Assessing the Effects of Opencast Coal Mining on the Quality of Surface Water: A Case Study in the Leeuwfonteinspruit, a Tributary of the Olifants River

Assessing the effects of opencast coal mining on surface water is often complex, requiring integrated approaches that combine guideline-based assessments, water quality indices (WQIs), hydrochemical tools, and statistical analyses. This study evaluated multi-year water quality data (2018–2023) from five sites along the Leeuwfonteinspruit, a tributary of the Olifants River Catchment in the Mpumalanga Province, South Africa. Water quality parameters were assessed against the South African Water Quality Guidelines (SAWQGs) for domestic, agricultural, and aquatic ecosystem uses. The results indicate frequent exceedances of SAWQGs for key parameters such as sulfate (SO4), electrical conductivity (EC), total dissolved solids (TDS), magnesium (Mg), calcium (Ca), iron (Fe), aluminum (Al), and manganese (Mn), particularly at the downstream site. Weighted Arithmetic Water Quality Index (WA-WQI) and Canadian Council of Ministers of the Environment Water Quality Index (CCME-WQI) classified water quality from good at some upstream sites to consistently poor or marginal water quality at downstream sites. Irrigation assessments based on sodium adsorption ratio (SAR), sodium percentage (Na%), and the United States Salinity Laboratory (USSL) diagram indicated that surface water posed a low sodicity risk across the catchment, with salinity identified as the primary constraint to irrigation suitability. Hydrogeochemical evaluation using Gibbs diagrams suggested that the upstream water chemistry is dominated by natural rock–water interactions and dilution processes, while midstream to downstream sites increasingly exhibit characteristics consistent with evaporation–crystallization and anthropogenic influences. Principal Component Analysis (PCA) and correlation analysis further suggested that water quality patterns are largely associated with salinity and mineralization that may be influenced by mining activities, with secondary contributions from agricultural inputs. Overall, the findings suggest that opencast coal mining is an important contributor to surface water quality deterioration in the Leeuwfonteinspruit, although other catchment activities and natural geochemical processes may also contribute. These findings highlight the need for strengthened monitoring, proactive mine water management, and effective regulatory enforcement.