Towards a conceptual hydrogeological model of the Chimiwungo ore body, Lumwana Mine, Northwestern Zambia
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A deep and clear understanding of the hydrogeology of an area is paramount in any dewatering project. The development of a detailed conceptual hydrogeological model goes a long way in creating an appreciation of the prevailing groundwater situation in active and proposed mining areas. The influx of water during mining and the collapse of pit walls present perennial problems and dangers to those involved. Over recent years, dewatering has evolved into a highly specialized field of hydrogeology. The proposed Chimiwungo mine pit is expected to go to depths beyond 300m by the year 2040. A series of interconnected fractures are suspected to be the main conduits for groundwater movement and storage. In order to achieve pit slope stability, pore water depressurization will need to be carried out alongside the dewatering of the main fractures and faults. The study focused on the steps undertaken in the development of a conceptual hydrogeological model of the Chimiwungo ore body. The report also outlines the problems encountered in the process and the shortfalls in the final conceptual model developed. Existing information and projected mine plans were analysed as a basis for the field investigations undertaken. A hydrocensus was carried out to establish existing groundwater and surface water monitoring points. Suitable positions for the drilling of new monitoring boreholes were selected on the basis of the mine geological model and exploration drilling. Drilling was problematic from the onset due to difficulties in accessing drilling sites, the highly unstable weathered material near the surface. Only one of the three large diameter boreholes and four of the six small diameter boreholes were completed in a period of eight months. The intention was to carry out pumping tests on the three large diameter monitoring boreholes. The massive delays and the inability of the drilling contractor to complete the drilling programme forced the mine to defer pumping tests to a later date. Drilling was suspended in May 2011 and, by the time of submission of this thesis, still had not resumed. Aquifer parameters had to be estimated by consulting literature and using accepted approximations. The study confirmed the existence of two aquifers; an upper weathered zone and a lower fractured zone. The high level of correlation between the groundwater level elevation and topographic elevation (98%) is evidence that the groundwater level elevation mimics the topography. This observation suggests that the aquifers being investigated are either unconfined or semiconfined/ leaky. Analysis of the piezometry defines the south-western part of the study area as the recharge zone and the discharge is to the surface water drainages in the middle of the area. Boreholes drilled into the weathered zone have very shallow water levels and respond very quickly to rainfall events. This may signify that most of the groundwater recharge takes place in the upper weathered zone. The general groundwater type can be characterized as a calcium-magnesium-bicarbonate (Ca- Mg-HCO3) type, indicating recently recharged groundwater. This is indicative of good quality groundwater which has not undergone intensive ion exchange processes as evidenced by the low TDS. Generally surface water quality within the study area is good to moderate with low levels of salinity and limited or low concentrations of dissolved metal species. Groundwater recharge was estimated at 498 mm/yr (38% of Mean Annual Precipitation) for the study area. The Chloride Mass Balance Method was used to estimate the average recharge value. The specific yield for the weathered rock aquifer at Chimiwungo was estimated by calculating the average Sy of silt and clay; Sy at Chimiwungo = 6 Storativity was assumed to be 1 x 10-5 for the fractured rock aquifer, as calculated by Golder Associates in 2003. The transmissivity values obtained through estimation (transmissivity is approximately equal to five time the blow yield) are comparable to the values obtained by Golder in 2003. The Chimiwungo River was set as the northern and eastern boundary (constant head) for the Chimiwungo Main pit. The southern and western boundaries were set as no-flow boundaries due to a topographic high. Similarly for the Chimiwungo North pit, the Chimiwungo River was set as the southern and western (constant head) boundary and the northern and eastern boundaries were set as no-flow boundaries due to a topographic high that occurs in this area. All other boundaries follow the quaternary catchment boundary and are therefore also assigned no-flow conditions.