Hydrochemical Characterization of Flow Systems in the Lusaka Aquifer, Zambia

In data-scarce regions, environmental tracers emerge as a vital tool for characterising groundwater, especially where conventional monitoring methods are limited. This study employed hydrochemical techniques to characterise and develop a conceptual flow model for the Lusaka Aquifer, a crucial water source for the local population threatened by human activities and climate change. 
Preliminary findings indicate that the primary hydrochemical facies within the aquifer are calcium-bicarbonate (Ca-〖HCO〗_3) and calcium-magnesium-bicarbonate (Ca-Mg-〖HCO〗_3). Strong correlations (R² > 0.5) exist between calcium and bicarbonate, magnesium, and bicarbonate, and Sodium and Chloride. Further, carbonate rock weathering, particularly the dissolution of calcite and dolomite, predominantly influences groundwater chemistry, albeit with indications of some anthropogenic influences. Hydrochemical signatures suggest a predominant migration path of water from dolomite to schist and limestone, with some samples suggesting origins from limestone. Isotope data comparing δ²H and δ¹⁸O values for groundwater and precipitation indicate a strong meteoric origin of groundwater recharge. Seasonal variations in precipitation isotope signatures, observed from January to April, further highlight the temporal dynamics of recharge. The groundwater samples were classified into four clusters using hierarchical cluster analysis (HCA), a multivariate statistical method, to identify distinct hydrochemical endmembers. Cluster 1 consisted of groundwater rich in bicarbonate, calcium, and magnesium, influenced by limestone and dolomite. Cluster 2 represented a mix of natural and urban influences, while Cluster 3 indicated high-quality recharge water characterised by calcite dissolution. Cluster 4 displayed a unique ionic composition, likely shaped by schist and potential contamination. 
The subsequent phase of this study involves establishing a Mixing Cell Model (MCM) to elucidate the flow system and ascertain recharge water composition. This endeavour promises to enhance our understanding of the hydrogeological system, crucial for effective resource management and preservation.
Keywords: Hydrochemistry, groundwater, flow systems, Mixing Cell Model, HCA