0.2280.14

0.009

–0.27

0.614

–0.05

Perchlorate 0.014 0.27 0.214 –0.14 0.199 0.14 0.861 0.02 0.616 –0.06 0.021 –0.25 0.496 –0.08 0.201 –0.17 0.996 –0.01 <0.001 –0.54 0.276 –0.12

1Eleven wells did not have data for depth to the bottom of the screened or open interval, but did have data for well depth (table A2). Well depth data were used to represent data for the depth of the bottom 

of the screened or open interval for these wells.
58 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Table 12. Results of Spearman’s tests for correlations between concentrations of selected inorganic constituents in the 

U.S. Geological Survey (USGS)-grid wells, Cascade Range and Modoc Plateau study unit, 2010, California Groundwater Ambient 

Monitoring and Assessment Program (GAMA) Priority Basin Project.

[ρ (rho), Spearman’s correlation statistic. p values (significance level of the Spearman’s test) less than threshold value (α) of 0.05 are considered significant: bold 

black text, significant positive correlation; bold red text, significant negative correlation; non-bold, not significant. Abbreviations: TDS, total dissolved solids; 

<, less than]

p-value

(ρ) Boron Molybdenum Vanadium Uranium Manganese Iron TDS

Arsenic <0.001

0.72

<0.001

0.77

<0.001

0.53

<0.001

0.55

0.039

0.22

0.456

–0.08

<0.001

0.50

Boron <0.001

0.82

0.217

0.13

<0.001

0.40

<0.001

0.44

0.804

0.06

<0.001

0.73

Molybdenum 0.265

0.12

<0.001

0.42

<0.001

0.44

0.572

0.06

<0.001

0.62

Vanadium 0.001

0.34

0.002

–0.32

<0.001

–0.40

0.344

0.10

Uranium 0.433

–0.08

0.151

–0.15

<0.001

0.39

Manganese <0.001

0.45

0.003

0.31

Iron 0.606

0.06

Inorganic constituents with health-based benchmarks, 

as a group, were present at high RCs in 9.4 percent of the 

primary aquifer system and at moderate RCs in 14.7 percent 

(table 9A). The proportion of the primary aquifer system 

having high RCs of inorganic constituents with healthbased benchmarks was significantly greater for the HL study 

areas compared to all of the other study areas (table 10). 

Inorganic constituents with SMCL benchmarks (metals and 

salinity indicators), as a group, were present at high RCs in 

15.1 percent of the primary aquifer system and at moderate 

RCs in 4.9 percent (table 9B). 

Trace Elements

The trace elements constituent class includes a variety 

of metallic and non-metallic constituents that typically are 

present in groundwater at concentrations less than 1 mg/L 

(Hem, 1985). Trace elements with health-based benchmarks, 

as a class, had a high-RC aquifer-scale proportion of 

6.5 percent and a moderate-RC aquifer-scale proportion 

of 13.6 percent in the CAMP study unit (table 9A). The 

proportion of the primary aquifer system having high or 

moderate RCs of at least one trace element was significantly 

greater in the HL study area than in the ES, QV, and TV study 

areas (table 10). 

Arsenic was detected at high RCs in approximately 

3 percent of the CAMP study unit primary aquifer; boron, 

molybdenum, and vanadium each were detected at high RCs 

in approximately 2 percent of the system (table 8). These 

four trace elements are discussed in more detail in following 

sections. Three other trace elements (fluoride, lead, and 

strontium) were detected at high RCs in less than 2 percent of 

the primary aquifer system or were only detected at moderate 

RCs (table 8).

Arsenic

Arsenic is a semi-metallic trace element. Natural sources 

of arsenic in groundwater include dissolution of arsenicbearing minerals, desorption of arsenic from mineral surfaces, 

and mixing with hydrothermal fluids. Pyrite, an iron sulfide 

mineral that can contain up to several percent (by weight) 

of arsenic, and arsenopyrite (FeAsS) are common accessory 

minerals in aquifer materials (Welch and others, 2000; 

Smedley and Kinniburgh, 2002). Anthropogenic sources of 

arsenic can include copper ore smelting, coal combustion, 

arsenical pesticides, arsenical veterinary pharmaceuticals, 

and wood preservatives (Welch and Stollenwerk, 2003). 

In addition, mining for copper, gold, and other metals can 

increase the rate of dissolution of natural arsenic-bearing 

minerals (Smedley and Kinniburgh, 2002). The CAMP study 

unit has had relatively little mining activity compared to other 

areas of the State (U.S. Geological Survey, 2005) and little 

registered use of arsenical pesticides (California Department 

of Pesticide Regulation, 2013), thus, anthropogenic sources of 

arsenic are unlikely to be significant. 

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 
Status and Understanding of Groundwater Quality 59

been linked to increased cancer risk and to non-cancerous 

effects including skin damage and circulatory problems 

(U.S. Environmental Protection Agency, 2010). An estimated 

8 percent of groundwater resources used for drinking water 

in the United States have high RCs of arsenic (greater than 

10 μg/L) (Focazio and others, 1999; Welch and others, 2000), 

and high concentrations of arsenic in groundwater resources 

used for drinking water are a worldwide concern (Smedley 

and Kinniburgh, 2002; Welch and others, 2006).

Arsenic was present at high RCs in 2.7 percent of 

the primary aquifer system in the CAMP study unit and at 

moderate RCs in 7.4 percent (table 8). High or moderate 

RCs of arsenic were only present in the HL, SH, LU and QV 

study areas (figs. 11A, 12A, tables C1A–F). The proportion 

of the primary aquifer system having high or moderate RCs 

of arsenic and the median concentration of arsenic were 

significantly greater in the HL study area than in the ES, QV, 

and TV study areas (tables 10, 11A). Of the 10 USGS-grid 

wells with high or moderate RCs of arsenic, 6 were in the HL 

study area (figs. 11A, 12A).

Arsenic concentrations showed significant correlations 

with groundwater age, aquifer lithology, geochemical 

conditions, and other water-quality constituents (tables 11A,

12). Arsenic concentrations were greater in the HL study area 

than in the ES, QV, and TV study areas, and were greater 

in wells in sedimentary deposits than in wells in volcanic 

rocks (table 11A). The positive correlation between arsenic 

and percentage of agricultural land use and the negative 

correlation between arsenic and the aridity index (table 11B) 

likely reflect that the HL study area has a greater percentage 

of agricultural land use and lower aridity index than many 

of the other study areas (table 7A). Mixed and pre-modern 

age groundwater had higher arsenic concentrations than did 

modern groundwater (table 11A). Arsenic concentrations 

showed significant positive correlation with pH and negative 

correlation with DO (table 11B). Unlike many of the GAMA 

Priority Basin Project study units, in the CAMP study unit, 

arsenic showed no significant relations with measures of well 

depth (tables 11A, 11B). This was expected, given the lack 

of significant correlations between values of DO and pH and 

measures of well depth (tables 7A–C). Arsenic concentrations 

showed significant positive correlations with TDS, manganese, 

boron, molybdenum, vanadium, and uranium (table 12), which 

likely reflects a combination of the effects of geochemical 

conditions on concentrations of these constituents and the 

geology of the HL study area. For all of these constituents, 

except for molybdenum and vanadium, the HL study area had 

the greatest proportion of the primary aquifer system with high 

or moderate concentrations (tables 10, C1A–F).

All or nearly all of the wells in the HL study area having 

high or moderate RCs of arsenic, boron, molybdenum, 

vanadium, and TDS were in the area northwest of Honey Lake 

(fig. 13). A potential source of arsenic and other constituents to 

groundwater in this area is hydrothermal fluids. Groundwater 

is used for production of geothermal energy in several areas 

along the north side of the basin (California Department 

of Conservation, Division of Oil, Gas, and Geothermal 

Resources, 2009; fig. 13). The Wendel and Amedee areas near 

the northeast edge of Honey Lake have geothermal power 

plants; whereas, in the Litchfield and Susanville areas in 

the northwest part of the basin, thermal waters are pumped 

directly into distribution systems used for conductive heating. 

Arsenic concentrations of approximately 200 μg/L and boron 

concentrations of greater than 5,000 μg/L have been measured 

in water from hot springs in the Wendel and Amedee areas 

(Wormald, 1968). Geochemical modeling of the major-ion 

chemistry for groundwater samples from the Honey Lake 

basin indicated that groundwater on the northwest side of 

Honey Lake could consist of up to 40 percent geothermal 

water mixed with water recharged from precipitation (Mayo 

and others, 2010).

Another source of high arsenic concentrations in 

groundwater in the HL study area could be interaction 

with sediments in the Honey Lake playa. The geochemical 

modelling by Mayo and others (2010) indicated that 

groundwater on the northwest side of Honey Lake generally 

evolves from a mixed-cation, bicarbonate-dominated 

water with low TDS to a sodium-dominated, bicarbonate 

or bicarbonate-sulfate water with higher TDS. USGS-grid 

well samples from wells on the west side of the study area 

generally were mixed-cation-bicarbonate waters or calciumsodium-bicarbonate waters, and had low RCs of arsenic and 

relatively low TDS concentrations (hexagons; figs. 13, 14). 

Samples from wells further downgradient along the Susan 

River towards the northwest side of Honey Lake generally 

were sodium-bicarbonate or sodium-bicarbonate/-sulfate 

waters with higher TDS and high or moderate RCs of arsenic 

(triangles; figs. 13, 14). 

This evolution in major-ion composition was associated 

with changes in geochemical conditions that favor increased 

solubility of arsenic. The evolution in major-ion composition 

can be represented by the increasing proportion of sodium 

in the cations (fig. 14), and the proportion of sodium in the 

cations was positively correlated with pH (spearman’s test, 

p<0.001, ρ=0.95) and was negatively correlated with DO 

concentrations (spearman’s test, p=0.013, ρ=0.63). All of the 

HL study area samples with high or moderate RCs of arsenic 

had pH values between 7.7 and 8.8, and six of the seven 

samples also had DO less than or equal to 1.1 mg/L (Shelton 

and others, 2013). High pH conditions promote desorption 

of arsenic from aquifer sediments, and anoxic conditions 

promote release of arsenic from reductive dissolution of 

iron and manganese oxyhydroxide minerals (Smedley and 

Kinniburgh, 2002; Stollenwerk, 2003; Welch and others, 

2006). On the basis of comparison with studies in the nearby 

Carson Desert basin in Nevada, sediments in the Honey Lake 

basin, particularly in the playa area near the lake, appear likely 

to contain arsenic available for desorption. Like Honey Lake 

basin, the Carson Desert basin is filled with sediments from 

volcanic rocks associated with Basin and Range extension and 

from granitic rocks of the Sierra Nevada as well as lacustrine 

deposits. 
60 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Constituents are present at low relative-concentration unless

noted as present at moderate or high relative-concentration.

Susan River AMEDEE FAULT ZONE AMEDEE FAULT ZONE

LITCHFIELD FAULT ZONELITCHFIELD FAULT ZONE

HONEY LAKE FAULT ZONEHONEY LAKE FAULT ZONE

Honey Lake

WARM SPRINGS FAULT ZONEWARM SPRINGS FAULT ZONE

Long Valley Creek

As B

As B

As Mo

U B

As B

B

As V B

As V

U

HL-11

HL-12

0 4 8 MILES

0 4 8 KILOMETERS

Base modified from U.S. Geological Survey

and other Federal digital data, various scales 

Albers Equal Area Projection

North American Datum of 1983

Honey Lake Valley (HL)

Areas with wells used

for geothermal energy

County boundary

Fault zone

Along the Honey Lake fault zone

North of Honey Lake

West end of basin

East of Honey Lake

100–250 251–600 601–800

Total dissolved solids,

in milligrams per liter

Location Relative-concentration Constituent

Arsenic

Boron

Molybdenum

Vanadium

Uranium

As

B

As

B

Mo

V

U

Moderate High

EXPLANATION

2

2 6

6

120°37'30" 120°30' 120°22'30" 120°15' 120°7'30"

40°22'30"

40°15'

40°7'30"

40°

CALIFORNIA

Map

area

sac15-0550_fig 13

Figure 13. Locations of U.S. Geological Survey (USGS)-grid wells, total dissolved solids concentrations, selected trace element 

concentrations, and selected geologic and hydrologic features of the Honey Lake Valley study area, Cascade Range and Modoc Plateau

(CAMP) study unit, 2010, California Groundwater Ambient Monitoring and Assessment (GAMA) Program Priority Basin Project.
Status and Understanding of Groundwater Quality 61

U

As, Mo,

U, B

As, V, B

As, B

As, B

As, B

As, V

B

Calcium (Ca)

Sulfate (SO4

)

Chloride (Cl)

Bicarbonate (HCO3

)

0

100

80

60

40

20

0

20

40

60

80

100

20

40

60

80

100

Magnesium (Mg)

Sodium (Na) + Potassium (K)

100

0

20

40

60

80

100

80

60

40

20

0

80

60

40

20

0

Calcium (Ca) + Magnesium (Mg)

Sulfate (SO4

) + Chloride (Cl)

0

20

40

60

80

100

20

40

60

80

100

0

100

0

20

40

60

80

100

20

40

60

80

100

0

PERCENT

PERCENT

PERCENT

Ca

Ca + NaK

Ca + Mg

Mg

Mg + NaK

mixed mixed

NaK

HCO3

HCO3

 + SO4

SO4

 + Cl

HCO3 + Cl

Cl

SO4

HL-11

HL-12

Relative-concentration Total dissolved solids, in milligrams per liter

Moderate High

Arsenic

Boron

Molybdenum

Vanadium

Uranium

As

B

As

B

Mo

V

U

Constituent

Along the Honey Lake fault

North of Honey Lake

West end of basin

East of Honey Lake

100–250 251–600 601–800 Location (see fig. 13)

Constituents are present at low relativeconcentration unless noted as present at 

moderate or high relative-concentration.

EXPLANATION

sac15-0550_fig 14

Figure 14. Samples from the Honey Lake Valley study area, Cascade Range and Modoc Plateau (CAMP) study unit, 2010, California 

Groundwater Ambient Monitoring and Assessment (GAMA) Program Priority Basin Project.
62 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Welch and others (1997) and Welch and Lico (1998) 

inferred that high arsenic (and uranium and molybdenum) 

concentrations in Carson Desert groundwater were caused 

by interaction between high-pH, low-DO groundwater and 

lacustrine and riverine sediments. Arsenic associated with 

iron and manganese oxyhydroxide minerals and sedimentary 

organic matter in these sediments can become soluble under 

those groundwater geochemical conditions.

Outside of the HL study area, the only other areas with 

high RCs of arsenic were in the SH study area (USGS-grid 

well SH-02) and near Goose Lake in the LU study area 

(CDPH well). Several wells in the LU, QV, and SH study 

areas had moderate RCs of arsenic (figs. 11A, 12A).

The five CAMP study unit study areas in which the 

primary aquifer system consists primarily of volcanic rocks, 

the ES, LU, QV, SH, and TV study areas, had high-RC 

aquifer-scale proportions of arsenic ranging from 0 percent to 

6.7 percent (tables C1A, C–F). This proportion of high RCs 

of arsenic is similar to that observed in the volcanic aquifer 

systems of the Columbia River Plateau, the Snake River Plain, 

and the island of Oahu (Frans and others, 2012), but lower 

than the proportion observed in volcanic aquifer systems in the 

Martis Valley groundwater basin near Lake Tahoe, California, 

(Fram and Belitz, 2012) and the basalt aquifers of the Carson 

Valley in western Nevada (Lico and Seiler, 1994). Differences 

in the pH of groundwater among these aquifer systems in part 

may account for the differences in the proportion of high RCs 

of arsenic in groundwater. Approximately half of the samples 

from the Martis Valley groundwater basin had pH values 

greater than or equal to 8, and pH values in samples from the 

Carson Valley basalt aquifers were greater than 9. In contrast, 

less than 15 percent of samples from volcanic aquifers in 

the CAMP study unit (fig. 9B; table A6), the Columbia 

River Plateau, the Snake River Plain, and the island of Oahu 

(Toccalino and others, 2010) had pH values greater than or 

equal to 8. High pH conditions promote desorption of arsenic 

from aquifer materials (Smedley and Kinniburgh, 2002; 

Stollenwerk, 2003; Welch and others, 2006).

Boron

Boron is a naturally occurring semi-metallic element 

with high solubility in water. Natural sources of boron to 

groundwater include dissolution of evaporate minerals, 

such as borax, ulexite, and colemanite, and boron-bearing 

silicate minerals, such as tourmaline, that are primarily found 

in igneous rocks (Hem, 1985; Klein and Hurlbut, 1993). 

Seawater contains approximately 4,500 μg/L of boron, thus, 

interactions with marine sediments, connate fluids, or seawater 

also can be natural sources of boron to groundwater. Boron 

is associated with thermal springs and volcanic activity 

(Hem, 1985). Boron can occur 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 comparison benchmark 

used for boron in this study was the CDPH notification level 

(NL-CA) of 1,000 μg/L (California Department of Public 

Health, 2010). At concentrations greater than the HAL-US 

of 6,000 μg/L, boron can adversely affect fetal development 

(U.S. Environmental Protection Agency, 2008). 

Boron was present at high RCs in 2.5 percent and at 

moderate RCs in 7.5 percent of the primary aquifer system 

(table 8). Boron was only present at high RCs in the TV study 

area and was present at moderate RCs in the HL, LU, and SH 

study areas (figs. 11A, 12B; tables C1A–F). The proportion of 

the primary aquifer system having high or moderate RCs of 

boron was significantly greater in the TV study area than in 

the ES and QV study areas, and the proportion having high 

or moderate RCs was significantly greater in the HL study 

area than in the ES, LU, QV, and SH study areas (table 10). 

Although the only samples with high RCs of boron were 

from the TV study area, all of the other samples from the TV 

study area had low RCs of boron; thus, the proportion of the 

primary aquifer system having high or moderate RCs of boron 

in the TV study area (15.5 percent; table C1F) was less than 

the proportion in the HL study area (36.7 percent; table C1B), 

where over a third of the samples had moderate RCs of boron.

Boron showed nearly the same pattern of significant 

correlations with potential explanatory factors as arsenic 

did; however, some of the processes controlling boron 

concentrations appeared to be different than those controlling 

arsenic concentrations. Like arsenic concentrations, boron 

concentrations were significantly greater in samples from the 

HL study area than in samples from the ES, QV, and TV study 

areas; greater in pre-modern and mixed-age groundwater 

compared to modern groundwater; and greater in sites with 

sedimentary aquifer lithology compared to sites with volcanic 

aquifer lithology (table 11A). Boron and arsenic concentrations 

were positively correlated, and as with arsenic concentrations, 

boron concentrations showed positive correlations with the 

percentage of agricultural land use, pH, TDS, manganese, 

molybdenum, and uranium and showed negative correlations 

with the aridity index and DO (tables 11B, 12). As discussed 

in the section on arsenic, most of these correlations 

between boron and potential explanatory factors reflect that 

groundwater in the HL study area commonly has elevated 

boron concentrations. The source of the boron could be mixing 

with geothermal fluids (see ‘Arsenic’ section).

The only samples with high RCs of boron in the CAMP 

study unit were in the TV study area, north of the SH study 

area (TV-01 and TV-15; fig. 12B; Shelton and others, 2013). 

These two samples did not have high or moderate RCs of 

arsenic, indicating different mechanisms were responsible 

for the high boron concentrations in these samples than the 

mechanisms responsible for elevated boron concentrations in 

the HL study area. In addition to having a high RC of boron, 

the sample from the USGS-grid well TV-01 had high RCs for 

TDS and molybdenum, and it was the only USGS-grid well 

sample having a major-ion composition dominated by sodium 

and chloride (fig. B1). Saline water with high boron, sodium, 
Status and Understanding of Groundwater Quality 63

and chloride concentrations emanates from several springs in 

the Tertiary volcanic rocks north of the Shasta Valley (Mack, 

1960), and mixing with these spring waters could account for 

quality of groundwater from TV-01. Mack (1960) suggested 

that these saline waters could be derived from connate 

waters from Mesozoic and Paleozoic marine sedimentary 

rocks underlying the volcanic rocks. USGS-grid well TV-15 

had screened intervals in both the volcanic rocks and the 

underlying metamorphic rocks (table A1).

Molybdenum

Molybdenum is a metallic trace element used in highstrength steel alloys and other industrial products. High 

concentrations of molybdenum are found in organic-rich 

sediments and sedimentary rocks deposited in sulfide-reducing 

environments (Crusius and others, 1996). Most molybdenum 

ore deposits are associated with porphyry granite or quartz 

monzonite plutons, and the primary ore mineral is molybdenite 

(MoS2

) (for example, Misra, 2000). Potential anthropogenic 

sources include the manufacture and use of molybdenum 

steel alloys, dry lubricants, and other industrial products, and 

surface application of biosolids (Evans and Barabash, 2010). 

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, 2009b). High 

levels of molybdenum in animals (including humans) can 

interfere with uptake of copper. Molybdenum is an essential 

trace nutrient for biological nitrogen fixation and can even be 

a limiting factor for biological activity in molybdenum-poor 

environments (Goldman, 1961; Evans and Barabash, 2010).

Molybdenum was present at high RCs in 2.1 percent 

of the primary aquifer system of the CAMP study unit, and 

was not present at moderate RCs (table 8). High RCs of 

molybdenum were only present in the HL and TV study areas 

(figs. 11A, 12C; tables C1A–F). There were no significant 

differences in aquifer-scale proportions for molybdenum 

among the study areas (table 10). Molybdenum concentrations 

showed a similar pattern of correlations with explanatory 

factors as did boron concentrations (tables 11A, B), and, as 

with boron, these correlations largely reflected the greater 

molybdenum concentrations in samples from the HL study 

area (table 11A).

Vanadium

Vanadium is a metallic trace element used in highstrength steel alloys. Vanadium concentrations in igneous 

rocks generally are greater in mafic rocks (basalts and 

gabbros) than in ultramafic or felsic (granites and dacite/

rhyolite) rocks (Fischer and Ohl, 1970; Prytulak and 

others, 2013) because titanium-rich magnetite, which is 

most abundant in mafic rocks, has a higher mineral-liquid 

partition coefficient for vanadium than does other oxide or 

silicate minerals (for example, Righter and others, 2006). 

Because of this, sediments derived from mafic igneous rocks 

(or their metamorphosed equivalents) should have greater 

amounts of vanadium than sediments derived from other 

rock types. Organic-rich sedimentary rocks formed in anoxic 

environments and phosphorites can also have high vanadium 

concentrations (Evans and Barabash, 2010). Anthropogenic 

sources include combustion of fossil fuels and emissions 

from smelting, foundry, and other iron metallurgy industrial 

processes (Evans and Barabash, 2010). The USEPA included 

vanadium as part of the first, second, and third Contaminant 

Candidate Lists (U.S. Environmental Protection Agency, 

1998, 2005a, 2009b), and vanadium has been selected as a 

potential candidate for regulatory determination from the 

third list (U.S. Environmental Protection Agency, 2011). The 

CDPH defined a Notification Level (NL-CA) for vanadium 

of 50 μg/L based on potential adverse effects on fetal and 

neonatal development (California Office of Environmental 

Health Hazard Assessment, 2000). 

Vanadium was present at high RCs in 1.8 percent of 

the primary aquifer system of the CAMP study unit, and 

at moderate RCs in 11.3 percent (table 8). These high-RC 

and moderate-RC proportions were approximately double 

the percentages reported by Wright and Belitz (2010) for 

the Cascade Mountains and Modoc Plateau hydrogeologic 

province in their assessment of the distribution of vanadium 

in groundwater in California. Wright and Belitz (2010) 

used statewide data from the CDPH database and the 

USGS National Water Information System (NWIS) database 

(http://waterdata.usgs.gov/nwis), including data collected 

by the GAMA Priority Basin Project through 2008, and 

calculated raw occurrence frequencies of wells with high or 

moderate RCs of vanadium in the State’s 10 hydrogeologic 

provinces. Their results for the Cascade Range and Modoc 

Plateau hydrogeologic province were based largely on wells 

in a few basins in the LU study area. The water-quality data 

collected for this study in 2010 expanded the availability of 

data for vanadium concentrations to all six study areas, and the 

spatial weighting used to calculate high-RC and moderate-RC 

aquifer-scale proportions eliminated the influence of clustered 

wells on the resulting proportions. High RCs of vanadium 

occurred in the HL and SH study areas, and moderate RCs 

occurred in all study areas, except for the TV study area 

(figs. 11A, 12D; tables C1A–F). There were no significant 

differences in aquifer-scale proportions for vanadium among 

the study areas (table 10). 

The relations between vanadium and potential 

explanatory factors were quite different than the relations 

between arsenic, boron, molybdenum, and uranium and 

potential explanatory factors. Vanadium was the only trace 

element or radioactive constituent for which samples from the 

HL study area did not have significantly greater concentrations 

than samples from other study areas (table 11A). Samples 

from the ES study area had significantly greater concentrations 

of vanadium than did samples from the TV study area 

(table 11A). Vanadium was the only trace element or 

radioactive constituent that had significantly greater 

concentrations in oxic conditions (table 11A); the others all 

had a negative correlation with DO (table 11B). 
64 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

As expected from the association between greater 

concentrations of vanadium and oxic conditions, vanadium 

concentrations showed significant negative correlations 

with iron and manganese concentrations (table 12). The 

negative correlation between site elevation and vanadium 

concentrations (table 11B) reflects the significantly lower 

elevations in the ES study area compared to the HL, LU, QV, 

and TV study areas (table 7A).

