AN INVERSE MODELING APPROACH TO EXAMINE POREWATER MIXING WITHIN A SUBTERRANEAN ESTUARY

Submarine groundwater discharge (SGD) consists of both terrestrial and marine sources, and the zone where these sources mix has been aptly named the subterranean estuary. Distinguishing these sources is important for evaluation of freshwater and dissolved constituent inputs to coastal waterbodies. Porewater and surface water samples were collected in fall, winter, and late spring (2004-05) from a transect extending from 10 m onshore to approximately 30 m offshore into Indian River Lagoon (IRL), Florida. The transect consists of eight multilevel porewater samplers, each of which has eight regularly-spaced ports extending to depths of 115 to 230 cmbsf. Samples were analyzed for ²²²Rn and ²²⁶Ra to evaluate the magnitude of recirculating seawater on SGD estimates in the IRL. Both porewater and surface water ²²²Rn activities are in excess of sediment ²²⁶Ra activities, which suggest that the Surficial aquifer plays an active role in transporting ²²²Rn and freshwater to the shoreface. Porewater salinities show the subterranean estuary appears to extend to about 22 to 25 m offshore. Excess ²²²Rn and salinity reveal a well-mixed zone of terrestrial groundwater and infiltrated lagoon water in the upper 25-35 cm. Both steady-state analytical and transient finite difference approximation models for the advection-diffusion and non-local exchange (ADNL) equation were used to evaluate excess ²²²Rn activity through space and time. The non-local exchange parameter, along with advection and diffusion, were used to examine the exchange velocity (piston velocity) needed to mix the upper porewaters. Initial conditions, the advection parameter, and the non-local exchange parameter were determined using an inverse modeling approach with the best-fit to the analytical solution to the steady-state version of the equation. Upper and lower boundary conditions were established using polynomial fits to surface water and the deepest porewaters through time. Initial estimates of non-local exchange and advection reveal that an exchange rate of 0.9 to 5 cm day^-1 is needed to maintain the observed shallow mixed zone.