Private Well Water, County by County

Nobody, unless the owner arranges it. 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." There is no Consumer Confidence Report for a well, no federal sampling schedule, no violation record and no agency reviewing anything. USGS says the same from the science side: "The quality and safety of water from privately owned domestic wells is not regulated under Federal or, in most cases, state laws. Homeowners are primarily responsible for maintaining their domestic well systems and for any water-quality monitoring."

EPA: "Around 15 percent of the U.S. population (over 43 million) rely on private wells as their source of drinking water." And: "EPA estimates that more than 23 million households rely on private wells for drinking water in the United States." The county level compilation behind this site puts the 2015 self-supplied domestic population across the 50 states at 42 million, 13.2 percent of their population, drawing about 3.2 billion gallons of groundwater a day. The share ranges from 49.7 percent in Maine down to 2 percent in Utah.

Which county or counties your ZIP falls in, and for each of them: every principal aquifer USGS attributes that county's groundwater to, USGS's estimate of how many self-supplied residents draw from each, the county's self-supplied population and domestic groundwater withdrawal, and your state's well program and testing guidance links. About 29 percent of ZIP codes cross county lines, and the tool shows every one of them. None of it describes a particular well: an aquifer is a regional map unit and a well is a hole in one particular place, so its depth comes from the well record and its water from a laboratory test.

USGS's definition: "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." And of an aquifer generally: "An aquifer is a geologic formation, a group of formations, or a part of a formation that contains sufficient saturated permeable material to yield significant quantities of water to wells and springs." USGS assigns each one a code: "This list of national principal aquifer codes and names are fixed values assigned by the National Water Information System (NWIS). Aquifers are identified by a geohydrologic unit code - a three-digit number related to the age of the formation, followed by a 4 or 5 character abbreviation for the geologic unit or aquifer name." This site renders USGS's names for those codes and nothing else about them. The three that supply the most self-supplied Americans are the Sand and gravel aquifers (glaciated regions) (about 8.3 million people, 22 states), the Piedmont and Blue Ridge crystalline-rock aquifers (about 5 million people, 11 states), the New York and New England crystalline-rock aquifers (about 2.8 million people, 7 states).

It means USGS mapped no principal aquifer as the source there and put the county's groundwater in its own residual bucket. That is USGS's own category, and it covers a lot of the country: the code appears in 834 counties carrying people. Local aquifers that are productive enough for a household but too small or too discontinuous to map regionally are real and common. It says nothing about the water, only about the map.

No, and this is the single most important limit on everything here. Aquifer level findings are regional patterns from a few thousand sampled wells; a household is one well, one depth, one casing, one wellhead, one set of surroundings. USGS found that "Contaminants found in domestic wells usually co-occurred with other contaminants as mixtures, rather than alone, which is a potential concern because the total toxicity of a mixture can be greater than that of any single contaminant." and that the health concerns span "Health risks associated with contaminants in domestic well water include gastrointestinal illness related to bacteria and other pathogens and exposure to elevated concentrations of nitrate, arsenic, radon, lead, and organic compounds." None of that predicts a result at any tap. The aquifer tells you what to ask the laboratory about. The laboratory tells you what is in your water.

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." And: "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 the baseline, what is worth adding depends on where you are, which is why every state page here links the state program that publishes local guidance. Laboratory, test kit and treatment choices follow from a test result and a licensed installer, so the state program and the county health department are the next step.

They are the two halves of the same map. The drinking water quality pages cover community water systems: EPA's federal SDWIS record, violation history, and where each system's annual Consumer Confidence Report lives. Roughly nine in ten Americans are on one of those. These pages cover everyone else, where there is no record to look up because no reporting is required. Each set of pages links the other, since each stops at the property line of the other.

By share of population on self-supplied water in 2015: Maine, New Hampshire, Vermont, Pennsylvania, Michigan, Indiana, Wisconsin, Montana, Alaska are at or above 25 percent, and 15 states sit under 10 percent, including Utah, California and Arizona where the population is concentrated in served metropolitan areas. By raw numbers the order is different: Pennsylvania, Michigan and New York carry the largest well populations in the country, and none of them is in the top three by share.

Because that is the most recent year USGS has published a county by aquifer compilation for. The water use estimates were released June 2018 (v2.0) and the by-aquifer attribution February 2020 (v1.0). Both are final, versioned data releases rather than a live feed, which is why every page here states the year next to the numbers instead of implying they are current. Population shifts, but which aquifer sits under a county does not.