The relations between vanadium and geochemical 

conditions were weaker than expected. Thermodynamic data 

indicate that vanadium should be most soluble in alkaline, 

oxic conditions (Wanty and Goldhaber, 1992), and statistical 

analysis of data for 1,283 groundwater samples distributed 

across California showed that vanadium concentrations were 

indeed higher in oxic groundwater with pH greater than or 

equal to 8 than in anoxic groundwater with any pH or oxic 

groundwater with pH less than 8 (Wright and Belitz, 2010). In 

the CAMP study unit, oxic groundwater with pH greater than 

or equal to 8 had significantly greater vanadium concentrations 

than anoxic groundwater (Wilcoxon rank-sum test, 

p=0.008), but did not have significantly different vanadium 

concentrations than oxic groundwater with pH less than 8 

(Wilcoxon rank-sum test, p=0.071). Wright and Belitz (2010) 

also showed that groundwater with high or moderate RCs of 

vanadium was most frequently detected in areas associated 

with sediments derived from mafic and intermediate rocks. 

Nearly all of the groundwater samples from the CAMP study 

unit were from aquifers largely composed of mafic volcanic 

rocks or sediments derived from mafic volcanic rocks. The 

lack of strong correlation between vanadium and geochemical 

conditions in the CAMP study unit—where the source rocks 

are expected to have vanadium—indicates the controls on 

vanadium distribution are complex.

Nutrients

Nutrients, including nitrate, nitrite, and ammonia, both 

have natural and anthropogenic sources to groundwater 

(Dubrovsky and others, 2010). Natural sources include 

atmospheric deposition, animal waste, and dissolution of 

organic material in soils. Anthropogenic sources include 

fertilizer application, livestock and human waste, sewage 

and septic effluents, and combustion of fossil fuels (emits 

nitrogen oxides to the atmosphere). Nitrate and nitrite 

have MCL-US benchmarks (10 and 1 mg/L, as nitrogen, 

respectively), and high levels of either in drinking water can 

cause “blue baby” syndrome (U.S. Environmental Protection 

Agency, 2009a). Ammonia has a HAL-US benchmark 

of 24.7 mg/L, as nitrogen (table 4A). Nitrate is the most 

oxidized form of nitrogen and, thus, is the most common 

form in oxic groundwater systems, such as those in most 

of the CAMP study unit. Although evapotranspiration from 

shallow groundwater can increase concentrations of nutrients 

in groundwater, concentrations of nitrate greater than about 

1 mg/L (corresponds to an RC of 0.1) generally are the result 

of anthropogenic inputs (Nolan and others, 2002; Dubrovsky 

and others, 2010). 

Nutrients, as a class, were present at high RCs in 

0.9 percent of the primary aquifer system in the CAMP study 

unit and at moderate RCs in 2.5 percent (table 9A). Nitrate 

was found at high RCs in the HL study area and at moderate 

RCs in the SH study area (fig. 11B; tables C1A–F). Ammonia 

was found at moderate RCs in the LU study area (fig. 11B; 

tables C1A–F).

Uranium and Radioactive Constituents

Most of the radioactivity in groundwater comes from 

decay of naturally occurring uranium and thorium in the 

rocks or sediments that compose the aquifers. Radioactive 

decay of uranium and thorium isotopes produces long series 

of radioactive daughter products, including isotopes of 

radium, uranium, and radon. These elements have different 

chemical properties, and their solubility in groundwater varies 

with geochemical conditions, water chemistry, and aquifer 

mineralogy (for example, Hem, 1985). This study included 

data for the individual constituents uranium and radon-222 

and for gross alpha and gross beta particle activities, which 

are measures of the activities of all radioactive elements in the 

water sample that decay by alpha or beta particle emission, 

respectively. Uranium was compared to the MCL-US of 

30 μg/L rather than to the MCL-CA of 20 picocuries per liter 

(pCi/L), and gross alpha particle activities were not adjusted 

for uranium activity (see appendix D).

Radioactive constituents were present at high RCs 

2.9 percent of the primary aquifer system, and at moderate 

RCs in 0.6 percent (table 9A). The proportion of the primary 

aquifer system having high or moderate RCs of at least one 

radioactive constituent was significantly greater in the HL 

study area than in the other study areas (table 10). Uranium 

and gross alpha particle activity were present at high RCs in 

the same 1.8 percent of the primary aquifer system, and radon222 was present at high RCs in 0.9 percent of the system 

(table 8). All of the detections of high RCs of radioactive 

constituents occurred in the HL study area (figs. 11B, 12E).

Sources of uranium to groundwater include dissolution 

of uranium-bearing minerals, such as uraninite (UO2

), 

zircon, and titanite, and desorption of uranium from mineral 

surfaces in the presence of bicarbonate (Hem, 1985; Jurgens 

and others, 2010). Chronic exposure to uranium in drinking 

water at concentrations greater than the MCL-US (30 μg/L) 

or activities greater than the MCL-CA (20 pCi/L) can result 

in toxic effects to the kidneys or increased cancer risks 

(U.S. Environmental Protection Agency, 2000, 2009a). 

The pattern of significant relations between uranium 

and potential explanatory factors largely reflected that 

samples from the HL study area had significantly greater 

concentrations of uranium than did samples from the ES, LU, 

QV, and TV study areas (table 11A). Uranium concentrations 

showed significant positive correlations with percentage of 

agricultural land use, pH, and concentrations of arsenic, boron, 
Status and Understanding of Groundwater Quality 65

molybdenum, vanadium, and TDS and negative correlations 

with the aridity index and DO concentrations (tables 11B, 12). 

Despite the significant correlations between uranium 

and potential explanatory factors, the primary feature that 

the two USGS-grid wells having samples with high uranium 

concentrations (HL-11 and HL-12) shared was that both were 

in the HL study area. The groundwater from HL-12 was oxic 

(DO=4.4 mg/L), had neutral pH (7.3), did not have high or 

moderate RCs of other trace elements, and had a mixed cation 

composition. HL-12 was near the southeastern margin of 

the study area in the Long Valley Creek drainage (fig. 13). 

The primary rock type in the Long Valley Creek drainage 

is Mesozoic granite of the northern Sierra Nevada (fig. 2), 

indicating that the aquifer sediment in the vicinity of HL-12 

likely contains a high proportion of sediment derived from 

granite. Granitic rocks elsewhere in the Sierra Nevada are 

known to yield sediment containing uranium (Wollenberg and 

Smith, 1968; Thomas and others, 1993; Jurgens and others, 

2010). 

HL-11 was in the central part of the basin, northwest 

of Honey Lake (fig. 13). The groundwater from HL-11 

was anoxic (DO less than 0.2 mg/L), had higher pH (7.8) 

compared to other samples, had high RCs of arsenic and 

molybdenum and a moderate RC of boron, and had a cation 

composition dominated by sodium (fig. 14). These chemical 

characteristics were similar to those of groundwater in the 

Carson Desert basin in Nevada, where high concentrations of 

arsenic, uranium, boron, and TDS (and molybdenum) were 

attributed to a combination of evaporative concentration and 

oxidation-reduction reactions involving sedimentary organic 

matter and iron-oxide coatings (Lico and Seiler, 1994; Welch 

and Lico, 1998). The Honey Lake and Carson Desert basins 

are both closed basins containing alluvial and lacustrine 

sediments derived from granitic and mafic volcanic rocks.

Constituents with Secondary Maximum 

Contaminant Level (SMCL) Benchmarks

The class of constituents with SMCL benchmarks 

includes salinity indicators (TDS, specific conductance, 

chloride, and sulfate) and several trace metals (iron, 

manganese, and zinc) that are commonly present in 

groundwater. These constituents affect the aesthetic properties 

of water, such as taste, color, and odor, or may create technical 

problems, such as scaling and staining of water delivery 

systems and fixtures. The SMCL benchmarks are based on 

these aesthetic and technical concerns and are not healthbased benchmarks. Constituents with SMCL benchmarks 

were present at high RCs in 15.1 percent of the primary 

aquifer system in the CAMP study unit and at moderate 

RCs in 4.9 percent (table 9B). The constituents with SMCL 

benchmarks most commonly present at high or moderate 

RCs were the salinity indicator TDS and the trace metals 

manganese and iron (table 8; fig. 11C).

Total Dissolved Solids (TDS)

The salinity indicators with SMCL benchmarks found 

at high or moderate RCs in the CAMP study unit were 

chloride and TDS. All of the samples with high or moderate 

RCs of chloride also had high or moderate RCs of TDS. 

Natural sources of TDS to groundwater include weathering 

and dissolution of minerals in soils, sediments, and rocks; 

mixing with saline or brackish waters from the ocean, 

estuaries (not a factor in the CAMP study unit), or saline 

lakes; interactions with marine or lacustrine sediments; 

mixing with hydrothermal solutions; and concentration by 

evapotranspiration of shallow groundwater (Hem, 1985). 

Anthropogenic sources of TDS to groundwater include 

recharge of water used for irrigation, wastewater discharge, 

and evaporation (Hem, 1985). TDS was present at high RCs 

in 1.2 percent of the CAMP study unit primary aquifer system 

and at moderate RCs in 10.1 percent (table 8). The proportion 

of the primary aquifer system having high or moderate RCs of 

TDS was significantly greater in the HL study area than in the 

ES, LU, QV, and TV study areas (tables 10, C1A–F). 

TDS concentrations were significantly greater in the 

HL study area than in the ES, QV, and TV study areas 

(table 11A), although the only samples from wells having 

high RCs for TDS were from the TV (TV-01) and LU (a 

CDPH well east of Goose Lake near the Oregon border) study 

areas (fig. 12F). The other significant correlations between 

TDS and the potential explanatory factors largely reflected 

the greater prevalence of moderate-RC concentrations in 

the HL study area. TDS concentrations were greater in 

groundwater from sedimentary aquifers than in groundwater 

from volcanic aquifers, in pre-modern groundwater than in 

modern groundwater, in anoxic groundwater than in oxic 

groundwater, and in samples from wells than in samples from 

springs (table 11A). The TDS concentrations showed positive 

correlations with the percentage of agricultural land use, pH, 

and arsenic, boron, molybdenum, uranium, and manganese 

concentrations, and negative correlations with the percentage 

of natural land use, aridity index, elevation, and dissolved 

oxygen concentration (tables 11B, 12).

Most of the wells with moderate RCs of TDS in the HL 

study area were clustered just north of Honey Lake (fig. 13). 

As discussed in the “Arsenic” section, this region has areas 

that are mapped as low-temperature geothermal areas. The 

wells near the center of the basin also could tap shallow 

groundwater from the Honey Lake playa, which has elevated 

TDS due to evaporative concentration. Honey Lake shallow 

playa groundwater has a maximum TDS concentration of 

around 1,100 mg/L, much lower than the maximum TDS of 

over 25,000 mg/L in the Fish Lake playa on the Nevada side 

of the Honey Lake basin (Mayo and others, 2010). Mayo and 

others (2010) suggest that the groundwater divide between the 

Honey Lake and Fish Springs parts of the basin resulted from 

relatively recent deformation related to isostatic rebound after 

draining of glacial Lake Lahontan, and that previously, the 

only terminal sink in the basin was the Fish Springs playa. 
66 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

As discussed in the “Boron” section, high RCs of TDS in 

groundwater in the TV study area appeared to be associated 

with saline thermal spring waters.

Manganese and Iron

The trace metals with SMCL benchmarks found at 

high or moderate RCs in the CAMP study unit were iron 

and manganese. Natural sources of iron and manganese to 

groundwater include weathering and dissolution of minerals in 

soils, sediments, and rocks. Iron and manganese oxyhydroxide 

minerals commonly coat mineral and sediment grains. In 

addition, iron-bearing silicate, sulfide, and (or) oxide minerals 

are in most rocks and sediments, and manganese commonly 

substitutes for iron in silicate minerals. The solubilities of 

manganese and iron are strongly dependent on oxidationreduction conditions; the more reduced species are much more 

soluble (Hem, 1985). 

Manganese and iron were present at high RCs in 

10.4 percent and 4.0 percent, respectively, of the primary 

aquifer system in the CAMP study unit and at moderate RCs 

in 1.2 percent each (table 8). The proportion of the primary 

aquifer system with high RCs of manganese was significantly 

greater in the HL study area than in the ES or QV study areas 

(tables 10, C1A–F), and there were no significant differences 

among the study areas in the proportion with high RCs of iron 

(tables 10, C1A–F). High RCs of manganese were present in 

all study areas (fig. 12G, tables C1A–F). High RCs of iron 

were present in all study areas except for the QV study area 

(fig. 12H, tables C1A–F). 

The spatially weighted proportions of high RCs of 

manganese in the ES and TV study areas and of iron in the SH 

and TV study areas were between 2.2 and 5.6 percent, and the 

grid-based proportions of high RCs were 0 percent because 

high RCs were only observed in CDPH wells. This difference 

between the spatially weighted and grid-based results was 

not unexpected. If a constituents is present at high RCs in 

less than about 5 percent of the primary aquifer system, then 

there is less than a 50 probability that 1 well in a 15-well grid 

network will have a high RC of the constituent (Belitz and 

others, 2010).

Manganese and iron showed different patterns of 

correlations with potential explanatory factors and waterquality constituents. Manganese concentrations showed 

significant positive correlations with percentages of 

agricultural and urban land use, pH, and concentrations of 

arsenic, boron, molybdenum, iron, and TDS, and showed 

significant negative correlations with DO and vanadium 

concentration (table 11B, 12). Most of these correlations 

reflect that of the nine USGS-grid samples that had 

manganese-reducing conditions, four were from the HL 

study area (table A6). In contrast, iron concentrations only 

showed correlations with vanadium (negative) and manganese 

(positive) (table 12). Of the 90 USGS-grid well samples, 

6 samples had iron-reducing conditions (table A6). Neither 

iron nor manganese showed significant correlations with 

study area, groundwater age, or measures of well depth 

(tables 11A, 11B).

Organic and Special-Interest Constituents

The organic constituents included two constituent 

classes: volatile organic compounds (VOCs), and pesticides 

(table 3). VOCs are present in paints, solvents, fuels, fuel 

additives, refrigerants, fumigants, and disinfected water and 

are characterized by their tendency to volatilize (for example, 

Zogorski and others, 2006). Typically, VOCs persist longer 

in groundwater than in surface water because groundwater 

is more 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 (for 

example, Gilliom and others, 2006). The only special-interest 

constituent analyzed for the CAMP study unit was perchlorate. 

At the start of the GAMA Priority Basin Project in 2003, 

perchlorate was considered a constituent of special interest to 

the CDPH and SWRCB because it had recently been detected 

in groundwater at concentrations that may be relevant to 

human health concerns (Belitz and others, 2003). The CDPH 

established an MCL-CA for perchlorate in 2007, and although 

perchlorate is an inorganic constituent, the classification 

of special-interest constituent is retained in this report for 

consistency with other GAMA Priority Basin Project reports.

The GAMA Priority Basin Project included analyses of 

a large number of organic constituents, many of which are 

not currently (as of 2014) subject to regulation in California 

drinking water. USGS-GAMA analytical methods for organic 

constituents had lower reporting limits than required for 

sampling for compliance with CDPH regulations (table 5). 

In the CAMP study unit, the majority of organic constituents 

detected are subject to regulation in California drinking 

water. Of the 85 VOCs analyzed, 18 were detected at least 

once, and of these, 13 have U.S. Environmental Protection 

Agency maximum contaminant level (MCL-US), California 

Department of Public Health maximum contaminant level 

(MCL-CA), or California Department of Public Health 

notification level (NL-CA) benchmarks (table 5). Of the 

63 pesticides and pesticide degradates analyzed, 9 were 

detected at least once, and of these, 5 have health-based 

benchmarks (table 5). In all, of the 70 organic constituents 

analyzed that had no health-based regulatory or non-regulatory 

benchmarks, 8 were detected in groundwater (table 5; Shelton 

and others, 2013).

Figure 15 summarizes the maximum RCs for individual 

organic constituents and perchlorate detected in samples from 

USGS-grid wells and the area-weighted detection frequencies 

for these constituents in the CAMP study unit as a whole. No 

organic constituents were detected at high or moderate RCs in 

the samples from USGS-grid wells, and none were reported 

at high or moderate RCs in the CDPH database for samples 

collected between September 18, 2007, and September 16, 
Status and Understanding of Groundwater Quality 67

Maximum relative-concentration, dimensionless

0.00001

0.0001

0.001

0.01

0.1

1

Area-weighted detection frequency, in percent

1 10 100

1,1-DCE

Benzene

Carbon disulfide

Dichloromethane

MTBE

PCE

Chloroform

1,4-DCB

Atrazine

Hexazinone

Simazine

Perchlorate

Moderate-RC

Low-RC

CFC-113 1,1,1-TCA 1,3,5-TMB

Carbaryl

Isopropylbenzene

n-Propylbenzene

Tebuthiuron

Prometon

MTBE, methyl tert-butyl ether; PCE, tetrachloroethene; 1,1-DCE, 1,1-dichloroethene; 

1,4-DCB, 1,4-dichlorobenzene; 1,1,1-TCA, 1,1,1-trichloroethane; 1,3,5-TMB, 

1,3,5-trimethylbenzene; CFC-113, 1,1,2-trichloro-1,2,2-triuoroethane

Volatile organic compounds 

Pesticides (herbicides, insecticides, and fumigants) 

Special-interest constituent

EXPLANATION

sac15-0550_fig 15

Figure 15. Detection frequency and maximum relative-concentration (RC) of organic and special-interest constituents detected in 

U.S. Geological Survey (USGS)-grid wells in the Cascade Range and Modoc Plateau (CAMP) study unit, 2010, California Groundwater 

Ambient Monitoring and Assessment (GAMA) Program Priority Basin Project. (MTBE, methyl tert-butyl ether; PCE, tetrachloroethene; 

1,1-DCE, 1,1-dichloroethene; 1,4-DCB, 1,4-dichlorobenzene; 1,1,1-TCA, 1,1,1-trichloroethane; 1,3,5-TMB, 1,3,5-trimethylbenzene;

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

2010. The trihalomethane chloroform and the herbicide 

atrazine were the only individual organic constituents with 

area-weighted detection frequencies greater than 10 percent 

(fig. 15). Perchlorate was detected at moderate RCs and had 

an area-weighted detection frequency of 43 percent (fig. 15). 

Moran and others (2005) sampled 23 wells in the area of the 

CAMP study unit and had a 39 percent detection frequency 

for methyl tert-butyl ether (MTBE). However, the reporting 

limits used by Moran and others (2005) were lower than those 

used by USGS-GAMA. If the MTBE results from Moran and 

others (2005) were screened at the reporting limit used by 

USGS-GAMA, then the MTBE detection frequency would 

be 4.3 percent, which is similar to the area-weighted MTBE 

detection frequency in the CAMP study unit in this study 

(fig. 15).
68 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Organic constituents with benchmarks were not present at 

high or moderate RCs in the CAMP study unit primary aquifer 

system, but were present at low RCs in about 41 percent of 

the system (table 13). The VOCs and pesticides each were 

detected at low RCs in approximately one-quarter of the 

primary aquifer system (table 13). Relations between water 

quality and potential explanatory factors were evaluated 

for the two organic constituent classes (herbicides and 

trihalomethanes) having an individual constituent with a 

detection frequency greater than 10 percent in the study unit as 

a whole (tables 11A, 11B).

Table 13. Summary of aquifer-scale proportions calculated using the spatially weighted method for organic constituent classes with 

health-based benchmarks and perchlorate for study areas, Cascade Range and Modoc Plateau (CAMP) study unit, 2010, California 

Groundwater Ambient Monitoring and Assessment (GAMA) Program Priority Basin Project.

[Study area abbreviations: ES, Sacramento Valley Eastside study area; HL, Honey Lake Valley study area; LU, Cascade Range and Modoc Plateau LowUse Basins study area; QV, Quaternary Volcanic Areas study area; SH, Shasta Valley and Mount Shasta Volcanic Area study area; TV, Tertiary Volcanic Areas 

study area. Study unit: Aquifer-scale proportions are area weighted by the percent (%) of the study unit covered by each study area: ES=10.4%, HL=13.4%, 

LU=18.4%, QV=25.7%, SH=13.3%, TV=18.7%. Relative-concentration categories: Relative-concentration (RC) is defined as the concentration measured 

in the sample divided by the concentration of the selected benchmark. 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. Other abbreviations: VOC, volatile organic compound]

Constituent classes 

and 

study areas

Number 

of 

wells

Aquifer-scale proportions 

(percent)

Not detected Low-RC Moderate-RC High-RC

Pesticides (herbicides)1

ES 15 100 0 0 0

HL 15 86.7 13.3 0 0

LU 15 86.7 13.3 0 0

QV 15 80.0 20.0 0 0

SH 15 33.3 66.7 0 0

TV 15 73.3 26.7 0 0

CAMP study unit 90 76.7 23.3 0 0

Trihalomethane (THM)2

ES 23 53.3 46.7 0 0

HL 24 86.7 13.3 0 0

LU 21 93.3 6.7 0 0

QV 26 100 0 0 0

SH 24 86.7 13.3 0 0

TV 20 86.7 13.3 0 0

CAMP study unit 138 87.9 12.1 0 0

Any VOC3

ES 23 46.7 53.3 0 0

HL 24 60.0 40.0 0 0

LU 21 86.7 13.3 0 0

QV 26 86.7 13.3 0 0

SH 24 73.3 26.7 0 0

TV 20 80.0 20.0 0 0

CAMP study unit 138 75.9 24.1 0 0

Any organic constituent with a health-based benchmark4

ES 23 46.7 53.3 0 0

HL 24 60.0 40.0 0 0

LU 21 73.3 26.7 0 0

QV 26 73.3 26.7 0 0

SH 24 26.7 73.3 0 0

TV 20 53.3 46.7 0 0

CAMP study unit 138 58.8 41.2 0 0
Status and Understanding of Groundwater Quality 69

Herbicides

All but one of the pesticides with human health-based 

benchmarks detected in the CAMP study unit were herbicides 

(table 5). The CDPH database contained no data for pesticides 

for sites in the CAMP study unit. The detection frequency of 

herbicides ranged from 0 in the ES study area to 67 percent in 

the SH study area (figs. 16, 17A; table 13). All concentrations 

were very low; the maximum RC for any individual herbicide 

(fig. 15) or for the sum of the RCs of all herbicides present in a 

sample was 0.03 (fig. 16).

Atrazine was the most frequently detected herbicide, 

with an area-weighted detection frequency of 14 percent 

(fig. 15). It was detected in all of the study areas except the ES 

study area, and the maximum study area detection frequency 

was 53 percent in the SH study area (fig. 16). All of the 

USGS-grid well samples containing atrazine also contained 

the degradation product deethylatrazine, and deethylatrazine 

was detected in about twice the number of USGS-grid well 

samples than was atrazine (Shelton and others, 2013). The 

concentrations of deethylatrazine generally were higher 

than the concentrations of atrazine. Neither the USEPA nor 

CDPH have established a benchmark for deethylatrazine (as 

of 2014); thus, a RC cannot be calculated, thereby preventing 

it from being included in the status assessment. Atrazine and 

deethylatrazine are the most common pesticides or pesticide 

degradates detected in the groundwater sampled by the 

National Water-Quality Assessment (NAWQA) Program 

(Gilliom and others, 2006), and their co-occurrence could 

reflect the relatively high degree of persistence of atrazine 

in groundwater environments (Kolpin and others, 1998). 

Hexazinone had an area-weighted detection frequency of 

9.8 percent (fig. 15) and was detected in the LU, QV, and SH 

study areas. Simazine was detected in the HL and SH study 

areas, and prometon and tebuthiuron were detected only in the 

SH study area.

Herbicide concentrations were significantly correlated 

with study area and groundwater age. The total concentration 

of herbicides was significantly greater in the SH study area 

than in any of the other study areas, and significantly greater 

in modern and mixed age groundwater than in pre-modern 

groundwater (table 11A). The negative correlation between 

pH and herbicide concentration (table 11B) likely reflects the 

lower pH values in samples from the SH study area (table A6), 

and the greater herbicide concentrations in oxic compared to 

anoxic groundwater (table 11A) likely reflects the significant 

association between oxic groundwater and the SH study area 

(table 7C). 

Constituent classes 

and 

study areas

Number 

of 

wells

Aquifer-scale proportions 

(percent)

Not detected Low-RC Moderate-RC High-RC

Perchlorate5

ES 29 9.1 90.9 0 0

HL 35 40.0 53.3 6.7 0

LU 28 53.3 40.0 6.7 0

QV 45 78.6 21.4 0 0

SH 38 53.3 46.7 0 0

TV 24 71.4 28.6 0 0

CAMP study unit 199 56.7 41.0 2.2 0

1

Herbicides include atrazine, hexazinone, prometon, simazine, and tebuthiuron. The insecticide carbaryl was detected in one sample that also had detections of 

herbicides; therefore, the aquifer-scale proportions for herbicides are the same as those for pesticides.

2

Chloroform was the only trihalomethane detected.

3

Any VOC includes chloroform; the solvents dichloromethane, tetrachloroethene (PCE), 1,1,1-trichloroethane; the organic synthesis compound 1,1-dichloroethene; the fumigant 1,4-dichlorobenzene; the gasoline oxygenate methyl tert-butyl ether (MTBE); the gasoline hydrocarbons 1,3,5-trimethylbenzene, benzene, 

isopropylbenzene, n-propylbenzene; and the refrigerant 1,1,2-trichloro-1,2,2-trifluoroethane (CFC-113). 

4

Any organic constituent includes pesticides and VOCs.

5

Perchlorate was analyzed in 84 U.S. Geological Survey-grid samples: 11 in ES; 15 each in HL, LU, and SH; and 14 each in QV and TV.

