Chlorate Contamination

Chlorate in California Drinking Water

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

319

Cities Tested

201

Over HA

63%

% Over HA

488.9 µg/L

State Avg

▲ 5% vs national

vs National

620

Health Violations

Analysis

Chlorate in California: what the data shows

California has 319 cities with chlorate data from the EPA's UCMR 3 program (2013–2015). Chlorate exceeded the 210 µg/L EPA lifetime health advisory in 201 of those cities. The state average max detected level is 488.9 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)

California

63%

201 of 319 cities

→ ▲ 5% above national rate (worse)

National avg

58%

1812 of 3098 cities

City Map

Chlorate data across California

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 California

Highest Chlorate levels (µg/L)

All Cities

All California cities ranked by chlorate level

#	City	Level	Over HA?	Violations	Grade
1	Mcfarland	12000.0 µg/L	Over HA	42	C+
2	Salinas	6900.0 µg/L	Over HA	639	F
3	Lemoore	3100.0 µg/L	Over HA	112	D
4	Paso Robles	3100.0 µg/L	Over HA	42	F
5	Sacramento	2700.0 µg/L	Over HA	425	F
6	Lake Elsinore	2500.0 µg/L	Over HA	8	A
7	Ridgecrest	2500.0 µg/L	Over HA	26	A-
8	Clovis	2400.0 µg/L	Over HA	221	F
9	San Jose	2200.0 µg/L	Over HA	142	F
10	Grass Valley	1900.0 µg/L	Over HA	8	A
11	Perris	1800.0 µg/L	Over HA	0	A
12	Fresno	1800.0 µg/L	Over HA	523	F
13	Goleta	1600.0 µg/L	Over HA	11	B
14	Rancho Santa Margarita	1500.0 µg/L	Over HA	0	A
15	Shafter	1500.0 µg/L	Over HA	25	A-
16	Firebaugh	1500.0 µg/L	Over HA	89	D
17	Soquel	1400.0 µg/L	Over HA	1	A
18	Marina	1400.0 µg/L	Over HA	4	A+
19	Blythe	1400.0 µg/L	Over HA	143	D
20	Clearlake	1300.0 µg/L	Over HA	5	B+
21	Tracy	1200.0 µg/L	Over HA	45	C
22	San Luis Obispo	1200.0 µg/L	Over HA	27	F
23	Delano	1200.0 µg/L	Over HA	67	B
24	Brentwood	1100.0 µg/L	Over HA	91	F
25	San Fernando	1100.0 µg/L	Over HA	1	A+
26	Rio Linda	1100.0 µg/L	Over HA	2	A-
27	Seal Beach	1020.0 µg/L	Over HA	0	A-
28	San Dimas	1000.0 µg/L	Over HA	6	B-
29	Lamont	1000.0 µg/L	Over HA	40	B+
30	Parlier	1000.0 µg/L	Over HA	14	A-
31	San Andreas	990.0 µg/L	Over HA	17	B-
32	Hollister	980.0 µg/L	Over HA	132	F
33	Modesto	960.0 µg/L	Over HA	76	D
34	La Crescenta	960.0 µg/L	Over HA	3	B+
35	Newhall	940.0 µg/L	Over HA	1	B+
36	Bakersfield	920.0 µg/L	Over HA	690	F
37	Torrance	920.0 µg/L	Over HA	0	A-
38	Ventura	910.0 µg/L	Over HA	21	D+
39	29 Palms	890.0 µg/L	Over HA	3	B
40	Davis	870.0 µg/L	Over HA	47	B+
41	Imperial	870.0 µg/L	Over HA	36	C+
42	Pleasanton	850.0 µg/L	Over HA	3	B-
43	Ontario	810.0 µg/L	Over HA	1	B+
