U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2013–3088

May 2014 Printed on recycled paper

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

Groundwater Quality in the South Coast Interior 

Basins, 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 

untreated groundwater quality and increases public access to groundwater-quality information. The South 

Coast Interior Basins constitute one of the study units being evaluated. 

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.

2

96

29 34

37

2

36°

38°

122° 120°

Central

Valley

Sierra Nevada

Southern Coast

Ranges

SOUTH COAST

INTERIOR

BASINS

STUDY UNIT

Livermore study area

Gilroy study area

Cuyama study area

Bakersfield

San

Francisco Livermore

P a c i f i c O c e a n

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006. 

Albers Equal Area Conic Projection

0

0 20 40 60 80 KILOMETERS

20 40 60 80 100 MILES

100

Gilroy

Arroyo

de Laguna

Pajaro River

San Benito River

Cuyama River

The South Coast Interior Basins Study Unit

The South Coast Interior Basins (SCI) study unit covers approximately 653 square 

miles and consists of the Livermore, Gilroy-Hollister, Cuyama Valley, Castac Lake Valley, 

Cuddy Canyon Valley, Cuddy Ranch Area, Cuddy Valley, and Mil Potrero groundwater 

basins (California Department of Water Resources, 2003). These basins were grouped into 

three study areas based on geography: Livermore (LIV), Gilroy (GIL), and Cuyama (CUY) 

(Mathany and others, 2009).

 Average annual 

precipitation across 

the study unit varies 

among the three study 

areas—from more than 

20 inches (in.) along the 

northwest margin of the 

LIV study area to less 

than 8 in. in the northern 

parts of the CUY study 

area. The study areas are 

drained by the Arroyo de 

la Laguna canal in LIV, 

the Pájaro River and the 

San Benito River in GIL, 

and the Cuyama River 

in CUY. The primary 

aquifer system consists of 

Plio-Pleistocene sedimentary formations, Tertiary 

to Holocene-age alluvial 

sedimentary sequences, 

and unconfined Holocene-age and Pleistoceneage alluvium. The 

primary aquifer system 

is defined as those parts 

of the aquifers corresponding to the perforated intervals of the public-supply wells listed in 

the California Department of Public Health (CDPH) database. Public-supply wells in the 

study unit are typically drilled to depths between 200 and 600 feet, consist of solid casing 

from the land surface to a depth of about 50 to 350 feet, and are perforated below the solid 

casing. Water quality in the primary aquifer system may differ from that in the shallower 

and deeper parts of the aquifer system. 

Land use within 500-meter buffer areas around the wells in the study unit is approximately 44 percent (%) agricultural, 29% urban, and 27% natural. Agricultural lands are 

mostly orchards, row crops, vineyards, or pastures. The largest urban areas are the cities of 

Livermore and Gilroy. 

Recharge to groundwater basins occurs as infiltration of spring and streamflow, direct 

infiltration of precipitation and irrigation, subsurface inflow, and engineered recharge 

(percolation ponds and river regulation). The primary sources of groundwater discharge 

are water pumped for irrigation and municipal supply, discharge to rivers and streams, 

and evaporation.

Overview of Water Quality

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

However, for context, benchmarks established for drinking-water 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 in groundwater 

can be affected by natural processes as well 

as by human activities. In the SCI study 

unit, one or more inorganic constituents 

with human-health benchmarks were present at high concentrations in about 29% of 

the primary aquifer system and at moderate 

concentrations in about 37%.

Human-made organic constituents 

can be found in products used in the home, 

business, industry, and agriculture, and 

can enter the environment through normal 

usage, spills, or improper disposal. In the 

SCI study unit, one or more organic constituents were present at high concentrations in 

about 2% of the primary aquifer system and 

at moderate concentrations in about 2%.
 

