Contaminants and rules in Montana water systems
84 different rules and contaminants appear in the past three years of violation records for Montana's 2,353 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 | Montana systems with violations, 3 yr | Share | In violation, latest quarter |
|---|---|---|---|---|
| Revised Total Coliform Rule (RTCR) | 8000 | 620 | 26% | 204 |
| E. coli (Escherichia coli) | 3014 | 75 | 3% | 75 |
Inorganic Chemical
Naturally occurring or from fertilizer and industry. Each has a maximum contaminant level.
| Rule or contaminant | Code | Montana systems with violations, 3 yr | Share | In violation, latest quarter |
|---|---|---|---|---|
| Nitrate-Nitrite (Combined) | 1038 | 338 | 14% | 10 |
| Arsenic | 1005 | 33 | 1% | 0 |
| Fluoride | 1025 | 26 | 1% | 1 |
| Antimony | 1074 | 23 | 1% | 0 |
| Nickel | 1036 | 22 | 1% | 0 |
| Beryllium | 1075 | 22 | 1% | 0 |
| Thallium | 1085 | 22 | 1% | 0 |
| Barium | 1010 | 20 | 1% | 0 |
| Cadmium | 1015 | 20 | 1% | 0 |
| Chromium | 1020 | 20 | 1% | 0 |
| Mercury | 1035 | 20 | 1% | 0 |
| Selenium | 1045 | 20 | 1% | 0 |
| Cyanide | 1024 | 9 | 0% | 0 |
| Asbestos | 1094 | 4 | 0% | 0 |
| Nitrite | 1041 | 3 | 0% | 0 |
Disinfection Byproduct
Formed when chlorine reacts with organic matter in the water.
| Rule or contaminant | Code | Montana systems with violations, 3 yr | Share | In violation, latest quarter |
|---|---|---|---|---|
| Total Haloacetic Acids (HAA5) | 2456 | 89 | 4% | 3 |
| TTHM (Total Trihalomethanes) | 2950 | 87 | 4% | 5 |
Treatment Technique
Rules that require a process, such as corrosion control or disinfection, rather than a measured limit.
| Rule or contaminant | Code | Montana systems with violations, 3 yr | Share | In violation, latest quarter |
|---|---|---|---|---|
| Lead and Copper Rule (LCR) | 5000 | 122 | 5% | 122 |
| Groundwater Rule (GWR) | 0700 | 45 | 2% | 18 |
| Surface Water Treatment Rule (SWTR) | 0200 | 30 | 1% | 13 |
| Chlorine (MRDL) | 0999 | 10 | 0% | 2 |
| Long Term 2 Enhanced Surface Water Treatment Rule (LT2) | 0800 | 7 | 0% | 1 |
| Interim Enhanced Surface Water Treatment Rule | 0300 | 5 | 0% | 1 |
Monitoring and Reporting
Administrative requirements. No direct health risk from the violation itself.
| Rule or contaminant | Code | Montana systems with violations, 3 yr | Share | In violation, latest quarter |
|---|---|---|---|---|
| Consumer Confidence Rule (CCR) | 7000 | 121 | 5% | 121 |
| Public Notification Rule | 7500 | 2 | 0% | 2 |
Volatile Organic Compound
Solvents and fuel components, usually monitored as a group, which is why the counts are similar.
| Rule or contaminant | Code | Montana systems with violations, 3 yr | Share | In violation, latest quarter |
|---|---|---|---|---|
| Dichloromethane (Methylene Chloride) | 2964 | 41 | 2% | 0 |
| o-Dichlorobenzene | 2968 | 41 | 2% | 0 |
| 1,2,4-Trichlorobenzene | 2378 | 40 | 2% | 0 |
| cis-1,2-Dichloroethylene | 2380 | 40 | 2% | 0 |
| Xylenes, Total | 2955 | 40 | 2% | 0 |
| p-Dichlorobenzene | 2969 | 40 | 2% | 0 |
| Vinyl Chloride | 2976 | 40 | 2% | 0 |
| 1,1-Dichloroethylene | 2977 | 40 | 2% | 0 |
| trans-1,2-Dichloroethylene | 2979 | 40 | 2% | 0 |
| 1,2-Dichloroethane | 2980 | 40 | 2% | 0 |
| 1,1,1-Trichloroethane | 2981 | 40 | 2% | 0 |
| Carbon Tetrachloride | 2982 | 40 | 2% | 0 |
| 1,2-Dichloropropane | 2983 | 40 | 2% | 0 |
| Trichloroethylene (TCE) | 2984 | 40 | 2% | 0 |
| 1,1,2-Trichloroethane | 2985 | 40 | 2% | 0 |
| Tetrachloroethylene (PCE) | 2987 | 40 | 2% | 0 |
| Chlorobenzene | 2989 | 40 | 2% | 0 |
| Benzene | 2990 | 40 | 2% | 0 |
| Toluene | 2991 | 40 | 2% | 0 |
| Ethylbenzene | 2992 | 40 | 2% | 0 |
| Styrene | 2996 | 40 | 2% | 0 |
| Ethylene Dibromide (EDB) | 2946 | 9 | 0% | 0 |
Organic Chemical
Industrial and synthetic organic chemicals with a maximum contaminant level.
