Simulation of temperature effects on groundwater flow, contaminant dissolution, transport and biodegradation due to shallow geothermal use

Abstract

The quantitative prognosis and assessment of possible impacts of temperature changes in groundwater due to the geothermal use of the shallow subsurface in urban regions requires process-based numerical models of coupled non-isothermal groundwater flow, heat and mass transport processes and biogeochemical reactive processes. This work therefore aims at developing and implementing numerical methods as well as the required parameterizations to simulate the effects of temperature increases due to heat injection in a groundwater aquifer. Parameter and process models for fluid flow, solute transport, mass transfer processes between aqueous and non-aqueous phases, and microbial growth coupled to contaminant biodegradation are expressed as functions of temperature for this purpose. The developed model is implemented in the OpenGeoSys code and applied in a set of benchmark simulations, where thermal impacts of borehole heat exchangers (BHE) are simulated in an aquifer with a TCE contamination in a residual NAPL source zone. The thermal plumes emitted by the BHEs result in a focusing of groundwater flow due to a viscosity reduction of the heated water. The local increase in groundwater flow as well as an increase in TCE solubility with temperature leads to increased TCE emissions from the source zone. At the same time, increases in microbial growth rates allow for higher TCE degradation rates by reductive dechlorination. Results of the benchmark simulations allow insights into the interactions of the individual processes and potential benefits or conflicts of geothermal use of the subsurface and natural attenuation processes at contaminated sites. Also, the benchmark simulations can be used as test cases for intercomparison and validation of reactive transport codes.