ANALYZING GROUNDWATER DYNAMICS BELOW A CARLETON COLLEGE GEOTHERMAL WELL FIELD USING DISTRIBUTED TEMPERATURE SENSING (DTS)

Many colleges and universities are considering utilizing geothermal energy (in the form of ground-source heat pumps) to heat and cool campus buildings and to decrease their dependence on fossil fuels. From 2017 to 2019 Carleton College, in Northfield, Minnesota, installed 305 geothermal wells in three fields as part of a transition from fossil-fuel-driven steam-based climate control. These fields were designed based on the static thermal properties of bedrock and surficial units, not taking into account groundwater flow. However, the amount of groundwater flow beneath these well fields is largely unknown (though presumably high, especially in cavernous karst formations) and highly fluctuating due to variable stratigraphy, and may have large effects on the efficiency of the geothermal system. In order to learn more about subsurface flow dynamics, the college outfitted five 520 foot deep wells with fiber optic cables to monitor the temperature of the ground adjacent to the wells using Distributed Temperature Sensing (DTS). By looking at the temperature profiles for each well, the groundwater flow underneath the well field can be estimated.

Initial data from these monitored wells show some matches between temperature and stratigraphy. Also, different parts of the temperature profiles respond differently to seasonal variations, perhaps controlled by the use of the field for heating. We are currently analyzing results to estimate the impact of groundwater flow on the temperature profiles. This study will provide relevant information for other institutions who are looking to utilize geothermal systems in lieu of traditional fossil fuel heating and cooling systems.