PROTECTING PRIVATE WELLS FROM ARSENIC USING GEOLOGY AND GIS

Widespread arsenic contamination in Cerro Gordo County, Iowa’s private wells was first discovered in the 1990s. Due to the elevated arsenic concentrations, a research project was funded to correlate arsenic concentrations in well water with known geologic stratigraphic units, and to build a countywide GIS geologic model that predicts depths of stratigraphic units contributing arsenic to groundwater. From 2011-2013 a total of 372 groundwater arsenic samples were taken from 60 private wells. Additionally there were 79 rock chip arsenic samples taken from Devonian-age Lime Creek Formation and Cedar Valley Group aquifers. Sample results indicate statistically significant higher arsenic concentrations in Lime Creek Formation groundwater and rock chip samples compared to Cedar Valley Group groundwater and rock chip samples.

Using the stratigraphic-based arsenic correlation, a GIS digital geologic model was created to assist in well design by accurately predicting depths of important geologic layers in Cerro Gordo County, Iowa. The digital geologic model was generated using information available from county well logs, geologic strip logs, bedrock outcrops, and soils. The top of the model is LiDAR land surface elevation, and the base of the model is the top of the Ordovician-age Maquoketa Formation. Predicted geologic units in the model are: unconsolidated Pleistocene clay with sand and gravel, Dakota sandstone, Mississippian limestone, Devonian Aplington-Sheffield shale and limestone, Devonian Lime Creek Formation shale and limestone, and Devonian Cedar Valley Group limestone and dolomite. The model is a 30-meter resolution raster dataset. Stratigraphic units constructed for Cerro Gordo County were imaged using the “Topo to Raster” 3-D Analyst Extension in ArcMap 10.2.2. The “Topo to Raster” surface interpolation method is designed for the creation of digital elevation models (DEMs) using a combination of geospatial point, line and polygon data.