COMPARISON OF HYPORHEIC EXCHANGE IN THICK AND THIN SEDIMENT LAYERS USING SOLUTE TRACER TESTS AND GEOPHYSICAL RESISTIVITY MONITORING

Crabby Creek, an urbanized stream in Valley Forge, PA, has been recently reconstructed and numerous flow-inhibiting structures have been installed to stabilize the stream banks and support the stream’s restoration. A geophysical resistivity survey of the stream bed showed two very different sections of a 67 m stretch. The upper reach was underlain by a thin sediment layer (0.6 m) over carbonate bedrock while the lower reach was underlain by a thicker sediment layer (up to 3 m) and greater depth to bedrock. Fine sedimentation from bank erosion can discourage exchange between groundwater and surface water and be detrimental to stream quality, but it is difficult to map subsurface heterogeneity that impacts hyporheic exchange. Solute tracer tests and time-lapse geophysical resistivity surveys were used to study how sediment thickness and flow inhibitors affect hyporheic exchange.

Two separate tracer tracers were conducted on each reach, monitored with approximately 30 piezometers and resistivity surveys during the tests. Hyporheic water was sampled at five to fifteen minute intervals; surface water was also sampled, and conductivity loggers were used to continuously monitor surface water. Resistivity surveys of the stream bed were repeated every eleven minutes. Both the resistivity surveys and the wells provided breakthrough curves of the tracer. The breakthrough curves show evidence of storage, dispersion, and low hydraulic conductivity. In general, the upper reach (thin sediment layer) breakthrough curves had sharply rising and falling limbs, indicating little to no dispersion and rapid transport to the hyporheic zone. The lower reach curves had more gradual rising and falling limbs, indicating greater dispersion and storage. Time-lapse resistivity surveys showed storage areas, mostly around flow inhibitors, where the solute lingered. Resistivity surveys coupled with point sampling provided the continuous monitoring needed to understand the influence of the flow inhibitors and sediment distribution on hyporheic exchange.