Table 13. Summary of aquifer-scale proportions calculated using the spatially weighted method for organic constituent classes with 

health-based benchmarks and perchlorate for study areas, Cascade Range and Modoc Plateau (CAMP) study unit, 2010, California 

Groundwater Ambient Monitoring and Assessment (GAMA) Program Priority Basin Project.—Continued

[Study area abbreviations: ES, Sacramento Valley Eastside study area; HL, Honey Lake Valley study area; LU, Cascade Range and Modoc Plateau LowUse Basins study area; QV, Quaternary Volcanic Areas study area; SH, Shasta Valley and Mount Shasta Volcanic Area study area; TV, Tertiary Volcanic Areas 

study area. Study unit: Aquifer-scale proportions are area weighted by the percent (%) of the study unit covered by each study area: ES=10.4%, HL=13.4%, 

LU=18.4%, QV=25.7%, SH=13.3%, TV=18.7%. Relative-concentration categories: Relative-concentration (RC) is defined as the concentration measured 

in the sample divided by the concentration of the selected benchmark. 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. Other abbreviations: VOC, volatile organic compound]
70 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Moderate-RC

Low-RC

Perchlorate Chloroform Atrazine Sum of herbicides

Relative-concentration, dimensionless

0

0.05

0.10

0.15

0.20

0.25

Detection frequency, in percent

100

80

60

40

20

0

ES HL LU QV SH TV SU ES HL LU QV SH TV SU ES HL LU QV SH TV SU ES HL LU QV SH TV SU

ES HL LU QV SH TV SU ES HL LU QV SH TV SU ES HL LU QV SH TV SU ES HL LU QV SH TV SU

A

B

ES - Eastside Sacramento Valley

HL - Honey Lake Valley

LU - Low-Use Basins

QV - Quaternary Volcanics

SH - Shasta Valley

TV - Tertiary Volcanics

Study areas

SU - Study unit (area-weighted)

Grid well Detection frequency

EXPLANATION

sac15-0550_fig 16

Figure 16. Area-weighted detection frequency and graphs of relative-concentrations (RC) of selected organic and special-interest 

constituents detected in U.S. Geological Survey (USGS)-grid wells, Cascade Range and Modoc Plateau (CAMP) study unit, 2010, 

California Groundwater Ambient Monitoring and Assessment (GAMA) Program Priority Basin Project.
Status and Understanding of Groundwater Quality 71

Lake

Shasta

Goose

Lake

Upper

Alkali

Lake

Eagle

Lake

Honey

Lake Lake

Almanor

Sacramento River

North Fork Feather River

Klamath River

Pit River

Shasta River

122° 121° 120°

42°

41°

40°

CALIFORNIA

Study

area

0 10 20 30 40 KILOMETERS

Base modified from U.S. Geological Survey and other

Federal and State digital data, various scales 

Albers Equal Area Projection

North American Datum of 1983

0 10 20 30 40 MILES

EXPLANATION

A

Central

Valley

Province

Klamath

Mountains

Province

Cascade Range

and 

Modoc Plateau

Province

Sierra

Nevada

Northern Province

Coast

Ranges

Province

Not detected

Detected at low relative-

 concentrations

Herbicides in USGS-grid wells

County boundary

Hydrologic provinces

Tertiary Volcanic Areas (TV)

Quaternary Volcanic Areas (QV)

Shasta Valley and Mount Shasta Volcanic Area (SH)

Eastside Sacramento Valley (ES)

Cascade Range and Modoc Plateau Low-Use Basins (LU)

Honey Lake Valley (HL)

Study areas

Other areas in the Cascade Range

 and Modoc Plateau Province

sac15-0550_fig 17a

Figure 17. Relative-concentrations (RC) in groundwater samples from U.S. Geological Survey (USGS)-grid wells, Cascade Range and 

Modoc Plateau (CAMP) study unit, 2010, California Groundwater Ambient Monitoring and Assessment (GAMA) Program Priority Basin 

Project for A, herbicides; B, trihalomethanes; and C, perchlorate. (RL, reporting limit)
72 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Lake

Shasta

Goose

Lake

Upper

Alkali

Lake

Eagle

Lake

Honey

Lake Lake

Almanor

Sacramento River

North Fork Feather River

Klamath River

Pit River

Shasta River

Central

Valley

Province

Klamath

Mountains

Province

Cascade Range

and 

Modoc Plateau

Province

Sierra

Nevada

Northern Province

Coast

Ranges

Province

122° 121° 120°

42°

41°

40°

CALIFORNIA

Study

area

0 10 20 30 40 KILOMETERS

Base modified from U.S. Geological Survey and other

Federal and State digital data, various scales 

Albers Equal Area Projection

North American Datum of 1983

0 10 20 30 40 MILES

B

EXPLANATION

Concentration of chloroform, in micrograms per liter

Not detected 

 (RL=0.5)

Low RC (0.5 to 8)

Not detected

 (RL=0.01)

Low RC (0.01 to 8)

USGS-grid wells CDPH public-supply wells

County boundary

Hydrologic provinces

Tertiary Volcanic Areas (TV)

Quaternary Volcanic Areas (QV)

Shasta Valley and Mount Shasta Volcanic Area (SH)

Eastside Sacramento Valley (ES)

Cascade Range and Modoc Plateau Low-Use Basins (LU)

Honey Lake Valley (HL)

Study areas

Other areas in the Cascade Range

 and Modoc Plateau Province

sac15-0550_fig 17b

Figure 17. —Continued
Status and Understanding of Groundwater Quality 73

Lake

Shasta

Goose

Lake

Upper

Alkali

Lake

Eagle

Lake

Honey

Lake Lake

Almanor

Sacramento River

North Fork Feather River

Klamath River

Pit River

Shasta River

Central

Valley

Province

Klamath

Mountains

Province

Cascade Range

and 

Modoc Plateau

Province

Sierra

Nevada

Northern Province

Coast

Ranges

Province

122° 121° 120° 42°

41°

40°

CALIFORNIA

Study

area

0 10 20 30 40 KILOMETERS

Base modified from U.S. Geological Survey and other

Federal and State digital data, various scales 

Albers Equal Area Projection

North American Datum of 1983

0 10 20 30 40 MILES

C

EXPLANATION

Concentration of perchlorate, in micrograms per liter

Not detected 

 (RL=4)

Not detected

 (RL=0.1)

Low RC (0.1 to 0.6)

Moderate RC 

 (0.6 to 6)

USGS-grid wells CDPH public-supply wells

County boundary

Hydrologic provinces

Tertiary Volcanic Areas (TV)

Quaternary Volcanic Areas (QV)

Shasta Valley and Mount Shasta Volcanic Area (SH)

Eastside Sacramento Valley (ES)

Cascade Range and Modoc Plateau Low-Use Basins (LU)

Honey Lake Valley (HL)

Study areas

Other areas in the Cascade Range

 and Modoc Plateau Province

sac15-0550_fig 17c

Figure 17. —Continued
74 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Herbicide concentrations showed no significant 

correlations with depths to the top or the bottom of the 

screened or open interval in the well (table 11B) or to depth 

class (table 11A). Herbicide concentrations also showed 

no significant correlations with land use (table 11B). This 

absence of correlations was unexpected, given the positive 

correlation between herbicide concentrations and percentage 

of agricultural land use and negative correlation between 

herbicide concentrations and well depth or depth to the top of 

the screened or open interval observed in many other GAMA 

Priority Basin Project study units (for example, Landon and 

others, 2010). 

The high detection frequency of herbicides in the SH 

study area compared to the other study areas (table 13; 

fig. 16)—and compared to most other areas investigated by 

the GAMA Priority Basin Project—and the lack of correlation 

between herbicide concentrations and land use or measures 

of well depth could be related to non-agricultural applications 

of herbicides and to specific features of the hydrology of 

the SH study area. The California Department of Pesticide 

Regulation (CDPR) maintains a database of registered 

pesticide applications at the scale of 1-mi2

 sections (California 

Department of Pesticide Regulation, 2013). Of the herbicides 

analyzed in this study, hexazinone has the greatest rate of 

registered usage in the CAMP study unit. The primary uses 

of hexazinone in the CAMP study unit are for timber-stand 

improvement in U.S. Forest Service and private timberlands 

and for weed management in grazing lands and alfalfa grown 

for forage (Kegley and others, 2011; California Department 

of Pesticide Regulation, 2013). Between 1991 and 2005, an 

average of 328 pounds per year (lb/yr) of hexazinone were 

applied in the SH study area, and another 832 lb/yr were 

applied in the forested watershed surrounding the study area. 

Use of herbicides in timberlands is of concern to the SWRCB 

and the Regional Water Quality Control Boards (for example, 

North Coast Regional Water Quality Control Board, 2005).

The other frequently detected herbicides in the CAMP 

study unit, atrazine and simazine, had few registered pesticide 

applications in specific 1-mi2

 sections between 1991 and 

2005 (California Department of Pesticide Regulation, 

2013). However, the database reports many registered nonagricultural uses of pesticides at the county level, rather than 

at the section level. Between 1991 and 1996, approximately 

300 lb/yr of atrazine and 487 lb/yr of simazine were applied 

for weed control in rights-of-way in Siskiyou, Modoc, and 

Lassen counties (the three counties mostly or entirely within 

the CAMP study unit) (California Department of Pesticide 

Regulation, 2013). Herbicide use by the California Department 

of Transportation has decreased markedly since the mid-1990s 

as a result of changes in weed control strategies, and atrazine 

and simazine are no longer used (California Department of 

Transportation, 2014). No information is available about 

unregistered usage of herbicides in the CAMP study unit.

Of the six study areas, the SH study area had the greatest 

proportion of USGS-grid wells with modern or mixed age 

groundwater (fig. 8C). The combination of potential sources 

of herbicides (timberlands, agriculture, and roadside right-ofways) and presence of modern groundwater could account for 

the greater detection frequency of herbicides in the SH study 

area compared to the other study areas.

Trihalomethanes

Water used for drinking and other household uses 

in domestic and public systems commonly is disinfected 

with chlorine solutions (for example, sodium hypochlorite 

[bleach], hypochlorous acid, chlorine gas, chloramines, or 

chlorine dioxide). In addition to disinfecting the water, the 

chlorine compounds can react with organic matter to produce 

trihalomethanes (THMs) and other chlorinated and (or) 

brominated disinfection byproducts (for example, Ivahnenko 

and Barbash, 2004). Chloroform was the most frequently 

detected VOC in groundwater across the USA (Zogorski and 

others, 2006).

The only THM detected in USGS-grid wells in the 

CAMP study unit was chloroform (table 5). Chloroform 

was not detected at high or moderate RCs in the CAMP 

study unit in samples from USGS-grid wells or reported 

at high or moderate RCs in the CDPH database. The areaweighted detection frequency of low RCs of chloroform in the 

CAMP study unit was 12 percent (fig. 15), and the detection 

frequencies in the six study areas ranged from zero in the 

QV study area to 47 percent in the ES study area (table 13, 

figs. 16, 17B). 

Chloroform concentrations were significantly greater 

in the ES study area than in the LU and QV study areas 

(table 11A) and showed significant positive correlations with 

urban land use and septic-tank density (table 11B). The density 

of septic tanks was significantly greater in the ES study area 

than in the LU and QV study areas (table 7A). The significant 

negative correlations between chloroform concentrations and 

elevation and pH (table 11B) reflect that the ES study area 

has significantly lower elevation and pH compared to other 

study areas (table 7A). Chloroform concentrations showed no 

significant relations with groundwater age or with well depth 

(tables 11A–B); however, half of the wells with detections of 

chloroform were from the ES study area, which generally had 

deeper wells than the other study areas (fig. 7).

Urban land use and septic-tank density were not 

significantly correlated with each other (table 7B); thus, the 

fact that both are correlated with chloroform concentration 

indicates a more complex relation among the three variables. 

The 90 USGS-grid wells were divided into four groups on the 

basis of percentages of urban land use and septic-tank density. 

The detection frequency of chloroform in sites with urban land 

use greater than 10 percent and septic-tank density greater 

than 2 tanks/km2

 was significantly greater than the detection 
Status and Understanding of Groundwater Quality 75

Chloroform detection frequency, in percent

0

10

20

30

40

50

>10% >10% <10% <10%

Green text indicates percentage of urban

land use in the 500-meter buffer.

>2 <2 >2 <2

Purple text indicates density of septic 

tanks in the 500-meter buffer (in number 

of tanks per square kilometer).

13

28

7

42

Groups of wells with significantly different detection frequencies 

are different colors (contingency table test, p<0.001)

Black text indicates number of wells in

each detection group.

sac15-0550_fig 18

Figure 18. Detection frequencies of chloroform in groundwater 

samples grouped by percentage of urban land use and density 

of septic tanks in the 500-meter buffer around the well, Cascade 

Range and Modoc Plateau (CAMP) study unit, 2010, California 

Groundwater Ambient Monitoring and Assessment (GAMA)

Program Priority Basin Project. (>, greater than; <, less than, 

%, percent)

frequency in sites with less than 10 percent urban land use and 

septic-tank density less than 2 tanks/km2 (contingency table 

test, p<0.001; fig. 18). Detection frequencies of chloroform 

in sites with either urban land use greater than 10 percent or 

septic-tank density greater than 2 tanks/km2, but not both, 

were in between the detection frequencies of chloroform in the 

other two groups (fig. 18).

Nationally, THMs have been strongly correlated with 

the 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). Septic systems can be a source of 

THMs to groundwater because they recharge water used 

for domestic purposes to the aquifer system, and this water 

may be disinfected for household use, or bleach and other 

cleaning products containing chlorine could have been 

used in the house. In addition, there could be a correlation 

between septic systems and domestic or small system 

wells—areas not part of large public water-supply systems 

may also not be part of sewage systems. Shock chlorination 

(often carried out by pouring bleach down a well) is a 

recommended procedure for treating bacterial contamination 

and odor problems in domestic drinking-water supply wells 

(U.S. Centers for Disease Control and Prevention, 2006), 

and could cause a reservoir of chlorinated water to form 

in the well bore and surrounding aquifer material. Smaller 

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. 

Of the 75 USGS-grid wells listed in the CDPH database, 

53 (71 percent) served fewer than 1,000 people, and 37 

(49 percent) served fewer than 200 people. The dominance of 

relatively small water systems could account for the relation 

between chloroform detections and septic-tank densities. 

There were no detections of chloroform in samples from the 

nine USGS-grid wells that were springs.

Perchlorate

Perchlorate is an inorganic anion that is highly soluble 

in water. It was classified as a special-interest constituent 

because, at the inception of the GAMA Priority Basin 

Project in 2003, perchlorate had recently been detected in 

public-supply wells in several areas of the State, and the 

CDPH was evaluating whether or not an MCL-CA should 

be established. The MCL-CA of 6 μg/L was promulgated in 

2007. Perchlorate has both natural and anthropogenic sources 

to groundwater. It forms naturally in the atmosphere and is 

present at very low concentrations in precipitation (Dasgupta 

and others, 2005; Parker and others, 2009; Rajagopalan 

and others, 2009). Naturally deposited perchlorate salts in 

the soils and unsaturated zones of aquifers in areas with 

arid to semi-arid climates can be re-solubilized and carried 

into deeper groundwater by recharge of applied irrigation 

water (Rao and others, 2007; Fram and Belitz, 2011). 

Perchlorate is a component of solid rocket fuel and is used 

in explosives, fireworks, safety flares, and other products 

(U.S. Environmental Protection Agency, 2005b; Dasgupta 

and others, 2006). It also may be present in some fertilizers 

(Dasgupta and others, 2006; Böhlke and others, 2009) and 

can form in the chlorine solutions used for drinking water 

disinfection (Massachusetts Department of Environmental 

Protection, 2006; Greiner and others, 2008).

Perchlorate was not present at high RCs in the primary 

aquifer system of the CAMP study unit and was detected at 

moderate RCs in 2.2 percent and at low RCs in 41 percent 

of the system (table 13). The study-area detection frequency 

of perchlorate ranged from 21 percent in the QV study area 

to 91 percent in the ES study area (table 13; figs. 16, 17C). 

Concentrations of perchlorate showed significant negative 

correlations with the aridity index and pH and positive 

correlations with the percentage of agricultural land use 

(table 11B). Perchlorate concentrations were positively 

correlated with nitrate concentrations (spearman p<0.001, 

ρ=0.63).
76 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Perchlorate detection frequencies in the CAMP study 

unit mostly were consistent with those expected from the 

distribution of perchlorate in California groundwater under 

natural conditions (Fram and Belitz, 2011). In the Fram and 

Belitz (2011) study, the observed occurrence frequencies 

of perchlorate at concentrations greater than threshold 

concentrations of 0.1 and 0.5 μg/L were compared to the 

predicted probability of perchlorate occurring under natural 

conditions as a function of aridity index. For the CAMP study 

unit, the 84 USGS-grid wells with perchlorate data were 

divided into 4 groups of 21 wells by the aridity index, and, 

for each group, the average aridity index and the detection 

frequencies of perchlorate at concentrations greater than 

0.1 μg/L and greater than 0.5 μg/L were calculated. The 

observed detection frequencies were close to the predicted 

probabilities in all cases, except for the occurrence of 

perchlorate at concentrations greater than 0.1 μg/L in the 

group of samples from the wettest areas (greatest average 

aridity index) (fig. 19). This indicated that anthropogenic 

sources of perchlorate were not required to explain the pattern 

of perchlorate occurrence in most of the CAMP study unit, 

although contribution from anthropogenic sources cannot be 

ruled out.

The elevated occurrence frequency of low concentrations 

of perchlorate in the group of 21 USGS-grid wells from the 

wettest areas is not easily explained. There were no recorded 

sites of perchlorate production or use and no known sites 

of groundwater contamination from industrial sources of 

perchlorate near the nine USGS-grid wells in that group 

with perchlorate detections (U.S. Environmental Protection 

Agency, 2005b; California Department of Toxic Substances 

Control, 2007; California State Water Resources Control 

Board, 2007). There also were no significant differences in 

detection frequency of solvents and gasoline components 

among the four groups (contingency table test, p=0.36), 

indicating an absence of industrial sources of perchlorate. 

The group from the wettest areas had significantly lower 

percentages of agricultural land use in the 500-m buffers 

around the USGS-grid wells than did the other three groups 

(Kruskal-Wallis test, p<0.001), and there were no significant 

differences in nitrate concentrations among the four groups 

(Kruskal-Wallis test, p=0.26). This indicates absence of 

agricultural sources of perchlorate. There were no significant 

differences in chloroform detection frequency among the four 

groups (contingency table test, p=0.30), indicating absence 

of perchlorate derived from chlorine solutions. Of the nine 

samples with detections of perchlorate in the group of samples 

from the wettest areas, six samples were from the ES study 

area. Of the 11 samples from the ES study area with data 

for perchlorate, 10 samples (91 percent) had a detection of 

perchlorate (Shelton and others, 2013). Further investigation 

of the increased occurrence of low concentrations of 

perchlorate in groundwater in the ES study area is beyond the 

scope of this report.

Average aridity index, dimensionless

0 0.5 1.0 1.5 2.0

Predicted probability of perchlorate detection or observed

perchlorate detection frequency, in percent 

0

0.2

0.4

0.6

0.8

>0.1

>0.5

The 84 USGS-grid wells with perchlorate data were divided into 4 groups of 21 wells. 

Horizontal error bars equal plus or minus one standard deviation of the average 

aridity index. Vertical error bars are the 90-percent Jeffrey’s confidence interval 

for the observed detection frequency.

EXPLANATION

Predicted probability of 

detecting perchlorate at 

concentration greater 

than threshold

Threshold concentration, 

in micrograms per 

liter

Observed detection 

frequency of perchlorate 

concentration greater 

than threshold

Figure 19. 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 by aridity index, Cascade Range and Modoc 

Plateau (CAMP) study unit, 2010, California Groundwater Ambient 

Monitoring and Assessment (GAMA) Program Priority Basin 

Project.
Summary 77

Summary

Groundwater quality in the Cascade Range and Modoc 

Plateau (CAMP) study unit was investigated as part of the 

California State Water Resource Control Board’s Groundwater 

Ambient Monitoring and Assessment (GAMA) Program 

Priority Basin Project. The CAMP study unit covers an area 

of approximately 15,000 square miles (mi2) or 39,000 square 

kilometers (km2) in Siskiyou, Modoc, Shasta, Lassen, Tehama, 

Plumas, and Butte counties in the northeastern corner of the 

State.

The GAMA Priority Basin Project is designed to 

provide a statistically unbiased assessment of untreated 

groundwater quality in the primary aquifer system used for 

public drinking-water supplies statewide. The primary aquifer 

system was defined by the range of depths of the screened or 

open intervals of wells listed in the California Department 

of Public Health (CDPH) database of wells used for public 

drinking-water supply. [The CDPH Drinking Water Program 

which regulated water quality in public-supply wells was 

transferred to the SWRCB Division of Drinking Water on 

July 1, 2014, however the label “CDPH” is retained in this 

report for consistency with other GAMA Priority Basin Project 

publications and because the CDPH had jurisdiction over 

public-supply wells at the time that samples were collected 

for this study.] Two types of assessments were made for the 

CAMP study unit: (1) a status assessment yielding quantitative 

estimates of the current status of groundwater quality in the 

primary aquifer system and (2) an understanding assessment

consisting of evaluation of relations between water quality and 

potential explanatory factors describing land use, geography, 

depth, geochemical conditions, groundwater age, and other 

characteristics of the primary aquifer system.

The assessments were based on data collected by the 

(U.S. Geological Survey (USGS) for the GAMA Priority 

Basin Project (USGS-GAMA) and data compiled from the 

CDPH database. The study unit was divided into six study 

areas on the basis of regional geology: Eastside Sacramento 

Valley (ES), Honey Lake Valley (HL), Cascade Range and 

Modoc Plateau Low Use Basins (LU), Quaternary Volcanic 

Areas (QV), Shasta Valley and Mount Shasta Volcanic Area 

(SH), and Tertiary Volcanic Areas (TV). Each study area was 

divided into 15 equal-area cells. The sizes of the grid cells 

ranged from 19 km2

 in the ES study area to 47 km2 in the QV 

study area. Water-quality and ancillary data were collected 

by USGS-GAMA from a well or spring in each of the 90 grid 

cells during 2010 (referred to as USGS-grid wells in this 

report). Of the 90 USGS-grid wells, 82 were listed in the 

CDPH database of public-supply wells and springs, and the 

other 8 were screened or had open intervals similar to wells 

listed in the CDPH database. Samples from the USGS-grid 

wells were analyzed for organic constituents (volatile organic 

compounds and pesticides), inorganic constituents (major ions, 

trace elements, and radioactive constituents), special interest 

constituents (perchlorate) and geochemical and age-dating 

tracers. Additional water-quality data were obtained from 

177 wells in the CDPH database having data for samples 

collected between September 16, 2007, and September 16, 

2010. 

Relative-concentrations (defined as sample concentration 

divided by benchmark concentration for the constituent) 

were used to evaluate groundwater quality for constituents 

that have Federal and (or) California regulatory or nonregulatory benchmarks for drinking-water quality. A relativeconcentration (RC) greater than 1.0 indicates a concentration 

above a benchmark. Organic and special interest constituent 

RCs were classified as “high” (RCs greater than 1.0), 

“moderate” (RCs greater than 0.10, but less than or equal to 

1.0), or “low” (RCs less than or equal to 0.1). For inorganic 

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 to evaluate regional-scale 

groundwater quality. High-RC aquifer-scale proportion was 

defined as the areal percentage of the primary aquifer system 

that had a RC greater than 1.0 for a particular constituent 

or class of constituents; moderate-RC and low-RC aquiferscale proportions were defined as the areal percentages of 

the primary aquifer system that had moderate and low RCs, 

respectively. Aquifer-scale proportions were calculated for 

the primary aquifer systems in each of the six study areas 

and on an area-weighted basis for the study unit as a whole. 

Both grid-based, which used data from one well per grid cell, 

and spatially weighted, which used data from multiple wells 

per grid cell, approaches were used to calculate aquifer-scale 

proportions.

The status assessment showed that inorganic constituents 

had greater high-RC and moderate-RC aquifer-scale 

proportions than did organic constituents and that there were 

significant differences in aquifer-scale proportions for many 

constituents among the six study areas. In the CAMP study 

unit as a whole, one or more inorganic constituents with 

health-based benchmarks (U.S. Environmental Protection 

Agency [USEPA] and CDPH maximum contaminant 

levels and action levels, CDPH notification levels, and 

USEPA lifetime health advisory levels) had high RCs in 

9.4 percent of the primary aquifer system and moderate 

RCs in 14.7 percent. Arsenic and boron each were found at 

high RCs in approximately 3 percent of the primary aquifer 

system, and molybdenum, uranium, and vanadium each were 

found at high RCs in approximately 2 percent. The primary 

aquifer system of the HL study area had greater proportions 

with high or moderate RCs of arsenic, boron, molybdenum, 

and uranium than did primary aquifer systems in other study 

areas. In the CAMP study unit as a whole, one or more 

inorganic constituents with aesthetic-based benchmark (CDPH 

secondary maximum contaminant levels) had high RCs in 

15.1 percent of the primary aquifer system and moderate RCs 

in 4.9 percent. Manganese, iron, and total dissolved solids 

(TDS) were found at high RCs in approximately 12 percent, 

5 percent, and 2 percent of the primary aquifer system, 

respectively. 
78 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Organic constituents with health-based benchmarks 

were not found at high or moderate RCs in the CAMP 

study unit. Of the 148 organic constituents analyzed by 

USGS-GAMA, 27 were detected in at least one well, and of 

those 27 constituents, 18 have health-based benchmarks. One 

or more volatile organic compound (VOC) was detected at 

low RCs in 24 percent of the primary aquifer system, and one 

or more pesticide was detected at low RCs in 23 percent. Two 

organic constituents had detection frequencies greater than 

10 percent: the trihalomethane chloroform and the herbicide 

atrazine. The special-interest constituent perchlorate was not 

found at high RCs in the CAMP study unit, but was found at 

moderate RCs in 2.2 percent of the primary aquifer system.

