SURFACE-SUBSURFACE WATER INTERACTIONS IN AN ALLUVIATED MOUNTAIN STREAM CHANNEL

Data from an instream tracer experiment performed in North Fork Dry Run, Shenandoah National Park, Virginia, illustrate that the transport of water and conservative solute in the stream is greatly influenced by interaction with water present in both the relatively shallow gravel bed and in the relatively deeper alluvial infill material. The shape of concentration-time curves from the field tracer experiment exhibit "storage" of tracer in the subsurface-short-term storage imposed by exchange with the gravel bed and long-term storage by exchange with the deeper alluvial infill. Time series models fit the observed concentration data very well. Estimates of mixing volumes generated by the time series analysis indicate that a large volume of the gravel bed participates in rapid mixing with the surface stream. At one point along the study reach a second-order time series model is supported by the data, suggesting two independent, parallel flow paths. Observations of tracer in wells more than 10 m from the stream and the very extended recession of the tracer in the stream where it flows over a bedrock ledge support the contention that the deeper alluvium plays an important role in transient storage of solute in North Fork Dry Run. The work implies that physically-based models to predict transport and storage in alluviated mountain catchments may have to account for interactions between the stream and large portions of its floodplain.