Toro Energy Limited – Wiluna Uranium Project
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Hydrogeology
6.1 Regional Hydrogeology The regional groundwater flow is directed towards Lake Way from all sides. Groundwater flows in the Project Area are maintained by recharge following rainfall though gradients near the lake. Significant recharge is likely to occur during rare periods of heavy rainfall, often associated with aftermaths of cyclones. Recharge occurs by direct infiltration of surface water through calcrete, sandy stream-beds and fractured bedrock outcrops. The remainder of rainfall is either evaporated, utilised by native vegetation and/or forms runoff to streams and salt lakes (RPS Aquaterra, 2010b). The main aquifer systems in the Project Area are: • Low yielding, surficial alluvial and colluvial aquifers overlain by moderate to high yielding valley calcrete aquifers; • Moderate to high yielding sand aquifers developed within palaeochannels; • Fractured bedrock aquifers where permeability and porosity have been produced by fracturing; and • Extremely low yielding playa deposits comprised of evaporates, clay and sand (ibid). Groundwater levels are being monitored where possible from the same network of groundwater quality monitoring bores. Some of these bores are sealed around the installed pumps and cannot be accessed to ascertain water levels. There is no data on the volumes of water being extracted annually from these bores, nor any method to determine how long the bores have been pumped prior to water levels being taken. The data on water levels from these bores is useful to determine the range of current water level fluctuations due to water being extracted for stock and domestic requirements. Another network of specifically constructed bores has been established (2010 Toro drilling bores) to specifically monitor water levels around the margins of the Centipede and Lake Way deposits. The data from these bores will be used to understand natural water level fluctuations in these areas due to localised rainfall and evapotranspiration. The longer term data collection would also be used to determine natural trends in groundwater levels. During mining operations the data collection would be used to verify the extent of water level drawdown at the margin of the mining activities and be compared with predicted modelled drawdowns due to dewatering from these excavations. Groundwater quality is being monitored around Lake Way in specifically completed observation bores in addition to local stock and domestic supply bores. 6.1.1 Surficial alluvial-colluvial aquifers including Calcretes Surficial alluvial-colluvial aquifers have a watertable of generally less than 5m below ground level (mbgl). The hydraulic conductivity of these aquifers is low with bore yields varying from 50 to 600m3/day (RPS Aquaterra, 2010b). Calcrete forms localised, high-yielding aquifers due to secondary porosity and high permeability due to fractures. Calcrete generally occurs in the lower portions of the drainage system where the watertable is shallow (generally less than 5mbgl). Saturated thicknesses generally range from 0.5m to 24mm. The salinity of groundwater in calcrete is frequently brackish to saline. However it can be fresher soon after and where recharged directly from rainfall. Potential bore yields are reported to range between 100 and 4,400m3/day (ibid).
WATER MANAGEMENT
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