Density-dependent surface water-groundwater interaction and nutrient discharge in the Swan-Canning Estuary

Salinity in the Swan-Canning Estuary, Western Australia, varies seasonally from freshwater conditions in winter up to the salinity of seawater in summ er. Field observations show that the resulting seasonal density contrasts b etween the estuary and the adjacent fresh groundwater system are sufficient to drive mixed-convection cells that give rise to circulation of river wat er in the aquifer. In this study, we examine the role of steady density-dri ven convection as a mechanism that contributes to the exchange of dissolved nutrients, particularly ammonium, between the Swan-Canning Estuary and the local groundwater system. We present results from two-dimensional (section ) and three-dimensional density-coupled flow and mass transport modelling, in comparison with Glover's abrupt-interface solution for saltwater intrusi on. The modelling is focused on developing an understanding of the physical processes that influence the longterm or mean convective behaviour of grou ndwater beneath the estuary. It :is shown that the convective stability dep ends fundamentally on the interplay between two factors: (1) the downward d estabilizing buoyancy effect of density contrasts between the estuary and a quifer; and (2) the upward stabilizing influence of regional groundwater di scharge. The structure of convection cells beneath the estuary and recircul ation rates of estuary water within the groundwater system are shown to be related to a flow-modified Rayleigh number that depends critically on the a quifer anisotropy and estuary meander pattern. The recirculation of estuary water by these mechanisms is responsible for transport of high concentrati ons of ammonium, observed in pore fluids in the estuary bed sediments, into groundwater and its eventual return to the estuary. Copyright (C) 2001 Joh n Wiley & Sons, Ltd.