U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2014–3096 

December 2014

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

Groundwater Quality in the 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 (PBP) of the GAMA Program provides a comprehensive assessment of the 

State’s groundwater quality and increases public access to groundwater-quality information. The Sierra 

Nevada Regional study unit constitutes one of the study units being evaluated. 

The Sierra Nevada Regional Study Unit

The Sierra Nevada Regional (SNR) study unit covers approximately 25,000 square miles 

and includes the Sierra Nevada mountain and foothill regions. The study unit was divided into 

four study areas corresponding to the four major rock types present in the region: granitic rocks, 

metamorphic rocks, sedimentary deposits, and volcanic rocks (Shelton and others, 2010). In 

contrast to most other study units evaluated by the GAMA PBP, the SNR study unit consists 

primarily of fractured-bedrock aquifers. 

Most of the Sierra Nevada consists of 

granitic and metamorphic rocks. Groundwater is in fractures in the rocks. The 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 

Kern Co

Tulare Co

Fresno Co

Madera Co

Alpine Co

Plumas Co

Lassen Co

Butte Co Sierra Co

Yuba Co

Nevada Co

Placer Co

El Dorado Co

Amador Co

Calaveras Co

Mono Co

Inyo Co

Mariposa Co

Tuolumne Co

121° 120° 119° 118°

40°

39°

38°

37° 

36°

35°

0 25 50 MILES

0 5025 KILOMETERS

Granitic

Metamorphic

Sedimentary

Volcanic

Aquifer lithology

Pacific Ocean

Water Resources, 2003). Sedimentary basins 

contain glacial, alluvial fan, fluvial, and lake 

sediments. Aquifers composed of different 

materials commonly contain groundwater 

with different chemical compositions.

The primary aquifer is defined as 

those parts of the aquifer system tapped 

by wells and springs listed in the State of 

California database of public drinking-water 

supply sources. In the SNR study unit about 

25 percent (%) of these sources are springs. 

Most wells are drilled to depths of 150 to 

600 feet, consist of solid casing or a seal 

from the land surface to a depth of about 50 

to 200 feet, and are open or have perforated 

casing below that depth. Water quality in 

the primary aquifer system may differ from 

that in the shallower and deeper parts of the 

aquifer system.

Average annual precipitation ranges 

from 11 inches at low elevations to more than 

80 inches at high elevations, and the majority 

of precipitation falls as snow. Rivers flowing 

from the Sierra Nevada provide a large part 

of the surface water used elsewhere in the State. About 90% of the land in the study unit is natural, and about 60% consists of national forests, national parks, and other federally managed lands. 

Urban land use is about 5% and is rapidly growing, placing increasing demands on groundwater 

supplies. The remaining 5% is in agricultural use.

Recharge to fractured-bedrock aquifers is mainly from stream-channel infiltration and direct 

infiltration of precipitation and snow melt. Sedimentary basin aquifers also are recharged by 

mountain-front recharge at the margins of the basins. Groundwater exits the aquifer system when 

it is pumped for water supply, flows into streams and lakes, discharges from springs, or leaves 

areas with a shallow depth to groundwater by evapotranspiration.

Overview of Water Quality

Organic

constituents

Inorganic

constituents

High Moderate

CONSTITUENT CONCENTRATIONS

Low or not detected 

Values are a percentage of the area of the primary aquifer system

with concentrations in the three specified categories. Values on

piechart may not equal 100 due to rounding of percentages.

<1

97

16

63

21

3

GAMA’s Priority Basin Project evaluates the quality of untreated groundwater. 

However, for context, groundwater quality 

was compared to benchmarks established 

for drinking-water quality. Benchmarks 

and definitions of high, moderate, and low

relative-concentrations are discussed in the 

inset box on page 3.

Many inorganic constituents occur naturally in groundwater. The concentrations 

of inorganic constituents can be affected 

by natural processes as well as by human 

activities. In the SNR study unit, one or 

more inorganic constituents with humanhealth benchmarks were present at high 

concentrations in about 16% of the primary 

aquifer system and at moderate concentrations in about 21%.

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 the SNR study unit, one or 

more organic constituents were present at 

high concentrations in less than (<) 1% of 

the primary aquifer system and at moderate 

concentrations in about 3%.

