Designing a dewatering plan for the Ruashi mine in the Democratic Republic of Congo
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This dissertation describes the results of field based investigations on groundwater flow at Ruashi Mine located in Katanga Province of Democratic Republic of Congo (DRC). The core objectives of the study were to simulate groundwater flow, estimate flow into the pits and ultimately design a dewatering strategy for the mine. In order to understand how groundwater flows into and through the mine, a detailed conceptual hydrogeological model was constructed as framework for numerical groundwater flow modelling. The numerical model was used simulate groundwater flow and predict pit inflow volumes. At the time of this research, mining at Ruashi was being carried out in three pits that are expected to reach terminal depth of 180 metres below ground level (mbgl) in 16 years of continued mining. The mine is located along a faulted overturned syncline composed of composed of Siltstones, Argillites, Sandstones and Shales and covered by Laterite. Based on aquifer hydraulic testing results, the transmissivity of the shallow aquifer was estimated to be 10 m2/d. The specific yield for the deep aquifer was estimated to be 1 x 10-5. The Chloride Mass Balance Method was used to estimate recharge to the groundwater system as 280 mm per annum (14% of Mean Annual Precipitation). Water levels vary from 1.02 to 62.5 mbgl. The general groundwater type was analysed to be calcium-magnesium-bicarbonate (Ca-Mg- HCO3), typical of young groundwater. The numerical groundwater flow model area is 15.7 km2 and comprises 5 layers, 17 240 elements and 10 614 nodes. The model results indicated that groundwater flow to the pits is unlikely to exceed 42 000 m3/d. Using the pumping capacity (15 000 m3/d) for year 2012, a maximum water level drawdown of 55 m was estimated. However, the numerical model demonstrated that the existing pumping boreholes can be augmented by an additional set of 16 boreholes pumping 2 000 m3/d per borehole. This pumping rate can lower the groundwater level to about 1 188 mamsl which is about two meters below pit terminal elevation. This study made significant contribution to understanding the hydrogeological properties of aquifers at the mine. The aquifer hydraulic testing data was used to estimate aquifer hydraulic parameters. However based on the field evidence, it is suggested that Packer testing could improve the estimates of aquifer hydraulic parameters for each aquifer. The numerical model demonstrated the typical aquifer response to different pumping scenarios. The different pumping scenarios were run in order to determine the optimum pumping rates to dewater the mine.