Grass

Valley

Nevada City

Sierra City

Yuba River

watershed

Bear River

watershed

Lake

Oroville

New Bullards

Bar Reservoir

Yuba River

Bear River

121°30' 121°

39°

30'

39°

Base modified from U.S. Geological Survey and 

other Federal and State digital data, various 

scales; Albers Equal-Area Conic projection, 

standard parallels are 29° 30' and 45° 30'; North 

American Datum of 1983

People using domestic wells in 1990, 

in number of people per square

kilometer (Johnson and Belitz, 2017)

EXPLANATION

0–0.5

>0.5–5

>5–25

>25

121°30' 121° 120°30'

39°

30'

39° Geologic unit (simplified from

Saucedo and others, 2000)

Alluvial deposits

Volcanic rocks

Ultramafic/mafic intrusive 

 rocks

Granitic intrusive rocks

Metavolcanic and

 metasedimentary rocks

Cenozoic age

Mesozoic and Paleozoic age

EXPLANATION

Wells sampled

0 10 20 MILES

0 10 20 KILOMETERS

Map

area

CALIFORNIA

Sierra

Nevada

U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2017–1115

September 2017

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

Groundwater Quality in the Yuba River and Bear River 

Watersheds, 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 GAMA Program’s Priority Basin Project assesses the quality of groundwater resources used for drinking water supply and increases public access to groundwater-quality information. In the Yuba River and Bear 

River Watersheds of the Sierra Nevada, many rural households rely on private wells for their drinking water supplies.

The Yuba-Bear Watersheds Overview of Water Quality

The Yuba River and Bear River Watersheds study unit (Yuba-Bear Watersheds) 

covers approximately 4,400 square kilometers on the western slope of the Sierra Nevada. 

Groundwater composes about 10 percent of overall water use in the region, but is the sole 

supply for many individual homes beyond the limits of public water supply infrastructure 

(Cosumnes, American, Bear, Yuba Integrated Regional Water Management Group, 2014). 

Recent drought conditions highlighted the vulnerability of private wells to diminished 

groundwater supplies in the study area and many wells required deepening (California 

Department of Water Resources, 2014).

Well water in the Yuba-Bear Watersheds mostly comes from fractured-rock aquifers. 

The quality of groundwater in these aquifers primarily depends on the type of rock, the 

age of the groundwater, and the type 

of human activities at the land surface. 

Previous groundwater studies in this area 

found elevated concentrations of nitrate, 

microbial indicators, and 

some trace elements in 

some wells (California 

State Water Resources 

Control Board, 2010; Fram 

and Belitz, 2014).

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

For context, concentrations measured in 

groundwater are compared to benchmarks 

established for drinking-water quality, such 

as maximum contaminant levels (MCL). 

A concentration above a benchmark is 

defined as high. Benchmarks and definitions of moderate and low concentrations 

are discussed on page 3.

Many inorganic constituents are 

natural in groundwater, and their concentrations can be affected by natural processes as 

well as by human activities. In the YubaBear Watersheds, one or more inorganic 

constituents were present at high concentrations in about 4 percent of the groundwater 

resources used for domestic drinking water.

Organic constituents are found in 

products used in the home, business, 

industry, and agriculture, and can enter the 

environment through normal usage, spills, 

or improper disposal. Organic constituents 

were not present at high concentrations 

in the groundwater resources used for 

domestic drinking water in the Yuba-Bear 

Watersheds.

ISSN 0196-1497 (print)

ISSN 2331-1258 (online)

https://doi.org/10.3133/ofr20171115

CONSTITUENT CONCENTRATIONS

High Moderate Low or not detected

Values indicate percentages of the area of the 

groundwater resources used for domestic drinking 

water with concentrations in the three specified 

categories.

Inorganic

constituents

Organic

constituents

85

11 4

97

3

This study was designed to provide a statistically representative assessment of the 

quality of groundwater resources used for domestic drinking water in the Yuba-Bear Watersheds. A total of 71 wells and 4 springs were sampled between October 2015 and May 2016 

(Jasper and others, 2017). The wells in the study unit typically were 30–150 meters deep, 

and water levels typically were 7–25 meters below land surface.
 

