KELLY, Susan, skelly@mail.clemson.edu, and MURDOCH, L., lmurdoch@clemson.edu,
Department of Geological Sciences, Clemson University, Clemson, SC 29634-0976

Baseflow is a fundamental hydrologic process describing groundwater discharge to streams, however, little is known about how it is spatially or temporally distributed.  This is due in part to the difficulties involved in obtaining accurate measurements of the baseflow flux.  Baseflow is the primary contributor to stream runoff under drought conditions, so its distribution plays an important role in determining the vulnerability of streams during low flows. Other factors such as sediment and contaminant transport and the ecology and chemical exchange within the hyporheic zone may be influenced by baseflow contributions.  Seepage meters, which consist of a shallow inverted bucket attached to a collection bag by flexible tubing, were developed to measure groundwater discharge to lakes or streams.  The bucket is pushed into the streambed and baseflow is measured by timing the filling of the collection bag.  These simple devices are used to obtain direct, discrete measurements of groundwater discharge, although many users have reported significant measurement variability.  Preliminary field and theoretical results indicate that seepage meters are not as simple as they appear.  We installed seepage meters in Twelve Mile River (Pickens County, SC), a fourth-order stream contained entirely within the Piedmont physiographic province.  Initial field tests yielded highly variable measurements of baseflow (coeffient of variation = 5% to 200%). Two primary factors contributing to this variability have been identified.  The effective conductance of the collection bag measured, and we used a theoretical analysis to show that the resistance caused by opening the bag is great enough to divert a significant fraction of the groundwater flow.  Additionally, the water flowing over the bag causes the head in the bag to be reduced by a factor equal to the velocity of the water.  This generates an artificial increase of groundwater flow into the seepage meter.  We are currently working on ways to use these insights to reduce the variability of seepage meter measurements so that the spatial and temporal distribution of baseflow can be more accurately measured.