44	Lomita	810.0 µg/L	Over HA	1	A-
45	Galt	800.0 µg/L	Over HA	27	B+
46	Arroyo Grande	800.0 µg/L	Over HA	93	F
47	Manteca	770.0 µg/L	Over HA	39	B-
48	Moorpark	770.0 µg/L	Over HA	1	A-
49	Olivehurst	730.0 µg/L	Over HA	2	A
50	Brawley	730.0 µg/L	Over HA	5	A-
51	Huntington Park	720.0 µg/L	Over HA	3	B+
52	Irvine	710.0 µg/L	Over HA	84	F
53	Chula Vista	700.0 µg/L	Over HA	1	B+
54	Anaheim	691.0 µg/L	Over HA	8	C+
55	Indio	670.0 µg/L	Over HA	12	F
56	Covina	660.0 µg/L	Over HA	10	B
57	Calabasas	660.0 µg/L	Over HA	14	A-
58	Half Moon Bay	660.0 µg/L	Over HA	6	A
59	Avenal	660.0 µg/L	Over HA	35	B
60	Orange	657.0 µg/L	Over HA	3	B
61	Farmersville	640.0 µg/L	Over HA	20	B+
62	Hawthorne	630.0 µg/L	Over HA	3	B
63	Fort Irwin	630.0 µg/L	Over HA	60	B+
64	Downey	620.0 µg/L	Over HA	14	B
65	Temecula	620.0 µg/L	Over HA	12	C+
66	San Jacinto	620.0 µg/L	Over HA	9	B
67	Auburn	610.0 µg/L	Over HA	8	A
68	Lathrop	600.0 µg/L	Over HA	44	B
69	Adelanto	590.0 µg/L	Over HA	13	C
70	Tulare	580.0 µg/L	Over HA	61	C-
71	Reedley	580.0 µg/L	Over HA	12	B-
72	Santa Monica	570.0 µg/L	Over HA	0	B
73	Grover Beach	570.0 µg/L	Over HA	34	F
74	Camp Pendleton	560.0 µg/L	Over HA	13	B
75	Fillmore	560.0 µg/L	Over HA	13	B-
76	Lakewood	540.0 µg/L	Over HA	1	A-
77	Pico Rivera	540.0 µg/L	Over HA	4	B
78	Sonora	530.0 µg/L	Over HA	25	D+
79	Signal Hill	510.0 µg/L	Over HA	6	B
80	Pomona	500.0 µg/L	Over HA	6	C+
81	South Gate	500.0 µg/L	Over HA	2	B
82	Arvin	500.0 µg/L	Over HA	81	C+
83	Pacific Grove	490.0 µg/L	Over HA	15	B-
84	Oakland	480.0 µg/L	Over HA	13	B
85	Fremont	480.0 µg/L	Over HA	3	B-
86	Exeter	480.0 µg/L	Over HA	41	B
87	Lake Forest	470.0 µg/L	Over HA	38	B-
88	Lakeside	470.0 µg/L	Over HA	0	A-
89	Pala	470.0 µg/L	Over HA	30	C-
90	El Monte	460.0 µg/L	Over HA	79	F
91	Paramount	460.0 µg/L	Over HA	0	A-
92	Boulder Creek	460.0 µg/L	Over HA	119	F
93	Banning	450.0 µg/L	Over HA	15	B-
94	Stockton	440.0 µg/L	Over HA	261	F
95	South Lake Tahoe	440.0 µg/L	Over HA	15	C
96	Yucaipa	440.0 µg/L	Over HA	9	C+
97	Twentynine Palms	440.0 µg/L	Over HA	70	B+
98	Spring Valley	430.0 µg/L	Over HA	4	B
99	Corte Madera	420.0 µg/L	Over HA	0	A-
100	Point Mugu	420.0 µg/L	Over HA	11	A-
101	California City	420.0 µg/L	Over HA	22	A-
102	Brooks	420.0 µg/L	Over HA	2	B+
103	Scotts Valley	420.0 µg/L	Over HA	11	B-
104	Santa Barbara	410.0 µg/L	Over HA	52	F
105	Atascadero	410.0 µg/L	Over HA	1	B
106	Carpinteria	410.0 µg/L	Over HA	5	A
107	Nuevo	410.0 µg/L	Over HA	6	B+
108	Mendota	408.0 µg/L	Over HA	3	A
109	Rosemead	400.0 µg/L	Over HA	17	C-
110	Rancho Cucamonga	400.0 µg/L	Over HA	0	B+
111	Rancho Cordova	400.0 µg/L	Over HA	115	F
