Pore Water Exchange and Nutrient and Trace Metals Fluxes In a New England Salt Marsh

We use direct groundwater sampling and ancillary geophysical imaging to explore tidally induced pore water exchange and associated enrichment/removal of dissolved components over spring-neap tidal cycles in a mixed clastic and peat salt marsh bordering the Rowley River in the Plum Island estuary, Massachusetts. The Ra-224 and Rn-222 signatures of groundwater extracted at depths of 1 m and 2 m beneath the marsh surface reveal very young ages (~2 days old) in the banks of a tidal creek running perpendicular to the Rowley River and in the undammed and relatively undisturbed marsh to the north of the tidal creek. This finding is consistent with complete exchange of pore waters over short timescales. In contrast, anthropogenic activities on the south side of the tidal creek have perturbed natural drainage patterns, and pore waters have consistently older (by several days) pore waters at distances greater than 5 m from the creek than in the creek bank, except after a large precipitation event. Between the creek bank and 15 m distance from the creek, groundwater salinity at 2 m depth increases from 24 to 32 psu on the north side and 28 to 38 psu on the south side. To first order, these groundwater salinity variations are consistent with the results of a spatially coincident and simultaneous multinode resistivity tomography study, which also captured changes in creek bank pore water conductivities on both intertidal and intratidal scales. Application of a multitracer mass balance model to the geochemistry data permitted us to evaluate pore water residence time and water fluxes. Depending on the values assigned to the pore water and river water endmembers, the results show that 10 to 20% of the volume of the tidal prism in the creek is recirculated through sediments every tidal cycle.