ANALYSIS OF THE INFLUENCE OF FERRICRETE ON HYPORHEIC EXCHANGE FLOWS
To quantify flow through the hyporheic zone, we used time-lapse electrical resistivity of the streambed and banks of Cement Creek taken over the course of a day in conjunction with a four-hour salt injection tracer test. The solute was constrained within the streambed, with little flow through the banks, and had longer residence times in the hyporheic zone during high flow than at low flow. Slug test data suggested the presence of a zone of lower permeability at 44-cm depth that was likely made of precipitated ferricrete that cemented cobbles together. The comparison of apparent bulk conductivity from the geophysics to in-stream fluid conductivity allowed for the calculation of mass transfer parameters between the stream and hyporheic zone based on the difference in solute retardation patterns in the two breakthrough curves. During high flow, in-stream breakthrough curves displayed slower breakthrough and greater smoothing which is consistent with the geophysical inversion results that indicate higher residence times at high flow. Analyses of low flow data indicated decreased residence time within the subsurface and comparatively faster breakthrough. The hyporheic storage area within Cement Creek, estimated from the modeled capacity coefficient, decreased by two orders of magnitude between high and low flow, along with a corresponding decrease in residence times.