Arsenic is a naturally occurring metalloid element that dissolves into groundwater, contaminating private wells. Since it is odorless, tasteless, and colorless, contamination is undetectable without testing. Chronic exposure to inorganic arsenic, the type most common in water, is linked to serious health issues, including various cancers and cardiovascular problems. Private well owners are solely responsible for ensuring water safety, as these sources are not subject to federal regulations governing public water supplies. The U.S. Environmental Protection Agency sets the maximum contaminant level (MCL) for public water systems at 10 parts per billion (ppb), a benchmark private well owners should also use.
Identifying Arsenic and Necessary Water Preparation
The first step for any private well owner is to have the water professionally tested by a certified laboratory. Testing is the only way to determine arsenic’s presence and concentration. A proper analysis should reveal the total arsenic level and the ratio of its two primary forms: Arsenic III (arsenite) and Arsenic V (arsenate).
This distinction is important because the two forms behave differently during treatment. Arsenic V is a negatively charged anion, making it readily removable by common filtration methods like adsorption and ion exchange. Conversely, Arsenic III is typically uncharged in the near-neutral pH range of most groundwater, making it highly mobile and difficult to capture. Effective removal requires converting Arsenic III to Arsenic V through a chemical process called oxidation.
This pre-treatment oxidation step is accomplished by injecting a powerful oxidizing agent into the water line upstream of the filtration system. Common oxidizers include chlorine bleach, potassium permanganate, or hydrogen peroxide, which chemically react with Arsenic III to change its charge state. Once converted, Arsenic V can be captured by the downstream treatment technology. Without this preparation, the removal system will likely fail to meet safe drinking water standards, especially if Arsenic III is highly concentrated.
Whole-House Arsenic Removal Systems (Point-of-Entry)
Whole-house treatment systems, known as Point-of-Entry (POE), treat all water entering the home, providing purified water at every faucet. The two primary technologies used for high-volume arsenic removal are adsorption media and anion exchange. Both systems require the arsenic to be in its oxidized, Arsenic V form for high efficiency.
Activated Alumina/Adsorption Media
Adsorption media systems, most commonly using activated alumina (aluminum oxide), function by chemically attracting and holding arsenic onto the media surface. Negatively charged Arsenic V ions adhere to positively charged sites on the highly porous alumina particles through ligand exchange. The system remains effective until the media’s surface sites become saturated with arsenic and can no longer capture contaminant molecules.
Once the media is exhausted, the homeowner must either replace it or have it regenerated off-site. Activated alumina efficiency is highly sensitive to the water’s pH, with optimal removal occurring in the slightly acidic range of pH 5.5 to 7. Higher pH levels reduce the alumina’s positive surface charge, thereby reducing its ability to attract Arsenic V.
Anion Exchange (Ion Exchange)
Anion exchange operates similarly to a conventional water softener, but it targets negatively charged ions like Arsenic V instead of hardness minerals. The system contains specialized resin beads initially charged with a harmless ion, typically chloride. As contaminated water passes through, Arsenic V ions trade places with the chloride ions, effectively swapping the arsenic out.
This method is highly effective, but its performance is diminished by other common negatively charged contaminants, such as sulfate. Sulfate competes aggressively with Arsenic V for the exchange sites, significantly shortening the system’s lifespan and requiring more frequent regeneration. When the resin capacity is spent, the system is regenerated by flushing it with a concentrated salt brine solution, which forces the captured arsenic ions off the resin and restores the chloride charge.
Targeted Drinking Water Solutions (Point-of-Use)
Treating only the water used for drinking and cooking is a practical and cost-effective approach accomplished using Point-of-Use (POU) systems. The most widely used POU method for arsenic removal is Reverse Osmosis (RO) filtration. These compact units are typically installed under the kitchen sink, supplying purified water to a dedicated faucet.
Reverse Osmosis works by forcing water pressure through a semi-permeable membrane with extremely small pores. The RO membrane allows only pure water molecules to pass through, leaving behind larger dissolved solids, including arsenic ions. This method is effective because it is a physical separation process, making it less dependent on the chemical state of the arsenic than adsorption or ion exchange.
While RO is effective at removing contaminants, it is a slow process that produces a continuous stream of wastewater containing rejected impurities. Since the system is concentrated on a single faucet, the total volume of treated water is small, making it a viable option when whole-house treatment is cost-prohibitive.
Choosing, Operating, and Maintaining Your System
The decision between a whole-house (POE) and a targeted (POU) system depends on the initial arsenic concentration, the household’s budget, and water usage patterns. POE systems offer comprehensive protection but involve higher upfront costs, greater maintenance complexity, and high energy consumption. POU systems, such as under-sink RO units, are more affordable to install and maintain but only treat a small portion of the home’s water supply.
Beyond cost, the water’s chemistry is a factor in selecting the appropriate technology. Water high in sulfate or fluoride may rapidly exhaust an anion exchange or activated alumina system due to competitive binding. Systems should be certified to NSF/ANSI standards, such as Standard 53 or 58, which verify performance in reducing arsenic to safe levels.
Long-term maintenance involves regular monitoring and timely replacement of filter media or cartridges according to the manufacturer’s schedule. Failing to replace exhausted media can lead to “breakthrough,” where captured arsenic is released back into the water at high concentrations. For regenerable systems, such as anion exchange, the concentrated brine solution containing the stripped arsenic must be managed carefully. Spent solid media, like activated alumina, must undergo testing using the Toxicity Characteristic Leaching Procedure (TCLP) to determine if it should be disposed of as non-hazardous or hazardous waste.