Simulating the Impacts of Deep Geothermal Development on Shallow Hydrothermal Resources in a Rocky Mountain Rift Valley.

Numerical modeling has been widely used to assess the feasibility of geothermal energy development at sites across the world, but modeling applications simulating the potential impacts on shallow hydrothermal resources and surface water are relatively scarce. In this study, we apply the MODFLOW 6 groundwater energy (GWE) code to simulate fully coupled groundwater flow and heat transport in a Rocky Mountain rift valley. The site features a moderate temperature hydrothermal system with steeply dipping normal faults, fractures, and shear zones that convey upwelling geothermal water to the shallow subsurface and facilitate interaction between deep geothermal pumping and the shallow subsurface. Following calibration against a set of publicly available well water levels, streamflow observations, well and spring water temperatures, and thermal gradients, the model is used to simulate the impacts of deep geothermal development on surface water supplies and shallow hydrothermal resources, including a hot spring system. The model simulates significant hydrologic and thermal impacts of deep geothermal pumping on the shallow hydrothermal system, including large changes in groundwater levels (-1.2 to +3.1 m), temperatures (-5.6°C to +8.7°C), and groundwater flow to springs (-10.7% to +15.4%). Depletion of tributary groundwater is simulated in three scenarios (0.8-10.7% of the geothermal extraction rate), demonstrating that deep geothermal pumping can infringe upon surface water rights. Results broadly demonstrate that in rift valley systems, geologic structures conveying upwelling geothermal water can lead to surficial thermal and hydrologic impacts in response to deep geothermal pumping, highlighting the need for regulatory frameworks that integrate geothermal energy and water resources.