Peatlands as Filters for Polluted Mine Water?—A Case Study from an Uranium-Contaminated Karst System in South Africa—Part II: Examples from Literature and a Conceptual Filter Model

Abstract

Abstract:As the second part of a series of four, this paper reviews a number of case studies of natural uranium attenuation in peat, as well as underlying chemical mechanisms reported in literature. Based on this review, a generic, conceptual, model for peat to act as filter for dissolved uranium (U) is developed for guiding subsequent field investigations. The model consists of a chemical and an hydraulic component which is derived largely from data reported in literature as well as from limited field observations. For the chemical model component 10 different processes, each controlled by factors relating to water chemistry, have been identified to govern the attenuation of U in peat via a net balance of immobilization and remobilization. For the hydraulic aspect of the filter model, five different principal modes of U polluted water coming in contact with peat are discussed, focusing on the associated peat-water contact time as a crucial parameter controlling chemical U attenuation. Moreover, links between the two model components are discussed and, based on the integrated conceptual model, possible effects of natural and anthropogenic events on U attenuation in peatlands are outlined. Guided by the model, various site-specific field and laboratory investigations are finally designed to verify how far the identified generic factors and processes are indeed applicable to the Gerhard Minnebron Peatland.

Colloidal Size and Redox State of Uranium Species in the Porewater of a Pristine Mountain Wetland

“…Wetlands are natural sinks for trace metal elements, including uranium. − Natural or constructed wetlands have been used for decades for the remediation of uranium-contaminated water. − The removal process is based on the sorption of uranium to natural organic matter (NOM) as well as on the (bio)­reduction of soluble U­(VI) to less soluble U­(IV).…”

The impact of anthropogenically induced degradation on the vegetation and biochemistry of South African palmiet wetlands

“…Wetland degradation can be grouped loosely into four main categories: disturbance of wetland physical structure, land-cover changes, pollution and climate change (Zedler and Kercher 2005;Meng et al 2017). Disturbance affecting the physical structure of wetlands may include drainage; either by humans or as a result of other damage (Zedler and Kercher 2005;Watters and Stanley 2007;Krüger et al 2015), peat excavation (Nsor 2007;Winde 2011;Cabezas et al 2014), erosion (e.g. channel or gully erosion) (Boardman 2014;Rebelo et al 2015;de Haan 2016), mechanical disturbance, such as the building of roads or railways across wetlands, and altered fire regimes (Zedler and Kercher 2004).…”

“…Types of land-cover changes include vegetation changes (Brooks et al 2003), alien plant invasion (Zedler and Kercher 2004;Zedler and Kercher 2005) or land conversion, for example to agriculture (Rebelo et al 2015;de Haan 2016). Wetland pollution may be in the form of agricultural runoff, wetland fertilization, or point source pollution (Jordan et al 2003;Zedler and Kercher 2005;Carpenter and Bennett 2011;Winde 2011). Ultimately the degradation of wetlands results in a loss of biodiversity and ecosystem services (Zedler and Kercher 2005;Meli et al 2014) which may have economic consequences for local communities (Schuyt 2005).…”

Peatlands as Filters for Polluted Mine Water?—A Case Study from an Uranium-Contaminated Karst System in South Africa—Part III: Quantifying the Hydraulic Filter Component

“…Fed by dolomitic spring water that emerges from the Boskop-Turffontein compartment (BTC), the wetland is also part of a vast underground karst network that extents well into the upstream catchment of the Wonderfonteinspruit (WFS) where decade-old deep-level gold mining is responsible for ongoing U pollution of surface and groundwater [2]. Since 1964, most of the water discharged from the eye is diverted into an irrigation canal and bypasses the wetland on its way back into the upper Mooi River and the Boskop Dam further downstream.…”

“…Based on a literature review, a conceptual model on the U filter function of peat was developed that consists of a chemical component characterizing the mechanisms responsible for the attenuation and release of U in and from peat as well as a hydraulic component that addresses the rate and mode of contact between (polluted) water and the peat. In order to verify to what extent the general assumptions made in this model are applicable to local conditions several site-specific investigations were designed [2]. This part of the paper series focuses on the hydraulic component of the filter model.…”