Science, Technology and Engineering

Environmental Geoscience

Honours Projects

Hydrogeochemical study of Upper Bet Bet catchment

Katherine Coady

Supervisor: Dr John Webb

Severe dryland salinity in the Upper Bet Bet catchment requires a detailed hydraulic and chemical study of water resources.

Hydrograph data and Cl^- mass balance shows the catchment is being recharged primarily through the high metamorphic slopes of the St Arnaud meta-sediments during the winter months. Hydraulic pressure from lateral drainage into the less permeable Shepparton Formation alluvium has raised the watertable, exposing shallow groundwater to evapotranspirative concentration. This has been exacerbated over the last century by increased recharge due to clearing of native vegetation. Groundwaters and surface waters become more saline down the flow path towards the northeast of the catchment.

Major ion composition and stable isotopes show that groundwaters are homogenous across the catchment. Groundwater d¹⁸O and d²H signatures are very similar to that of the Melbourne winter rainfall average indicating recharge occurs predominantly in winter and that all groundwaters are of meteoric origin.

The headwaters of Bet Bet Creek show high salinities in the range of 1000-10,000 µS/cm and are severely incised by gully erosion. Bet Bet Creek shows large seasonal fluctuations in flow and salinity, increasing from ~200 µS/cm in winter and spring up to ~17,000 µS/cm in late summer and autumn. Influx of saline groundwater baseflow is identified by streamflow modeling.

The salt budget for the catchment indicates it is severely salinised and there is a net export of salt occurring; ~14,000 tonnes/year of salt leave the Upper Bet Bet via Bet Bet Creek. This is due to changes in the water balance mobilising cyclic salt from the unsaturated zone into groundwater.

Salinity in the Upper Bet Bet catchment is controlled by evapotranspiration of shallow groundwaters. Progressive infiltration of concentrated salts in soil water increases groundwater salinity down the flow path.