Chlorate Contamination

Chlorate in California Drinking Water

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

321

Cities Tested

202

Over HA

63%

% Over HA

489.2 µg/L

State Avg

▲ 4% vs national

vs National

624

Health Violations

Analysis

Chlorate in California: what the data shows

California has 321 cities with chlorate data from the EPA's UCMR 3 program (2013–2015). Chlorate exceeded the 210 µg/L EPA lifetime health advisory in 202 of those cities. The state average max detected level is 489.2 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%

202 of 321 cities

→ ▲ 4% above national rate (worse)

National avg

59%

1866 of 3166 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	San Juan Capistrano	880.0 µg/L	Over HA	5	B+
41	Davis	870.0 µg/L	Over HA	47	B+
42	Imperial	870.0 µg/L	Over HA	36	C+
43	Pleasanton	850.0 µg/L	Over HA	3	B-
44	Ontario	810.0 µg/L	Over HA	1	B+
45	Lomita	810.0 µg/L	Over HA	1	A-
46	Galt	800.0 µg/L	Over HA	27	B+
47	Arroyo Grande	800.0 µg/L	Over HA	93	F
48	Manteca	770.0 µg/L	Over HA	39	B-
49	Moorpark	770.0 µg/L	Over HA	1	A-
50	Olivehurst	730.0 µg/L	Over HA	2	A
51	Brawley	730.0 µg/L	Over HA	5	A-
52	Huntington Park	720.0 µg/L	Over HA	3	B+
53	Irvine	710.0 µg/L	Over HA	84	F
54	Chula Vista	700.0 µg/L	Over HA	1	B+
55	Anaheim	691.0 µg/L	Over HA	8	C+
56	Indio	670.0 µg/L	Over HA	12	F
57	Covina	660.0 µg/L	Over HA	10	B
58	Calabasas	660.0 µg/L	Over HA	14	A-
59	Half Moon Bay	660.0 µg/L	Over HA	6	A
60	Avenal	660.0 µg/L	Over HA	35	B
61	Orange	657.0 µg/L	Over HA	3	B
62	Farmersville	640.0 µg/L	Over HA	20	B+
63	Hawthorne	630.0 µg/L	Over HA	3	B
64	Fort Irwin	630.0 µg/L	Over HA	60	B+
65	Downey	620.0 µg/L	Over HA	14	B
66	Temecula	620.0 µg/L	Over HA	12	C+
67	San Jacinto	620.0 µg/L	Over HA	9	B
68	Auburn	610.0 µg/L	Over HA	8	A
69	Lathrop	600.0 µg/L	Over HA	44	B
70	Adelanto	590.0 µg/L	Over HA	13	C
71	Tulare	580.0 µg/L	Over HA	61	C-
72	Reedley	580.0 µg/L	Over HA	12	B-
73	Santa Monica	570.0 µg/L	Over HA	0	B
74	Grover Beach	570.0 µg/L	Over HA	34	F
75	Camp Pendleton	560.0 µg/L	Over HA	13	B
76	Fillmore	560.0 µg/L	Over HA	13	B-
77	Lakewood	540.0 µg/L	Over HA	1	A-
78	Pico Rivera	540.0 µg/L	Over HA	4	B
79	Sonora	530.0 µg/L	Over HA	25	D+
80	Signal Hill	510.0 µg/L	Over HA	6	B
81	Pomona	500.0 µg/L	Over HA	6	C+
82	South Gate	500.0 µg/L	Over HA	2	B
83	Arvin	500.0 µg/L	Over HA	81	C+
84	Pacific Grove	490.0 µg/L	Over HA	15	B-
85	Oakland	480.0 µg/L	Over HA	13	B
86	Fremont	480.0 µg/L	Over HA	3	B-
87	Exeter	480.0 µg/L	Over HA	41	B
88	Lake Forest	470.0 µg/L	Over HA	38	B-
89	Lakeside	470.0 µg/L	Over HA	0	A-
90	Pala	470.0 µg/L	Over HA	30	C-
91	El Monte	460.0 µg/L	Over HA	79	F
92	Paramount	460.0 µg/L	Over HA	0	A-
93	Boulder Creek	460.0 µg/L	Over HA	119	F
94	Banning	450.0 µg/L	Over HA	15	B-
