Delivery of marine chloride in precipitation and removal by rivers in the Murray-Darling Basin, Australia

Abstract

Rising groundwater tables and increasing river salinities are major problems in the semi-arid Murray-Darling (M-D) Drainage Basin of SE Australia. Chemical data from precipitation, rivers and groundwaters are integrated here to estimate chloride fluxes in the context of dramatic alteration of the hydrologic balance in the basin initiated during European settlement approximately 150 years ago. Our analysis indicates that about half of Cl deposition from the atmosphere in the western Murray Basin is derived from resuspended regional continental dust, resulting in high Ca/Na ratios in bulk precipitation samples. Estimates of recent marine deposition of Cl to the M-D Basin were made by excluding rainfall data from sites having high Ca/Na ratios. Basin-wide budgets for Cl delivery from the atmosphere and removal by rivers indicate that the Darling Basin retains about 90% of current annual input of marine Cl whereas the River Murray exports two to three times current annual input to the Murray Basin (exclusive of the Darling). Some implications from this study are as follows: 1. (1) Re-evaluation of net marine Cl input by rain implies that recent model calculations of 36 Cl Cl inputs from the atmosphere to the Murray Basin may differ significantly from actual values. In particular, net 36 Cl Cl input ratios from the atmosphere to the western Murray Basin may be a factor of two higher than those derived assuming all Cl in rain is of recent marine origin. 2. (2) The very large inventories of salt within the two catchments, especially the saline groundwaters of the Murray Basin, accumulated over very long periods, probably of the order of a few million years. The current rearrangements of stored salinity as a result, at least in part, of human perturbations of the hydrologic balance are likely to continue for an extended period because of the massive quantities of salts involved, and thus present major limitations on options for long-term management of the M-D Basin.