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Groundwater study at the Bog charts the invisible

The approximate size and shape of the Cedarburg Bog’s watershed are easy to determine. A lot is known, too, about its water chemistry and about the buffering min¬erals that underground springs deposit in the water, minerals that keep its pH (the measure of its acidity) largely in the neutral zone.

In the fall of 2011, the Friends of the Cedarburg Bog received a grant from the Wisconsin Coastal Management Program to chart the invisible – the supply of groundwater that so pro¬foundly influences the Bog’s ecology. How big is the groundwater “pool” that directly influences the Bog? How deep is it? Which way is it flowing? What’s the shape of the rock layer beneath it?

Supporting scientific studies to protect the Bog is a major goal of the Friends. So, working with the Wisconsin Geological and Natural History Survey, the Wisconsin Department of Natural Resources, Ozaukee and Washington Counties, and the University of Wisconsin-Milwaukee the Friends undertook a year-long study to try to answer these questions.

The project is being managed by Joanne Kline, longtime Bog Friend and adviser and an Environmental Analysis and Review Specialist with the DNR.

Land use and development in areas where groundwater is recharged have the potential to change the quality and quantity of water reaching the Cedarburg Bog. Determining how groundwater flows to and through the Bog and identifying the groundwater recharge areas is essential to protecting this critical habitat.

Some of the water that feeds the surface systems through runoff, rainfall or snowmelt sinks into the soil and ends up as groundwater. Moisture that doesn’t stick to soil particles or get taken up by plants may continue to travel down through the dirt to collect in a saturated zone called the water table. The water table surface in this area has a slight slope so that most of the groundwater that reaches the Bog comes from the north and west.

The location and quality of the Bog’s groundwater is very important to the plants and animals that live there and may explain why the federally-endangered Hines Emerald dragonfly chooses this spot as one of the three locations it is found within the state. Another rare species of particular interest is the eastern prairie white-fringed orchid.

The first goals were to map the water table and determine the path(s) of the groundwater flow. Several residents of the 42-square-mile study area allowed their wells to be tested, data on water levels and stratigraphy were collected from 800 water supply wells, along with historic accounts from drillers’ logs. Stratigraphic studies confirm the types of rocks present and their layering, and the data that have been collected will result in a 3-D map of the water table and of the different kinds of rock layers under and around the Bog.

Mild weather allowed researchers an early start in 2012, and groundwater data study points were established across the study area. Beginning in March, students of UWM Geosciences Professor Bill Kean measured the thick¬ness of various ground layers using electromagnetic induction and electrical resistivity. Weak electric currents are sent into the ground and the differences in resistance from point to point are recorded. The differences can be used to create contour maps of the rock lay¬ers.

By mid-summer, new permanent monitoring wells had been drilled at several sites on the north border of the Bog and temporary “push piezometers” were installed in the String Bog, Watts Lake, and Long Lake. A piezometer is a tube that allows scientists to measure groundwater pressure and movement. Mystified fishermen at Watts Lake watched as yards and yards of PVC pipe were sunk into the muck to make a piezometer.

In addition, geological data were analyzed in the area that is the suspected habitat of the Hine’s Emerald dragonfly. The naiad (immature stage) of the Hine’s Emerald is a habitat specialist that lives for two to four years in certain kinds of cool, shallow, spring-fed marshes and sedge meadows. Under¬standing groundwater characteristics where the naiads are known to occur guides the search for other populations.

And the next steps? The “muscle work” is over, and the numbers are being crunched. The study will give the Friends a much more accurate picture of the shape of the Bog’s basin and the direction of flow in the Bog, and a better idea of the chemical composition of its water. An extension of the study might lead to a groundwater flow model that would have broad application to groundwater planning in Southeastern Wisconsin.

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