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 [sup.15]N[O.sup.-.sub.3] was created upgradient of a mesohaline salt marsh. By measuring the changes in concentration and isotopic enrichment of N[O.sup.-.sub.3], [N.sub.2]O, N[H.sup.+.sub.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, N[O.sup.-.sub.3] removal was 90% complete within the 50 cm of marsh nearest the upland border. The peak N[O.sup.-.sub.3] loss rate from the plume ranged from 208 to 645 [micro]M [d.sup.-1]. Rates of DNRA (180 [micro]M [d.sup.-1]) and DNF (387-465 [micro]M [d.sup.-1]) processed 30% and 70% of the N[O.sup.-.sub.3] load, respectively. Terminal [N.sub.2]O production was approximately equal to [N.sub.2] production rates during DNE Comparison of [sup.15]N lost from the [sup.15]N[O.sup. -.sub.3] pool and [sup.15]N gained in each of the reduced products accounted for only 22% of the reduced [sup.15]N, thus indicating N export from the system. Despite high rates of DNRA, the N[H.sup.+.sub.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 [sup.15]N in the [N.sub.2]O and [N.sub.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 N[H.sup.+.sub.4] and evasion of N gases dictated the extent of export versus retention of the groundwater N[O.sup.-.sub.3] load.