U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2011–3150

June 2012 Printed on recycled paper

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

Groundwater Quality in the Kern County

Subbasin, 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. The Kern 

County Subbasin constitutes 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 aquifers

with concentrations in the three specified categories. Values

on piechart may not equal 100 due to rounding of percentages. 

48 29

23

89

8

2

KINGS CO TULARE CO

KERN CO

SAN LUIS

OBISPO CO

SANTA 

BARBARA CO VENTURA

CO

Poso

Kern

River

California

Aqueduct 

Creek

Bakersfield

Delano

0 10 20 Miles

0 10 20 Kilometers

Sierra Nevada

Coast Ranges

Tehachapi

Mts

San Emigdio Mts

EXPLANATION

Corcoran clay

Kern County subbasin

Friant-Kern Canal

58

99

5

5

The Kern County Subbasin Study Unit

The Kern County Subbasin (KERN) study unit is located at the southern end of California’s 

San Joaquin Valley and consists of the Kern County groundwater subbasin (California Department of Water Resources, 2003; Shelton and others, 2008). The study unit covers about 3,000 

square miles in Kern County. It is bounded by the Kern, Kings, and Tulare County lines to the 

north, the granitic bedrock of the Sierra Nevada and Tehachapi Mountains to the east and southeast, and the marine sediments of the San Emigdio Mountains and Coast Ranges to the southwest 

and west. The KERN study unit has long, hot summer days and cool nights, and mild, damp winters with dense fog. Average annual rainfall is about 6 inches. The Kern River, which originates 

in the Sierra Nevada, is the primary stream flowing through the study unit. 

 The primary aquifers consist of alluvial sediments (mixtures of sand, silt, clay, cobbles, 

and boulders), and marine and continental deposits in the deeper portion of the aquifers. Downward flow of groundwater is impeded by a subsurface clay layer, known as the Corcoran clay, in 

the central part of the KERN study unit. The primary aquifers are defined as those parts of the 

aquifers corresponding to the perforated intervals of wells listed in the California Department of 

Public Health database. 

Public-supply wells 

typically are drilled to 

depths between 600 

and 800 feet below 

land surface, consist of 

solid casing from the 

land surface to a depth 

of about 275–450 feet, 

and are perforated or 

screened below the 

solid casing. Water 

quality in the primary 

aquifers may differ 

from that in the shallower and deeper parts 

of the aquifer system. 

Land use in the 

study unit is approximately 66 percent 

(%) agricultural, 3% 

urban, and 31% natural. 

The primary agricultural uses are for field crops (such as vegetables and hay) and fruit and nut 

orchards. The largest urban area is the City of Bakersfield.

The primary sources of recharge are from the Kern River and artificial recharge at groundwater banking facilities that exist throughout most of the study unit (Tom Haslebacher, Kern 

County Water Agency, written commun., June 15, 2007). Secondary sources of recharge include 

return flows from agricultural and municipal irrigation and infiltration of flows from intermittent 

streams along the edge of the subbasin. The primary sources of groundwater discharge are water 

pumped for irrigation and municipal supply (California Department of Water Resources, 2003).

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 inorganic constituents can be 

affected by natural processes as well as 

by human activities. In the KERN study 

unit, one or more inorganic constituents 

were present at high concentrations in 

23% of the primary aquifers and at moderate concentrations in 29%.

Organic constituents are 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, one or more organic constituents were present at high concentrations 

in 2% of the primary aquifers and at 

moderate concentrations in 8%.
 

 

Trace and 

minor elements

Total

dissolved

solids 

Nutrients

Uranium and

radioactive

constituents

Iron or

manganese

Perchlorate

INORGANIC CONSTITUENTS

SPECIAL-INTEREST CONSTITUENTS

54 27

20

81

13 5

82

6

94

14

17

69

13 6

81

13

6

RESULTS: Groundwater Quality in the Kern County Subbasin 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 

KERN study unit, trace elements were present at high concentrations in 20% of 

the primary aquifers, and at moderate concentrations in 27%. Arsenic, antimony, 

boron, and vanadium were the trace elements that occurred at high concentrations in more than 2% of the primary aquifers. Lead, thallium, and selenium were 

detected at high concentrations, but in less than (<) 2% of the primary aquifers. 

