Chlorate Contamination

Chlorate in Connecticut Drinking Water

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

110

Cities Tested

Over HA

85%

% Over HA

629.7 µg/L

State Avg

▲ 26% vs national

vs National

134

Health Violations

Analysis

Chlorate in Connecticut: what the data shows

Connecticut has 110 cities with chlorate data from the EPA's UCMR 3 program (2013–2015). Chlorate exceeded the 210 µg/L EPA lifetime health advisory in 93 of those cities. The state average max detected level is 629.7 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

85%

93 of 110 cities

→ ▲ 26% above national rate (worse)

National avg

59%

1866 of 3166 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	2	D
2	Ansonia	1476.0 µg/L	Over HA	12	F
3	Ansonia	1476.0 µg/L	Over HA	18	F
4	Ansonia	1476.0 µg/L	Over HA	1	C+
5	Ansonia	1476.0 µg/L	Over HA	28	F
6	Ansonia	1476.0 µg/L	Over HA	4	D
7	Ansonia	1476.0 µg/L	Over HA	4	C+
8	Ansonia	1476.0 µg/L	Over HA	0	C-
9	Ansonia	1476.0 µg/L	Over HA	4	D+
10	Ansonia	1476.0 µg/L	Over HA	0	B+
11	Ansonia	1476.0 µg/L	Over HA	0	B+
12	Ansonia	1476.0 µg/L	Over HA	0	B+
13	Ansonia	1476.0 µg/L	Over HA	0	B+
14	Ansonia	1476.0 µg/L	Over HA	25	F
15	Ansonia	1476.0 µg/L	Over HA	0	B+
16	Ansonia	1476.0 µg/L	Over HA	0	B+
17	Ansonia	1476.0 µg/L	Over HA	0	B+
18	Ansonia	1476.0 µg/L	Over HA	0	B+
19	Ansonia	1476.0 µg/L	Over HA	0	B+
20	Ansonia	1476.0 µg/L	Over HA	0	B+
21	East Granby	900.0 µg/L	Over HA	85	F
22	Windham	900.0 µg/L	Over HA	11	B+
23	Groton	770.0 µg/L	Over HA	23	D+
24	Groton	770.0 µg/L	Over HA	17	F
25	Groton	770.0 µg/L	Over HA	6	B-
26	Beacon Falls	660.0 µg/L	Over HA	4	F
27	Beacon Falls	660.0 µg/L	Over HA	7	F
28	Beacon Falls	660.0 µg/L	Over HA	5	F
29	Beacon Falls	660.0 µg/L	Over HA	7	D
30	Beacon Falls	660.0 µg/L	Over HA	7	D
31	Beacon Falls	660.0 µg/L	Over HA	7	D
32	Beacon Falls	660.0 µg/L	Over HA	27	F
33	Beacon Falls	660.0 µg/L	Over HA	2	C-
34	Beacon Falls	660.0 µg/L	Over HA	3	C-
35	Beacon Falls	660.0 µg/L	Over HA	46	F
36	Beacon Falls	660.0 µg/L	Over HA	7	F
37	Beacon Falls	660.0 µg/L	Over HA	2	F
38	Beacon Falls	660.0 µg/L	Over HA	2	F
39	Beacon Falls	660.0 µg/L	Over HA	2	F
40	Beacon Falls	660.0 µg/L	Over HA	2	C-
41	Beacon Falls	660.0 µg/L	Over HA	2	C-
42	Beacon Falls	660.0 µg/L	Over HA	2	C-
43	Beacon Falls	660.0 µg/L	Over HA	2	C-
44	Beacon Falls	660.0 µg/L	Over HA	2	C-
45	Berlin	620.0 µg/L	Over HA	5	C
46	Berlin	620.0 µg/L	Over HA	8	C
