GROUNDWATER TEMPERATURE DISTRIBUTIONS AT THE AQUIFER SCALE: AN ANALYSIS OF SURFACE AND NEAR SURFACE PROCESSES AND HYDROGEOLOGIC CONTROLS ON HEAT TRANSPORT

Groundwater temperatures in shallow and nested flow systems show considerable variability due to changes in air temperature. Air temperature is propagated into the ground primarily by conduction. In shallow groundwater flow systems, where the vadose zone is thin, temperature signals are advected and dispersed with groundwater and may reach considerable depths. Heterogeneous groundwater flow regimes complicate heat transport processes and result in variable temperature distributions in the shallow subsurface at the aquifer scale.

We present analysis that documents a hydrogeologic control on the seasonal behavior of groundwater temperatures using distributed times series analysis. The analysis demonstrates the importance of air-ground coupling effects on the resulting subsurface thermal regime. The temperature signal undergoes amplitude and phase changes in both the vadose and saturated zones, and these changes are influenced by factors such as soil moisture, surface land use/cover, snow cover, thickness of the vadose zone, and advection. Using field data, we show that the air temperature signal is attenuated by 25% at the ground surface, and the presence of a persistent snow pack during the winter modifies the ground surface temperature signal and results in mean groundwater temperatures that are significantly warmer than the mean air temperature. Understanding hydrogeologic controls and air-ground coupling effects is needed to quantify the groundwater thermal regime at the aquifer scale.

In addition to conduction of the air temperature signal into the ground surface, the total energy budget of the ground surface includes solar and black-body radiative fluxes. We present site data from an arid climate demonstrating the importance of radiative energy fluxes. Numerical modeling is used to further explore the importance of the radiative components of the ground energy budget relative to conduction under a variety of conditions. Despite hydrogeological complications, it is a promising line of research to use groundwater temperature signals to understand groundwater flow conditions and hydrogeology at the aquifer scale. Our work also demonstrates that hydrogeology needs to be addressed in studies that evaluate the effects of changing air temperature signals on groundwater thermal regimes.