DISTRIBUTION OF N AND OTHER REDOX-SENSITIVE SPECIES IN TWO ADJACENT WETLAND STREAMS DRAINING AN AGRICULTURAL FIELD IN THE GEORGIA PIEDMONT

This study investigates how mixing of source waters affects the distribution and cycling of N and other redox-sensitive species in two adjacent wetland streams draining a cow pasture and crop field at the USDA Agricultural Research Station in Watkinsville, GA. One stream originates at a headcut spring and is protected from surface runoff by a man-made berm. Flow is contributed almost entirely by ground water emerging from the headcut spring and ground water flux through the streambed. The adjacent stream (14m away at headcut) is not protected by a berm and does not have a flowing spring at its headwaters. It is open to surface runoff and input of soil and organic matter from the upgradient pasture and fields. Both streams are shallow, narrow, and slow. The two streams combine at a confluence 57m downstream in the protected, spring-fed stream and 70m downstream in the runoff stream.

Several sampling stations have been established in both streams and downstream of their confluence. Since September 2003, samples have been collected every 4-6 weeks and are being analyzed for several parameters, including temperature; pH; conductivity; alkalinity; NO3-; NO2-; total N; total dissolved N; NH4+; urea; Fe (II, III and total); total organic C; and the dissolved gases O2, N2O, CH4, CO2 and H2.

NO3--N concentrations decrease along both streams from headcut to confluence - from 7-12 to 0.2-1.5mg/L in the runoff channel and from 6-8 to 3-5mg/L in the spring-fed channel. Discharge and concentration measurements show that dilution by ground water flux does not account for all of the nitrate loss. Tracer experiments within the spring-fed stream show lower percent recovery for NO3- than Br- - an indication that denitrification may be occurring. Ammonia, alkalinity, TOC and NO2- are consistently higher in the runoff stream than in the spring-fed stream, but trends along each flow path are sometimes weak and seem to vary with season. Fe2+ is usually, but not always, more concentrated than Fe3+ at each station, especially in the runoff stream. Thus, the adjacent streams have different distributions of certain dissolved chemical species in a setting where climate, relief, land use and underlying geology are the same.