EFFECTS OF GEOLOGIC HETEROGENEITY AND TRANSIENT BOUNDARY CONDITIONS ON GROUNDWATER-SURFACE WATER EXCHANGE FLUX ESTIMATES FROM VERTICAL TEMPERATURE PROFILES

Analytical solutions to the one-dimensional heat transport equation for steady-state conditions can provide simple means to quantify groundwater surface water exchange. The error in exchange flux estimates introduced when the underlying assumptions of homogeneous sediments and constant temperature boundary conditions are violated is systematically evaluated in a simulation study. Temperatures in heterogeneous sediments are simulated using a numerical model for coupled water and heat flow. Heterogeneity in the sediments is represented by indicator fields of discrete, binary geologic units as well as multi-gaussian fields of hydraulic conductivity (K) based on geostatistical simulations. The contrast in K between the geologic units is identified as the most crucial parameter, leading to large errors for a log(K)-contrast of three orders of magnitude, while the influence of the structural arrangement of the units is smaller. The effects of transient temperature boundary conditions are also investigated using an analytical equation for heat transport. In the period near maximum and minimum surface water temperatures, errors introduced by a seasonal and diurnal variation of the surface water temperature are small for Darcy-velocities > 0.1m/d. For smaller fluxes, however, errors can be large. The steady-state assumption underlying simple analytical solutions to the heat flow equation is acceptable at certain times of the year and for medium to high flux rates, but pronounced geologic heterogeneity leads to large errors. Over all the range of conditions for which the simple analytical methods yield acceptable results is surprisingly wide.