U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2012–3136

March 2013 Printed on recycled paper

California Groundwater Ambient Monitoring and Assessment (GAMA) Program 

Priority Basin Project: Shallow Aquifer Assessment

GAMA

The California State Water 

Resources Control Board’s (SWRCB) 

GAMA Program is a comprehensive 

assessment of statewide groundwater 

quality in California. From 2004 to 

2012, the GAMA Program’s Priority Basin Project focused on assessing groundwater resources used for 

public drinking-water supplies. More 

than 2,000 public-supply wells were 

sampled by U.S. Geological Survey (USGS) for this effort. Starting 

in 2012, the GAMA Priority Basin 

Project began an assessment of water 

resources in shallow aquifers in 

California. These shallow aquifers 

provide water for domestic and small 

community-supply wells, which are 

often drilled to shallower depths in 

the groundwater system than publicsupply wells. Shallow aquifers are of 

interest because shallow groundwater 

may respond more quickly and be 

more susceptible to contamination 

from human activities at the land 

surface, than the deeper aquifers.

The SWRCB’s GAMA Program 

was developed in response to the 

Groundwater Quality Monitoring Act 

of 2001 (Water Code sections 10780-

10782.3): a public mandate to assess 

and monitor the quality of groundwater resources used for drinking-water 

supplies, and to increase the availability of information about groundwater quality to the public. The U.S. 

Geological Survey is the technical 

lead of the Priority Basin Project.

Stewardship of California’s 

groundwater resources is a responsibility shared between well owners, 

communities, and the State. Participants and collaborators in the GAMA 

Program include Regional Water 

Quality Control Boards, Department 

of Water Resources, Department of 

Public Health, local and regional 

groundwater management entities, 

county and local water agencies, 

community groups, and private citizens. Well-owner participation in the 

GAMA Program is entirely voluntary.

What Will Participants 

Gain from the GAMAPBP Shallow Aquifer 

Assessment?

The project will provide 

many benefits to State, local, 

and community participants 

and to individual well owners. It will:

•	 Provide well owners with 

information about the constituents in their water;

•	 Establish baseline groundwater quality for comparison with future conditions;

•	 Identify emergent constitu- ents in groundwater;

•	 Identify areas of California 

where domestic households 

or small communities rely 

upon poor-quality groundwater for drinking supply; 

•	 Improve understanding of 

local, regional, and statewide hydrogeology;

•	 Produce groundwater-quality reports for each study 

area;

•	 Provide new information 

for local, regional, and 

statewide groundwater 

programs to help manage 

groundwater resources;

•	 Provide new data for the 

SWRCB’s GeoTracker 

GAMA publicly accessible 

online database; and

•	 Compare water quality in 

the shallow and deeper 

aquifer systems.

Radon sampling in Temecula, California

What is the GAMA Priority Basin Project (GAMA-PBP)?
0 100 200 Miles

0 150 300 Kilometers

PACIFIC OCEAN

Areas that account for 50%

Additional areas that bring

 total to 75%

Additional areas that bring

 total to 90%

Additional areas that bring

 total to 99%

Areas accounting

 for final 1%

Cumulative percentage of the

number of California households

using domestic wells

Table 1. List of analytes.

GAMA Priority Basin Project Shallow Aquifer Assessment Study Design

Sampled constituents Why constituents are of interest 

The ability of a water sample to transmit an electrical current; related to the salinity of the water 

The availability of charged ions and dissolved oxygen affects the mobility of dissolved chemical compounds in water

Temperature can influence the concentration of dissolved gases (such as oxygen, nitrogen, carbon dioxide) in water 

Naturally occurring elements that influence water quality 

Elevated concentrations of nutrients (such as nitrogen, phosphorous) can degrade water quality 

Naturally occurring elements that help define water characteristics and affect water quality 

Include human-made compounds such as solvents, gasoline additives, and organic synthesis products that can degrade water quality 

A compound added to gasoline to improve air quality. At high concentrations, it can degrade water quality 

Used to control weeds, insects, and other pests and can degrade water quality 

A byproduct of water treatment from the reaction of dimethylamine and chloramines which can degrade water quality 

Inorganic constituent found in rocket fuel, fireworks, and flares, and also occurs naturally at low concentrations in groundwater.

 At high concentrations, it can degrade water quality 

Naturally occurring atoms and radioactivity used to track water movement; may degrade water quality at high concentrations 

Isotope of hydrogen used to identify water recharged since 1952 

Dissolved gases used to trace groundwater movement, for chronology, and to study climate change. Noble gases are analyzed at the 

 Lawrence Livermore National Laboratory

Naturally occurring isotopes of oxygen and hydrogen used to track water through the hydrologic cycle

A naturally occurring radioactive gas which, at high levels, can contribute to health problems such as lung cancer 

Specific conductance

Alkalinity, pH, and dissolved oxygen

Temperature

Major and minor ions

Nutrients

Trace elements

Volatile organic compounds (VOCs)

Methyl tert-butyl ether (MTBE)

Pesticides

N-Nitrosodimethylamine (NDMA)

Perchlorate (ClO4

-

)

Isotopes, radioactivity

Tritium (3H)

Noble gases

Stable isotopes (δ18O, δD)

Radon (Rn) 

The locations of domestic and small 

system wells across California were identified from well-drillers’ logs and U.S. 

Census data, and the areas of the State 

with the greatest densities of households 

that rely on domestic wells were prioritized for sampling for this project. The 

high-priority areas (see figure below) will 

be grouped into study units. Sixty to 120 

shallow wells will be sampled in each 

study unit to provide a spatially unbiased 

assessment of the groundwater quality 

in the shallow aquifer systems. 

Domestic and small communitysupply wells typically tap shallower parts of the aquifer system 

than do the public-supply wells 

sampled in the previous phase of 

the GAMA Priority Basin Project. Shallow parts of the aquifer 

system may be more susceptible 

to contamination from human 

activities at the land surface, and 

shallow groundwater typically 

responds more rapidly to landsurface activities than the deeper 

systems. 

Two types of assessments 

will be conducted for each study 

unit: 

Status: the assessment of 

current groundwater quality and 

Understanding: the assessment of natural and human factors that affect groundwater quality.

Water-quality assessments for the 

GAMA Priority Basin Project will combine existing water-quality data (such as 

data from the SWRCB’s Domestic Well 

Project) with new data collected for this 

project. Samples will be analyzed for 

a large suite of chemical constituents, 

including nutrients, human-made organic 

constituents, naturally occurring trace 

elements, and tracers used to help identify the source and age of the sampled 

groundwater (table 1). 

Reporting Results

The GAMA Priority Basin Project 

provides new water-quality data and 

assessment of water resources to participants. Before releasing data or reports to 

the public, the SWRCB and USGS will 

provide results to individual well owners. Well owner information will be kept 

confidential.

Data summary and interpretive 

reports prepared by USGS will be posted 

on GAMA Program websites. Waterquality data also will be available through 

the SWRCB’s GeoTracker GAMA 

database: http://geotracker.waterboards.

ca.gov

GAMA Priority Basin Project reports and 

data can be obtained from:

GAMA Project Chief

U.S. Geological Survey

California Water Science Center

6000 J Street, Placer Hall

Sacramento, CA 95819-6129

Telephone: (916) 278-3000

http://ca.water.usgs.gov/gama

GAMA Program Unit Chief

State Water Resources Control Board

Division of Water Quality

P.O. Box 2231

Sacramento, CA 95812

Telephone: (916) 341-5779

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