TEMPERATURE AS A HYDROLOGIC TRACER IN LAYERED SUBSURFACE ENVIRONMENTS

Groundwater flow induces advective heat transfer, and thus the deviation of subsurface temperatures from an expected conduction-dominated regime can be analysed to estimate the direction and magnitude of vertical water fluxes. A number of analytical approaches have been proposed for using heat as a hydrologic tracer, and these have typically invoked the assumption of homogeneous thermal properties. Heterogeneous thermal properties are ubiquitous in subsurface environments, both at the scale of geologic strata as well as at finer (cm) scales in streambeds. Herein, we demonstrate how an analytical solution, previously developed for determining vadose zone fluxes, can also be applied to estimate water fluxes in layered, saturated subsurface environments. The solution allows for n-layers with each layer characterized by a distinct thermal conductivity. One primary limitation of the solution is that it assumes temperature profiles are at steady-state. This condition may be violated in streambeds due to diurnal surface temperature change or in deeper (borehole) temperature profiles due to multi-decadal change and inter-borhole flow, and thus caution should be employed. However, in streambeds, upward flow strongly reduces the downward propagation of complicating surface signals, as does ice cover, and the effects of climatic shifts on near-surface borehole temperatures can be readily identified. The application of the solution for studying groundwater-surface water interactions is demonstrated using temperature data collected from a streambed with an abrupt change in thermal properties at the interface between layers of sand and organic soil. Also, a deeper subsurface temperature profile recorded in a layered environment in southern Australia is analysed to estimate deeper vertical water fluxes. A simple spreadsheet program is presented to allow users to enter data from shallow or deep temperature-depth profiles and run the solution to infer vertical flows.