ABSTRACTDoor 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 the influx of tourists and fruit‐canning operations create additional wastes.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 bacterial, from individual waste‐disposal systems, agricultural, industrial, and municipal sources. Contamination is in zones that originate from point sources and move in the direction of ground‐water flow, either naturally or as induced by pumping wells. The contaminated areas include only a small percentage of the total ground‐water system and are separated by large areas of ground water free of contamination. Tests based on indicator bacteria suggest that the periods of highest contamination potential occur during or immediately following rapid ground‐water recharge periods. Increasing the depth of casing and pressure grouting the casing into firm bedrock are two well‐construction procedures that reduce the conta