COUPLED TEMPERATURE AND GEOCHEMICAL MONITORING IN THE BALL STATE UNIVERSITY DISTRICT-SCALE, GROUND-SOURCE GEOTHERMAL HEAT PUMP

In 2015, Ball State University completed a district-scale, ground-source geothermal heat pump comprised of two district energy stations in two phases with more than 3,600 borehole heat exchangers (BHEs) to depths that range between 137-152 m. These BHEs utilize Pleistocene glacial till and the Silurian and Ordovician carbonates and shale, as well as groundwater, as a thermal reservoir. 72 monitoring points surround and lie within the BHEs fields, some as single wells and others as nests of four or five wells positioned in key stratigraphic intervals. Semi-continuous records of temperature profiles from these monitoring points have documented changes in ground temperature, in at least one case to above 30°C, and illustrate the migration of thermal plumes in relation to conduction and groundwater advection.

What happens to groundwater geochemistry during sustained geothermal loading? We seek to answer this question through strategic monitoring of field chemistry, principle ions, and stable isotopes of hydrogen, oxygen, and carbon. Samples collected from several wells and from different depths in the summer and fall of 2015 establish a baseline upon which changes can be compared. Depth of groundwater is the principle control on geochemistry, with Ca-Mg-HCO3-type waters in the shallow aquifers in the glacial till and Silurian bedrock and a trend toward Na-K-Cl-type waters in the Ordovician strata at depth. The d18O and d2H of the samples are tightly clustered and all lie along a water line with deuterium excess greater that the GMWL. Deeper groundwater has a slight positive shift in δ18O without a corresponding shift in δ2H. In the more-recently developed phase 2 BHE field, samples from wells with warmer groundwater are more depleted in 13C, although this same trend does not hold when considering the accumulated samples form the warmer groundwater in the phase 1 BHE field.