The hydrochemical and isotopic characterization of groundwater in southern Mozambique
A groundwater chemistry sampling campaign was run over two sampling seasons in 2018. Groundwater samples were taken across the region of southern Mozambique, between the towns of Namaacha, Catuane, Ponta do Ouro and Marracuene. Major anions and cations were analyzed by two labs at Stellenbosch University, and stable isotopes of oxygen and hydrogen, and radiogenic isotopes of strontium were analyzed at the Department of Geological Sciences at the University of Cape Town. The aim of this study was to characterize the isotopic and hydrochemical composition of groundwater in the southern Mozambique study area, using major ions and stable isotopes. Samples were categorized into different zones based on the underlying geology in which the borehole was sited. Overall, the dominant anions and cations are: Cl>HCO3>SO4 and Na> Ca=Mg>K. 68% of samples plotted in the Na-Cl water type, whilst 30% plotted in the Na-HCO3 water type of the Piper and Chadha diagrams. Salinization is the mechanism controlling the Na-Cl water types, whilst recharge is mechanism controlling the Na-HCO3 water types. Saturation indices of calcite, dolomite, halite, and gypsum. Saturation indices were calculated using the thermodynamic software PHREEQC. All samples were undersaturated with respect to gypsum and halite, suggesting that the conditions were thermodynamically favored for their dissolution. Samples that were from boreholes that plotted in the limestone layers and >40m depth in basalts, and quaternary sediments were oversaturated with respect to calcite and dolomite, indicating that the conditions were thermodynamically favored for their precipitation out of solution. Samples that plotted in the rhyolite, shallow basalts and quaternary sediments were undersaturated with respect to calcite and dolomite, suggesting dissolution into solution. Sixty-two percent of sample had an NA/Cl ration greater than one, indicating silicate weathering as a major process affecting the chemical character of the water. Samples in the basalts however had Na/Cl ratios less than one, and δ 2H and δ 18O values out of the range of seawater, suggesting ion exchange as a process affecting the chemistry of groundwater in these areas. Since there is no established Local Meteoric Water Line (LMWL) in the area, LMWL from Pretoria and Durban were used as proxies, and the Global Meteoric Water Line (GMWL) was also plotted. Samples in the rhyolites, basalts and quaternary sediments has high d-excess values, and showed a strong evaporation trend. The mechanisms for groundwater salinization in these areas is strongly influenced by evapoconcentaion effects. Geographical features such as seasonal variation, latitude, elevation and distance from coast do not appear to be a major factor affecting the isotopic composition of the groundwaters. The strontium isotopes and elemental strontium concentrations indicate that groundwater mixing likely occurred in each zone, however no end-members were established.
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