Towards a conceptual hydrogeological model of the Chimiwungo ore body, Lumwana Mine, Northwestern Zambia

[摘要] A deep and clear understanding of the hydrogeology of an area is paramount in anydewatering project. The development of a detailed conceptual hydrogeological model goes along way in creating an appreciation of the prevailing groundwater situation in active andproposed mining areas. The influx of water during mining and the collapse of pit wallspresent perennial problems and dangers to those involved. Over recent years, dewatering hasevolved into a highly specialized field of hydrogeology.The proposed Chimiwungo mine pit is expected to go to depths beyond 300m by the year2040. A series of interconnected fractures are suspected to be the main conduits forgroundwater movement and storage. In order to achieve pit slope stability, pore waterdepressurization will need to be carried out alongside the dewatering of the main fracturesand faults.The study focused on the steps undertaken in the development of a conceptualhydrogeological model of the Chimiwungo ore body. The report also outlines the problemsencountered 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 fieldinvestigations undertaken. A hydrocensus was carried out to establish existing groundwaterand surface water monitoring points. Suitable positions for the drilling of new monitoringboreholes 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, thehighly unstable weathered material near the surface. Only one of the three large diameterboreholes and four of the six small diameter boreholes were completed in a period of eightmonths.The intention was to carry out pumping tests on the three large diameter monitoringboreholes. The massive delays and the inability of the drilling contractor to complete thedrilling programme forced the mine to defer pumping tests to a later date. Drilling wassuspended 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 acceptedapproximations.The study confirmed the existence of two aquifers; an upper weathered zone and a lowerfractured zone.The high level of correlation between the groundwater level elevation and topographicelevation (98%) is evidence that the groundwater level elevation mimics the topography. Thisobservation 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 asthe recharge zone and the discharge is to the surface water drainages in the middle of thearea. Boreholes drilled into the weathered zone have very shallow water levels and respondvery quickly to rainfall events. This may signify that most of the groundwater recharge takesplace 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 goodquality groundwater which has not undergone intensive ion exchange processes as evidencedby the low TDS. Generally surface water quality within the study area is good to moderatewith 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) forthe study area. The Chloride Mass Balance Method was used to estimate the average rechargevalue.The specific yield for the weathered rock aquifer at Chimiwungo was estimated bycalculating the average Sy of silt and clay; Sy at Chimiwungo = 6 Storativity was assumed tobe 1 x 10-5 for the fractured rock aquifer, as calculated by Golder Associates in 2003. Thetransmissivity values obtained through estimation (transmissivity is approximately equal tofive 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 theChimiwungo Main pit. The southern and western boundaries were set as no-flow boundariesdue to a topographic high. Similarly for the Chimiwungo North pit, the Chimiwungo Riverwas set as the southern and western (constant head) boundary and the northern and easternboundaries 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 alsoassigned no-flow conditions.

[发布机构] University of the Free State