REACH SCALE DETERMINATION OF WATER FLUXES AT THE STREAM-GROUNDWATER INTERFACE USING MAPPED STREAMBED TEMPERATURES

The quantification of water fluxes through the streambed with fine spatial resolution on a large scale can be crucial to understanding near stream flow dynamics and accurately assessing the distribution of contaminant transport across the groundwater/surface-water interface. Our approach takes advantage of the temperature gradient between groundwater (whose temperature remains nearly constant throughout the year) and stream water (whose temperatures vary seasonally) to determine the magnitude of groundwater discharge. Shallow streambed temperatures can be easily and inexpensively measured at hundreds of locations along a stream reach in a short period of time and can be used for the delineation of different zones of groundwater-stream water interactions (Conant 2004). The current study quantified water fluxes for each temperature observation point by applying a very simple steady-state analytical solution of the one-dimensional heat diffusion-advection equation. This method was successfully applied to three different streams in Germany and Canada on reaches that were between 60 and 750 m in length. A key underlying assumption for quantification using this method is that the streambed temperatures are measured at a depth where quasi-steady state temperatures exist throughout the time required to map the temperatures (i.e., avoid diurnal variations). To evaluate this assumption, the streambed temperature variations over time were numerically modeled with the USGS VS2DH program by using the observed fluctuations in surface water temperatures. The assumption appeared to be valid, but quasi-steady state conditions occurred at greater depths for low discharge zones than for high discharge zones. The magnitudes and spatial patterns of water flux obtained from applying the analytical model to the mapped streambed temperatures agreed well with the fluxes obtained using piezometers, Darcy's law, and seepage meters. The main advantages of this quantification method when compared to the prior method of obtaining fluxes empirically from streambed temperatures (Conant 2004), is that it does not require the installation and testing of large numbers of piezometers, has minimal data input requirements, and so is an even quicker way of estimating groundwater discharge on the reach scale.