Research · published 2026-07-07 · download the county-level CSV

1 in 9 Wisconsin well records sits within a quarter mile of a documented fuel leak

Wisconsin does not require anyone to test a private well after it is drilled — sampled once at construction, then never again unless the owner pays for it. The state also carries one of the country’s largest backlogs of underground fuel-tank leaks it has documented but not finished cleaning up. We mapped where those two facts meet.

Comparing 800,317 Wisconsin well records (WI DNR) against the U.S. EPA’s UST Finder database of underground-storage-tank releases — documented leaks of gasoline, diesel, and heating oil at 20,471 Wisconsin sites:

• 85,442 well records (10.7%) lie within 1,320 ft (a quarter mile) of at least one documented petroleum release.
• 7,880 (1.0%) lie within a quarter mile of a release the state still lists as open — reported but not yet cleaned up (EPA snapshot, 2026-06-12).

None of these households is required to test their water. Public water systems near the same leaks are monitored on a federal schedule; private wells fall entirely outside that law.

Counties with the most records near an open leak

Download the full county table (CSV)

What this does and does not say

• Proximity is not contamination. Distance from a documented release is a screening signal, not evidence any specific well is affected. Whether a plume reaches a well depends on hydrogeology, depth, direction, and remediation history. Testing is the only way to know — which is the point: no one is required to.
• What we counted. Wisconsin well records — drilling events, which include some replacement and abandoned wells.
• “Open” status lags. The EPA release snapshot dates to 2026-06-12; states close releases every year, so the current open count is somewhat lower. The “any documented release” figure — cumulative history — is not affected.

Methodology

• Wells: 800,317 Wisconsin well records with coordinates from WI DNR (refreshed weekly; see our general methodology).
• Releases: 20,471 Wisconsin release sites with coordinates from EPA UST Finder (920 listed open in the snapshot).
• Join: great-circle (haversine) distance, well-to-release, threshold 1,320 ft. Each well is counted once no matter how many releases are nearby.
• Why a quarter mile: peer-reviewed work behind UST Finder used 1,500 ft as the outer bound of observed petroleum plume lengths (Weaver et al. 2017); we use a tighter 1,320 ft, so our threshold is the more conservative of the two.
• Checks: results were spot-verified at the address level (flagged wells and their nearest release share streets and ZIP codes) and screened for coordinate artifacts (no bulk co-located records).

Sources

• Wisconsin well records: WI DNR
• EPA UST Finder (release locations and status)
• EPA, Semiannual Report of UST Performance Measures, FY2025 (state cleanup backlogs)
• Weaver, Murray & Kremer (2017), Science of the Total Environment 609:1589–1599 (plume-length basis for the distance threshold)
• Murray, Hall, Weaver & Kremer (2021), JAWRA 57(5):828–843 (EPA’s domestic-well proximity estimates)
• ASTSWMO/EPA UST Finder briefing (state-by-state wells-near-releases counts)