QUANTIFYING DOWNFLOW THROUGH CREEK SEDIMENTS USING TEMPERATURE TIME-SERIES - ONE-DIMENSIONAL SOLUTION INCORPORATING MEASURED SURFACE-TEMPERATURE

Several authors have addressed the question of identifying the locatio n of and quantifying the volume of groundwater/surface water interacti on. Utilizing measurement of temperature within the sediments has been suggested as a means of estimating water flux through the sediments. A qualitative version of this method has been applied to identifying l ocations of communication between a creek and groundwater based upon t emperature time series measurements in the water column and the sedime nts. The discussion presented earlier is extended to a more general so lution which allows for incorporation of measured surface temperature (rather than an assumed surface temperature). The mathematical formula tion presented is targeted on the quantification of flux across the se diment for conditions of one-dimensional downflow with a constant flux over periods of days to weeks. This technique, based on the assumptio n that the temperature of the surface water is the primary thermal inf luence on sediment temperature, is shown to provide an estimate of flu id flux (volume of water per area per time) for flux rates above a cri tical threshold, The flux threshold depends on a number of factors inc luding thermal diffusivity of the sediments and depth of burial of the temperature measuring device, Based on use of three simple forcing fu nctions for temperature at the surface of the sediments, it is shown t hat the amplitude of the temperature response in the sediments to a ch ange in temperature in the overlying water column decreases with incre asing depth and decreasing flux, Further, the timing of the peak respo nse in the sediments becomes increasingly delayed as depth increases a nd flux decreases. These observations, combined with consideration of the assumption of one-dimensional flow, lead to suggesting that field design be based on relatively shallow burial (e.g. 5-15 cm) of the dev ice to measure sediment temperatures. Based on these observations, one of the data sets presented earlier is reanalyzed to derive flux estim ates, It is shown that a reasonable fit is obtained with a downflow fl ux of less than 0.03 cm day(-1). Independent measurement of hydraulic gradient and hydraulic conductivity provide a range of flux estimates for the same site (although measured in a different year) which are co nsistent with this value. Based on these results, it is argued that th is technique is a reasonable screening tool for use in situations wher e relatively inexpensive, point estimates of water flux are required w ithin losing reaches of a creek.