| Abstract |
Groundwater plays a vital role in supporting water security. Its strategic significance is amplified in water-scarce and climate-vulnerable regions, where it serves as a buffer during droughts. However, its sustainability is threatened by growing demand, land use changes, inadequate regulation and climate variability. Moreover, in rapidly urbanizing areas, the risk of contamination further exacerbates the situation. Despite its importance, processes governing groundwater availability and response to anthropogenic and climatic drivers remain insufficiently understood. The aim of this Ph.D. study is to characterise the groundwater system of the Lusaka Aquifer in Zambia with a view to assess urban water security. This aquifer underlies a rapidly urbanizing city and is hosted in carbonate formations, which are vulnerable to anthropogenic influences. To achieve the goal of the study, the following specific objectives were set: (i) to determine the groundwater flow system and mixing processes using hydrochemistry and stable isotopes (hydrogen and oxygen); (ii) to evaluate impacts of landcover on groundwater recharge potential zones from 1990 to 2022 (iii) to estimate recharge in the Lusaka aquifer using multiple methods and assess the impact of landcover change from 1990 to 2022; (iv) to evaluate the response of the groundwater system to precipitation through trend analysis and GRACE data. The study adopted an integrated approach that combined classical hydrochemical methods, stable isotopes and Hierarchical Cluster Analysis (HCA) to characterise groundwater sources, flow paths and quantify recharge contributions. Geospatial techniques were combined with multi-criteria decision analysis to delineate Groundwater Recharge Potential Zones (GRPZ) based on hydrogeological and climatic factors. Recharge was quantified with the Water Balance Method (WBM) in Google Earth Engine (GEE), complemented by the Water Table Fluctuation Method (WTFM) and empirical techniques, while the Mann Kendall (MK) test and Sen’s slope were used to determine trends in groundwater levels. The response of groundwater levels to precipitation was evaluated with Wavelet analysis to quantify lags and periodicities. Hydrochemical, isotopic and hierarchical cluster analysis revealed four distinct clusters corresponding to recharge zones and aquifer compartments in dolomite, schist, and limestone. The evolution of hydrochemical facies dominated by Calcium-Bicarbonate and Calcium-Magnesium-Bicarbonate indicated the presence of flow regimes.
Stable isotopic signatures (δ¹⁸𝑂 and δ2𝐻) of precipitation and groundwater indicated a meteoric origin of groundwater, suggesting that recharge primarily occurs through direct infiltration of precipitation.
Delineation of GRPZs revealed that the areal coverage of low and moderate recharge zones increased by 316.67 km² and 16.57 km² respectively. In contrast, high and very high zones declined by 316.12 km² and 17.18 km². The impact of built-up on GRPZ’s revealed notable encroachments on low, moderate and high recharge zones by 127.28 km², 332.02 km² and 106.65 km² respectively. The transition matrix showed that built-up areas had a gain of 25.26 % of the total study area, largely
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sourced from cropland (10.37%), forest (8.13%), and vegetation (5.55%). Forest cover underwent notable transformations with a loss of 25.75%. Recharge quantification revealed a decline in recharge of 30.23 % between 1990 and 2022. The most severe deficits occurred in 2007/08 and 2016/17, with recharge falling to 110.50 mm/season. Lithology and Landuse/Landcover (LULC) strongly influenced recharge, with the highest recharge values (>350 mm) recorded in carbonate formations under forested and vegetated cover. In contrast, granitic terrains exhibited low recharge (<10 mm) regardless of LULC. Schists showed moderate recharge, with values exceeding 150 mm under vegetation but declined in built-up areas (<50 mm). Comparison of WBM with the WTF recharge estimates revealed consistent patterns across the major geological formations. Trend analysis of groundwater levels from 1951 to 2023 revealed that approximately 70.6% of boreholes in dolomite, 75% in Limestone, and 59% in schist formations exhibited declining trends. Cross wavelet analysis revealed a groundwater response lag of approximately 2 to 6 months relative to precipitation, indicating a rapid recharge process. A dominant periodicity of 300 to 400 days was determined which suggested a strong annual cycle, characteristic of seasonally driven recharge dynamics. GRACE (Gravity Recovery and Climate Experiment satellite) derived groundwater storage anomalies over Lusaka between 2002 and 2022 revealed a pronounced decline with short-term recoveries between 2009 and 2014 followed by sustained negative trends after 2016. Despite its coarse resolution, the GRACE signal reflected a long-term depletion of groundwater resources likely driven by increasing abstraction, land use change, and climate variability. These findings highlight the need for integrated groundwater management approaches that account for the aquifer’s structural complexity, preferential flow and vulnerability to urbanization. This research offers new insights into the complex hydrogeological framework of Lusaka’s aquifer system. It identifies zones with high groundwater recharge potential and demonstrates how urban expansion is progressively altering these critical areas. Recharge rates were quantified across different geological formations, revealing spatial variability linked to lithology and land cover change. The study further shows that urbanization is significantly impacting recharge processes. Finally, temporal analysis of groundwater storage trends indicates a declining trajectory, highlighting the combined influence of over-abstraction and climate variability on aquifer sustainability. |
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