GAUGING THE INFLUENCE OF HYDROGEOLOGY ON THE GROUND-SOURCE GEOTHERMAL SYSTEM AT BALL STATE UNIVERSITY

In 2009, Ball State University (BSU) began constructing the nation’s largest ground-source geothermal system. The district-wide system will span over 731 acres; however, the geothermal boreholes will be concentrated into two sites consisting of 3,600 boreholes known as Phase 1 and Phase 2.

There is little published research on the effects of large-scale geothermal system upon the groundwater environment or the effects of the groundwater flow environment on the geothermal field. For the Etten-Leur development in the Netherlands, computer generated models predicted significant groundwater cooling while, conversely, at Stockton College, increases in groundwater temperatures were observed. To maintain high performance coefficients, geothermal systems are designed to maintain an annual thermal balance. Due to concern over the impact of large-scale geothermal systems, a series of groundwater monitoring wells were installed in the Phase 1 fields, allowing students and faculty to collect hydrogeologic and temperature data. Since BSU initiated Phase 1 in late November 2011, the campus energy needs have been cooling dominated. As of July 2012, after constant thermal loading, a temperature increase of 4-5 C° has been observed in the center of the South Field with concurrent increases in three other monitoring wells.

Three of the monitoring wells that have increased temperatures overall are also exhibiting thermal variations correlated to changes in stratigraphic units. There is a temperature ‘spike’ between 14-19.5 m in depth in a highly transmissive zone within the surficial aquifer of glacial till and a temperature ‘dip’ at a depth of 70 m within a poorly conductive shale/clay aquitard of Ordovician age. The stratigraphy and subsequent hydrogeology in the geothermal field may be affecting the temperature profiles by either increasing or reducing heat transfer within the borehole field.