47	Berlin	620.0 µg/L	Over HA	40	F
48	Berlin	620.0 µg/L	Over HA	0	B-
49	East Granby	620.0 µg/L	Over HA	5	C-
50	East Granby	620.0 µg/L	Over HA	9	D
51	East Granby	620.0 µg/L	Over HA	25	F
52	East Granby	620.0 µg/L	Over HA	24	F
53	East Granby	620.0 µg/L	Over HA	12	C-
54	East Granby	620.0 µg/L	Over HA	1	C+
55	East Granby	620.0 µg/L	Over HA	0	B
56	Berlin	600.0 µg/L	Over HA	6	F
57	Franklin	598.0 µg/L	Over HA	143	F
58	Franklin	598.0 µg/L	Over HA	27	F
59	Franklin	598.0 µg/L	Over HA	32	F
60	Franklin	598.0 µg/L	Over HA	19	B-
61	Franklin	598.0 µg/L	Over HA	23	D+
62	Franklin	598.0 µg/L	Over HA	13	B
63	Franklin	598.0 µg/L	Over HA	13	B
64	Bethel	520.0 µg/L	Over HA	135	F
65	Bethel	520.0 µg/L	Over HA	59	F
66	Bethel	520.0 µg/L	Over HA	25	F
67	Mashantucket	470.0 µg/L	Over HA	7	B
68	Berlin	464.0 µg/L	Over HA	4	B+
69	New London	440.0 µg/L	Over HA	16	C-
70	New London	440.0 µg/L	Over HA	0	C+
71	Harwinton	340.0 µg/L	Over HA	8	F
72	Harwinton	340.0 µg/L	Over HA	19	F
73	Harwinton	340.0 µg/L	Over HA	3	B+
74	Harwinton	340.0 µg/L	Over HA	5	D
75	Uncasville	300.0 µg/L	Over HA	1	A
76	Berlin	250.0 µg/L	Over HA	3	D+
77	Berlin	250.0 µg/L	Over HA	1	D
78	Berlin	250.0 µg/L	Over HA	0	B+
79	Berlin	250.0 µg/L	Over HA	1	B-
80	Berlin	250.0 µg/L	Over HA	9	C
81	Berlin	250.0 µg/L	Over HA	33	F
82	Berlin	250.0 µg/L	Over HA	1	B
83	Berlin	250.0 µg/L	Over HA	0	A-
84	Berlin	250.0 µg/L	Over HA	0	A-
85	Berlin	250.0 µg/L	Over HA	0	A-
86	Berlin	250.0 µg/L	Over HA	0	A-
87	Middlebury	244.0 µg/L	Over HA	14	C
88	Middlebury	244.0 µg/L	Over HA	18	C-
89	Middlebury	244.0 µg/L	Over HA	11	D+
90	Plainville	240.0 µg/L	Over HA	3	C+
91	East Granby	240.0 µg/L	Over HA	9	F
92	Berlin	214.0 µg/L	Over HA	4	C
93	Berlin	214.0 µg/L	Over HA	0	B-
94	Bristol	200.0 µg/L	No	4	C+
95	Ansonia	190.0 µg/L	No	6	B-
96	East Windsor	180.0 µg/L	No	6	C+
97	Hampton	170.0 µg/L	No	12	B-
98	Hampton	170.0 µg/L	No	1	B
99	Bristol	150.0 µg/L	No	18	C
100	Bristol	150.0 µg/L	No	9	C
101	Clinton	110.0 µg/L	No	18	C+
102	Clinton	110.0 µg/L	No	20	D
103	Clinton	110.0 µg/L	No	4	D+
104	Clinton	110.0 µg/L	No	7	C+
105	Clinton	110.0 µg/L	No	0	B-
106	East Granby	110.0 µg/L	No	3	C+
107	Chester	52.0 µg/L	No	10	D
108	Chester	52.0 µg/L	No	12	B-
109	Chester	52.0 µg/L	No	5	C
110	Beacon Falls	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. 93 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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