OCCURRENCES OF ARSENIC IN INDIANA GROUND WATER

Beginning in 2008, the Indiana Department of Environmental Management (IDEM) Drinking Water Branch, Ground Water Section has collected ground water samples across the State of Indiana as part of a Statewide Ground Water Monitoring Network (GWMN). The GWMN seeks to statistically determine background ground water quality in hydrogeologically-defined settings of the state. The sites sampled as part of the GWMN consist of non-community public water supply and private residential drinking water wells.

Since 2013, 1162 samples were collected from unique sample sites randomly distributed within twenty generalized hydrogeologic settings across the state, based on a stratified sampling procedure. The sample locations were selected in an effort to represent geographic areas of the state, as well as hydrogeologic sensitivities and generalized geologic settings. The samples were analyzed for around 200 parameters; including alkalinity, anions/cations, metals, nitrate-nitrite, synthetic organic compounds, volatile organic compounds, and unregulated pesticide degradates.

Dissolved arsenic was detected in 44% of the samples and exceeded the EPA Maximum Contaminant Level (MCL) of 10 ug/L in nearly 11% of the samples. 119 of the 127 samples that exceed the arsenic MCL were collected in Indiana’s glaciated area north of the Wisconsin Glacial Boundary. In general, the hydrogeologic settings that were rich in glacial till had higher levels of arsenic. Ground water samples showing reducing redox conditions (with negative field ORP values) showed significantly higher levels of Arsenic (n=847, average of 5.30 ug/L) than samples with oxidizing conditions (n=315, average 1.61 ug/L) (Mann-Whitney U=70,584, p<0.05). Wells screened in unconsolidated material had a significantly higher average arsenic concentration (n=812, average 4.77 ug/L) than bedrock wells (n=348, average 3.23 ug/L) (Mann-Whitney U=123,250, p<0.05). The highest average arsenic levels were found in wells screened between 100-150 ft below the ground surface, although high levels were found at all depths sampled in this study.

Additional sampling is being conducted at previously sampled sites to determine the geochemical species of the arsenic (As III vs. As V) to further examine the relationship between arsenic mobility and geochemical conditions.