Well water in Mercer County, New Jersey
1in5
Testing strongly recommended
private wells tested between 2002 and 2024 in Mercer County had total coliform above the New Jersey limit (any detection), according to NJDEP.
The strongest signal is PFOA: 63% of 19 local samples were above the limit, too few samples to carry the headline but first on the test list below.
3,471 private wells tested 2002 to 2024 in state summaries; 1,614 test results across 14 contaminants from at least 164 wells, 1925 to 2024; sodium has the most at 273. Data refreshed September 15, 2026.
Compiled by Nick at Well Water Nerd from public records. How these pages are built
An estimated 40,381 people in Mercer County get their drinking water from a private well, per USGS 2015 county water use. That is about 11% of residents.
Separately, the USGS national arsenic model estimates that 1% of the 40,381 people on private wells in Mercer County draw water above 10 ug/L (the model's range is 1% to 2%), from geology and well depth rather than local tests.
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.
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 | Above limit | Samples | Detected | Median | Highest | Limit |
|---|---|---|---|---|---|---|
| PFOA 2021 to 2024 | 19 | 74% | 5.8 | 35 | 4 ng/L EPA limit | |
| PFOS 2021 to 2024 | 19 | 63% | 2.4 | 19 | 4 ng/L EPA limit | |
| Nitrate 1980 to 2024 | 205 | 88% | 1.8 | 72.4 | 10 mg/L EPA limit | |
| Uranium 1985 to 2022 | 55 | 82% | 1.5 | 49 | 30 ug/L EPA limit | |
| Arsenic 1983 to 2024 | 168 | 69% | 0.4 | 23 | 10 ug/L EPA limit | |
| Lead 1972 to 2024 | 201 | 35% | 0 | 70 | 15 ug/L EPA limit | |
| Radium (226 + 228) 1985 to 2022 | 52 | 54% | 0 | 39 | 5 pCi/L EPA limit | |
| Sodium 1949 to 2024 | no limit | 273 | 100% | 12 | 251 | 20 mg/L health guideline |
| Iron 1925 to 2024 | no limit | 265 | 79% | 22 | 15,700 | 300 ug/L nuisance level |
| Manganese 1950 to 2024 | no limit | 256 | 80% | 21.9 | 3,210 | 300 ug/L health guideline |
| Radon 1985 to 2012 | no limit | 94 | 99% | 1,020 | 39,200 | 2,000 pCi/L health guideline |
Sampled fewer than 10 times, too few to summarize: Copper (4), Fluoride (1), Chloride (2).
Over time
Share of samples above the limit by period. A cell needs ten samples to show; a comparison needs twenty in each period. Later periods sample fewer wells, and often the ones already known to have a problem.
| Contaminant | before 2000 | 2000s | 2010s | 2020s |
|---|---|---|---|---|
| Nitrate | 11%123 samples | 4%28 samples | 3%35 samples | 0%19 samples |
| Lead | 3%124 samples | 0%25 samples | 0%34 samples | 0%18 samples |
| Arsenic | 8%78 samples | 0%35 samples | 0%36 samples | 0%19 samples |
| Uranium | 5%40 samples | n/a | 0%11 samples | n/a |
| Radium (226 + 228) | 0%32 samples | n/a | 8%13 samples | n/a |
Nitrate, lead, arsenic have held within a few points between the two most recent periods with enough samples.
Closer than the county
See the sampled wells near your address
The same records, drawn around one point: which wells within a few miles were sampled and what they found, how deep the wells next to you were drilled, and any leaking tank or cleanup site within a mile.
Check my addressWhat 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
- 1PFOA63% of 19 local samples were above the limit.
- 2PFOS42% of 19 local samples were above the limit.
- 3Total coliform20% of 3,465 local wells tested were above the New Jersey limit (any detection).
- 4Arsenic18% of 3,466 local wells tested were above the New Jersey limit.
- 5Nitrate7% of 205 local samples were above the limit.
- 6Uranium4% of 55 local samples were above the limit.
- 7E. coli3% of 3,454 local wells tested were above the New Jersey limit (any detection).
- 8Lead2% of 201 local samples were above the limit.
Test your Mercer County well for PFOA, PFOS and total coliform
Certified lab analysis of your own tap water, bottles and prepaid return shipping included. The Essential well panel covers arsenic, uranium, nitrate, lead, copper, manganese, iron, sodium and bacteria; radon and PFAS are add-ons.
