LATE QUATERNARY AQUIFER SALINIZATION WITHIN THE MURRAY BASIN, AUSTRALIA

Groundwater salinization is the most pressing environmental problem facing farmers and cities across the Murray Basin, Australia. However, the origin of its dissolved salts remains enigmatic. Possible aquifer salinization mechanisms include: 1) reflux of lake brines, 2) evaporative concentration of salts within the vadose zone, and 3) diffusion of connate salts from a marine confining unit. Here we present a coupled surface/subsurface paleo-hydrochemical model in order to asses the relative importance of these salinization mechanisms that operated since the Late Quaternary. The study was conducted using an idealized representation of an east-west section of the eastern half Murray Basin. This idealized section represents general stratigraphy, which includes upper and lower Renmark aquifers and the extent of Geera clay confining unit. It also includes a series of equally spaced three lakes and a discharge area at downstream end representing surface and base-flow discharge to River Murray. Cosine functions were used to represent Late Quaternary climatic oscillations and to access the magnitude of the evaporative concentration of solutes during the dry climatic episodes of the Late Quaternary. Additionally, sensitivity analysis was conducted in which we systematically varied hydraulic conductivities of different geologic units, and initial salinity conditions. Our results indicate that evaporative concentration of salts from lakes and associated brine reflux was the dominant aquifer salinization mechanism that operated since the Late Quaternary period. There was a time lag of about 1000 years between upstream and downstream lakes in response to a given climatic forcing which is consistent with the lake-groundwater hydrodynamics. Reflux of brines from the upstream lake contributed to salinity built-up in confined aquifer where as reflux of brines from the downstream lakes contributed to salinity built-up in unconfined aquifer. Although, there was significant salinity build-up in the vadose zone, its contribution to aquifer salinization was less dramatic owing to lower flux rates. Our computations indicate that the seawater within confining units would have been flushed within 105 years by vertical discharge and it is unlikely that the Murray Basin has achieved dynamic equilibrium condition with respect to solute transport.