Abstract
Given the increasing pressure of man-made activities on riparian zones, the capacity of the riparian vegetation along the Upper Olifants River, South Africa, to phytoextract and phytostabilize aluminium (Al), manganese (Mn) and iron (Fe) from the soil was investigated. The aim of the study was to gain better understanding of the capacity of indigenous vegetation in riparian zones to immobilize metals in the soil, thereby improving river water quality and ecosystem services. Seven commonly-occurring pollution-tolerant riparian plant species were evaluated to establish their potential as bioaccumulators for Fe, Al and Mn. Species included: Cyperus haspan, Schoenoplectus corymbosus, Typha capensis, Phragmites australis, Cynodon dactylon, Cyperus marginatus and Juncus effusus, which were sampled in five riparian areas in the Upper Olifants catchment. The bioconcentration factor (BCF) for Mn was > 1 for all species investigated with a maximum of 5 for Typha capensis, which also showed the highest accumulation of Al (10.26) and Fe (7.03). The remaining species presented with Al and Fe BCF between 0.11 and 2.00, with minimal transfer from root to shoot. When measured against an ideal hypothetical buffer zone, the buffer zones under investigation varied between intact and severely compromised. Intact riparian zones showed elevated metal concentrations in the soil, yet significantly lower concentrations in the river water compared to areas with insufficient vegetative cover. A polluted riparian area overgrown by P. australis effectively phytoextracted 204 960 g/m2 Al, 204 400 g/m2 Fe and 27 887 g/m2 Mn. The two indigenous Cyperus spp. were not ideal for metal immobilization with low bioaccumulation and transfer factors as well as low biomass. High biomass and Al, Fe and Mn phytostabilizing species: P. australis, T. capensis, S. corymbosus and J. effusus, should be considered in the rehabilitation of South African buffer areas.Keywords: riparian vegetation, heavy metals, acid mine drainage, phytostabilization, bioaccumulation
Highlights
Riparian vegetation increases resistance and surface roughness, slowing, intercepting and otherwise influencing overland, subsurface and groundwater flows (Welsch, 1991)
Phragmites australis, Typha capensis, Juncus effusus, Schoenoplectus corymbosus and Cynodon dactylon have been previously recorded to take up various heavy metals (Batty et al, 2000; Deng et al, 2004; Demirezen and Aksoy, 2006; Fitamo and Leta, 2002; Tangahu et al, 2011), warranting their analysis for Al, Fe and Mn uptake; while Cyperus haspan and Cyperus marginatus are two locally common species but are unstudied in terms of their bioaccumulation potential
A plant’s ability to accumulate or stabilize metals from soils to its root can be estimated by using the bioconcentration factor (BCF), translocation factor (TF) and transfer coefficient (TC) ratios
Summary
Riparian vegetation increases resistance and surface roughness, slowing, intercepting and otherwise influencing overland, subsurface and groundwater flows (Welsch, 1991). Many riparian zones worldwide are being degraded by infringing land use practices such as livestock grazing, agriculture or urbanization, alien plant invasions or increased pollution levels in the catchment (Clericia et al, 2014; Wantzen et al, 2013). These activities reduce the capacity of riparian zones to perform their ecosystem services, further degrading the integrity of the surrounding ecosystem. For the Olifants River catchment, the riparian vegetation provides bank stabilization and retains pollutants from surface runoff, it is exposed to increasing metal concentrations accumulating in the soil, deposited from upstream mining practices.
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