Tobias, C. R.. Virginia Institute of Marine Science, School of Marine Science, College of William and Mary, ctobias@vims.edu
Macko, S. R.. University of Virginia, sam8f@uva.pcmail.virginia.edu
Anderson, I. A.. Virginia Institute of Marine Science, School of Marine Science, College of William and Mary, iris@vims.edu

 
TRACKING THE FATE OF GROUNDWATER NITRATE DURING DISCHARGE TO A MESOHALINE MARSH: RESULTS FROM AN ISOTOPICALLY ENRICHED INSITU TRACER EXPERIMENT
 
In Spring 1998, during peak groundwater discharge, a subsurface natural gradient tracer test was performed in a groundwater discharge zone located in a fringing marsh along the York River; a subestuary to the Chesapeake Bay. A plume consisting of slightly 15N enriched nitrate (7500 per mil) and bromide (a conservative tracer) was introduced into three wells located along the marsh/upland border. Plume transport was monitored for over 100 days in a shallow piezometer grid encompassing a marsh area of eight square meters. Concentration and isotopic (15N) abundance was determined for the nitrate, ammonium, nitrous oxide, molecular nitrogen, and particulate organic nitrogen pools over this time. These measurements were used to estimate rates of denitrification, dissimilatory reduction to ammonium, and nitrate uptake/retention and asses the relative proportion of allochthonous nitrogen retained in the marsh. Total nitrate reduction, determined from the decrease in the nitrate /bromide ratio was dominated by denitrification. Considerably less nitrate was converted to ammonium or trapped in particulates. However isotopic abundance and the elevated concentrations remained high in the reduced nitrogen pools well after all the nitrate was consumed. This suggests that nitrate reduction does not equate with immediate nitrogen export from the system.
 
Day: Wednesday, Feb. 3
Time: 08:30 - 08:45am
Location: Eldorado Hotel
 
Code: SS32WE0830E