Private Well Water in California

USGS estimated 1.4 million people in California were on self-supplied domestic water in 2015, which is 3.7 percent of the state population and ranks 48 of 50 by share. Nationally the figure is 13.2 percent. Those are estimates of population, not a count of wells: no national well registry exists, and most states do not have a complete one either. The largest single concentration is in Fresno County at about 102,000 people. Figures come from USGS's county level water use compilation for 2015, published June 2018 (v2.0).

Nobody can answer that about a well without testing that specific well. What is known is structural: in EPA's words, "The quality and safety of drinking water from private domestic wells are not regulated by the Federal Government under the Safe Drinking Water Act nor by most state governments and laws." and "Private well owners are responsible for delivering safe drinking water to their households." So the answer comes from a laboratory test on your own tap, arranged by you, and the people who can tell you what to test for locally are California State Water Resources Control Board, GAMA well owner resources and your county or district health department. CDC Private Wells and EPA Private Drinking Water Wells publish general guidance.

EPA's baseline, verbatim: "Test your private well annually for total coliform bacteria, nitrates, total dissolved solids, and pH levels. If you suspect the presence of other contaminants, you should test for those also. You can also contact your local health department to find out what substances may be common in your area’s groundwater." EPA adds: "You may want to test more frequently if small children or elderly adults live in your house or if someone in your house is pregnant or nursing. These segments of the population are often more vulnerable to pollutants than others." Beyond that baseline the list is local, which is why the state program link matters: California State Water Resources Control Board, GAMA well owner resources publishes what is worth testing for in California specifically, and a county health department will often know the local answer better still. Which contaminants to expect from a particular well, and any treatment choice, follow from a test result.

Enter your ZIP in the lookup on this page and it will show every principal aquifer USGS attributes to your county, with USGS's own estimate of how many self-supplied residents draw from each. Statewide the leading one in California is the Central Valley aquifer system, which USGS credits with about 563,000 self-supplied residents across 23 counties. A principal aquifer is, in USGS's words, "A principal aquifer is defined as a regionally extensive aquifer or aquifer system that has the potential to be used as a source of potable (drinkable) water. For study or mapping purposes, aquifers are often combined into aquifer systems." That is a regional map, not a survey of your property: your well's actual depth and completion are on the driller's log, if one exists.

Not the way it regulates a water utility. EPA's summary of the national picture: "While most states have zero or limited regulations for private drinking water wells, states do have programs to assist well-owners." EPA's federal directory of state programs points California at California State Water Resources Control Board, GAMA well owner resources, which is the link here. What state programs typically do cover is construction: who may drill, how a well must be sealed and cased, and what record gets filed. Ongoing water quality is almost never covered, and no state issues a private well the annual report a utility customer gets.

Because Consumer Confidence Reports are a Safe Drinking Water Act requirement on community water systems, and your well is not one. There is no federal testing schedule for it, no violation record, no agency reviewing results, and nobody to notice if something changes. USGS puts the consequence plainly: "Typically, the water supplied by domestic wells is not routinely tested. As a result, people using domestic-supply wells could be drinking water with elevated concentrations of some contaminants." If you want to know what the utility side looks like in California, and what EPA's database does list for the systems that do have to report, see the California drinking water quality page, which is the other half of this story.

USGS sampled about 2,100 domestic wells for its national assessment: "This study from the National Water-Quality Assessment (NAWQA) Program of the U.S. Geological Survey (USGS) assesses water-quality conditions for about 2,100 domestic wells. The sampled wells are located in 48 states and in parts of 30 regionally extensive aquifers used for water supply in the United States." EPA summarises the headline finding this way: "A U.S. Geologic Survey study of 2,100 private wells found that water pumped from about one in five wells contained one or more contaminants at a concentration greater than a human-health benchmark for drinking water." USGS notes what those contaminants tended to be: "The contaminants most often found at these elevated concentrations were inorganic chemicals, such as metals , radionuclides , and nitrate ; all of these but nitrate are derived primarily from natural sources." None of that is a finding about California: USGS published no state by state result from that study and sampled no particular county. Read it as a reason to test California well water rather than as an answer that replaces testing it.

From two USGS data releases. Population and withdrawal figures are from USGS's estimated use of water in the United States, county level data for 2015, published June 2018 (v2.0). The county to aquifer attribution is from USGS's estimated groundwater withdrawals from principal aquifers, county level data for 2015, published February 2020 (v1.0): 10,520 county and aquifer rows covering 67 principal aquifer codes. Both are US Government works in the public domain. What neither release is: a registry of wells, a measurement of water quality, or anything at all about a single property.