MODELING DENSITY-DEPENDENT FLOW AND SOLUTE TRANSPORT AT THE LAKE TUTCHEWOP SALINE DISPOSAL COMPLEX, VICTORIA

Intercepted saline groundwaters and drainage effluent from irrigation are commonly stored in both natural and artificial saline disposal bas ins throughout the Murray-Darling Basin of Australia. Their continued use as wastewater evaporation sites requires an understanding of exist ing groundwater dynamics. The useful lifetimes of individual basins, t heir sustainability and possible environmental impacts remain largely unknown. In this work, the movement of salt to the underlying groundwa ter system from Lake Tutchewop, a saline disposal complex in north-cen tral Victoria, was modelled in cross-section. Due to the salinity cont rast between the hypersaline basin waters and the regional groundwater , it was necessary to simulate density dependent flow behaviour. Under certain conditions, these density-stratified systems may become unsta ble leading to the onset of convective behaviour, which greatly increa ses the movement of salt from the basin to the groundwater system. Mod elled concentration profiles in the aquifer system and calculated seep age rates from the basin show that Lake Tutchewop is stable under its present operating regime. The downward movement of salt is mainly cont rolled by diffusion and dispersion. The calibrated model was used to a ssess the impact of several management scenarios using time-dependent boundary conditions for lake salinity and water levels. The influence of heterogeneous basin linings on ensuing salt flux rates is examined, and results show that increased solute transport will occur under suc h conditions. A sensitivity analysis performed on governing variables showed that salt fluxes were most sensitive to lake salinity levels. A solute Rayleigh number defined in terms of basin salinity and hydroge ologic parameters is seen to be an effective tool for predicting the l ong term behaviour of such saline disposal basins. The models and conc epts developed in this work may find application in the design and man agement of saline disposal complexes. (C) 1998 Published by Elsevier S cience B.V. All rights reserved.