U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2012–3010

 April 2012

U.S. Geological Survey and the California State Water Resources Control Board

Groundwater Quality in the Central Sierra 

Nevada, California 

Groundwater provides more than 40 percent of California’s drinking water. To protect this vital resource, the 

State of California created the Groundwater Ambient Monitoring and Assessment (GAMA) Program. The Priority Basin Project of the GAMA Program provides a comprehensive assessment of the State’s groundwater quality and increases public access to groundwater-quality information. Two small watersheds of the Fresno and San 

Joaquin Rivers in the Central Sierra Nevada constitute one of the study units being evaluated.

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The Central Sierra Nevada Study Unit

The Central Sierra Nevada study unit is approximately 370 square miles and includes no 

California Department of Water Resources defined groundwater basins. The study unit was 

divided into two study areas: the Coarse Gold study area corresponding to the watershed of 

the Fresno River above Hensley Lake, and the Wishon study area corresponding to the watershed of Willow Creek, a tributary of the San Joaquin River (Ferrari and others, 2008).

The primary aquifers in the study unit are fractured bedrock. Most of the bedrock is 

granitic rock, which is the most abundant rock type in the Sierra Nevada, with the remainder 

being metamorphic rock. Groundwater occurs in open joints, fractures, and exfoliation planes 

in the bedrock. These fracture systems may be interconnected or isolated, resulting in variability in water levels, well yields, and water quality on local and regional scales (California 

Department of Water Resources, 2003). 

The primary aquifers in the study unit are defined as those parts of the aquifers corresponding to the open intervals of wells listed in the California Department of Public Health 

database. In the Central Sierra Nevada study unit, these wells typically are drilled to depths 

between 300 and 750 feet, consist of solid casing from land surface to a depth of about 50 to 

75 feet, and are open below the solid casing. 

In the Sierra Nevada, ore 

deposits commonly are associated 

with the contacts between granitic 

and metamorphic rocks. The Central 

Sierra Nevada study unit contains 

many small mining prospect sites 

(nearly all inactive), mainly for gold 

or tungsten (U.S. Geological Survey, 2005). Minerals associated with 

naturally-occurring ore deposits commonly contain high concentrations of 

constituents that may adversely affect 

groundwater quality. 

 The Central Sierra Nevada 

study unit has warm, dry summers 

and cold, wet winters. Average annual 

precipitation ranges from 20 inches 

at Hensley Lake to 50 inches in the 

mountains in the northern part of the 

study unit. Land use in the Central Sierra Nevada study unit is approximately 95 percent (%) 

undeveloped (forests and grasslands), and 5% urban. The undeveloped lands are used mostly 

for recreation and open-range grazing. The largest urban area is the city of Oakhurst.

Municipal and community water supply accounts for nearly all of the total water use in 

the Central Sierra Nevada study unit. Groundwater provides nearly all of the water supply, 

with limited use of surface water in some areas. Recharge to the groundwater flow system 

is mainly from stream-channel infiltration and direct infiltration of precipitation. Groundwater leaves the aquifer system mainly when it is pumped for water supply or flows into 

streams or lakes.

Overview of Water Quality

GAMA’s Priority Basin Project 

evaluates the quality of untreated 

groundwater. However, for context, 

benchmarks established for drinkingwater quality are used for comparison. 

Benchmarks and definitions of high, 

moderate, and low concentrations are 

discussed in the inset box on page 3.

 Many inorganic constituents 

occur naturally in groundwater. The 

concentrations of the inorganic constituents can be affected by natural 

processes as well as by human activities. In the Central Sierra Nevada study 

unit, one or more inorganic constituents 

were present at high concentrations in 

about 41% of the primary aquifers and 

at moderate concentrations in 36%.

Human-made organic constituents 

are found in products used in the home, 

business, industry, and agriculture. 

Organic constituents can enter the environment through normal usage, spills, 

or improper disposal. In this study unit, 

organic constituents were not present 

at high concentrations in the primary 

aquifers, and were present at moderate 

concentrations in about 4%.