For the understanding assessment, statistical tests 

were used to evaluate relations between concentrations of 

constituents and potential explanatory factors descriptive 

of geologic, hydrologic, land use, and geochemical 

characteristics of the primary aquifer system at the 

90 USGS-grid wells. The potential explanatory factors 

evaluated were study area; aquifer lithology class; the 

percentages of agricultural, natural, and urban land use within 

a radius 500-meters (m) around the USGS-grid well (500-m 

buffers); the densities of septic tanks and leaking or formerly 

leaking underground fuel tanks in the 500-m buffers; the 

aridity index and elevation at the well site; depths to the top 

and bottom of the screened, or open interval for wells, and 

depth class (spring, shallow well, overlapping well, deep 

well); groundwater age class (modern, mixed, pre-modern); 

and the water-quality indicators pH, dissolved oxygen 

concentration, and oxidation-reduction class (oxic, anoxic).

Concentrations of arsenic, boron, molybdenum, uranium, 

and TDS were significantly greater in USGS‐grid well samples 

from the HL study area than in samples from other study 

areas. Many of the correlations between the concentrations of 

these constituents and values of other potential explanatory 

factors reflect associations between potential explanatory 

factors and location in the Honey Lake study area. 

Concentrations of arsenic, boron, molybdenum, uranium, 

and TDS generally were greater in pre-modern and mixed 

age groundwater compared to modern age groundwater and 

in sites with sedimentary aquifer lithology compared to sites 

with volcanic aquifer lithology, and generally were positively 

correlated with percentage of agricultural land use and pH 

and negatively correlated with aridity index and dissolved 

oxygen concentration, consistent with the associations 

between values of these explanatory factors and location in 

the Honey Lake study area. The source of uranium was likely 

sediments derived from granitic rocks in the Sierra Nevada 

adjacent to the Honey Lake study area, and mixing with fluids 

from hydrothermal systems in the Honey Lake study area is 

likely a source of arsenic, boron, and other constituents to 

groundwater. Interaction between groundwater and Honey 

Lake playa sediments under high pH and low dissolved 

oxygen conditions may result in desorption of arsenic, 

molybdenum, and uranium from the sediments. Cumulative 

dissolution of aquifer materials over long time periods and 

evaporative concentration of groundwater in the playa may 

also contribute to increased concentrations of trace elements 

and TDS in Honey Lake groundwater.

Thermal springs are present in many parts of the 

CAMP study unit and could account for locally elevated 

concentrations of arsenic, boron, molybdenum, and TDS in 

the other study areas. Vanadium concentrations in USGS-grid 

wells were greater in oxic samples than in anoxic samples, but 

were not correlated with pH, contrary to expectations based on 

previous studies.

The SH study area had significantly greater occurrence of 

low RCs of herbicides than all of the other study areas, which 

could reflect the greater prevalence of modern groundwater 

in the SH study area and the presence of potential sources 

of herbicides from applications to timberlands and roadside 

rights-of-way. Chloroform occurrence was associated with 

the combination of septic-tank density greater than two tanks 

per square kilometer (km2

) and urban land use greater than 

10 percent within a radius of 500-meters of the well. These 

conditions were most prevalent in the ES study area. The 

detection frequency of low concentrations (0.1–0.5 μg/L) 

of perchlorate was consistent with probability of occurrence 

expected under natural conditions, except for in the ES study 

area where detection frequencies were much higher than 

expected, but could not be explained by known anthropogenic 

sources of perchlorate.
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January 2014, http://water.epa.gov/action/advisories/

drinking/upload/dwstandards2012.pdf.

U.S. Geological Survey, 2005, Mineral Resources Data 

System (MRDS): U.S. Geological Survey online spatial 

data, accessed August 2011, http://tin.er.usgs.gov/mrds/.

Wanty, R.B., and Goldhaber, M.B., 1992, 

Thermodynamics and kinetics of reactions involving 

vanadium in natural systems, Accumulation of 

vanadium in sedimentary rocks: Geochimica et 

Cosmochimica Acta, v. 56, no. 4, p. 1471–1483, 

http://dx.doi.org/10.1016/0016-7037(92)90217-7.

Webster, J.G., and Nordstrom, D.K., 2003, Geothermal 

arsenic, in Welch, A.H., and Stollenwerk, K.G., eds., 

Arsenic in ground water: Geochemistry and occurrence: 

Boston, Massachusettes, Kulwer Academic Publishers, 

p. 101–126.

Welch, A.H., and Lico, M.S., 1998, Factors controlling As 

and U in shallow ground water, southern Carson Desert, 

Nevada: Applied Geochemistry, v. 13, no. 4, p. 521–539, 

http://dx.doi.org/10.1016/S0883-2927(97)00083-8.

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ground water, Geochemistry and occurrence: Springer, 

475 p.

Welch, A.H., Lawrence, S.J., Lico, M.S., Thomas, J.M., and 

Schaefer, D.H., 1997, Ground-water quality assessment 

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Wanty, R.B., 2000, Arsenic in ground water of 

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Watkins, S.A., 2006, Arsenic in ground water, A review of 

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studies in the central part of the Sierra Nevada batholiths, 

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December 2014, http://www.water.ca.gov/pubs/

waterquality/honey_lake_water_quality_investigation/

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controlling the regional distribution of vanadium in 

groundwater: Groundwater, v. 48, no. 4, p. 515–525, 

http://dx.doi.org/10.1111/j.1745-6584.2009.00666.x.

Zogorski, J.S., Carter, J.M., Ivahnenko, Tamara, 

Lapham, W.W., Moran, M.J., Rowe, B.L., Squillace, P.J., 

and Toccalino, P.L., 2006, Volatile organic compounds 

in the Nation’s ground water and drinking-water supply 

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http://pubs.er.usgs.gov/publication/cir1292.
88 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Appendix A: Attribution of Potential Explanatory Factors

The 90 U.S. Geological Survey (USGS)-grid wells 

(figs. A1A–G) were attributed with values for the following 

potential explanatory factors: aquifer lithology and study 

area, land use, densities of leaking (or formerly leaking) 

underground storage tanks and septic tanks, hydrologic 

conditions, well depth, groundwater age, and geochemical 

conditions. These factors were assumed to be the ones most 

likely to have causative relations with the concentrations of 

water-quality constituents in the groundwater.

Aquifer Lithology and Study Area

Aquifer lithology was classified on the basis of lithologic 

information from California Departement of Water Resources 

(CDWR) well completion reports 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 

CDWR well completion report, the category from the CDWR 

well completion report was used. The 90 wells sampled by 

Groundwater Ambient Monitoring and Assessment Program 

(USGS-GAMA) have screened or open intervals in 11 of the 

66 geologic units defined on the California State geologic map 

(Jennings, 1977; Saucedo and others, 2000). These geologic 

units were grouped into four lithologic categories on the basis 

of rock type and age:

• G: Mesozoic granitic rocks (California State geologic 

map unit grMz).

• M: Mesozoic and Paleozoic metamorphic and marine 

sedimentary rocks. Metamorphic rocks are not exposed 

at the surface in the CAMP study unit, however, 

lithologic descriptions in CDWR well completion 

reports indicate that four wells in the SH study area 

have screened or open intervals in metamorphic rocks 

below the surface (table A1). The part of the Klamath 

Mountains adjacent to these four wells is mapped on 

the State geologic map as the following geologic units: 

m, undivided Mesozoic, Paleozoic, and Precambrian 

metavolcanic and metasedimentary rocks; Ku, 

Mesozoic marine sedimentary and metasedimentary 

rocks; and SO, Paleozoic marine sedimentary and 

metasedimentary rocks.

• S: Cenozoic sedimentary deposits, all of which are 

Quaternary age non-marine sedimentary deposits, 

including alluvial, consolidated alluvium, and sand 

dune sediments (California State geologic map units 

Q, QPc, and Qs). These deposits consist of Holocene 

age alluvial (Q) and eolian (Qs) deposits, and PlioPleistocene and Pliocene alluvial (QPc) deposits. 

The alluvial deposits include alluvial fan, fluvial, and 

lacustrine sediments.

• V: Cenozoic volcanic rocks. The Tertiary lava flows 

(Tv) and pyroclasic deposits (Tvp) are primarily 

Miocene and Pliocene in age, and the Quaternary lava 

flows are Pleistocene (Qv) and Holocene (Qrv) in age. 

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.

The lithologic categories assigned to the USGS-grid 

wells are listed in table A1: GS, 3 wells; MS, 3 wells; M, 

1 well; S, 18 wells; V, 47 wells; VM, 1 well; and VS, 17 wells. 

Wells assigned to two categories (GS, MS, VM, or VS) have 

screened or open intervals in two different lithologic units. 

The CAMP study unit was divided into six study areas on 

the basis of geologic characteristics to investigate potential 

differences in groundwater quality among these geologically 

distinct aquifer systems.
Appendix A: Attribution of Potential Explanatory Factors 89

sac15-0550_fig A1a.ai

Base modified from U.S. Geological Survey and other

Federal and State digital data, various scales 

Albers Equal Area Projection

North American Datum of 1983

Lake

Shasta

Goose

Lake

Upper

Alkali

Lake

Eagle

Lake

Honey

Lake Lake

Almanor

Sacramento River

North Fork Feather River

Klamath River

Pit River

Shasta River

Central

Valley

Province

Klamath

Mountains

Province

Cascade Range

and 

Modoc Plateau

Province

Sierra

Nevada

Province

Northern

Coast

Ranges

Province

97

395

395

89

299

99

70

36

44

139

139

"

"

"

"

"

"

"

"

0 10 20 MILES

0 10 20 KILOMETERS

Fig. A1F

Fig. A1E

Fig. A1D

Fig. A1G

Fig. A1B Fig. A1C 

5

EXPLANATION

Tertiary Volcanic Areas (TV)

Quaternary Volcanic Areas (QV)

Shasta Valley and Mount Shasta Volcanic Area (SH)

Eastside Sacramento Valley (ES)

Cascade Range and Modoc Plateau Low-Use Basins (LU)

Honey Lake Valley (HL)

Study areas Other areas in the Cascade Range and

 Modoc Plateau Low-Use Basins

National monument and park

Hydrogeologic provinces

County boundary

U.S. Geological Survey-grid well

Other California Department of Public 

Health well

Study-area buffers

A 122° 121° 120° 42°

41°

40°

Figure A1. Boundaries of the six study areas and study area buffers and locations of U.S. Geological Survey (USGS)-grid wells and 

California Department of Public Health (CDPH) wells for A, the entire study unit; B, the northwestern; C, north–central; D, northeastern; 

E, west–central; F, southwestern; and G, southeastern parts of the Cascades Range and Modoc Plateau (CAMP) study unit, 2010, 

California Groundwater Ambient Monitoring and Assessment (GAMA) Program Priority Basin Project.
90 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

sac15-0550_fig A1b.ai

"

"

"

"

"

TV-15 TV-01

SH-15

SH-14

SH-13

SH-12

SH-11

SH-10

SH-09

SH-08

SH-07

SH-06

SH-05

SH-04

SH-03

SH-02

SH-01

Weed

Yreka Montague

Dunsmuir unsmuir

SH-15

SH-13

SH-12

SH-11

SH-10

SH-09

SH-01

SISKIYOU CO

Yreka Montague

Little Shasta River

Shasta River

96

89

TV-15

Klamath River

IRON

GATE

DAM

Copco

Lake

Mount Shasta

Mount Shasta City

SH-14

SH-08

SH-07

SH-06

SH-05

SH-04

SH-03

SH-02

TRINITY CO

Weed

Parks Creek

Willow

Creek

Trinity River

Sacramento River

Mount Shasta

Mount Shasta City

97

B

0 5 MILES

0 5 KILOMETERS

5

Lake

Shastina

EXPLANATION

SH-12

Tertiary Volcanic Areas (TV)

Quaternary Volcanic Areas (QV)

Shasta Valley and Volcanics (SH)

Study areas

Other areas in the Cascade Range and Modoc 

Plateau Low-Use Basins

Other hydrogeologic provinces

Fault (Saucedo and others, 2000)

County boundary

Dam

U.S. Geological Survey-grid well

Other California Department of Public Health well

Study-area buffers

Base modified from U.S. Geological Survey and other

Federal and State digital data, various scales 

Albers Equal Area Projection

North American Datum of 1983

122°30' 122°15'

41°45'

41°30'

41°15'

Figure A1. —Continued
Appendix A: Attribution of Potential Explanatory Factors 91

sac15-0550_fig A1c.ai

Base modified from U.S. Geological Survey and other

Federal and State digital data, various scales 

Albers Equal Area Projection

North American Datum of 1983

"

"

"

"

QV-04

QV-02

QV-01

LU-10

LU-08

LU-07

LU-02

LU-01

Dorris

McCloud

Macdoel

Tulelake

122°15' 122° 121°45' 121°30'

41°45'

41°30'

41°15' 89

139

97

QV-02

QV-01

LU-08

SISKIYOU CO

McCloud

Ash Creek

McCloud River

LU-10 QV-04

LU-07

LU-02

LU-01

MODOC CO

Dorris

Macdoel

Tulelake

C

0 5 10 MILES

0 5 10 KILOMETERS

Meiss

Lake

Lower

Klamath

Lake Tule

Lake

EXPLANATION

Other areas in the Cascade Range and LU-08

Modoc Plateau Low-Use Basins

National monument and park

Fault (Saucedo and others, 2000)

County boundary

U.S. Geological Survey-grid well

Other California Department of 

 Public Health well

Study-area buffers Tertiary Volcanic Areas (TV)

Quaternary Volcanic Areas (QV)

Shasta Valley and Mount Shasta Volcanic Area (SH)

Cascade Range and Modoc Plateau Low-Use Basins (LU)

Study areas

LAVA BEDS

NATIONAL

MONUMENT

Medicine Lake

Highlands

Figure A1. —Continued
92 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

sac15-0550_fig A1d.ai

"

"

"

"

TV-07

QV-03

LU-11

LU-09 LU-06

LU-05

LU-04

LU-03

Canby

Likely

Alturas

Cedarville

Warner

Mountains

121° 120°45' 120°30' 120°15'

41°30'

41°15'

139

139

299

299

395

395

Canby

Alturas

Cedarville

MODOC CO

TV-07

QV-03

LU-11

LU-09 LU-06

LU-05

LU-04

LU-03

Likely

LASSEN CO

North Fork Pit River

Fitzhugh Creek

Pit Riv er

Pine Creek

Big Sage

Reservoir

Middle

Alkali

Lake

Ash

Creek

Mill Creek

Willow Creek

East Creek

Cedar Creek Cedar Creek

Lower

Alkali

Lake

Moon

Lake

Warner

Mountains

D

South Fork Pit River

0 5 10 MILES

0 5 10 KILOMETERS

SURPRISE FAULT

Base modified from U.S. Geological Survey and other

Federal and State digital data, various scales 

Albers Equal Area Projection

North American Datum of 1983

EXPLANATION

TV-07 Other areas in the Cascade Range and

 Modoc Plateau Low-Use Basins

Fault (Saucedo and others, 2000)

County boundary

U.S. Geological Survey-grid well

Other California Department of Public 

 Health well

Study-area buffers

Tertiary Volcanic Areas (TV)

Quaternary Volcanic Areas (QV)

Cascade Range and Modoc Plateau Low-Use Basins (LU)

Study areas

Figure A1. —Continued
Appendix A: Attribution of Potential Explanatory Factors 93

sac15-0550_fig A1e.ai

"

"

"

"

122° 121°45' 121°30' 121°15'

41°

40°45'

40°30'

TV-14 TV-11

TV-05

TV-04

TV-03

TV-02

QV-15

QV-13

QV-12

QV-11

QV-10

QV-08

QV-07

QV-05

LU-13

LU-12

QV-15

QV-13

QV-12

QV-11

QV-10

LU-12

LASSEN VOLCANIC

NATIONAL

PARK

299

89

89

44

Burney

Manzanita Lake

Fall River Mills

TEHAMA CO PLUMAS CO

LASSEN VOLCANIC

NATIONAL

PARK

Pit

River

TV-14 TV-11

TV-02

QV-08

QV-07

QV-05

Burney

SHASTA CO

Pit River

TV-05

TV-04

Fall River Mills

LASSEN CO

Hat Creek

Manzanita Lake

89

E

Fall River

0 5 10 MILES

0 5 10 KILOMETERS

Base modified from U.S. Geological Survey and other

Federal and State digital data, various scales 

Albers Equal Area Projection

North American Datum of 1983

EXPLANATION

Tertiary Volcanic Areas (TV)

Quaternary Volcanic Areas (QV)

Eastside Sacramento Valley (ES)

Cascade Range and Modoc Plateau

 Low-Use Basins (LU)

QV-13

Other areas in the Cascade Range and

 Modoc Plateau Low-Use Basins

Other hydrogeologic provinces

National monument and park

Fault (Saucedo and others, 2000)

U.S. Geological Survey-grid well

Other California Department of Public 

 Health well

Study-area buffers

Study areas County boundary

Figure A1. —Continued
94 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

sac15-0550_fig A1f.ai

Base modified from U.S. Geological Survey and other

Federal and State digital data, various scales 

Albers Equal Area Projection

North American Datum of 1983

"

"

"

"

North Fork Feather River

Chico Monocline

TV-13

TV-12

TV-10

TV-09

QV-14 TV-06 QV-09 LU-15

LU-14

ES-15

ES-14

ES-13

ES-12

TV-13

TV-12

ES-11

ES-10

ES-09

ES-08

ES-07

ES-06

ES-05

ES-04

ES-03

ES-08

ES-07

ES-02

ES-01

Chico

Magalia

Chester

TV-10

TV-09

QV-14 TV-06 QV-09 LU-15

LU-14

PLUMAS CO

TEHAMA CO

LASSEN CO

Chester

Paradise ES-15

ES-14

ES-11

ES-09 ES-06

ES-05

ES-04

ES-02

ES-01

BUTTE CO

Chico

Magalia

Paradise

121°45' 121°30' 121°15' 121°

40°15'

40°

39°45'

36

Lake

Almanor

Lake

Oroville

70 99

Mill Creek

Butte Creek

Deer Creek

Big

Chico Creek

Mud Creek

Butt Creek

Rock Creek

Paynes Creek

South Fork Battle Creek

North Fork Antelope Creek

Little Dry

Creek

F

0 5 10 MILES

0 5 10 KILOMETERS

EXPLANATION

Tertiary Volcanic Areas (TV)

Quaternary Volcanic Areas (QV)

Eastside Sacramento Valley (ES)

Cascade Range and Modoc Plateau

 Low-Use Basins (LU)

ES-04

Other areas in the Cascade Range and

 Modoc Plateau Low-Use Basins

Other hydrogeologic provinces

Fault (Saucedo and others, 2000)

County boundary

U.S. Geological Survey-grid well

Other California Department of Public 

 Health well

Study-area buffers

Study areas

Figure A1. —Continued
Appendix A: Attribution of Potential Explanatory Factors 95

sac15--0550_fig A1g.ai

"

"

Eagle

Lake

Honey Lake

TV-08

QV-06

HL-15

HL-14

HL-13

HL-12

HL-11

HL-10

HL-09

HL-08

HL-07

HL-06

HL-05

HL-04

HL-03

HL-02

HL-01

TV-08

QV-06

HL-15

HL-14

HL-13

HL-12

HL-11

HL-10

HL-09

HL-08

HL-07

HL-06

HL-05

HL-04

HL-03

HL-02

HL-01

HONEY LAKE FAULT ZONE

LASSEN CO

PLUMAS CO

Herlong

Susanville

Herlong

Susanville

120°45' 120°30' 120°15' 120°

40°45'

40°30'

40°15'

40°

395

395

139

Willow Creek

Indian Creek

Pete Creek

Smoke Creek

Secret Creek

DeepCreek

Long Valley Creek

Balls Canyon

Bull Creek

Rush Creek

36

G

Last Chance Creek

Susan River

0 5 10 MILES

0 5 10 KILOMETERS

EXPLANATION

Other areas in the Cascade Range and HL-12

 Modoc Plateau Low-Use Basins

Other hydrogeologic provinces

Fault (Saucedo and others, 2000)

U.S. Geological Survey-grid well

Other California Department of Public 

 Health well

Study-area buffers

Study areas

County boundary Tertiary Volcanic Areas (TV)

Quaternary Volcanic Areas (QV)

Cascade Range and Modoc Plateau

 Low-Use Basins (LU)

Honey Lake Valley (HL)

Base modified from U.S. Geological Survey and other

Federal and State digital data, various scales 

Albers Equal Area Projection

North American Datum of 1983

Figure A1. —Continued
96 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Table A1. Land use percentages, septic-tank density, underground storage tank density and geology classification for U.S. Geological 

Survey (USGS)-grid wells, Cascade Range and Modoc Plateau (CAMP) study unit, 2010, California Groundwater Ambient Monitoring and 

Assessment Program (GAMA) Priority Basin Project.

[Well identification numbers: ES, Sacramento Valley Eastside study area; HL, Honey Lake Valley study area; LU, Cascade Range and Modoc Plateau LowUse Basins study area; QV, Quaternary Volcanic Areas study area; SH, Shasta Valley and Mount Shasta Volcanic Area study area; TV, Tertiary Volcanic Areas 

study area. Aquifer lithology class: G, granitic rocks; M, metamorphic rocks; S, sedimentary deposits; V, volcanic rocks. Other abbreviations: tanks/km2, 

number of tanks per square kilometer; UST, leaking or formerly leaking underground-storage tank; WCR, well completion report]

Well 

identification 

number

Geology classification1 Land-use percentages2

Septic density3

(tanks/km2)

UST density4

(tanks/km2) Aquifer 

lithology class Source of data1 Agricultural 

(percent)

Natural 

(percent) Urban (percent)

Sacramento Valley Eastside study area

CAMP-ES-01 V WCR 0 44 56 112 0.06

CAMP-ES-02 V WCR 0 36 64 39.8 0.04

CAMP-ES-03 V field 0 92 8 0.4 0.00

CAMP-ES-04 S WCR 21 58 21 4.7 0.01

CAMP-ES-05 S WCR 0 0 100 256 0.87

CAMP-ES-06 V WCR 0 99 1 3.1 0.01

CAMP-ES-07 V WCR 0 99 1 0.4 0.00

CAMP-ES-08 V WCR 0 100 0 0.4 0.00

CAMP-ES-09 VS WCR 0 100 0 1.7 0.02

CAMP-ES-10 S WCR 0 100 0 4.0 0.04

CAMP-ES-11 V WCR 0 92 8 2.9 0.01

CAMP-ES-12 VS WCR 0 76 24 1.7 0.02

CAMP-ES-13 VS map 0 100 0 1.7 0.02

CAMP-ES-14 VS map 0 100 0 1.7 0.02

CAMP-ES-15 V WCR 5 61 34 1.7 0.03

Honey Lake Valley study area

CAMP-HL-01 S WCR 0 100 0 0.4 0.00

CAMP-HL-02 GS WCR 2 93 5 2.2 0.01

CAMP-HL-03 S map 27 43 30 1.7 0.01

CAMP-HL-04 S WCR 63 14 23 4.2 0.01

CAMP-HL-05 S WCR 27 42 31 4.2 0.01

CAMP-HL-06 S WCR 37 54 9 4.2 0.00

CAMP-HL-07 S WCR 4 94 2 1.3 0.00

CAMP-HL-08 S WCR 12 88 0 1.1 0.00

CAMP-HL-09 S WCR 0 42 58 0.0 0.00

CAMP-HL-10 S WCR 0 66 34 0.0 0.00

CAMP-HL-11 S WCR 27 52 21 1.3 0.00

CAMP-HL-12 GS WCR 1 69 30 0.7 0.00

CAMP-HL-13 GS map 0 100 0 1.1 0.00

CAMP-HL-14 S map 36 64 0 1.1 0.00

CAMP-HL-15 S WCR 1 63 36 0.5 0.00

Cascade Range and Modoc Plateau Low-Use Basins study area

CAMP-LU-01 V WCR 44 10 46 0.2 0.61

CAMP-LU-02 V WCR 64 1 36 0.2 0.00

CAMP-LU-03 VS WCR 0 13 87 1.2 0.00

CAMP-LU-04 S WCR 38 20 43 0.4 0.00

CAMP-LU-05 VS WCR 40 7 53 1.2 1.82
Appendix A: Attribution of Potential Explanatory Factors 97

Table A1. Land use percentages, septic-tank density, underground storage tank density and geology classification for U.S. Geological 

Survey (USGS)-grid wells, Cascade Range and Modoc Plateau (CAMP) study unit, 2010, California Groundwater Ambient Monitoring and 

Assessment Program (GAMA) Priority Basin Project.—Continued

[Well identification numbers: ES, Sacramento Valley Eastside study area; HL, Honey Lake Valley study area; LU, Cascade Range and Modoc Plateau LowUse Basins study area; QV, Quaternary Volcanic Areas study area; SH, Shasta Valley and Mount Shasta Volcanic Area study area; TV, Tertiary Volcanic Areas 

study area. Aquifer lithology class: G, granitic rocks; M, metamorphic rocks; S, sedimentary deposits; V, volcanic rocks. Other abbreviations: tanks/km2, 

number of tanks per square kilometer; UST, leaking or formerly leaking underground-storage tank; WCR, well completion report]

Well 

identification 

number

Geology classification1 Land-use percentages2

Septic density3

(tanks/km2)

