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Appendix B Groundwater Recharge and Discharge on Wisconsin's Door Peninsula Based on a talk given at the workshop by Kenneth R. Bradbury Wisconsin Geological and Natural History Survey The workshop on "Groundwater fluxes across interfaces" was held in Egg Harbor, Wisconsin, which is situated on the Door Peninsula (Figure B-1~. The hydrogeology of the Door Peninsula illus- trates the complexity and importance of flow across interfaces - groundwater recharge and discharge processes. A rocky peninsula between Lake Michigan and Green Bay, the Door Peninsula is character- ized by rugged rocky shorelines and sandy beaches. The scenery along with the mild climate, abundant natural resources, and a small-town feel have made the Door Peninsula one of the most popular tourist destinations in the midwestern United States. All residents of the county depend on groundwater, but groundwater quality problems have plagued the county for many years. Bacteria and nitrate exceed drinking water standards in about 30 percent of the private wells in the county, and private well owners often report turbid or muddy water in their wells during certain times of the year (Bradbury and Mul- doon, 19921. Other groundwater contaminants include agricultural chemicals, pesticide residues from cherry and apple orchards, and petroleum and other non-aqueous phase liquids. Much of the charm of the Door Peninsula, and its groundwater problems, are directly related to its unique geology - a combination of Paleozoic bedrock and Pleistocene modifications. SiTurian-age dolomites form the backbone of the peninsula and dip gently eastward into the Michigan Basin (Figure B-2~. In the Late Pleistocene, continental glaciers covered the area, and when they retreated they left behind a unique landscape. On the western side of the county the Silurian escarpment forms high cliffs along the Green Bay shoreline; only a few miles to the east the land meets Lake Michigan with sandy beaches and diverse wetlands. In between, in the uplands of the county, glaciers removed most of the soil, so that in most places the bedrock is less than two meters below the surface and in many places it is exposed at the land surface. The dolomite contains both near-horizontal and vertical fractures (Mul- doon et al., 2001~. These fractures are extensive, and the vertical Fractures are easily visible from the air, particularly under alfalfa fields in dry weather. The combination of thin soils and fractured rock makes groundwater in the county extremely vulnerable to contamination. Groundwater flow in the Door Peninsula is conceptually simple, with recharge occurring in the uplands along the crest of the peninsula, and discharge occurring in springs and seeps along the Green Bay and Lake Michigan shorelines. In detail, however, both the recharge and discharge processes are 71
72 Apperld~c B G:reen: B~ >',~ 2 ~~ 5~ ) av ~ lie H ~ ~..~.CMQ.~.r.,~...,..,C..,~o~.,. am. A: -- - N ~ - in.. at.: ~ Lake MI~GhI~9a:n ~ ~2- 4: :.~: 8:: :10~ Miles ~17 H I 1~ Q 2~:4::6::~:10 Kilometers 1_ H H 1 Canada ~ I FIGURE B-1 Location of the Door Peninsula in eastern Wisconsin, extending out into Lake Michigan. E.H. is Egg Harbor, the site of the workshop. SOURCE: Adapted from Kenneth R. Bradbury, Wisconsin Geological and Natural History Survey, written commun., 2002. ~ -D- ~ quite complex. Groundwater recharge is rapid, transient, and often focused in discrete depressions where open vertical fractures or solution features occur at the land surface. Most annual recharge occurs over a few weeks following spring rains and snowmelt; during this period groundwater levels often rise by tens of meters. Thermal and geochemical data collected during specific recharge events show that recharge water can move from the land surface to tens of meters below the water table in a matter of hours or days. Once in the aquifer, groundwater flow is generally horizontal along bedding-plane fractures that have been widened by solution (Rayne et al., 2001~. Flow rates in this fractured dolomite can be extremely rapid - on the order of 10 km/yr. These rapid flow rates mean that local private and municipal water supply wells are extremely vulnerable to contamination from surface sources, and that contributing areas for local wells can be very large. Groundwater not captured by wells in the Door Peninsula discharges to discrete shoreline springs, local wetlands, and as diffuse flow through offshore lake sediments. In general, springs occur where conductive horizontal or vertical factures intersect the shoreline. Locally, hydraulic gradients in the discharge zone can be high enough to create flowing wells, sand boils and measurable thermal anomalies,
Appendix B 73 Cross se~ion:.thro.u~g~h~th:~.e. Door Penins:u:la. recharge Green l l l :La~ke Michi:g~a:n:- "sandstone"~: aqu~ifer discharge idea FIGURE B-2 West-to-east cross section of the Door Peninsula. SOURCE: Kenneth R. Bradbury, Wisconsin Geological and Natural History Survey, written common., 2002. but over large areas of the lakebed the upward groundwater discharge through lake sediments is slow and difficult to measure. In this environment, closing the water budget - balancing recharge processes with dis- charge processes - is extremely challenging, yet understanding recharge and discharge processes is essential for solving local groundwater contamination problems and for making wise land-use management decisions for the area.