Watershed boundary

40 8 Miles

0 4 8 Kilometers

Mariposa

County

Madera

County

Fresno

County

Chowchilla River Chowchilla River

Fresno River Fresno River

San JoaquinSan Joaquin River

Willow

Creek

Willow

Creek

Bass

Lake

Hensley Lake

Wawona

Oakhurst

Coarse Gold

study area

Wishon

study area

Organic

constituents

Inorganic

constituents

High Moderate

CONSTITUENT CONCENTRATIONS

Low or not detected 

Values are a percentage of the area of the primary aquifers

with concentrations in the three specified categories. Values

on pie chart may not equal 100 due to rounding of percentages.

23 41

36 96

4
 

 

RESULTS: Groundwater Quality in the Central Sierra Nevada Study Unit

Inorganic Constituents with Human-Health Benchmarks

Trace and minor elements are naturally present in the minerals in rocks and 

soils, and in the water that comes into contact with those materials. In the Central 

Sierra Nevada study unit, trace elements were present at high concentrations in 

about 25% of the primary aquifers, and in moderate concentrations in about 32%. 

Arsenic, molybdenum, and fluoride were the trace elements that most frequently 

occurred at high and moderate concentrations. Chromium and selenium also were 

detected at high concentrations, but in less than (<) 1% of the primary aquifers.

Radioactivity is the emission of energy or particles during spontaneous decay 

of unstable atoms. Humans are exposed to small amounts of natural radioactivity 

every day. Most of the radioactivity in groundwater comes from decay of naturallyoccurring isotopes of uranium and thorium in minerals in the rocks or sediments of 

the aquifers. Radioactive constituents occurred at high levels in about 31% of the 

primary aquifers, and at moderate levels in about 26%. Gross alpha particle activity, 

and uranium were the radioactive constituents that most frequently occurred at high 

and moderate levels. 

Nutrients, such as nitrogen and phosphorous, are naturally present at low concentrations in groundwater. High and moderate concentrations generally occur as a 

result of human activities. Common sources of nutrients include fertilizer applied to 

crops and landscaping, seepage from septic systems, and human and animal waste. 

In the Central Sierra Nevada study unit, nutrients were not detected at high concentrations in the primary aquifers, and were present at moderate concentration in 

about 3%.

Inorganic Constituents with Non-Health Benchmarks

(Not included in water-quality overview charts shown on the front page) 

Some constituents affect the aesthetic properties of water, such as taste, 

color, and odor, or may create nuisance problems, such as staining and scaling. 

The State of California has a recommended and an upper limit for total dissolved 

solids (TDS) in drinking water. All water naturally contains TDS as a result of the 

weathering and dissolving of minerals in soils and rocks. Iron and manganese are 

naturally-occurring constituents that commonly occur together in groundwater. 

Anoxic conditions in groundwater (very low amounts of dissolved oxygen) may 

result in release of manganese and iron from minerals into groundwater. 

In the Central Sierra Nevada study unit, TDS were present at high concentrations (greater than the upper limit) in about 2% of the primary aquifers, and at 

moderate concentrations (between the upper and recommended limits) in about 3%. 

Manganese, with or without iron, was present at high concentrations in about 32% 

of the primary aquifers, and at moderate concentrations in about 9% of the primary 

aquifers. 

 Perchlorate 

(Not included in water-quality overview charts shown on the front page)

Perchlorate is an inorganic constituent that has been regulated in California 

drinking water since 2007. It is an ingredient in rocket fuel, fireworks, safety flares, 

and other products, may be present in some fertilizers, and occurs naturally at low 

concentrations in groundwater. Perchlorate was not detected in the primary aquifers. 

Trace 

elements

Total 

dissolved 

solids

Radioactive

constituents

Nutrients

Manganese

Perchlorate

INORGANIC CONSTITUENTS

INORGANIC CONSTITUENTS SPECIAL-INTEREST CONSTITUENTS

43

25

32

43

26

31

95

100

97

58

32

9

2 4

3
RESULTS: Groundwater Quality in the Central Sierra Nevada Study Unit

Organic Constituents

The Priority Basin Project uses laboratory methods that can detect the presence 

of low concentrations of volatile organic compounds (VOCs) and pesticides, far below 

human-health benchmarks. VOCs and pesticides detected at these very low concentrations can be used to help trace water from the landscape into the aquifer system.

