HR: 17:15h
AN: B32D-06    [PDF]
TI: Measuring The Flux of Nitrogen From Watersheds, Errors and The Temporal Resolution Problem
AU: * Showers, W J
EM: w_showers@ncsu.edu
AF: N. Carolina State Univ., Dept MEAS, Raleigh, NC 27695 United States
AB: Agricultural and urban land use has increased the fluxes of nutrients and sediments into surface waters and ground waters. Transport of nitrogen through watersheds into coastal waters has resulted in eutrophication and water quality degradation. Management actions aimed at reducing nitrate contamination of surface waters need a better understanding of fundamental processes that control water quality on a watershed scale. RiverNet, a high resolution (hourly) in situ nitrate monitoring program has found significant concentration variations associated with point sources in the Neuse River Basin, NC. Nutrient inputs from agricultural watersheds are highly correlated to discharge. Nutrient inputs from waste application fields are variable and most important during falling hydrographs. Nitrogen, carbon and oxygen isotopes of nitrate and POM indicate that in-stream nutrient consumption is not an important process in riverine nutrient transport to the estuary in the river mainstem, but can be important in smaller streams and creeks. This water depth/nitrogen in-stream loss data indicates that the contribution of point sources have been underestimated on a watershed scale. In addition large fluxes of nitrate from contaminated groundwater to surface waters adjacent to WAF occur over a 1 to 3 day period after large rain events. The flux of contaminated groundwater has not been taken into account in the NPDES permitting process. $^{17}$O of river and groundwater nitrate indicates that atmospheric deposition can be a significant contributor (90 %) to nitrate flux in urban watersheds and to a lesser extent in forested watersheds, but contributes less than 10% of the nitrate flux exported from the basin as a whole. Nitrate fluxes calculated from hourly measurements differ from daily calculated fluxes by up to 20% during high flow conditions and 80% during low flow conditions. These findings indicate that significant errors can be produced by monitoring programs that try to determine long term trends of basin scale nitrogen flux without high resolution data sets produced by in situ monitoring. These data indicate that current monitoring efforts have tremendously underestimated the amount of nitrogen that is exported from watersheds to the coastal ocean.
UR: http://rivernet.ncsu.edu
DE: 1040 Isotopic composition/chemistry
DE: 1803 Anthropogenic effects
DE: 1831 Groundwater quality
DE: 1871 Surface water quality
SC: Biogeosciences [B]
MN: 2003 Fall Meeting