Investigation of pore water residence times and drainage velocities in salt marshes using short-lived radium isotopes

Pore water residence times and drainage velocities were investigated in Jamaica Bay (NY, USA), a highly urbanized estuary in New York City where salt marsh degradation and loss has accelerated in recent years. Five different marshes were sampled for shallow (25 cm) and deep (≥ 50 cm) dissolved 224Ra over a several year period. Pore water residence times were estimated using a one-dimensional advective transport model, assuming that the pore water 224Ra grows into an equilibrium 224Ra activity, and that flow is vertical. Equilibrium between dissolved 224Ra and the marsh sediment was assessed with respect to solid-phase 228Th, 228Ra, reactive solid-phase Fe and Mn, all of which except Mn scaled as a function of grain size. Pore water 224Ra and estimated residence times did not display any seasonality between the marshes. The shortest residence times were estimated for the highest elevation, fine-grained-sand marsh (1.4 ± 0.9 d at 25 cm), which has the highest percent vegetation cover. We hypothesize that the faster drainage velocity (25.9 ± 17.8 cm d− 1 at 25 cm) of this marsh, combined with a higher mean elevation above sea-level, together limit exposure of the marsh plants to the build-up of H2S, a phytotoxin, in the pore fluid from the spring through summer. In contrast, shallow pore water residence times were greater for marshes with a lower elevation above sea-level, from (3.9 ± 1.8) d to (9.2 ± 3.6) d, where sediments were finer-grained, and throughput fluxes of Fe and H2S were higher throughout spring and summer seasons. This work demonstrates the utility of 224Ra as a tracer of salt marsh flow dynamics, and highlights the importance of sediment heterogeneity and marsh platform elevation on the dynamic geochemistry of salt marshes.