Health alert: The EPA maximum contaminant level (MCL) for arsenic in public drinking water is 10 parts per billion (ppb). Private well owners are responsible for testing their own water — there is no federal monitoring requirement for private wells.
Arsenic is a naturally occurring element found in rock and soil throughout the United States. It dissolves into groundwater through natural geological processes and from certain agricultural and industrial uses — including wood preservatives and pesticides applied to cotton and other crops. For millions of Americans on private well water, arsenic contamination is a real and often invisible risk.
The challenge: not all arsenic is the same, and the wrong filtration system won’t protect you. This guide explains the two types of arsenic found in drinking water, the five proven removal methods, and exactly how to choose the right system for your home.
What Is Arsenic and Why Is It Dangerous?
Arsenic (chemical symbol: As) is a naturally occurring metalloid that is odorless and tasteless in water — meaning you cannot detect contamination without testing. Long-term exposure to arsenic above safe levels is linked to serious health effects including bladder, lung, and skin cancers, as well as cardiovascular disease, diabetes, and developmental problems in children.
Arsenic enters drinking water two ways: naturally, as groundwater flows through arsenic-bearing rock and soil, and from human activity, including mining runoff, agricultural chemicals, and industrial waste. Well water in the western United States, New England, the Midwest, and parts of the Southeast carries the highest risk, though contamination occurs in all 50 states.
The Two Types of Arsenic in Water: Why It Matters for Treatment
Arsenic in household water supplies exists in two primary chemical forms called “valences” or “species.” Identifying which type — or what ratio of both — is in your water is the single most important factor in choosing the right filtration system.
Arsenic (V) — Arsenate
Arsenic (V) carries a negative electrical charge. It is most commonly found in shallow wells and surface water sources with higher dissolved oxygen content. Because it is charged, Arsenic (V) is the easier of the two forms to remove — most standard filtration technologies handle it effectively.
Arsenic (III) — Arsenite
Arsenic (III) carries a neutral charge. It is more common in deeper wells with lower dissolved oxygen levels. Its neutral charge makes it significantly harder to remove — many filtration technologies that work well on Arsenic (V) have little to no effect on Arsenic (III). This is why a proper water test is essential before choosing any system.
Get an arsenic speciation test. A standard arsenic test only tells you the total arsenic level. An arsenic speciation test tells you the breakdown of Arsenic (III) vs. Arsenic (V) in your water — this is the test you need to choose the right treatment system. Contact a state-certified water testing lab in your area to request one.
5 Proven Methods to Remove Arsenic from Drinking Water
Each method below has different strengths depending on your arsenic type, water chemistry, pH, and other contaminants present. No single solution is right for every situation — use this as a starting point and work with a certified water treatment professional to confirm the best fit.
1. Reverse Osmosis (RO)Point-of-use
A semi-permeable membrane filters water at the molecular level, installed under the kitchen sink to treat drinking and cooking water. Highly effective on negatively charged Arsenic (V) but has limited effectiveness on neutral Arsenic (III) without pre-oxidation.
Arsenic (V): highly effectiveArsenic (III): limitedBest for: As(V)-dominant water
2. Anion ExchangeWhole-house or POU
Anion exchange resin attracts and holds negatively charged ions, swapping them for harmless chloride ions. Works well on Arsenic (V) because it is negatively charged. Will not remove neutral Arsenic (III).
Arsenic (V): effectiveArsenic (III): not effectiveBest for: As(V)-dominant water
3. Anion Hybrid Resins (Iron Oxide-Impregnated)Both types
A specialized resin that combines standard anion exchange with iron oxide embedded in the resin beads. The iron oxide creates a magnetic-like attraction for the neutrally charged Arsenic (III), significantly expanding what the resin can remove. One of the few technologies effective on both forms.
Arsenic (V): highly effectiveArsenic (III): good (high-percentage removal)
4. Granular Ferric Oxide (GFO) MediaBoth types
GFO media uses adsorption — arsenic bonds directly to the surface of iron oxide granules as water passes through. Because iron oxide naturally attracts neutral Arsenic (III) as well as negative Arsenic (V), this is an excellent choice for water supplies containing both forms. Media must be replaced periodically when exhausted.
