Contaminants and rules in California water systems
92 different rules and contaminants appear in the past three years of violation records for California's 7,384 active public water systems. The tables show how many systems each one affected and how many of those violations are still unresolved. Most entries are monitoring, reporting, or notification lapses rather than confirmed contamination.
Microbial
Bacteria and the rules that test for them. Health-based when a sample is positive.
| Rule or contaminant | Code | California systems with violations, 3 yr | Share | Unresolved |
|---|---|---|---|---|
| Revised Total Coliform Rule (RTCR) | 8000 | 774 | 10% | 100 |
| E. coli (Escherichia coli) | 3014 | 22 | 0% | 21 |
Inorganic Chemical
Naturally occurring or from fertilizer and industry. Each has a maximum contaminant level.
| Rule or contaminant | Code | California systems with violations, 3 yr | Share | Unresolved |
|---|---|---|---|---|
| Nitrate | 1040 | 526 | 7% | 41 |
| Arsenic | 1005 | 114 | 2% | 24 |
| Nitrite | 1041 | 82 | 1% | 0 |
| Selenium | 1045 | 29 | 0% | 3 |
| Fluoride | 1025 | 28 | 0% | 0 |
| Mercury | 1035 | 25 | 0% | 0 |
| Antimony | 1074 | 24 | 0% | 0 |
| Barium | 1010 | 23 | 0% | 0 |
| Cadmium | 1015 | 23 | 0% | 0 |
| Chromium | 1020 | 23 | 0% | 0 |
| Beryllium | 1075 | 23 | 0% | 0 |
| Thallium | 1085 | 23 | 0% | 0 |
| Nickel | 1036 | 19 | 0% | 0 |
| Cyanide | 1024 | 6 | 0% | 0 |
| Asbestos | 1094 | 5 | 0% | 0 |
| Lead | 1030 | 1 | 0% | 1 |
| Nitrate-Nitrite (Combined) | 1038 | 1 | 0% | 0 |
Disinfection Byproduct
Formed when chlorine reacts with organic matter in the water.
| Rule or contaminant | Code | California systems with violations, 3 yr | Share | Unresolved |
|---|---|---|---|---|
| Total Haloacetic Acids (HAA5) | 2456 | 122 | 2% | 6 |
| TTHM (Total Trihalomethanes) | 2950 | 120 | 2% | 4 |
| Stage 1 Disinfectants and DBP Rule | 0400 | 5 | 0% | 5 |
Treatment Technique
Rules that require a process, such as corrosion control or disinfection, rather than a measured limit.
| Rule or contaminant | Code | California systems with violations, 3 yr | Share | Unresolved |
|---|---|---|---|---|
| Lead and Copper Rule Revisions (LCRR) | 5200 | 844 | 11% | 844 |
| Lead and Copper Rule (LCR) | 5000 | 405 | 5% | 404 |
| Chlorine (MRDL) | 0999 | 57 | 1% | 12 |
| Surface Water Treatment Rule (SWTR) | 0200 | 56 | 1% | 28 |
| Groundwater Rule (GWR) | 0700 | 35 | 0% | 33 |
| Interim Enhanced Surface Water Treatment Rule | 0300 | 15 | 0% | 1 |
| Long Term 2 Enhanced Surface Water Treatment Rule (LT2) | 0800 | 10 | 0% | 9 |
Monitoring and Reporting
Administrative requirements. No direct health risk from the violation itself.
| Rule or contaminant | Code | California systems with violations, 3 yr | Share | Unresolved |
|---|---|---|---|---|
| Consumer Confidence Rule (CCR) | 7000 | 70 | 1% | 70 |
| Public Notification Rule | 7500 | 40 | 1% | 40 |
Volatile Organic Compound
Solvents and fuel components, usually monitored as a group, which is why the counts are similar.
| Rule or contaminant | Code | California systems with violations, 3 yr | Share | Unresolved |
|---|---|---|---|---|
| Ethylene Dibromide (EDB) | 2946 | 30 | 0% | 1 |
| Carbon Tetrachloride | 2982 | 30 | 0% | 2 |
| Toluene | 2991 | 30 | 0% | 2 |
| 1,2,4-Trichlorobenzene | 2378 | 29 | 0% | 2 |
| cis-1,2-Dichloroethylene | 2380 | 29 | 0% | 2 |
| Xylenes, Total | 2955 | 29 | 0% | 2 |
| Dichloromethane (Methylene Chloride) | 2964 | 29 | 0% | 2 |
| o-Dichlorobenzene | 2968 | 29 | 0% | 2 |
| p-Dichlorobenzene | 2969 | 29 | 0% | 2 |
| Vinyl Chloride | 2976 | 29 | 0% | 2 |
| 1,1-Dichloroethylene | 2977 | 29 | 0% | 2 |
| trans-1,2-Dichloroethylene | 2979 | 29 | 0% | 2 |
| 1,2-Dichloroethane | 2980 | 29 | 0% | 2 |
| 1,1,1-Trichloroethane | 2981 | 29 | 0% | 2 |
| 1,2-Dichloropropane | 2983 | 29 | 0% | 2 |
| Trichloroethylene (TCE) | 2984 | 29 | 0% | 2 |
| 1,1,2-Trichloroethane | 2985 | 29 | 0% | 2 |
| Tetrachloroethylene (PCE) | 2987 | 29 | 0% | 2 |
| Chlorobenzene | 2989 | 29 | 0% | 2 |
| Benzene | 2990 | 29 | 0% | 2 |
| Ethylbenzene | 2992 | 29 | 0% | 2 |
| Styrene | 2996 | 29 | 0% | 2 |
Organic Chemical
Industrial and synthetic organic chemicals with a maximum contaminant level.
