Arsenic in Ohio's Ground Water Ambient Ground Water Monitoring Wells

Ambient Ground Water Monitoring wells which have elevated concentrations of arsenic (defined here as at least one sample greater than 10 micrograms/liter) are plotted against the background of major aquifer types in Ohio. Of the 450 wells in the AGWMP (200 actively sampled), approximately 25 percent have at least one arsenic result greater than 10 ug/L. Within this untreated ground water data set, arsenic is found at detected levels more consistently in unconsolidated aquifers (glacial tills, sand and gravels) than in the sandstone or carbonate bedrock aquifers of Ohio. The map clearly indicates that ground water arsenic, at these levels, appears to occur in a wide band across central Ohio, roughly following the southern extent of glaciation.

The occurrence of detectable arsenic in ground water results from a series of geologic, lithologic, and geochemical controls acting on arsenic bound in the rock and sediment matrix through which the ground water moves. The result of this interaction is that ground water arsenic detections remain locally controlled, leaving accurate prediction as to where arsenic may be found in ground water necessarily vague. Nonetheless, several geochemical indicators are identified which clarify conditions conducive to the appearance of arsenic in ground water. The primary indicator is that the water is lacking in oxygen (i.e., is "reduced"). The second indicator is an elevated concentration of iron in the water (e.g., leading to "iron staining"). The presence of these two pieces of evidence suggests an increased probability that the ground water may contain detectable levels of arsenic.