FIELD EVALUATION OF ELECTROMAGNETIC INDUCTION FOR CHARACTERIZING THE VARIABILITY OF VERTICAL HYDRAULIC CONDUCTIVITY IN STREAMBED SEDIMENTS

The quality and quantity of ground water and surface water are directly dependent on the exchanges and mixing that occur in streambeds. These processes are controlled by the hydraulic conductivity (K) of streambed sediments, so measurement of K and its variability are critically important in hydrological sciences. Methods for accurately measuring K are not well-suited for rapid, high-resolution assessment of its variability at large scales. Electromagnetic induction (EMI) is a geophysical method that can be used for rapid, high-resolution mapping of physical parameters that may also control K, but few studies have evaluated the use of this method in streambeds. Here we present a field application of EMI to the beds of two streams in Nebraska, USA. Over 100 simultaneous, co-located measurements of apparent conductivity and vertical K were used to investigate the electrical-hydraulic relationship in the streambeds. Results show statistically significant correlation, but the strength and direction of that correlation vary between the sites. We attribute these differences to a greater abundance of silt + clay and the presence of upwelling gas at one of the sites. Despite these differences, EMI is shown to be useful for gross characterization of changes in K in shallow streams with uniform temperature and water conductivity, when supplemented by direct measurements. Our application of EMI further reveals the complex distribution of K in a sandy meandering stream and we relate these patterns to the fluvial geomorphology of the studied reach.