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

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. The low 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 is available 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 impacts on the volume of water available for recharge to the aquifer in the area. Despite the high rainfall volume and high rainfall intensity only one perennial surface run-off structure exists 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 Ipumbu Dam. The volume of water that evaporates from the soil and vegetation before it can reach the underlying aquifer will impact on the volume of water that can be abstracted from the aquifer 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 calculated potential total evaporation volume with the measured net evaporation and annual rainfall indicates a discrepancy between the evaporation and rainfall data. The rainfall data is considered 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 the surface run-off characteristics. Brachystegia Miombo woodlands cover the study area with areas of widespread grass cover between the trees. The main aquifer consists of Upper Roan Formation limestone and dolomite. The main joint set (160º strike direction) controls the strike direction of the solution cavities in the area. Schist and quartzite of the Lower Roan Formation border the limestone and dolomite. Extensive fracturing in the dolomite is found in the geological logs of the high yielding boreholes drilled in the area. The soil in the study area displays infiltration rates higher than the observed rainfall intensity. Combined with the low topographical gradients that retard run-off the high infiltration rate leads to high recharge percentages. The aquifer hydraulic characteristics were determined during the investigation. It is considered that Lake Nampamba forms part of an extensive, well-connected solution cavity network. No compartmentalisation of the dolomite is evident. Three prominent karstic features (Lake Nampamba, Lake Kashiba and the “Chibili Pavement”) occur in the area. Both Lake Nampamba and Lake Kashiba are sinkholes in the dolomite. The author has no scientific evidence to conclude that the solution cavity network stretches 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 in the west that feed Ipumbu dam are interconnected. The groundwater displays a calcium-magnesium dominant character, as is expected from a dolomitic aquifer. Plotting the chemistry data on a trilinear (Piper) diagram indicates that the groundwater recently recharged. This corresponds to the theory of high recharge due to the relatively high rainfall, flat topography, and high soil infiltration rate. Depth to groundwater varies on a seasonal basis due to fluctuations in rainfall and thus recharge, and abstraction volumes. The groundwater level shows an immediate response to recharge, but does indicate a one to two month delay between maximum rainfall 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 were identified as high yielding boreholes. Constant rate aquifer tests were performed on these 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 is expected in karstic aquifers where the high transmissivity is associated with solution cavities and fractured areas. The low transmissivities are associated with the fine crystalline, competent, unweathered rock. Sustainable yields from the borehole calculated 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 CRD methods. 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 annual rainfall. 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 western aquifers is calculated to be 136Mm3/annum. The current and proposed future annual abstraction volumes are 25.3Mm3 and 44.42Mm3 respectively. This indicates that the combined eastern and western aquifers are capable of sustaining the abstraction volumes. A numerical model was constructed to evaluate the assumptions made, and correlates the calculated values of the manual calculations. The model was initially constructed in steady state without taking the abstraction from the aquifer into account. Once the model was successfully calibrated, the abstraction volumes were incorporated into the numerical model and the model was further calibrated against time series observed rainfall, groundwater levels and abstraction volumes by comparing the groundwater levels with time against those calculated using the numerical model. Once the model was calibrated, the model was applied to evaluate the sustainability of the current and proposed abstraction programs. The numerical model confirms that the combined eastern and western aquifers are capable of sustaining the abstraction programs.