Volatile Organic Compounds with Human-Health Benchmarks

VOCs are in many household, commercial, industrial, and agricultural products, 

and are characterized by their tendency to volatilize (evaporate) into the air. 

Gasoline additives and oxygenates are used to increase the efficiency of fuel 

combustion. In the Central Sierra Nevada study unit, gasoline additives and oxygenates 

were not present at high concentrations, but were detected at moderate concentrations in 

about 4% of the primary aquifers, and at low concentrations (or not detected) in about 

96%.

Other VOCs include trihalomethanes, solvents, refrigerants, and organic synthesis 

reagents. Trihalomethanes form during disinfection of water supplies, and may enter 

groundwater by the infiltration of landscape irrigation water, the leaking of distribution 

lines, or the disinfection of wells. Solvents are used for a number of purposes, including manufacturing and cleaning. Other VOCs were not detected at high or moderate 

concentrations in the primary aquifers. Trihalomethanes and solvents, when detected, 

were at low concentrations.

Pesticides with Human-Health Benchmarks

Pesticides, including herbicides, insecticides, fungicides, and fumigants, are 

applied to crops, gardens, lawns, around buildings, and along roads to help control 

unwanted vegetation (weeds), insects, fungi, and other pests. In the Central Sierra 

Nevada study unit, pesticides were not detected at high or moderate concentrations in 

the primary aquifers. Herbicides occasionally were detected at low concentrations. 

BENCHMARKS FOR EVALUATING GROUNDWATER QUALITY

 GAMA’s Priority Basin Project uses benchmarks established for drinking water to 

provide context for evaluating the quality of untreated groundwater. After withdrawal, 

groundwater may be disinfected, filtered, mixed, and exposed to the atmosphere before 

being delivered to consumers. Federal and California regulatory benchmarks for protecting human health (Maximum Contaminant Level, MCL) were used when available. 

Nonregulatory benchmarks for protecting aesthetic properties, such as taste and odor 

(Secondary Maximum Contaminant Level, SMCL), and nonregulatory benchmarks for 

protecting human health (Notification Level, NL, and Lifetime Health Advisory, HAL) 

were used when Federal or California regulatory benchmarks were not available.

High, moderate, and low concentrations are defined relative to benchmarks

Concentrations are considered high if they are greater than a benchmark. For inorganic constituents, concentrations are moderate if they are greater than one-half of a benchmark. For organic constituents and perchlorate, concentrations are moderate if they are 

greater than one-tenth of a benchmark; this lower threshold was used because organic constituents are generally less prevalent and 

have smaller concentrations relative to benchmarks than inorganic constituents. Low values include nondetections and values less than 

moderate concentrations. Methods for evaluating water quality are discussed in Fram and Belitz (2012).

Gasoline

additives and

oxygenates

Pesticides

Other volatile

organic

compounds 

ORGANIC CONSTITUENTS

4

96

100

100

High concentrations

Moderate concentrations

EX

Low concentrations

AMPLE Values are a percentage of the area of the primary aquifers

with concentrations in the three specified categories. Values

on pie chart may not equal 100 due to rounding of percentages.

CONSTITUENT CONCENTRATIONS
Factors that Affect Groundwater Quality

In the Central Sierra Nevada study unit, inorganic constituents were present at high 

or moderate concentrations in about 77% of the primary aquifers. Arsenic, boron, fluoride, 

molybdenum, and uranium were the constituents most frequently present at high or moderate 

concentrations (pie charts above the graph show porpotions for study unit). These constituents 

are derived from dissolution of naturally-occurring minerals in the granitic and metamorphic 

rocks of the aquifers. Dissolution requires appropriate groundwater geochemical conditions 

(such as pH and dissolved oxygen concentration), enough time for dissolution to occur, and 

presence of minerals containing these constituents. Mining activities may increase dissolution 

rates by exposing rocks to air and increasing porosity and permeability, thereby increasing 

contact between minerals and groundwater.

Tritium and carbon-14 are radioactive tracers produced in the atmosphere and present in 

precipitation. They are used to estimate the time since groundwater recharged into aquifers. 

