Quantifying and Modeling Stream-aquifer Interactions in the Elkhorn River Basin, Nebraska

This study combined statistical analyses, field investigations and numerical groundwater flow modeling to quantify the connectivity between the Elkhorn River and its adjacent aquifers in Nebraska. The Mann-Kendall trend tests were conducted to detect increasing or decreasing tendencies on the time series data of streamflow, which were collected from eighteen gauging stations in the Elkhorn River and its tributaries. Decreasing trends were not found in the annual streamflow data. Field investigation of streambed hydraulic properties was performed in the Elkhorn River near Winslow and Norfolk. Vertical hydraulic conductivities (K v) of the shallow streambed sediments were obtained by in-situ permeameter tests. Geoprobe ® techniques were employed for electrical conductivity (EC) logging and sediments coring. The K v profiles of the sediment cores were determined using in-lab permeameter tests. The K v values of streambed sediments were generally higher at the Norfolk site than at the Winslow site. Two numerical groundwater flow models were developed for the Upper Elkhorn River Basin near Atkinson and the Lower Elkhorn River Basin near Winslow using MODFLOW-2005. The aquifer system was divided into five hydrostratigraphic units and the external hydrologic processes were incorporated in the models. The two models were calibrated under transient conditions based on the observation records from the U.S. Geological Survey groundwater level database. Hypothetical wells having different locations, pumping schedules and screen depths were placed in the model for streamflow depletion analyses. The results suggested that a well which was located closer to the river with shallow screen was more susceptible in causing stream depletion within the Atkinson model. The thick layer of silt/clay/till deposits in the Winslow area may reduce the hydrological connection between the river and aquifers and result in low and steady stream depletion rate. The stream depletion caused by the spatially allocated wells with the same distance to the river varied, which could result from the anisotropy and the heterogeneity of the aquifer system. This study would aid the administrative authorities to make informed decisions on integrated water resources management. ACKNOWLEDGEMENT This work would not have been possible without my adviser Dr. Xun-Hong Chen. During my study at University of Nebraska Lincoln, I have always been supported by Dr. Chen in my study and life. He encouraged me to think independently, initially and academically. I appreciate all his great advices and continuous patience during my learning procedure. I would also like to thank my other two committee …