Seeps regulate stream nitrate concentration in a forested appalachian catchment

Received for publication March 6, 2009. Surface seeps can be defined as locations where upwelling ground water saturates the surface for most of the year and excess ground water can be delivered to the stream channel via surface flowpaths. If a stream is predominantly fed by seeps, then ground water added to the stream via these surface flowpaths may result in reduced interactions with the subsurface riparian zone. It is generally believed that seep ground water that upwells and then flows along surface flowpaths can be subject to diminished denitrification and biologic uptake processes. Seep effects on stream nitrate (NO3) concentration were studied in Baldwin Creek (5.35 km2), southwestern Pennsylvania. Nitrate retention within seep zones was evaluated over a 1-yr period (May 2002–2003) using a monthly, nested (top and bottom of seep) sampling approach along 15 individual seeps. Seep samples were analyzed for NO3–N, NH3–N, and dissolved organic carbon, along with stream waters and streamflow measurements at seven stream stations. Seeps were generally NO3 sinks with concentrations decreasing downseep: 31% median annual reduction and 73% maximum monthly reduction. During cold and wet periods, seeps frequently behaved as NO3 sources to the stream (NO3 concentrations increased or remained constant downseep). Seep temperature and discharge were related to seasonal variability in seep NO3 retention. Seasonal variations in stream NO3 concentration have been attributed to upland soil and vegetation processes in numerous watersheds. At Baldwin Creek, seep NO3 processing regulated the seasonal variability of stream NO3 concentrations. These results suggest that seeps provide important water quality functions and can modulate the effects of elevated regional N deposition in Appalachian catchments.

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