U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2011–3089

July 2011 Printed on recycled paper

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

Groundwater Quality in the Monterey Bay and 

Salinas Valley 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 of the GAMA Program provides a comprehensive assessment of the State’s 

groundwater quality and increases public access to groundwater-quality information. The basins around 

Monterey Bay, and in the Salinas and Paso Robles Valleys, 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 aquifers

with concentrations in the three specified categories. Values

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

49 36

15

93

7 <1

Salinas River

San Benito

River

Salinas

River

San Joaquin River

Fresno River

Arroyo Seco

Los Gatos Creek

Carmel River

Panoche Creek

Cholame

Creek

Nacimiento River

Chowchilla River

San Juan Creek

Pajaro

River

San Antonio River

San Lorenzo Creek

Llagas Creek

MADERA CO

Salinas

Monterey

King City

Santa Cruz Watsonville

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

Monterey

Bay

Base from U.S. Geological Survey National Elevation

Dataset, 2006, Albers Equal-Area Conic Projection

EXPLANATION

STUDY AREA 

Santa Cruz

Monterey Bay

Salinas Valley

Paso Robles

Groundwater

 basin

 boundary

Groundwater

 subbasin

 boundary

0 20 MILES

0 20 KILOMETERS

The Monterey-Salinas Study Unit

The Monterey-Salinas study unit is nearly 1,000 square miles and consists of the Santa 

Cruz Purisima Formation Highlands, Felton Area, Scotts Valley, Soquel Valley, West Santa 

Cruz Terrace, Salinas Valley, Pajaro Valley, and Carmel Valley groundwater basins (California 

Department of Water Resources, 2003; Kulongski and Belitz, 2011). These basins were grouped 

into four study areas based primarily on geography. Groundwater basins in the north were 

grouped into the Santa Cruz study area, and those to the south were grouped into the Monterey 

Bay, the Salinas Valley, and the Paso Robles study areas (Kulongoski and others, 2007). 

The study unit has warm, dry summers and cool, moist winters. Average annual rainfall 

ranges from 31 inches in Santa Cruz in the north to 13 inches in Paso Robles in the south. The 

study areas are drained by several rivers and their principal tributaries: the Salinas, Pajaro, and 

Carmel Rivers, and San Lorenzo Creek.

The Salinas Valley is a large intermontane valley that extends southeastward from Monterey Bay to Paso Robles. It has been filled, up to a thickness of 2,000 feet, with Tertiary and 

Quaternary marine and terrestrial sediments that overlie granitic basement. The Miocene-age 

Monterey Formation and Pliocene- to 

Pleistocene-age Paso Robles Formation, and Pleistocene to Holocene-age 

alluvium contain freshwater used for 

supply. The primary aquifers in the 

study unit 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 

are typically drilled to depths of 200 

to 650 feet, consist of solid casing 

from the land surface to depths of 

about 175 to 500 feet, and are perforated below the solid casing. Water 

quality in the primary aquifers may 

differ from that in the shallower and 

deeper parts of the aquifer system. 

Groundwater movement is generally 

from the southern part of the Salinas 

Valley north towards the Monterey 

Bay.

Land use in the study unit is 

about 44 percent (%) natural (mostly grassland and forests), 43% agricultural, and 13% urban. 

The primary agricultural uses are row crops, pasture, hay, and vineyards. The largest urban 

areas are the cities of Santa Cruz, Watsonville, Monterey, Salinas, King City, and Paso Robles.

Recharge to the groundwater system is primarily from stream-channel infiltration from the 

major rivers and their tributaries, and from infiltration of water from precipitation and irrigation. The primary sources of discharge are water pumped for irrigation and municipal supply, 

evaporation, and discharge to streams.

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 can 

be affected by natural processes as well 

as by human activities. In the MontereySalinas study unit, one or more inorganic 

constituents were present at high concentrations in about 15% of the primary 

aquifers and at moderate concentrations 

in about 36% of the primary aquifers.