112	Napa	400.0 µg/L	Over HA	53	F
113	Santa Paula	400.0 µg/L	Over HA	5	A-
114	Patterson	400.0 µg/L	Over HA	35	B+
115	Rancho Santa Fe	400.0 µg/L	Over HA	3	A-
116	Rialto	380.0 µg/L	Over HA	6	B+
117	Chino	380.0 µg/L	Over HA	9	B
118	Palm Springs	380.0 µg/L	Over HA	16	B
119	Woodland	380.0 µg/L	Over HA	52	F
120	Glendora	380.0 µg/L	Over HA	0	A-
121	Kerman	380.0 µg/L	Over HA	13	B
122	Vacaville	370.0 µg/L	Over HA	26	F
123	Brownsville	360.8 µg/L	Over HA	0	A-
124	Corona	360.0 µg/L	Over HA	81	D
125	Camarillo	360.0 µg/L	Over HA	4	B
126	Desert Hot Springs	360.0 µg/L	Over HA	4	A
127	Discovery Bay	360.0 µg/L	Over HA	3	A+
128	Martinez	350.0 µg/L	Over HA	0	A
129	Sanger	350.0 µg/L	Over HA	16	C
130	Windsor	350.0 µg/L	Over HA	2	A+
131	Bermuda Dunes	348.0 µg/L	Over HA	0	A+
132	Oceanside	340.0 µg/L	Over HA	2	A-
133	Encinitas	340.0 µg/L	Over HA	55	F
134	Lompoc	340.0 µg/L	Over HA	18	C
135	Carmichael	340.0 µg/L	Over HA	1	B+
136	Manhattan Beach	340.0 µg/L	Over HA	1	A-
137	Corcoran	330.0 µg/L	Over HA	3	A
138	Costa Mesa	328.0 µg/L	Over HA	2	A-
139	Los Osos	327.5 µg/L	Over HA	0	B+
140	Santa Cruz	320.0 µg/L	Over HA	38	F
141	Ramona	320.0 µg/L	Over HA	36	C
142	Edwards Afb	320.0 µg/L	Over HA	8	A-
143	Anderson	315.0 µg/L	Over HA	14	C-
144	Laguna Hills	310.0 µg/L	Over HA	1	B+
145	Palmdale	310.0 µg/L	Over HA	15	C+
146	Ceres	310.0 µg/L	Over HA	129	F
147	Los Banos	310.0 µg/L	Over HA	29	B
148	Elk Grove	310.0 µg/L	Over HA	28	C
149	Vernon	310.0 µg/L	Over HA	3	A
150	Lindsay	310.0 µg/L	Over HA	240	F
151	Los Angeles	307.0 µg/L	Over HA	94	F
152	Redding	305.0 µg/L	Over HA	49	F
153	Alhambra	300.0 µg/L	Over HA	7	A-
154	Glendale	300.0 µg/L	Over HA	0	A-
155	Placerville	300.0 µg/L	Over HA	6	A-
156	Victorville	300.0 µg/L	Over HA	1	B+
157	Santa Clara	300.0 µg/L	Over HA	5	B
158	Lodi	300.0 µg/L	Over HA	16	C+
159	San Clemente	300.0 µg/L	Over HA	3	A-
160	Livermore	300.0 µg/L	Over HA	53	B
161	Norco	300.0 µg/L	Over HA	39	F
162	El Verano	300.0 µg/L	Over HA	0	B+
163	Kirkwood	300.0 µg/L	Over HA	0	A
164	Lynwood	290.0 µg/L	Over HA	0	B+
165	Fallbrook	290.0 µg/L	Over HA	3	A
166	Arcadia	290.0 µg/L	Over HA	2	B
167	Colton	290.0 µg/L	Over HA	5	B-
168	Ukiah	290.0 µg/L	Over HA	14	C+
169	Oak View	290.0 µg/L	Over HA	12	B+
170	Trabuco Canyon	290.0 µg/L	Over HA	2	A-
171	Newport Beach	288.0 µg/L	Over HA	4	B+
172	Placentia	282.0 µg/L	Over HA	0	B+
173	Hemet	280.0 µg/L	Over HA	31	B-
174	Lincoln	280.0 µg/L	Over HA	6	A-
175	Beverly Hills	280.0 µg/L	Over HA	18	B
176	Burlingame	270.0 µg/L	Over HA	14	A-
177	Fontana	270.0 µg/L	Over HA	0	A-
178	Milpitas	260.0 µg/L	Over HA	2	A-
179	Palo Alto	260.0 µg/L	Over HA	8	A-
