The Cubango–Okavango River Basin (CORB) feeds a large endorheic delta with one of the most pristine wetlands on the planet, the Okavango Delta in Northern Botswana. This study presents an overview of the sediment production and distribution system in the CORB, and aims at expanding the current knowledge of anthropogenic impact on the Okavango Delta. Here, together with detailed characterization of surficial sediments from the CORB, we present the first results of MP sediment concentrations for the upper Okavango Delta (Panhandle), considered as a major sink for most of the anthropogenic pollutants in the basin. At the upstream end (Mohembo), discharge varies between 100 m3 s-1 during low flow and 700 m3 s-1 during peak flow (March–April). Sediment samples were collected at 20 sites across the CORB for the characterization of sediment variability in the system. An additional 7 sites were targeted in the Panhandle, between Mohembo and Sepupa, for a preliminary assessment of the MP content in Delta sediments. Samples were collected by scooping from active channels, point bars and oxbow lakes. Grain size distribution is decreasing from 0.5 mm to 0.3 mm moving downstream with virtually no components < 0.15 mm. The average organic matter content in the sands is always below 1%. Fluorescent microscopy showed MP sediment concentrations from around 57 MP/kg dry weight in some of the oxbows to 400 MP/kg in the main channel. About 92% of the identified MP were classified as fragments, varying in length between 64 and 1065 µm. The remainder of the particles was identified as fibres. Raman Spectroscopy revealed that the composition of the MP is dominated by polyethene terephthalate (PET), polypropylene (PP), polyethene (PE), polystyrene (PS), and polyvinyl chloride (PVC). Overall, abandoned meanders have less MP than the active channels, and no MP were found entangled in the papyrus roots that cover most of the floodplains. From this novel data set it was possible to estimate that 10.9–336.2 billion particles could be annually transported into the Okavango Delta, indicating that the region represents a significant sink for MP, raising concerns for the unique wetland ecosystem. Future studies will focus on determining basin-wide MP distribution and their relation to sediment transport patterns.
New sedimentological data from the Okavango wetlands (Botswana): and endorheic sink for microplastic particles of the Cubango Okavango River Basin
Fulvio FranchiConceptualization
;
2023-01-01
Abstract
The Cubango–Okavango River Basin (CORB) feeds a large endorheic delta with one of the most pristine wetlands on the planet, the Okavango Delta in Northern Botswana. This study presents an overview of the sediment production and distribution system in the CORB, and aims at expanding the current knowledge of anthropogenic impact on the Okavango Delta. Here, together with detailed characterization of surficial sediments from the CORB, we present the first results of MP sediment concentrations for the upper Okavango Delta (Panhandle), considered as a major sink for most of the anthropogenic pollutants in the basin. At the upstream end (Mohembo), discharge varies between 100 m3 s-1 during low flow and 700 m3 s-1 during peak flow (March–April). Sediment samples were collected at 20 sites across the CORB for the characterization of sediment variability in the system. An additional 7 sites were targeted in the Panhandle, between Mohembo and Sepupa, for a preliminary assessment of the MP content in Delta sediments. Samples were collected by scooping from active channels, point bars and oxbow lakes. Grain size distribution is decreasing from 0.5 mm to 0.3 mm moving downstream with virtually no components < 0.15 mm. The average organic matter content in the sands is always below 1%. Fluorescent microscopy showed MP sediment concentrations from around 57 MP/kg dry weight in some of the oxbows to 400 MP/kg in the main channel. About 92% of the identified MP were classified as fragments, varying in length between 64 and 1065 µm. The remainder of the particles was identified as fibres. Raman Spectroscopy revealed that the composition of the MP is dominated by polyethene terephthalate (PET), polypropylene (PP), polyethene (PE), polystyrene (PS), and polyvinyl chloride (PVC). Overall, abandoned meanders have less MP than the active channels, and no MP were found entangled in the papyrus roots that cover most of the floodplains. From this novel data set it was possible to estimate that 10.9–336.2 billion particles could be annually transported into the Okavango Delta, indicating that the region represents a significant sink for MP, raising concerns for the unique wetland ecosystem. Future studies will focus on determining basin-wide MP distribution and their relation to sediment transport patterns.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