In the Central Sierra Nevada study unit, high and moderate concentrations of arsenic, boron, 

fluoride, molybdenum, and uranium were associated with longer time since recharge. Modern 

groundwater (recharged in the last 60 years) had one or none of the five constituents present at 

high or moderate concentrations. Ancient groundwater (recharged thousands of years ago) and 

mixtures of modern and ancient groundwater had two or more constituents present at high or 

moderate concentrations.

Arsenic

MCL (Federal)

10 ppb

Boron

NL (State)

1 ppm

Fluoride

MCL (State)

2 ppm

Molybdenum

HAL (Federal)

40 ppb

Uranium

MCL (State)

20 picocurie per liter

TRITIUM ACTIVITY, IN TRITIUM UNITS

0 1 2 3 4

UNCORRECTED CARBON-14 AGE, IN YEARS

modern

4,000

8,000

12,000

16,000

66

19

15

87

13

84

9 7

60 29

11

71

15

14

modern

groundwater

ancient

groundwater

mixed

groundwater

Number of constituents

present at moderate or

high concentration

Wishon Coarse Gold

0 or 1

3 or 4

2

ppm, parts per million; ppb, parts per billion

By Miranda S. Fram and Kenneth Belitz

SELECTED REFERENCES

California Department of Water Resources, 2003, California’s groundwater: California Department of 

Water Resources Bulletin 118, 246 p. http://www.water.ca.gov/groundwater/bulletin118/update2003.

cfm.

Ferrari, M.J., Fram, M.S., and Belitz, Kenneth, 2008, Groundwater quality data in the Central Sierra 

study unit, 2006—Results from the California GAMA Program: U.S. Geological Survey Data Series 

335, 59 p. (Also available at http://pubs.usgs.gov/ds/335/.)

Fram, M.S., and Belitz, Kenneth, 2012, Status and understanding of groundwater quality in the TahoeMartis, Central Sierra, and Southern Sierra study units, 2006–2007—California GAMA Program 

Priority Basin Project: U.S. Geological Survey Scientific Investigations Report 2011-5216, 222 p. 

(Also available at http://pubs.usgs.gov/sir/2011/5216.)

U.S. Geological Survey, 2005, Mineral Resources Data System: U.S. Geological Survey. http://tin.

er.usgs.gov/mrds/

Priority Basin Assessments 

GAMA’s Priority Basin Project 

(PBP) assesses water quality in that 

part of the aquifer system used for 

drinking water, primarily public supply. 

Water quality in the primary aquifers, 

assessed by the PBP, may differ from 

that in the deeper parts of the aquifer, 

or from the shallower parts, which are 

being assessed by GAMA’s Domestic 

Well Project. Ongoing assessments 

are being conducted in more than 120 

basins throughout California.

The PBP assessments are based on 

a comparison of constituent concentrations in untreated groundwater with 

benchmarks established for protection of human health and for aesthetic 

concerns. The PBP does not evaluate 

the quality of drinking water delivered 

to consumers.

The PBP uses two scientific 

approaches for assessing groundwater 

quality. The first approach uses a network of wells to statistically assess the 

status of groundwater quality. The second approach combines water-quality, 

hydrologic, geographic, and other data 

to help assess the factors that affect 

water quality. In the Central Sierra 

Nevada study unit, data were collected 

by the PBP in 2006, and from the 

CDPH database for 2003–2006. The 

PBP includes chemical analyses generally not available as part of regulatory 

compliance monitoring, including 

measurements at concentrations much 

lower than human-health benchmarks, 

and measurement of constituents that 

can be used to trace the sources and 

movement of groundwater.

For more information

Technical reports and hydrologic 

data collected for the GAMA PBP Program may be obtained from:

GAMA Project Chief

U.S. Geological Survey

California Water Science Center

4165 Spruance Road, Suite 200

San Diego, CA 92101

Telephone number: (619) 225-6100

WEB: http://ca.water.usgs.gov/gama

GAMA Program Unit

State Water Resources Control Board

Division of Water Quality

PO Box 2231, Sacramento, CA 95812

Telephone number: (916) 341-5779

WEB: http://www.waterboards.ca.gov/gama