UST density4

(tanks/km2) Aquifer 

lithology class Source of data1 Agricultural 

(percent)

Natural 

(percent) Urban (percent)

Cascade Range and Modoc Plateau Low-Use Basins study area—Continued

CAMP-LU-06 V WCR 55 19 26 0.1 0.00

CAMP-LU-07 V WCR 72 4 24 0.2 0.01

CAMP-LU-08 VS map 0 96 4 0.1 0.02

CAMP-LU-09 VS map 24 75 1 0.2 0.00

CAMP-LU-10 VS map 1 78 21 0.1 0.05

CAMP-LU-11 VS map 27 44 29 0.2 0.00

CAMP-LU-12 V WCR 0 87 13 1.3 0.00

CAMP-LU-13 S WCR 13 42 46 0.2 0.00

CAMP-LU-14 VS WCR 0 93 7 0.7 0.00

CAMP-LU-15 VS map 0 73 27 1.9 0.01

Quaternary Volcanic area study area

CAMP-QV-01 V field 0 100 0 0.0 0.00

CAMP-QV-02 V field 0 100 0 0.1 0.02

CAMP-QV-03 V map 0 93 7 0.1 0.00

CAMP-QV-04 VS WCR 0 100 0 0.1 0.05

CAMP-QV-05 V map 0 17 83 39.1 0.00

CAMP-QV-06 V map 1 87 11 1.2 0.04

CAMP-QV-07 V WCR 0 97 3 0.3 0.01

CAMP-QV-08 V WCR 0 66 34 0.5 0.00

CAMP-QV-09 V field 0 84 16 1.0 0.00

CAMP-QV-10 V WCR 0 100 0 1.1 0.01

CAMP-QV-11 V WCR 0 62 38 1.8 0.00

CAMP-QV-12 VS WCR 0 100 0 1.5 0.00

CAMP-QV-13 V WCR 0 100 0 0.7 0.01

CAMP-QV-14 V WCR 0 100 0 0.4 0.00

CAMP-QV-15 V WCR 0 95 5 1.4 0.00

Shasta Valley and Mount Shasta Volcanic Area study area

CAMP-SH-01 M WCR 0 45 55 6.3 0.29

CAMP-SH-02 V WCR 1 81 18 1.2 0.02

CAMP-SH-03 MS map 55 43 2 1.6 0.01

CAMP-SH-04 V WCR 0 84 16 2.1 0.01

CAMP-SH-05 V field 0 100 0 1.8 0.36

CAMP-SH-06 S WCR 0 7 93 18.8 0.62

CAMP-SH-07 V field 0 100 0 1.9 0.03

CAMP-SH-08 V map 15 74 11 1.1 0.02

CAMP-SH-09 MS WCR 14 63 23 2.4 0.02

CAMP-SH-10 VS WCR 0 97 3 1.3 0.02
98 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Table A1. Land use percentages, septic-tank density, underground storage tank density and geology classification for U.S. Geological 

Survey (USGS)-grid wells, Cascade Range and Modoc Plateau (CAMP) study unit, 2010, California Groundwater Ambient Monitoring and 

Assessment Program (GAMA) Priority Basin Project.—Continued

[Well identification numbers: ES, Sacramento Valley Eastside study area; HL, Honey Lake Valley study area; LU, Cascade Range and Modoc Plateau LowUse Basins study area; QV, Quaternary Volcanic Areas study area; SH, Shasta Valley and Mount Shasta Volcanic Area study area; TV, Tertiary Volcanic Areas 

study area. Aquifer lithology class: G, granitic rocks; M, metamorphic rocks; S, sedimentary deposits; V, volcanic rocks. Other abbreviations: tanks/km2, 

number of tanks per square kilometer; UST, leaking or formerly leaking underground-storage tank; WCR, well completion report]

Well 

identification 

number

Geology classification1 Land-use percentages2

Septic density3

(tanks/km2)

UST density4

(tanks/km2) Aquifer 

lithology class Source of data1 Agricultural 

(percent)

Natural 

(percent) Urban (percent)

Shasta Valley and Mount Shasta Volcanic Area study area—Continued

CAMP-SH-11 VS map 75 22 3 1.3 0.02

CAMP-SH-12 V map 37 63 0 0.8 0.01

CAMP-SH-13 V field 7 93 0 1.3 0.01

CAMP-SH-14 V WCR 0 98 2 1.1 0.01

CAMP-SH-15 MS map 90 10 1 3.3 0.02

Tertiary Volcanic Areas study area

CAMP-TV-01 V map 24 76 0 0.6 0.00

CAMP-TV-02 V map 0 100 0 0.3 0.00

CAMP-TV-03 V field 0 100 0 1.8 0.01

CAMP-TV-04 V map 19 81 0 0.3 0.00

CAMP-TV-05 V WCR 2 72 26 0.1 0.00

CAMP-TV-06 V WCR 0 94 6 10.8 0.02

CAMP-TV-07 V WCR 0 100 0 0.0 0.00

CAMP-TV-08 V map 0 96 4 1.3 0.00

CAMP-TV-09 VS WCR 0 73 27 18.4 0.03

CAMP-TV-10 V map 0 66 34 5.7 0.00

CAMP-TV-11 V WCR 0 88 12 1.5 0.00

CAMP-TV-12 V WCR 0 100 0 1.2 0.02

CAMP-TV-13 V map 0 100 0 1.2 0.02

CAMP-TV-14 V WCR 0 98 2 1.2 0.00

CAMP-TV-15 VM WCR 0 98 2 0.5 0.01

1

Classification based on lithology of screened or open interval as described in the California Department of Water Resources well completion report (WCR), 

if available. For wells without WCRs, lithology was inferred from geologic map of California (Jennings, 1977; Saucedo and others, 2000) and well depth 

information, if available (map). For most springs, lithology was estimated from field observations (field). Wells can have screened or open intervals in more than 

one lithology.

2

Land-use percentages within 500 meters of well site (Johnson and others, 2009).

3

Septic-tank density within 500 meters of well site, based on 1990 U.S. Census data.

4

Leaking (or formerly leaking) underground-storage tank density within a 500-meter radius of well site (California State Water Resources Control Board, 

2007).
Appendix A: Attribution of Potential Explanatory Factors 99

Land Use

Land use was classified by using an enhanced version of 

the satellite-derived (30-meter 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. About two-thirds 

of the Cascade Range and Modoc Plateau (CAMP) study-unit 

wells had groundwater classified as mixed or pre-modern age, 

indicating presence of groundwater recharged many decades 

to perhaps thousands of years ago (see ‘Well Depth and 

Groundwater Age’ section); therefore, land use patterns from 

several decades ago were thought more likely to be relevant 

to the groundwater samples than current land-use patterns. 

The imagery was classified into 25 land-cover classifications 

(Nakagaki and Wolock, 2005). For this study, these 25 landcover classifications were condensed into 3 principal land-use 

categories: urban, agricultural, and natural. Land-use statistics 

for the study unit, study areas, and areas within a 500-meter 

radius around each well (500-meter buffers) were calculated 

for each category by using ArcGIS. Buffers consisting of 

a 500-meter radius circle centered on each well have been 

shown to be effective at correlating land use with occurrence 

of volatile organic compounds (VOCs), pesticides, and nitrate 

for the purposes of statistical characterization (for example, 

Rupert, 2003; Johnson and Belitz, 2009). Land-use data for 

USGS-grid wells are listed in table A1.

The 25 land-cover classes defined by Nakagaki and 

Wolock (2005) were 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 includes cultivated 

crops and land used for pasture or hay. Natural land includes 

everything else. In this classification, open-range grazing, such 

as that practiced on U.S. Forest Service and U.S. Bureau of 

Land Management lands in the Cascade Range and Modoc 

Plateau (CAMP) study unit, was classified as natural land, not 

agricultural land.

Septic Systems and Underground Storage Tanks 

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 A1).

The density of leaking or formerly leaking underground 

storage tanks (USTs) was determined from the locations 

of tanks in the State Water Resources Control Board’s 

(SWRCB’s) GeoTracker database of environmental cleanup 

sites (California State Water Resources Control Board, 

2007). The density of USTs was calculated by using Theissen 

polygons (Tyler Johnson, USGS, written commun., 2007). 

The boundaries of the Theissen polygon around a particular 

UST was created by bisecting the linear distances between the 

UST and all the surrounding USTs. The density of USTs in 

the polygon was the number of tanks in the polygon (nearly 

always one) divided by the area of the polygon in square 

kilometers. A well was assigned the UST density of the 

Theissen polygon in which it was located (table A1).

Well Construction Information and Hydrologic 

Conditions

Well-construction data were obtained primarily from 

CDWR well completion reports. In locations where CDWR 

well completion reports were not available, well-construction 

data were obtained from ancillary records of well owners 

or the USGS National Water Information System database 

(http://waterdata.usgs.gov/nwis). Well-identification 

verification procedures are described by Shelton and others 

(2013). Well depths and depths to the top and bottom of the 

screened or open interval for USGS-grid wells are listed in 

table A2. 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 not assigned a value for well depth or 

depths to top and bottom of the screened or open interval. 

Eleven wells had data for well depth, but not for depth 

to the bottom of the screened or open interval (table A2). For 

these wells, the depth to the bottom of the screened interval 

was assumed to be the same as the well depth for statistical 

tests and for graphical presentations of the data. Of the 

63 wells having data for well depth and depth to the bottom of 

the screened or open intervals, the two depths were equal for 

48 of the wells (76 percent). 

Wells were classified as production wells or springs, 

and production wells were further classified as shallow, 

overlapping, or deep wells (table A2). Production wells pump 

the groundwater from the aquifer to a distribution system. 

Wells with depths less than 200 feet below land surface (ft bls) 

were defined as shallow; wells with depth to the top of the 

screened or open interval greater than 200 ft bls were defined 

as deep; and wells with depth to the top of the screened or 

open interval less than 200 ft bls and total depth of greater 

than 200 ft bls were defined as overlapping. The name 

‘overlapping’ refers to the fact that these wells were screened 

or open in both the shallow and deep parts of the primary 

aquifer system. 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. 
100 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Table A2. Hydrologic conditions and well construction information for U.S. Geological Survey (USGS)-grid wells, Cascade Range and 

Modoc Plateau (CAMP) study unit, 2010, California Groundwater Ambient Monitoring and Assessment Program (GAMA) Priority Basin 

Project.

[Depth class: deep, top of screened or open interval >200 ft; overlapping, top of screened or open interval <200 ft and bottom of screened or open interval 

>200 ft; shallow, bottom of screened or open interval <200 ft; unknown, no construction information available for the well. Other abbreviations: >, greater 

than; <, less than; ft; feet; LSD, land-surface datum; na, no data available; —, depths not defined for springs]

Well 

identification 

number

Hydrologic conditions Well construction

Aridity index1

(dimensionless)

Elevation of LSD 

(ft above 

NAVD88)2

Depth class3 Well depth 

(ft below LSD)

Top of screened 

or open interval 

(ft below LSD)

Bottom of 

screened or open 

interval 

(ft below LSD)4

Eastside Sacramento Valley study area (ES)

CAMP-ES-01 1.35 2,351 Overlapping 400 65 400

CAMP-ES-02 1.14 1,686 Overlapping 325 60 325

CAMP-ES-03 0.75 2,267 Spring — — —

CAMP-ES-04 0.62 383 Overlapping 475 160 475

CAMP-ES-05 1.25 1,934 Deep 550 395 525

CAMP-ES-06 1.37 2,602 Deep 930 823 930

CAMP-ES-07 0.83 3,089 Shallow 80 60 80

CAMP-ES-08 0.87 3,328 Deep 386 346 386

CAMP-ES-09 0.99 1,524 Deep 730 500 730

CAMP-ES-10 1.02 1,520 Deep 770 495 760

CAMP-ES-11 1.33 2,306 Deep 480 440 480

CAMP-ES-12 1.28 2,821 Overlapping 995 93 995

CAMP-ES-13 1.16 2,054 Shallow 96 na na

CAMP-ES-14 0.67 413 Overlapping or deep 5

>285 na na

CAMP-ES-15 0.62 294 Deep 480 206 470

Honey Lake Valley study area (HL)

CAMP-HL-01 0.19 4,031 Overlapping 300 50 300

CAMP-HL-02 0.38 4,344 Overlapping 240 140 240

CAMP-HL-03 0.36 4,144 Unknown 5

>180 na na

CAMP-HL-04 0.33 4,097 Overlapping 600 55 600

CAMP-HL-05 0.36 4,139 Shallow 190 190 190

CAMP-HL-06 0.32 4,106 Shallow 56 40 56

CAMP-HL-07 0.21 4,011 Overlapping 200 60 200

CAMP-HL-08 0.20 4,006 Overlapping 208 60 208

CAMP-HL-09 0.22 4,133 Deep 530 220 520

CAMP-HL-10 0.22 4,123 Overlapping or deep 545 na na

CAMP-HL-11 0.23 4,050 Overlapping 225 165 205

CAMP-HL-12 0.26 4,223 Overlapping 246 50 246

CAMP-HL-13 0.41 4,474 Shallow 130 na na

CAMP-HL-14 0.27 4,093 Shallow 102 na na

CAMP-HL-15 0.24 4,058 Shallow 120 110 120

Cascade Range and Modoc Plateau Low-Use Basins study area (LU)

CAMP-LU-01 0.39 4,247 Deep 1,236 840 1,236

CAMP-LU-02 0.29 4,038 Deep 2,664 2,546 2,664

CAMP-LU-03 0.30 4,379 Deep 670 350 670

CAMP-LU-04 0.32 4,666 Overlapping 353 126 350
Appendix A: Attribution of Potential Explanatory Factors 101

Well 

identification 

number

Hydrologic conditions Well construction

Aridity index1

(dimensionless)

Elevation of LSD 

(ft above 

NAVD88)2

Depth class3 Well depth 

(ft below LSD)

Top of screened 

or open interval 

(ft below LSD)

Bottom of 

screened or open 

interval 

(ft below LSD)4

Cascade Range and Modoc Plateau Low-Use Basins study area (LU)—Continued

CAMP-LU-05 0.30 4,391 Deep 500 220 500

CAMP-LU-06 0.40 4,317 Overlapping 220 180 220

CAMP-LU-07 0.32 4,259 Shallow 87 59 87

CAMP-LU-08 1.53 4,184 Spring — — —

CAMP-LU-09 0.34 4,400 Overlapping 310 100 310

CAMP-LU-10 0.29 4,052 Overlapping 343 100 338

CAMP-LU-11 0.39 4,225 Shallow 184 na na

CAMP-LU-12 1.11 4,127 Shallow 146 54 146

CAMP-LU-13 0.45 4,124 Shallow 6

345 95 110

CAMP-LU-14 0.80 4,549 Overlapping 235 162 222

CAMP-LU-15 0.80 4,528 Deep 371 228 360

 Quaternary Volcanic Areas study area (QV)

CAMP-QV-01 1.38 6,887 Spring — — —

CAMP-QV-02 1.74 4,919 Spring — — —

CAMP-QV-03 0.32 5,014 Deep 740 400 740

CAMP-QV-04 0.31 4,090 Deep 325 211 325

CAMP-QV-05 0.54 3,200 Deep 226 226 226

CAMP-QV-06 0.37 4,197 Deep 5

≥800 5800 5≥800

CAMP-QV-07 0.67 3,258 Deep 300 250 300

CAMP-QV-08 0.48 3,218 Overlapping 244 184 244

CAMP-QV-09 0.81 4,985 Spring — — —

CAMP-QV-10 1.01 2,260 Shallow 174 47 174

CAMP-QV-11 0.93 3,338 Shallow 170 110 170

CAMP-QV-12 0.85 2,588 Overlapping 232 3 232

CAMP-QV-13 0.90 2,957 Shallow 184 164 184

CAMP-QV-14 1.31 4,715 Deep 465 240 465

CAMP-QV-15 1.09 3,728 Shallow 150 22 150

Shasta Valley and Mount Shasta Volcanic Area study area (SH)

CAMP-SH-01 0.48 2,744 Shallow 120 12 120

CAMP-SH-02 0.82 2,952 Overlapping 232 80 232

CAMP-SH-03 0.81 2,779 Shallow 107 na na

CAMP-SH-04 0.90 3,721 Deep 450 200 450

CAMP-SH-05 0.79 3,778 Spring — — —

CAMP-SH-06 1.07 3,684 Deep 328 288 328

CAMP-SH-07 1.32 4,465 Spring — — —

CAMP-SH-08 0.70 3,302 Unknown 5

>40 na na

Table A2. Hydrologic conditions and well construction information for U.S. Geological Survey (USGS)-grid wells, Cascade Range and 

Modoc Plateau (CAMP) study unit, 2010, California Groundwater Ambient Monitoring and Assessment Program (GAMA) Priority Basin 

Project.—Continued

[Depth class: deep, top of screened or open interval >200 ft; overlapping, top of screened or open interval <200 ft and bottom of screened or open interval 

>200 ft; shallow, bottom of screened or open interval <200 ft; unknown, no construction information available for the well. Other abbreviations: >, greater 

than; <, less than; ft; feet; LSD, land-surface datum; na, no data available; —, depths not defined for springs]
102 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Well 

identification 

number

Hydrologic conditions Well construction

Aridity index1

(dimensionless)

Elevation of LSD 

(ft above 

NAVD88)2

Depth class3 Well depth 

(ft below LSD)

Top of screened 

or open interval 

(ft below LSD)

Bottom of 

screened or open 

interval 

(ft below LSD)4

Shasta Valley and Mount Shasta Volcanic Area study area (SH)—Continued

CAMP-SH-09 0.55 2,576 Overlapping 230 30 225

CAMP-SH-10 0.71 2,841 Overlapping 309 132 267

CAMP-SH-11 0.65 2,762 Shallow or overlapping 9

>52 52 na

CAMP-SH-12 0.47 2,642 Shallow 150 na na

CAMP-SH-13 0.49 2,604 Spring — — —

CAMP-SH-14 1.02 3,844 Shallow 140 120 140

CAMP-SH-15 0.49 2,568 Shallow 7

121 na na

Tertiary Volcanic Areas study area (TV)

CAMP-TV-01 0.51 2,690 Overlapping 218 175 218

CAMP-TV-02 0.67 2,953 Overlapping or deep 7

>450 na na

CAMP-TV-03 0.68 978 Spring8 — — —

CAMP-TV-04 0.83 3,455 Overlapping or deep 290 na na

CAMP-TV-05 0.44 4,266 Deep 300 210 290

CAMP-TV-06 0.90 4,823 Deep 380 305 380

CAMP-TV-07 0.63 6,262 Overlapping 230 50 230

CAMP-TV-08 0.43 5,173 Overlapping 200 na na

CAMP-TV-09 0.89 4,564 Overlapping 403 158 400

CAMP-TV-10 0.95 4,551 Overlapping or deep 5

>300 na na

CAMP-TV-11 1.60 3,767 Overlapping 202 87 202

CAMP-TV-12 1.79 4,321 Overlapping 210 170 210

CAMP-TV-13 1.73 4,220 Shallow 5

135 na na

CAMP-TV-14 1.61 3,284 Shallow 175 155 170

CAMP-TV-15 0.43 2,273 Overlapping 275 54 275

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.

3

Springs have no vertically drilled hole, and groundwater reaches the surface without a pump. The classes shallow, overlapping, and deep apply to wells. 

Depth class is given as ‘overlapping or deep’ when the well depth is greater than 200 ft, and no data are available for depth to top of screened or open interval. 

Depth class is given as ‘overlapping or shallow’ when depth to top of screened or open interval is less than 200 ft, and no data are available for well depth.

4

Eleven wells did not have data for depth to the bottom of the screened or open interval, but did have data for well depth. Well-depth data were used to represent data for the depth of the bottom of the screened or open interval for these wells for statistical tests and for graphical presentations.

5

Estimated from well-owner records; no driller’s log available.

6

Driller’s log (1978) gives initial well depth as 500 ft; well owner provided video log (2005) showing depth of 345 ft.

7

Estimated from measurement of well depth by USGS; no driller’s log available.

8

Conveyance from the TV-03 spring source to the point of use is an open channel.

9

Driller’s log (1972) reports installation of sanitary seal to depth of 52 ft. Owner reports hole depth and well depth greater than 52 ft.

Table A2. Hydrologic conditions and well construction information for U.S. Geological Survey (USGS)-grid wells, Cascade Range and 

Modoc Plateau (CAMP) study unit, 2010, California Groundwater Ambient Monitoring and Assessment Program (GAMA) Priority Basin 

Project.—Continued

[Depth class: deep, top of screened or open interval >200 ft; overlapping, top of screened or open interval <200 ft and bottom of screened or open interval 

>200 ft; shallow, bottom of screened or open interval <200 ft; unknown, no construction information available for the well. Other abbreviations: >, greater 

than; <, less than; ft; feet; LSD, land-surface datum; na, no data available; —, depths not defined for springs]
Appendix A: Attribution of Potential Explanatory Factors 103

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; table A2):

aridity index averageannual precipitation

averageannual 

= evapotranspiration (A1)

Greater values of the index correspond to wetter 

conditions. Values less than 0.05 are defined as hyper-arid, 

0.05–0.20 as arid, 0.20–0.50 as semi-arid, 0.50–0.65 as dry 

sub-humid, 0.65–1.00 as humid, and greater than1.00 as wet. 

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, California Water Science Center, 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. The study unit had a large range 

in elevation of well sites—approximately 300 ft to 7,000 ft. 

Elevations were obtained from U.S. Geological Survey digital 

elevation GIS coverage and are reported in feet relative to 

NAVD 88 (table A2).

Groundwater Age Classification

Groundwater dating techniques provide estimates of 

the time elapsed since a given parcel of groundwater entered 

the saturated zone and was no longer in contact with the 

atmosphere. The techniques used in this report to estimate 

groundwater residence times or ‘age’ were those based on 

tritium (3

H) (for example, Tolstikhin and Kamensky, 1969; 

Torgersen and others, 1979), and carbon-14 (14C) activity (for 

example, Vogel and Ehhalt, 1963; Plummer and others, 1993). 

Tritium (3H) is a short-lived radioactive isotope 

of hydrogen with a half-life of 12.32 years (Lucas and 

Unterweger, 2000). It is produced naturally in the atmosphere 

from the interaction of cosmogenic radiation with nitrogen 

(Craig and Lal, 1961), or, anthropogenically, by above-ground 

nuclear weapons testing (peak 1952 to 1963, Michel, 1989) 

and by the operation of nuclear reactors. Tritium enters the 

hydrologic cycle following exchange with protium (1

H) in 

water to form tritiated water. Above-ground nuclear testing 

resulted in a large increase in tritium values in precipitation, 

beginning in about 1952 and peaking in 1963 at values of over 

1,000 tritium units (TU) in the northern hemisphere (Michel, 

1989).

Previous investigations have used a range of tritium 

values from 0.2 to 1.0 TU as minimum thresholds for 

indicating the presence of water that has been in contact with 

the atmosphere since 1952 (Michel, 1989; Plummer and 

others, 1993; Michel and Schroeder, 1994; Clark and Fritz, 

1997; Manning and others, 2005; Landon and others, 2010). 

For samples collected for the CAMP study unit in 2010, 

tritium values greater than 0.5 TU were defined as indicating 

a predominance of groundwater recharged since 1952. The 

threshold value of 0.5 TU was selected because background 

tritium values in California precipitation at the latitudes 

and longitudes corresponding to the CAMP study unit are 

approximately 4 to 5 TU (Robert Michel, USGS, written 

commun., 2012). Radioactive decay of tritium in water with a 

tritium value of 5 TU in 1950 would result in a tritium value 

of 0.2 TU in 2010. 

Carbon-14 (14C) is a widely used chronometer that is 

based on the radiocarbon content of organic and inorganic 

carbon. 14C is formed in the atmosphere by the interaction 

of cosmic-ray neutrons with nitrogen and, to a lesser degree, 

with oxygen and carbon. 14C is incorporated into carbon 

dioxide and mixed throughout the atmosphere. The carbon 

dioxide dissolves in precipitation, which eventually recharges 

the aquifer. As a result, dissolved inorganic carbon species 

(primarily carbonic acid, bicarbonate, and carbonate) are 

commonly used for 14C dating of groundwater. 14C activity in 

groundwater reflects the time elapsed since groundwater was 

last exposed to the atmospheric 14C source. 14C has a half-life 

of 5,730 years and can be used to estimate groundwater ages 

ranging from 1,000 to approximately 30,000 years before the 

present (Clark and Fritz, 1997). 14C data can be reported in units of percent Modern 

(pM) or in units of percent modern carbon (pmc). 14C data 

for the CAMP study unit in Shelton and others (2013) are 

given in pM units, as reported by the analyzing laboratory, 

and have been normalized for carbon isotopic fractionation 

based on a δ13C value of –25 per mil (parts per thousand). The 

un-normalized 14C data in pmc units are used in this report. 

Data were converted from pM to pmc by using this equation, 

derived from Plummer and others (2004):

pmC

pM C

=

+





 



 1

1000

0 975

13 2

2



. (A2)

where

pM is the 14C value in units of pM, and

δ13C is the measured 13C composition in units of 

per mil
104 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

The 14C pmc values in this study are referred to as 

“uncorrected” because they are not adjusted to consider waterrock interactions, such as exchanges with sedimentary sources 

of carbon (Fontes and Garnier, 1979). Measured values of 

percent modern carbon can be greater than 100 pmc because 

the definition of the 14C content in “modern” carbon does not 

include the excess 14C produced in the atmosphere by aboveground nuclear weapons testing. For samples collected for 

the CAMP study unit in 2010, 14C values greater than 90 pmc 

were defined as indicating the presence of some groundwater 

recharged since 1952. This threshold of 90 pmc was selected 

because nearly all samples with tritium values less than 

0.50 TU also had 14C values less than 90 pmc. 

The age distributions in groundwater samples were 

classified as pre-modern, modern, or mixed, by using the 

thresholds for tritium and 14C values as shown in table A3. 