 INORGANIC CONSTITUENTS

RESULTS: Groundwater Quality in the Yuba River and Bear River Watersheds, Sierra Nevada, California

Inorganic Constituents with Human-Health Benchmarks

Historical gold-mining activity in the Yuba-Bear Watersheds has resulted in 

elevated concentrations of mercury and other trace elements in water and sediments 

in rivers and reservoirs. Trace elements also are naturally present in the minerals 

of rocks and sediments and in the groundwater that comes into contact with those 

materials. About 4 percent of the groundwater resources used for domestic drinking 

water had high concentrations of one or more trace elements and 8 percent had 

moderate concentrations. Four trace elements were present at high concentrations 

(arsenic, barium, molybdenum, and strontium), and three were present at moderate 

concentrations (arsenic, molybdenum, and vanadium). Mercury was detected at low 

concentrations in 4 percent of the groundwater resources.

Most of the radioactivity in groundwater comes from the decay of naturally 

radioactive isotopes of uranium, thorium, and potassium in minerals in aquifer 

materials. Radioactive constituents were not present at high levels in the 

groundwater resources used for domestic drinking water. Gross alpha-particle 

activity and gross beta-particle activity were present at moderate levels in about 

3 percent.

Nutrients, including nitrate, are naturally present at low concentrations in 

groundwater, but moderate and high concentrations generally indicate contamination 

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

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

Nitrate was not present at high concentrations in the groundwater resources used for 

domestic drinking water.

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 can create nuisance problems, such as staining and scaling. The 

benchmarks used for these constituents were non-regulatory secondary maximum 

contaminant level benchmarks. 

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

the groundwater, and all water naturally contains dissolved solids 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. Total 

dissolved solids were not present at high concentrations (greater than the upper 

limit), but was present at moderate concentrations (between the recommended and 

upper limits) in about 1 percent of the groundwater resources used for domestic 

drinking water. 

Anoxic conditions (low amounts of dissolved oxygen) can result in the release 

of natural manganese, iron, and other associated trace elements from minerals 

into groundwater. Manganese or iron was present at high concentrations in about 

20 percent of the groundwater resources used for domestic drinking water. In the 

entire Sierra Nevada, groundwater from wells in metamorphic rocks more commonly 

had high concentrations of manganese or iron than groundwater from wells in 

granitic, volcanic, or sedimentary rocks (Fram and Belitz, 2014). Over half of the 

wells sampled for the Yuba-Bear Watersheds were in metamorphic rocks.

sac16-0631_fig pies

Trace elements

Uranium and radioactive

constituents

Nitrate

Total dissolved solids

Manganese or iron

88

8 4

99

1

20

7

73

99

1

97

3
ORGANIC CONSTITUENTS

RESULTS: Groundwater Quality in the Yuba River and Bear River Watersheds, Sierra Nevada, California

Organic Constituents with Human-Health Benchmarks

The Priority Basin Project used laboratory methods that can detect concentrations of volatile 

organic compounds (VOCs) and pesticides that are below human-health benchmarks. The VOCs 

and pesticides detected at these very low concentrations can be used to help trace movement of 

water from the land surface into the aquifer system. 

Volatile organic compounds, including solvents, gasoline components, and refrigerants, are 

contained in many household, commercial, and industrial products. No VOCs were detected at 

high concentrations in the groundwater resources used for domestic drinking water in the YubaBear Watersheds, and VOCs were detected at moderate concentrations in about 3 percent. The 

VOCs detected at moderate concentrations were tetrachloroethene (PCE), trichloroethene (TCE), 

and toluene. 

Pesticides, including herbicides, insecticides, and fumigants, are applied to crops, gardens, 

lawns, around buildings, and along roads to help control unwanted vegetation, insects, fungi, and 

other pests. Pesticides were not detected at high or moderate concentrations in the groundwater 

resources used for domestic drinking water. Low concentrations of herbicides or degradates of 

herbicides were detected in about 4 percent of the groundwater resources.

Microbial Indicators 

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

Microbial indicators are used to evaluate the potential for fecal contamination of water 

sources. In the Yuba-Bear Watersheds, total coliforms and Escherichia coli (E. coli) were 

detected in 5 percent of the wells sampled, and total coliforms alone were detected in another 

17 percent of the wells samples. Total coliforms are present naturally in soils and in digestive 

tracts of animals, whereas E. coli specifically indicate contamination with animal (or human) 

fecal waste (California State Water Resources Control Board, 2016). 

