ARSENIC IN GROUNDWATER IN SOUTHEASTERN WISCONSIN: SOURCES OF ARSENIC AND MECHANISMS CONTROLLING ARSENIC MOBILITY

Moderately high arsenic concentrations occur in groundwater in glacial and upper bedrock aquifers throughout southeastern Wisconsin. There is no evidence of anthropogenic sources of arsenic in the area. The objective of this study is to determine the source(s), and geochemical controls on the mobility, of the naturally occurring arsenic in this region. Approximately 10% of the wells in the study area have arsenic concentrations above the new U.S. Environmental Protection Agency drinking water standard of 10 ppb. Maximum arsenic concentrations are on the order of 80 ppb. Detailed chemical, mineralogical, and hydrogeological studies are being conducted near the city of Lake Geneva where a cluster of arsenic-impacted wells occurs. Geochemical analysis of 330 ft of rotosonic core from Lake Geneva shows arsenic concentrations ranging from less than detection to 5 ppm in most samples. Higher concentrations of arsenic occur in an organic horizon in the glacial aquifer (21 ppm) and in weathered bedrock (15 ppm) at the base of the glacial aquifer. Based on these data, there are three possible sources of arsenic in the core 1) organic material, 2) weathered bedrock, 3) low levels of arsenic dispersed throughout the clay and silt units in the glacial deposits. X-ray diffraction is being used to identify potential arsenic-bearing minerals in core samples. Selective extractions on core samples are being conducted to further investigate the solid phase associations of arsenic. Groundwater chemistry data show that dissolved arsenic (<0.45 um) is dominantly As (III) (65%-100%). Up to 65% of the total arsenic measured in groundwater samples is in the particulate (>0.45um) fraction. Time series sampling during a 24-hour pump test in an arsenic-impacted well showed relatively little change in water chemistry, suggesting that processes local to the well bore have little influence on arsenic concentrations. Our geochemical, mineralogical, and groundwater chemistry data will be summarized, and hypotheses regarding the cause(s) of arsenic contamination in southeastern Wisconsin will be discussed.