###### Abstract
The resulting adsorption/desorption behavior of metals within these compartments in aquatic environments might over time lead to their long-distance transport within river systems and thus cause contamination far from the pollution source [14; 15; 16]. In the case of the South African gold mines of the Witwatersrand basin, a lack of larger quantities of neutralizing minerals and the high abundance of iron sulfides favored the formation of AMD. Scientists had been well aware of the issue in the region and had predicted the overflow of a borehole and an old shaft of the Harmony Gold Mine near Krugersdorp years before it happened in the South African winter months of 2002 [17] and references therein. The pH of the MIW entering the environment was extremely low and it contained high loads of dissolved metals, and metalloids as well as high sulfate concentrations. An estimation of initially 7 ML per day of MIW in summer and 12.5 ML per day in winter were released into the Tweelopiespruit [18], a small river in the upper reaches of the Crocodile River (West) system. Once the MIW was diluted with water from Tweelopiespruit and was exposed to air, the pH increased, followed by the precipitation of various metals. The water was polluted and stained orange, and the sediments were covered with a thick orange-red deposit of ferrihydrite. This phenomenon is typically seen as a consequence of AMD. A number of studies [9; 19; 20] examined the water of Tweelopiespruit after the spill and found the metal and metalloid loads to be extremely high and toxic for the aquatic environment and for the local communities. A neutralization treatment of the MIW was therefore implemented to substantially improve the water quality. This treatment facility is, in a modified form, still operating today. The so-called \"chemical process\" used, utilizes limestone as a neutralization agent. First, the MIW is treated with limestone to a slightly alkaline pH and subsequently aerated with oxygen. Thereby, a ferric hydroxide precipitate is produced, which is put into a settling chamber, where metals and metalloids are removed from the solution [21]. For an increased settling efficiency of particles, the treatment might also include coagulants (such as aluminum sulfate) [22]. Almost 20 years passed since the spill of MIW in the West Rand was first reported, and the treatment facility has been operating for many years as well. Nonetheless, the post-treatment water and sediment quality in the upper reaches of the Crocodile River (West) system was rarely discussed in a scientific setting. This calls for examining Tweedipiespruit and the connected rivers to investigate the general water quality of the river system. For this study, we had the unique opportunity to take water as well as sediment samples over a 13-month period. Thereby making it the first study looking at sediment and water quality in the area over a longer period of time, after the implementation of water treatment. This yielded insights into both: the momentary situation of the water and sediment composition as well as the potential seasonal changes. The aim of this study was to determine the physico-chemical parameters, including the chemical composition of dissolved components in the water phase, as well as the metal content in the sediment. This should enable a characterization of the post-treatment state of the system but also an estimation of possible hazards for the aquatic environment in the region.