U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2011–3055

June 2011 Printed on recycled paper

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

Groundwater Quality in the Santa Clara River 

Valley, California

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

resource, the State of California has 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 Santa Clara River Valley is one of the study units being evaluated. 

Pacific Ocean

OXNARD

FOREBAY

County

Los Angeles

Santa Monica Mountains

Topatopa Mountains

San Gabriel Mountains

River

River

Santa

Clara

Ventura

CalleguasCreek

Ventura

County

Santa Clara River Valley

Santa Clara River Valley

Las Posas Valley

Simi Valley

Pleasant Valley

Santa Ynez Mts

Ojai

Valley

Upper

Ojai

Valley

Ventura

River

Valley

Ventura

Newhall

Camarillo

Simi Valley

Oxnard

Santa

Clarita

Arroyo Santa

Rosa Valley

0 5 10 MILES

0 5 10 KILOMETERS

Organic

constituents

Inorganic

constituents

High Moderate

CONSTITUENT CONCENTRATIONS

Low or not detected 

Inorganics 

Values are a percentage of the area of the primary aquifer

system with concentrations in the three specified categories.

49 30

21

95

5

The Santa Clara River Valley Study Unit

The Santa Clara River Valley (SCRV) study unit is located in Los Angeles and 

Ventura Counties, California, and is bounded by the Santa Monica, San Gabriel, Topatopa, 

and Santa Ynez Mountains, and the Pacific Ocean. The 460-square-mile study unit includes 

eight groundwater basins: Ojai Valley, Upper Ojai Valley, Ventura River Valley, Santa Clara 

River Valley, Pleasant Valley, Arroyo Santa Rosa Valley, Las Posas Valley, and Simi Valley 

(California Department of Water Resources, 2003; Montrella and Belitz, 2009). The SCRV 

study unit has hot, dry summers and cool, moist winters. Average annual rainfall ranges 

from 12 to 28 inches. The study unit is drained by the Ventura and Santa Clara Rivers, and 

Calleguas Creek.

The primary aquifer system in the Ventura River Valley, Ojai Valley, Upper Ojai 

Valley, and Simi Valley basins is largely unconfined alluvium. The primary aquifer system in the remaining groundwater basins mainly consists of unconfined sands and gravels 

in the upper portion and 

partially confined marine 

and nonmarine deposits 

in the lower portion. The 

primary aquifer system 

in the SCRV study unit is 

defined as those parts of the 

aquifers corresponding to 

the perforated intervals of 

wells listed in the California Department of Public 

Health (CDPH) database. 

Public-supply wells typically are completed in the 

primary aquifer system to 

depths of 200 to 1,100 feet 

below land surface (bls). 

The wells contain solid casing reaching from the land 

surface to a depth of about 60-700 feet, and are perforated below the solid casing to allow 

water into the well. Water quality in the primary aquifer system may differ from the water 

in the shallower and deeper parts of the aquifer.

Land use in the study unit is approximately 40 percent (%) natural (primarily shrubs, 

grassland, and wetlands), 37% agricultural, and 23% urban. The primary crops are citrus, 

avocados, alfalfa, pasture, strawberries, and dry beans. The largest urban areas in the study 

unit are the cities of Ventura, Oxnard, Camarillo, Simi Valley, Newhall, and Santa Clarita.

Currently, groundwater pumping for agricultural use accounts for the greatest amount 

of discharge from the aquifer system in the SCRV study unit, followed by municipal use. 

Recharge to the groundwater system is through stream-channel infiltration from the three 

main river systems and by direct infiltration of precipitation and irrigation. Recharge facilities in the Oxnard forebay play an important role in recharging the local aquifer systems.

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 are 

naturally present in groundwater. The 

concentrations of inorganic constituents 

can be affected by natural processes, as 

well as by human activities. In the SCRV 

study unit, one or more inorganic constituents were present at high concentrations 

in about 21% of the primary aquifers, and 

at moderate concentrations in about 30%.

Organic constituents are present 

in products used in the home, business, 

industry, and agriculture. In the SCRV 

study unit, organic constituents can enter 

the environment through normal usage, 

spills, or improper disposal. Organic 

constituents were not present at high concentrations in the primary aquifer system, 

but one or more organic constituents were 

present at moderate concentrations in 

about 5% of the primary aquifer system.
 