Samples with tritium greater than 0.5 TU and 14C greater 

than or equal to 90 pmc were classified as modern; samples 

with tritium less than 0.5 TU were classified as premodern; all other samples were classified as mixed. Tritium 

concentrations, uncorrected 14C percent modern carbon, and 

age classifications for the samples are listed in table A4.

Table A3. Tritium and carbon-14 threshold values used for 

groundwater age classification for the Cascade Range and 

Modoc Plateau study unit, 2010, California Groundwater Ambient 

Monitoring and Assessment Program (GAMA) Priority Basin 

Project.

[Groundwater age classification: mixed, groundwater containing components recharged before and after 1952; modern, groundwater recharged since 

1952; pre-modern, groundwater recharged before 1952. Abbreviations: pmc, 

percent modern carbon; TU, tritium units; <, less than; ≥, greater than or equal 

to]

Age class Number of 

samples

Threshold values

Tritium 

(TU)

Carbon-14 

(pmc)

Pre-modern 33 <0.5 Any

Mixed 20 ≥0.5 <90

Modern 34 ≥0.5 ≥90

Modern or mixed 3 ≥0.5 No data

Well ID 

number Tritium (TU) Carbon-14 (pmc) 

Groundwater age 

classification

Eastside Sacramento Valley study area (ES)

CAMP-ES-01 2.03 106 Modern

CAMP-ES-02 2.13 112 Modern

CAMP-ES-03 0.15 79 Pre-modern

CAMP-ES-04 1.56 88 Mixed

CAMP-ES-05 0.37 91 Pre-modern

CAMP-ES-06 0.47 97 Pre-modern

CAMP-ES-07 1.06 94 Modern

CAMP-ES-08 0.84 94 Modern

CAMP-ES-09 0.34 86 Pre-modern

CAMP-ES-10 0.15 89 Pre-modern

CAMP-ES-11 1.44 100 Modern

CAMP-ES-12 1.91 115 Modern

CAMP-ES-13 2.78 115 Modern

CAMP-ES-14 0.12 95 Pre-modern

CAMP-ES-15 0.87 98 Modern

Honey Lake Valley study area (HL)

CAMP-HL-01 0.00 26 Pre-modern

CAMP-HL-02 0.50 55 Pre-modern

CAMP-HL-03 0.47 68 Pre-modern

CAMP-HL-04 0.21 65 Pre-modern

CAMP-HL-05 1.78 81 Mixed

CAMP-HL-06 2.63 102 Modern

CAMP-HL-07 <0.03 34 Pre-modern

CAMP-HL-08 0.12 56 Pre-modern

CAMP-HL-09 <0.03 59 Pre-modern

CAMP-HL-10 0.31 69 Pre-modern

CAMP-HL-11 1.50 83 Mixed

CAMP-HL-12 0.53 92 Modern

CAMP-HL-13 0.59 101 Modern

CAMP-HL-14 0.03 51 Pre-modern

CAMP-HL-15 0.43 86 Pre-modern

Table A4. Tritium, carbon-14 data, and groundwater age classes, 

Cascade Range and Modoc Plateau (CAMP) study unit, 2010, 

California Groundwater Ambient Monitoring and Assessment 

Program (GAMA) Priority Basin Project.

[Well identification numbers: ES, Sacramento Valley Eastside study area; 

HL, Honey Lake Valley study area; LU, Cascade Range and Modoc Plateau 

Low Use Basins study area; QV, Quaternary Volcanic Areas study area; SH, 

Shasta Valley and Mount Shasta Volcanic Area study area; TV, Tertiary Volcanic Areas study area. Groundwater age classification: mixed, groundwater 

containing components recharged before and after 1952; modern, groundwater 

recharged since 1952; pre-modern, groundwater recharged before 1952. Other 

abbreviations: ID, identification; na, not available; pmc, percent modern 

carbon; TU, tritium units; <, less than]
Appendix A: Attribution of Potential Explanatory Factors 105

Table A4. Tritium, carbon-14 data, and groundwater age classes, Cascade Range and Modoc Plateau (CAMP) study unit, 2010, 

California Groundwater Ambient Monitoring and Assessment Program (GAMA) Priority Basin Project.—Continued

[Well identification numbers: ES, Sacramento Valley Eastside study area; HL, Honey Lake Valley study area; LU, Cascade Range and Modoc Plateau Low 

Use Basins study area; QV, Quaternary Volcanic Areas study area; SH, Shasta Valley and Mount Shasta Volcanic Area study area; TV, Tertiary Volcanic Areas 

study area. Groundwater age classification: mixed, groundwater containing components recharged before and after 1952; modern, groundwater recharged since 

1952; pre-modern, groundwater recharged before 1952. Other abbreviations: ID, identification; na, not available; pmc, percent modern carbon; TU, tritium 

units; <, less than]

Well ID 

number Tritium (TU) Carbon-14 (pmc) 

Groundwater age 

classification

Shasta Valley and Mount Shasta Volcanic Area study area (SH)

CAMP-SH-01 3.07 84 Mixed

CAMP-SH-02 0.87 60 Mixed

CAMP-SH-03 2.66 101 Modern

CAMP-SH-04 0.12 49 Pre-modern

CAMP-SH-05 0.75 50 Mixed

CAMP-SH-06 1.50 73 Mixed

CAMP-SH-07 2.53 110 Modern

CAMP-SH-08 1.12 69 Mixed

CAMP-SH-09 2.13 98 Modern

CAMP-SH-10 2.00 76 Mixed

CAMP-SH-11 2.63 94 Modern

CAMP-SH-12 1.69 94 Modern

CAMP-SH-13 1.75 48 Mixed

CAMP-SH-14 3.57 101 Modern

CAMP-SH-15 1.88 100 Modern

Tertiary Volcanic Areas study area (TV)

CAMP-TV-01 14.32 43 Mixed

CAMP-TV-02 1.09 89 Mixed

CAMP-TV-03 1.69 56 Mixed

CAMP-TV-04 0.15 72 Pre-modern

CAMP-TV-05 0.03 78 Pre-modern

CAMP-TV-06 2.13 80 Mixed

CAMP-TV-07 <0.03 87 Pre-modern

CAMP-TV-08 0.28 53 Pre-modern

CAMP-TV-09 0.09 52 Pre-modern

CAMP-TV-10 1.15 98 Modern

CAMP-TV-11 2.88 94 Modern

CAMP-TV-12 2.72 na Mixed or modern

CAMP-TV-13 2.31 104 Modern

CAMP-TV-14 2.31 109 Modern

CAMP-TV-15 0.25 54 Pre-modern

Well ID 

number Tritium (TU) Carbon-14 (pmc) 

Groundwater age 

classification

Cascade Range and Modoc Plateau Low-Use Basins study area (LU)

CAMP-LU-01 0.00 25 Pre-modern

CAMP-LU-02 0.09 10 Pre-modern

CAMP-LU-03 0.50 59 Pre-modern

CAMP-LU-04 3.19 95 Modern

CAMP-LU-05 0.37 58 Pre-modern

CAMP-LU-06 0.03 33 Pre-modern

CAMP-LU-07 0.94 99 Modern

CAMP-LU-08 4.10 112 Modern

CAMP-LU-09 <0.03 71 Pre-modern

CAMP-LU-10 0.09 85 Pre-modern

CAMP-LU-11 0.68 90 Modern

CAMP-LU-12 3.32 na Mixed or modern

CAMP-LU-13 1.97 92 Modern

CAMP-LU-14 2.35 85 Mixed

CAMP-LU-15 2.47 98 Modern

 Quaternary Volcanic Areas study area (QV)

CAMP-QV-01 2.85 112 Modern

CAMP-QV-02 2.72 113 Modern

CAMP-QV-03 <0.03 6 Pre-modern

CAMP-QV-04 1.84 86 Mixed

CAMP-QV-05 0.62 na Mixed or modern

CAMP-QV-06 3.19 51 Mixed

CAMP-QV-07 1.88 91 Modern

CAMP-QV-08 0.90 89 Mixed

CAMP-QV-09 1.03 92 Modern

CAMP-QV-10 0.47 95 Pre-modern

CAMP-QV-11 1.19 96 Modern

CAMP-QV-12 2.57 110 Modern

CAMP-QV-13 1.25 49 Mixed

CAMP-QV-14 0.06 59 Pre-modern

CAMP-QV-15 3.63 78 Mixed

Although more sophisticated lumped parameter models 

that incorporate mixing could have been used for analyzing 

groundwater age distributions (for example, Cook and Böhlke, 

2000; Jurgens and others, 2012), use of these models to 

understand age mixtures was beyond the scope of this report. 

Instead, classification into modern (primarily recharged after 

1952), mixed, and pre-modern (primarily recharge before 

1952) groundwater age classes was sufficient to provide 

an appropriate and useful characterization for examining 

groundwater quality at the study area and study unit scale.
106 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Geochemical Conditions

Geochemical conditions investigated as potential 

explanatory factors in this report include oxidation-reduction 

characteristics and pH. Oxidation-reduction (redox) conditions 

can 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). As electron acceptors are 

depleted along groundwater flow paths, the typical TEAP 

sequence is oxygen reduction (oxic), followed, in turn, by 

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 significant 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 (Stumm and Morgan, 1996; Appelo and Postma, 

2005). The microbes couple reduction of these inorganic 

species to oxidation of organic matter to produce energy 

needed for metabolism and growth. Groundwater samples can 

contain redox-active chemical species that indicate 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 

sometimes are not be consistent with one another, indicating 

the presence of electrochemical disequilibrium, complicating 

the assessments of redox conditions (Lindberg and Runnels, 

1984; Appelo and Postma, 2005).

In this report, oxidation-reduction conditions were 

represented in two ways: by dissolved oxygen (DO) 

concentrations and by classified oxidation-reduction condition. 

The DO concentrations were measured at USGS-grid wells 

(Shelton and others, 2013). Oxidation-reduction conditions 

were classified on the basis of the DO, nitrate, manganese, 

and iron concentrations by using a modified version of the 

classification scheme of McMahon and Chapelle (2008) and 

Jurgens and others (2009) (tables A5, A6). The modification 

was that the DO threshold for separating oxic from anoxic 

groundwater was increased from 0.5 milligrams per liter 

(mg/L) to 1 mg/L. Anoxic conditions were further classified as 

suboxic, nitrate-reducing, manganese-reducing, or manganeseand iron-reducing. Samples were classified as mixed if the 

DO concentration was greater than or equal to 1 mg/L, and 

manganese or iron concentrations were greater than the 

thresholds for indicating manganese-reducing or iron-reducing 

conditions (table A5, A6).

Table A5. Oxidation-reduction classification system applied to U.S. Geological Survey (USGS)-grid wells from the Cascade Range and 

Modoc Plateau study unit, 2010, California Groundwater Ambient Monitoring and Assessment Program (GAMA) Priority Basin Project.

[Anoxic sub-classes: NO3-red, nitrate-reducing; Mn-red, manganese-reducing; Fe-red, iron-reducing. Units: mg/L, milligrams per liter; μg/L, micrograms per 

liter. Other abbreviations: ≥, greater than or equal to; >, greater than; ≤, less than or equal to; <, less than; Any, any concentration]

Category Number of samples Dissolved oxygen 

(mg/L)

Nitrate, as nitrogen 

(mg/L)

Manganese 

(μg/L)

Iron 

(μg/L)

Oxic class

Oxic 72 ≥1 Any <50 <100

Anoxic classes

Suboxic 7 <1 <0.5 <50 <100

NO3

-red 0 <1 ≥0.5 <50 <100

NO3

-red, Mn-red 1 <1 ≥0.5 ≥50 <100

Mn-red 2 <1 <0.5 ≥50 <100

Fe-red 1 <1 <0.5 <50 ≥100

Mn-red, Fe-red 2 <1 <0.5 ≥50 <100

Mixed classes

Mixed (oxic - anoxic Mn-red) 2 ≥1 Any ≥50 <100

Mixed (oxic - anoxic Fe-red) 1 ≥1 Any Any ≥100

Mixed (oxic - anoxic Mn-red, Fe-red) 2 ≥1 Any ≥50 ≥100
Appendix A: Attribution of Potential Explanatory Factors 107

Table A6. Oxidation-reduction classification, dissolved oxygen concentration, pH, and oxidation-reduction species ratios for arsenic 

and iron, Cascade Range and Modoc Plateau (CAMP) study unit, 2010, California Groundwater Ambient Monitoring and Assessment

(GAMA) Program Priority Basin Project.

[Well identification numbers: CAMP, Cascade Range and Modoc Plateau study unit; ES, Sacramento Valley Eastside study area; HL, Honey Lake Valley study 

area; LU, Cascade Range and Modoc Plateau Low-Use Basins study area; QV, Quaternary Volcanic Areas study area; SH, Shasta Valley and Mount Shasta 

Volcanic Area study area; TV, Tertiary Volcanic Areas study area. Oxidation-reduction classification: See table A5. Anoxic sub-classes: Mn-red, manganesereducing; Fe-red, iron-reducing; NO3

-red, nitrate-reducing. Units: mg/L, milligrams per liter; μg/L, micrograms per liter. Ratios of oxidized to reduced species 

of metals: As(V)/As(III), ratio of arsenic(V) to arsenic(III); Fe(III)/Fe(II), ratio of iron(III) to iron(II). Other abbreviations: >, greater than; <, less than; 

—, concentration too low to measure ratio]

Well 

identification 

number

pH 

(standard units)

Dissolved oxygen 

(mg/L)

Oxidation-reduction classification1 Ratios of oxidized and 

reduced species of metals

Redox class Subclass of 

anoxic/mixed Fe(III)/ Fe(II) As(V)/ As(III)

Eastside Sacramento Valley study area (ES)

CAMP-ES-01 6.2 5.6 Oxic — — >10

CAMP-ES-02 6.2 9.8 Oxic — — —

CAMP-ES-03 7.2 4.7 Oxic — — >10

CAMP-ES-04 7.2 1.3 Oxic — — >10

CAMP-ES-05 6.7 9.8 Oxic — — —

CAMP-ES-06 6.8 10.3 Oxic — — —

CAMP-ES-07 6.6 6.6 Oxic — — >10

CAMP-ES-08 6.6 6.2 Oxic — — —

CAMP-ES-09 7.5 6.0 Oxic — — >10

CAMP-ES-10 6.7 7.4 Oxic — na na

CAMP-ES-11 6.6 7.4 Oxic — — —

CAMP-ES-12 5.8 5.3 Mixed Fe-red >10 —

CAMP-ES-13 6.0 5.5 Oxic — — —

CAMP-ES-14 7.2 6.2 Oxic — — >10

CAMP-ES-15 7.1 6.8 Oxic — — >10

Honey Lake Valley study area (HL)

CAMP-HL-01 8.8 6.7 Oxic — — 2

>10

CAMP-HL-02 7.6 1.0 Oxic — <0.01 >10

CAMP-HL-03 7.7 0.9 Anoxic Suboxic — 2

>10

CAMP-HL-04 7.6 4.2 Oxic — — >10

CAMP-HL-05 7.0 8.3 Oxic — — >10

CAMP-HL-06 6.4 7.8 Oxic — — >10

CAMP-HL-07 7.9 0.6 Anoxic Mn-red 0.03 2

0.4

CAMP-HL-08 9.2 0.4 Anoxic Suboxic <0.01 <0.01

CAMP-HL-09 7.6 6.4 Mixed Mn-red >10 0.8

CAMP-HL-10 7.0 1.5 Mixed Mn-red, Fe-red 0.13 >10

CAMP-HL-11 7.8 <0.2 Anoxic NO3

-red, Mn-red 0.08 3>10

CAMP-HL-12 7.3 4.4 Oxic — — >10

CAMP-HL-13 6.5 7.5 Oxic — — —

CAMP-HL-14 7.8 0.4 Anoxic Suboxic — >10

CAMP-HL-15 8.0 1.1 Oxic — — 3

>10

Cascade Range and Modoc Plateau Low-Use Basins study area (LU)

CAMP-LU-01 8.4 2

0.2 Anoxic Suboxic — —

CAMP-LU-02 7.3 <0.2 Anoxic Fe-red <0.01 —

CAMP-LU-03 8.0 1.5 Oxic — — >10
108 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Table A6. Oxidation-reduction classification, dissolved oxygen concentration, pH, and oxidation-reduction species ratios for arsenic 

and iron, Cascade Range and Modoc Plateau (CAMP) study unit, 2010, California Groundwater Ambient Monitoring and Assessment 

(GAMA) Program Priority Basin Project.—Continued

[Well identification numbers: CAMP, Cascade Range and Modoc Plateau study unit; ES, Sacramento Valley Eastside study area; HL, Honey Lake Valley study 

area; LU, Cascade Range and Modoc Plateau Low-Use Basins study area; QV, Quaternary Volcanic Areas study area; SH, Shasta Valley and Mount Shasta 

Volcanic Area study area; TV, Tertiary Volcanic Areas study area. Oxidation-reduction classification: See table A5. Anoxic sub-classes: Mn-red, manganesereducing; Fe-red, iron-reducing; NO3

-red, nitrate-reducing. Units: mg/L, milligrams per liter; μg/L, micrograms per liter. Ratios of oxidized to reduced species 

of metals: As(V)/As(III), ratio of arsenic(V) to arsenic(III); Fe(III)/Fe(II), ratio of iron(III) to iron(II). Other abbreviations: >, greater than; <, less than; 

—, concentration too low to measure ratio]

Well 

identification 

number

pH 

(standard units)

Dissolved oxygen 

(mg/L)

Oxidation-reduction classification1 Ratios of oxidized and 

reduced species of metals

Redox class Subclass of 

anoxic/mixed Fe(III)/ Fe(II) As(V)/ As(III)

Cascade Range and Modoc Plateau Low-Use Basins study area (LU)—Continued

CAMP-LU-04 7.8 8.5 Oxic — — >10

CAMP-LU-05 7.5 1.9 Oxic — — 2

>10

CAMP-LU-06 7.9 0.2 Anoxic Suboxic <0.01 —

CAMP-LU-07 7.0 5.7 Oxic — — —

CAMP-LU-08 6.3 10.9 Oxic — — —

CAMP-LU-09 8.0 1.6 Oxic — 1.22 >10

CAMP-LU-10 7.8 0.2 Anoxic Mn-red, Fe-red 0.19 —

CAMP-LU-11 7.0 7.0 Oxic — — >10

CAMP-LU-12 6.6 8.9 Oxic — — >10

CAMP-LU-13 6.9 1.0 Mixed Mn-red, Fe-red >10 >10

CAMP-LU-14 7.1 7.9 Oxic — — >10

CAMP-LU-15 7.3 7.2 Oxic — — >10

 Quaternary Volcanic Areas study area (QV)

CAMP-QV-01 5.7 12.0 Oxic — — —

CAMP-QV-02 6.3 11.0 Oxic — — —

CAMP-QV-03 7.3 0.8 Anoxic Mn-red, Fe-red 0.20 —

CAMP-QV-04 7.9 1.0 Mixed Mn-red 0.09 2

1.8

CAMP-QV-05 7.5 10.4 Oxic — — —

CAMP-QV-06 8.5 3.0 Oxic — — >10

CAMP-QV-07 7.6 9.8 Oxic — — —

CAMP-QV-08 7.4 7.3 Oxic — — >10

CAMP-QV-09 7.2 10.0 Oxic — — —

CAMP-QV-10 6.8 7.4 Oxic — — —

CAMP-QV-11 6.4 6.3 Oxic — — —

CAMP-QV-12 6.3 4.6 Oxic — — —

CAMP-QV-13 6.7 8.7 Oxic — — >10

CAMP-QV-14 8.5 3.6 Oxic — 4.55 >10

CAMP-QV-15 6.5 9.8 Oxic — — >10

Shasta Valley and Mount Shasta Volcanic Area study area (SH)

CAMP-SH-01 6.8 5.0 Oxic — — —

CAMP-SH-02 6.6 4.7 Oxic — — 3

>10

CAMP-SH-03 7.3 4.7 Oxic — — >10
Appendix A: Attribution of Potential Explanatory Factors 109

Table A6. Oxidation-reduction classification, dissolved oxygen concentration, pH, and oxidation-reduction species ratios for arsenic 

and iron, Cascade Range and Modoc Plateau (CAMP) study unit, 2010, California Groundwater Ambient Monitoring and Assessment 

(GAMA) Program Priority Basin Project.—Continued

[Well identification numbers: CAMP, Cascade Range and Modoc Plateau study unit; ES, Sacramento Valley Eastside study area; HL, Honey Lake Valley study 

area; LU, Cascade Range and Modoc Plateau Low-Use Basins study area; QV, Quaternary Volcanic Areas study area; SH, Shasta Valley and Mount Shasta 

Volcanic Area study area; TV, Tertiary Volcanic Areas study area. Oxidation-reduction classification: See table A5. Anoxic sub-classes: Mn-red, manganesereducing; Fe-red, iron-reducing; NO3

-red, nitrate-reducing. Units: mg/L, milligrams per liter; μg/L, micrograms per liter. Ratios of oxidized to reduced species 

of metals: As(V)/As(III), ratio of arsenic(V) to arsenic(III); Fe(III)/Fe(II), ratio of iron(III) to iron(II). Other abbreviations: >, greater than; <, less than; 

—, concentration too low to measure ratio]

Well 

identification 

number

pH 

(standard units)

Dissolved oxygen 

(mg/L)

Oxidation-reduction classification1 Ratios of oxidized and 

reduced species of metals

Redox class Subclass of 

anoxic/mixed Fe(III)/ Fe(II) As(V)/ As(III)

Shasta Valley and Mount Shasta Volcanic Area study area (SH)—Continued

CAMP-SH-04 7.2 4.4 Oxic — — >10

CAMP-SH-05 6.4 10.6 Oxic — — >10

CAMP-SH-06 6.5 9.3 Oxic — — —

CAMP-SH-07 6.3 10.8 Oxic — — —

CAMP-SH-08 6.7 4.0 Oxic — — >10

CAMP-SH-09 6.9 5.8 Oxic — — —

CAMP-SH-10 6.8 3.2 Oxic — — >10

CAMP-SH-11 7.2 0.3 Anoxic Suboxic — >10

CAMP-SH-12 7.1 7.2 Oxic — — 2

>10

CAMP-SH-13 6.6 9.3 Oxic — — >10

CAMP-SH-14 6.3 10.5 Oxic — 0.32 —

CAMP-SH-15 7.3 0.2 Anoxic Mn-red 0.21 —

Tertiary Volcanic Areas study area (TV)

CAMP-TV-01 7.9 0.8 Anoxic Suboxic — >10

CAMP-TV-02 7.2 8.2 Oxic — <0.01 >10

CAMP-TV-03 6.9 11.2 Oxic — — >10

CAMP-TV-04 7.6 7.4 Oxic — — >10

CAMP-TV-05 7.5 6.9 Oxic — — —

CAMP-TV-06 8.0 9.4 Oxic — — —

CAMP-TV-07 7.8 7.5 Oxic — — —

CAMP-TV-08 8.3 5.9 Oxic — — >10

CAMP-TV-09 7.5 4.9 Oxic — — >10

CAMP-TV-10 6.2 8.1 Oxic — — —

CAMP-TV-11 7.0 7.9 Oxic — — —

CAMP-TV-12 6.8 8.6 Oxic — 0.15 —

CAMP-TV-13 6.6 8.6 Oxic — — —

CAMP-TV-14 5.8 7.8 Oxic — 1.48 —

CAMP-TV-15 9.3 4.9 Oxic — — 0.29

1

Oxidation-reduction classification criteria are given in table A5.

2

Arsenic concentration between 5 and 10 μg/L (moderate relative-concentration).

3

Arsenic concentration greater than 10 μg/L (high relative-concentration).
110 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Arsenic and iron 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 can provide information about the 

progress of the TEAP involving the constituent. The following 

ratios are reported in table A6:

As As + + 5 3 / (A3)

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 Fe + + 3 2 / (A4)

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).

Total concentrations of As and Fe and concentrations of 

As+3 and Fe+2 were reported by Shelton and others (2013). The 

concentrations of As+5 and Fe+3 were calculated from these 

data by difference. As+5/As+3 was reported as greater than 

10 if the total arsenic concentration was above the reporting 

limit and As+3 was not detected and as less than 0.01 if the 

total arsenic concentration equaled the As+3 concentration. 

Similarly, Fe+3/Fe+2 was reported as greater than 10 if the 

total iron concentrations was above the reporting limit and 

Fe+2 was not detected and as less than 0.01 if the total iron 

concentration equaled the Fe+2 concentration.

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GAMA Priority Basin Project: U.S. Geological Survey 

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Lindberg, R.D., and Runnels, D.D., 1984, Ground water 

redox reactions, An analysis of equilibrium state applied 

to Eh measurements and geochemical modeling: 

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H/3He age data in assessing the susceptibility of wells 

to contamination: Groundwater, v. 43, no. 3, p. 353–367, 

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processes and water quality of selected principal aquifer 

systems: Groundwater, v. 46, no. 2, p. 259–271, 

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agricultural pesticide use in drainage basins using land 

cover maps and county pesticide data: U.S. Geological 

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data, http://water.usgs.gov/lookup/getspatial?nlcde92.

