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Arsenic is a naturally occurring carcinogen found in groundwater across the US. Long-term exposure causes cancer, cardiovascular disease, and developmental problems in children. Testing is essential as arsenic is undetectable by taste or smell.
Arsenic is a naturally occurring element found in rocks, soil, water, air, and living organisms. It exists in both organic and inorganic forms, with inorganic arsenic being the more toxic variety and the primary concern in drinking water. Arsenic has been used historically in pesticides, wood preservatives, and various industrial processes, though most of these uses have been banned or restricted.
Unlike many water contaminants that result from human pollution, arsenic in groundwater often comes from natural geological sources. As groundwater flows through arsenic-bearing rock formations, it can dissolve arsenic into the water. This process has occurred over millions of years, and certain regions have naturally high arsenic levels due to their specific geological makeup.
Arsenic contamination is a global health issue. The World Health Organization estimates that over 140 million people worldwide drink water containing arsenic above safe levels. In the United States, an estimated 2.1 million Americans are exposed to arsenic levels exceeding the EPA limit, with millions more exposed to levels that may pose long-term health risks.
Silent Threat: Arsenic is completely colorless, tasteless, and odorless at drinking water concentrations. You cannot detect it through your senses. Testing is the only way to know if arsenic is present in your water. This invisible nature makes arsenic particularly dangerous because contamination can go undetected for years.
The more toxic and harder-to-remove form of arsenic. Commonly found in oxygen-poor (reducing) groundwater conditions, often associated with deeper wells.
The oxidized form of arsenic, generally easier to remove through treatment. Common in oxygen-rich (oxidizing) conditions, often associated with shallower wells.
The type of arsenic present affects treatment effectiveness. Many treatment systems work better on arsenate than arsenite. For water with high arsenite levels, pre-oxidation (using chlorine, ozone, or aeration) can convert arsenite to arsenate for more effective removal. Laboratory testing can identify which form is present.
Arsenic is classified as a Group 1 carcinogen by the International Agency for Research on Cancer (IARC), meaning there is sufficient evidence that it causes cancer in humans. Health effects depend on exposure level, duration, and individual factors including genetics, age, and nutritional status.
Arsenic-related health effects often develop slowly over years or decades of exposure. Cancers may not appear until 20-40 years after initial exposure. This long latency period means that current exposure may not cause symptoms for many years, making prevention through testing and treatment essential.
The EPA lowered the arsenic limit from 50 ppb to 10 ppb in 2006, acknowledging the significant cancer risk at higher levels. The previous standard had been in place since 1975 and was based on outdated science. Even the current limit is considered by many experts to be too high, as studies show increased cancer risk at levels as low as 5 ppb.
Many health experts and organizations argue that the 10 ppb standard is still too high. The Natural Resources Defense Council and other groups have called for lowering the limit to 3-5 ppb. Studies from Bangladesh and other highly exposed populations have shown clear health effects at levels below 10 ppb. The EPA acknowledges that the MCLG of zero reflects the fact that there is no known safe level.
Arsenic occurs naturally in certain rock formations, particularly volcanic deposits and sedimentary rocks. As groundwater flows through these formations over thousands of years, it dissolves arsenic into the water. This is the primary source of arsenic in most affected US regions, especially the Southwest.
Historical and current mining operations, particularly for gold, copper, and other metals, can release arsenic into groundwater. Mine tailings and waste rock often contain elevated arsenic levels that leach into water supplies over time.
Lead arsenate and other arsenic-based pesticides were widely used in orchards and cotton fields through the mid-20th century. Contaminated soil continues to release arsenic into groundwater decades later, particularly in former agricultural areas.
Chromated copper arsenate (CCA) was widely used to preserve wood until 2003. Old decks, playground equipment, and other structures may leach arsenic into surrounding soil and potentially groundwater.
Glass manufacturing, semiconductor production, and other industrial processes use arsenic compounds. While regulated, historical contamination may persist near industrial sites.
Groundwater sources are much more affected than surface water. Private wells drawing from bedrock or deep aquifers are at higher risk than shallow wells or municipal systems using surface water. However, some municipal systems in high-arsenic areas have also exceeded limits. Check your water utility's Consumer Confidence Report or test your private well.
Cost: $20-100
Cost: $15-40
Below 5 ppb: Generally considered low risk, but zero is the goal
5-10 ppb: Below EPA limit but elevated; consider treatment, especially for vulnerable populations
Above 10 ppb: Exceeds EPA limit; treatment required; do not use for drinking or cooking
Effectiveness: 95-98% reduction
The most practical and effective option for home use. Point-of-use RO systems installed under the kitchen sink treat drinking and cooking water. Removes both arsenite and arsenate effectively. Requires regular membrane and filter replacement.
Cost: $200-600 for point-of-use systems
Effectiveness: 90-95% reduction (for arsenate)
Uses specialized resin to remove arsenate. Less effective for arsenite unless pre-oxidation is used. Can be whole-house or point-of-use. Requires periodic regeneration or resin replacement.
Cost: $500-2,000+ depending on size
Effectiveness: 85-95% reduction
Aluminum oxide media adsorbs arsenic as water passes through. Works best for arsenate and requires optimal pH (5.5-6.0). May need pH adjustment for best performance. Media requires periodic replacement.
Cost: $400-1,000 for point-of-use systems
Effectiveness: 95%+ reduction
Boils water and condenses the steam, leaving arsenic behind. Very effective but slow, produces limited quantities, and uses significant energy. Best for small volumes.
Cost: $100-400 for countertop units
Effectiveness: 90-99% reduction
Newer technology using iron oxide or iron hydroxide media. Effective for both arsenite and arsenate. Growing option for point-of-use and whole-house treatment.
For most homeowners, a point-of-use reverse osmosis system offers the best combination of effectiveness, reliability, and cost. Consider having your water tested for arsenic speciation (arsenite vs arsenate) to help select the most effective treatment. If arsenite is predominant, some systems may benefit from pre-oxidation. Test your treated water periodically to ensure the system is performing properly.
Arsenic absorption through skin during normal bathing is minimal and not considered a significant exposure route. The primary concern is ingestion through drinking water and food preparation. However, if you have very high arsenic levels, avoid soaking in bath water for extended periods and do not swallow any water while bathing.
Plants can absorb arsenic from irrigation water and accumulate it in their tissues, particularly in leaves and roots. Rice is especially efficient at arsenic uptake. If your water has elevated arsenic, consider using an alternative water source for food gardens, or focus on fruits (which accumulate less arsenic than leafy vegetables or root crops).
Not necessarily. Arsenic distribution depends on local geology, and deeper wells may actually have higher arsenic levels in some areas. Deep bedrock wells in New England, for example, often have higher arsenic than shallow wells. Consult with a local hydrogeologist or well driller who knows your area's specific conditions before drilling deeper.
Follow manufacturer recommendations, which typically range from 6 months to 2 years depending on the system type and water usage. Test your treated water periodically (every 6-12 months) to verify the system is still performing effectively. Filter capacity depends on incoming arsenic levels and water usage.
Yes, arsenic in food can contribute to total exposure. Rice and rice products are particularly high in arsenic. Apple juice, some wines, and seafood can also contain arsenic. While food arsenic is a valid concern, drinking water is typically the larger exposure source for people with contaminated wells. Address water contamination first, then consider dietary sources.