The Eastern Basin chemical acid mine drainage (AMD) treatment plant is one of the world’s largest high-density sludge (HDS) plants, and came into operation in August 2016. The plant is situated near the inoperative Grootvlei Mine in Springs, South Africa, and upstream of the Blesbokspruit Wetland, a former Ramsar Wetland of International Importance, now on the Montreux Record. Since being in operation it has had a major influence on surface water quality along the wetland area. The plant was constructed to mitigate the anticipated decant of AMD water from the abandoned Grootvlei Mine into the Blesbokspruit Wetland. Making use of the BACI (Before-After-Control-Impact) design, this study compares surface water quality of the Blesbokspruit upstream (control site) and downstream (impact site) of the treatment plant for 3-year periods before and after it came into operation. Quarterly water quality data (aluminium, ammonia, chloride, conductivity, dissolved oxygen, fluoride, iron, magnesium, manganese, nitrate, pH, phosphate, sodium and sulphate) from 2013–2019 were used for 5 historical Rand Water monitoring sites along the Blesbokspruit. The current HDS treatment process has negatively influenced conductivity, chloride, magnesium, sodium and sulphate levels downstream. Since the commissioning of the treatment plant, the levels of these parameters have increased significantly. Notably, conductivity and sulphate have reached the management range defined as ‘unacceptable’ within the framework set out by the Blesbokspruit Forum (which is less stringent than the national guidelines for aquatic ecosystems), with potential impacts on salinization of the Vaal Barrage downstream. However, the significant reduction of iron, ammonia and phosphate concentrations downstream of the plant may be a combined beneficial effect of dilution by increased discharge from the plant and the wetland removing these contaminants. These results highlight the need for further research into possible secondary treatment and desalinisation mechanisms and the potential ecological and downstream water supply implications of increasing salinity within the area.