MIXED CONVECTION PROCESSES BELOW A SALINE DISPOSAL BASIN

Saline groundwater and irrigation drainage are commonly diverted and s tored in both natural and artificial depressions throughout the Murray -Darling Basin of Australia. The disposal basin brines that are formed are often denser than ambient groundwater, Under certain conditions t hese dense brines may become unstable causing them to mix with groundw ater over distances several orders of magnitude greater than due to di ffusion alone. A model is developed to study the mixed convection proc esses below a saline disposal basin located between a recharge and dis charge zone. Numerical simulations are performed in cross-section usin g the 2-D density dependent model SUTRA (saturated-unsaturated transpo rt). It is shown that the salt front movement is related to both the a bility of the dense saline brines to mix convectively with underlying groundwaters (Rayleigh convection) and the strength of the regional ad vective velocity. Both homogeneous and heterogeneous aquifer systems a re studied and the effects of anisotropy are considered. Our numerical results suggest that the behaviour of a dense brine plume overlying l ess dense groundwater in a homogeneous porous medium depends on the ma gnitude of at least two non-dimensional numbers, a Rayleigh number and modified Peclet number, defined in terms of basin scale hydrogeologic parameters including dispersion, It is shown that the onset of gravit ational instabilities and the formation of free convective cells begin s when the magnitude of a non-dimensional parameter combining the Rayl eigh and modified Peclet number exceeds a certain critical value. (C) 1997 Elsevier Science B.V. All rights reserved.