95	Stockton	440.0 µg/L	Over HA	261	F
96	South Lake Tahoe	440.0 µg/L	Over HA	15	C
97	Yucaipa	440.0 µg/L	Over HA	9	C+
98	Twentynine Palms	440.0 µg/L	Over HA	70	B+
99	Spring Valley	430.0 µg/L	Over HA	4	B
100	Corte Madera	420.0 µg/L	Over HA	0	A-
101	Point Mugu	420.0 µg/L	Over HA	11	A-
102	California City	420.0 µg/L	Over HA	22	A-
103	Brooks	420.0 µg/L	Over HA	2	B+
104	Scotts Valley	420.0 µg/L	Over HA	11	B-
105	Santa Barbara	410.0 µg/L	Over HA	52	F
106	Atascadero	410.0 µg/L	Over HA	1	B
107	Carpinteria	410.0 µg/L	Over HA	5	A
108	Nuevo	410.0 µg/L	Over HA	6	B+
109	Mendota	408.0 µg/L	Over HA	3	A
110	Rosemead	400.0 µg/L	Over HA	17	C-
111	Rancho Cucamonga	400.0 µg/L	Over HA	0	B+
112	Rancho Cordova	400.0 µg/L	Over HA	115	F
113	Napa	400.0 µg/L	Over HA	53	F
114	Santa Paula	400.0 µg/L	Over HA	5	A-
115	Patterson	400.0 µg/L	Over HA	35	B+
116	Rancho Santa Fe	400.0 µg/L	Over HA	3	A-
117	Rialto	380.0 µg/L	Over HA	6	B+
118	Chino	380.0 µg/L	Over HA	9	B
119	Palm Springs	380.0 µg/L	Over HA	16	B
120	Woodland	380.0 µg/L	Over HA	52	F
121	Glendora	380.0 µg/L	Over HA	0	A-
122	Kerman	380.0 µg/L	Over HA	13	B
123	Vacaville	370.0 µg/L	Over HA	26	F
124	Brownsville	360.8 µg/L	Over HA	0	A-
125	Corona	360.0 µg/L	Over HA	81	D
126	Camarillo	360.0 µg/L	Over HA	4	B
127	Desert Hot Springs	360.0 µg/L	Over HA	4	A
128	Discovery Bay	360.0 µg/L	Over HA	3	A+
129	Martinez	350.0 µg/L	Over HA	0	A
130	Sanger	350.0 µg/L	Over HA	16	C
131	Windsor	350.0 µg/L	Over HA	2	A+
132	Bermuda Dunes	348.0 µg/L	Over HA	0	A+
133	Oceanside	340.0 µg/L	Over HA	2	A-
134	Encinitas	340.0 µg/L	Over HA	55	F
135	Lompoc	340.0 µg/L	Over HA	18	C
136	Carmichael	340.0 µg/L	Over HA	1	B+
137	Manhattan Beach	340.0 µg/L	Over HA	1	A-
138	Corcoran	330.0 µg/L	Over HA	3	A
139	Costa Mesa	328.0 µg/L	Over HA	2	A-
140	Los Osos	327.5 µg/L	Over HA	0	B+
141	Santa Cruz	320.0 µg/L	Over HA	38	F
142	Ramona	320.0 µg/L	Over HA	36	C
143	Edwards Afb	320.0 µg/L	Over HA	8	A-
144	Anderson	315.0 µg/L	Over HA	14	C-
145	Laguna Hills	310.0 µg/L	Over HA	1	B+
146	Palmdale	310.0 µg/L	Over HA	15	C+
147	Ceres	310.0 µg/L	Over HA	129	F
148	Los Banos	310.0 µg/L	Over HA	29	B
149	Elk Grove	310.0 µg/L	Over HA	28	C
150	Vernon	310.0 µg/L	Over HA	3	A
151	Lindsay	310.0 µg/L	Over HA	240	F
152	Los Angeles	307.0 µg/L	Over HA	94	F
153	Redding	305.0 µg/L	Over HA	49	F
154	Alhambra	300.0 µg/L	Over HA	7	A-
155	Glendale	300.0 µg/L	Over HA	0	A-
156	Placerville	300.0 µg/L	Over HA	6	A-
157	Victorville	300.0 µg/L	Over HA	1	B+
158	Santa Clara	300.0 µg/L	Over HA	5	B
159	Lodi	300.0 µg/L	Over HA	16	C+
160	San Clemente	300.0 µg/L	Over HA	3	A-
161	Livermore	300.0 µg/L	Over HA	53	B
162	Norco	300.0 µg/L	Over HA	39	F
163	El Verano	300.0 µg/L	Over HA	0	B+
164	Kirkwood	300.0 µg/L	Over HA	0	A
165	Lynwood	290.0 µg/L	Over HA	0	B+
