USING HYDROGEOPHYSICS TO MONITOR CHANGE IN HYPORHEIC FLOW AROUND STREAM RESTORATION STRUCTURES

In 2008 Crabby Creek, an urbanized stream in Valley Forge, Pa, was partially reconstructed to prevent incision into a sewer line. A 0.8 km reach of the creek was moved and structures installed to reduce erosion. Hyporheic exchange is important because the flux of water through the stream bed influences microbial communities and water quality, but little is known about how it changes over time, particularly in response to stream restoration. In 2009 a solute tracer test with geophysical resistivity monitoring was conducted to study how sediment thickness above and below a restoration structure affected hyporheic exchange. One year later a second solute tracer test and resistivity survey was done on the same reach to see what changed and what stayed the same. During each tracer test hyporheic water was collected at 5 to 15 minute intervals from wells at 20 and 40 cm depth, along with resistivity surveys along a 13.5 meter reach. A geophysical resistivity line with sensors every 0.5 m was placed in the thalweg of the reach and continually measured the resistivity over a 10 minute cycle. The time-lapse resistivity surveys show conductivity changes by subtracting out the geologic heterogeneity. Sediment thickness was measured by probing the depth to bedrock. Above the restoration structure, the sediment thickness has not changed. Below the restoration structure, though, the tile probe data show that the sediment loss ranges from 20 to 40cm. Sediment from below the structure is swept away during strong storm events. The solute tracer test produced breakthrough curves showing the fluid conductivity of the hyporheic water during the course of the injection. The 2010 breakthrough curves are surprisingly similar to their 2009 counterparts. In a few instances tracer lingered in the sediment during the tail of the sampling period in the 2010 data. This tailing, associated with longer storage in sediment, was associated with a range of locations and depths. Above the structure the time-lapse resistivity data showed similar plumes both years. Below the structure the plume during the 2010 tracer test was half as deep, and the conductivity change was about 25% lower. A shallower tracer plume and more tailing may be explained with a scouring of the sediment and a change in the sediment type below the restoration structure.