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Glynn, P.D., 1993, Environmental tracers for age-dating 

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Plummer, L.N., Bexfield, L.M., Anderholm, S.K., Sanford, 

W.E., and Busenberg, Eurybiades, 2004, Geochemical 

characterization of ground-water flow in the Santa Fe 

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Groundwater-quality data in the Cascade Range and Modoc 

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Appendix B: Comparison of CDPH and USGS-GAMA Program Data 113

Appendix B: Comparison of California Department of Public Health and 

U.S. Geological Survey-Groundwater Ambient Monitoring and Assessment 

Program Data

Appendix B: Comparison of CDPH and USGS-GAMA Program Data

Major-ion data for the 90 U.S. Geological Survey 

(USGS)-grid wells were compared with major ion data from 

wells in the California Department of Public Health (CDPH) 

database for the Cascade Range and Modoc Plateau (CAMP) 

study unit to evaluate whether the USGS-grid wells were 

representative of the range of groundwater types pumped 

by wells used for municipal and community drinking-water 

supply in the CAMP study unit. The CDPH well dataset 

consisted of all wells with a complete major ion analysis 

between September 16, 2007, and September 16, 2010. If 

multiple analyses were available, the most recent one with an 

acceptable cation-anion balance was selected. The datasets 

were compared by using Piper diagrams (Piper, 1944; Hem, 

1985). Groundwater types were defined on a Piper diagram 

according to the cations and anions present in the greatest 

proportions (fig. B1A).

For electroneutrality, the total concentrations of positive 

charges in a water sample, expressed as milliequivalents of 

cations per liter, must equal the total concentration of negative 

charges, expressed as milliequivalents of anions per liter; thus, 

cation-anion balance is a test of the internal consistency of a 

major-ion analysis (Hem, 1985). An acceptable cation-anion 

balance was defined as one for which the difference between 

the total cation and anion concentrations, both expressed in 

milliequivalents per liter, was no greater than 10 percent of 

the total. Of the 234 CDPH wells with data available in the 

3-year period, 45 had major-ion analyses, and 44 of those had 

major-ion analyses with acceptable cation-anion balance. The 

major-ion analyses for all 90 USGS-grid wells had acceptable 

cation-anion balance.

The anion compositions of most of the CDPH wells 

(93 percent) and the USGS-grid wells (92 percent) were 

classified as bicarbonate-type (HCO3) waters (fig. B1B). The 

median fraction of sulfate (SO4

) in the anions was greater 

in the USGS-grid wells than in the CDPH wells (Wilcoxon 

rank-sum test, p=0.019; Helsel and Hirsch, 2002), reflecting 

sampling of HCO3-SO4-type groundwater in the HL study 

area by USGS-GAMA that was not present in the CDPH 

dataset. The cation compositions of a majority of the CDPH 

wells (82 percent) and the USGS-grid wells (69 percent) 

were classified as calcium-magnesium (Ca-Mg)-type or 

mixed-cation-type waters (fig. B1B). A greater proportion of 

USGS-grid wells had more than 40 percent sodium-potassium 

(NaK) in the cations than did CDPH wells (contingency table 

test, p=0.029; Helsel and Hirsch, 2002), primarily reflecting 

sampling of NaK-type and Ca-NaK-type groundwaters in 

the HL, LU, QV, and TV study areas by USGS-GAMA that 

were not present in the CDPH dataset. The reason for the 

differences between the two datasets is unknown.

Trace element, nutrient, and radioactive constituent data 

from the USGS-grid wells were compared to the most recent 

data from the CDPH database for samples collected between 

September 16, 2007, and September 16, 2010, from the 

same wells. The purpose of this comparison was to evaluate 

the quality of the data in the CDPH database. If data from 

USGS and CDPH for the same wells showed acceptable 

agreement, then the quality of the CDPH data from wells 

without USGS data were considered acceptable for use in the 

spatially weighted calculations of aquifer-scale proportions. 

Only constituents having one or more detections with a RC 

greater than 0.5 in either dataset were examined. Sixty-one 

USGS-grid wells had data for one or more constituents in the 

CDPH database; however, the number of wells with data in the 

CDPH database varied considerably among the constituents. 

Nitrate was the only constituent for which there were more 

than 35 wells with data from both datasets. Both datasets 

were recensored to the most common reporting limit used in 

the CDPH database (table B1) for each constituent for these 

comparisons.

The comparisons for each constituent were made in 

two ways: linear regression and percentage of wells with 

concentration above a threshold (table B1). The slope, 

intercept, and coefficient of determination (r2

) of the linear 

regression were compared to values for one-to-one line (slope 

= 1, intercept = 0, r2 = 1) to evaluate the agreement between 

the two datasets. The percentage of wells with concentrations 

above a threshold in each dataset was compared to evaluate 

whether use of one dataset or the other would affect results 

for aquifer-scale proportions. In most cases, the threshold 

used for this test corresponded to a RC equal to 0.5. A 

threshold corresponding to a RC equal to 0.333 was used for 

the trace-element lead in order to have a sufficient number 

of samples with concentrations above the threshold. The 

90 percent confidence interval for the percentage above the 

threshold calculated from USGS data was determine by using 

the Jeffrey’s interval for the binomial proportion (Belitz and 

others, 2010).

Sufficient data were available to make comparisons 

for 10 constituents. For nitrate, arsenic, vanadium, chloride, 

manganese, and TDS, the two datasets had acceptable 

agreement. The percentages of wells with concentration above 

the threshold based on CDPH data were within the 90 percent 

confidence intervals around the percentages of wells with 

concentration above the threshold based on USGS data 

(table B1). The slopes of the linear regressions were between 

0.75 and 1.1, the intercepts were close to zero, and the r2

values were greater than 0.85 (table B1; fig. B2). 
114 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

100

80

60

40

Calcium (Ca) + Magnesium (Mg)

Sulfate (SO4

) + Chloride (Cl)

60

80

100

Sulfate (SO4

)

Bicarbonate (HCO3

)

0

20

80

100

Magnesium (Mg)

Sodium (Na) + Potassium (K)

60

Chloride (Cl)

0

20

40

Calcium (Ca)

80

100

0

20

40

60

0

20

0

20

40

60

100

80

40

0

20

60

80

100

40

0

20

40

60

100

80

100

80

60

40

20

0

0

20

60

80

100

40

100

80

40

20

0

60

PERCENT

PERCENT

PERCENT

Ca

Ca + Mg

Mg

mixed

Ca +NaK

NaK

Mg + NaK

HCO3

HCO3

 + SO4

C + Cl

mixed

Cl

HCO3

 + Cl

SO4

TV-01

ES - Eastside Sacramento Valley

HL - Honey Lake Valley

LU - Low-Use Basins

QV - Quaternary Volcanics

SH - Shasta Valley and Mount Shasta Volcanic Area

TV - Tertiary Volcanic Area

USGS-grid wells

EXPLANATION

sac15-0550_fig B1a

A

Figure B1. Piper diagrams showing A, samples collected by the U.S. Geological Survey (USGS) for the study unit; and B, wells in the 

California Department of Public Health (CDPH) database having major-ion chemical analyses with acceptable cation-anion balance 

during the time period September 16, 2007, through September 17, 2010, Cascade Range and Modoc Plateau (CAMP) study unit, 2010, 

California Groundwater Ambient Monitoring and Assessment Program (GAMA) Priority Basin Project.
Appendix B: Comparison of CDPH and USGS-GAMA Program Data 115

100

80

60

40

Calcium (Ca) + Magnesium (Mg)

Sulfate (SO4

) + Chloride (Cl)

60

80

100

Sulfate (SO4

)

Bicarbonate (HCO3

)

0

20

80

100

Magnesium (Mg)

Sodium (Na) + Potassium (K)

60

Chloride (Cl)

0

20

40

Calcium (Ca)

80

100

0

20

40

60

0

20

0

20

40

60

100

80

40

0

20

60

80

100

40

0

20

40

60

100

80

100

80

60

40

20

0

0

20

60

80

100

40

100

80

40

20

0

60

ES - Eastside Sacramento Valley

HL - Honey Lake Valley

LU - Low-Use Basins

QV - Quaternary Volcanics

SH - Shasta Valley

TV - Tertiary Volcanics

CDPH wells

EXPLANATION

sac15-0550_fig B1b

Ca

Ca + Mg

Mg

mixed

Ca +NaK

NaK

Mg + NaK

HCO3

HCO3

 + SO4

SO4

 + Cl

mixed

HCO Cl 3

 + Cl

SO4

PERCENT

PERCENT

PERCENT

B

Figure B1. —Continued
116 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Table B1. Results of comparisons between concentrations of constituents in samples from wells with U.S. Geological Survey (USGS)

data and data in the California Department of Public Health (CDPH) database during the time period September 16, 2007, through 

September 17, 2010, Cascade Range and Modoc Plateau (CAMP) study unit, 2010, California Groundwater Ambient Monitoring and 

Assessment (GAMA) Program Priority Basin Project

[Both datasets were censored to the most common reporting limit for data from the CDPH database (RL). Abbreviations: mg/L, milligrams per liter; na, not 

available; pCi/L, picocuries per liter; r2, coefficient of determination; SMCL, secondary maximum contaminant level; μg/L, micrograms per liter]

Constituent

Number of wells 

with a detection 

greater than RL/total 

number of wells with 

USGS and CDPH data

RL Linear regression Threshold 

concentration

Percentage 

of wells with 

concentration 

above threshold

90-percent 

confidence

 interval 

for USGS 

Slope (percent) Intercept r2 USGS CDPH

Nutrients

Ammonia 0/0 na na na na na na na na

Nitrate 39/59 0.1 mg/L 0.87 0.1 0.86 5 mg/L 1.7 3.4 0.3–6.4

Trace elements and minor ions with health-based benchmarks

Arsenic 8/32 2 μg/L 0.75 0 0.86 5 μg/L 6.3 9.4 1.8–16

Boron 0/9 100 μg/L na na na na na na na

Fluoride 16/28 0.1 mg/L 0.96 0 0.48 2 mg/L 0 0 0–4.7

Lead 13/21 0.2 μg/L 0.09 0.2 0.13 5 μg/L 0 9.5 0–6.2

Molybdenum 0/0 na na na na na na na na

Strontium 0/0 na na na na na na na na

Vanadium 8/9 2 μg/L 0.99 0.7 0.99 25 μg/L 11 11 2.0–36

Radioactive constituents

Gross-alpha particle activity 6/25 3 pCi/L 0.86 –0.5 0.61 7.5 pCi/L 4.0 12 0.7–12

Radon-222 0/0 na na na na na na na na

Uranium 3/5 na na na na na na na na

Inorganic constituents with SMCL benchmarks

Chloride 13/20 1 mg/L 1.1 –0.2 0.88 250 mg/L 0 0 0–6.5

Iron 8/27 50 μg/L 0.41 –7 0.91 150 μg/L 7.4 19 2.2–19

Manganese 6/24 10 μg/L 0.98 4 0.96 50 μg/L 17 17 7.2–32

Total dissolved solids (TDS) 23/23 na 0.94 14 0.86 500 mg/L 0 0 0–5.7

Agreement between the two datasets was also considered 

acceptable for fluoride, despite the comparatively poorer linear 

regression fit than the others, because the percentage of wells 

with concentrations above the threshold was the same for both 

datasets (table B1; fig. B2). Thus, poor agreement between the 

USGS and CDPH data would be unlikely to affect results of 

aquifer-scale proportion calculations.

The USGS and CDPH datasets did not show acceptable 

agreement for lead, gross-alpha particle activity, or iron. 

For these three constituents, the percentages of wells with 

concentrations above the thresholds based on CDPH data 

were at or above the upper 90 percent confidence interval 

limit for the percentages of wells with concentrations above 

the thresholds based on USGS data (table B1). This indicated 

that use of CDPH data could bias the results for aquiferscale proportions toward greater high-RC and moderate-RC 

proportions. The linear regressions for these three constituents 

diverged markedly from the one-to-one line (table B1; 

fig. B3). Because of the poor agreement between USGS and 

CDPH data for lead, gross-alpha particle activity, and iron, 

the high-RC and moderate-RC aquifer-scale proportions 

calculated with the spatially weighted method were considered 

less reliable than those calculated with the grid-based method.
Appendix B: Comparison of CDPH and USGS-GAMA Program Data 117

CDPH fluoride, in milligrams per liter

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

USGS-GAMA fluoride, in milligrams per liter

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

USGS-GAMA nitrate-N, milligrams per liter

0

2

4

6

8

10

12

0

2

4

6

8

USGS-GAMA arsenic, in micrograms per liter

0

10

20

30

40

USGS-GAMA vanadium, in micrograms per liter

100

200

300

400

500

USGS-GAMA manganese, in micrograms per liter

0

CDPH vanadium, in micrograms per liter

0 10 20 30 40

12 0 100 200 300 400 500

CDPH nitrate-N, milligrams per liter CDPH manganese, in micrograms per liter

0 8642 10

CDPH arsenic, in micrograms per liter

0 2 4 6 8

A B

C D

E

Concentration below threshold (table B1)

1:1 line

Linear regression line

Well with USGS and CDPH data

EXPLANATION

sac15-0550_fig B2a-e

Figure B2. Comparison of data collected by the U.S. Geological Survey (USGS) and data reported in the California Department of 

Public Health (CDPH) database during the time period September 16, 2007, through September 17, 2010, for A, nitrate; B, manganese; 

C, arsenic; D, vanadium; E, flouride, F, chloride, and; G, total dissolved solids, Cascade Range and Modoc Plateau (CAMP) study unit, 

2010, California Groundwater Ambient Monitoring and Assessment (GAMA) Program Priority Basin Project.
118 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010 USGS-GAMA chloride, in milligrams per liter USGS-GAMA total dissolved solids, in milligrams per liter

CDPH chloride, in milligrams per liter CDPH total dissolved solids, in milligrams per liter

F G

0 5 10 15 20

0

5

10

15

20

0 100 200 300 400

0

100

200

300

400

Concentration below threshold (table B1)

1:1 line

Linear regression line

Well with USGS and CDPH data

EXPLANATION

sac15-0550_fig B2f-g

Figure B2. —Continued

Lead detections (greater than or equal to 1 μg/L) were 

reported in 10 CDPH wells during the period September 16, 

2007, through September 17, 2010. Two wells had high-RCs, 

and three had moderate-RCs of lead. Three of the CDPH wells 

with detections of lead also were sampled by USGS, and 

the agreement between the two datasets was poor (table B1; 

fig. B3A), with concentrations reported by CDPH as much 

higher than those measured by USGS. This pattern indicates 

that use of the CDPH data could result in observations of highRCs or moderate-RCs of lead where none would be observed 

if USGS data were used. In addition, of the 10 CDPH wells 

with lead detections, 7 wells had historical data for lead, and, 

of those 7 wells, only 1 had a previous detection of lead (at a 

low-RC). Based on this poor agreement between the USGS 

and CDPH results for wells with data in both datasets and the 

lack of confirmation of detections of lead in the CDPH dataset, 

the CDPH data for lead were considered suspect. Use of the 

CDPH data for lead in the status assessment could result in 

erroneous results. Spatially weighted aquifer-scale proportions 

for lead are listed in tables 8 and C1A–F, but lead was not 

included in the calculations of aquifer-scale proportions for 

trace elements as a class or for inorganic constituents as a 

class (table 9A).

The higher concentrations of iron and lead reported in 

the CDPH database compared to the concentrations measured 

by the USGS could reflect the difference in sample collection 

and analysis methods. Samples for analysis of trace elements 

by USGS are filtered during sample collection, whereas, the 

analytical methods used for regulatory compliance sampling 

for trace elements can use unfiltered samples. The source of 

particulate iron and lead in water samples could be particles 

from piping in wells and distribution systems.
Appendix B: Comparison of CDPH and USGS-GAMA Program Data 119

USGS-GAMA lead, micrograms per liter USGS-GAMA iron, in micrograms per liter

USGS-GAMA gross alpha, in picocuries per liter

CDPH lead, micrograms per liter CDPH gross alpha, in picocuries per liter

CDPH iron, in micrograms per liter

A B

C

0

10

20

30

0 10 20 30

0 300 600 900 1,200 1,500 1,800

0

300

600

900

1,200

1,500

1,800

0 2 4 6 8

0

2

4

6

8

Concentration below threshold (table B1)

1:1 line

Linear regression line

Well with USGS and CDPH data

EXPLANATION

sac15-0550_fig B3a-c

Figure B3. Comparison of data collected by the U.S. Geological Survey (USGS) and data reported in the California Department of 

Public Health (CDPH) database during the time period September 16, 2007, through September 17, 2010, for A, lead; B, gross-alpha 

particle activity; and C, iron, Cascade Range and Modoc Plateau (CAMP) study unit, 2010, California Groundwater Ambient Monitoring 

and Assessment (GAMA) Program Priority Basin Project.

References Cited 

Belitz, Kenneth, Jurgens, Bryant, Landon, M.K., Fram, M.S., 

and Johnson, Tyler, 2010, Estimation of aquifer scale 

proportion using equal area grids, Assessment of regional 

scale groundwater quality: Water Resources Research, v. 46, 

no. 11, http://dx.doi.org/10.1029/2010WR009321.

Helsel, D.R., and Hirsch, R.M., 2002, Statistical methods in 

water resources: U.S. Geological Survey Techniques of 

Water-Resources Investigations, book 4, chap. A3, 510 p., 

http://water.usgs.gov/pubs/twri/twri4a3/.

Hem, J.D., 1985, Study and interpretation of the chemical 

characteristics of natural water: U.S. Geological 

Survey Water Supply Paper 2254, 263 p., 

http://pubs.er.usgs.gov/publication/wsp2254.

Piper, A.M., 1944, A graphic procedure in the 

geochemical interpretation of water-analyses: EOS, 

American Geophysical Union Transactions, v. 25, no. 6, 

p. 914–923, http://onlinelibrary.wiley.com/doi/10.1029/

TR025i006p00914/abstract.
120 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Appendix C: Aquifer-Scale Proportions in Study Areas

Grid-based and spatially weighted aquifer-scale 

proportions in the six study areas for individual relative 

concentrations (RC) inorganic constituents detected at high or 

moderate in the U.S. Geological Survey (USGS)-grid wells or 

reported at high or moderate RCs in the California Department 

of Public Health (CDPH) database in samples collected 

between September 2007 and September 2010 are listed in 

tables C1A–F.

Aquifer-scale proportion results are not tabulated for 

organic constituents because there were no high-RC or 

moderate-RC detections of these constituents; hence, the 

proportions were 100 percent low-RCs. Four herbicides 

(atrazine, hexazinone, prometon, and simazine), the 

trihalomethane chloroform, the solvents 1,1-dichloroethene 

and tetrachloroethene (PCE), and the gasoline oxygenate 

methyl tert-butyl ether (MTBE) each were detected in greater 

than 10 percent of the 15 USGS-grid wells in 1 or more of the 

6 study areas (Shelton and others, 2013). All detections had 

low RCs. The CDPH database contained no pesticide data for 

samples collected in the Cascade Range and Modoc Plateau 

(CAMP) study unit between September 2008 and September 

2010. The CDPH database contained data for volatile 

organic compounds (VOCs) in samples from 71 wells, and, 

of those 71 wells, 24 also had USGS-Groundwater Ambient 

Monitoring and Assessment (GAMA) VOC data. Chloroform 

was the only VOC with a detection reported in the CDPH 

database between September 2008 and September 2010. One 

CDPH well had one detection of chloroform at a low RC.

References Cited 

Shelton, J.L., Fram, M.S., and Belitz, Kenneth, 2013, 

Groundwater-quality data in the Cascade Range and Modoc 

Plateau study unit, 2010, Results from the California 

GAMA Program: U.S. Geological Survey Data Series 688, 

124 p., http://pubs.er.usgs.gov/publication/ds688.
Appendix C: Aquifer-Scale Proportions in Study Areas 121

Table C1A. Aquifer-scale proportions for the Sacramento Valley Eastside (ES) study area for inorganic constituents detected at 

high or moderate relative-concentrations in the Cascade Range and Modoc Plateau study unit, 2010, California Groundwater Ambient 

Monitoring and Assessment Program (GAMA) Priority Basin Project.

[Sacramento Valley Eastside study area covers 10.38 percent of the Cascade Range and Modoc Plateau study unit. Relative-concentration (RC) categories 

for inorganic constituents: High, RC 1.0, RC less than or equal to 1.0 and greater than 0.5; low, RC less than 0.5. RC defined as measured value divided by 

benchmark value. Inorganic constituents not listed in this table either do not have benchmarks or were detected only at low RCs. Benchmark types and values 

listed in table 4A. Other abbreviations: SMCL, secondary maximum contaminant level]

Constituent

Number

Raw occurrence 

frequency 

(percent)

Aquifer-scale proportions 

(percent)

Spatially weighted Grid-based 

Wells Cells Moderate-RC High-RC Moderate-RC High-RC Moderate-RC High-RC

Nutrients

Ammonia 15 15 0 0 0 0 0 0

Nitrate 30 15 0 0 0 0 0 0

Trace elements

Arsenic 27 15 0 0 0 0 0 0

Boron 22 15 0 0 0 0 0 0

Fluoride 24 15 0 0 0 0 0 0

Lead 23 15 4.3 0 3.3 0 0 0

Molybdenum 15 15 0 0 0 0 0 0

Strontium 15 15 0 0 0 0 0 0

Vanadium 23 15 8.7 0 10.0 0 13.3 0

Radioactive constituents

Gross alpha particle activity 24 15 0 0 0 0 0 0

Adjusted gross alpha particle activity 24 15 0 0 0 0 0 0

Radon-222 activity 15 15 0 0 0 0 0 0

Uranium 15 15 0 0 0 0 0 0

Inorganic constituents with SMCL benchmarks

Chloride 22 15 0 0 0 0 0 0

Iron 24 15 8.3 8.3 3.6 8.9 0 6.7

Manganese 23 15 0 8.7 0 4.4 0 0

Total dissolved solids (TDS) 24 15 0 0 0 0 0 0

1

Based on the most recent analyses for each California Department of Public Health (CDPH) well during September 16, 2007, through September 16, 2010, 

combined with data from U.S. Geological Survey (USGS)-grid wells.

2

Grid-based aquifer-scale proportions are based on samples collected by the USGS from 15 grid wells during September 13, 2007, through October 12, 2010.
122 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Table C1B. Aquifer-scale proportions for the Honey Lake Valley (HL) study area for inorganic constituents detected at high or 

moderate relative concentrations in the Cascade Range and Modoc Plateau study unit, 2010, California Groundwater Ambient 

Monitoring and Assessment Program (GAMA) Priority Basin Project.

[Honey Lake Valley study area covers 13.41 percent of the Cascade Range and Modoc Plateau study unit. Relative-concentration (RC) categories for 

inorganic constituents: high, RC greater than 1.0, RC less than or equal to 1.0 and greater than 0.5; low, RC less than 0.5. RC defined as measured value 

divided by benchmark value. Inorganic constituents not listed in this table either do not have benchmarks or were detected only at low RCs. Benchmark types 

and values listed in table 4A. Other abbreviations: SMCL, secondary maximum contaminant level]

Constituent

Number

Raw occurrence 

frequency 

(percent)

Aquifer-scale proportions (percent)

Spatially weighted Grid-based 

Wells Cells Moderate-RC High-RC Moderate-RC High-RC Moderate-RC High-RC

Nutrients

Ammonia 15 15 0 0 0 0 0 0

Nitrate 35 15 2.9 2.9 1.1 6.7 0 6.7

Trace elements

Arsenic 31 15 25.8 19.4 21.7 16.4 26.7 13.3

Boron 21 15 28.6 0 36.7 0 40.0 0

Fluoride 23 15 8.7 0 10.0 0 13.3 0

Lead 21 15 0 0 0 0 0 0

Molybdenum 15 15 0 6.7 0 6.7 0 6.7

Strontium 15 15 0 0 0 0 0 0

Vanadium 27 15 7.4 3.7 8.3 3.3 6.7 6.7

Radioactive constituents

Gross alpha particle activity 22 15 0 13.6 0 15.6 6.7 13.3

Adjusted gross alpha particle activity 22 15 4.5 4.5 2.2 4.5 6.7 0

Radon-222 activity 15 15 0 6.7 0 6.7 0 6.7

Uranium 19 15 0 15.8 0 15.0 0 13.3

Inorganic constituents with SMCL benchmarks

Chloride 26 15 0 0 0 0 0 0

Iron 31 15 0 3.2 0 3.3 0 6.7

Manganese 31 15 0 22.6 0 28.9 0 26.7

Total dissolved solids (TDS) 30 15 23.3 0 34.4 0 40.0 0

1

Based on the most recent analyses for each California Department of Public Health (CDPH) well during September 16, 2007, through September 16, 2010, 

combined with data from U.S. Geological Survey (USGS)-grid wells.

2

Grid-based aquifer-scale proportions are based on samples collected by the USGS from 15 grid wells during July through October 2010.
Appendix C: Aquifer-Scale Proportions in Study Areas 123

Table C1C. Aquifer-scale proportions for the Cascade Range and Modoc Plateau Low Use Basins (LU) study area for inorganic 

constituents detected at high or moderate relative concentrations in the Cascade Range and Modoc Plateau study unit, 2010, California 

Groundwater Ambient Monitoring and Assessment Program (GAMA) Priority Basin Project.

