Characterisation of the dolomitic aquifer in the Copperbelt Province, Northern Zambia

[摘要] English: External factors that influence the aquifer characteristics of, and sustainable yield from,the dolomitic aquifer include topography, rainfall, surface drainage, evapotranspiration,plant growth, geology, and soils.The topography is gently undulating with a gradient of between 1:400 and 1:800. Thelow topographical gradients inhibit surface run-off and promote high recharge rates.The annual rainfall will provide the volume of water present in the catchment area that isavailable for potential recharge to the aquifer. The average rainfall is 1 115mm/annum.Water that leaves the hydrological system of the study area as surface run-off impactson the volume of water available for recharge to the aquifer in the area. Despite the highrainfall volume and high rainfall intensity only one perennial surface run-off structureexists in the study area. The Kafue River is on average 10 to 20m wide and 3m deep.Only one non-perennial surface run-off feature exists. This feature feeds into the IpumbuDam.The volume of water that evaporates from the soil and vegetation before it can reach theunderlying aquifer will impact on the volume of water that can be abstracted from theaquifer without negatively impacting on the volume of water in storage in the aquifer.Very little evaporation data is available for the study area. Comparing the calculatedpotential total evaporation volume with the measured net evaporation and annual rainfallindicates a discrepancy between the evaporation and rainfall data. The rainfall data isconsidered to be accurate, and based on perceived inaccuracy of the evaporation data,it is recommended that the evaporation data not be used in water balance calculations.The natural plant growth influences the evapotranspiration and in some cases thesurface run-off characteristics. Brachystegia Miombo woodlands cover the study areawith areas of widespread grass cover between the trees.The main aquifer consists of Upper Roan Formation limestone and dolomite. The mainjoint set (160º strike direction) controls the strike direction of the solution cavities in thearea. Schist and quartzite of the Lower Roan Formation border the limestone anddolomite. Extensive fracturing in the dolomite is found in the geological logs of the highyielding boreholes drilled in the area.The soil in the study area displays infiltration rates higher than the observed rainfallintensity. Combined with the low topographical gradients that retard run-off the highinfiltration rate leads to high recharge percentages.The aquifer hydraulic characteristics were determined during the investigation. It isconsidered that Lake Nampamba forms part of an extensive, well-connected solutioncavity network. No compartmentalisation of the dolomite is evident. Three prominentkarstic features (Lake Nampamba, Lake Kashiba and the 'Chibili Pavement) occur inthe area. Both Lake Nampamba and Lake Kashiba are sinkholes in the dolomite.The author has no scientific evidence to conclude that the solution cavity networkstretches as far as the Kafue River. However, analysis of the water level, abstraction and water level data does confirm that Lake Nampamba in the east and the dambo areas inthe west that feed Ipumbu dam are interconnected.The groundwater displays a calcium-magnesium dominant character, as is expectedfrom a dolomitic aquifer. Plotting the chemistry data on a trilinear (Piper) diagramindicates that the groundwater recently recharged. This corresponds to the theory ofhigh recharge due to the relatively high rainfall, flat topography, and high soil infiltrationrate.Depth to groundwater varies on a seasonal basis due to fluctuations in rainfall and thusrecharge, and abstraction volumes. The groundwater level shows an immediateresponse to recharge, but does indicate a one to two month delay between maximumrainfall and peak groundwater level.A total of 65 boreholes were drilled in the study area between 1978 and 2004.Preliminary aquifer tests were performed on 38 boreholes, and 14 boreholes wereidentified as high yielding boreholes. Constant rate aquifer tests were performed onthese boreholes. The aquifer test data was analysed to obtain the aquifer transmissivity.The aquifer transmissivity ranges between 1 and 6 900m2/day. This wide range isexpected in karstic aquifers where the high transmissivity is associated with solutioncavities and fractured areas. The low transmissivities are associated with the finecrystalline, competent, unweathered rock. Sustainable yields from the boreholecalculated from the aquifer test data range between 5 and 100l/s (432 and 8 640m3/day).Recharge calculations were performed using the Chloride, SVF, Equal Volume and CRDmethods. The chloride method is used only as an indication of the recharge percentage.The SVF, Equal Volume, and CRD methods calculated recharge as 25% of the annualrainfall.Aquifer storativity is calculated using the SVF and CRD methods at 0.02 (2%).The total volume of water that can be abstracted from the combined eastern and westernaquifers is calculated to be 136Mm3/annum. The current and proposed future annualabstraction volumes are 25.3Mm3 and 44.42Mm3 respectively. This indicates that thecombined eastern and western aquifers are capable of sustaining the abstractionvolumes.A numerical model was constructed to evaluate the assumptions made, and correlatesthe calculated values of the manual calculations. The model was initially constructed insteady state without taking the abstraction from the aquifer into account.Once the model was successfully calibrated, the abstraction volumes were incorporatedinto the numerical model and the model was further calibrated against time seriesobserved rainfall, groundwater levels and abstraction volumes by comparing thegroundwater levels with time against those calculated using the numerical model.Once the model was calibrated, the model was applied to evaluate the sustainability ofthe current and proposed abstraction programs. The numerical model confirms that thecombined eastern and western aquifers are capable of sustaining the abstractionprograms.

[发布机构] University of the Free State