METHODS FOR EVALUATING GROUNDWATER QUALITY

This study used comparison to benchmarks established for drinking water to provide 

context for evaluating the quality of groundwater. The quality of drinking water can differ 

from the quality of groundwater because of contact with household plumbing, exposure to 

the atmosphere, or water treatment. U.S. Environmental Protection Agency and California 

State Water Resources Control Board Division of Drinking Water regulatory benchmarks set 

for the protection of human health (maximum contaminant level, MCL) were used for comparison when available. Otherwise, non-regulatory benchmarks set for protection of aesthetic 

and technical properties, such as taste and odor (secondary maximum contaminant level, 

SMCL), and non-regulatory benchmarks set for the protection of human health (notification 

levels, NL, and lifetime health advisory levels, HAL) were used. Water quality in domestic 

wells is not regulated in California.

Benchmark type and value for selected constituents.

[Benchmark type: CA, State Water Resources Control Board Division of Drinking Water; EPA, U.S. Environmental Protection Agency; HAL, lifetime health 

advisory level; MCL, maximum contaminant level; NL, notification level; SMCL, secondary maximum contaminant level. Abbreviations: pCi/L, picocuries per 

liter; ppb, parts per billion or micrograms per liter (μg/L); ppm, parts per million or milligrams per liter (mg/L)]

Constituent Benchmark Constituent Benchmark

Type Value Type Value

Arsenic EPA MCL 10 ppb Nitrate, as nitrogen EPA MCL 10 ppm

Barium CA MCL 1,000 ppb Total dissolved solids (TDS),

 upper and recommended CA SMCL 1,000 ppm

Molybdenum EPA MCL 40 ppb 500 ppm

Strontium EPA HAL 4,000 ppb Manganese CA SMCL 50 ppb

Vanadium CA NL 50 ppb Iron CA SMCL 300 ppb

Mercury EPA MCL 2 ppb Tetrachloroethene (PCE) EPA MCL 5 ppb

Gross alpha-particle activity EPA MCL 15 pCi/L Trichloroethene (TCE) EPA MCL 5 ppb

Gross beta-particle activity CA MCL 50 pCi/L Toluene CA MCL 150 ppb EX

AMPLE CONSTITUENT CONCENTRATIONS

Pie diagrams are used to summarize groundwater quality results. 

The pie slices represent the percentages of the groundwater 

resources with high, moderate, and low concentrations of a 

constituent. Methods for calculating these percentages are 

discussed by Fram and Belitz (2014).

High: Concentrations are greater 

 than the benchmark

Low: Concentrations are less than moderate

 or the constituent was not detected

Moderate: Concentrations 

 are less than the benchmark, 

 but greater than one-half 

 (for inorganic constituents)

 or one-tenth (for organic 

 constituents) 

sac16-0602_fig pies

VOCs

Pesticides

100

E. Coli and total coliforms

78

5

17

97

3
ISSN 2327-6916 (print)

ISSN 2327-6932 (online)

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

GAMA’s Priority Basin Project 

(PBP) assesses water quality in 

groundwater resources used for 

public and domestic drinking-water 

supplies. This study in the Yuba and 

Bear River Watersheds in the Sierra 

Nevada focused on groundwater 

resources used for domestic drinking 

water. Ongoing assessments are being 

carried out in more than 120 basins 

and areas outside of basins throughout 

California. The PBP assessments 

compare 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.

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 waterquality, hydrologic, geographic, and 

other data to help assess the factors 

that affect water quality. In the 

Yuba-Bear Watersheds study unit, 

data were collected by the PBP in 

2015–16. 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) 278-3000

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) 341-5855

WEB: 

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

By Miranda S. Fram, Monica Jasper, and Kim A. Taylor

REFERENCES CITED

California Department of Water Resources, 2014, Public update for drought response: Groundwater basins with potential water 

shortages and gaps in groundwater monitoring, http://groundwater.ca.gov/docs/Drought_Response-Groundwater_Basins_

April30_Final_BC.pdf.

