Seasonal variation of hydrochemical characteristics of open-pit groundwater near a closed metalliferous mine in O'Kiep, Namaqualand Region, South Africa

In this study, the hydrochemical characteristic changes of open-pit groundwater (OPGW) near a closed metalliferous mine proposed for drinking and irrigation usage were investigated. Water samples from an OPGW in O'Kiep were collected in April (dry season) and August (wet season), 2017. Data analysis methods, such as the coefficient of variation, correlation coefficient, piper trilinear plot analysis and sodium adsorption ratio (SAR) among others were used to assess of the seasonal variation in the OPGW quality. The hydrochemical characteristics of the OPGW were also compared with drinking water guidelines. Based on the results, it was evident that the OPGW quality varied seasonally. The most abundant cation and anions were Ca2+ and SO42−, with recorded concentrations of 631 and 6020 mg/L, (dry season); 541 and 4450 mg/L (wet season), respectively. The OPGW was relatively rich in SO42− > Cl− > Mg2+ > Ca2+ > Na+ > K+ during the dry season; however, these cations and anions were reduced during the wet season with Cl− and Na+ subsequently increasing. PHREEQC indicated that cation exchanges played a significant role in the OPGW chemistry with Sb and Ca appearing to be the minerals with the highest precipitation potential followed by Cu7S4 and Blaubleil, respectively. Covellite, CdSe2 and NiSe were likely to be in equilibrium while Ca3Sb2 will remain in a dissolution state. Furthermore, the seasonal fluctuation in the OPGW quality characteristics was attributed to the water table level fluctuations and the water quality did not satisfy the guidelines stipulated by the South African National Standard, SANS241-1 (2015) and World Health Organisation, WHO (2011). The trilinear piper plot classified the OPGW into three types: Ca2+–Mg2+–Cl−–SO42; Ca2+–Mg2+–F−; and Ca2+–Mg2+–SO42−. Overall, the SAR values were within the permissible levels for irrigation purposes; however, long-term use of the OPGW might be detrimental for plant growth.