Well water in Mercer County, New Jersey
In Mercer County, 20% of 3,465 private wells tested between 2002 and 2024 had total coliform above the New Jersey limit (any detection), according to NJDEP.
3,471 private wells tested 2002 to 2024 in state summaries; 168 individual samples from 93 wells, 1958 to 2024. Data refreshed September 9, 2026.
What homeowner tests found
NJDEP Private Well Testing Act summaries: private wells tested and reported to the state, 2002 to 2024. Shares are against the level the state applies to each contaminant, named in the last column; several are stricter than EPA.
| Contaminant | Wells tested | Above state level | Median | 95th pct | Highest | State level | |
|---|---|---|---|---|---|---|---|
| Manganese | 3,471 | 23% | 50 ug/L | ||||
| Iron | 3,470 | 21% | 300 ug/L | ||||
| Total coliform | 3,465 | 20% | |||||
| Arsenic | 3,466 | 18% | 5 ug/L | ||||
| PFOS | 302 | 12% | 13 ng/L | ||||
| PFOA | 302 | 11% | 14 ng/L | ||||
| Gross alpha | 3,179 | 6% | 15 pCi/L | ||||
| E. coli Fecal coliform or | 3,454 | 3% | |||||
| Nitrate | 3,471 | 2% | 10 mg/L | ||||
| PFNA | 302 | <1% | 13 ng/L | ||||
| Uranium | 453 | <1% | 30 ug/L |
What individual samples found
Each row summarizes every public sample for that contaminant inside Mercer County. Above limit means above the EPA maximum contaminant level; where EPA has no enforceable limit, the row shows a health guideline or nuisance level instead.
| Contaminant | Samples | Detected | Above limit | Median | Highest | Limit | |
|---|---|---|---|---|---|---|---|
| PFOA 2021 to 2024 | 19 | 74% | 63% | 5.8 | 35 | 4 ng/L EPA limit | |
| PFOS 2021 to 2024 | 19 | 63% | 42% | 2.4 | 19 | 4 ng/L EPA limit | |
| Uranium 1985 to 2022 | 55 | 82% | 4% | 1.5 | 49 | 30 ug/L EPA limit | |
| Arsenic 1983 to 2024 | 168 | 69% | 4% | 0.4 | 23 | 10 ug/L EPA limit | |
| Radium (226 + 228) 1985 to 2022 | 52 | 54% | 2% | 0 | 39 | 5 pCi/L EPA limit | |
| Radon 1985 to 2012 | 94 | 99% | no limit | 1,020 | 39,200 | 2,000 pCi/L health guideline |
Sampled fewer than 10 times, too few to summarize: Copper (4), Fluoride (1), Sodium (5), Chloride (2), Manganese (1), Iron (1).
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What this means
Among homeowners in Mercer County who sent a sample to the state lab, the health-based limits most often exceeded were total coliform (20% of 3,465 wells), arsenic (18% of 3,466 wells), and PFOS (12% of 302 wells). These are ordinary wells tested by their owners, so the shares are the best available picture of a typical well here, with one caveat: people test more often when they suspect a problem.
Manganese and iron exceed the state's advisory level in a sizable share of wells; these are mostly a taste, staining or diet issue rather than a toxicity one.
What to test for in Mercer County
- Manganese23% of 3,471 local wells tested were above the New Jersey nuisance level.
- Iron21% of 3,470 local wells tested were above the New Jersey nuisance level.
- Total coliform20% of 3,465 local wells tested were above the New Jersey limit (any detection).
- Gross alpha6% of 3,179 local wells tested were above the New Jersey limit.
- E. coli3% of 3,454 local wells tested were above the New Jersey limit (any detection).
- Radium (226 + 228)2% of 52 local samples were above the limit.
- Nitrate2% of 3,471 local wells tested were above the New Jersey limit.
- PFNA<1% of 302 local wells tested were above the New Jersey limit.
