Chlorate in New Hampshire Drinking Water
Ranked by max chlorate detected (µg/L) · UCMR 3 data (2013–2015) · Data from EPA SDWIS & UCMR
Chlorate in New Hampshire: what the data shows
New Hampshire has 20 cities with chlorate data from the EPA's UCMR 3 program (2013–2015). Chlorate exceeded the 210 µg/L EPA lifetime health advisory in 11 of those cities. The state average max detected level is 480.8 µg/L. Chlorate is a disinfection byproduct that forms when chlorine dioxide or hypochlorite solutions are used to disinfect drinking water. It is most common in systems that use chlorine dioxide for taste-and-odor control or that store hypochlorite for extended periods. There is no federal MCL — the EPA lifetime health advisory is 210 µg/L. Chlorate can interfere with thyroid function by blocking iodide uptake, an effect of particular concern for pregnant women, infants, and people with thyroid conditions. Reverse osmosis and ion exchange (anion-specific) are effective at removing chlorate; standard activated carbon filters provide only modest reduction.
Cities exceeding 210 µg/L EPA lifetime HA (no MCL)
Chlorate data across New Hampshire
Each dot is a city with UCMR 3 chlorate testing data. Cities where chlorate exceeds the 210 µg/L EPA lifetime health advisory are highlighted. Size reflects population served.
Top 10 cities by chlorate level in New Hampshire
Highest Chlorate levels (µg/L)
All New Hampshire cities ranked by chlorate level
| # | City | Level | Level | Over HA? | Violations | Grade |
|---|---|---|---|---|---|---|
| 1 | Littleton | 2415.6 µg/L | Over HA | 16 | A- | |
| 2 | Durham | 1360.0 µg/L | Over HA | 26 | B | |
| 3 | Dover | 750.0 µg/L | Over HA | 10 | B+ | |
| 4 | Exeter | 652.3 µg/L | Over HA | 60 | F | |
| 5 | Rochester | 650.0 µg/L | Over HA | 49 | F | |
| 6 | Concord | 614.6 µg/L | Over HA | 1 | D+ | |
| 7 | Laconia | 451.2 µg/L | Over HA | 13 | C+ | |
| 8 | Salem | 420.0 µg/L | Over HA | 37 | D | |
| 9 | Manchester | 380.0 µg/L | Over HA | 5 | D+ | |
| 10 | Somersworth | 380.0 µg/L | Over HA | 35 | A- | |
| 11 | Merrimack | 290.0 µg/L | Over HA | 13 | B+ | |
| 12 | Nashua | 190.0 µg/L | No | 3 | A- | |
| 13 | Keene | 190.0 µg/L | No | 13 | A- | |
| 14 | Seabrook | 184.0 µg/L | No | 29 | C+ | |
| 15 | Portsmouth | 180.0 µg/L | No | 12 | B+ | |
| 16 | Derry | 160.0 µg/L | No | 63 | F | |
| 17 | Hudson | 150.0 µg/L | No | 27 | B | |
| 18 | Walpole | 92.5 µg/L | No | 6 | D | |
| 19 | Lebanon | 77.0 µg/L | No | 33 | B- | |
| 20 | Bartlett | 29.0 µg/L | No | 193 | D |
Frequently asked questions about chlorate in New Hampshire
Is chlorate in New Hampshire tap water dangerous?
Chlorate has no federal MCL. The EPA lifetime health advisory is 210 µg/L. 11 cities in New Hampshire exceed this level. Chlorate can interfere with thyroid iodide uptake, which is a particular concern for infants, pregnant women, and people with hypothyroidism.
Where does chlorate in New Hampshire water come from?
Chlorate is a byproduct of chlorine-based disinfectants — particularly chlorine dioxide and hypochlorite (bleach) solutions. Levels tend to be higher in systems that use chlorine dioxide for taste-and-odor treatment or store sodium hypochlorite at high concentrations or for long periods. Levels vary seasonally with disinfectant use.
How can I reduce chlorate exposure?
Reverse osmosis is the most effective home treatment for chlorate, typically removing 80–95%. Anion exchange systems also work but require regeneration. Standard activated carbon filters provide only limited chlorate reduction. Boiling does NOT remove chlorate.