Controls on nitrate degradation in two adjacent wetland streams with different geochemistry and flow-source terms, Watkinsville, GA
Schroer, Katherine Lynn
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A two-year sampling program was conducted to elucidate controls on nitrate N removal and retention in two small wetland streams that drain portions of a 10ha cattle pasture and crop field. Cow manure and synthetic fertilizer were the main sources of nitrate N to the wetland. A man-made earthen berm prevented surface runoff from entering the Protected Stream (63m long), whereas the adjacent Runoff Stream (70m) received surface runoff from the pasture. The Protected Stream had been dredged and had a perennial spring flowing from its headcut; the Runoff Stream did not. The two streams are 14m apart at headcut. Four seasonal tracer injection tests (Chapter 3) were conducted to evaluate wetland stream response to a pulse of high but realistic concentration of [NO3- -N]. In addition, samples were collected ~monthly for two years at 8-10 sampling stations and samples were analyzed for the several redox parameters using field probes, laboratory instruments and wet chemistry methods. Residence time exerted a primary control on N removal processes. Denitrification and/or nitrate reduction to ammonia were enhanced when residence times were longer. Organic carbon was the main electron donor in both streams. Sulfide may be an important secondary reductant. Temperature exerted control on stream redox processes above 18-20°C . Stream baseflow exerted less control than baseflow-plus-runoff on the wetland’s capacity to remove nitrate N. These results constrain processes that affect nitrate N removal efficiency in agricultural wetland streams.