Printed on recycled paper
RESULTS: Groundwater Quality in the Sierra Nevada Regional Study Unit

 

 

Trace

elements 

Total

dissolved

solids 

Nutrients

Uranium and

radioactive

constituents

Manganese

and/or iron

Perchlorate

INORGANIC CONSTITUENTS

SPECIAL-INTEREST CONSTITUENT

11

16

73

72

8

96

3

77

18

5

1

99

<1

1

96

1 3

20

Inorganic Constituents with Health-Based Benchmarks

Trace elements are naturally present in the minerals in rocks and soils, and in 

the water that comes into contact with those materials. In the SNR study unit, trace 

elements were present at high concentrations in about 11% of the primary aquifer 

system and at moderate concentrations in about 16%. Arsenic, boron, fluoride, and 

molybdenum were the trace elements that most frequently occurred at high and 

moderate concentrations. Antimony, beryllium, cadmium, copper, lead, selenium, 

and strontium also were detected at high concentrations, but in <1% of the primary 

aquifer system. 

Radioactivity is the release of energy or energetic 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 the decay of 

naturally occurring isotopes of uranium and thorium in minerals in aquifer materials. 

Radioactive constituents occurred at high levels in about 8% of the primary aquifer 

system and at moderate levels in about 20%. Uranium, radon-222, and gross alpha 

particle activity were the constituents that most frequently occurred at high and moderate levels.

Nutrients are naturally present at low concentrations in groundwater, and high 

and moderate concentrations generally occur as a result of human activities. Common sources of nutrients include fertilizers applied to crops and landscaping, seepage 

from septic systems, and human and animal waste. Nutrients were present at high 

concentrations in about 1% of the primary aquifers, and at moderate concentrations 

in about 3%. Nitrate was the only nutrient detected at high and moderate concentrations. 

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. 

Total dissolved solids (TDS) concentration is a measure of the salinity of the 

groundwater, and all water naturally contains TDS as a result of the weathering and 

dissolution of minerals in rocks and sediments. The State of California has a recommended and an upper limit for TDS in drinking water. TDS concentrations were high 

(greater than the upper limit) in about 1% of the primary aquifer system and moderate (between the recommended and upper limits) in about 3%. 

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

result in release of naturally occurring manganese and iron from minerals into 

groundwater. Manganese, iron, or both were present at high concentrations in about 

18% of the primary aquifer system and at moderate concentrations in about 5%.

Constituent of Special Interest: 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 used in solid rocket fuel, fireworks, safety flares, and 

other products, may be present in some fertilizers, and occurs naturally at low concentrations in groundwater. Perchlorate was detected at high concentrations in <1% 

of the primary aquifer system and at moderate concentrations in about 1%. 
RESULTS: Groundwater Quality in the Sierra Nevada Regional Study Unit

Solvents

Other

VOCs

ORGANIC CONSTITUENTS

Pesticides

Trihalomethanes

and gasoline

oxygenates

96

3 <1

100

>99

<1

100

Organic Constituents

The Priority Basin Project uses laboratory methods that can detect volatile organic 

compounds (VOCs) and pesticides at low concentrations far below human-health 

benchmarks. VOCs and pesticides detected at these low concentrations can be used to 

trace water from the landscape into the aquifer system.

Volatile Organic Compounds with Human-Health Benchmarks

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

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

Solvents are used for a number of purposes, including manufacturing and cleaning. In the SNR study unit, solvents were present at high concentrations in <0.1% of 

the primary aquifer system and at moderate concentrations in about 3%. The solvents 

detected at high concentrations were tetrachloroethene (PCE) and 1,2-dichloroethane. 

Trihalomethanes can form during disinfection of water supplies and may enter 

groundwater by the infiltration of landscape irrigation water or leakage of distribution 

systems. Trihalomethanes were not detected at high concentrations and were present at 

moderate concentrations in <0.1% of the primary aquifer system.

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

help meet air-quality standards. Gasoline oxygenates were not detected at high concentrations and were present at moderate concentrations in <0.1% of the primary aquifer 

system.

Other VOCs, including organic synthesis reagents and refrigerants, were not 

detected at either high or moderate concentrations.

Pesticides with Human-Health Benchmarks

Pesticides, including herbicides, insecticides, 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 SNR study unit, pesticides were not 

detected at high or moderate concentrations in the primary aquifer system. 

