Sedimentology, hydrogeology and hydrogeochemistry of Machile Basin, Zambia

Abstract

An important environmental problem in arid to semi-arid regions of the world is salinisation of water resources. Moreover, most of these regions are inhabited by poor communities in developing countries, with limited or no financial resources to develop suitable alternatives to meet their basic water needs. With increasing world population and increasing effects of climate change, groundwater resources are seen as a suitable alternative. However, the exploitation of groundwater in these regions requires knowledge of aquifer structure and genesis, and mechanisms that control hydrochemistry. Water resources practioners in arid and semi-arid regions accept that future development of groundwater resources depends largely on the degree and rate of salinisation. This PhD study focuses on investigating the sedimentological and hydro-geochemical conditions that have shaped the groundwater environment in Western Zambia to the present hydrogeology, in particular the origin and dynamics of groundwater salinity. The case study is the Machile Basin of south-western Zambia, a semi-arid region on the northern extension of a desiccated lake system – Palaeo-lake Makgadikgadi (PLM), southern Africa. PLM is a palaeo-mega endorheic lake system that was formed due to tectonic disruptions in the Early Pleistocene (speculated at c. 1.4 Ma). The lake has sustained several lake levels, with the highest level at ~995 m above mean sea level (amsl) and became largely desiccated in the Late Pleistocene (probably by c. 500 ka). Methods used in the field and laboratory included: deep borehole drilling, geophysical borehole logging, groundwater table measurements, sediment dating (with optically stimulated luminescence - OSL), mineralogical analysis (XRD and SEM), groundwater tracers (18O, 2H, 3H/3He, 14C), hydrochemistry (including pore-water), cation exchange capacity and sediment dilution experiments. Unconsolidated sediment samples at intermittent depths up to 50 m below ground level (bgl) were retrieved from a 100 m bgl research borehole. Sediment XRD analysis shows that the sediment has predominantly silicate minerals (quartz and feldspars) with whitish nodule like structures detected to be bassanite (dehydrated gypsum), whereas carbonates (calcite and dolomite) were below detection by XRD; a simple acid test, however, validated the presence of carbonates. OSL dating showed that the sediments are old (> 300 ka) and cannot be accurately constrained as the quartz mineral grains are fully or nearly saturated. The sediment pack shows conditions of palaeo-environmental changes of wet and dry conditions (based on microfossils and facies changes) depositing fluvio-lacustrine sediments (sand, silt and clay intercalations); geophysical logging delineated and resolved these sediments well, showing high salinity down to the basement rocks at 100 m bgl. Within the Machile Basin, the groundwater table has a low hydraulic gradient in the central palaeo-lake sediment region, characterized by brackish-saline water, and higher gradients in the fresh water fringe region. The fresh water is typically Ca-Mg-HCO3 and Na-HCO3 dominated due to silicate weathering, whereas, the saline groundwater is Na-SO4-Cl dominated, the result of sustained dedolomitization and ion exchange processes as modelled using the geochemical code PHREEQC. The groundwater ages along a flow line using 14C and 3H/3He are < 10 ka and generally < 50 ka at a regional scale over the PLM, compared to very old sediments (> 300 ka), suggesting pluvial climatic recharge process during Late Pleistocene (> 30-20 ka) and Holocene (8-4.5 ka) that induced partial leaching of the sediments. Stable water isotopes (2H and 18O) data suggest that groundwater receives meteoric recharge; however most of this water is lost to evapo-transpiration rendering the saline environment a virtually stagnant groundwater zone without any through flow. Fresh groundwater is therefore hosted in the near-surface zone (such as river channels, palaeo drainages and alluvial deltaic features) within the PLM region. This study supports a tectonically controlled evolution of PLM in which hydrological feedbacks were secondary processes during the PLM development; climatic variation during pluvial events was, however, critical in flushing the sediment. The present hydrogeology of the desiccated lake, includes, clay, silt and sand sediments that host almost stagnant saline water. Freshwater from the regional groundwater is generally lost to evapo-transpiration or superficial drainage. These findings suggest that the salinisation of the groundwater resources in this system is primarily the result of the palaeo processes that formed PLM in an arid climate leaving evaporitic minerals and high salinity in the sediments. Because of the generally arid climate since the formation, combined with low hydraulic conductivity the PLM sediments are still not flushed after c. 500 ka. This implies that investigations for fresh groundwater must be targeted at deeper aquifers or other technologies such as groundwater desalination and rain-water harvesting must be implemented.