Contamination sources on record in Mercer County
NJDEP records 2,132 regulated sites in Mercer County, among 2,132 regulated sites in total. Groundwater does not follow property lines, so a well within a few hundred feet of one of these is worth testing for what that site could release.
- 1,598
- registered underground storage tank sitesRegistered tanks are regulated and inspected; they matter most when they are old or near a shallow well.
- 9
- lined landfills and transfer facilities
- 525
- other remediation projects
Treatment that fits the local profile
- Total coliform
- Inspect and repair the well cap and casing, then shock chlorinate and retest; UV disinfection whole-house; Chlorination system
- Arsenic
- Point-of-use reverse osmosis; Adsorptive media (iron-based) whole-house; Anion exchange
- PFOS
- Granular activated carbon whole-house; Point-of-use reverse osmosis; Anion exchange
- PFOA
- Granular activated carbon whole-house; Point-of-use reverse osmosis; Anion exchange
- Gross alpha
- Point-of-use reverse osmosis; Water softener (cation exchange) for radium; Anion exchange for uranium
- Uranium
- Point-of-use reverse osmosis; Anion exchange; Whole-house anion exchange
Get your Mercer County water tested at home
A licensed local water treatment company tests your tap water in your home, walks you through the numbers, and quotes treatment only if something needs fixing. Free, about 30 minutes, no obligation. Tell us what you need and we match you with a company that serves Mercer County.
Common questions
Is there arsenic in well water in Mercer County?
Arsenic was detected in 69% of 168 public well samples in Mercer County, and 4% were above the EPA limit of 10 ug/L. Arsenic has no taste or smell; a lab test is the only way to know your level.
How high is radon in well water in Mercer County?
The median radon level across 94 public samples in Mercer County was 1,020 pCi/L, with a high of 39,200 pCi/L. New Hampshire recommends treatment above 2,000 pCi/L.
How often should I test a private well in Mercer County?
Test for bacteria and nitrate every year. Test once for arsenic, uranium, radon, fluoride, manganese and PFAS, then again every three to five years or after any change in taste, color, or plumbing.
Towns in Mercer County
- East Windsor township19% total coliform
- Ewing township16% total coliform
- Hamilton township11% total coliform
- Hopewell township26% arsenic
- Lawrence township17% total coliform
- Princeton29% total coliform
- Robbinsville township18% total coliform
- West Windsor township12% total coliform
4 towns have fewer than 30 public samples and are included in the Mercer County totals above rather than shown separately: Hightstown borough, Hopewell borough, Pennington borough, Trenton.
Sources
- USGS Water Quality Portal: 421 samples in Mercer County. Data from the Water Quality Portal, a service of USGS, EPA and USDA; groundwater samples collected by USGS and cooperators. Retrieved September 3, 2026.
- NJDEP Private Well Testing Act summaries: summaries for 11 contaminants in Mercer County, up to 3,471 wells each. Share of private wells exceeding each standard in tests required at sale or lease under the New Jersey Private Well Testing Act, September 2002 to December 2024, published by NJDEP. Tests are untreated water at the time of sale or lease, so every well is counted once per transaction. Municipalities with fewer than ten wells are suppressed. Standards are New Jersey's: arsenic 5 ug/L, PFOA 14 ng/L, PFOS 13 ng/L, PFNA 13 ng/L, manganese and iron secondary standards. Uranium and PFAS were added to the required panel in 2021 and 2020, so their well counts are smaller. Retrieved September 9, 2026.
- NJDEP regulated sites: 2,132 regulated sites located in Mercer County. Known Contaminated Sites, regulated underground storage tank facilities and solid waste landfills from the New Jersey Department of Environmental Protection (NJ-GeoWeb). Retrieved September 9, 2026.
Non-detects count as samples and are treated as zero when computing medians. Exactly at the limit counts as not above it. A sample belongs to the town containing the well's reported coordinates.