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 and special-interest constituents, concentrations are moderate if they 

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

have smaller concentrations relative to benchmarks than inorganic constituents. Low concentrations include non-detections and values 

less than moderate concentrations. Methods for evaluating water quality are discussed by Fram and Belitz (2014). 

High concentrations

Moderate concentrations

Low concentrations

or not detected

EX

AMPLE Values are a percentage of the area of the primary aquifer system

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

Primary aquifer systems in the SNR study unit are composed of granitic rocks, 

metamorphic rocks, sediments, or volcanic rocks and have different groundwater quality. These differences in water quality reflect differences in the composition of the rocks 

and sediments that compose the aquifers, in the occurrence of developed land uses, in 

the types of wells found in different rock types, and in other factors (Fram and Belitz, 

2012, 2014).

The sedimentary aquifers have the greatest proportion with high or moderate concentrations of nitrate. Areas near wells in sediments have greater amounts of urban and 

agricultural land 

use than areas near 

wells in other rock 

Uranium

MCL = 20 pCi/L

or 30 μg/L

Nitrate

MCL = 10 mg/L

Arsenic

MCL = 10 μg/L

Granitic

(51%) 

Metamorphic

(30%) 

Sedimentary

(7%) 

Volcanic

(12%) 

Pie diagrams show proportions of primary aquifer system with

low concentrations of the constituent.

Rock type 

(percent of primary

aquifer system) 

high, moderate,

types, and thus, may 

have more potential anthropogenic 

sources of nitrate to 

groundwater. The 

sedimentary aquifers compose 7% 

of the SNR study 

unit primary aquifer 

system, but contain 

more than 20% of 

the public-supply 

wells in the study 

unit.

The granitic 

and sedimentary 

aquifers have 

greater proportions 

with high or moderate concentrations 

of uranium than do 

the metamorphic 

and volcanic aquifers, reflecting that minerals containing uranium and thorium are typically more common in granitic rocks than in other rock types and that granitic rocks are 

the dominant source of sediments in most of the SNR study unit.

Arsenic was the trace element most commonly present at high or moderate concentrations in all four aquifer systems, and the greatest proportion of high concentrations occurred in the volcanic aquifers. High arsenic concentrations in groundwater in 

volcanic rocks typically were associated with oxic, high-pH conditions, whereas high 

arsenic concentrations in groundwater in other rock types typically were associated with 

anoxic conditions and location in the southwestern part of the SNR study unit where the 

granitic rocks typically have higher concentrations of arsenic and other trace elements 

than in other parts of the study unit. 

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 shallower and 

deeper parts may differ from that in 

the primary aquifer system. GAMA’s 

Domestic Well Project assesses water 

quality in the shallower parts of the 

aquifer system. 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 the 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 SNR study unit, 

data were collected by the PBP in 

2008 and from the State of California 

database for 2006–2008. The PBP 

includes chemical analyses not generally 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 Program 

may be obtained from

GAMA Project Chief

U.S. Geological Survey

California Water Science Center

6000 J Street, Placer Hall

Sacramento, CA 95819

Telephone number: (916) 278-3100

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

GAMA Program Unit Chief

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

SELECTED REFERENCES

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

Resources Bulletin 118, 246 p. 

Fram, M.S., and Belitz, K., 2012, Status and understanding of groundwater quality in the Tahoe-Martis, 

Central Sierra, and Southern Sierra study units, 2006–2007—California GAMA Priority Basin Project: 

U.S. Geological Survey Scientific Investigations Report 2011-5216, 222 p. (Also available at http://pubs.

usgs.gov/sir/2011/5216/.)

Fram, M.S., and Belitz, K., 2014, Status and understanding of groundwater quality in the Sierra Nevada 

Regional study unit, 2008—California GAMA Priority Basin Project: U.S. Geological Survey Scientific 

Investigations Report 2014-5174, 118 p. (Also available at http://pubs.usgs.gov/sir/2014/5174/.) 

 Shelton, J.L., Fram, M.S., Munday, C.M., and Belitz, K., 2010, Groundwater-quality data for the Sierra 

Nevada study unit, 2008—Results from the California GAMA Program: U.S. Geological Survey Data 

Series 534, 106 p. (Also available at http://pubs.usgs.gov/ds/534/.)

By Miranda S. Fram and Kenneth Belitz

ISSN 2327-6916 (print)

ISSN 2327-6932 (online)

http://dx.doi.org/10.3133/fs20143096