Investigating groundwater salinity in the Machile-Zambezi Basin (Zambia) with hydrogeophysical methods

Abstract

The importance of knowing the current state of groundwater resources cannot be over emphasized in rural areas in the developing countries with limited water resources. As such, innovative geophysical techniques are now part of the norm for quick and effective characterization of groundwater resources worldwide. This thesis presents the application of geo-electrical and electromagnetic methods for the investigation of groundwater salinity in the Machile-Zambezi Basin in south western Zambia, southern central Africa. Aerial and ground based transient electromagnetic measurenments were used to map the spatial distribution of apparent electrical resistivity on a regional scale in order to obtain a regional overview of groundwater salinization based on electrical resistivity correlation. Furthermore, ground based transient electromagnetic soundings and direct current and induced polarization measurements were used to investigate on a local scale, indications of surface water/ groundwater exchange from electrical resistivity anomalies coincident with alluvial fans and flood plains as deduced from the aerial electrical resistivity result. New and innovative geophysical data inversion schemes were also developed and used to gain a better explanation of the data collected. These include a new scheme for the joint inversion of direct current and induced polarization data, and transient electromagnetic data; and a new coupled hydrogeophysical inversion setup to allow for the first time the joint use of direct current and transient electromagnetic data in one optimization. The result from the regional mapping with transient electromagnetic measurenments showed a spatial distribution of electrical resistivity that indicated block faulting in the Machile-Zambezi Basin. Saline groundwater was found to occur predominantly in the low lying graben areas that are essentially an extension of the Palaeo Lake Makgadikgadi system into south western Zambia. In addition, surface water from the Zambezi River was found to interact with saline aquifers to such an extent that the surficial physical form of alluvial fans and flood plains was visible in the spatial distribution of electrical resistivity from the aerial survey up to a depth of about 40 m. Interpretation of direct current and induced polarization, and transient electromagnetic data using the new joint inversion scheme revealed a fresh water lens overlying the saline aquifer at Kasaya in Kazungula District, Zambia. The freshwater lens appeared to be in hydraulic contact with the Zambezi River where it was thickest (60 m) and had the highest electrical resistivity values (about 200 Ωm) which steadily declined to about 30 Ωm whereas the thickness reduced to around 22 m at the end of the 6 600 m long transect line measured perpendicular to the Zambezi River towards the North. The distribution of chargeability along the Kasaya transect line was found to be correlated with the distribution of electrical resistivity thus giving a strong indication of the intrusion of fresh surface water into a pre-existing saline aquifer. It is postulated that the intrusion of fresh surface water in to the saline aquifer was driven by evapotranspiration. Finally, the new coupled hydrogeophysical inversion approach resulted in sharp estimates of hydrogeological model parameters. This was for a coupled flow and solute transport model setup for the Kasaya transect under the forcing of evapotranspiration. Performance of the coupled hydrogeophysical inversion was better with the inclusion of direct current data in comparison to the use of transient electromagnetic data alone. The broader implications of these findings is that groundwater salinization in the Machile Zambezi Basin is now known to be strongly influenced by the tectonism of the Palaeo Lake Makgadikgadi system and is therefore not expected to increase over time. Rather, surface water tends to interact with the saline aquifers in places to create freshwater lenses that are an important source of clean drinking water. Therefore, the findings of this thesis will need to be augmented with data and further research from other geoscience disciplines such as surface water hydrology, geochemistry, petrology and meteorology in order to come up with sustainable water resources management practices that are applicable to arid and semi-arid sedimentary basins in general and the Machile-Zambezi Basin in particular within the broader context of the Kalahari Basin of southern Africa. This will make it possible to project the effects of climate variability and anthropogenic activities on the water resources with a view of disaster preparedness and mitigation for which this part of the world is particularly vulnerable.