Massachusetts well water by county: arsenic and nitrate
Modelled arsenic and nitrate for private-well users in every Massachusetts county, what to test for, and where to find a certified lab. Private wells aren’t covered by the Safe Drinking Water Act.
All 14 counties
- Worcester CountyWell users 109,400Arsenic about 13 in 100Nitrate 0.2 mg/L
- Middlesex CountyWell users 52,300Arsenic about 17 in 100Nitrate 0.1 mg/L
- Plymouth CountyWell users 46,400Arsenic about 4 in 100Nitrate 0.6 mg/L
- Bristol CountyWell users 44,200Arsenic about 6 in 100Nitrate 0.2 mg/L
- Hampden CountyWell users 32,400Arsenic about 10 in 100Nitrate 0.2 mg/L
- Hampshire CountyWell users 25,100Arsenic about 9 in 100Nitrate 0.1 mg/L
- Barnstable CountyWell users 24,000Arsenic about 4 in 100Nitrate 0.4 mg/L
- Berkshire CountyWell users 23,900Arsenic about 4 in 100Nitrate 0.1 mg/L
- Franklin CountyWell users 21,600Arsenic about 5 in 100Nitrate 0.1 mg/L
- Essex CountyWell users 21,500Arsenic about 17 in 100Nitrate 0.1 mg/L
- Norfolk CountyWell users 19,400Arsenic about 9 in 100Nitrate 0.1 mg/L
- Dukes CountyWell users 4,900Arsenic about 5 in 100Nitrate 0.1 mg/L
- Suffolk CountyWell users 3,100Arsenic about 11 in 100Nitrate 0.1 mg/L
- Nantucket CountyWell users 2,200Arsenic about 4 in 100Nitrate 0.1 mg/L
Each mark = 1 in 100 well users the model places above 10 µg/L arsenic (scale 0–10)10 mg/L nitrate: the federal limit (MCL) for public systems
- These figures describe the area, not your well. Only a lab test shows what’s in your water.
- CDC says the Well Water Index figures “do not describe risk to individual wells.” (CDC Tracking Network)
- The arsenic model is better at spotting low-arsenic areas than at confirming high ones.
- Well-user counts are 2010 USGS estimates; tract values are averages over each census tract.
- Total coliform bacteriaEvery year
- NitrateEvery year
- Total dissolved solidsEvery year
- pHEvery year
- ArsenicAsk your local health department
EPA: Private Drinking Water Wells · CDC: Guidelines for Testing Well Water
Use a state-certified lab
Massachusetts Department of Environmental Protection lists the laboratories certified to test drinking water in Massachusetts.
Screening tells you if something is worth a closer look
A lab result is the number you can act on.
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Lab-certified results
ETR Labs Basic Water Quality Test Kit
ETR Laboratories
50+ substances, including lead, copper, arsenic, nitrate, hardness and coliform bacteria
- Analyzed by Environmental Testing and Research Laboratories (ETR Labs), Leominster, MA · ISO/IEC 17025:2017, ANAB certificate AT-2638
- Turnaround: 2–4 business days after the lab receives it
What it can't tell you: It does not test PFAS, and not every result on the panel falls under the lab's accredited scope.
Screening: quick check, not lab-grade
Watersafe Drinking Water Test Kit
Silver Lake Research
Lead, bacteria, nitrate, copper, iron, chlorine, hardness and pH
- Turnaround: Minutes; the bacteria test can take up to 48 hours
What it can't tell you: Each test gives a yes/no or a rough range, not a lab number, and it cannot detect PFAS or arsenic.
A mail-in kit’s lab is accredited (ISO/IEC 17025) but may not be certified by your state. For a home sale, a loan, or a health-department follow-up, use a state-certified lab.
Massachusetts well water
Sources: CDC Well Water Index (2024) · USGS arsenic and nitrate models · USGS domestic well populations · EPA and CDC testing guidance. Data last updated 2026-10-08. All sources →
How many people use private wells in Massachusetts?
About 430,400 people across 14 Massachusetts counties, by the USGS 2010 estimate. USGS models place about 41,900 of them in areas where wells are likely to be above the 10 µg/L arsenic limit. These are area estimates, not well results.
Are private wells regulated in Massachusetts?
The quality and safety of drinking water from private domestic wells are not regulated by the Federal Government under the Safe Drinking Water Act nor by most state governments and laws. (EPA) Testing and treatment are the owner's responsibility.
Where can I get my well tested in Massachusetts?
Massachusetts Department of Environmental Protection keeps the list of laboratories certified to test drinking water in Massachusetts. Only use laboratories that are certified to do drinking water testing. (EPA)