Application of 224Ra and 222Rn for evaluating seawater residence times in a tidal subterranean estuary

Abstract

The length of time in which seawater circulates through the beach face and mixes with terrestrial groundwater will have a profound influence on the geochemical cycling of nutrient deliveries to surface waters through submarine groundwater discharge. The goal of this work is to utilize pore water measurements of 222Rn and Ra isotopes to evaluate the time-scale of tidal seawater circulation in a coastal bluff and a barrier beach subterranean estuary (STE) on Long Island, NY. Intertidal pore water profiles sampled monthly between 2014 and 2015 revealed distinct geochemical differences between the two sites that were driven by the primary hydrogeologic forcing mechanisms. The coastal bluff site was dominated by a large hydraulic gradient that drove terrestrial groundwater discharge, while the barrier beach site had saline pore waters and was physically controlled by tidal forcing mechanisms. These mechanisms resulted in notably different pore water radionuclide (222Rn, 223,224,226,228Ra) and trace metal (Fe, Mn, Ba) distributions in the STE. A one-dimensional advective transport model was applied to estimate the residence time of circulated seawater within the STE using measured radionuclide activities. There are inherent uncertainties within the model, set by the activity of deep groundwater that has mixed with the circulated seawater, the initial activity of seawater that infiltrated the coastal aquifer and the radionuclide production rates. Residence times generally increased with depth from 0.1 to ≥15d and were greater in the low tide well of the barrier beach (4.9±3.4d) compared to the coastal bluff (1.1±0.4d). Residence times were fairly constant over time within the coastal bluff STE but varied seasonally in the barrier beach STE, suggesting that the barrier beach may have not been in “steady-state.” Variable pore water residence times within the STE may have a profound effect on the cycling of carbon, nutrient and trace metal fluxes to the coastal ocean.