Reducing Uncertainty in Characterization of the Vadose Zone for Modeling Groundwater Vulnerability

Groundwater is the principal source of drinking water for nearly two billion people. Modeling aquifer susceptibility to pollution is critical for implementing programs to protect groundwater quality. Such models typically involve geospatial analysis of the inter-relationships between landscape characteristics (e.g. depth-to-water, soils, aquifer properties, and recharge) that impact pollution risks. It has, however, been especially difficult to characterize the vadose zone, the unsaturated zone between the soil surface and the water table. Working in the Elkhorn River Basin, Nebraska, we demonstrate a novel GIS approach for characterizing the vadose zone such that the uncertainty in groundwater pollution risk assessment modeling is reduced. The method is implemented using standard, widely-available national and state geospatial data. The groundwater level data from 2000-2008 in the study area, retrieved by using Microsoft Excel based web-query from USGS Active Groundwater Level Network, were processed and stored in a geodatabase with uniform horizontal and vertical coordinate systems. GIS queries using an interpolated groundwater level raster layer determined the vadose zone depth at each test-hole location of the study area. The thickness of low-permeability materials within the vadose zone depth, like silt and clay, was calculated at each test-hole location, and then interpolated into the whole study area using the optimized variograph and kriging in GIS. This raster map resulting from geostatistical simulation was used to characterize the vadose zone for the aquifer susceptibility modeling. Different from the traditional qualitative approach, this one captures both horizontal spatial variability and vertical structure of the vadose zone regarding its pollution prevention properties. The results showed that the Lower Elkhorn River Basin has a thick (averages about 14 feet) low-permeability vadose zone generally associated with low pollutant-leaching potential. They also showed that the Upper Elkhorn River Basin has a very thin or no low-permeability vadose zone with high pollution risk. The resulting vadose zone layer can be fed into a DRASTIC model to assess the aquifer vulnerability.