166	Fallbrook	290.0 µg/L	Over HA	3	A
167	Arcadia	290.0 µg/L	Over HA	2	B
168	Colton	290.0 µg/L	Over HA	5	B-
169	Ukiah	290.0 µg/L	Over HA	14	C+
170	Oak View	290.0 µg/L	Over HA	12	B+
171	Trabuco Canyon	290.0 µg/L	Over HA	2	A-
172	Newport Beach	288.0 µg/L	Over HA	4	B+
173	Placentia	282.0 µg/L	Over HA	0	B+
174	Hemet	280.0 µg/L	Over HA	31	B-
175	Lincoln	280.0 µg/L	Over HA	6	A-
176	Beverly Hills	280.0 µg/L	Over HA	18	B
177	Burlingame	270.0 µg/L	Over HA	14	A-
178	Fontana	270.0 µg/L	Over HA	0	A-
179	Milpitas	260.0 µg/L	Over HA	2	A-
180	Palo Alto	260.0 µg/L	Over HA	8	A-
181	Escondido	257.0 µg/L	Over HA	10	B
182	Pasadena	250.0 µg/L	Over HA	92	F
183	Sunnyvale	250.0 µg/L	Over HA	3	A
184	Roseville	250.0 µg/L	Over HA	11	B
185	Folsom	250.0 µg/L	Over HA	1	A-
186	Laguna Beach	250.0 µg/L	Over HA	2	B
187	Santa Ana	249.0 µg/L	Over HA	8	C
188	Vista	240.0 µg/L	Over HA	0	A-
189	Vallejo	240.0 µg/L	Over HA	6	A
190	Merced	240.0 µg/L	Over HA	9	C+
191	Novato	240.0 µg/L	Over HA	3	B+
192	Fair Oaks	240.0 µg/L	Over HA	0	B+
193	Port Hueneme	240.0 µg/L	Over HA	0	B+
194	Paradise	240.0 µg/L	Over HA	2	A
195	Daly City	230.0 µg/L	Over HA	4	A
196	Redlands	230.0 µg/L	Over HA	2	B
197	Cutten	230.0 µg/L	Over HA	5	A+
198	Carlsbad	220.0 µg/L	Over HA	0	A+
199	Compton	220.0 µg/L	Over HA	8	B
200	Quartz Hill	220.0 µg/L	Over HA	1	A-
201	Rosamond	220.0 µg/L	Over HA	205	F
202	Mammoth Lakes	220.0 µg/L	Over HA	24	B
203	Mountain View	210.0 µg/L	No	3	A
204	Stanford	210.0 µg/L	No	3	A+
205	Menlo Park	210.0 µg/L	No	2	A-
206	San Diego	200.0 µg/L	No	43	F
207	San Marcos	200.0 µg/L	No	27	D
208	Montebello	200.0 µg/L	No	5	B
209	Morgan Hill	200.0 µg/L	No	92	F
210	Bellflower	200.0 µg/L	No	0	B+
211	San Bruno	200.0 µg/L	No	0	A+
212	Belmont	200.0 µg/L	No	3	A+
213	Millbrae	200.0 µg/L	No	1	A-
214	Santa Rosa	190.0 µg/L	No	177	F
215	Santee	190.0 µg/L	No	14	B
216	Wasco	190.0 µg/L	No	26	B+
217	Cudahy	190.0 µg/L	No	3	B+
218	Ripon	190.0 µg/L	No	10	A-
219	Garden Grove	186.0 µg/L	No	2	B
220	Redwood City	180.0 µg/L	No	10	B+
221	Porterville	180.0 µg/L	No	74	F
222	Cupertino	180.0 µg/L	No	3	A+
223	Healdsburg	180.0 µg/L	No	12	B+
224	La Palma	172.0 µg/L	No	0	A-
225	San Bernardin0	170.0 µg/L	No	0	A-
226	Jurupa Valley	170.0 µg/L	No	62	B
227	San Bernardino	170.0 µg/L	No	192	F
228	Imperial Beach	170.0 µg/L	No	1	A
229	Simi Valley	170.0 µg/L	No	3	A
230	Pacifica	170.0 µg/L	No	2	A
231	Monrovia	170.0 µg/L	No	0	B+
232	Riverbank	170.0 µg/L	No	75	D
233	Taft	170.0 µg/L	No	0	A-
234	Greenfield	170.0 µg/L	No	17	B
235	Livingston	170.0 µg/L	No	29	C-
236	Apple Valley	160.0 µg/L	No	57	D
237	Foster City	160.0 µg/L	No	2	A+
238	Norwalk	160.0 µg/L	No	1	A-
239	Hillsborough	160.0 µg/L	No	7	A
240	Montara	160.0 µg/L	No	3	A-
241	Oxnard	150.0 µg/L	No	49	F
242	La Habra	150.0 µg/L	No	0	A-
