Chlorate Contamination

Chlorate in Washington Drinking Water

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

Cities Tested

Over HA

42%

% Over HA

230.6 µg/L

State Avg

▼ 17% vs national

vs National

261

Health Violations

Analysis

Chlorate in Washington: what the data shows

Washington has 71 cities with chlorate data from the EPA's UCMR 3 program (2013–2015). Chlorate exceeded the 210 µg/L EPA lifetime health advisory in 30 of those cities. The state average max detected level is 230.6 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)

Washington

42%

30 of 71 cities

← ▼ 17% below national rate (better)

National avg

59%

1866 of 3166 cities

City Map

Chlorate data across Washington

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 Washington

Highest Chlorate levels (µg/L)

All Cities

All Washington cities ranked by chlorate level

#	City	Level	Over HA?	Violations	Grade
1	Wenatchee	1400.0 µg/L	Over HA	41	D
2	Snoqualime	940.0 µg/L	Over HA	4	A-
3	Lacey	870.0 µg/L	Over HA	125	F
4	Kennewick	850.0 µg/L	Over HA	78	F
5	Cashmere	470.1 µg/L	Over HA	21	F
6	Vancouver	470.0 µg/L	Over HA	36	F
7	Renton	419.0 µg/L	Over HA	11	D+
8	Port Orchard	410.0 µg/L	Over HA	753	F
9	Kelso	407.0 µg/L	Over HA	15	F
10	Marysville	365.0 µg/L	Over HA	12	D
11	Maple Valley	350.0 µg/L	Over HA	7	C+
12	Bremerton	320.0 µg/L	Over HA	13	C
13	Pasco	311.0 µg/L	Over HA	137	D
14	Tacoma	310.0 µg/L	Over HA	232	F
15	Sunnyside	310.0 µg/L	Over HA	2	A-
16	Longview	304.0 µg/L	Over HA	0	B+
17	Puyallup	300.0 µg/L	Over HA	124	F
18	Shelton	300.0 µg/L	Over HA	158	F
19	Stanwood	299.0 µg/L	Over HA	31	D
20	Kent	290.0 µg/L	Over HA	31	F
21	Pullman	290.0 µg/L	Over HA	0	B+
22	Prosser	280.0 µg/L	Over HA	42	C+
23	West Richland	266.0 µg/L	Over HA	99	F
24	Sammamish	265.0 µg/L	Over HA	6	C-
25	Silverdale	258.0 µg/L	Over HA	17	F
26	Carbonado	250.0 µg/L	Over HA	41	B
27	Spanaway	240.0 µg/L	Over HA	9	C+
28	Mount Vernon	232.0 µg/L	Over HA	191	F
29	Bainbridge Island	220.0 µg/L	Over HA	3	A
30	Joint Base Lewis-Mcchord	216.0 µg/L	Over HA	3	B+
31	Redmond	210.0 µg/L	No	4	B-
32	Auburn	200.0 µg/L	No	18	D+
33	Birch Bay	179.4 µg/L	No	14	A
34	Camas	164.0 µg/L	No	2	C
35	Lynnwood	160.0 µg/L	No	0	B-
36	Ferndale	153.0 µg/L	No	110	F
37	Spokane Valley	151.0 µg/L	No	10	C-
38	Buckley	150.0 µg/L	No	5	D+
39	Poulsbo	148.7 µg/L	No	95	F
40	Arlington	136.0 µg/L	No	83	F
41	Moses Lake	130.0 µg/L	No	132	F
42	Battle Ground	130.0 µg/L	No	12	B-
43	Snohomish	123.0 µg/L	No	11	D
44	Sequim	122.0 µg/L	No	64	F
45	Clarkston	122.0 µg/L	No	16	B+
46	Federal Way	120.0 µg/L	No	2	B
47	Lakewood	120.0 µg/L	No	5	B+
48	Bucoda	120.0 µg/L	No	5	B+
49	Monroe	112.0 µg/L	No	4	C
50	Bothell	110.0 µg/L	No	7	F
51	Centralia	110.0 µg/L	No	5	B-
52	Edmonds	100.0 µg/L	No	0	A-
53	Manchester	100.0 µg/L	No	7	A
54	Yakima	97.0 µg/L	No	162	F
55	Everett	93.0 µg/L	No	16	D
56	Othello	87.3 µg/L	No	102	D
57	Mountlake Terrace	79.0 µg/L	No	0	B+
58	Mukilteo	77.6 µg/L	No	0	A-
59	Seattle	61.0 µg/L	No	138	F
60	Chehalis	59.0 µg/L	No	12	F
61	East Wenatchee	57.0 µg/L	No	19	B
62	Dupont	52.0 µg/L	No	0	A
63	Issaquah	49.0 µg/L	No	10	C-
64	Lynden	49.0 µg/L	No	235	F
65	Tumwater	48.0 µg/L	No	24	D+
66	Port Angeles	41.0 µg/L	No	19	F
67	Richland	34.0 µg/L	No	7	C+
68	Royal City	29.2 µg/L	No	1	A-
69	Washougal	28.0 µg/L	No	2	B+
70	Yacolt	25.3 µg/L	No	0	A
71	Spokane	25.0 µg/L	No	56	F

FAQ

Frequently asked questions about chlorate in Washington

Is chlorate in Washington tap water dangerous?

Chlorate has no federal MCL. The EPA lifetime health advisory is 210 µg/L. 30 cities in Washington 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 Washington 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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