Geology and ground water in Door County, Wisconsin, with emphasis on contamination potential in the Silurian dolomite

Door County, a recreational and fruit-growing area bordering Lake Michigan in northeastern Wisconsin, has had a long history of ground-water contamination from surface and near-surface sources. Contamination is most severe in late summer when fruit-canning operations and the influx of tourists create additional wastes. Silurian dolomite is the upper bedrock unit in the county and yields generally adequate supplies of very hard water with locally objectionable concentrations of iron and nitrate. Thin soil cover and well-fractured dolomitic bedrock give easy entry to ground-water contaminants throughout large parts of Door County. Many contaminants enter the dolomite by surface or near-surface seepage. There is little attenuation of contamination concentrations in the well-jointed dolomite, and contaminants may travel long distances underground in a relatively short time. The major source of ground-water contamination is bacteria, from individual waste-disposal systems, agricultural, industrial, and municipal wastes. Areas of the county underlain by contaminated zones include only a small percentage of the total ground-water system and are separated by large volumes of ground water free of contamination. (Woodard-USGS)