| Rule or contaminant | Code | California systems with violations, 3 yr | Share | Unresolved |
|---|---|---|---|---|
| 1,2-Dibromo-3-Chloropropane (DBCP) | 2931 | 34 | 0% | 3 |
| Pentachlorophenol | 2326 | 28 | 0% | 1 |
| Hexachlorocyclopentadiene | 2042 | 27 | 0% | 1 |
| 2,3,7,8-TCDD (Dioxin) | 2063 | 26 | 0% | 1 |
| Hexachlorobenzene | 2274 | 26 | 0% | 1 |
| Di(2-ethylhexyl) Adipate | 2035 | 25 | 0% | 1 |
| Di(2-ethylhexyl) Phthalate | 2039 | 25 | 0% | 1 |
| Benzo(a)pyrene | 2306 | 25 | 0% | 1 |
| Polychlorinated Biphenyls (PCBs) | 2383 | 25 | 0% | 0 |
Pesticide
Pesticides and herbicides, monitored as a group.
| Rule or contaminant | Code | California systems with violations, 3 yr | Share | Unresolved |
|---|---|---|---|---|
| Glyphosate | 2034 | 30 | 0% | 1 |
| Dalapon | 2031 | 28 | 0% | 1 |
| Diquat | 2032 | 28 | 0% | 1 |
| Endothall | 2033 | 28 | 0% | 1 |
| Oxamyl (Vydate) | 2036 | 28 | 0% | 1 |
| Picloram | 2040 | 28 | 0% | 1 |
| Dinoseb | 2041 | 28 | 0% | 1 |
| Carbofuran | 2046 | 28 | 0% | 1 |
| 2,4-D | 2105 | 28 | 0% | 1 |
| 2,4,5-TP (Silvex) | 2110 | 28 | 0% | 1 |
| Simazine | 2037 | 27 | 0% | 1 |
| Atrazine | 2050 | 27 | 0% | 1 |
| Endrin | 2005 | 26 | 0% | 1 |
| BHC-Gamma (Lindane) | 2010 | 26 | 0% | 1 |
| Methoxychlor | 2015 | 26 | 0% | 1 |
| Toxaphene | 2020 | 26 | 0% | 1 |
| Alachlor (Lasso) | 2051 | 26 | 0% | 1 |
| Heptachlor | 2065 | 26 | 0% | 1 |
| Heptachlor Epoxide | 2067 | 26 | 0% | 1 |
| Chlordane | 2959 | 26 | 0% | 1 |
Radionuclide
Naturally occurring radioactive elements in groundwater.
| Rule or contaminant | Code | California systems with violations, 3 yr | Share | Unresolved |
|---|---|---|---|---|
| Combined Uranium | 4006 | 55 | 1% | 9 |
| Gross Alpha, Excl. Radon and U | 4000 | 15 | 0% | 1 |
| Combined Radium (-226 and -228) | 4010 | 13 | 0% | 2 |
| Radium-226 | 4020 | 1 | 0% | 0 |
| Radium-228 | 4030 | 1 | 0% | 0 |
Other rules
| Rule or contaminant | Code | California systems with violations, 3 yr | Share | Unresolved |
|---|---|---|---|---|
| Not applicable | 9 | 0% | 3 | |
| Coliphage | 3028 | 3 | 0% | 1 |
| Bromate | 1011 | 2 | 0% | 0 |
| Copper, Free | 1022 | 1 | 0% | 1 |
| Enterococci | 3002 | 1 | 0% | 0 |
About this data
A system is counted once per rule if any of its violations in the past three years involved that rule, including monitoring and reporting lapses. Unresolved means still in violation at EPA's latest data refresh. Share is of the 7,384 active California systems.
Source: the U.S. EPA's Enforcement and Compliance History Online system (ECHO), which publishes the federal Safe Drinking Water Information System. EPA's terms and values are shown as published. The California records are from EPA's data refresh of Oct 7, 2026; EPA refreshes this data quarterly, and states report to EPA with a lag of three to six months, so the newest violations may not appear yet. Retrieved from ECHO on Oct 11, 2026.