Arsenic is a naturally occurring metalloid found in soil and rock formations throughout the United States. It dissolves into groundwater through the natural weathering of arsenic-containing minerals — a process that happens independently of any industrial activity, though industrial sources can add to natural background levels. In Illinois, arsenic levels in groundwater are elevated in a number of areas relative to national averages, particularly in deeper aquifer systems.

Industrial and agricultural sources can also contribute arsenic to water supplies. Certain pesticides historically used in farming contained arsenic compounds, and these can persist in soil and leach into groundwater decades after use. Wood preservatives and certain industrial processes are additional potential sources in areas with relevant industrial history.

The EPA's MCL for arsenic in drinking water is 10 parts per billion, a standard set in 2001 that represented a significant tightening of the previous 50 ppb limit. The revision was controversial at the time because of the compliance costs it imposed on small water systems, but the scientific basis was well-established — arsenic is a known human carcinogen, and research had clearly shown that the previous standard was inadequately protective.

It's worth noting that some health researchers consider even 10 ppb to carry residual cancer risk with long-term consumption. The standard represents a regulatory balance between health protection and feasibility, not a threshold below which arsenic is considered completely safe. Arsenic naturally present in private well water can vary considerably from the municipal supply in the same area, which is one reason well owners should test independently of any area-wide assessment.

Arsenic is classified as a known human carcinogen by the EPA, the International Agency for Research on Cancer, and the National Toxicology Program. Long-term exposure to arsenic in drinking water has been most strongly linked to bladder, lung, and skin cancers. Kidney and liver cancers have also been associated with arsenic exposure in epidemiological studies.

Non-cancer effects of chronic arsenic exposure include skin damage (discoloration, hyperkeratosis), circulatory system problems, and neurological effects. As with most chronic contaminants, the risk is cumulative — it's the total lifetime exposure that matters, not any single measurement.

Arsenic in drinking water is effectively treated by several technologies. Reverse osmosis is the most practical point-of-use approach and achieves high removal rates for both the trivalent (arsenic III) and pentavalent (arsenic V) forms. Activated alumina filtration is also effective and is commonly used in point-of-use and point-of-entry applications specifically targeting arsenic. Iron-based adsorption media is another option, particularly for whole-home applications.