Geophysics to assess impacts of a proposed pit dewatering disposal into a sensitive local creek and on shallow and deep aquifer interactions

Abstract:

With local drilling access restrictions, a confidential options study for disposal of a Pilbara mine pit dewatering discharge in Western Australia, employed geophysics to help understand the fate of water discharged into a local ephemeral creek tributary of a main arid ephemeral creek system.

Of particular interest was a potential altered hydrological regime from these discharges, for ephemeral stream flows and the instream botanical environment, the capacity of the stream bed sands to absorb and buffer the discharges, the nature and geometry of the sands and the interaction of these sands with possible deeper paleo-valley and bedrock aquifers and shallow flood plain aquifers. An airborne EM centre-of-stream transect, limited ERI across-stream transects and NMR soundings, plus interpretation of existing regional geophysics and remote sensed data, within the limitations of each method, mapped the local flood plain, the shallow stream bed sands and the underlying mainstream paleo-valley and bedrock of the tributary.

The tributary and mainstream stream bed sediments are interpreted (from ERI and airborne EM), to be low resistivity, low water content and showing frequent shallow induration and (from NMR) low in permeability and increasingly lower downstream. Shallow sediments, not indurated over time, show higher permeabilities, some possibility to buffer discharge waters, are up to 10 m and typically < 2 to 5 m thick and appear to extend onto the surrounding terraces of the flood plain in places.

ERI transects show the main creek paleo-channel contains semi-continuous cemented layers (aquitards?) and is up to 130 m deep and 200 m or more west of the current stream. Radiometric, DTM and ASTER data indicate a broader recent floodplain also. The DTM shows the mainstream as aggrading, variable gradient and up to 2 m above the floodplain. Cross cutting structure evident on regional geophysics has probably influenced valley sediment development over time.

Airborne EM indicates mostly continuous low conductivity, clays or weathered bedrock allowing the occasional connectivity of the shallow stream sands and potential bedrock aquifers (such as dolomites).

In conjunction with proposed discharge / buffer trials, pre- and post-trial shallow ERI transects and confirmatory drilling and piezometer monitoring is recommended.