Door County is in northeastern Wisconsin and is an area of 491 square miles. The county forms the main body of the peninsula between Green Bay and Lake Michigan. The land surface is an upland ridge controlled by the underlying bedrock. The west edge of the ridge forms an escarpment facing Green Bay. Silurian dolomite is the upper bedrock unit throughout most of the county and is the most important aquifer. This bedrock is exposed in much of the county, particularly north of Sturgeon Bay, elsewhere, it is covered by a generally thin mantle of soil or drift. The bedrock units are divided into two major aquifer systems in Door County; the Silurian dolomite aquifer system and the sandstone aquifer system, consisting of Ordovician and Cambrian bedrock units. These two major systems are separated by the Maquoketa Shale of Ordovician age, a nearly impermeable, generally nonproductive unit. The Silurian dolomite aquifer system is itself divided into the Niagaran aquifer and the underlying Alexandrian aquifer. Water occurs in the Silurian dolomite aquifer system in two types of openings nearly vertical joints (fractures) and horizontal to slightly dipping bedding-plane joints. Vertical joints are more common in the upper part of the Niagaran aquifer. These yield small amounts of water to wells. Bedding-plane joints transmit most of the water in the lower part of the Niagaran aquifer and in the Alexandrian aquifer. The bedding-plane joints, because they are poorly interconnected, act as semiartesian conduits separated by impermeable rock. Eight water-bearing zones in generally continuous bedding-plane joints have been mapped. The dolomite is recharged from direct precipitation and snowmelt. It discharges water to pumping wells and by natural springs discharge to Lake Michigan and Green Bay and to interior lakes and streams. Wells in the Silurian dolomite aquifer system have adequate yields to meet most needs, except in the southwest corner of the county, where the dolomite is thin or absent. Transmissivity values range from a low of 4.0 feet squared per day in the Niagaran aquifer near Sturgeon Bay to more than 13 000 feet squared per day for the Alexandrian aquifer near Fish Creek. Water from Silurian dolomite is a very hard calcium magnesium bicarbonate type, with objectionable concentrations of iron and nitrate in water from some wells. Sanitary quality, as indicated by tests for total coliform bacteria, has been a chronic problem in certain areas. Concentrations of indicator organisms are greatest during or immediately after rapid groundwater recharge, with concentrations rapidly decreasing after periods of recharge. Wells close to septic systems and in areas underlain by

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 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 contamination potential in wells.

Available ground water in much of central Wisconsin is limited to discharge through wells of low yield. Aquifers that yield small amounts of water to wells include fractured crystalline rock at or near surface in the eastern part of the area, sandstone overlying crystalline rock in the southern and western parts, and glacial till that covers the area north and west of the Marshfield moraine. Many wells in crystalline rock yield less than 2 gpm (gallons per minute). About 90 percent of the wells in sandstone and most wells in glacial till yield 5-20 gpm. Outwash sand and gravel in segments of some bedrock channels, however, yield large supplies of water to wells. Wells in surficial sand and gravel in the lower valleys of major tributaries to the Wisconsin River yield as much as 450 gpm. Sand and gravel in segments of bedrock channels, many of which were delineated during this project, are covered by till or alluvium; wells in these sand and gravel deposits yield 100 400 gpm. Induced recharge to buried aquifers by infiltration of water through the beds of overlying streams is feasible at six sites within 8 miles of Marshfield. Infiltration through the streambed of Little Eau Pleine River about 7 mil^s northeast of Marshfield was about 200 gpm when the groundwater level was lowered temporarily. Additional recharge through ponding is possible at other sites. Streamflows in the area generally are not dependable sources of municipal or industrial supplies without storage. Nearly one-third of the flows measured in August 1969 were less than 0.01 cubic foot per second per square mile. Annual flows, however, would provide adequate within-year storage. Chemical quality of water in the area is suitable for most uses. Ground water is hard, contains objectionable concentrations of iron, and locally is high in nitrate content. Surface water has an average dissolved-solids content of about 100 milligrams per liter. DESCRIPTION OF AREA The area of investigation (fig. 1) in central Wisconsin comprises parts of Clark, Jackson, Lincoln, Marathon, Portage, Tsylor, and Wood Counties. The area is bounded on the east by the Wisconsin River, on the west by the west edge of the Black River basin, on the south by the south township line 22 N., and on the north by the north township line 31 N. Outside those boundaries problems of water supply are less acute than within the defined study area. The study area, 3,010 square miles, lies in parts of two of Martin's (1932) geographical provinces, the Northern Highland and the Central Plain. As 4 WATER AVAILABILITY IN CENTRAL WISCONSIN geology was outlined by Leverett (1929), Hole (1943), and Thwaites (1946). Information on the water resources of the area is included in reports by Kirchoffer (1905), Weidman and Schultz (1915), Wisconsin Bureau of Sanitary Engineering (1935), Drescher (1956), Holt (1965), and Devaul and Green (1971). COOPERATION AND ACKNOWLEDGMENTS This study is part of the statewide cooperative program of the U.S. Geological Survey and the University Extension the University of W...

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