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