Hydrologic connections of a stream-aquifer-vegetation zone in south-central Platte River valley, Nebraska

Field investigation and numerical modeling approaches were used to examine the hydrologic relations between the Platte River and the adjacent alluvial aquifer and riparian zone in south-central Nebraska. Field methods include direct-push techniques for coring and electric logging in the river channel, permeameter tests for estimating the hydraulic conductivity of the streambed, and monitoring of groundwater levels responding to changes in stream stages and to groundwater evapotranspiration. The channel sediments consist mostly of coarse sand and gravels at the study site with large values of horizontal and vertical hydraulic conductivity. Groundwater in the riparian zone responded nearly simultaneously to the changes in stream stages, and diurnal fluctuations of the water table are correlated with fluctuations of stream stage in summer. All these indicate a well-connected river-aquifer-vegetation hydrologic system. Numerical models, based on the Galerkin finite element method, were developed to construct detailed flow nets for examining the changes in the patterns of groundwater flow dynamics resulting from the use of groundwater and stream water by riparian vegetation. Simulation results suggest that a number of hydrologic factors, such as the thickness of the aquifer and vertical anisotropy of aquifer hydraulic conductivity, also affect the flow patterns. Vertical flow is a major component, more significant than the horizontal flow below the river and the vegetation zone in the growing season. Groundwater evapotranspiration can bring deeper groundwater to the water table by hydraulic lift. This function of riparian vegetation could cause a complicated situation in an investigation of groundwater quality in riparian zones.