Groundwater protection is vital to states like Wisconsin, where two thirds of the people draw their drinking water from underground aquifers.  Unfortunately, these waters face several threats:
 

1. Excessive Withdrawals --- In several areas, groundwater levels have dropped dramatically because high-capacity wells are pumping too much water and the aquifers aren't being given enough time to recharge naturally.   In urban areas, recharge is difficult because much of the land surface is covered with pavement and roofs, causing rainwater and snowmelt to flow away rather than seep into the ground.   In Wisconsin, our regulation of groundwater quantity is extremely weak and has resulted in major concerns when large water withdrawals are attempted, such as a recent proposal by Perrier for a bottled water plant.   Despite fears that Perrier would damage local spring-fed streams and private wells, the Wisconsin DNR did not have the authority to stop the project.  Perrier left because of public opposition, but still owns property in a sensitive area and is likely to try again.  Our groundwater law loopholes must be closed and a groundwater conservation plan enacted. 2. Toxic Contamination --- In some localities, our groundwater becomes contaminated because of insufficient topsoil layers to filter rainwater as it trickles down to recharge the groundwater. Livestock manure, human sewage sludge, fertilizers and pesticides can also seep down into groundwater supplies.

In some areas of Northeast Wisconsin, contaminated surface runoff can directly enter underground water supplies through fractured rock outcroppings (see below), sink holes, quarries and abandoned wells. To make matters worse, large areas of Northeast Wisconsin suffer from naturally high levels of toxic minerals and contaminants - such as arsenic, lead, flouride, iron and radium - in certain layers of the underground aquifer.  When these toxic layers are drilled through or pumped, the contaminants can spread into clean aquifer layers resulting in wider groundwater contamination problems. 

Fractured Limestone in Northeast Wisconsin

Open most basic groundwater textbooks and you’ll find hydrogeology discussed in terms of porous media, with aquifers composed of well-sorted sand, and groundwater moving through pores between sand grains. In the dolomite terrain of northeastern Wisconsin, however, the story is much different. Over much of Wisconsin’s Door Peninsula, fractured dolomite is exposed at or near the land surface. Rain or snow falling on this landscape enters the groundwater system through an interconnected network of vertical and horizontal bedrock fractures. Once in the aquifer, the water flows laterally, through horizontal fractures, until it discharges to local lakes, springs or streams or is captured by water-supply wells. 

Open most basic groundwater textbooks and you’ll find hydrogeology discussed in terms of porous media, with aquifers composed of well-sorted sand, and groundwater moving through pores between sand grains. In the dolomite terrain of northeastern Wisconsin, however, the story is much different. Over much of Wisconsin’s Door Peninsula, fractured dolomite is exposed at or near the land surface. Rain or snow falling on this landscape enters the groundwater system through an interconnected network of vertical and horizontal bedrock fractures. Once in the aquifer, the water flows laterally, through horizontal fractures, until it discharges to local lakes, springs or streams or is captured by water-supply wells. 

In such systems, rapid groundwater movement and minimal contaminant attenuation are common, and so the land-use practices in the areas where the water originates — often called the capture zones or contributing areas — highly influence the quality of groundwater produced by local wells.

Determining these capture zones and understanding how groundwater moves from recharge to local wells are critical to protecting groundwater in fractured-rock terranes. 

The Wisconsin Geological and Natural History Survey (WGNHS) has carried out a series of field-based, hydrogeologic research investigations with the goal of improving our understanding of fluid movement in fractured carbonate rocks throughout Wisconsin. The other aim of this research is to find the best methods for protecting groundwater and making sure contaminants are not carried to the wells that supply drinking water. Undertaking wellhead-protection studies in fractured-rock settings is a challenging endeavor, yet it is in these very vulnerable settings that wellhead-protection programs are most essential. 

The vertical and horizontal fractures in this Silurian dolomite in Door County are typical of the fractures that underlie the county and affect the groundwater. Photos courtesy Wisconsin Geological and Natural History Survey, University of Wisconsin-Extension.

