EFFECT OF STREAM BED THICKNESS ON GROUNDWATER-SURFACE WATER INTERACTION USING A TRACER TEST AT CRABBY CREEK, CHESTER COUNTY, PENNSYLVANIA

This study characterized the role streambed sediment thickness has on groundwater-surface water interaction as shown in a stream tracer test. Crabby Creek is an urban stream in Chester County, Pennsylvania, 25 miles west of Philadelphia that was reconstructed and is being studied to see how streams respond to engineered alterations. Initial studies included a geophysical resistively survey and tile probing (using a metal rod to measure depth to bedrock). These studies showed a large variation in sediment thickness from 0.6 to 3 m thick along a 67 m reach. The streambed is underlain by Ordovician carbonate bedrock that has karst topography where weathering formed sinkholes infilled by sediment. Two sodium chloride tracer tests were conducted in June 2009 in the two contrasting reaches with thick and thin sediment. Streambed wells were sampled to measure conductivity of the tracer along seven stream cross-sections. Breakthrough curves of the tracer concentration in wells showed differences along both the stream length and width, indicating variability in the streambed properties.

One important property in the streambed is depth to bedrock because it affects the storage zone for groundwater-surface water interaction. The variation in bedrock depth was measured using the tile probe on a smaller scale across the 2 m width of the stream. In the reach with thick sediment, depths varied from 0.15 to greater than 2.4 m. The breakthrough curves from the tracer test in this reach had gradually rising and falling conductivity with variable peaks ranging from 0.75 to 0.95 mS/cm. Where sediment was thin, the sediment thickness was more uniform across the stream width, from 0.1 to 0.7 m. Breakthrough curves showed the shallow sediment had sharp rising and falling limbs with uniform peak conductivities (0.7 to 0.75 mS/cm). The greater sediment thickness provided a storage zone that can lower breakthrough curve concentrations. The variation in breakthrough curves was associated with variation in sediment thickness. However, individual breakthrough curves could not be matched to sediment thickness across the stream, probably because of three dimensional flowpaths and variation in other streambed properties such as hydraulic conductivity.