Tracking the fate of a high concentration groundwater nitrate plume through a fringing marsh: A combined groundwater tracer and in situ isotope enrichment study

A groundwater plume enriched in 15 NO 3 − was created upgradient of a mesohaline salt marsh. By measuring the changes in concentration and isotopic enrichment of NO 3 − , N 2 O, N 2 , NH 4 + , and particulate organic nitrogen (PON) during plume transport through the marsh, in situ rates of dissimilatory nitrate reduction to ammonium (DNRA) and denitrification (DNF) were estimated, as well as N storage in the reduced N pools. For groundwater discharge within the top 10 cm of marsh, NO 3 − removal was 90% complete within the 50 cm of marsh nearest the upland border. The peak NO 3 − loss rate from the plume ranged from 208 to 645 µM d −1 . Rates of DNRA (180 µM d −1 ) and DNF (387–465 µM d −1 ) processed 30% and 70% of the NO 3 − load, respectively. Terminal N 2 production was approximately equal to N 2 production rates during DNF. Comparison of 15N lost from the 15NO 3 − pool and 15 N gained in each of the reduced products accounted for only 22% of the reduced 15 N, thus indicating N export from the system. Despite high rates of DNRA, the NH 4 + produced was not a long‐term repository for the groundwater‐derived N but was instead rapidly immobilized into marsh PON and retained on longer timescales. The small inventory of 15 N in the N 2 and N 2 pools relative to DNF rates, coincident with an undersaturation of dissolved argon, indicated that denitrified N was exported to the atmosphere on short timescales. The relative magnitudes of DNF and DNRA in conjunction with the immobilization of NH 4 + and evasion of N gases dictated the extent of export versus retention of the groundwater NO 3 − load.