California State Water Resources Control Board, 2010, GAMA Domestic Well Project—Groundwater quality data report, Yuba 

County focus area, http://www.waterboards.ca.gov/water_issues/programs/gama/docs/yubareportsummary.pdf.

California State Water Resources Control Board, 2016, Groundwater Information Sheet: Bacterial Indicators, http://www.waterboards.ca.gov/gama/docs/coc_bacteria_indicators.pdf.

Cosumnes, American, Bear, Yuba Integrated Regional Water Management Group (CABY), 2014, CABY Integrated Regional 

Water Management Plan 2014 Update, http://cabyregion.org/caby-plan/.

Fram, M.S., and Belitz, Kenneth, 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, https://

doi.org/10.3133/sir20145174.

Jasper, M., Bennett, G.L., and Fram, M.S., 2017, Groundwater-quality data in the Yuba and Bear Watersheds Shallow Aquifer 

Study Unit, 2015-2016: Results from the California GAMA Priority Basin Project: U.S. Geological Survey Data Release, 

https://doi.org/10.5066/F73F4MS9. 

Johnson, T.D., and Belitz, Kenneth, 2017, Domestic well locations and populations served in the contiguous U.S.: 1990: Science 

of the Total Environment, v. 607–608, p. 658–668, https://doi.org/10.1016/j.scitotenv.2017.07.018.

Jurgens, B.C., Böhlke, J.K., and Eberts, S.M., 2012, TracerLPM (Version 1): An Excel® workbook for interpreting groundwater 

age distributions from environmental tracer data: U.S. Geological Survey Techniques and Methods Report 4-F3, 60 p. http://

pubs.usgs.gov/tm/4-f3/.

Saucedo, G.J., Bedford, D.R., Raines, G.L., Miller, R.J., and Wentworth, C.M., 2000, GIS data for the geologic map of California: 

California Department of Conservation, Division of Mines and Geology, CD-ROM 2000-007.

Factors that Affect Groundwater Quality

Groundwater is generally a mixture of waters that recharged the aquifer system at different 

times. Groundwater ‘age’ — the time since the water was at the land surface — can range 

from less than 1 year to more than 50,000 years. The distribution of ages in a groundwater 

sample affects the water quality. Some constituents can be present at higher concentrations 

in older groundwater because of slow dissolution of minerals. Some constituents can be 

present at higher concentrations in groundwater with ages corresponding to particular times 

study unit had tritium concentration greater than 1.3 tritium units (TU), indicating they were 

“modern” (recharged after 1950). A few samples had tritium concentrations less than 0.2 TU, 

indicating they were “pre-modern” groundwater recharged before 1950, and the rest were 

mixtures of water recharged before and after 1950. Because most samples indicated relatively 

recent groundwater recharge, groundwater resources used for domestic supply in the YubaBear Watersheds study unit could be vulnerable to water-quality changes resulting from human 

activities at the land surface and to relatively rapid depletion during periods of drought.

when human activities 

introduced contaminants to the 

recharged water. Preliminary 

groundwater ages based on 

tritium data for samples from 

the Yuba-Bear Watershed are 

summarized here.

Tritium is a radioactive 

isotope of hydrogen that forms 

in the atmosphere naturally 

and in nuclear explosions 

and reactors. Atmospheric 

nuclear-weapons testing in the 

1950s and 1960s increased 

tritium concentrations to 

far above natural levels. 

The tritium concentration 

in groundwater can be used 

to estimate whether the 

groundwater recharged before 

or after about 1950 (Jurgens 

and others, 2012). Twothirds of the samples from 

the Yuba-Bear Watershed 

Priority Basin Assessments

121°30' 121° 120°30'

39°

30'

39°

0 10 20 MILES

0 10 20 KILOMETERS

<0.2 (premodern)

0.2 to 1.3 (mixed)

>1.3 (modern)

EXPLANATION

Tritium activity, in tritium units 

(groundwater-age class)

Base modified from U.S. Geological Survey and other Federal and State digital data,

various scales; Albers Equal-Area Conic projection, standard parallels are 29° 30' 

and 45° 30'; North American Datum of 1983

Grass

Valley

Nevada

City

Sierra

City

Yuba River

watershed

Bear River

watershed

Lake

Oroville New Bullards

Bar Reservoir

Yuba

River

Bear

River