How to read well water test results

What each line on the report tells you

A well water report is a list of substances, each with a number and a unit. The EPA describes what to expect: your results should include the concentration of each substance you tested for, and may also say whether that concentration exceeds a national primary or secondary drinking water standard. If your lab did not compare the results with EPA standards, the EPA points you to its list of drinking water contaminants and their maximum contaminant levels for reference.

That leaves three things to read on every line: the result, the unit it is written in, and the level it is being compared with. Most confusion comes from mixing up the last two. A number can look alarming or reassuring only because it sits in a different unit from the standard beside it, or because the standard is about taste and staining rather than health.

Units: mg/L, ppm, µg/L and ppb

Milligrams per liter is the working unit of the EPA's standards table, which notes that milligrams per liter are equivalent to parts per million. A lab that writes ppm and a lab that writes mg/L are giving you the same number.

Some results come in a unit a thousand times smaller. The Minnesota Department of Health explains that one microgram per liter is the same as one part per billion, and gives the federal arsenic standard as 10 micrograms per liter. The EPA's table writes that same standard as 0.010 mg/L. Uranium appears in the EPA table directly in micrograms, at 30 µg/L.

Before comparing, put the result and the standard in the same unit. A line that reads 8 ppb arsenic is 0.008 mg/L. Read carelessly next to a standard of 0.010, the 8 can look either far over or far under, depending on which unit you assumed.

Primary standards: the health-based numbers

The EPA's National Primary Drinking Water Regulations set two numbers for each contaminant. The maximum contaminant level goal (MCLG) is the level below which there is no known or expected risk to health. The maximum contaminant level (MCL) is the highest level allowed in drinking water, set as close to the goal as feasible using the best available treatment technology and taking cost into consideration. MCLs are enforceable, but the EPA states that these regulations apply to public water systems. A private well is not one, so for well owners the MCL is a reference point rather than a legal limit.

A few values from that table: nitrate 10 mg/L and nitrite 1 mg/L, both measured as nitrogen; arsenic 0.010 mg/L; fluoride 4.0 mg/L; uranium 30 µg/L. Lead and copper work differently. They are regulated by a treatment technique with an action level, 1.3 mg/L for copper and 0.015 mg/L for lead, and the EPA notes that the lead action level changes to 0.010 mg/L from November 1, 2027.

Watch the nitrate basis. The EPA's 10 mg/L is for nitrate measured as nitrogen, and the Minnesota and Washington health departments write it the same way. California's State Water Resources Control Board notes that its earlier nitrate standard was written as nitrate (NO3) and is now written as nitrate as nitrogen. Check which basis your report uses, and compare the result only with a level written on the same basis.

Secondary standards: taste, color and staining

A second EPA table covers 15 contaminants under the National Secondary Drinking Water Standards. The EPA calls them non-mandatory and does not enforce them; they exist to help public water systems manage aesthetic qualities such as taste, color and odor, and the EPA says these contaminants are not considered to present a risk to human health at the secondary level.

Several of the most familiar items on a well report live here. pH has a secondary range of 6.5 to 8.5, and the EPA's table links low pH with a bitter metallic taste and corrosion and high pH with a slippery feel, a soda taste and deposits. Iron is 0.3 mg/L (rusty color, sediment, metallic taste, reddish or orange staining), manganese 0.05 mg/L (black to brown color, black staining), total dissolved solids 500 mg/L, and chloride and sulfate 250 mg/L each.

Fluoride is the one to read twice. It has a secondary standard of 2.0 mg/L, which the EPA's table connects with tooth discoloration, and a separate primary MCL of 4.0 mg/L. A result between the two is above one EPA number and below the other.

Bacteria and hardness read differently

Coliform bacteria are not a concentration you hold up against a number. The sample either tests positive for coliform bacteria or it does not, which is how the Minnesota Department of Health describes the result. In the EPA's primary table the goal for total coliforms is zero, and the EPA notes that coliforms are not a health threat in themselves but are used to indicate whether other potentially harmful bacteria may be present.

Hardness has no EPA standard at all; it is in neither the primary table nor the list of 15 secondary contaminants. The U.S. Geological Survey classifies water by hardness in mg/L as calcium carbonate: 0 to 60 is soft, 61 to 120 moderately hard, 121 to 180 hard, and more than 180 very hard. The USGS describes hardness as a property of water that is not a health concern but can be a nuisance, through mineral buildup in plumbing and water heaters and poor performance of soaps and detergents.

Common mistakes when reading a report

Comparing across units, or across kinds of standard. A ppb result held against a mg/L standard is off by a factor of a thousand, so convert first. And a pH of 6.2 is outside the EPA's secondary range, which is about corrosion and taste, while a nitrate result over 10 mg/L is above a health-based MCL. They are different kinds of finding, and they belong in different conversations.

Acting on one high number without confirming it. The EPA's advice for a result that exceeds a health standard is to contact your public health department for the steps to follow and to have the well re-tested to confirm the contaminant's presence and concentration.

Reading each report alone. One year's 3.8 mg/L nitrate says less than the same number next to 3.4 the year before and 2.1 four years earlier. WellLedger keeps the kinds of result apart when you type a report in: items with a health-based EPA level can light red, secondary items stop at amber, and items without an EPA number, such as hardness, get a trend line and no color.

This guide explains how well water reports are laid out and what the EPA's standards mean. It is not medical advice and cannot tell you whether a particular result is safe for your household; ask the certified lab that ran the test or your local or state health department.

Frequently asked questions