Arsenic is a naturally occurring element found in rock, soil, and water throughout the Earth’s crust. It becomes a concern when it dissolves into water sources used for drinking and cooking. Due to its toxicity, even at very low concentrations, arsenic in drinking water is a global public health issue. Establishing a safe level requires rigorous scientific review and regulatory standards to mitigate long-term health risks.
Regulatory Standards for Arsenic in Water
The safety of arsenic levels is determined by enforceable government standards, which set the maximum allowable concentration in public water systems. The U.S. Environmental Protection Agency (EPA) has set the Maximum Contaminant Level (MCL) for arsenic at 10 micrograms per liter (\(\mu\)g/L), or 10 parts per billion (ppb). This standard applies to all public water systems, mandating corrective action if the level is exceeded.
The EPA also establishes the non-enforceable Maximum Contaminant Level Goal (MCLG), representing the concentration posing no known health risk. Since arsenic is a known human carcinogen, the MCLG is set at zero. The difference between the MCL and the MCLG reflects a balance between health protection and the technical feasibility of water treatment.
For people relying on private wells, the EPA standard is not legally binding; the owner is responsible for ensuring the water is safe. Although the MCL provides a strong guideline, private well water is unregulated and may contain levels above the public standard depending on local geology.
Sources of Arsenic Contamination
Arsenic enters drinking water through natural processes and human activities, primarily contaminating groundwater sources. The main source is geological weathering, where arsenic-containing minerals and rocks erode and leach the element into soil and water. This natural mobilization is pronounced in areas with specific rock formations, such as volcanic rock or sulfide mineral deposits.
Groundwater is more susceptible to high concentrations of naturally occurring arsenic than surface water sources like rivers and lakes. Human-caused (anthropogenic) sources also contribute, including runoff from historical mining operations, smelting, and the use of older arsenic-based pesticides in agriculture.
Although the use of many arsenic compounds has been reduced, the element is persistent and cannot be destroyed in the environment. Industrial processes, such as coal combustion or manufacturing, can release arsenic into the air, which settles onto land and water. However, the most widespread exposure pathway remains the ingestion of contaminated drinking water.
Health Consequences of Exposure
Health risks depend heavily on the chemical form of the element; inorganic arsenic is significantly more toxic than its organic counterpart. Inorganic arsenic compounds, commonly found in contaminated water, are classified as human carcinogens. The primary concern is chronic, long-term, low-level exposure from drinking water, which takes years to manifest serious health issues.
Chronic ingestion of inorganic arsenic is strongly linked to cancers of the bladder, lungs, and skin. Initial non-cancer effects often appear as skin lesions, such as hyperpigmentation and keratosis on the palms and soles. These dermatological changes are considered the most characteristic sign of chronic arsenic poisoning and can eventually progress into skin cancer.
Long-term exposure can affect multiple organ systems, leading to non-cancer effects. These include cardiovascular issues, such as heart disease and high blood pressure, and neurological problems like peripheral neuropathy. Arsenic exposure is also associated with the development of diabetes and various pulmonary diseases.
Children and pregnant women are particularly vulnerable, as early-life exposure can have developmental consequences. Exposure during the womb or early childhood has been linked to negative impacts on cognitive development and an increased risk of cancer later in life.
Testing Your Water and Effective Removal Methods
For individuals with private wells, regular water testing is the only way to determine arsenic presence and concentration, as these sources are not monitored by the EPA. Testing should be performed by a state-certified laboratory to provide an accurate measurement of the total arsenic concentration. It is prudent to test water at least once a year, or after any significant changes to the well or surrounding environment.
If testing reveals arsenic levels above the 10 \(\mu\)g/L standard, a suitable treatment technology must be implemented. The two common approaches are point-of-entry (POE) systems, which treat all water entering the home, and point-of-use (POU) systems, which treat water at a single tap, typically the kitchen sink.
Reverse Osmosis (RO) is a highly effective POU method, utilizing a semi-permeable membrane that can remove up to 99% of arsenic from the water. Adsorption media filtration is another effective method, often used in whole-house POE systems, which involves passing water through a tank filled with media like granular ferric oxide or activated alumina that chemically bind to the arsenic.
The effectiveness of any removal method is influenced by the arsenic species present. Arsenite (Arsenic III) is often more difficult to remove than arsenate (Arsenic V), sometimes requiring an initial oxidation step to convert it to the more easily filterable form.