Testing California's Domestic Wells: The GAMA Priority Basin Project

The Groundwater Ambient Monitoring and Assessment Priority Basin Project (GAMA-PBP) is a statewide assessment of groundwater quality in California. Its first phase, in 2004–15, examined groundwater used for public drinking-water supplies. The second phase, which the U.S. Geological Survey began sampling in 2012, looks at groundwater used by domestic wells.

Why domestic wells

An estimated 2 million Californians rely on their own domestic wells or on state small-system wells for drinking water, and far less is known about that water than about public supplies. Domestic wells are usually shallower than public-supply wells, and shallow groundwater may respond faster, and be more vulnerable, to contamination from activities at the land surface.

The State Water Resources Control Board created the program under the Groundwater Quality Monitoring Act of 2001, a public mandate to assess and monitor groundwater used for drinking water and make the information public. The USGS leads the project in cooperation with the board. Participation by well owners is voluntary, and collaborators include regional water quality boards, the Department of Water Resources, the board's Division of Drinking Water, local groundwater and water agencies, community groups and private citizens.

A USGS scientist samples a well in the Coachella Valley, California. U.S. Geological Survey.

How the data will help

The project will:

• give well owners water-quality and water-level data for their wells, and help find well-depth information;
• set a baseline of groundwater quality for future comparison;
• identify where households rely on poor-quality groundwater;
• improve understanding of local, regional and statewide hydrogeology;
• produce a groundwater-quality report for each study area and add data to the State Water Board's public GAMA Groundwater Information System;
• compare water quality in shallow and deeper aquifers.

How wells are chosen

Domestic wells were located from well drillers' logs, and the areas with the most households relying on them were prioritized and grouped into 20 to 25 study units. A grid is laid over each unit, one well is sampled per cell, and about 30 to 100 wells are sampled per unit. Wells are found by going door to door.

Areas of California relying on domestic wells. U.S. Geological Survey.

What is measured

At each well, the team measures the groundwater level and samples for:

• field properties — specific conductance, pH, dissolved oxygen and temperature, which affect how dissolved chemicals move;
• microbial indicators — total coliforms, E. coli and enterococci, signs that a well is vulnerable to contamination from surface water or human or animal waste;
• PFAS — human-made compounds used in firefighting foams, industry and many consumer products;
• major ions, alkalinity and total dissolved solids;
• nutrients — high nitrate or phosphorus generally point to fertilizer, animal waste or septic systems;
• trace elements such as arsenic, hexavalent chromium, uranium and manganese, mostly natural;
• volatile organic compounds — fumigants, solvents, gasoline additives, hydrocarbons and trihalomethanes;
• pesticides and their breakdown products;
• perchlorate, from rocket fuel, fireworks, flares and some fertilizers, and naturally present at low levels;
• tritium, carbon-14, noble gases and stable isotopes of water, used to date groundwater and trace its movement — tritium identifies water recharged since about 1950, carbon-14 water hundreds to tens of thousands of years old.

Results

Well owners receive their own results, and their information is kept confidential in public data and reports. Summaries, reports and data tools are available from the State Water Board and USGS websites.

Sources

• U.S. Geological Survey, California Water Science Center: "California Groundwater Ambient Monitoring and Assessment (GAMA) Program Priority Basin Project: Domestic-Supply Assessment," Fact Sheet, in cooperation with the California State Water Resources Control Board.