Man-made 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 this study unit, one or more 

organic constituents were present at high 

concentrations in less than (<) 1% of the 

primary aquifers and at moderate concentrations in about 7%.
 

 

69

25

6

10

88

Trace and 

minor elements

Total

dissolved

solids 

Nutrients

Radioactive

constituents

Iron or

manganese

NDMA

2

INORGANIC CONSTITUENTS

SPECIAL-INTEREST CONSTITUENTS

8

5

87

9

31

60

21

11 67

7

93

RESULTS: Groundwater Quality in the Monterey-Salinas 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 

Monterey-Salinas study unit, one or more of the trace elements were present at 

high concentrations in about 6% of the primary aquifers. Arsenic, boron, and 

molybdenum were the trace elements that most frequently occurred at high concentrations. Aluminum, cadmium, and lead also were detected at high concentrations, but in less than (<) 1% of the primary aquifers. 

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. Gross-alpha radioactivity occurred at high 

levels in about 2% of the primary aquifers, and at moderate levels in about 10%.

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 

8% of the primary aquifers, and at moderate concentrations in about 5% of the 

primary aquifers.

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 may create nuisance problems, such as staining and scaling.

The State of California has a recommended and an upper limit for total dissolved 

solids (TDS) in drinking water. Groundwater naturally contains TDS as a result of 

the weathering and dissolution of minerals in soils. Anoxic conditions in groundwater (very low amounts of dissolved oxygen) may result in release of manganese 

and iron from minerals into groundwater. 

In the Monterey-Salinas study unit, TDS concentrations were high (greater 

than the upper limit) in about 9% of the primary aquifers. About 31% of the 

primary aquifers had moderate TDS concentrations (between the recommended 

and upper limit). Iron and manganese are naturally occurring elements, and one or 

both were present at high concentrations in about 21% of the primary aquifers, at 

moderate concentrations in about 11% of the primary aquifers, and at low concentrations in about 67% 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 

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

recently been detected in, or are considered to have the potential to affect, 

drinking-water supplies. Their presence in groundwater is monitored by the California Department of Public Health. NDMA was detected at moderate concentrations in about 7% of the primary aquifers (2 of the 29 wells sampled). Perchlorate 

was not detected in any of the wells sampled.
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

Trihalomethanes

Herbicides and

fumigants 

ORGANIC CONSTITUENTS

100

100

Insecticides

99

<1 3

97

1

Gasoline

additives 98

<1 2

<1

RESULTS: Groundwater Quality in the Monterey-Salinas Study Unit

Organic Constituents

The Priority Basin Project uses laboratory methods that can detect the presence 

of volatile organic compounds (VOCs) and pesticides at very low concentrations, far 

below human-health benchmarks. VOCs and pesticides detected at these very 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 (evaporate) into the air.

Solvents are used for a number of purposes, including manufacturing and cleaning. In the Monterey-Salinas study unit, only one of the solvents, tetrachloroethene, was 

found at a high concentration, and that concentration represented less than (<) 1% of the 

primary aquifers. Solvents were present at moderate concentrations in about 3% of the 

primary aquifers and were at low concentrations, or not detected, in about 97%.

Gasoline additives increase the efficiency of fuel combustion. The gasoline additive methyl-tert-butyl-ether (MTBE) was detected at high concentrations in <1%, and at 

moderate concentrations in about 2% of the primary aquifers.

Trihalomethanes may form during municipal water purification and enter groundwater by the infiltration of landscape irrigation water. Trihalomethanes were not 

detected at high concentrations and were detected at moderate concentrations in <1% of 

the primary aquifers.

Other organic compounds, which include organic synthesis reagents and refrigerants, were not detected at either high or moderate concentrations.

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 

Monterey-Salinas study unit, insecticides were not detected at high concentrations in 

the primary aquifers, but the insecticide dieldrin was detected at moderate concentrations in about 1% of the primary aquifers. Herbicides and fumigants, when detected, 

were at low concentrations.