180	Escondido	257.0 µg/L	Over HA	10	B
181	Pasadena	250.0 µg/L	Over HA	92	F
182	Sunnyvale	250.0 µg/L	Over HA	3	A
183	Roseville	250.0 µg/L	Over HA	11	B
184	Folsom	250.0 µg/L	Over HA	1	A-
185	Laguna Beach	250.0 µg/L	Over HA	2	B
186	Santa Ana	249.0 µg/L	Over HA	8	C
187	Vista	240.0 µg/L	Over HA	0	A-
188	Vallejo	240.0 µg/L	Over HA	6	A
189	Merced	240.0 µg/L	Over HA	9	C+
190	Novato	240.0 µg/L	Over HA	3	B+
191	Fair Oaks	240.0 µg/L	Over HA	0	B+
192	Port Hueneme	240.0 µg/L	Over HA	0	B+
193	Paradise	240.0 µg/L	Over HA	2	A
194	Daly City	230.0 µg/L	Over HA	4	A
195	Redlands	230.0 µg/L	Over HA	2	B
196	Cutten	230.0 µg/L	Over HA	5	A+
197	Carlsbad	220.0 µg/L	Over HA	0	A+
198	Compton	220.0 µg/L	Over HA	8	B
199	Quartz Hill	220.0 µg/L	Over HA	1	A-
200	Rosamond	220.0 µg/L	Over HA	205	F
201	Mammoth Lakes	220.0 µg/L	Over HA	24	B
202	Mountain View	210.0 µg/L	No	3	A
203	Stanford	210.0 µg/L	No	3	A+
204	Menlo Park	210.0 µg/L	No	2	A-
205	San Diego	200.0 µg/L	No	43	F
206	San Marcos	200.0 µg/L	No	27	D
207	Bellflower	200.0 µg/L	No	0	B+
208	Montebello	200.0 µg/L	No	5	B
209	San Bruno	200.0 µg/L	No	0	A+
210	Belmont	200.0 µg/L	No	3	A+
211	Millbrae	200.0 µg/L	No	1	A-
212	Santa Rosa	190.0 µg/L	No	177	F
213	Santee	190.0 µg/L	No	14	B
214	Wasco	190.0 µg/L	No	26	B+
215	Cudahy	190.0 µg/L	No	3	B+
216	Ripon	190.0 µg/L	No	10	A-
217	Garden Grove	186.0 µg/L	No	2	B
218	Redwood City	180.0 µg/L	No	10	B+
219	Porterville	180.0 µg/L	No	74	F
220	Cupertino	180.0 µg/L	No	3	A+
221	Healdsburg	180.0 µg/L	No	12	B+
222	La Palma	172.0 µg/L	No	0	A-
223	San Bernardin0	170.0 µg/L	No	0	A-
224	Jurupa Valley	170.0 µg/L	No	62	B
225	San Bernardino	170.0 µg/L	No	192	F
226	Imperial Beach	170.0 µg/L	No	1	A
227	Simi Valley	170.0 µg/L	No	3	A
228	Pacifica	170.0 µg/L	No	2	A
229	Monrovia	170.0 µg/L	No	0	B+
230	Riverbank	170.0 µg/L	No	75	D
231	Taft	170.0 µg/L	No	0	A-
232	Greenfield	170.0 µg/L	No	17	B
233	Livingston	170.0 µg/L	No	29	C-
234	Apple Valley	160.0 µg/L	No	57	D
235	Foster City	160.0 µg/L	No	2	A+
236	Norwalk	160.0 µg/L	No	1	A-
237	Hillsborough	160.0 µg/L	No	7	A
238	Montara	160.0 µg/L	No	3	A-
239	Oxnard	150.0 µg/L	No	49	F
240	La Habra	150.0 µg/L	No	0	A-
241	Bell Gardens	150.0 µg/L	No	0	B
242	Buena Park	140.0 µg/L	No	1	B+
243	Rowland Heights	140.0 µg/L	No	0	B+
244	Susanville	140.0 µg/L	No	34	A-
245	Westminster	139.0 µg/L	No	2	A-
246	Tustin	132.0 µg/L	No	8	B
247	Riverside	130.0 µg/L	No	13	C
248	Hayward	130.0 µg/L	No	0	A-
249	Fullerton	130.0 µg/L	No	8	B
250	Azusa	130.0 µg/L	No	2	B+
251	Burbank	130.0 µg/L	No	1	A-
252	Upland	130.0 µg/L	No	4	B+
253	East Palo Alto	130.0 µg/L	No	8	A