The focus of our work is the Silurian dolomite aquifer of Door County, in northeastern Wisconsin. A rocky peninsula between Lake Michigan and Green Bay, Door County has rugged shorelines, mild climate, abundant natural resources and a small-town feel that together have made it 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 U.S. Environmental Protection Agency and Wisconsin 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. Other groundwater contaminants include agricultural chemicals, pesticide residues from cherry and apple orchards, and petroleum and other non-aqueous phase liquids such as gasoline and solvents. 

Much of Door County’s charm, and its groundwater problems, are directly related to its unique geology — a combination of Paleozoic bedrock and Pleistocene modifications. Silurian-age dolomites form the backbone of the peninsula and dip gently eastward into the Michigan Basin. In the Late Pleistocene, continental glaciers covered the area, and, when they retreated, left behind a fascinating 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, the 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. 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. 

Over the past 20 years, WGNHS has carried out a number of scientific studies of the hydrogeology of Door County at the request of, and with substantial funding from, local, state and even national regulatory agencies. Each of these studies has involved the collection and analysis of new field data and has included extensive cooperation with local landowners, citizens’ groups and decision makers. Each project builds on previous work and has led to a broader understanding of fractured-rock hydrogeology that WGNHS scientists have shared in national and international venues. For example, Maureen Muldoon, now at the University of Wisconsin-Oshkosh, carried out detailed groundwater monitoring and tracer experiments in an active dolomite quarry, producing some of the most detailed data on dolomite hydrogeology ever collected.

A fast trip to the Bay

The City of Sturgeon Bay (population 9,100) lies midway up the Door Peninsula and is the county seat as well as an important industrial and recreational center. Sturgeon Bay’s water supply comes entirely from municipal wells drawing water from the fractured dolomite aquifer. In 1998, the Sturgeon Bay Water Utility, with support and funding from the Wisconsin Department of Natural Resources, requested that the WGNHS provide assistance in delineating the capture zones for its municipal wells in support of a wellhead-protection plan. The motivation for this project was twofold: to assist the City of Sturgeon Bay and the county, and to gain a better understanding of how groundwater moves and of techniques for delineating the capture zone. This understanding could be applied to other fractured carbonate aquifers in the state. 

Since the turn of the century, the city has installed 12 municipal wells within the city limits; nine of these wells have shown signs of bacterial contamination, and seven of the wells have been shut down and abandoned. Currently the city operates five wells. Water from three of these wells is disinfected on site. Only two wells have remained free of bacterial contamination. 

We used our observations and field measurements from the Sturgeon Bay area to create a model showing how rapidly groundwater moves near the city. The existing wellhead protection guidelines are based on a five-year travel time from infiltration at the land surface to capture by the municipal wells. But we found that travel times to municipal wells are so rapid — less than two years — and distances traveled are so large — up to 10 kilometers — that the traditional criteria for wellhead protection are meaningless.

Vertical fractures visible in an alfalfa field, central Door County, reflect fractures below in the carbonate rocks. The alfalfa grows greener and more vigorously above sediment-filled fractures. 

The predicted short travel times of the flow system are consistent with our conceptual model and with numerical modeling results. Additional geochemical and isotopic data collected for this study are consistent with the results of those models. These isotopes can act as effective natural groundwater tracers and should be used routinely in groundwater studies in fractured carbonate rocks.

Armed with the technical understanding that municipal well water is coming from several kilometers away and from outside the city limits, the city, water utility and county officials are working together to develop a wellhead-protection plan that will identify and perhaps reduce potential contaminant sources at the land surface in the contributing areas for the city wells.

(from information provided by the University of Wisconsin - Madison)

Links to More Groundwater Information

Groundwater Management Program - Wisconsin Dept. of Natural Resources

http://www.dnr.state.wi.us/org/water/dwg/gw/index.htm
 

Groundwater - U.S. Environmental Protection Agency

http://www.epa.gov/ebtpages/wategroundwater.html
 

Wisconsin Groundwater - U.S. Geological Survey

http://wi.water.usgs.gov/

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