PHYSIOCHEMICALLY DISTINGUISHING HYDROLOGICAL EXCHANGE BETWEEN STREAMS AND STRATIFIED GLACIAL DRIFT WITH MULTIPLE GROUNDWATER SOURCES

Physical methods of observing groundwater surface water interaction in streams, such as minipiezometers and seepage, have a long history of success in quantifying flux exchanges in the streambed. In highly heterogeneous physical scenarios, however, physical methods are incapable of distinguishing multiple sources of groundwater inputs into the streambed. Within western New York, legacy stratified glacial drift aquifers create a heterogeneous landscape where streams experience multiple sources of groundwater input. Fine-grained lacustrine and coarse gravel bodies interfinger amongst a matrix of till in this area producing a large contrast of hydraulic conductivities in the groundwater environment. Using in-stream minipiezometers, tracer tests, and measurements of surface flow in 3 stream reaches of the Cattaraugus Creek Watershed, the stream aquifer relationships were determined to investigate the impact of the complex geologic setting on groundwater flow paths. Physiochemical sampling of the streambed was collected from minipiezometers in each stream reach and included temperature, dissolved oxygen, nitrate, conductivity, and pH. These indicators in addition to hydraulic head and tracer experiments yielded the importance of different physical flow scenarios within the glacial heterogeneity. Sources of groundwater identified include water that originated from the surface water, groundwater that discharges diffusely from stream bank sediments and focused input from coarse gravel bodies. In the fine-grained lacustrine substrate surface alluvium is thin, only allowing significant exchange with the top 30cm of sediment. In this case water quality of groundwater resembles mostly surface water and subsurface nitrate concentration is controlled by subsurface infiltration. In contrast the coarse gravel sediments are highly permeable, producing a connection between stream and coarse gravel aquifers. In this case incoming groundwater discharges causing high nitrate concentrations in the stream bed. Shallow subsurface water samples in the coarse gravel substrate case indicate that groundwater is physiochemically distinct from hyporheic water and thus there are more than a single type of gaining zone in the stream.