Groundwater Seepage and Dissolved Organic Carbon Flux in an Appalachian Catchment

Riparian zones with extensive groundwater seeps may allow short-circuiting of groundwater to stream channels and modify groundwater contact with shallow organic soil layers in riparian areas. The controls on dissolved organic carbon (DOC) export to an Appalachian headwater stream were examined at Baldwin Creek (southwestern Pennsylvania), where baseflow is derived from multiple surface seeps. DOC inputs from near-stream seeps and DOC transformations along these zones and the main channel were monitored monthly over a one year period (May 2002-2003) using a nested sampling approach. A total of 264 seep samples were collected at the top and the bottom of 15 seeps. Seep samples were analyzed for DOC and nitrogen species along with streamwater and stream discharge at 7 stream stations. After discharging to the land surface at the top of seeps median annual DOC concentrations increased downseep by 41 %. DOC gains occurred largely in seeps fed by deep groundwater flowpaths, as indicated by stable temperatures and sustained discharge throughout the year, rather than in surface-water dominated seeps. Along the main stream channel median annual DOC concentrations were 14% lower than seep bottom concentrations. DOC budgets for seeps and the main stream channel suggested that DOC is released by seep zones but retained by the main channel. During periods of high flow there was a tendency for greater DOC concentrations at the watershed outlet and greater DOC export. These data suggest that seeps provide a source of DOC to the main stream channel and alteration of seep zones or groundwater flowpaths to these zones can affect the delivery of DOC to the channel. Best management practices for headwater streams with dominant seep contributions should consider protection of near seep zones and seep recharge areas. KEY TERMS: Stream-groundwater interactions; seeps; riparian zone; DOC; headwater streams