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

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.