Hydrogeology and salinity processes in the Mt Mercer-Illabarook area

Hydrogeology and salinity processes in the Mt Mercer-Illabarook area

Mockunas, Grant Douglas (University of Ballarat, School of Science & Engineering, Department of Geology)

The Illabarook salinity target area is identified in the Corangamite Catchment Management Authority's Salinity Action Plan 2005 - 2008 as contributing to the rising salinity in the Woady Yaloak River, threatening the river ecosystem and Lake Corangamite. This investigation of the hydrogeology and salinity processes occurring in the target area involved geophysical surveys, soil analysis, installation of groundwater monitoring bores, downhole salinity assessment, aquifer recovery testing and surface water sampling.|The geophysical survey delineated salinity and saline discharge across three sites at Illabarook and Mount Mercer. The mapping identified salinity is primarily associated with drainage areas underlain y Palaeozoic (Ordovician) sedimentary rocks of the Castlemaine Group. Installation of groundwater investigation and monitoring bores at Illabarook and Mount Mercer, determined that groundwater flow systems are variable across the target area. Neogene sediments originally considered the main source of saline discharge, were found to be spatially variable and contributing to a lesser extent that previously hypothesised. Flow systems in the weathered Palaeozoic (Ordovician) rocks were identified by geophysical mapping and piezometer installations to be the principal source of saline discharge.| The influence of groundwater flow within Deep Lead systems was also investigated and it is concluded that the influence of these highly variable systems on saline discharge is considered greater than previously hypothesised. Although the exact extent and nature of these systems is yet to be determined, it is believed they have a major influence on the location of saline groundwater discharge in some areas. Surface water sampling was conducted at ten sites across the target area and enabled greater definition of stream salt loads eminating from subcatchments within the target area. The data indicated a strong relationship between rainfall and stream electrical conductivity (salinity). The analysis indicated that in certain areas, groundwater-surface water interaction is contributing to stream salt loads.| This investigation concludes that the occurrence of salinity is strongly controlled by the groundwater flow systems driving it. Revised conceptual hydrogeological models were developed based on the research findings. In line with flow system characteristics contributing to saline groundwater discharge, the plantation of broadscale tree belts targeting recharge and discharge areas is believed an appropriate management response.

Corangamite Catchment Management Authority, National Action Plan for Salinity and Water Quality, University of Ballarat