Science, Technology and Engineering

Environmental Geoscience

Honours Projects

Dryland salinity in the Creswick area, Victoria

Sarah McAliece

Dryland salinity has become a major problem in Victoria, particularly in the south east and central north of the state. Although dryland salinity in Victoria has been investigated for a long time and solutions and treatment have been the topic of much research, the hydrochemistry has not been studied in detail. The area around Creswick in the Loddon Catchment affected by dryland salinity was selected for detailed study. This area is characterized by volcanic cones and basalt lava flows overlying Ordovician sedimentary bedrock. Recharge occurs on the slopes of the volcanic cones and through fractures in the basalt. Discharge occurs at the base of slopes.

This study aims to develop a conceptual model for the process of salinisation around Creswick. Over 20 bores were selected from a large area from a variety of rock types and topographic settings. In addition to collecting water samples, the soil conductivity and salinity were measured at a number of sites.

Results show that, in general, groundwater salinity increases downslope, probably as a result of the progressive input of highly saline water infiltrating slowly through the very low permeability, highly saline soils on the Paleozoic bedrock. Recharge occurs largely through the thin, higher permeability soils on the volcanic cones and sedimentary hills. Because fracture permeability characterizes both basalts and Paleozoic sediments, flow pathways for groundwater are not always predictable. Thus, fresher groundwater is sometimes found in the higher elevation Paleozoic outcrops, underlying more saline groundwater in the basalt lavas.

In areas where salinity increases downslope, preferred pathways must be allowing water transmission to bypass the upslope saline groundwater. In general, soil permeability appears to have a controlling influence on groundwater salinity. Soils with a low hydraulic conductivity have slower water movement through the soil, allowing evapotranspiration to concentrate the infiltrating rainwater. Any aeolian accessions of salt will be readily flushed away. Thus, the underlying groundwater will have a higher salinity.