 

58

38

4

14

11

75

15

9

76

12

88

Trace and 

minor elements

Total

dissolved

solids 

Sulfate 

Nutrients

Radioactive

constituents

Iron or 

manganese

Perchlorate

INORGANIC CONSTITUENTS

SPECIAL INTEREST CONSTITUENTS

44

56

35

56

9

51

27

22

RESULTS: Groundwater Quality in the Santa Clara River Valley 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 SCRV 

study unit, one or more trace elements were present at high concentrations in about 

4% of the primary aquifer system. Arsenic, boron, and vanadium were the trace 

elements that occurred at high concentrations in 2, 3, and 3% of the primary aquifer system, respectively.

 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 present in 

minerals in the aquifer sediments. In the SCRV study unit, radioactive constituents 

were present at high concentrations in 14% of the primary aquifer system, and 11% 

at moderate concentrations. 

Nutrients, such as nitrate, are naturally present at low concentrations in 

groundwater. High and moderate concentrations generally occur as a result 

of human activities such as the application of fertilizer for agriculture. Livestock, when in concentrated numbers, and septic systems also can affect nitrate 

concentrations in groundwater. In the SCRV study unit, nitrate was present at high and moderate concentrations in 15% and 9%, respectively, of the 

primary aquifer system.

Inorganic Constituents with Non-Health Benchmarks

(Not included in overview of water-quality 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 scaling and staining.

Total dissolved solids (TDS, an indicator of salinity) and sulfate are naturally 

occurring constituents. The State of California has a recommended and an upper 

limit for both TDS and sulfate in drinking water. In the SCRV study unit, TDS 

and sulfate were present at high concentrations (greater than the upper limit) in 

35 and 22% of the primary aquifer system, respectively. Moderate concentrations 

(between the recommended and upper limit) for TDS and sulfate were 56 and 

27%, respectively.

Iron and manganese are naturally occurring elements, and either or both were 

present at high concentrations in about 44% of the primary aquifer system. Iron 

and (or) manganese were not present at moderate concentrations but were present 

at low cencentrations or not detected in 56% of the primary aquifer system.

Perchlorate 

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

Perchlorate, an inorganic constituent, is of special interest in California 

because this constituent has recently been found in groundwater and is considered 

to have the potential to affect drinking-water supplies. Its presence in groundwater 

has been regulated by the California Department of Public Health since 2007. Perchlorate is an ingredient in rocket fuel, fireworks, safety flares, and other products, 

may be present in some fertilizers, and also occurs naturally at low concentrations 

in groundwater. In the SCRV study unit, perchlorate was present at moderate concentrations in 12% of the primary aquifer system and was low or not detected in 

88% of the primary aquifer system.
High concentrations

Moderate concentrations

Low concentrations

EX

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

system with concentrations in the three specified categories.

CONSTITUENT CONCENTRATIONS

Solvents

Trihalomethanes

Herbicides and

insecticides

(including

fumigants) 

Other volatile

organic

compounds 

ORGANIC CONSTITUENTS

100

100

2

98

2

98

RESULTS: Groundwater Quality in the Santa Clara River Valley 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 very low concentrations can be 

used to trace the pathway of water from the land surface 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 into the air. 

Solvents are used for a number of purposes, including manufacturing and cleaning. In the SCRV study unit, solvents were not present at high concentrations but were 

present at moderate concentrations in about 2% of the primary aquifer system. Solvents were present at low concentrations or not detected in about 98% of the primary 

aquifer system. 

Trihalomethanes may form during municipal disinfection of water supplies, and 

may enter groundwater by infiltration of landscape irrigation water. Trihalomethanes 

were not detected at high concentrations in the primary aquifer system, but were present at moderate concentrations in 2% of the primary aquifer system. The trihalomethane, chloroform, was detected at low concentrations in 17% of the primary aquifer 

system.

Other VOCs include organic synthesis reagents and gasoline additives. Other 

VOCs were not detected at high or moderate concentrations in the primary aquifer 

system.

Pesticides with Human-Health Benchmarks

Pesticides (herbicides, insecticides, and fumigants) are applied to crops, gardens, 

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

other pests. In the SCRV study unit, herbicides and insecticides (including fumigants) 

were not detected at high or moderate concentrations in the primary aquifer system. 

