Arsenic in drinking water: limits, risks & removal.
Arsenic is a mostly natural contaminant that erodes into groundwater from geologic deposits. The EPA limit (MCL) is 0.010 mg/L — 10 ppb — and the health goal (MCLG) is zero. Because arsenic harms over years, not days, a confirmed exceedance is a chronic Tier 2 violation: the system must notify customers within 30 days, not 24 hours.
Arsenic is a chronic contaminant — not an acute one.
An exceedance is not a same-day emergency — but it is still a violation, judged on the running annual average and answered with a 30-day public notice.
- 1 · Running annual average ≥ 0.010 mg/L (10 ppb)
- 2 · MCL violation confirmed under § 141.62 · § 141.23
- 3 · Tier 2 public notice within 30 days chronic — with the required health-effects language
Source: 40 CFR §§ 141.62 · 141.23 · 141.203 · Appendix A to Subpart Q
What long-term arsenic exposure does
Long-term cancer risk
Long-term exposure is linked to cancer of the bladder, lungs, and skin. Because arsenic is a carcinogen with no known safe threshold, EPA sets the MCLG — the health goal — at zero (EPA, 2006; 40 CFR § 141.62).
Skin and circulatory effects
Chronic arsenic exposure can cause skin changes such as lesions and pigmentation, plus circulatory and cardiovascular problems. EPA's own health-effects language cites "skin damage or problems with circulatory systems" (EPA, National Primary Drinking Water Regulations).
Cardiovascular and developmental effects
Beyond cancer, long-term arsenic is associated with cardiovascular disease and developmental effects. The risk accrues over years of drinking water above the limit — not from a single glass — which is what makes the standard strict.
Chronic, not acute
Unlike nitrate or E. coli, arsenic does not cause same-day illness. The 10 ppb standard "protects consumers from the effects of long-term, chronic exposure" (EPA, 2006), which is why an MCL exceedance is a Tier 2 notice, not a 24-hour Tier 1 one.
Where arsenic in drinking water comes from
Natural geologic deposits
Most arsenic in U.S. drinking water is natural — it erodes from rock and sediment into groundwater. EPA lists "erosion of natural deposits" as the primary source, and it dominates in groundwater systems.
Regional hot spots
Arsenic above 10 ppb is most common in parts of the Southwest (volcanic formations), the upper Midwest (glacial aquifers in Minnesota, Michigan, Wisconsin, and South Dakota), and New England (granite bedrock) (USGS).
Industrial and agricultural
Smaller contributions come from mining, smelting, and past use of arsenical pesticides. EPA also names "runoff from orchards" and "runoff from glass and electronics production wastes" among the sources.
How water systems remove arsenic
Adsorptive media
Activated alumina and iron-based adsorptive media bind arsenic as water passes through. Activated alumina is an EPA best available technology under 40 CFR § 141.62(c); iron-based adsorptive media is a common small-system compliance technology in EPA's Arsenic Rule guidance.
Reverse osmosis & ion exchange
Reverse osmosis and anion exchange are BATs that strip dissolved arsenic from the water. Both are practical at point-of-entry or small-system scale, which matters for the small groundwater systems where arsenic is most common (40 CFR § 141.62(c)).
Coagulation, oxidation & softening
Coagulation/filtration, oxidation/filtration, and lime softening are BATs for larger systems. Coagulation/filtration and lime softening are not BAT for systems under 500 service connections (40 CFR § 141.62(c)).
Convert arsenite first
Arsenite (As III) is harder to remove than arsenate (As V), so systems often pre-oxidize it before adsorption or filtration. For oxidation/filtration, EPA notes an iron-to-arsenic ratio of at least 20:1 to reach high removal (40 CFR § 141.62(c)).
An arsenic exceedance is a Tier 2 notice — 30 days, not 24 hours.
Arsenic is a chronic contaminant, so a confirmed MCL exceedance is a Tier 2 public notification under 40 CFR § 141.203 — as soon as practical, and no later than 30 days after the system learns of it, carrying the EPA-mandated health-effects language. Compliance turns on the running annual average at each sampling point (§ 141.23), so a single high quarter can put you over.
1water’s CCR agent reports your arsenic results each year, and the compliance validator checks them against the § 141.62 MCL. A public-notification generator with the verbatim health-effects language is rolling out — so if the annual average crosses 10 ppb, the right Tier 2 notice is drafted, not scrambled. Tell us your state →
Arsenic in drinking water — common questions
- U.S. EPA — National Primary Drinking Water Regulations, MCL for arsenic 0.010 mg/L (10 ppb) effective January 23, 2006, MCLG 0, and best available technologies, 40 CFR § 141.62.
- U.S. EPA — inorganic monitoring and running-annual-average compliance, 40 CFR § 141.23.
- U.S. EPA — Public Notification Rule, 40 CFR § 141 Subpart Q (§ 141.201–141.204) and Appendix A to Subpart Q; arsenic MCL violations are Tier 2 (30-day notice).
- U.S. Geological Survey — occurrence of arsenic in U.S. groundwater (Southwest, upper Midwest, and New England).
Keep reading
Turbidity in drinking water: NTU limits & the SWTR
The 0.3 NTU / 1 NTU Surface Water Treatment Rule limits for filtered surface-water and GWUDI systems, why turbidity shields pathogens from disinfection, and what an exceedance triggers.
The Ground Water Rule: what it requires of ground water systems
How 40 CFR 141 Subpart S protects ground water systems from fecal contamination — sanitary surveys, triggered source-water monitoring after a coliform-positive, and corrective action.
Fluoride in drinking water: limits, notices & 0.7 mg/L
The three fluoride numbers — the enforceable 4.0 mg/L MCL, the 2.0 mg/L secondary standard and its special notice, and the 0.7 mg/L fluoridation recommendation — plus where fluoride comes from.
Catch an exceedance before it becomes a rejection.
1water helps small and mid-size water systems stay ahead of arsenic, nitrate, coliform, lead, PFAS, and CCR requirements — self-serve, and priced for the small end.