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

Abstract

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

Norovirus Outbreak Caused by a New Septic System in a Dolomite Aquifer

“…The Door Peninsula has long been known as an area having high vulnerability to groundwater contamination (Sherrill 1978;Bradbury 2003). Over broad areas of the peninsula soils are thin or absent, and groundwater recharge can be rapid, with little attenuation of surface contaminants.…”

“…Numerous past studies have characterized the dolomite aquifer in the Door Peninsula (Sherrill 1978;Muldoon et al 2001;Rayne et al 2001). The dolomite is densely fractured, and secondary dissolution has enlarged both fracture apertures and secondary porosity.…”

“…The natural gamma log (Figure 4) obtained from the restaurant well correlates to an earlier gamma log over the depth interval between 50 and 132 m in a nearby core hole (Dr-394) described in detail by Muldoon et al (2001) 15.2 to more than 27 cm. In Door County, such caliper and/or temperature "kicks" usually occur where major bedding-plane fractures intersect boreholes (Sherrill 1978), and these bedding-plane fractures or fracture zones can be traced laterally from many kilometers . In Door County, such caliper and/or temperature "kicks" usually occur where major bedding-plane fractures intersect boreholes (Sherrill 1978), and these bedding-plane fractures or fracture zones can be traced laterally from many kilometers .…”

Site Characterization in Densely Fractured Dolomite: Comparison of Methods

“…The dolomite forms a prominent escarpment along the western edge of the peninsula, adjacent to Green Bay. The Silurian strata dip gently into the Michigan Basin to the east‐southeast at ∼0.5° or 8 to 9 m/km and are underlain by Ordovician‐age shale, which forms a regionally extensive confining unit (Sherrill 1978). The dolomite is >150 m thick along the eastern shore of the peninsula and thins to the southwest.…”

“…Ground water recharge does not occur uniformly throughout the year; the primary recharge period is during spring snowmelt, and additional recharge usually occurs in the fall of the year when vegetation has gone quiescent (Rayne et al 2001). Ground water flow is characterized by recharge through vertical fractures and rapid lateral movement along horizontal high‐permeability zones (Sherrill 1978; Bradbury and Muldoon 1992) that appear to be laterally continuous on the scale of miles (Muldoon et al 2001a). Accounts of contaminant releases suggest that ground water flow rates can be very rapid.…”