ANALYSIS OF THE INFLUENCE OF FERRICRETE ON HYPORHEIC EXCHANGE FLOWS

The area of confluence between surface water and groundwater, known as the hyporheic zone, is a natural biogeochemical filter that is dependent on channel morphology and hydraulic conductivity, pressure-driven downwelling and upwelling currents, and stream discharge. In Cement Creek near Silverton, Colorado, deposition of amorphous iron minerals reduces the permeability of the streambed and limits flow through the hyporheic zone. This limited exchange may lower the potential for pollutant attenuation from the metals-loaded waters of Cement Creek within the hyporheic zone. This study found that hyporheic exchange in this system is limited in spatial extent and reduces during low flow when compared to what we would expect from streams without ferricrete.

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.