Washington County Well Water Testing
Private-well guidance for Washington County, Wisconsin: USGS area-risk estimates for arsenic and nitrate, a recommended test panel, and how to get your own well tested at a Wisconsin-certified lab.
Washington County groundwater risk (area estimates)
These are modeled USGS estimates for the county area — not a measurement of your specific well.
Arsenic
area estimate3%
modeled chance a well in this area exceeds 10 µg/L (the EPA limit).
5% chance of exceeding 5 µg/L.
Most-probable concentration category: <=5 ug/L.
Nitrate
area estimate0.20 mg/L
predicted nitrate (as N) for domestic-supply depth.
This estimate is below the 10 mg/L EPA limit, but individual wells can still exceed it.
Private wells
area estimate37,204
people on private wells in Washington County.
Roughly 14,882 households, estimated from the USGS modeled domestic-supply population.
Area context
Additional state-only or optional layers for Washington County, shown where the data exists.
Uranium
area estimateA state-only uranium model is not available for Wisconsin. See observed samples and test your own well to learn its uranium level.
Radon
area estimateA state-only radon model is not available for Wisconsin. See observed samples and test your own well to learn its radon level.
Primary aquifer
area contextWells in Washington County most likely draw from the Silurian-Devonian aquifers (USGS national aquifer code 410). This is the dominant principal aquifer mapped under the county area, not a determination for any single well.
Agricultural land use
area contextAbout 23% of the Washington County area is row-crop farmland (USDA Cropland Data Layer). Intensively farmed row-crop land raises the likelihood of elevated nitrate in nearby groundwater, so it is a reason to include nitrate in your test panel. This is land-use context for the county area — not a measurement of nitrate in any well.
PFAS
public-system proxyPFAS were detected in 4 of 8 public water systems tested in Washington County under the EPA's national PFAS monitoring program (UCMR5).
This reflects PFAS detections in public water systems near Washington County — a regional proxy, not a measurement of private wells. If you rely on a private well, only a lab test of your own water reveals its PFAS level.
These figures are USGS area estimates: statistical groundwater models describing how likely elevated contaminant levels are across a county. They are not designed to predict the concentration in any single well. Only testing your own well reveals its water quality.
Recommended test panel for Washington County
- coliform
- nitrate
Coliform bacteria is recommended for every private well as the universal baseline test. Nitrate is recommended because about 23% of this county area is row-crop land (USDA CDL), a land-use predictor of elevated groundwater nitrate — this is area context, not a measured concentration. The federal loan minimum (FHA, VA, USDA) tests for coliform, nitrate, nitrite, lead.
Federal FHA, VA, and USDA home loans require testing for coliform, nitrate, nitrite, lead at the point of a federally-backed loan. See the program-specific rules: FHA well test, VA well test, USDA well test.
Already have lab results? Interpret your well water test results to see how your own numbers compare to EPA limits.
How Washington County compares across Wisconsin
Where Washington County's modeled area estimates sit relative to the other Wisconsin counties in our analysis.
- Arsenicbelow the Wisconsin median
Washington County: 3% · Wisconsin median: 4% · flagged in 3 of 72 counties.
See all Wisconsin arsenic data → - Nitratebelow the Wisconsin median
Washington County: 0.20 mg/L · Wisconsin median: 0.50 mg/L · flagged in 0 of 72 counties.
See all Wisconsin nitrate data →
Municipal (public) water in Washington County
Most Washington County residents are served by a public water system, not a private well. If that's you, open your city's tap-water quality report:
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Find a state-certified lab
Test your Washington County well through a lab certified by the Wisconsin Department of Natural Resources (chemistry) and Department of Agriculture, Trade, & Consumer Protection (microbiology) — find one in the official directory (Excel spreadsheet for chemistry certifications, searchable database for microbiology).
Recognize & research well-water problems
Notice a problem? Diagnose it by symptom
Learn about these contaminants in drinking water
Data sources
Washington County's estimates are modeled from public federal datasets. See the data & methodology for how we build, source, and validate them.
- USGS arsenic probability-of-exceedance model
- USGS nitrate predicted concentration, domestic-supply depth
- USGS domestic (private) well population density
- USGS principal aquifers
- USDA NASS Cropland Data Layer (CDL) 30m
- EPA UCMR5 PFAS occurrence in public water systems (Fifth Unregulated Contaminant Monitoring Rule)
- EPA/USGS Water Quality Portal (waterqualitydata.us)
- US Census TIGER/Line county polygons
By TapWaterData Editorial · Last updated June 26, 2026.
Washington County well water FAQ
How do I test my well water in Washington County?
Order a sample kit covering the recommended panel for Washington County (coliform, nitrate), then send it to a Wisconsin-certified drinking-water laboratory. Certified labs use EPA-approved methods, so the results are defensible for a federally-backed home loan. You can also use a mail-in test kit for a convenient EPA-certified analysis.
What does the USGS arsenic estimate mean for my well?
It is an area estimate, not a prediction for your specific well. The USGS model describes how likely elevated arsenic is across Washington County as a whole; an individual well in the county can be much higher or much lower. Only testing your own well reveals its actual arsenic level.
How often should a private well be tested?
The CDC recommends testing private wells at least once a year for total coliform bacteria and nitrate, and more often if you notice a change in taste, color, or odor, or after flooding or nearby construction.
Does Wisconsin require well testing?
Federal FHA, VA, and USDA home-loan programs require a private-well water test at the point of a federally-backed loan. A state sale, rental, or recurring testing mandate was not independently verified for this state — consult your state's program.
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