Transient hydrological conditions implied by chloride mass balance in southeast Australian rivers

Abstract

A robust correlation between electrical conductivity (EC) values and Cl concentrations in river water from southeast Australia allows detailed Cl fluxes to be calculated from continuous EC and river discharge records. Many Victorian rivers export significantly more Cl than is delivered to their catchments by rainfall. Cl* is defined as the mass of Cl exported in the rivers relative to that input by rainfall over a multi-year period (Cl*=100% indicates that the river exports the same mass of Cl as is input by rainfall). There is a systematic relationship between catchment type and Cl*. Rivers draining cleared plains have Cl* values between 50 and 750%, rivers draining volcanic plains have Cl* values of 770–1600%, whereas rivers with large forested upland catchments have Cl* values of 50–110%. These values are minima as they do not account for Cl exported by groundwater from the catchments. The calculations are based on long-term (up to 22year) records that span drought and high rainfall periods. The magnitude of Cl* is far higher than can be explained by errors in the calculations or variability in rainfall and runoff, and Cl/Br ratios preclude halite dissolution as a source of Cl. The excess Cl reflects hydrological changes in the catchments. Land clearing on the cleared plains has caused the rise of regional water tables which results in the export of Cl from saline groundwater via increased baseflow to the river systems. Drainage systems on the volcanic plains are re-establishing following impoundment by recent (<4.5Ma) lava flows; Cl which accumulated in shallow groundwater around saline lakes and marshes developed on these volcanic plains is now being exported via the rivers. The upland catchments have undergone less landscape change and may be in chemical balance. The methodology outlined here provides a straightforward assessment of whether catchments are in chemical balance that may in turn indicate whether they are undergoing hydrological changes.