Geothermal and Hydrogeologic Controls on Regional Groundwater Temperatures

Abstract:

A 1-D analytic solution for combined heat and groundwater flow through an aquifer system accounts for geothermal heating at the base of the aquifer, recharge of cooler water along the groundwater flow path, advection of heat within the aquifer, conduction of heat through the vadose zone, and viscous heating. The 1-D solution, which uses a freely available Python script, can be applied to moderately complex geometries by solving the heat flow equation for piece-wise linear or constant properties and boundary conditions. Analysis of the Eastern Snake River Plain regional aquifer system demonstrates that viscous heating, normally neglected by numerical solutions, is variably important along the groundwater flow path, and that heat conduction to the land surface and cool recharge are the primary thermal perturbations causing deviation from a steady, slow heating along the flow path. Because viscous heating is sometimes important, a general anisotropic form of the viscous heat-generation term has been derived and can be included in more complex 2-D and 3-D numerical solvers of the coupled heat and groundwater flow equations. The 1-D solution allows quick and easy determination of whether this term needs to be included. The rate at which thermal perturbations equilibrate with distance is controlled by the Peclet Number (the ratio of advective to conductive heat transport), which can be used to estimate the distance over which thermal perturbations (e.g., cool recharge or local geothermal hotspots) will be detectable.