Concerns were held of a possible transport mechanism for saline contaminants detected in a freshwater stream north of a water storage dam on the power station site. GBG Australia was contracted to undertake a multi-discipline geophysical investigation.
Localised boreholes provided limited information on the stratigraphy of the area. A broader understanding of the greater area was necessary to understand how fluid may potentially be transported toward the fresh water stream. To gain this broader understanding, two geophysical techniques were employed, Frequency-Domain Electromagnetics (FDEM) and Electrical Resistivity Tomography (ERT).
Prior to the geophysical investigation these sand and gravel lenses were thought to have offered the permeability required for fluid transport and therefore were deemed to likely be the transport mechanism.
The requirements of the geophysical investigation were to identify the location and extent of sedimentary layers and to aid in the definition of more permeable layers. The data would then be used to generate a groundwater conceptual model. GBG Australia carried out FDEM to cover the areas broadly between the water storage dam and a freshwater stream to the north. ERT was conducted concurrently. Three ERT profiles were positioned perpendicular to the expected direction of flow towards the freshwater stream, and at equal distances moving away from the power stations water storage dam. The locations the survey are shown in Figure 1. The rationale to these positions was to attempt to intersect any sediment lenses or migration paths, as well as to understand how they may change between the power station and the freshwater stream.
The FDEM results revealed a high conductivity response that appeared to be originating and migrating from the water storage dam in question. The results showed the response decreasing in intensity towards the freshwater stream. This was interpreted as either a deepening migration path to the north and being beyond the penetration of the instrument or some sort of barrier between the power station site and property to the north separated by an unsealed road.
The ERT results show small lenses of low resistivity, or conversely, higher conductivity, that deepen towards the freshwater stream. This corroborated the theory that a possible migration path is deepening towards the North.