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National Drinking Water Database


Total trihalomethanes (TTHMs) in California


Total trihalomethanes constitute the sum of four disinfection byproducts: chloroform, bromodichloromethane, dibromochloromethane, and bromoform. [read more]

The Most Polluted Communities in California

936 water utilities reported detecting Total trihalomethanes (TTHMs) in tap water since 2004, according to EWG's analysis of water quality data supplied by state water agencies

Ranked by highest average Total trihalomethanes (TTHMs) level

RankSystem Population Served Positive test results of total reported tests Average Level
(Range)
1Snp-Wolverton
Three Rivers, CA
2,9001 of 1130 ppb
(130 ppb)
2Loma Mar Mutual
Loma Mar, CA
1086 of 6125.44 ppb
(76.12 to 229.25 ppb)
3Csa No. 31 Stonegate Ws
Hollister, CA
2521 of 1120 ppb
(120 ppb)
4LA Jolla Farms
Firebaugh, CA
302 of 2116 ppb
(80 to 152 ppb)
5City of Huron
Huron, CA
5,8601 of 1109 ppb
(109 ppb)
6Riviera West Mutual Water Co.
Kelseyville, CA
50017 of 17107.73 ppb
(39.3 to 185 ppb)
7Acwa Buckhorn Plant
Jackson, CA
9,0001 of 1100 ppb
(100 ppb)
8Bolinas Community Pud
Bolinas, CA
1,20021 of 2199.02 ppb
(53 to 240 ppb)
9Ramona Municipal Wd
Ramona, CA
30,0003 of 395.67 ppb
(89 to 100 ppb)
10Winterhaven Wd
Winterhaven, CA
1,05010 of 1192.85 ppb
(0 to 144 ppb)

Health Based Limits for Total trihalomethanes (TTHMs)

StandardDescriptionLevel
One in one million (10-6) Cancer RiskThe concentration of a chemical in drinking water corresponding to an excess estimated lifetime cancer risk of 1 in 1,000,000. Source: U.S. Environmental Protection Agency.9.8 ppb
EPA Human Health Water Quality CriteriaWater quality criteria set by the US EPA provide guidance for states and tribes authorized to establish water quality standards under the Clean Water Act (CWA) to protect human health. These are non-enforceable standards based upon exposure by both drinking water and the contribution of water contamination to other consumed foods. Source: U.S. Environmental Protection Agency.11.5 ppb
Maximum Contaminant Limit (MCL)The enforceable standard which defines the highest level of a contaminant that is allowed in drinking water. MCLs are set as close to health-based limits (Maximum Contaminant Level Goals, or MCLGs) as feasible using the best available analytical and treatment technologies and taking cost into consideration. Source: U.S. Environmental Protection Agency.80 ppb
Lifetime health-based limit, non-cancer riskConcentration of a chemical in drinking water that is not expected to cause any adverse, noncarcinogenic health effects for a lifetime of exposure. The Lifetime health-based limit (or Health Advisory, HA) is based on exposure for a a 70-kg adult consuming 2 liters of water per day. Source: U.S. Environmental Protection Agency.130 ppb
One in ten thousand (10-4) Cancer RiskThe concentration of a chemical in drinking water corresponding to an excess estimated lifetime cancer risk of 1 in 10,000. Source: U.S. Environmental Protection Agency.980 ppb
Drinking Water Equivalent LevelA lifetime exposure concentration protective of adverse, noncarcinogenic health effects, that assumes all of the exposure to a contaminant is from drinking water. Source: U.S. Environmental Protection Agency.1835 ppb
Children's health-based limit for 10-day exposureConcentration of a chemical in drinking water that is not expected to cause any adverse, noncarcinogenic effects for up to ten days of exposure. The Ten-Day health-based limit (or Health Advisory, HA) is typically set to protect a 10-kg child consuming 1 liter of water per day. Source: U.S. Environmental Protection Agency.4860 ppb
Children's health-based limit for 1-day exposureConcentration of a chemical in drinking water that is not expected to cause any adverse, noncarcinogenic health effects for up to one day of exposure. The One-Day health-based limit (or Health Advisory, HA) is typically set to protect a 10-kg child consuming 1 liter of water per day. Source: U.S. Environmental Protection Agency.10600 ppb

Violation Summary for Total trihalomethanes (TTHMs) in California

Data from the U.S. Environmental Protection Agency includes the following violations of federal standards in California since 2004

Violation TypeNumber of Violations
Over maximum contaminant level, Average172
Monitoring and Reporting Disinfection Byproduct Rule157
Failure to monitor regularly15
Failure to notify state agency3
Failure to notify public of violation1
Over maximum contaminant level, Single Sample1