Submarine groundwater discharge to a small estuary estimated from radon and salinity measurements and a box model

Submarine groundwater discharge was quantified by a variety of methods in Salt Pond, adjacent to Nauset Marsh on Cape Cod, USA. Discharge estimates based on radon and salinity took advantage of the presence of the narrow channel connecting Salt Pond to Nauset Marsh, which allowed constructing whole-pond mass balances as water flowed in and out due to tidal fluctuations. A box model was used to estimate discharge separately to Salt Pond and to the channel by simulating the timing and magnitude of variations in the radon and salinity data in the channel. Discharge to the pond is estimated to be 2200±1100 m³d^-1, while discharge to the channel is estimated to be 300±150 m³d^-1, for a total discharge of 2500±1250 m³d^-1 to the Salt Pond system. This translates to an average groundwater flow velocity of 3±1.5 cm d^-1 Seepage meter flow estimates are broadly consistent with this figure, provided discharge is confined to shallow sediments (water depth <1 m). The radon data can be modeled assuming all groundwater fluxes to both the channel and to the pond are fresh, with no need to invoke a saline component. The absence of a saline component in the radon flux may be due to removal of radon from saline groundwater by recent advection of seawater or it may to due to the presence of impermeable sediments in the center of the pond that limit seawater recirculation. This groundwater flux estimated from the radon and salinity data is comparable to a value of 3200-4500 m³d^-1 predicted by a recent hydrologic model (Masterson, 2004; Colman and Masterson, 2004). Additional work is needed to determine if the measured rate of discharge is representative of the long-term average, and to determine the rate of groundwater discharge seaward of Salt Pond. Data also suggest a TDN flux from groundwater to Salt Pond of ~2.6 mmol m^-2d^-1, a figure comparable to fluxes observed in other eutrophic settings.