MCL for arsenic in drinking water and EPA compliance rules
The EPA MCL for arsenic is 0.010 mg/L, or 10 ppb. The MCLG is zero. The limit covers community and non-transient non-community systems, not every well.
MCL for arsenic in drinking water
The federal EPA MCL for arsenic in drinking water is 0.010 mg/L, equivalent to 10 µg/L or 10 ppb. The separate maximum contaminant level goal, or MCLG, is zero, a non-enforceable health goal. EPA lists both values in its National Primary Drinking Water Regulations table.
MCL means maximum contaminant level. It is an enforceable standard for covered water systems. EPA sets MCLs as close to health goals as feasible, considering treatment technology and cost, so arsenic's enforceable limit is 0.010 mg/L rather than the zero goal.
The arsenic limit in laboratory units
Check the unit before comparing a laboratory result with the arsenic maximum contaminant level. These values express the same drinking-water limit:
| Unit on the report | Equivalent arsenic MCL |
|---|---|
| Milligrams per liter, mg/L | 0.010 |
| Micrograms per liter, µg/L | 10 |
| Parts per billion, ppb | 10 |
| Parts per million, ppm | 0.010 |
EPA gives the standard as 10 ppb or 0.010 ppm. Converting milligrams to micrograms gives 0.010 mg/L = 10 µg/L. Miss the decimal and the result is off by 1,000: 0.010 mg/L is not 0.010 µg/L, and the standard is not 10 mg/L. 40 CFR 141.23(i)(4) says arsenic sampling results are reported to the nearest 0.001 mg/L.
Which water systems must meet the limit?
The federal arsenic limit applies to community water systems and non-transient non-community water systems. The legal value is 40 CFR 141.62(b)(16). Paragraph (b) applies that inorganic MCL to those systems, not to every public water system.
EPA adopted the 10 ppb standard on January 22, 2001, replacing 50 ppb. Covered systems had to comply by January 23, 2006, according to EPA's arsenic rule history.
Private domestic wells are different. Their drinking-water quality and safety are not regulated by the federal government under the Safe Drinking Water Act, nor by most state governments and laws. Most is not all, so check the state or local rule instead of treating the federal MCL as a private-well compliance number.
Is 10 ppb a safe level of arsenic?
Calling 10 ppb a safe level of arsenic, or an acceptable level of arsenic in drinking water, overreads the rule. Ten ppb is the federal enforceable limit, not a promise of zero health risk. The zero MCLG is a separate public-health goal.
EPA sets goals for known cancer-causing contaminants at zero because any chemical exposure could present a cancer risk. For long-term exposure above the arsenic MCL, EPA lists skin damage or problems with circulatory systems, and a possible increased risk of getting cancer. Those are EPA's stated potential effects, not a prediction about one person's exposure.
Does one result above 10 ppb mean a violation?
Under 40 CFR 141.23(i), systems monitoring more often than annually use a running annual average at any sampling point. If that average is greater than the MCL, the system is out of compliance. A sample that would cause the annual average to be exceeded puts the system out of compliance immediately.
For annual or less-frequent monitoring, a result greater than the MCL means noncompliance. But confirmation is not automatic. The state may require one confirmation sample at the same point, as soon as possible and within two weeks. Where the state requires it, 40 CFR 141.23(f)(3) says to average the initial result and the confirmation result, and that average is what compliance uses. Where the state does not require it, don't wait for a second sample.
RegAffairs AI's answer on the arsenic drinking-water limit cites these EPA figures.