U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2013–3015

September 2013

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

Groundwater Quality in the South Coast Range 

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

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

The coastal basins in the Southern Coast Ranges constitute one of the study units being evaluated. 

Printed on recycled paper

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

pie chart may not equal 100 due to rounding of percentages.

<1

98

2

33

35

31

101

101

154

1

1

1

Nipomo

Lompoc

Solvang Buellton

Morro Bay

Santa Maria

Arroyo Grande

San Luis

Obispo

Santa

Santa

Ynez

San

Creek Antonio

Maria River

River

120°30’ 120°

35°

34°

30’

0 5 10 Miles

0 10 20 Kilometers

EXPLANATION

Groundwater basins

Study areas

Santa Ynez River Valley

San Antonio Creek Valley

Basins

Uplands

Santa Maria River Valley

San Luis Obispo Valley

Los Osos Valley

PACIFIC

OCEAN

The South Coast Range–Coastal Study Unit

The South Coast Range–Coastal (SCRC) study unit is located in California’s Southern Coast 

Ranges hydrogeologic province and includes parts of Santa Barbara and San Luis Obispo Counties 

(Mathany and others, 2010). The SCRC study unit includes five groundwater basins—Los Osos 

Valley, San Luis Obispo Valley, Santa Maria River Valley, San Antonio Creek Valley, and Santa 

Ynez River Valley (California Department of Water Resources, 2003)—as well as upland areas 

of Plio-Pleistocene non-marine and Pliocene marine sediments that are adjacent to the groundwater basins. The 766-square-mile study unit was separated into the two study areas (Basins and 

Uplands) on the basis of alluvial 

and terrace deposits found in the 

five groundwater basins.

The climate in the SCRC 

study unit is influenced in large 

part by the Pacific Ocean as 

well as by the topography of the 

surrounding areas. The coastal 

lowlands and valleys near the 

Pacific Ocean have a coastal 

climate, with warm, dry summers and cool, wet winters. The 

slopes of the coastal valleys and 

within the upland areas have 

a Mediterranean climate with 

hot, dry summers and cool, wet 

winters. Average annual rainfall 

ranges from 13 inches in the 

coastal lowlands to 20 inches in 

the uplands. The Santa Maria 

and Santa Ynez Rivers and San 

Antonio Creek are the major 

streams in the SCRC study unit. 

The primary aquifer 

system consists of alluvial sediments (mixture of gravel, clay, silt, and sand), the Paso Robles 

Formation (unconsolidated to poorly consolidated coarse sand and gravel), and the Careaga Sand 

(unconsolidated fine- to medium-grained marine sand). The primary aquifer system is defined as 

that part of the aquifer corresponding to the perforated depth intervals of wells listed in the California Department of Public Health (CDPH) database. CDPH wells are typically drilled to depths 

between 265 and 630 feet below land surface and are perforated or screened at depths below 

115–350 feet (Burton and others, 2013). Water quality in the primary aquifer system may differ 

from that in the shallower and deeper parts of the aquifers. 

Land use in the study unit is approximately 61 percent (%) natural, 29% agricultural, and 

10% urban. The primary agricultural uses are for vegetables and flowers, vineyards, and pasture 

and hay. The largest urban areas are the cities of Lompoc, Santa Maria, Arroyo Grande, and San 

Luis Obispo (Burton and others, 2013).

The primary source of recharge to the groundwater flow system is from percolation of precipitation, irrigation waters, seepage from streams and rivers, subsurface inflow, and engineered 

recharge (California Department of Water Resources, 2003). The primary sources of groundwater 

discharge are water pumped for irrigation and municipal supply.

Overview of Water Quality

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

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 SCRC study unit, 

one or more inorganic constituents with 

human-health benchmarks were present at 

high concentrations in 33% of the primary 

aquifer system and at moderate concentrations in 35%.

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 SCRC 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 2%.
 s h

Trace and

minor elements 

Total

dissolved

solids 

Nutrients

(nitrate plus

nitrite)

Uranium and

radioactive

constituents

Iron or

manganese

Perchlorate

INORGANIC CONSTITUENTS

SPECIAL-INTEREST CONSTITUENT

27

29

44

95

5

76

16

66

62

29

71

34

4

10

14

18

RESULTS: Groundwater Quality in the South Coast Range–Coastal 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 SCRC 

study unit, trace elements were present at high concentrations in 27% of the primary 

aquifer system and at moderate concentrations in 29%. Arsenic and molybdenum 

were present at high concentrations in 7% and 25% of the primary aquifer system, 

respectively. Barium, copper, and selenium also were detected at high concentrations, 

but in <1% of the primary aquifer system. 

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. Radioactive constituents were not present at high concentrations 

in the primary aquifer system, but were present at moderate concentrations in 5%.

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 plus nitrite was present at high concentrations in 10% of 

the primary aquifer system and at moderate concentrations in 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. Other constituents, such as iron and 

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

The State of California has a recommended and an upper limit for TDS in drinking water. TDS is a naturally occurring constituent that results from the weathering 

and dissolution of minerals in soils and rocks. In the SCRC study unit, TDS was 

present at high concentrations (above the upper limit) in 16% of the primary aquifer 

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

in 66% of the primary aquifer system. 

Iron and manganese are naturally occurring elements that may be released from 

minerals in soils and rocks under very low dissolved oxygen conditions. Iron, manganese, or both were present at high concentrations in 34% of the primary aquifer 

system and at moderate concentrations in 4%.