243	Bell Gardens	150.0 µg/L	No	0	B
244	Buena Park	140.0 µg/L	No	1	B+
245	Rowland Heights	140.0 µg/L	No	0	B+
246	Susanville	140.0 µg/L	No	34	A-
247	Westminster	139.0 µg/L	No	2	A-
248	Tustin	132.0 µg/L	No	8	B
249	Riverside	130.0 µg/L	No	13	C
250	Hayward	130.0 µg/L	No	0	A-
251	Fullerton	130.0 µg/L	No	8	B
252	Azusa	130.0 µg/L	No	2	B+
253	Burbank	130.0 µg/L	No	1	A-
254	Upland	130.0 µg/L	No	4	B+
255	East Palo Alto	130.0 µg/L	No	8	A
256	Altadena	130.0 µg/L	No	2	B+
257	Sierra Madre	130.0 µg/L	No	5	A
258	Highland	120.0 µg/L	No	3	B+
259	Santa Fe Springs	120.0 µg/L	No	1	B+
260	Rohnert Park	120.0 µg/L	No	21	F
261	Atwater	120.0 µg/L	No	3	B+
262	Arcata	120.0 µg/L	No	3	A
263	American Canyon	120.0 µg/L	No	15	A-
264	Long Beach	110.0 µg/L	No	2	C
265	Santa Maria	110.0 µg/L	No	36	F
266	Inglewood	110.0 µg/L	No	3	A-
267	Beaumont	110.0 µg/L	No	2	A-
268	Montclair	110.0 µg/L	No	0	B+
269	Oakley	110.0 µg/L	No	60	D
270	South Pasadena	110.0 µg/L	No	5	A
271	Thousand Oaks	100.0 µg/L	No	1	A
272	Yucca Valley	100.0 µg/L	No	7	A-
273	Crescent City	100.0 µg/L	No	14	C
274	Nipomo	100.0 µg/L	No	1	A
275	Gonzales	100.0 µg/L	No	53	F
276	Huntington Beach	99.9 µg/L	No	5	C-
277	Gilroy	99.0 µg/L	No	28	C+
278	Monterey Park	98.0 µg/L	No	1	B
279	La Verne	98.0 µg/L	No	6	B+
280	Newman	97.0 µg/L	No	3	A-
281	Hamilton City	97.0 µg/L	No	0	A+
282	Phelan	95.0 µg/L	No	0	B+
283	Fountain Valley	94.6 µg/L	No	0	A
284	Baldwin Park	90.0 µg/L	No	1	A
285	Concord	88.0 µg/L	No	19	B-
286	Coachella	86.0 µg/L	No	11	C-
287	Madera	85.0 µg/L	No	564	D
288	Westlake Village	84.0 µg/L	No	8	F
289	Whittier	81.0 µg/L	No	4	C+
290	Brea	80.0 µg/L	No	0	A
291	Valley Center	80.0 µg/L	No	15	F
292	Soledad	78.0 µg/L	No	28	C+
293	Walnut	74.0 µg/L	No	1	A-
294	El Segundo	74.0 µg/L	No	0	A-
295	Coalinga	68.0 µg/L	No	267	D
296	Fairfield	67.0 µg/L	No	7	B-
297	Grand Terrace	67.0 µg/L	No	1	B
298	Cerritos	66.0 µg/L	No	0	B
299	Chino Hills	65.0 µg/L	No	2	A
300	Truckee	65.0 µg/L	No	7	A-
301	Citrus Heights	64.0 µg/L	No	2	A
302	Morro Bay	64.0 µg/L	No	12	A-
303	Kingsburg	64.0 µg/L	No	17	B
304	Chowchilla	58.0 µg/L	No	25	A-
305	Red Bluff	54.0 µg/L	No	23	F
306	Palm Desert	52.0 µg/L	No	12	C+
307	Carmel Valley	45.2 µg/L	No	45	C+
308	Antioch	44.0 µg/L	No	3	A
309	Sonoma	43.0 µg/L	No	5	B-
310	Temescal Valley	39.0 µg/L	No	2	A
311	La Mesa	36.0 µg/L	No	0	A+
312	Dinuba	34.0 µg/L	No	24	A-
313	Chico	32.0 µg/L	No	306	F
314	Poway	30.0 µg/L	No	0	A-
315	Loma Linda	30.0 µg/L	No	3	A
316	Petaluma	27.0 µg/L	No	2	B
317	Eureka	25.0 µg/L	No	3	A-
318	Oakdale	25.0 µg/L	No	34	C-
319	El Centro	24.0 µg/L	No	14	C-
320	Benicia	22.0 µg/L	No	1	A-
321	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. 202 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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