2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 9
Presentation Time: 10:30 AM

TRACKING THE FATE OF A HIGH CONCENTRATION GROUNDWATER NITRATE PLUME THROUGH A FRINGING MARSH: A COMBINED GROUNDWATER TRACER AND IN SITU ISOTOPE ENRICHMENT STUDY


TOBIAS, Craig, Geography and Geology, University of North Carolina Wilmington, 601 South College Road, Wilmington, NC 28403-5944, MACKO, Stephen, Environmental Sciences, University of Virginia, Clark Hall, Charlottesville, VA 22903, HARVEY, Judson, USGS, Reston, VA 20192 and CANUEL, Elizabeth A., VIMS, Colleg of William and Mary, Gloucester Point, VA 23062, sam8f@virginia.edu

The groundwater-wetland ecotone is a common and often highly reactive interface. We conducted a flowpath-scale in situ stable isotope enrichment study to track the fate of groundwater nitrate during discharge to a coastal marsh. A groundwater plume enriched in 15N-nitrate was created upgradient of a mesohaline marsh. Changes in concentration and isotopic enrichment of nitrate, nitrous oxide, dinitrogen gas, ammonia, and particulate N were measured during plume transit. The data was used to calculate rates of denitrification (DNF), dissimilatory nitrate reduction to ammonium (DNRA), and N storage. Of the groundwater that discharged within the top 10 cm, nearly 90% of the load was removed within the fist 0.5 meters of marsh. About 70% of the initial nitrate reduction occurred via DNF. Terminal nitrous oxide production was approximately equal to that of dinitrogen gas during DNF. DNRA accounted for roughly the remaining 30% of initial nitrate reduction. Total 15N recovery was low and attributed to export of labeled dinitrogen gas to the atmosphere. The labeled ammonium produced via DNRA was similarly not a long-term repository for the nitrate-derived N. The ammonium was quickly immobilized into marsh PON. DNF followed by atmospheric evasion of dinitrogen gas represented attenuation of N loads to the adjacent water mass. DNRA, however merely changed the flavor of the incoming N but did not represent a true removal mechanism. This study represents an early attempt to use in situ isotopic tracers, on a flowpath scale to derive in situ rates across a terrestrial aquatic interface. As such, it demonstrated the utility of a promising approach, as well as some room for improvement.