Arsenic (V): highly effectiveArsenic (III): highly effectiveBest for: mixed or unknown speciation
5. Titanium-Based Adsorption MediaBoth types
Titanium dioxide or titanium oxide media uses adsorption to bind arsenic to the media surface. Like GFO, it is effective on both arsenic species and is often chosen for its stability across a wide pH range and its resistance to competing ions that can reduce other media’s performance.
Arsenic (V): highly effectiveArsenic (III): effectiveBest for: wide pH range, mixed speciation
Quick Comparison: Which Method Is Right for Your Water?
| Method | Arsenic (III) | Arsenic (V) | Key consideration |
|---|---|---|---|
| Reverse osmosis | Limited | Highly effective | Point-of-use only; add pre-oxidation for As(III) |
| Anion exchange | Not effective | Effective | Affected by competing sulfate/phosphate ions |
| Hybrid resin (iron oxide) | Good | Highly effective | Iron and manganese in water can foul resin |
| Granular ferric oxide | Highly effective | Highly effective | pH range and competing ions affect performance |
| Titanium media | Effective | Highly effective | Stable across wider pH; higher upfront cost |
Important: Every method above has performance limits depending on your water’s pH, the presence of iron, manganese, sulfates, phosphates, silica, and other competing contaminants. A treatment system that works perfectly in one household may underperform in another. Always have your full water chemistry analyzed before selecting a system.
How to Get Started: A Step-by-Step Action Plan
- 1Test your water — request a full water analysis plus an arsenic speciation test from a state-certified lab. This reveals total arsenic, the As(III)/As(V) ratio, pH, iron, hardness, and other factors that affect treatment.
- 2Review your results with a licensed water treatment professional who can interpret the chemistry and recommend the appropriate system for your specific conditions.
- 3Choose a certified system — look for NSF/ANSI 53 or NSF/ANSI 58 certification for arsenic reduction, which confirms the system performs as claimed under standardized testing conditions.
- 4Install and maintain — follow the manufacturer’s service schedule. Exhausted media or expired membranes will allow arsenic to pass through even a high-quality system.
- 5Re-test annually — water chemistry can change seasonally or as geological conditions shift. Annual testing confirms your system continues to perform.
Frequently Asked Questions
Arsenic is colorless, odorless, and tasteless — you cannot detect it without testing. If you have a private well, especially in a high-risk region, have your water tested by a state-certified laboratory. The EPA recommends private well owners test annually for common contaminants including arsenic.
The EPA’s maximum contaminant level (MCL) for arsenic in public water systems is 10 micrograms per liter (µg/L), also expressed as 10 parts per billion (ppb). This standard does not legally apply to private wells, but the same health-based limit is the widely accepted safe target for well water treatment.
Standard carbon block or pitcher filters (like Brita or PUR) are not designed to remove arsenic and are not certified to do so. You need a system specifically certified to NSF/ANSI 53 (for adsorption media) or NSF/ANSI 58 (for reverse osmosis) with an arsenic reduction claim. Always verify the certification before purchasing.
No. Boiling water concentrates arsenic rather than removing it, because water evaporates while arsenic remains. Boiling is effective for killing bacteria and viruses but should never be used as a treatment for chemical contaminants like arsenic, lead, or nitrates.
Arsenic (III), or arsenite, carries a neutral electrical charge and is typically found in deeper, low-oxygen groundwater. Arsenic (V), or arsenate, carries a negative charge and is more common in shallow wells and surface water. This difference in charge is critical for filtration — technologies that use ion exchange work well on negatively charged Arsenic (V) but cannot capture neutral Arsenic (III) without additional treatment or specialized media.
Yes. Pre-oxidation using chlorine, chlorine dioxide, ozone, or potassium permanganate can convert Arsenic (III) to Arsenic (V), making it easier to remove with downstream technologies like reverse osmosis or anion exchange. This approach adds complexity and cost and must be carefully designed to avoid creating other treatment problems. A water treatment professional can assess whether pre-oxidation is appropriate for your water chemistry.
Replacement frequency depends on the media type, your arsenic concentration, and your household water usage. Granular ferric oxide and titanium media have a finite adsorption capacity and must be replaced when exhausted — typically every 1–3 years. RO membranes generally last 2–3 years. Anion exchange resin may be regenerable or may need replacement depending on the system design. Regular water testing is the only reliable way to confirm your system continues to perform.