 

Trace and

minor elements 

Total

dissolved

solids 

Nutrients

Uranium and

radioactive

constituents

Iron or

manganese

Perchlorate

INORGANIC CONSTITUENTS

SPECIAL-INTEREST CONSTITUENT

20

57 23

82

18

71

14

14

43

29 29

71

23

6

34

66

<1

RESULTS: Groundwater Quality in the South Coast Interior Basins Study Unit

Inorganic Constituents with Health-Based 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 SCI study unit, 

trace and minor elements were detected at high concentrations in about 20% of the 

primary aquifer system and at moderate concentrations in about 23%. Arsenic, boron, 

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

concentrations.

Radioactivity is the release of energy or energetic particles during spontaneous decay of unstable atoms. Most of the radioactivity in groundwater comes from 

the decay of naturally occurring isotopes of uranium and thorium in minerals in the 

sediments of the aquifer. Radioactive constituents were not present at high concentrations in the primary aquifer system and were present at moderate concentrations in 

about 18%.

Nutrients, such as nitrate and nitrite, can be naturally present at low concentrations in groundwater. High and moderate concentrations generally occur as a result 

of human activities, such as applying fertilizer to crops. Livestock, when in concentrated numbers, and septic systems also produce nitrogenous waste that can leach 

into groundwater. Nitrate was present at high concentrations in about 14% of the 

primary aquifer system and at moderate concentrations in about 14%.

Inorganic Constituents with Non-Health Benchmarks

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

Some constituents, such as total dissolved solids (TDS), can affect the aesthetic 

properties of water, such as taste, color, and odor, or may create nuisance problems, 

such as staining and scaling. All water naturally contains TDS as a result of the 

weathering and dissolution of minerals in soils. The State of California has a recommended and an upper limit for TDS in drinking water. In the SCI study unit, TDS 

was present at high concentrations (greater than the upper limit) in 29% of the primary aquifer system and at moderate concentrations (between the upper and recommended limits) in 43%. Most of the sites with high concentrations of TDS also had 

high concentrations of sulfate.

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

result in release of naturally occurring elements such as iron and manganese from 

minerals into groundwater. Iron, or manganese, or both were present at high concentrations in 23% of the primary aquifer system.

Constitutient 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 an ingredient in rocket fuel, fireworks, safety flares 

and other products, may be present in some fertilizers, and also occurs at low concentrations under natural conditions in groundwater. Perchlorate was detected at high 

concentrations in <1% of the primary aquifer system and at moderate concentrations 

in about 34%. 
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

Solvents

Other

VOCs

ORGANIC CONSTITUENTS

Herbicides and

insecticides

(including

fumigants)

Trihalomethanes

97

2 <1

100

99

<1

100

RESULTS: Groundwater Quality in the South Coast Interior Basins Study Unit

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, which include solvents and trihalomethanes, are 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 SCI study unit, solvents were present at high concentrations in <1% of the 

primary aquifer system. The solvent detected at high concentrations was tetrachloroethene (PCE). Solvents were present at moderate concentrations in about 2% of the 

primary aquifer system and were low or not detected in about 97%.

Trihalomethanes may form during municipal water purification and can enter 

groundwater by the infiltration of landscape irrigation water. One trihalomethane, 

bromoform, was detected at high concentrations in <1% of the primary aquifer system. 

Trihalomethanes were present at low concentrations, or were not detected, in about 

99% of the primary aquifer system.

Other volatile organic compounds, including organic synthesis reagents, refrigerants, and gasoline additives, were not detected at either high or moderate concentrations.