| Rule or contaminant | Code | Montana systems with violations, 3 yr | Share | In violation, latest quarter |
|---|---|---|---|---|
| Di(2-ethylhexyl) Phthalate | 2039 | 39 | 2% | 1 |
| Di(2-ethylhexyl) Adipate | 2035 | 37 | 2% | 1 |
| Hexachlorocyclopentadiene | 2042 | 37 | 2% | 1 |
| Hexachlorobenzene | 2274 | 37 | 2% | 1 |
| Benzo(a)pyrene | 2306 | 37 | 2% | 1 |
| Pentachlorophenol | 2326 | 37 | 2% | 1 |
| Polychlorinated Biphenyls (PCBs) | 2383 | 9 | 0% | 0 |
| 1,2-Dibromo-3-Chloropropane (DBCP) | 2931 | 9 | 0% | 0 |
Pesticide
Pesticides and herbicides, monitored as a group.
| Rule or contaminant | Code | Montana systems with violations, 3 yr | Share | In violation, latest quarter |
|---|---|---|---|---|
| Endrin | 2005 | 37 | 2% | 1 |
| BHC-Gamma (Lindane) | 2010 | 37 | 2% | 1 |
| Methoxychlor | 2015 | 37 | 2% | 1 |
| Toxaphene | 2020 | 37 | 2% | 1 |
| Dalapon | 2031 | 37 | 2% | 1 |
| Oxamyl (Vydate) | 2036 | 37 | 2% | 1 |
| Simazine | 2037 | 37 | 2% | 1 |
| Picloram | 2040 | 37 | 2% | 1 |
| Dinoseb | 2041 | 37 | 2% | 1 |
| Carbofuran | 2046 | 37 | 2% | 1 |
| Atrazine | 2050 | 37 | 2% | 1 |
| Alachlor (Lasso) | 2051 | 37 | 2% | 1 |
| Heptachlor | 2065 | 37 | 2% | 1 |
| Heptachlor Epoxide | 2067 | 37 | 2% | 1 |
| 2,4-D | 2105 | 37 | 2% | 1 |
| 2,4,5-TP (Silvex) | 2110 | 37 | 2% | 1 |
| Chlordane | 2959 | 37 | 2% | 1 |
| Diquat | 2032 | 9 | 0% | 0 |
| Endothall | 2033 | 9 | 0% | 0 |
| Glyphosate | 2034 | 9 | 0% | 0 |
Radionuclide
Naturally occurring radioactive elements in groundwater.
| Rule or contaminant | Code | Montana systems with violations, 3 yr | Share | In violation, latest quarter |
|---|---|---|---|---|
| Gross Alpha, Excl. Radon and U | 4000 | 22 | 1% | 3 |
| Combined Radium (-226 and -228) | 4010 | 16 | 1% | 1 |
| Combined Uranium | 4006 | 6 | 0% | 1 |
| Radium-226 | 4020 | 4 | 0% | 1 |
| Radium-228 | 4030 | 4 | 0% | 1 |
Other rules
| Rule or contaminant | Code | Montana systems with violations, 3 yr | Share | In violation, latest quarter |
|---|---|---|---|---|
| Bromate | 1011 | 8 | 0% | 0 |
| Carbon, Total | 2920 | 2 | 0% | 2 |
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. In violation, latest quarter counts systems whose contaminant list for the latest official quarter in EPA's data, or new violations reported since, includes this entry. Share is of the 2,353 active Montana 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 Montana 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.