Order the Tap Score well water test kit, from $209Already tested? Upload the report
Just want bacteria checked first? Tap Score coliform and E. coli test, $99. Coliform and E. coli are the tests every well needs every year.
Want a quick look before paying for a lab? Home well water test strips on Amazon check hardness, iron, pH and nitrate in minutes. They are a screen, not a result: arsenic, bacteria, uranium and PFAS need a certified lab.
Affiliate links: the merchant pays us a commission and the price is the same for you. It does not change what the data above says. As an Amazon Associate we earn from qualifying purchases.
Where to get a well tested in New Jersey: certified labs, state programs and prices.
Already have results back? Get your lab report read against the Mercer County numbers, $29. Every line explained, ranked by what matters, with the fix for each.
The rock under Mercer County
Northern Atlantic Coastal Plain aquifer system
Semiconsolidated sand. Coastal plain sands like these give steady yields at moderate depth. Salt water and sodium are the usual issues near the coast, and nitrate where the water table is shallow; arsenic and radon from bedrock are less common.
Classification from the USGS Principal Aquifers of the United States (national map, 2003 edition, GIS release 2015), read at the county centroid. A single well can sit in a different unit, especially near a boundary.
Official help in New Jersey
Two places to start that are not selling anything: the state program that answers private well questions, and its list of labs certified to run drinking water tests. A certified lab is the difference between a result you can act on and a number.
- Private Wells, NJDEP
- Certified Laboratories, NJDEP
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
Nitrate
Point-of-use reverse osmosis; Anion exchange; Distillation
Systems we would look at first
Some links below are affiliate links. If you buy through one, the merchant pays us a commission at no extra cost to you. It does not change which products we list or what the data says.
For total coliform
Aquasana Rhino Well Water with UV
whole-house, at the point of entry, about $2,871
Whole-house system built for wells: sediment pre-filter, KDF and carbon, and a UV lamp rated to kill 99.99% of bacteria and viruses. It does not remove arsenic, nitrate, uranium or radon.
For arsenic
Under-sink reverse osmosis system
under-sink or countertop, about $300
Look for NSF/ANSI 58 certification listing the contaminant you need removed.
For arsenic
Whole-house arsenic and uranium filter
whole-house, at the point of entry, about $2,500
Adsorptive iron-based media; sized to your well yield.
For arsenic
Aquasana SmartFlow reverse osmosis
under-sink or countertop, about $225
Certified to NSF/ANSI 58 and 53 to reduce arsenic (96%, pentavalent), fluoride (90%), lead (99%), nitrate, nitrite and PFOA/PFOS at the kitchen tap, with a remineralizer.
For PFOS
Aquasana Claryum 3-Stage under-sink filter
under-sink or countertop, about $175
Certified to NSF/ANSI 53 to reduce 99% of lead and 99.7% of PFOA and PFOS at the tap. Not a fix for arsenic, nitrate or bacteria.
For PFOS
Whole-house PFAS carbon system
whole-house, at the point of entry, about $3,000
Granular activated carbon certified to NSF/ANSI 53 for PFOA and PFOS; replace media on schedule.
For PFOS
Aquasana HealthShield whole-house filter
whole-house, at the point of entry, about $2,099
Whole-house carbon system the maker certifies to reduce PFAS (PFOA and PFOS) and VOCs at every tap; a UV option adds bacteria protection.
Free, no obligation
Get on the list for a free in-home water test in Mercer County
No water treatment company serves Mercer County through Well Water Nerd yet. We are signing licensed companies on county by county: they test at your tap for free, walk you through the numbers, and quote treatment only if something needs fixing. Leave your details and you are first in line. Nothing is shared until a company joins for Mercer County, and you get an email naming it first.
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.
Counties near Mercer County
Closest first. Groundwater does not stop at a line on a map, so the next county over is a fair second opinion.
Want the numbers themselves? Download the New Jersey dataset as CSV, every county and town, with the sources named.
Sources
- USGS Water Quality Portal: 1,614 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.
- USGS national arsenic model (Ayotte 2017): estimated share of 40,381 people on private wells in Mercer County above 10 ug/L of arsenic, from geology, well depth and land use. County estimates of domestic well users with arsenic above 10 ug/L from the USGS national logistic regression model (Ayotte, Medalie and Qi, 2017), with private well population from the USGS 2015 county water-use compilation (Dieter and others, 2018). Retrieved September 15, 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.