Assessment of Environmental Impacts of Geothermal Source Heat Exchangers

Background/Need: A growing number of large-scale public institutions and corporations have begun to rely on ground-source heat exchangers (GSHE), used in combination with heat pumps, to heat, cool, and ventilate their interior spaces. Essentially, these systems transfer heat to and from the ground as needed by circulating a fluid through a loop of polyethylene pipe. GSHE systems rely on ground temperatures, which
remain relatively stable year round, to provide an efficient heat source in the winter and serve as a heat sink in the summer. Geothermal energy is considered clean, renewable, and sustainable, thanks to the Earth’s nearly unlimited thermal storage capacity. Most recently, large-scale GSHEs have been installed at West Madison High School, the Wisconsin Institutes of Discovery (at the University of Wisconsin-Madison), and Epic Systems Corporation. Epic’s system relies on one of the largest GSHE fields in the nation, with more than 5,000 bore shafts drilled as of 2015. There is a growing need for research to monitor whether additional levels of arsenic may be released from the bedrock, as Epic’s field adds heat to the ground adjacent to the borefield. As a number of recent studies have demonstrated, temperature increases might raise the rate of scorodite
(FeAsO4·2H2O) dissolution by up to a half of magnitude. Field data collected at the Brinton site in Floyd County, Virginia, also show that arsenic concentrations in groundwater are positively correlated with temperature. In Wisconsin, the zone of highest arsenic is near Lake Winnebago where the St. Peter sandstone forms most of the bedrock, but high arsenic concentrations have also been found in the Prairie du Chien, the bedrock layer into which Epic’s geothermal boring shafts have been drilled. If in fact enhanced scorodite dissolution occurs at higher temperatures, thereby affecting arsenic mobility, there exists a potential for harmful groundwater contamination in the geothermal field and in downstream wells, wetlands and streams.
Objectives: The purpose of this study is to address the research need associated with the potential release of arsenic into groundwater due to temperature increases in a ground-source heat exchange field. Based on the study’s outcomes, we will determine the best design and operation practices that take potential environmental impacts into account and develop a set of recommendations. The study is also designed to provide mitigation strategies that large-scale GSHE installations could use to minimize potential health risks and environmental impacts.