Estimation of Dispersion and Characteristic Mixing Times in Plum Island Sound Estuary

Salinity distributions, dye release studies, and parameter estimation techniques are used to determine the longitudinal (or tidal) dispersion coefficient in the Plum Island Sound estuary in north-eastern Massachusetts, U.S.A. A one-dimensional, intertidal, advection–dispersion model is used to test the validity of the steady-state assumption employed to estimate tidal dispersion from salinity. It is found that salinity distributions in the upper Parker River can take months to relax to steady state under low freshwater discharge conditions, so that dispersion cannot be reliably estimated using the steady-state assumption. To overcome this problem, dye release studies and parameter estimation techniques are used to determine the functionality of the dispersion coefficient that produces the best fit between the observed salinity profiles, dye plumes and model output under transient conditions for the period between 28 April 1992 and 12 April 1995. The estimated dispersion coefficient is found to be a hyperbolic function of distance from the head of the estuary and varies from an average of 3•6 m2 s−1in the upper Parker River (0–5•2 km) to an average of 670 m2 s−1in Plum Island Sound (14•3–24 km). The advection–dispersion model is also used to investigate three characteristic mixing-time scales involving average age, average residence time and average transit time (also called turnover or flushing time) for freshwater and saltwater sources in defined subsections of the estuary. It is found that these time scales can vary from a few hours to 3 months depending on location and freshwater discharge. Transport in the upper Parker River can be either advection or dispersion dominated depending on discharge, while Plum Island Sound is always dominated by dispersive terms. Tidal channel morphometry leads to a very dynamic system in the upper Parker River that significantly impacts ecological processes in the region. We discuss the relevance and appropriate application of the three characteristic mixing-time scales to ecological processes.