DETERMINING ORIGIN AND CHEMISTRY OF GROUNDWATER DISCHARGE TO THE MITCHELL RIVER, AUSTRALIA

Many large rivers are perennial, supplying vast quantities of fresh water for current and potential future irrigation development. The flow of these rivers is often maintained, during drier months of the year, by groundwater discharge. Determining the locations, fluxes and chemistry of groundwater discharge to these rivers is therefore of paramount importance to maintain their ecological value under future management decisions.

The Mitchell River, located in tropical north Queensland, Australia, was sampled at the end of two prolonged dry seasons to investigate whether its perennial flow was sustained by discharge of local or regional groundwater. Preliminary reconnaissance sampling of the upper half of the catchment in October 2010 provided ion chemistry, electrical conductivity (EC), radon (²²²Rn), chlorofluorocarbons (CFCs), helium (⁴He), water stable isotopes (²H/¹H and ¹⁸O/¹⁶O) and strontium isotopic ratio (⁸⁷Sr/⁸⁶Sr) data that were interpreted to infer locations – and potentially different sources – of groundwater discharging into the river. Then in October 2011 the Mitchell River was synoptically sampled by helicopter every 5 km along a 400 km reach. Water samples were taken from the main river channel and major tributaries for ion chemistry, EC, ²²²Rn, and ⁸⁷Sr/⁸⁶Sr, and many of these sites were flow gauged either using an ADCP (Acoustic Doppler Current Profiler) and/or a portable flowmeter. Simultaneously, groundwater samples were taken from existing bores completed in different geologies.

Hydrochemical, isotopic and hydrometric results all indicate that water from different sources, related to different geologies, contributes to maintain the river flow at the end of the dry season. Surface water or groundwater with very low residence time mostly contributes to dry season flow in the upper part of the catchment, with no significant contribution of deep and old groundwater anywhere along the studied reach. A one-dimensional, longitudinal river mass balance model, calibrated using PEST, was used to infer the spatial variability of groundwater discharge flux and its chemistry along a 400 km river transect. In doing so, we were able to link groundwater inflow with contrasted chemistry to different geologies found in the catchment.