ALMOST EVERYWHERE: NATURALLY OCCURRING ARSENIC IN WISCONSIN’S AQUIFERS

During the past decade, researchers have expended over $800,000 in state and federal monies to investigate the sources and distribution of arsenic in Wisconsin's groundwater. This body of work shows that trace levels of arsenic are almost ubiquitous in groundwater from across the state, and that arsenic is present in a variety of geologic and hydrogeologic settings.

In the Fox River valley of east-central Wisconsin, the primary source of arsenic to groundwater is sulfide mineralization in the St. Peter sandstone. Up to 30 percent of wells in some parts of this region have arsenic concentrations greater than 5 mg/L. In areas south and west of the Fox River valley, arsenic concentrations in groundwater from the St. Peter sandstone exceed 5 mg/L in 3 to 10 percent of the wells tested.

In southeast Wisconsin, groundwater in the sand and gravel aquifer and some shallow bedrock formations is susceptible to arsenic contamination. Research at a site near Lake Geneva demonstrated that where groundwater is under sufficiently reducing conditions, low solid-phase concentrations of arsenic in Quaternary sediments are sufficient to cause arsenic in groundwater at concentrations up to 100 µg/L. Data from water-sampling programs in northern Wisconsin showed that similar geologic and geochemical conditions likely exist in shallow aquifers in other glaciated areas of the state, resulting in similar levels of arsenic in groundwater.

A limited number of water samples from other regions of Wisconsin suggests arsenic sources are also present in those places. However, little detailed hydrogeologic information exists as to the exact nature of arsenic sources and their location in the local stratigraphy. The research performed to date provides a picture of the distribution and mechanisms of release of arsenic to Wisconsin's groundwater, but Wisconsin has no requirement to test new or existing domestic wells for arsenic. Continuing efforts in the state to address this issue should include educating homeowners about testing well water for arsenic.