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Salinity Processes

Sources of salt

There are at least five sources of salt in the landscapes of the Corangamite region, viz:

• Cyclic salt. Cyclic salt is being added through rainfall, which contains small quantities of dissolved salt from the oceans. Studies have shown that the quantity of salt in rainfall is proportional to the distance from the ocean.
• Depositional salt. Salts may be deposited with marine sediments (termed 'connate salt') or be accumulated by wind-blown salts from salt lakes, coastal flats, etc. Recent studies suggest that dust storms during the arid conditions of the last glacial period contributed significant quantities of salt to eastern Australian landscapes.
• Mineral dissolution. The dissolution of minerals by groundwater and their alteration during weathering can be a source of salt.
• Groundwater evaporation. Almost all of the groundwater in the Australian landscape contains salts, which can be concentrated by evaporation of discharge. Significant amounts can be added to the soil during centuries of groundwater discharge, even where the salt in the groundwater is present in low concentrations.
• Anthropogenic. Salts can be added to the landscape through the application of fertilisers, stock manure and urine, irrigation waters, etc.

Hydrologic change

The onset of secondary salinity has been associated with changes to the hydrologic budget. This is often related to the assumed widespread clearing of native vegetation and its replacement with exotic species that use less water. The most widely accepted model is that more water has recharged groundwater systems since widespread land-use change causing water tables to rise to the near-surface, mobilising the salts stored in the regolith.

However, some doubt exists on whether this model holds for all areas of south west Victoria. In some areas (e.g. Western Victorian volcanic plains, Heytesbury region) the depth of the groundwater tables below the surface may be relatively unchanged over the past 200 years and other factors - such as changes to soil waterlogging and regolith hydrology - are implicated in the spread of salinity. In these areas, where the rate of groundwater movement is very slow (e.g. Heytesbury) or the aquifers thin (e.g. volcanic plains) and the rainfall is high, the groundwater systems are regulated by the rate of discharge and it is believed that the recharge was always in excess of discharge. The groundwater systems have been relatively ‘full’ for centuries. The land-use change has resulted in excessive water in the near surface and the spread of salinity may be due to excessive soil waterlogging and shallow, temporal water flows in the near-surface.