Assessment of groundwater quality for irrigation and drinking purposes in the Limpopo Granulite-Gneiss region, Limpopo Province, South Africa

Abstract

The global freshwater availability studies have classified South Africa as water stressed in 1999 and approaching water scarcity by the year 2025. While groundwater is available everywhere, it is not always in suitable quality meaning some groundwater dependent rural communities lack adequate information about the water quality status of their groundwater supplies. While there may be significant groundwater research studies done, the outputs never reach the impacted communities. Groundwater accounts for nearly 70% of rural domestic water supply in Limpopo Province. Furthermore, rapid population growth and more frequent drought events have led to reduced surface water supply and increased in groundwater abstraction, putting groundwater resources under enormous pressure. As communities grow, more groundwater is abstracted, and land use changes to more paved roads, houses, shopping centres, and parking lots, decreasing groundwater recharge. A total of 319 groundwater samples, from 17 monitoring sites located within Limpopo granulite-gneiss region, collected between 2000 and 2017, were analysed in order to evaluate its suitability for drinking and irrigation purposes. The abundance of cations and anions are showing Na+>Ca2+>Mg2+>K and HCO3->Cl->SO42->NO3-+NO2->F-, respectively. The weighted average values show major anions are dominant over the major cations. Na+ accounts for 48% of the cations and HCO3- accounts for 41% of the anions. The pH of groundwater in the study area ranges from 6,9 to 9,2; with over 83,7% of the samples falling within the pH 7,5 to 8,5 range; while 5,0% of the samples are within the pH 6,5 to <7,5 range; and 11,3% samples are within the pH >8,5 to 9,2 range. TDS values range from 107 to 2426 mg/L with a weighted average of 1122 mg/L. At least 57% of the samples are categorically brackish and 43% are fresh. On the Piper diagramme, the water samples are mainly plotting on Ca-Mg-HCO3 type (zone 5) and mixed (Ca-Mg-Cl-SO4 and Ca-Mg-HCO3) type (zone 9), Ca-Mg-HCO3 type indicating carbonate (temporary) hardness. The Gibbs diagramme indicates that groundwater chemistry is controlled mainly by evaporation dominance mechanisms, while high Ca2++Mg2+ concentrations relative to HCO3- concentrations, indicate silicate weathering involving reverse ion exchange. Pearson correlation analysis shows a very strong positive correlation of Na+ with Cl- (0,80), SO42- (0,81), and F- (0,78). Suitability for irrigation assessment results show that all the groundwater samples were suitable for irrigation purposes based on sodium adsorption ratio, residual sodium carbonate, and permeability index. However, for the Kelly ratio index, seven monitoring sites are unsuitable for irrigation purposes. Over 71% of the water samples have concentrations of nitrate higher than the WHO and SANS241 recommended guideline value of ≤11 mg/L, making 15 out of 17 monitoring sites unsuitable for drinking. However, the remaining two sites failed drinking water suitability in TDS, Na, Cl, SO4, and F based on SANS241and WHO, guidelines. To mitigate methemoglobinemia, a simpler, cheaper, and more immediate approach that is within the capabilities of communities is recommended.