Chlorate Contamination

Chlorate in Connecticut Drinking Water

Ranked by max chlorate detected (µg/L) · UCMR 3 data (2013–2015) · Data from EPA SDWIS & UCMR

Cities Tested

Over HA

86%

% Over HA

667.3 µg/L

State Avg

▲ 28% vs national

vs National

105

Health Violations

Analysis

Chlorate in Connecticut: what the data shows

Connecticut has 92 cities with chlorate data from the EPA's UCMR 3 program (2013–2015). Chlorate exceeded the 210 µg/L EPA lifetime health advisory in 79 of those cities. The state average max detected level is 667.3 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.

Comparison

Cities exceeding 210 µg/L EPA lifetime HA (no MCL)

Connecticut

86%

79 of 92 cities

→ ▲ 28% above national rate (worse)

National avg

58%

1812 of 3098 cities

City Map

Chlorate data across Connecticut

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.

Worst Offenders

Top 10 cities by chlorate level in Connecticut

Highest Chlorate levels (µg/L)

All Cities

All Connecticut cities ranked by chlorate level

#	City	Level	Over HA?	Violations	Grade
1	Ansonia	1476.0 µg/L	Over HA	0	B+
2	Bethany	1476.0 µg/L	Over HA	4	C-
3	Branford	1476.0 µg/L	Over HA	0	A-
4	Cheshire	1476.0 µg/L	Over HA	28	F
5	Derby	1476.0 µg/L	Over HA	0	B+
6	East Haven	1476.0 µg/L	Over HA	0	A-
7	Hamden	1476.0 µg/L	Over HA	0	A-
8	Milford	1476.0 µg/L	Over HA	0	B+
9	New Haven	1476.0 µg/L	Over HA	0	A-
10	North Branford	1476.0 µg/L	Over HA	25	F
11	North Haven	1476.0 µg/L	Over HA	0	C-
12	Orange	1476.0 µg/L	Over HA	0	B+
13	Prospect	1476.0 µg/L	Over HA	1	C+
14	Seymour	1476.0 µg/L	Over HA	2	D+
15	West Haven	1476.0 µg/L	Over HA	0	B+
16	Woodbridge	1476.0 µg/L	Over HA	0	B+
17	Meriden	1476.0 µg/L	Over HA	4	C+
18	Southington	1476.0 µg/L	Over HA	12	F
19	Wallingford	1476.0 µg/L	Over HA	4	D
20	Wolcott	1476.0 µg/L	Over HA	18	F
21	Mansfield	900.0 µg/L	Over HA	85	F
22	Groton	770.0 µg/L	Over HA	17	F
23	Stonington	770.0 µg/L	Over HA	6	B
24	Ledyard	770.0 µg/L	Over HA	23	D+
25	Beacon Falls	660.0 µg/L	Over HA	3	C-
26	Bridgeport	660.0 µg/L	Over HA	2	C-
27	Darien	660.0 µg/L	Over HA	7	F
28	Fairfield	660.0 µg/L	Over HA	2	C-
29	New Canaan	660.0 µg/L	Over HA	7	F
30	Redding	660.0 µg/L	Over HA	2	C-
31	Shelton	660.0 µg/L	Over HA	2	C-
32	Stamford	660.0 µg/L	Over HA	4	F
33	Stratford	660.0 µg/L	Over HA	2	C-
34	Westport	660.0 µg/L	Over HA	2	C-
35	Wilton	660.0 µg/L	Over HA	7	D
36	Norwalk	660.0 µg/L	Over HA	7	D
37	Newtown	660.0 µg/L	Over HA	46	F
38	Greenwich	660.0 µg/L	Over HA	5	F
39	South Windsor	620.0 µg/L	Over HA	5	C
40	Windsor Locks	620.0 µg/L	Over HA	0	B-
41	East Windsor	620.0 µg/L	Over HA	25	F
42	Ellington	620.0 µg/L	Over HA	0	B
43	Suffield	620.0 µg/L	Over HA	1	C+
44	Manchester	620.0 µg/L	Over HA	8	C
45	Enfield	620.0 µg/L	Over HA	9	D
46	Tolland	620.0 µg/L	Over HA	24	F
47	Stafford	620.0 µg/L	Over HA	12	C
48	Berlin	600.0 µg/L	Over HA	6	F
49	Bozrah	598.0 µg/L	Over HA	13	B
50	Franklin	598.0 µg/L	Over HA	13	B
51	Lisbon	598.0 µg/L	Over HA	27	F
52	Montville	598.0 µg/L	Over HA	143	F
53	Norwich	598.0 µg/L	Over HA	19	B-
54	Preston	598.0 µg/L	Over HA	23	D+
55	Danbury	520.0 µg/L	Over HA	59	F
56	Bethel	520.0 µg/L	Over HA	25	F
57	Brookfield	520.0 µg/L	Over HA	135	F
58	Mashantucket	470.0 µg/L	Over HA	7	B
59	Glastonbury	464.0 µg/L	Over HA	4	B+
60	Waterford	440.0 µg/L	Over HA	16	C-
61	New London	440.0 µg/L	Over HA	0	C+
62	Harwinton	340.0 µg/L	Over HA	5	D
63	Torrington	340.0 µg/L	Over HA	3	B+
64	Uncasville	300.0 µg/L	Over HA	1	A
65	East Hartford	250.0 µg/L	Over HA	0	A-
66	Farmington	250.0 µg/L	Over HA	3	D+
67	Hartford	250.0 µg/L	Over HA	0	A-
68	Newington	250.0 µg/L	Over HA	1	D
69	Rocky Hill	250.0 µg/L	Over HA	1	B
70	West Hartford	250.0 µg/L	Over HA	0	B+
71	Wethersfield	250.0 µg/L	Over HA	0	A-
72	Windsor	250.0 µg/L	Over HA	0	A-
73	Cromwell	250.0 µg/L	Over HA	1	B
74	Portland	250.0 µg/L	Over HA	9	C
75	Waterbury	244.0 µg/L	Over HA	14	C
76	Watertown	244.0 µg/L	Over HA	11	D+
77	Avon	240.0 µg/L	Over HA	3	C+
78	New Britain	214.0 µg/L	Over HA	0	B-
79	Plainville	214.0 µg/L	Over HA	4	C
80	Bristol	200.0 µg/L	No	4	C+
81	Oxford	190.0 µg/L	No	6	B
82	Somers	180.0 µg/L	No	6	C+
83	Hampton	170.0 µg/L	No	1	B
84	Plymouth	150.0 µg/L	No	18	C
85	Clinton	110.0 µg/L	No	4	D+
86	Guilford	110.0 µg/L	No	20	D
87	Madison	110.0 µg/L	No	18	C+
88	Old Saybrook	110.0 µg/L	No	0	B-
89	Westbrook	110.0 µg/L	No	7	C+
90	Granby	110.0 µg/L	No	3	C+
91	Deep River	52.0 µg/L	No	5	C
92	Naugatuck	39.0 µg/L	No	1	A

FAQ

Frequently asked questions about chlorate in Connecticut

Is chlorate in Connecticut tap water dangerous?

Chlorate has no federal MCL. The EPA lifetime health advisory is 210 µg/L. 79 cities in Connecticut 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 Connecticut 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.

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