Spatial and temporal analysis of water and sediment quality in relation to land-use in the Limpopo River Basin

Abstract

The Limpopo River Basin is an international river basin shared between Botswana, Zimbabwe, South Africa and Mozambique. This basin serves as an important water resource for social, economic and ecological functioning. The pressures on this subtropical savannah basin to supply the quantity and quality of water required for the basic human needs and natural ecology are inevitable. Rapid population growth across all four countries, and subsequently a growing economy is increasing the anthropogenic impacts, which are reflected in the water- and sediment quality. Therefore, it is important to manage and protect the basin in a sustainable manner for future utilisation. Although several studies have been conducted in the basin, limited data on the spatial and temporal distribution of water- and sediment quality in relation to land-use are available. Thus, this study aimed to determine the change of water- and sediment quality within the LRB, from 2021 to 2024, and link the changes with possible land-use activities. To achieve the aims of the study in situ water quality parameters were measured, as well as nutrient and anion concentrations using ion chromatography, and element concentrations through quadrupole inductively coupled plasma-mass spectroscopy. These results were assessed against the fitness for water use values determined for the LRB according to the species sensitivity distribution model used in the 2013 Monograph study for the LRB, and the target water quality range values for water quality parameters. Additionally, Mauchino diagrams were used to present the ionic composition for each site and principal component analyses were performed to interpret the spatial and temporal water quality distribution. Furthermore, the sediment quality assessment included sediment grain size analysis, total organic carbon (TOC) measurements, and element analysis using ICP-MS. Grouped element charts were used to compare the 2021 and 2024 overlapping sites, whereas PCA plots were used to interpret the spatial and temporal sediment quality distribution for all the sites, as well as the overlapping sites. Lastly, the change in landuse/land-cover (LULC) classes were determined using the LULC data obtained from ESRI Sentinel-2 Land Cover Explorer for 2021 and 2024, whereafter it was analysed and associated with changes in water- and sediment quality. The water quality assessment indicated elevated pH, electrical conductivity, dissolved oxygen, nutrient concentrations and some selected element concentrations. It was also observed that the nutrient concentrations were specifically higher in the upper region of the LRB, and elements such as zinc and copper throughout the whole basin. Additionally, most of the sites were dominated by higher alkalinity and high concentrations of major anions and cations. The sediment quality assessment indicated higher TOC during both years. Furthermore, the sediment grain size analysis indicated that most of the sites during 2021 and 2024 were dominated by coarse sand ii and secondly, a combination of course and medium sand during 2021 and a combination of very coarse, coarse and medium sand during 2024. Some sites also indicated higher element concentrations due to anthropogenic impacts. The LULC assessment indicated an increase in rangelands, crop fields and built areas from 2021 to 2024. The increase in LULCand the corresponding anthropogenic impacts on water quality were reflected in the increase in elements, as well as nutrients, salts and alkalinity at some overlapping sites. However, it must be noted that the element concentrations were higher in the sediment and lower in the water during 2021, whereas the element concentrations were higher in the water and lower in the sediment during 2024. This indicates the potential effect of higher rainfall from 2021 to 2024, potentially releasing the elements from the sediment during high flow events. Ultimately, the impacts of existing anthropogenic activities including agriculture, mining, urbanisation, sewage effluent and industrial runoff were evident. Consequently, further water quality deterioration was observed from 2021 to 2024, indicating the impacts of an increase in LULC.