CHARACTERIZING HYPORHEIC FLOW IN AN URBAN STREAM USING HYDROGEOPHYSICS AND HEAT FLOW

Valley Creek, in Southeastern Pennsylvania, has undergone changes typical of streams in urbanized areas, such as bank erosion, channel redirection, and habitat disruption. Hyporheic exchange sustains both benthic and stream ecosystems, because the flux of water provides for necessary biochemical reactions to take place in the streambed. Monitoring and characterization of hyporheic flow remains a difficult and time consuming process, but a better understanding of depth and location of the hyporheic zone may be available with a geophysics coupled tracer test as well as heat flux measurements. We characterized the hyporheic flow of a 13.5 m reach bounded by two step structures installed for erosion. A two-hour solute tracer injection was monitored at four different well depths along four different distances of the reach, with samples taken every 5 to 15 minutes to capture the passing of the salt tracer through the hyporheic zone. Well sampling was concurrent with a 28-electrode, dipole-dipole, resistivity survey over the same 13.5 m reach repeated in ten minute intervals to create time-lapse tomographs. In addition, temperature probes were inserted into the streambed at each well nest location. By modeling the diurnal temperature changes of stream water compared to subsurface water, we calculated the direction and magnitude of groundwater of the stream water-groundwater flux. Freeze core samples were taken at each line to quantify heterogeneities in streambed sediments. Temperature modeling of heat flux showed groundwater discharge at two locations. Well samples indicated no hyporheic flux at these two locations, but showed hyporheic flow only where heat flux indicated groundwater discharge was not occurring. Water samples collected at 15 cm depth intervals showed only shallow penetration of the tracer, up to 20 cm. The locations of hyporheic flow did not seem to be related to the step structures, suggesting that sediment heterogeneity controls the pattern of hyporheic flow at this site.