Arsenic doesn’t announce itself. It has no taste, no smell, and no color — the only way to know if it’s in your water is to test for it. That makes it one of the more important contaminants to understand, especially if you’re on a private well.
Where Arsenic Comes From
Arsenic occurs naturally in rock and soil, and it dissolves into groundwater as water passes through arsenic-bearing minerals. Levels vary significantly by region — some areas have naturally high background arsenic in groundwater, while others have almost none. It can also enter water supplies from certain agricultural and industrial sources, though naturally occurring arsenic is the more common cause in most private wells.
Because it’s a groundwater issue, private wells are the primary concern here. Municipal water systems are required to test for and treat arsenic to meet federal limits, so if you’re on city water, this is one of the contaminants your Consumer Confidence Report already covers.
The Health Risks
The EPA’s Maximum Contaminant Level (MCL) for arsenic in public drinking water is 10 parts per billion (ppb), also written as 0.010 mg/L. The World Health Organization uses the same guideline value.
Health effects depend heavily on the level and duration of exposure:
- Long-term exposure to arsenic above the safe threshold is linked to increased risk of certain cancers (skin, bladder, and lung), as well as cardiovascular disease and skin lesions. These risks develop over years to decades of exposure, not from a single glass of water.
- Short-term, high-dose exposure (well above drinking water levels) can cause acute symptoms, but this scenario is rare in typical residential water supplies.
The practical takeaway: arsenic risk is about cumulative, long-term exposure. This is exactly why testing matters — you can’t feel or taste elevated arsenic, so the only way to know your actual exposure level is a proper water test.
Should You Test for It?
If you’re on a private well, yes — arsenic should be part of your regular well-testing panel, particularly if you’re in a region known for naturally occurring groundwater arsenic. A local health department or state geological survey can often tell you whether your area has a known arsenic history. If you haven’t tested your well recently, see our guide on how often to test your well water and what to test for.
If you’re on municipal water, your CCR will show whether arsenic has been detected and at what level relative to the MCL — check there first before assuming you need additional testing.
Removal Options
Not all filtration methods remove arsenic effectively, so it’s worth knowing which ones actually work before choosing a system — and just as important, which form of arsenic they work best against.
Arsenic occurs in water in two chemical forms: arsenate (As(V)) and arsenite (As(III)). This isn’t a minor technicality — it directly affects how well a given treatment method performs. Most technologies that handle arsenate well perform noticeably worse against arsenite unless the water is pre-treated to convert it first.
| Method | Effectiveness | Notes |
|---|---|---|
| Reverse Osmosis (RO) | High for arsenate (As(V)); meaningfully lower for arsenite (As(III)) unless pre-oxidized | Most reliable whole-house or under-sink option; if your test shows a significant proportion of As(III), ask about a pre-oxidation step to improve removal |
| Activated Alumina | High for arsenate; similarly reduced for arsenite without pre-treatment | Common in point-of-use systems specifically marketed for arsenic |
| Anion Exchange | Effective for arsenate only | Removes essentially none of the arsenite form unless it’s first converted to arsenate via oxidation |
| Standard Carbon Filtration | Not effective | Carbon filters (including most pitcher filters) do not reliably remove either form of arsenic |
| Water Softeners (ion exchange for hardness) | Not designed for this | A softener addresses hardness minerals, not arsenic — don’t rely on one for arsenic removal |
This is exactly why a proper water test — one that ideally identifies which form of arsenic is present, not just a total arsenic number — should guide your treatment choice, rather than assuming any “arsenic-rated” system will perform identically regardless of your water’s specific chemistry. If your well test comes back with elevated arsenic and a significant arsenite fraction, ask a water treatment professional whether a pre-oxidation step ahead of your chosen system is worth adding. If your levels come back elevated, look for a treatment system specifically certified for arsenic reduction (NSF/ANSI Standard 58 for RO systems, or NSF/ANSI Standard 53 for other arsenic-rated systems) rather than assuming your existing filter already handles it.
The Bottom Line
Arsenic is a serious but manageable water quality concern. The risk comes from long-term exposure, not a single test result being slightly elevated — but it’s not something to ignore either. If you’re on a private well and haven’t tested for it, that’s the first step. If your levels come back elevated, RO and activated alumina systems are your most reliable options — just confirm which form of arsenic you’re dealing with, since that determines whether a pre-oxidation step is worth adding, and confirm any system you consider is specifically certified for arsenic removal rather than assuming general filtration covers it.
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