Fluoride is a minor element that was present at high concentrations in 4% of the 

primary aquifers and at moderate concentrations in 2% of the primary aquifers.

Radioactivity is the release of energy or energetic particles during structural 

changes in the nucleus of an atom. Most of the radioactivity in groundwater 

comes from decay of naturally occurring isotopes of uranium and thorium in minerals in the aquifer sediments. In the KERN study unit, radioactive constituents 

were present at high concentrations in 6% of the primary aquifers, and at moderate concentrations in 13% of the primary aquifers. Radium and uranium were 

detected at high concentrations.

Nutrients, such as nitrate and nitrite, are 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 the nutrient present at high concentrations in 5% of the primary aquifers and at moderate concentrations in 13% of the 

primary aquifers.

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), sulfate, and chloride, affect the aesthetic properties of water, such as taste, color, and odor. Other 

constituents, such as iron and manganese, can create nuisance problems, such as 

scaling and staining. 

The State of California has recommended and upper limits for TDS, sulfate, 

and chloride in drinking water. In the KERN study unit, TDS was present at high 

concentrations (greater than the upper limit) in about 14% of the primary aquifers, 

and at moderate concentrations (between the recommended and upper limit) in 

17% of the primary aquifers. Sulfate was present at high concentrations in about 

8% of the primary aquifers, and at moderate concentrations in 6% of the primary 

aquifers. Chloride was present at high concentrations in 2% of the primary aquifers, and at moderate concentrations in 4% of the primary aquifers. 

Iron and manganese are naturally occurring elements, and were present at 

high concentrations in 13% of the primary aquifers. Iron, or manganese, or both 

were present at moderate concentrations in 6% of the primary aquifers.

 Perchlorate and N-Nitrosodimethylamine (NDMA) 

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

Perchlorate, an inorganic constituent, and NDMA, a semi-volatile organic 

compound, are of special interest in California because these constituents have 

recently been found in or are considered to have the potential to affect drinkingwater supplies. Their presence in groundwater is monitored by the California 

Department of Public Health (http://www.cdph.ca.gov). In the KERN study unit, 

perchlorate was not present at high concentrations in the primary aquifers, but 

was present at moderate concentrations in 6%. NDMA was not present at high or 

moderate concentrations in the primary aquifers.
High concentrations

Moderate concentrations

Low concentrations

EX

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

Solvents

Other volatile

organic

compounds

Fumigants 

ORGANIC CONSTITUENTS

Pesticides

Trihalomethanes

<1 4

95

96

4

2

98

100

93

2 4

RESULTS: Groundwater Quality in the Kern County Subbasin Study Unit

Organic Constituents

The Priority Basin Project uses laboratory methods that can detect low concentrations of volatile organic compounds (VOCs) and pesticides, far below human-health 

benchmarks. VOCs and pesticides detected at these 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 into the air.

Solvents are used for a number of purposes, including manufacturing and cleaning. 

In the KERN study unit, solvents were present at high concentrations in <1% of the primary aquifers. The solvents found at high concentrations were carbon tetrachloride and 

trichloroethene. Solvents were present at moderate concentrations in 4% of the primary 

aquifers.

Trihalomethanes may form during municipal water purification and can enter 

groundwater by the infiltration of landscape irrigation water. Trihalomethanes were 

present at moderate concentrations in 4% of the primary aquifers.

Other VOCs include organic synthesis reagents and gasoline hydrocarbons. Other 

VOCs were not present at high concentrations but were present in moderate concentrations in about 2% of the primary aquifers. The VOC found at moderate concentrations 

was benzene, which is a gasoline hydrocarbon.

Pesticides with Human-Health Benchmarks

Pesticides are applied to crops, gardens, lawns, around buildings, and along roads 

to help control unwanted vegetation (weeds), insects, fungi, and other pests. In the 

KERN study unit, pesticides (not including fumigants) were not detected at high or 

moderate concentrations in the primary aquifers. 

