Arsenic Removal from Well Water

Volume I  ·  May 2026  ·  488 words

Arsenic is a naturally occurring metalloid present in groundwater across large regions of the United States — particularly the Southwest, upper Midwest, and New England — at concentrations that exceed the EPA maximum contaminant level of 10 parts per billion (ppb). It is tasteless, odorless, and colorless in solution, and chronic exposure at concentrations above 50 ppb is associated with increased risk of bladder, lung, and skin cancer. Removing arsenic from well water is chemically straightforward but operationally demanding: arsenic exists in two oxidation states with different removal requirements, and the choice of treatment technology must account for the water's pH, iron concentration, and competing ions.

Arsenic(III) vs. Arsenic(V) and the oxidation requirement. Arsenic in groundwater exists predominantly as arsenite — As(III), the trivalent form — under reducing (low-oxygen) conditions, and as arsenate — As(V), the pentavalent form — under oxidizing conditions. This distinction matters because As(III) is uncharged at neutral pH (H₃AsO₃) and passes through reverse osmosis membranes and adsorptive media with substantially lower removal efficiency than the charged As(V) species (H₂AsO₄⁻ and HAsO₄²⁻). Reverse osmosis achieves 95–99% rejection of As(V) but only 50–70% rejection of As(III). Any arsenic treatment system relying on RO or adsorptive media must therefore include an oxidation pretreatment step — chlorine injection, potassium permanganate, or a manganese dioxide catalytic medium — to convert As(III) to As(V) before the removal stage.

Reverse osmosis for arsenic. Point-of-use reverse osmosis systems, such as the APEC ROES-50, reduce arsenic V to below 5 ppb when the feed water is pretreated and the pH is below 8.0. At pH above 8.5, arsenate exists primarily as HAsO₄²⁻, which carries a −2 charge and is actually better rejected by the thin-film composite RO membrane — but scaling (calcium carbonate precipitation on the membrane surface) becomes the limiting factor. RO produces approximately 3–5 gallons of reject water for every gallon of treated water, and the reject stream contains concentrated arsenic — a consideration for homes on septic systems where the brine discharge enters the leach field.

Adsorptive media: iron-enhanced and titanium-based. Iron-enhanced adsorptive media — granular ferric oxide (GFO) or granular ferric hydroxide (GFH) — remove arsenic through ligand exchange, where arsenate ions displace hydroxide groups on the iron oxide surface. These media achieve 90–99% As(V) removal at pH 6.0–7.5, with performance declining at higher pH where hydroxide competition increases. Media lifespan is determined by the empty bed contact time (EBCT), typically 3–5 minutes, and the arsenic loading capacity — typically 3–10 mg of arsenic per gram of media. A whole-house adsorptive arsenic system like the iSpring whole house filtration system can be configured with an arsenic-specific media tank, though the media must be replaced when saturated — it cannot be regenerated. Spent arsenic media is classified as hazardous waste in some states and must be disposed of accordingly.

Testing and monitoring. Raw well water should be tested for both total arsenic and speciated As(III)/As(V) before selecting a treatment system. Arsenic concentrations can fluctuate seasonally with changes in the water table, so testing at both high-water (spring) and low-water (late summer) conditions provides a more complete picture. After treatment installation, quarterly testing for the first year establishes a baseline, and annual testing thereafter confirms continued performance. A post-treatment arsenic concentration consistently above 5 ppb — half the EPA limit — warrants investigation, as it may indicate As(III) breakthrough from an exhausted oxidation stage or media approaching saturation.