SIMULATION OF SUBSURFACE TEMPERATURE FLUCTUATIONS IN RESPONSE TO RECHARGE AND SURFACE TEMPERATURE VARIATIONS IN A SYNTHETIC, MULTI-SCALE TOTHIAN GROUNDWATER FLOW SYSTEM

Interaction between surface conditions and the underlying groundwater system is an important aspect of heat transport within the critical zone, which in turn can affect nutrient cycles and other biochemical processes that impact water quality. Heat-transport simulations that include the groundwater-flow system are useful for assessing the impacts of climate change and the efficacy of human intervention to promote water quality and public health. The hydrologic simulator MODFLOW 6, which integrates a groundwater-flow (GWF) model, a groundwater solute-transport (GWT) model, and the newly developed groundwater energy-transport (GWE) model into a unified framework, is used to explore the links between groundwater temperatures in the critical zone and climate change. To this end, MODFLOW 6 is applied in this study to simulate heat transport in a synthetic, Tothian flow system that includes local, sub-regional, and regional flow cells. The Toth flow model is adapted to include both the saturated and unsaturated zones. The model is used to investigate how the propagation of heat in the subsurface is affected by lateral recharge variations due to topographical undulations, variations in surface temperature, and the presence of an unsaturated zone. Although the flow system is idealized, it incorporates some of the complexities found in real-world environments. Results suggest that the new GWE module in MODFLOW 6 is useful for understanding the effects of surface and near-surface conditions on fluctuations of subsurface temperature in complex groundwater flow systems. Potential real-world applications include regional-scale simulations that include an unsaturated zone.

Keywords: energy modeling, thermal transport, geothermal, climate change, interfaces.