[Cascade Range and Modoc Plateau Low Use Basins study area covers 18.44 percent of the Cascade Range and Modoc Plateau study unit. Relativeconcentration (RC) categories for inorganic constituents: high, RC greater than 1.0, RC less than or equal to 1.0 and greater than 0.5; low, RC less than or 

equal to 0.5. RC defined as measured value divided by benchmark value. Inorganic constituents not listed in this table either do not have benchmarks or were 

detected only at low RCs. Benchmark types and values listed in table 4A. Other abbreviations: SMCL, secondary maximum contaminant level]

Constituent

Number

Raw occurrence 

frequency 

(percent)

Aquifer-scale proportions (percent)

Spatially weighted Grid-based 

Wells Cells Moderate-RC High-RC Moderate-RC High-RC Moderate-RC High-RC

Nutrients

Ammonia 15 15 6.7 0 6.7 0 6.7 0

Nitrate 44 15 2.3 0 1.3 0 0 0

Trace elements

Arsenic 25 15 8.0 4.0 8.9 3.3 6.7 0

Boron 17 15 5.9 0 6.7 0 6.7 0

Fluoride 22 15 0 0 0 0 0 0

Lead 22 15 0 0 0 0 0 0

Molybdenum 15 15 0 0 0 0 0 0

Strontium 15 15 0 0 0 0 0 0

Vanadium 17 15 5.9 0 6.7 0 6.7 0

Radioactive constituents

Gross alpha particle activity 27 15 3.7 0 3.3 0 0 0

Adjusted gross alpha particle activity 27 15 0 0 0 0 0 0

Radon-222 activity 15 15 0 0 0 0 0 0

Uranium 15 15 0 0 0 0 0 0

Inorganic constituents with SMCL benchmarks

Chloride 24 15 0 0 0 0 0 0

Iron 23 15 13.0 21.7 12.2 15.6 6.7 13.3

Manganese 23 15 17.4 17.4 13.3 15.6 6.7 13.3

Total dissolved solids (TDS) 30 15 3.3 3.3 6.7 3.3 6.7 0

1

Based on the most recent analyses for each California Department of Public Health (CDPH) well during September 16, 2007, through September 16, 2010, 

combined with data from U.S. Geological Survey (USGS)-grid wells.

2

Grid-based aquifer-scale proportions are based on samples collected by the USGS from 15 grid wells during July through October 2010.
124 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Table C1D. Aquifer-scale proportions for the Quaternary Volcanic Areas (QV) study area for inorganic constituents detected at high 

or moderate relative concentrations in the Cascade Range and Modoc Plateau study unit, 2010, California Groundwater Ambient 

Monitoring and Assessment Program (GAMA) Priority Basin Project.

[Quaternary Volcanic Areas study area covers 25.72 percent of the Cascade Range and Modoc Plateau study unit. Relative-concentration (RC) categories for 

inorganic constituents: High, RC greater than 1.0, RC less than or equal to 1.0 and greater than 0.5; low, RC less than or equal to 0.5. RC defined as measured 

value divided by benchmark value. Inorganic constituents not listed in this table either do not have benchmarks or were detected only at low RCs. Benchmark 

types and values listed in table 4A. Other abbreviations: SMCL, secondary maximum contaminant level]

Constituent

Number

Raw occurrence 

frequency 

(percent)

Aquifer-scale proportions (percent)

Spatially weighted Grid-based 

Wells Cells Moderate-RC High-RC Moderate-RC High-RC Moderate-RC High-RC

Nutrients

Ammonia 15 15 0 0 0 0 0 0

Nitrate 64 15 0 0 0 0 0 0

Trace elements

Arsenic 26 15 3.8 0 6.7 0 6.7 0

Boron 22 15 0 0 0 0 0 0

Fluoride 30 15 0 0 0 0 0 0

Lead 25 15 4.0 8.0 2.2 4.4 0 0

Molybdenum 15 15 0 0 0 0 0 0

Strontium 15 15 0 0 0 0 0 0

Vanadium 21 15 14.3 0 15.0 0 20.0 0

Radioactive constituents

Gross alpha particle activity 28 15 0 0 0 0 0 0

Adjusted gross alpha particle activity 28 15 0 0 0 0 0 0

Radon-222 activity 15 15 0 0 0 0 0 0

Uranium 15 15 0 0 0 0 0 0

Inorganic constituents with SMCL benchmarks

Chloride 27 15 0 0 0 0 0 0

Iron 28 15 0 0 0 0 0 0

Manganese 28 15 0 7.1 0 13.3 0 13.3

Total dissolved solids (TDS) 27 15 3.7 0 6.7 0 6.7 0

1

Based on the most recent analyses for each California Department of Public Health (CDPH) well during September 16, 2007, through September 16, 2010, 

combined with data from U.S. Geological Survey (USGS)-grid wells.

2

Grid-based aquifer-scale proportions are based on samples collected by the USGS from 15 grid wells during July through October 2010.
Appendix C: Aquifer-Scale Proportions in Study Areas 125

Table C1E. Aquifer-scale proportions for the Shasta Valley and Mount Shasta Volcanic Area (SH) study area for inorganic constituents 

detected at high or moderate relative concentrations in the Cascade Range and Modoc Plateau study unit, 2010, California Groundwater 

Ambient Monitoring and Assessment Program (GAMA) Priority Basin Project.

[Shasta Valley and Mount Shasta Volcanic Area study area covers 13.35 percent of the Cascade Range and Modoc Plateau study unit. Relative-concentration 

(RC) categories for inorganic constituents: High, RC greater than 1.0, RC less than or equal to 1.0 and greater than 0.5; low, RC less than or equal to 0.5. RC 

defined as measured value divided by benchmark value. Inorganic constituents not listed in this table either do not have benchmarks or were detected only at low 

RCs. Benchmark types and values listed in table 4A. Other abbreviations: SMCL, secondary maximum contaminant level]

Constituent

Number

Raw occurrence 

frequency 

(percent)

Aquifer-scale proportions (percent)

Spatially weighted Grid-based 

Wells Cells Moderate-RC High-RC Moderate-RC High-RC Moderate-RC High-RC

Nutrients

Ammonia 15 15 0 0 0 0 0 0

Nitrate 42 15 7.1 0 11.1 0 13.3 0

Trace elements

Arsenic 29 15 3.4 3.4 6.7 6.7 6.7 6.7

Boron 15 15 6.7 0 6.7 0 6.7 0

Fluoride 29 15 0 0 0 0 0 0

Lead 28 15 0 0 0 0 0 0

Molybdenum 15 15 0 0 0 0 0 0

Strontium 15 15 6.7 0 6.7 0 6.7 0

Vanadium 15 15 20.0 6.7 20.0 6.7 20.0 6.7

Radioactive constituents

Gross alpha particle activity 27 15 0 0 0 0 0 0

Adjusted gross alpha particle activity 27 15 0 0 0 0 0 0

Radon-222 activity 15 15 0 0 0 0 0 0

Uranium 15 15 0 0 0 0 0 0

Inorganic constituents with SMCL benchmarks

Chloride 26 15 0 0 0 0 0 0

Iron 27 15 3.7 3.7 1.7 2.2 0 0

Manganese 27 15 0 3.7 0 6.7 0 6.7

Total dissolved solids (TDS) 26 15 7.7 0 13.3 0 13.3 0

1

Based on the most recent analyses for each California Department of Public Health (CDPH) well during September 16, 2007, through September 16, 2010, 

combined with data from U.S. Geological Survey (USGS)-grid wells.

2

Grid-based aquifer-scale proportions are based on samples collected by the USGS from 15 grid wells during July through October 2010.
126 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

Table C1F. Aquifer-scale proportions for the Tertiary Volcanic Areas (TV) study area for inorganic constituents detected at high 

or moderate relative concentrations in the Cascade Range and Modoc Plateau study unit, 2010, California Groundwater Ambient 

Monitoring and Assessment Program (GAMA) Priority Basin Project.

[Tertiary Volcanic Areas study area covers 18.70 percent of the Cascade Range and Modoc Plateau study unit. Relative-concentration (RC) categories for 

inorganic constituents: High, RC greater than 1.0, RC less than or equal to 1.0 and greater than 0.5; low, RC less than or equal to 0.5. RC defined as measured 

value divided by benchmark value. Inorganic constituents not listed in this table either do not have benchmarks or were detected only at low RCs. Benchmark 

types and values listed in table 4A. Other abbreviations: SMCL, secondary maximum contaminant level]

Constituent

Number

Raw occurrence 

frequency 

(percent)

Aquifer-scale proportions 

(percent)

Spatially weighted Grid-based 

Wells Cells Moderate-RC High-RC Moderate-RC High-RC Moderate-RC High-RC

Nutrients

Ammonia 15 15 0 0 0 0 0 0

Nitrate 38 15 0 0 0 0 0 0

Trace elements

Arsenic 21 15 0 0 0 0 0 0

Boron 18 15 5.6 11.1 2.2 13.3 0 13.3

Fluoride 23 15 0 0 0 0 0 0

Lead 18 15 0 0 0 0 0 0

Molybdenum 15 15 0 6.7 0 6.7 0 6.7

Strontium 15 15 0 0 0 0 0 0

Vanadium 17 15 0 0 0 0 0 0

Radioactive constituents

Gross alpha particle activity 22 14 0 0 0 0 0 0

Adjusted gross alpha particle activity 22 15 0 0 0 0 0 0

Radon-222 activity 15 15 0 0 0 0 0 0

Uranium 16 15 0 0 0 0 0 0

Inorganic constituents with SMCL benchmarks

Chloride 21 15 0 4.5 0 6.7 0 6.7

Iron 22 15 0 9.1 0 5.6 0 0

Manganese 22 15 0 4.5 0 3.3 0 0

Total dissolved solids (TDS) 22 15 0 4.5 0 6.7 0 6.7

1

Based on the most recent analyses for each California Department of Public Health (CDPH) well during September 16, 2007, through September 16, 2010, 

combined with data from U.S. Geological Survey (USGS)-grid wells.

2

Grid-based aquifer-scale proportions are based on samples collected by the USGS from 15 grid wells during July through October 2010.
Appendix D. Radioactivity 127

Appendix D. Radioactivity

The class of radioactive constituents includes constituents 

with abundances commonly measured as activities rather than 

concentrations. Activity is measured in units of picocuries 

per liter (pCi/L), and one picocurie equals approximately 

two atoms decaying per minute. When atoms decay, they 

release alpha or beta particles, and (or) gamma radiation. 

Gross alpha particle activity is a measure of the total activity 

of non-volatile isotopes decaying by alpha emission. The 

U.S. Environmental Protection Agency maximum contaminant 

level (MCL-US) (15 pCi/L) for gross alpha particle activity 

applies to adjusted gross alpha particle activity, which is equal 

to the measured gross alpha particle activity minus uranium 

activity (U.S. Environmental Protection Agency, 2000). Data 

collected by the U.S. Geological Survey (USGS) Groundwater 

Ambient Monitoring and Assessment (GAMA) and data 

compiled in the California Department of Public Health 

(CDPH) database are reported as gross alpha particle activity 

without correction for uranium activity (“unadjusted”). Gross 

alpha particle activity is used as a screening tool to determine 

whether other radioactive constituents must be analyzed. For 

regulatory purposes, analysis of uranium is only required 

if gross alpha particle activity is greater than 15 pCi/L 

(California Department of Public Health, 2013b); therefore, 

the CDPH database contains substantially more data for gross 

alpha particle activity than for uranium. As a result, it is not 

always possible to calculate adjusted gross alpha particle 

activity. For this reason, results for unadjusted gross alpha 

particle activity (that is, without correction for uranium) are 

the primary data used in the status assessments made by the 

USGS-GAMA for Priority Basin Project study units. Results 

for adjusted gross alpha particle activity also are given in this 

report for comparison (tables 8 and C1A–F).

USGS-GAMA reports data for two measurements of 

gross alpha particle activity, counted 72 hours and 30 days 

after sample collection. Regulatory sampling for gross alpha 

particle activity permits use of quarterly composite samples 

(California Department of Public Health, 2013b). The 

composite samples sent by water agencies to laboratories for 

analysis may be mixtures of four samples collected 9 months, 

6 months, 3 months, and a few days before submission. 

Because of these long holding times for CDPH data, the 

USGS-GAMA gross alpha 30-day count data could be more 

appropriate to use when combining USGS-GAMA and CDPH 

datasets. The 30-day count data were used in this study. Gross 

alpha particle activity in a groundwater sample can change 

with time after sample collection because of the radioactive 

decay of parent isotopes and ingrowth and subsequent decay 

of radioactive daughter isotopes (activity can increase or 

decrease depending on sample composition and holding time) 

(Arndt, 2010).

Most uranium results in the CDPH databases are 

reported as activities because the maximum contaminant level 

(MCL-CA) for uranium is 20 pCi/L. Uranium activities for 

CDPH wells, reported in pCi/L, were converted to uranium 

concentrations, reported in micrograms per liter (μg/L), by 

using a conversion factor derived from data collected by 

USGS-GAMA for the CAMP study unit. Activities of the 

three dominant uranium isotopes, uranium-234, -235, and 

-238, were measured in 20 CAMP study unit USGS-grid wells 

(Shelton and others, 2013). Total uranium activity is equal to 

the sum of the activities of these three isotopes (Arndt, 2010). 

Linear regression of the uranium concentration and uranium 

activity data from these 20 samples yield the following 

relation with r2

=0.998, and the slope of the regression equation 

was used as the conversion factor.

U x mass = − Uact 1 3. . 5 0 28 (D1)

where

Umass is the concentration of uranium in μg/L, and

Uact is the activity of uranium in pCi/L. 

Total uranium activity is assumed to equal the sum of the 

activities of the three uranium isotopes, uranium-234, -235, 

and -238.
128 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

References Cited 

Arndt, M.F., 2010, Evaluation of gross alpha and 

uranium measurements for MCL compliance: 

Water Research Foundation, Denver, Colorado, 299 p., 

accessed January 10, 2012, 

http://www.waterrf.org/PublicReportLibrary/3028.pdf.

California Department of Public Health, 2013, California 

Code of Regulation, Title 22, Division 4 Environmental 

Health, Chapter 15 Domestic water quality and monitoring 

regulations, Article 4 Primary standards—Inorganic 

chemicals, §64431 Maximum contaminant levels—

Inorganic chemicals; Article 5 Radioactivity, §64442 MCLs 

and monitoring—Gross alpha particle activity, radium-226, 

radium-228, and uranium; Article 5.5 Primary standards—

Organic chemicals, §64444 Maximum contaminant levels—

Organic chemicals; and Article 16 Secondary drinking water 

standards, §64449 Secondary maximum contaminant levels 

and compliance, accessed December 2013, 

http://www.waterboards.ca.gov/laws_regulations/.

Shelton, J.L., Fram, M.S., and Belitz, Kenneth, 2013, 

Groundwater-quality data in the Cascade Range and Modoc 

Plateau study unit, 2010, Results from the California 

GAMA Program: U.S. Geological Survey Data Series 688, 

124 p., http://pubs.er.usgs.gov/publication/ds688.

U.S. Environmental Protection Agency, 2000, National 

primary drinking water regulations; radionuclides; final 

rule, accessed January 2014 https://www.federalregister.gov/

articles/2000/12/07/00-30421/national-primary-drinkingwater-regulations-radionuclides-final-rule.
Appendix E: Additional Water-Quality Data 129

Appendix E: Additional Water-Quality Data

Noble gas data provided by the Lawrence Livermore 

National Laboratory that were not available at the time of 

publication of the Data-Series Report (Shelton and others, 

2013) are tabulated in this appendix (table E1).

References Cited

Shelton, J.L., Fram, M.S., and Belitz, Kenneth, 2013, 

Groundwater-quality data in the Cascade Range and Modoc 

Plateau study unit, 2010, Results from the California 

GAMA Program: U.S. Geological Survey Data Series 688, 

124 p., http://pubs.er.usgs.gov/publication/ds688.

Table E1. Results for analyses of noble gases by Lawrence Livermore National Laboratory for samples collected for the Cascade 

Range and Modoc Plateau (CAMP) study unit, 2010, California Groundwater Ambient Monitoring and Assessment Program (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. Other abbreviations: cm3STP/gH20, cubic centimeters at standard temperature and pressure per gram of water; x, times; na, not available]

GAMA 

identification 

number

Helium-3/

Helium-4 

(atom ratio)

(61040)

x 10–6

Helium-4

(85561)

x 10–7

Neon

(61046)

x 10–7

Argon

(85563)

x 10–4

Krypton

(85565)

x 10–8

Xenon

(85567)

x 10–8

(cm3STP/gH2O)

Eastside Sacramento Valley study area (ES)

CAMP-ES-01 1.51 0.42 1.84 3.39 7.76 1.12

CAMP-ES-02 1.07 19.80 41.09 7.94 27.21 2.51

CAMP-ES-03 1.30 0.22 1.00 2.43 5.34 0.84

CAMP-ES-04 0.84 10.02 14.48 8.59 13.55 1.50

CAMP-ES-05 1.46 0.52 2.10 3.82 8.40 1.18

CAMP-ES-06 0.96 0.91 2.18 3.37 8.24 1.18

CAMP-ES-07 1.42 0.48 2.06 3.43 8.35 1.17

CAMP-ES-08 1.39 0.52 2.19 3.64 8.16 1.18

CAMP-ES-09 1.26 0.61 1.97 3.11 6.94 0.97

CAMP-ES-10 1.38 0.49 2.11 3.29 7.25 1.03

CAMP-ES-11 1.40 0.48 2.10 3.57 7.92 1.14

CAMP-ES-12 1.38 0.42 1.84 3.33 7.91 1.12

CAMP-ES-13 1.40 0.46 2.10 3.48 7.94 1.15

CAMP-ES-14 1.32 0.49 1.91 3.12 7.10 0.99

CAMP-ES-15 1.19 0.62 1.94 3.17 7.13 1.01

Honey Lake Valley study area (HL)

CAMP-HL-01 1.68 3.77 7.25 5.46 9.27 1.02

CAMP-HL-02 0.90 0.98 2.65 4.05 8.78 1.17

CAMP-HL-03 2.66 44.18 1.99 3.37 7.91 1.11

CAMP-HL-04 1.15 0.62 2.13 3.57 8.22 1.13

CAMP-HL-05 1.62 0.46 1.91 3.25 7.50 1.04

CAMP-HL-06 0.96 3.10 5.30 4.62 9.14 1.15

CAMP-HL-07 na na na na na na

CAMP-HL-08 1.26 0.84 2.56 7.07 6.21 1.21

CAMP-HL-09 1.03 3.30 8.36 5.77 11.15 1.27

CAMP-HL-10 0.94 0.87 2.18 3.30 7.33 0.97

CAMP-HL-11 1.64 1.88 2.91 4.46 7.73 1.09
130 Status and Understanding of Groundwater Quality in the Cascade Range and Modoc Plateau Study Unit, 2010

GAMA 

identification 

number

Helium-3/

Helium-4 

(atom ratio)

(61040)

x 10–6

Helium-4

(85561)

x 10–7

Neon

(61046)

x 10–7

Argon

(85563)

x 10–4

Krypton

(85565)

x 10–8

Xenon

(85567)

x 10–8

(cm3STP/gH2O)

Honey Lake Valley study area (HL)

CAMP-HL-12 0.99 0.69 2.07 3.26 7.35 1.06

CAMP-HL-13 1.33 0.94 3.39 4.01 8.46 1.14

CAMP-HL-14 2.38 3.23 1.97 3.31 7.72 1.03

CAMP-HL-15 1.35 0.60 2.30 3.49 7.63 1.03

Cascade Range and Modoc Plateau Low-Use Basins study area (LU)

CAMP-LU-01 2.61 0.74 2.02 3.51 8.01 1.15

CAMP-LU-02 2.38 0.51 1.53 3.09 5.58 0.81

CAMP-LU-03 1.68 2.19 2.15 3.37 7.36 1.04

CAMP-LU-04 2.10 0.69 2.87 4.12 8.68 1.17

CAMP-LU-05 1.76 2.58 1.94 3.34 6.90 0.99

CAMP-LU-06 2.97 4.81 2.10 3.62 8.06 1.21

CAMP-LU-07 1.39 0.63 2.72 4.17 9.28 1.22

CAMP-LU-08 1.32 0.49 1.80 2.27 4.92 0.65

CAMP-LU-09 2.30 3.00 1.79 3.10 7.19 1.01

CAMP-LU-10 1.58 0.52 2.34 3.55 8.13 1.14

CAMP-LU-11 1.39 0.47 1.83 3.14 7.09 1.03

CAMP-LU-12 1.93 0.43 1.84 3.39 7.97 1.17

CAMP-LU-13 1.22 0.52 1.98 3.79 8.17 1.14

CAMP-LU-14 2.41 0.83 2.30 3.79 8.55 1.33

CAMP-LU-15 1.85 0.53 2.24 3.76 8.83 1.26

 Quaternary Volcanic Areas study area (QV)

CAMP-QV-01 1.40 0.37 1.73 3.54 8.69 1.28

CAMP-QV-02 1.35 0.39 1.78 3.59 8.92 1.33

CAMP-QV-03 2.44 1.48 2.03 3.51 7.99 1.17

CAMP-QV-04 na na na na na na

CAMP-QV-05 1.61 0.46 1.92 3.62 8.72 1.29

CAMP-QV-06 1.00 0.78 1.83 3.07 6.97 0.97

CAMP-QV-07 1.39 0.42 1.91 3.60 8.81 1.26

CAMP-QV-08 2.56 0.53 1.98 3.50 8.34 1.19

CAMP-QV-09 na na na na na na

CAMP-QV-10 1.46 0.55 2.10 3.57 8.18 1.19

CAMP-QV-11 1.50 0.54 2.42 4.07 8.79 1.27

CAMP-QV-12 1.77 0.71 3.02 4.28 9.28 1.29

CAMP-QV-13 3.76 0.59 1.85 3.58 7.49 1.15

CAMP-QV-14 1.04 0.97 2.71 4.07 9.23 1.31

CAMP-QV-15 1.62 0.41 1.87 3.25 8.47 1.29

Table E1. Results for analyses of noble gases by Lawrence Livermore National Laboratory for samples collected for the Cascade 

Range and Modoc Plateau (CAMP) study unit, 2010, California Groundwater Ambient Monitoring and Assessment Program (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. Other abbreviations: cm3STP/gH20, cubic centimeters at standard temperature and pressure per gram of water; x, times; na, not available]
Appendix E: Additional Water-Quality Data 131

GAMA 

identification 

number

Helium-3/

Helium-4 

(atom ratio)

(61040)

x 10–6

Helium-4

(85561)

x 10–7

Neon

(61046)

x 10–7

Argon

(85563)

x 10–4

Krypton

(85565)

x 10–8

Xenon

(85567)

x 10–8

(cm3STP/gH2O)

Shasta Valley and Mount Shasta Volcanic Area study area (SH)

CAMP-SH-01 0.68 1.43 2.49 3.81 8.42 1.15

CAMP-SH-02 1.58 5.09 1.98 3.25 7.23 1.04

CAMP-SH-03 1.20 0.96 2.27 3.61 8.08 1.13

CAMP-SH-04 6.61 2.92 2.75 3.91 8.79 1.25

CAMP-SH-05 1.45 0.40 1.80 3.45 8.12 1.22

CAMP-SH-06 7.14 1.40 1.97 3.65 8.75 1.24

CAMP-SH-07 1.35 0.41 1.88 3.68 8.68 1.30

CAMP-SH-08 6.56 1.39 2.41 4.60 6.41 1.11

CAMP-SH-09 1.00 0.99 2.07 3.33 7.60 1.02

CAMP-SH-10 3.20 1.18 2.41 3.75 8.17 1.19

CAMP-SH-11 2.95 1.01 2.62 3.85 8.18 1.19

CAMP-SH-12 1.34 0.69 1.90 3.43 7.55 1.08

CAMP-SH-13 1.62 0.47 1.76 3.17 7.57 1.07

CAMP-SH-14 1.57 0.74 3.38 4.80 10.99 1.63

CAMP-SH-15 0.15 8.25 2.91 4.04 8.76 1.24

Tertiary Volcanic Areas study area (TV)

CAMP-TV-01 2.31 104.0 1.93 3.33 7.33 1.02

CAMP-TV-02 1.39 0.42 1.83 3.19 6.70 1.01

CAMP-TV-03 1.78 0.46 1.82 3.28 7.72 1.17

CAMP-TV-04 5.51 1.34 1.95 3.55 8.34 1.20

CAMP-TV-05 1.32 0.52 1.99 3.21 7.39 0.98

CAMP-TV-06 2.77 1.01 1.94 3.59 8.57 1.28

CAMP-TV-07 1.43 0.54 2.28 3.65 8.25 1.24

CAMP-TV-08 1.34 0.38 1.55 2.65 5.94 0.87

CAMP-TV-09 1.99 3.25 2.26 3.84 8.96 1.38

CAMP-TV-10 1.54 0.73 3.08 4.54 10.12 1.42

CAMP-TV-11 1.36 5.06 15.93 8.68 16.51 1.95

CAMP-TV-12 1.59 0.47 2.04 3.75 8.98 1.31

CAMP-TV-13 1.60 0.68 2.67 4.18 9.19 1.33

CAMP-TV-14 1.40 0.80 3.39 4.39 9.80 1.36

CAMP-TV-15 0.90 7.71 1.73 8.55 5.80 0.89

Table E1. Results for analyses of noble gases by Lawrence Livermore National Laboratory for samples collected for the Cascade 

Range and Modoc Plateau (CAMP) study unit, 2010, California Groundwater Ambient Monitoring and Assessment Program (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. Other abbreviations: cm3STP/gH20, cubic centimeters at standard temperature and pressure per gram of water; x, times; na, not available]

Publishing support provided by the U.S. Geological Survey 

Science Publishing Network, Sacramento Publishing Service Center

For more information concerning the research in this report, contact the

 Director, California Water Science Center

U.S. Geological Survey

6000 J Street, Placer Hall

Sacramento, California 95819

http://ca.water.usgs.gov
Author—Spine Title—Report Series Name 0000

ISBN

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ISSN 2328-0328 (online)

ISSN 2328-031X (print)

http://dx.doi.org/10.3133/sir20145238

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