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 an ingredient in rocket fuels, fireworks, and other 

products, may be present in some fertilizers, and can occur naturally at low concentrations in groundwater. In the SCRC study unit, perchlorate was not present at high 

concentrations in the primary aquifer system, but was present at moderate concentrations in about 29%. 
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. Values on

pie chart may not equal 100 due to rounding of percentages.

CONSTITUENT CONCENTRATIONS

Solvents

ORGANIC CONSTITUENTS

Pesticides

Trihalomethanes

Other volatile

organic

compounds

100

100

100

<1

98

<1 2

RESULTS: Groundwater Quality in the South Coast Range–Coastal 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. The presence of 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 used 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 SCRC study unit, solvents were present at high concentrations in <1% of the primary aquifer system. The solvent detected at high concentrations was perchloroethene 

(PCE). Solvents were present at moderate concentrations in about 2% of the primary 

aquifer system and were low or not detected in 98%. 

Trihalomethanes may form during water purification and can enter groundwater by 

the infiltration of landscape irrigation water. Trihalomethanes were present at moderate 

concentrations in <1% of the primary aquifer system and at low concentrations or not 

detected in about 100%.

Other VOCs include organic synthesis reagents, refrigerants, and gasoline hydrocarbons. Other VOCs were not detected or were present at low concentrations in 100% 

of the primary aquifer system. 

Pesticides with Human-Health Benchmarks

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

to help control unwanted vegetation, insects, fungi, and other pests. In the SCRC study 

unit, pesticides were not detected or were present at low concentrations in 100% of 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 non-regulatory 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 includes nondetections and values less than moderate concentrations. Methods for evaluating water quality are discussed in Burton and others (2013).
Dissolved oxygen, in milligrams per liter

Molybdenum concentration,

in micrograms per liter

0.1

1

10

100

1,000

< 2 > 2

High

Moderate

Low

Groundwater age

Nitrate concentration,

in milligrams per liter

0.01

0.1

1

10

100

High

Moderate

Low

Modern Mixed Pre-modern

26 Number of wells

 with data

75th percentile

Median

25th percentile

10th percentile

90th percentile

Value greater than

90th percentile

Value less than

10th percentile

EXPLANATION

15 11

19 17

27

Factors that Affect Groundwater Quality

In the SCRC study unit, molybdenum and nitrate are two constituents that were 

detected at high concentrations in the primary aquifer system (Burton and others, 

2013). The non-regulatory U.S. Environmental Protection Agency (USEPA) HAL for 

molybdenum is 40 micrograms per liter (μg/L), and the regulatory USEPA MCL for 

nitrate is 10 milligrams per liter (mg/L) as nitrogen. 

Molybdenum is a trace element that occurs naturally in aquifer sediments. Molybdenum can be mobilized under geochemical conditions indicated by low dissolved 

oxygen (DO) or high pH (Goldberg, 2009). High and moderate concentrations of 

molybdenum in the SCRC study unit were prevalent when DO was <2 mg/L. Molybdenum concentrations also tended to increase as pH increased when DO was >2 mg/L, 

likely the result of pH-dependent desorption from aquifer sediments (Burton and others, 2013). 

 Nitrate is a naturally occurring constituent, although concentrations greater than 2 mg/L 

generally indicate the 

presence of nitrate from 

human activities such as 

application of fertilizer, 

the use of septic systems, 

and domesticated animal 

waste. The correlation 

between nitrate concentrations and groundwater 

age suggests elevated 

nitrate concentrations in 

SCRC study unit groundwater are the result of 

human activities (Burton 

and others, 2013). High 

concentrations of nitrate 

were detected in wells 

with groundwater of 

modern age (groundwater recharged since 

1952). Low concentrations of nitrate were 

detected in wells with 

groundwater of pre-modern age (groundwater recharged before 1952). Nitrate concentrations also can be affected by geochemical conditions indicated by DO concentration. 

In contrast to molybdenum, nitrate concentrations generally were low in groundwater 

having DO <2 mg/L, likely because nitrate can be degraded in low DO conditions 

(Kendall, 1998). 

SELECTED REFERENCES

Burton, C.A., Land, M.T., and Belitz, Kenneth, 2013, Status and understanding of groundwater quality in the 

South Coast Range–Coastal study unit, 2008—California GAMA Priority Basin Project: U.S. Geological 

Survey Scientific Investigations Report 2013-5053, 85 p. Available at http://pubs.usgs.gov/sir/2013/5053.

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/

bulletin118update2003.cfm.

Goldberg, Sabine, 2009, Influence of soil solution salinity on molybdenum adsorption by soils: Soil Science, 

v. 174, p. 9–13. 

Kendall, C., 1998, Tracing nitrogen sources and cycling in catchments, in Kendall, C., and McDonnell, J.J., 

eds., Isotope tracers in catchment hydrology: Amsterdam, Elsevier Science, chapter 16, p. 519–576.

Mathany, T.M., Burton, C.A., Land, Michael, and Belitz, Kenneth, 2010, Groundwater-quality data in the 

South Coast Range–Coastal study unit, 2008—Results from the California GAMA Program: U.S. Geological Survey Data Series 504, 106 p. Available at http://pubs.usgs.gov/ds/504/.

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 aquifer system, 

assessed by the PBP, may differ from 

water quality in the 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 water-quality, 

hydrologic, geographic, and other data 

to help assess the factors that affect 

water quality. In the SCRC 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 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

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) 341-5779

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

By Carmen A. Burton and Kenneth Belitz