Science, Technology and Engineering

Environmental Geoscience

Honours Projects

Groundwater recharge and flow on a small scale (paddock) in the Willaura area, western Victoria

Kate Bailue

Supervisor: Dr John Webb

Very few studies have investigated small-scale (paddock) hydrogeological processes and how they interact with dryland salinisation. In order to improve our understanding of groundwater recharge and flow on this scale, one paddock in western Victoria was analysed using detailed hydrograph data, groundwater chemistry and soil characteristics.

Overall a topographic difference of just 2 m across the drainage line of the paddock is responsible for the large spatial variation in groundwater recharge, groundwater salinity and soil salinity.

The hydrographs examined for the 12 month study period show that recharge only occurs in the drainage line of the paddock as a consequence of surface runoff to the drainage line following large rainfall events (>15 mm), with no infiltration identified for the other areas of the paddock. Over the 12 months all hydrographs show a long term drop in groundwater pressure (30-100 cm), whereby infiltration was exceeded by evaporation and lateral groundwater flow, moving groundwater in a northwesterly direction out of the paddock, where it joins the regional groundwater system. Lateral groundwater flow is very slow (~53 cm/year) and reflects the low permeability of the sediment.

Groundwater salinity varies considerably across the paddock (2-29 mS/cm) and is moderately correlated to watertable depth (R² = 0.73). The most saline groundwater is found underneath the drainage line of the paddock and is the direct result of increased evaporation from the shallow watertable within ~ 2 m of the surface. This is also reflected in the unsaturated zone, which contains a higher concentration of salt underneath the drainage line. The groundwater composition shows nutrient uptake by plants is responsible for the depletion of Ca²⁺, Mg²⁺ and K⁺ in the groundwater, while ion exchange onto clays controls Na⁺ and Mg²⁺ levels.

The soil composition and texture do not vary significantly across the site and are therefore not responsible for the large spatial variation in soil salinity. All samples have a high silt content increasing to over 70% at ~ 1.5 m depth, indicating windblown component probably the result of aeolian deposition in the Pleistocene.

Soil Cl^- profiles from the soil samples collected from the paddock show that Cl^- concentration does not become constant with depth, due to the shallow watertable contributing salts to the soil by evaporation above the watertable. Salt budget calculations show that the groundwater contains ~2.5 times more salt than the unsaturated zone, and the paddock a slight net exporter of salt.

The project was co-supervised by Dr Baden Williams and supported by the Glenelg-Hopkins Catchment Management Authority.