254	Altadena	130.0 µg/L	No	2	B+
255	Sierra Madre	130.0 µg/L	No	5	A
256	Highland	120.0 µg/L	No	3	B+
257	Rohnert Park	120.0 µg/L	No	21	F
258	Atwater	120.0 µg/L	No	3	B+
259	Arcata	120.0 µg/L	No	3	A
260	American Canyon	120.0 µg/L	No	15	A-
261	Santa Fe Springs	120.0 µg/L	No	1	B+
262	Long Beach	110.0 µg/L	No	2	C
263	Santa Maria	110.0 µg/L	No	36	F
264	Inglewood	110.0 µg/L	No	3	A-
265	Beaumont	110.0 µg/L	No	2	A-
266	Montclair	110.0 µg/L	No	0	B+
267	Oakley	110.0 µg/L	No	60	D
268	South Pasadena	110.0 µg/L	No	5	A
269	Thousand Oaks	100.0 µg/L	No	1	A
270	Yucca Valley	100.0 µg/L	No	7	A-
271	Crescent City	100.0 µg/L	No	14	C
272	Nipomo	100.0 µg/L	No	1	A
273	Gonzales	100.0 µg/L	No	53	F
274	Huntington Beach	99.9 µg/L	No	5	C-
275	Gilroy	99.0 µg/L	No	28	C+
276	Monterey Park	98.0 µg/L	No	1	B
277	La Verne	98.0 µg/L	No	6	B+
278	Newman	97.0 µg/L	No	3	A-
279	Hamilton City	97.0 µg/L	No	0	A+
280	Phelan	95.0 µg/L	No	0	B+
281	Fountain Valley	94.6 µg/L	No	0	A
282	Baldwin Park	90.0 µg/L	No	1	A
283	Concord	88.0 µg/L	No	19	B-
284	Coachella	86.0 µg/L	No	11	C-
285	Madera	85.0 µg/L	No	564	D
286	Westlake Village	84.0 µg/L	No	8	F
287	Whittier	81.0 µg/L	No	4	C+
288	Brea	80.0 µg/L	No	0	A
289	Valley Center	80.0 µg/L	No	15	F
290	Soledad	78.0 µg/L	No	28	C+
291	Walnut	74.0 µg/L	No	1	A-
292	El Segundo	74.0 µg/L	No	0	A-
293	Coalinga	68.0 µg/L	No	267	D
294	Fairfield	67.0 µg/L	No	7	B-
295	Grand Terrace	67.0 µg/L	No	1	B
296	Cerritos	66.0 µg/L	No	0	B
297	Chino Hills	65.0 µg/L	No	2	A
298	Truckee	65.0 µg/L	No	7	A-
299	Citrus Heights	64.0 µg/L	No	2	A
300	Morro Bay	64.0 µg/L	No	12	A-
301	Kingsburg	64.0 µg/L	No	17	B
302	Chowchilla	58.0 µg/L	No	25	A-
303	Red Bluff	54.0 µg/L	No	23	F
304	Palm Desert	52.0 µg/L	No	12	C+
305	Carmel Valley	45.2 µg/L	No	45	C+
306	Antioch	44.0 µg/L	No	3	A
307	Sonoma	43.0 µg/L	No	5	B-
308	Temescal Valley	39.0 µg/L	No	2	A
309	La Mesa	36.0 µg/L	No	0	A+
310	Dinuba	34.0 µg/L	No	24	A-
311	Chico	32.0 µg/L	No	306	F
312	Poway	30.0 µg/L	No	0	A-
313	Loma Linda	30.0 µg/L	No	3	A
314	Petaluma	27.0 µg/L	No	2	B
315	Eureka	25.0 µg/L	No	3	A-
316	Oakdale	25.0 µg/L	No	34	C-
317	El Centro	24.0 µg/L	No	14	C-
318	Benicia	22.0 µg/L	No	1	A-
319	Hesperia	21.0 µg/L	No	2	B

FAQ

Frequently asked questions about chlorate in California

Is chlorate in California tap water dangerous?

Chlorate has no federal MCL. The EPA lifetime health advisory is 210 µg/L. 201 cities in California 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 California 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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