## 2 Materials and Methods  
### Study Area  
This research focuses on the Zaalklapspruit wetland in Mpumalanga province, South Africa, situated within the transboundary Olifants River catchment (Quaternary catchment B2 OG), as depicted in Figure 2. This catchment is vital, supplying water to a wide array of ecosystems, communities, and stakeholders. Nonetheless, it faces significant challenges due to contamination from domestic, agricultural, industrial, and mining sources, marking it as one of the most polluted catchments in Southern Africa ([12; 30; 31]). Despite growing demands on the Olifants River for freshwater, its quality is compromised by pollution from these varied anthropogenic activities ([8; 32; 33; 34]). The Zaalklapspruit wetland, significantly impacted by AMD from nearby mines, is recognized as a highly sensitive ecosystem. It is designated as a \"National Freshwater Ecosystem Priority Area\" and is listed among the \"critically endangered wetland types\" [35; 36; 37]. Furthermore, it is categorized as a vulnerable \"Mesic Highveld Grassland group 4 wetland vegetation\" as per the South African National Biodiversity Institute (SANBI) [35; 36; 37]. Historically, large volumes of AMD from upstream coal mines have altered Zaalklapspruit from an unchanneled to a channeled valley bottom wetland, reducing its water retention capabilities and diminishing its capacity to improve the water quality [3]. Nonetheless,  
Figure 1: Explanatory schematic representation of the source, pathway and receptor approach to the ecologically engineered wetland that received acid mine drainage. (**A**) Wastewater is introduced into the wetland from an abandoned coal mine (red square) upstream. (**B**) Pathways: flocculation of particles in the wetland that ensure the transfer of contaminants from the surface water to the bottom sediment. (**C**) Receptors: scanning electron microscopy (SEM) image of microorganisms in the bottom sediment of the wetland, which is at the receiving end of the ecosystem (Source: adopted from [29]). adopting ecological engineering principles has facilitated gradually restoring the wetland's functional integrity. The Council for Scientific and Industrial Research (CSIR), in partnership with the SANBI and the Working for Wetlands (WfWET) program of the Department of Environmental Affairs (DEA), implemented several interventions in 2014 to address the challenges the wetland system encountered. Previously, the wetland had ceased functioning as a sponge, allowing storm waters to erode gullies and dry the floodplain. The rehabilitation efforts aimed to raise the streambed and employ structures to slow and diffuse the water flow. This included the construction of concrete structures, earth berms, and weirs, with funding provided by Coaltech [38]. The concrete and earth berm structures were strategically placed throughout the wetland to decelerate the water flow and enhance water retention for effective AMD treatment, as illustrated in Figure 3. These ecologically engineered interventions transformed the wetland back to an unchanneled valley bottom configuration and successfully decreased the water velocity to below 0.2 m/s, prolonging the system's water retention time [8]. Furthermore, these modifications expanded the wetland's surface area by 9.4 ha, bringing the total to 135.3 ha. Subsequent to these ecological engineering efforts, notable enhancements in surface water quality were observed. The improvements included an increase in the water pH and alkalinity to within the South African Water Quality Guidelines for aquatic ecosystems (Department of Water Affairs and Forestry [DWaF]) [39], a 50% reduction in the total dissolved solids (TDSs), and a 65% reduction in the sulfate levels. Additionally, the concentrations of Al, Fe, and Mn in the surface water decreased significantly, recording reductions of approximately 99%, 76%, and 96%, respectively, after the rehabilitation efforts were evaluated [8; 38].