The fumigant 1,2-dibromo-3-chloropropane (DBCP) was present at high concentrations in 2% of the primary aquifers. DBCP and other fumigants were present at 

moderate concentrations in 4% of the primary aquifers. Use of DBCP as a soil fumigant 

was discontinued in California in 1977.

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 are generally 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 Burton and others (2012).
S i e r r a N e v a d a

C o a s t R a n g e s

Pacific Ocean

0 20 40 Miles

0 30 60 Kilometers

CALIFORNIA

NEVADA

Bakersfield

Fresno

Modesto

Stockton

Northern

San Joaquin

Valley

Central-Eastside

San Joaquin

Valley

Southeast

San Joaquin

Valley

Kern County

subbasin

Madera-Chowchilla

High

concentration

Moderate

concentration 

Arsenic

Nitrate

Total dissolved solids (TDS)

0

10

20

30

40

PERCENTAGE OF SAMPLES WITH HIGH OR MODERATE CONCENTRATIONS

0

10

20

30

40

0

10

20

30

40

Northern San

Southeast San

Joaquin ValleyJoaquin Valley

MaderaChowchilla

Central-Eastside

San Joaquin Valley

Kern County subbasin

San Joaquin Valley

Inorganic Constituent Concentrations in the Eastern San 

Joaquin Valley

Arsenic, nitrate, and TDS were inorganic constituents that were present at high concentrations in KERN and four other study units in the eastern San Joaquin Valley (ESJV) 

(Bennett and others, 2010; Landon and others, 2010; Burton and others, 2012; J. Shelton, 

USGS, written commun., 2011). Arsenic was the trace element most frequently present 

at high concentrations in KERN (20%) and other ESJV study units (9% to 20%). High 

concentrations of arsenic result from the interaction of groundwater with naturally occurring minerals. Nitrate was present at high concentrations in 5% of the primary aquifers 

in KERN, and ranged from 0% to 9% in other study units in the ESJV. High concentrations of nitrate can result from human activities such as applicaton of fertilizer, animal 

waste, and septic systems. TDS, a constituent with a non-health-based benchmark, was 

present at high concentrations in 

14% of the primary aquifers in 

KERN, the largest proportion in 

the ESJV. High concentrations 

of TDS result from natural and 

human sources.

By Carmen A. Burton and Kenneth Belitz

SELECTED REFERENCES

Bennett, G.L.,V, Fram, M.S., Belitz, Kenneth, and Jurgens, B.C., 2010, Status and understanding of 

groundwater quality in the Northern San Joaquin Basin, 2005—California GAMA Priority Basin 

Project: U.S. Geological Survey Scientific Investigations Report 2010-5175, 82 p. Available at 

http://pubs.usgs.gov/sir/2010/5175.

Burton, C.A., Shelton, J.L., and Belitz, Kenneth, 2012, Status and understanding of groundwater quality in the two Southern San Joaquin Valley study units, 2005–2006—California GAMA Priority 

Basin Project: U.S. Geological Survey Scientific Investigations Report 2011-5218, 150 p. Available 

at http://pubs.usgs.gov/sir/2011/5218.

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.

Landon, M.K., Belitz, Kenneth, Jurgens, B.C., Kulongoski, J.T., and Johnson, T.D., 2010, Status and 

understanding of groundwater quality in the Central–Eastside San Joaquin Basin, 2006—California 

GAMA Priority Basin Project: U.S. Geological Survey Scientific Investigations Report 2009-5266, 

97 p. Available at http://pubs.usgs.gov/sir/2009/5266.

Shelton, J.L., Pimentel, Isabel, Fram, M.S., and Belitz, Kenneth, 2008, Ground-water quality data in 

the Kern County subbasin study unit, 2006—Results from the California GAMA Program: U.S. 

Geological Survey Data Series Report 337, 75 p. Available at http://pubs.usgs.gov/ds/337.

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 

may differ from water quality in shallower and deeper parts of the aquifers. 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 waterquality, hydrologic, geographic, and 

other data to help assess the factors that 

affect water quality. In the KERN study 

unit, data were collected by the PBP in 

2006 and from the CDPH database for 

2003–2005. 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 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