EPISODIC STREAM-AQUIFER INTERACTIONS IN A PENNSnVANIA URBAN/AGRICULTURAL WATERSHED

It is hypothesized that overland flow from urbanizing and agricultural land uses will enhance stream-groundwater interactions. Near-channel groundwater chemistry and hydraulics were studied during runoff events to test this hypothesis and to increase the understanding of hyporheic processes important to stream chemistry and ecology. Twenty-four piezometers (0.3to 1-meter depth) were installed at two central Pennsylvania sites in active, channel sediments of an alluvial aquifer overlying carbonate bedrock, Measurements were taken in piezometers and the stream for five rainfall events between October 2001 and April 2002. This paper discusses the November 30, 2001 event. Surface water and groundwater samples were analyzed to find chemical parameters appropriate for tracing surface water-groundwater interactions. Regression analyses with '*O indicate that calcium may be an appropriate tracer in this watershed. Water chemistry and head varied significantly with time, depth and location in channel sediments during the event. Field data indicate that runoff can result in increased stream losses and an increase in shallow, subsurface flow downstream. KEY TERMS isotopic tracers, piezometers, stormflow, hyporheic processes, hydraulic conductivity