Ask Mr. Smarty Plants

First of all, our focus and expertise are with plants native to North America and we are in no way heating/cooling experts.  However, as I understand your question, you want to know if the heat exchange loop for your geothermal heating/cooling system will heat up the soil enough to adversely affect the native prairie plants that you want to grow on its surface.  I am not going to be able to give you a definitive answer to that question, but I can give you some thoughts about possibilities.  I have learned from GeoExchange.org that there are several ways to accomplish the heat exchange for your system:  1) horizontal ground closed loops, 2) vertical ground closed loops, 3) pond closed loops,  4) open loop system, and 5) standing column well system.  There is a good discussion of the various Ground Loop Configurations from Virginia Tech and the Virginia Department of Mines, Minerals and Energy.  I am assuming from what you have said in your question that you are using option #1—horizontal ground closed loop.  There are several variables that will affect how much heat is transferred to the soil above the ground loop. Number one is the depth that the loop is buried.  According to the GeoExchange.org sites, the trench for the pipes of the loop are usually constructed 3 to 6 feet below the surface or 4 to 10 feet according to the Virginia Tech site. The deeper they are buried, the less they will affect the surface and subsurface temperature.  Secondly, the composition of the soil and its moisture content will affect how much heat will be transferred to the surface.  The greatest heat transfer will be in the summer when you are cooling the house; but, if you use that heat from the air conditioning mode for the hot water heater in your home, it will reduce the heat transfer to the soil. You can read an  excellent discussion of all the soil parameters that affect soil temperature in Earth Temperature and Site Geology, again from Virginia Tech and the Virginia Department of Mines, Minerals and Energy.  You can see the soil maps and soil data for Milwaukee County online from the USDA Natural Resources Conservation Service (NRCS) on the Web Soil Survey (WSS) page to determine many of your particular soil's parameters.

So, you see, with all these variables there isn't a simple way to figure what the increase in soil temperature might be.  I can tell you, though, that many of the prairie grasses and plants that grow in your region in Wisconsin also grow in regions further south such as Oklahoma and Texas where the soil temperatures are higher.  You can see the  soil temperature differences in the two regions on the map, Soil Temperature Regimes of the Contiguous United States from the USDA. Wisconsin is in the "Mesic" regime while Oklahoma and most of Texas are in the "Thermic" regime.  Here are the descriptions of the two areas from the Wilkes University Center for Environmental Quality web page:

"Mesic—The mean annual soil temperature is 8° C or higher but lower than 15° C, and the difference between mean summer and mean winter soil temperatures is more than 6° C either at a depth of 50 cm from the soil surface or at a densic, lithic, or paralithic contact, whichever is shallower.

Thermic—The mean annual soil temperature is 15° C or higher but lower than 22° C, and the difference between mean summer and mean winter soil temperatures is more than 6° C either at a depth of 50 cm from the soil surface or at a densic, lithic, or paralithic contact, whichever is shallower."

There are no doubt some genetic differences between a prairie grass growing in Wisconsin and the same species growing in Oklahoma that make each better adapted to growing in its own region, but it is doubtful that those genetic differences are enough to keep them from growing in the other region.  Thus, even if the ground loop does heat the soil, there are Wisconsin prairie species that should be able to grow in a soil that is warmer than normal.

Here are examples of species that have a widespread distribution from as far north as Wisconsin to as far south as Oklahoma and Texas.  You can see their county distributions in each state by clicking on that state on the map on the USDA web page.

Andropogon gerardii (big bluestem), and the USDA distribution map

Elymus canadensis (Canada wildrye), and the USDA distribution map

Schizachyrium scoparium (little bluestem), and the USDA distribution map

Monarda punctata (spotted beebalm), and the USDA distribution map

Solidago gigantea (giant goldenrod), and the USDA distribution map

Phlox pilosa (downy phlox), and the USDA distribution map

There will no doubt be some native Wisconsin prairie plants that will not thrive in warmer soil.  You will have to experiment to see which they might be.  There are, however, many more species that grow in both the mesic and thermic temperature regimes than those shown above.  As you choose the prairie plants you want to grow on your land, you can check them against our Native Plant Database and/or the USDA Plant Database to be sure they will grow in both temperature regions.