Metals · CAS 7440-38-2
Arsenic in Tap Water
Arsenic is a naturally occurring semi-metal that dissolves into groundwater from rock and soil, and can also enter water from mining and legacy agricultural chemicals.
Substance identity
- Scientific name
- Arsenic (As)
- Family
- Metals
- CAS number
- 7440-38-2
This guide explains the compound. Only a water-system record or household test can tell you whether it appears in your water.
- Federal reference
- 10 µg/L (MCL)Enforceable maximum contaminant level
- Health-based goal
- 10 µg/L federal limit (MCL); MCLG of 0US EPA Arsenic Rule
- Measurement context
- Stated in the recordThis contaminant is tracked through federal and state compliance monitoring. Private wells are not covered by the federal rule and should be tested directly. PurityMap does not yet publish a per-system measured value for Arsenic; this page is a reference explainer. To learn what is in your own water, check your utility’s Consumer Confidence Report (CCR) or use a certified home test.
- Cities on record
- None yetUS cities in PurityMap's published record where Arsenic appears
A reference is a published threshold, not a safety line drawn by PurityMap. Where a result sits against it is a finding; whether that constitutes a violation is a separately published compliance field.
What is Arsenic?
Arsenic is a naturally occurring semi-metal that dissolves into groundwater from rock and soil, and can also enter water from mining and legacy agricultural chemicals.
Where Arsenic comes from
Natural erosion of arsenic-bearing rock into groundwater; also mining, smelting, and past pesticide use.
- Natural mineral deposits
- Mining and smelting
- Legacy agricultural chemicals
Most affected: Anyone on an affected community system or private well; developing children and pregnant people are of particular concern.
How Arsenic is measured
This contaminant is tracked through federal and state compliance monitoring. Private wells are not covered by the federal rule and should be tested directly. PurityMap does not yet publish a per-system measured value for Arsenic; this page is a reference explainer. To learn what is in your own water, check your utility’s Consumer Confidence Report (CCR) or use a certified home test.
Why context mattersHow PurityMap reads a Arsenic result
Evidence exceeds a reference level. Used sparingly and only when warranted.
This substance was found in the monitoring record. Being present is not the same as being above a reference level, and this signal does not say which.
Somebody looked and the laboratory reported nothing above its reporting limit. This is a result, and it is not the same as nobody looking.
No sampling for this substance has been reported for this system. Nothing is known either way, and an empty row is not a clean one.
Where the numbers come from
- A laboratory result for one sampling point
Collected under a monitoring programme, analysed by an approved method, and reported with its own detection limits.
- Read against a published reference
The reference comes from the regulator, with its own effective date. PurityMap does not set thresholds and does not adjust them.
- Attached to a system's Water Profile
Where it contributes to the published assessment alongside the evidence quality of the record it came from.
The full evidence chain →
Where Arsenic is found across the U.S.
These are the places in PurityMap’s published record where Arsenic appears on a water system’s monitoring results. It reflects the systems and sampling we track, not a national prevalence estimate.
PurityMap has no city on record with Arsenic on its monitoring results yet. This fills in as monitoring data is published — a substance that was never tested is shown as pending, never as a pass.
Treatment capability, if a finding establishes a need
The formats that commonly hold a verified reduction claim for Arsenic are Reverse osmosis, Anion exchange, Activated alumina / adsorptive media. Holding the format is not holding the claim — the certification record is what is checked.
Boiling does not remove a dissolved chemical compound and can concentrate it. Many pitcher and refrigerator filters carry no verified claim for a compound like this at all.
Whether a system-level measurement or a household-tap measurement is the relevant scope depends on how the compound enters the water — a published characteristic of the record, not an assumption drawn here.
A finding establishes the need, the need names the capability, the capability requires a verified certification. No product appears before that sequence completes.
Filter Finder →Frequently asked questions
What is Arsenic?
Arsenic is a naturally occurring semi-metal that dissolves into groundwater from rock and soil, and can also enter water from mining and legacy agricultural chemicals.
What is the limit for Arsenic in drinking water?
The enforceable federal limit is 10 µg/L (MCL). The health-based goal is 10 µg/L federal limit (MCL); MCLG of 0. PurityMap reads every measurement against its stated benchmark and never sets its own.
Where does Arsenic in tap water come from?
Natural mineral deposits, Mining and smelting, Legacy agricultural chemicals.
Why does Arsenic matter?
Long-term arsenic exposure is linked to skin, bladder, and lung cancers and to cardiovascular and developmental effects. Because the risk is cumulative, the EPA set the limit near the lowest feasible level. Boiling does not remove arsenic and concentrates it.
How do you remove Arsenic from water?
Treatment technologies commonly certified to reduce Arsenic include Reverse osmosis, Anion exchange, Activated alumina / adsorptive media. Match the exact certified claim to your water before buying — holding a format is not the same as holding the claim.
Sources and methodology
Reference values on this page come from the governed contaminant record. Local occurrence claims stay on the linked water-system evidence pages, where sampling scope, period and source remain visible.
How PurityMap analyzes water data →