MAPPING CARBONATE BEDROCK SURFACES IN GLACIATED LANDSCAPES

Land application of agricultural, municipal, and industrial wastes in carbonate bedrock landscapes covered with glacial deposits poses a threat to groundwater quality if application rates are inappropriate or inappropriately timed. Contaminant attenuation is limited in carbonate bedrock, and the thickness and texture of the overlying unlithified materials are critical for attenuating contaminants in percolating waters. Unfortunately, most existing maps do not provide sufficient detail for field-scale waste-disposal decisions. Detailed soil surveys provide textures of materials to a depth of approximately 2m; maps of glacial material completed at scales of 1:100,000 do not provide sufficient detailed information on texture or thickness of unlithified materials for field-by-field decisions regarding land applications of wastes. Careful examination of well construction reports (WCRs), combined with geostatistical modeling can provide good information for land-use decisions, as demonstrated in Calumet County in northeastern Wisconsin. From over 4000 WCRs, 1900 were precisely located and utilized as surrogate geologic logs to construct a best-fit statistical model of the “depth to bedrock” using universal kriging in ArcGIS Geostatistical Analyst. This model was used to produce three-dimensional representations of the thickness of unlithified material between bedrock and the land surface. The identification and delineation of internally drained watersheds reveals large upland areas draining to lower lying lands with very thin unlithified deposits, and in these areas contaminated wells are common.Where glacial features are comprised of bedrock at their core, initial seismic refraction data indicate depth to bedrock may in fact be even less than indicated by the models. All these factors contribute to internally drained landscapes exhibiting extreme variability in susceptibility to groundwater contamination by surface pollutants. As a result of this study, maps can be generated at scales appropriate for decision makers involved in land application of wastes.