The pesticides atrazine and simazine were detected at low concentrations in 17 and 

26% of the primary aquifer system, respectively. 

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) are used for this evaluation 

when available. Otherwise, nonregulatory benchmarks for protecting aesthetic properties 

(Secondary Maximum Contaminant Level, SMCL), such as taste and odor, and nonregulatory benchmarks for protecting human health (Notification Level, NL, and Lifetime Health Advisory, HAL) are used.

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 Burton and others (2011).
0 

100 

200 

300 

400 

500 

600 

700 

800 

900 

1,000 

0 10 20 30 

DEPTH TO TOP-OF-PERFORATIONS, IN FEET 

NITRATE AS NITROGEN, IN MILLIGRAMS PER LITER 

Modern

Mixed

Pre-modern

CDPH grid wells

 (no age data) 

EXPLANATION

Low Moderate High

Factors That Affect Groundwater Quality

In the SCRV study unit, nitrate is the constituent with a human-health benchmark 

that was most frequently present at high concentrations. High concentrations of nitrate 

were detected in about 15% of the primary aquifer system. Nitrate concentrations in 

groundwater can be affected by both natural and human factors. Certain bacteria and 

algae naturally convert nitrogen from the atmosphere to nitrate, which is an important 

nutrient for plants. Anthropogenic sources of nitrate include its application as a fertilizer for agriculture and 

landscape maintenance, 

livestock, and septic systems (Hem, 1985). The 

human-health regulatory 

benchmark for nitrate (as 

nitrogen) is 10 milligrams 

per liter.

In the SCRV study 

unit, nitrate was present 

at high concentrations 

only in the upper part 

of the primary aquifer 

system. Water from wells 

with high concentrations 

of nitrate occurred in the 

upper 300 feet. At depths 

below 300 feet, nitrate is 

detected only at moderate 

or low concentrations. 

Nitrate also was associated with young groundwater. High and moderate 

nitrate concentrations were found only in modern (entered the aquifer since about 1953) 

or mixed-age groundwater. In pre-modern-age groundwater (entered the aquifer before 

1953), nitrate is only detected at low concentrations. Nitrate concentrations generally 

were low in groundwater that has low dissolved oxygen (Burton and others, 2011). Previous groundwater studies within the SCRV study unit have confirmed that degradation 

of nitrate occurs in some parts of the aquifer that have low dissolved oxygen (Izbicki 

and others, 2005). 

By Carmen A. Burton, Matthew K. Landon, and Kenneth Belitz

SELECTED REFERENCES

 Burton, C.A., Montrella, Joseph, Landon, M.K., and Belitz, Kenneth, 2011, Status and 

understanding of groundwater quality in the Santa Clara River Valley, 2007—California GAMA Priority Basin Project: U.S. Geological Survey Scientific Investigations 

Report 2011-5052, 86 p. Available at http://pubs.usgs.gov/sir/2011/5052/ 

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

Department of Water Resources Bulletin 118, 246 p., accessed August 22, 2007, at 

http://www.water.ca.gov/groundwater/bulletin118/update2003.cfm

Hem, J.D., 1985, Study and interpretation of the chemical characteristics of natural 

water (3d ed.): U.S. Geological Survey Water-Supply Paper 2254, 263 p.

Izbicki, J.A., Christensen, A.H., Newhouse, M.N., and Aiken, G.R., 2005, Inorganic, 

isotopic, and organic composition of high-chloride water from wells in a coastal 

southern California aquifer: Applied Geochemistry, v. 20, p. 1496–1517.

Montrella, Joseph, and Belitz, Kenneth, 2009, Ground-water quality data in the Santa 

Clara River Valley study unit, 2007—Results from the California GAMA Program: 

U.S. Geological Survey Data Series 408, 84 p. Available at http://pubs.usgs.gov/

ds/408

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 water quality in these primary 

aquifers. 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 to 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 uses additional wells to help 

assess the factors that affect water quality. Both approaches use data routinely 

collected for regulatory compliance, as 

well as data collected by the PBP. Data 

were collected by the PBP in 2007, and 

were compiled 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 humanhealth 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