Characterization of the Stream-Aquifer Hydrologic Connection in the Elkhorn River Basin

In this study, the hydrologic relationship between the Elkhorn River and the surrounding aquifer at eight study sites were studied. The calculated hydraulic gradient showed that the adjacent aquifer recharges the Elkhorn River all year. Reversed gradient was only found at Atkinson site during pumping seasons from 2008 to 2010. Geoprobe log data and in-situ permeameter test data were combined to calculate the equivalent hydraulic conductivity (K) and unit-length streambed conductance (C) at eight sites, where the greatest values were found at Neligh and the lowest values found at Hadar. Accordingly, the result of cross correlation analysis on the lag effect between stream stage and groundwater level was consistent with the indication of K and C. A geological model of multimillion grids was built based on well log data of test holes and registered wells using IDW interpolation method. The grids were grouped into eight hydrofacies and their corresponding hydraulic conductivity values were assigned based on empirical value from books and former studies in this area. Then the geological model was upscaled to three aquifer units by coarser grid using an averaging technique (Li, 1999) and bound method (Cardwell, 1945). A groundwater flow model (ULEN) was also built using MODFLOW to calculate the stream depletion ratio at Neligh and Hadar site based on the hydrostratigraphic model. A hypothetical well was created and pumped water at a rate of 1000 GPM under three scenarios in June, July and August: the well is located from the river at 1000 ft, 1 mile and 3 miles. The modeling results are consistent with the data analysis which showed that the stream depletion ratio is more pronounced at Neligh than Hadar. Advisor: Xun-Hong Che