CHARACTERISTICS OF GROUNDWATER – STREAM WATER INTERACTION IN A DISTURBED HEADWATER CATCHMENT WITHIN THE SOUTHERN APPALACHIAN MOUNTAINS

Headwater streams and their interaction with groundwater play a large role in the discharge and water quality of larger stream systems. Gribble Gap, a 0.4 km²watershed where this study was conducted, is currently forest covered, but had excessive soil erosion during much of the 1900s when the land was used for logging and as pasture. This study was undertaken to better describe the types and characteristics of groundwater and stream water interaction in the Gribble Gap catchment.

Data collection was from October to December 2016, mostly during a significant drought. Multiple approaches were used to identify stream reaches as gaining or losing, and as connected or not connected with groundwater. Primary methods to identify reach types were hydraulic head data from riparian and in-stream wells, thermal traits of water (FLIR infrared camera), and diversion of stream water from the channel. For the diversion trial, upstream surface water was captured and diverted so groundwater seeps could be visually identified. Once stream zones were identified as gaining or losing, additional studies of groundwater and surface conductivity and temperature, stream chemistry, riparian stratigraphy, and geomorphology were used to characterize stream reaches.

Consistent data from multiple data sets confirms there are distinct gaining and losing reaches. At the time of the study, most groundwater inputs to the stream were not sourced from the stream bed vertically, but were lateral inputs at the base of the channel bank. In some places, the stream was not connected to the groundwater (it was perched), reflecting a low hydraulic conductivity layer just below the gravel bed of the stream. Perhaps reflecting this, the streams have pervasive undercutting of the banks. Temperature traits of the stream water proved effective at locating active groundwater seepage zones. Water chemistry, however, showed minimal variation with reach types—this may reflect the strictly baseflow conditions during sampling. The interaction of groundwater and stream water is very complex in this watershed. Future work characterizing where and why there is down valley groundwater gaining compared to where there is groundwater driven into the stream from slopes would be helpful to understand these interactions at large.