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. Otherwise, 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.

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 Kulongoski and Belitz (2011).
Maximum

Contaminant

Level (MCL)

0

DEPTH TO TOP OF PERFORATIONS, IN FEET BELOW LAND SURFACE

10 20 30 40

NITRATE PLUS NITRITE AS NITROGEN, IN MILLIGRAMS PER LITER 

0.5 * MCL

0

400

800

1,200

Urban or Agricultural

 land use

Mixed land use 

Urban or Agricultural

 land use

Mixed land use 

Urban or Agricultural

 land use

Mixed land use 

Natural land

EXPLANATION

Modern-age groundwater

Mixed-age groundwater

Pre-modern-age groundwater

Low Moderate High

Factors that Affect Groundwater Quality

In the Monterey-Salinas study unit, nitrate is the constituent that most frequently 

exists at high concentrations in the primary aquifers (about 8%), and trace elements 

occur at high concentrations in 6% of the primary aquifers. In contrast, trace elements 

occur at high 

concentrations 

in 14% of the 

primary aquifers in the North 

San Francisco 

Bay study unit 

(Kulongoski 

and others, 

2010), and 17% 

of the Central 

Eastside San 

Joaquin Valley study unit 

(Landon and 

others, 2010).

Nitrogen 

in groundwater can occur 

as dissolved 

nitrate, nitrite, or ammonia. Human activities, such as the application of nitrate as a 

fertilizer for agriculture, or the production of nitrogen-rich waste from livestock, generate nitrate that can leach into groundwater. Septic systems may also introduce nitrogenrich waste into groundwater. The human-health regulatory benchmark for nitrate plus 

nitrite, as nitrogen, is 10 milligrams per liter.

In the Monterey-Salinas study unit, high concentrations of nitrate were found in 

the shallower wells sampled (top of well perforations less than 400 feet depth) that are 

located in agricultural and urban areas (see figure above). The groundwaters with high 

and moderate concentrations of nitrate also were predominantly classified as modern 

age or mixed age, which indicates that a component of the groundwater was recharged 

in the past 50 years.

The elevated nitrate concentrations in recently recharged groundwaters from agricultural areas are most likely the result of farming practices that introduced nitrogen at 

the land surface; however, additional data are needed to confirm the sources of nitrate. 

By Justin T. Kulongoski and Kenneth Belitz

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

Kulongoski, J.T., and Belitz, Kenneth, 2007, Ground-water quality data in the Monterey Bay and 

Salinas Valley Basins, California, 2005: Results from the California GAMA Program: U.S. 

Geological Survey Data Series Report 258, 84 p. Available at http://pubs.usgs.gov/ds/2007/258

Kulongoski, J.T., and Belitz, Kenneth, 2011, Status and understanding of groundwater quality in 

the Monterey Bay and Salinas Valley Basins, 2005: California GAMA Priority Basin Project: 

U.S. Geological Survey Scientific Investigations Report 2011-5058, 85 p. Available at http://

pubs.usgs.gov/sir/2011/5058/

Kulongoski, J.T., Belitz, Kenneth, Landon, M.K., and Farrar, Christopher, 2010, Status and understanding of groundwater quality in the North San Francisco Bay Groundwater Basins, 2004: 

California GAMA Priority Basin Project: U.S. Geological Survey Scientific Investigations 

Report 2010-5089, 88 p. Available at http://pubs.usgs.gov/sir/2010/5089/

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/

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, assessed 

by the PBP, may differ from that in the 

deeper parts of the aquifer, or from the 

shallower parts, which are being assessed 

by GAMA’s Domestic Well Project. 

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 with 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 combines water-quality, 

hydrologic, geographic, and other data to 

help assess the factors that affect water 

quality. In the Monterey-Salinas study 

unit, data were collected by the PBP in 

2005 and from the CDPH database for 

2002–2005. 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

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