UNDERSTANDING WATER TABLE RESPONSE IN A LOESS-DOMINATED HILLSLOPE

Water tables and their response to precipitation and pumping can show a range of complexity that is controlled in part by the degree of geologic heterogeneity. However, water table measurement points are often sparsely distributed in space and time, and as a result water table representations are not reflective of the underlying geologic composition. This is particularly true of water tables in less hydraulically conductive units, which are often poorly studied. The objective of this study is to map the water table surface in detail for a loess-dominated hill slope across a seven-acre area during steady state conditions and before, during, and after precipitation events. The site has 12 continuously cored boreholes that were instrumented with conventional wells ranging between 4.5 and 15 m deep. These wells are distributed across the site and capture conditions at the peak, middle, and foot of the slope. Pressure heads in these wells were monitored once every five minutes over the past two years using submersible pressure transducers. There are also three Continuous Multichannel Tubing multilevel systems (MLSs) aligned on a longsect parallel to the slope. Each MLS provides seven depth-discrete monitoring intervals ranging between 2 and 15 m deep in a single borehole, and hydraulic head data is collected using a coaxial water level tape.We are observing sharp changes in head with depth in the MLSs, likely due to changes in vertical hydraulic conductivity in the subsurface. In the conventional wells installed along the western margin of the site we are seeing a delayed and very weak response, if any at all, to precipitation. The wells installed along the slope are more reactive, particularly those screened at shallow depths, as the deeper wells display relatively muted responses. These dramatic changes in water table response to precipitation cause short term changes in flow direction. Additionally, the water table is relatively deep considering the area receives roughly 92 cm of annual precipitation and that loess dominates the surficial sediments. Core logs indicate a sand unit at depth that may be draining the local flow system. This contrast in hydraulic conductivity suggests that the sand unit may be the primary lateral groundwater flow pathway but additional analysis of subsurface lithology and its relationship to groundwater flow is on-going.