Physical and Numerical Modeling of Unstable Flow Due to Heavier Saltwater Overlying Freshwater

It is well-known that unstable flows occur when denser saltwater overlies lighter freshwater. One such situation occurs in the Indian River Lagoon (IRL), a coastal estuary, located on the east coast of Florida. The IRL water is brackish as it is a mixture of the saltwater from the Atlantic Ocean flowing through inlets and the freshwater flowing from canals, rivers and groundwater. This brackish water overlies the freshwater that flows in the unconfined aquifer below the IRL. All cases of unstable density stratification result in a rapid and erratic redistribution of the salt. The rate of salt movement is much different than predicted by ordinary molecular diffusion alone. The dynamics of this unstable flow was investigated using a physical model made of Plexiglas which is 1.83m high and has a base of 0.61m by 0.61m. The model was filled with 1.64m of sand and the part above the sand, the source area, was used to add seawater or brackish water. The model consisting of five ports located at 0.255, 0.560, 0.865, 1.170, and 1.474m from the source was filled with 40F sand which was saturated with freshwater. This paper shows breakthrough curves from two experiments, conducted over five days and over 28 days in which the source concentration was kept constant at 36,000 mg/L at a depth of 4.5cm. Results indicate that the flow was unstable in the beginning (did not yield smooth breakthrough curves) but stabilized after approximately 23 days. Measured breakthrough curves were compared with numerically simulated curves using SEAWAT. It was found that the salt concentrations, under unstable conditions, can be predicted with reasonable accuracy if (1) the selected molecular diffusion coefficients are several orders of magnitude higher than values obtained through traditional experiments and (2) the selected molecular diffusion coefficients are time-dependent.