Pesticides with Human-Health Benchmarks

Pesticides, which include 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 SCI study unit, herbicides 

and insecticides (including fumigants) were not detected at either 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 for the evaluation 

when available. Otherwise, non-regulatory 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 Level, HAL) were used when Federal and 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 (except perchlorate), concentrations are moderate if they are greater than one-half of a benchmark. For organic and special-interest constituents (including perchlorate), 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

includes nondetections and values less than moderate concentrations. Methods for evaluating water quality are discussed in Parsons 

and others (2014).
35°

34°

30’

120° 119°30’ 119°

0 5 10 MILES

0 5 10 KILOMETERS

SAN BENITO

CO

SANTA CLARA CO

MONTEREY CO

SANTA

CRUZ

CO

Gilroy

Hollister

Morgan Hill Coyote Creek

121°45’

37°

36°

45’

121°30’ 121°15’

0 1 2 3 4 5 MILES

0 1 2 3 4 5 KILOMETERS

Frick Lake

Patterson Reservoir

ALAMEDA CO

CONTRA COSTA CO

Dublin

Livermore

Pleasanton

Laurel Creek

121°50’ 121°40’

37°

40’

37°

45’

EXPLANATION

Cuyama

0 10 20 MILES

0 10 20 KILOMETERS

KERN CO

VENTURA CO

SANTA BARBARA CO

SAN LUIS OBISPO CO

Lebec

Cuyama New Cuyama

California Aqueduct

Santa Ynez River

Urban

LAND-USE CLASSIFICATION

Agricultural

Natural

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

Livermore

STUDY AREA

Gilroy

USGS-understanding well USGS or CDPHgrid well 

Measured concentration,

in milligrams per liter

Relativeconcentration

Low

Moderate

High

5.1 to 10

> 10

5.1 to 10

> 10

CDPHother well 

5.1 to 10

> 10

–

< 5.0 <

– 5.0 <– 5.0

Nitrate in the South Coast Interior Basins Study Unit

In the SCI study unit, nitrate was the constituent with a health-based benchmark 

that was most commonly present at high concentrations. About 14% of the primary 

aquifer system had nitrate concentrations greater than the Federal MCL of 10 milligrams per liter as nitrogen. High and moderate concentrations of nitrate in groundwater 

generally indicate that anthropogenic sources of nitrate are present. Potential anthropogenic sources include fertilizer applied to crops and landscaping, animal waste, seepage 

from septic systems, and wastewater discharge.

High and moderate concentrations of nitrate were detected in all three study areas 

of the SCI study unit. Groundwater with high and moderate concentrations of nitrate 

was predominantly classified as oxic and as modern age (Parsons and others, 2013), 

which indicates the groundwater was recharged since 1952. Nitrate concentrations were 

greater in areas with agricultural or urban land use than in areas with natural land use. 

Elevated nitrate concentrations have previously been observed in the Cuyama Valley 

(Everett and others, 2011) and were attributed to irrigation return flows. 

SELECTED REFERENCES

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

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

update2003.cfm

Everett, R.R., Hanson, R.T., and Sweetkind, D.S., 2011, Kirschenmann Road multi-well monitoring site, 

Cuyama Valley, Santa Barbara County, California: U.S. Geological Survey Open-File Report 2011-1292, 

4 p. Available at http://pubs.usgs.gov/of/2011/1292/.

Mathany, T.M., Kulongoski, J.T., Ray, M.C., and Belitz, Kenneth, 2009, Groundwater-quality data in the 

South Coast Interior Basins study unit, 2008—Results from the California GAMA Program: U.S. Geological Survey Data Series Report 463, 82 p. Available at http://pubs.usgs.gov/ds/463/.

Parsons, M.C., Kulongoski, J.T., and Belitz, K., 2014, Status and understanding of groundwater quality in 

the South Coast Interior groundwater basins, 2008—California GAMA Priority Basin Project: U.S. Geological Survey Scientific Investigations Report 2014-5023, 68 p. Available at http://dx.doi.org/10.3133/

sir20145023.

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 PBP 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 for drinking water. 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 South Coast Interior 

Basins study unit, data were collected 

by the PBP in 2008 and compiled from 

the CDPH database for 2005–2008. The 

PBP includes chemical analyses not 

generally required 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

By Mary C. Parsons and Kenneth Belitz

ISSN 2327-6916 (print)

ISSN 2327-6932 (online)

http://dx.doi.org/10.3133/fs20133088