Assessment of Environmental Impacts of Geothermal Source Heat Exchangers

Many buildings and homes are heated and cooled by geothermal systems in Wisconsin, where the geothermal industry has experienced exponential growth over the last two decades. Large-scale versions of the system have recently been installed in Dane County, including the largest Geothermal Source Heat Exchanger field in the nation. Clearly, geothermal systems have acquired a potential to conserve a significant amount of energy and reduce the current rate of greenhouse gas emissions immediately. It remains only to be demonstrated that geothermal systems can be environmentally sound and sustainable over the long term; consequently, to partially meet that need, we propose to address two significant environmental concerns: (i) the potential release of arsenic into groundwater as a result of temperature increases in a geothermal field and (ii) the subsequent increases in the temperature of surface water in trout streams. Using computational tools, we propose to estimate the effects of a large-scale geothermal field by quantifying the amount of heat transferred from vertical heat exchanger arrays and also to determine both the direction and the rate of groundwater flow. At selected wells, we will monitor groundwater temperature continuously and also arsenic concentration. The proposed modeling and field studies will assess the presence, concentration level and spread of the thermal and arsenic pollutants produced by ground source heat exchangers. The results of the study should help in any effort to create regulatory guidelines for dealing with the threats these outcomes may pose to humans and environment. In addition, the assessment protocol to be developed using computational tools could also be employed to plan, develop and operate sustainable geothermal energy systems in Wisconsin and elsewhere.