Investigation of hydrogeochemical processes and groundwater quality in the Chókwè district, Mozambique
Saveca, Paulo Sergio Lourenco
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Groundwater has been recognised in Sub-Saharan countries as the main source of potable water in rural areas. In semi-arid regions, the climatic and anthropogenic factors both significantly affect groundwater quality. The present study was carried out in the Chókwè district, one of the semi-arid regions in Mozambique within the Limpopo River basin. About 33 water sources (27 groundwater, five surface water and one rainwater) were sampled from July to December 2015 for physicochemical parameters. This study focused on investigating hydrogeochemical processes in groundwater chemistry and their influence on water quality, as well as spatial variability in the Chókwè district. The hydrogeological approaches (WISH) and geospatial tool (Quantum GIS), combined with statistical analyses, were used to assess the groundwater quality. Geochemical ratios, correlation, graphical methods were also applied to understand the local hydrogeology on groundwater hydrochemistry. In addition, the Mozambique standards for drinking water and those of the World Health Organization were used for the assessment of groundwater quality. The analytical results of groundwater chemistry indicated that the order of abundance of cation concentration were Na+ > Mg2+ > Ca2+ > K+, while those of anions were Cl– > HCO3– > SO42–. There is a dominance of Na-Cl hydrochemical facies, and high mineralised groundwater occurs in aquifers underlined by two geological units, namely: alluvium, sand, silt, gravel geological units and eluvial floodplain clayey sand geological units. The alluvium, sand, silt, gravel showed that the content of electrical conductivity (EC) ranged from 603 to 12 000 μS/cm with an average value of 2 364 μS/cm, while for total dissolved solids (TDS) it ranged from 488 to 7 626 mg/L, with an average value of 1 621 mg/L. In the eluvial floodplain clayey sand geological unit the content of EC ranged from 522 to 5 530 μS/cm with an average value of 2 300 μS/cm, while for TDS it ranged from 406 to 3 537 mg/L with an average value of 1 562 mg/L. It was also observed that 15% and 30% of groundwater samples were classified as poor and unacceptable for drinking. For hardness, 7% and 30% of groundwater was hard and very hard, respectively. All parameters in the surface water are within the desirable limits, unlike that of groundwater. Weathering, ion exchange, dissolution and precipitation are the main hydro- geochemical processes. In aquifer mineralogy there is a dominance of sodic plagioclase (Albite), calcic plagioclase (Anorthite), halite, dolomite and calcite. Generally, the groundwater is saline and the land use, chemical evolution, as well as the local hydrogeology, are the factors affecting the spatial variability of water quality. Therefore, groundwater of the Chókwè district would not be safe to use for irrigation over the long term, due to a sodium and salinity hazard.