Tracing the dominant sources of sediment flowing towards Lake Victoria using geochemical tracers and a Bayesian mixing model

Abstract

Purpose Lake Victoria has been increasingly silting over the past decades, impacting water quality and loss of biodiversity. Sediment control strategies require information on the relative and absolute contributions of sediment from different sources. However, to date, there is no continuous monitoring of sediment flux or water quality in any of the tributaries, prohibiting an assessment of the scale of the problem. The aim of this study was to trace the dominant sources of riverine sediment using geochemical fingerprinting, thereby generating a knowledge base for improving land management and reducing sediment yields in Simiyu River catchment, one of the main contributing rivers to Lake Victoria. Materials and methods Geochemical tracer concentrations were analyzed in transported sediment from the main river and two tributaries (riverbed sediments) and from soils in five dominant land use types (agricultural land, bush land, forest land, channel banks, and main river banks). Dominant sources to the Simiyu main river sediment were attributed using the Bayesian MixSIAR model. Results and discussion The mixing model outputs showed that the Simiyu tributary was the dominant source of sediment to the Simiyu main river with 63.2%, while the Duma tributary accounted for 36.8%. Cultivated land was shown to be the main land use source of riverine sediment, accounting for 80.0% and 86.4% in Simiyu and Duma sub-tributaries, respectively, followed by channel banks with 9.0% in both sub-tributaries. Direct unmixing of the Simiyu main river sediment to the land use sources yielded 64.7% contribution of cultivated land and 26.5% of channel banks. Conclusion The demonstrated application of sediment source tracing provides an important pathway for quantifying the dominant sources of sediment in the rivers flowing towards Lake Victoria. Eroded soil from agricultural areas is the biggest contributor to transported sediment in the Simiyu River. This information is vital for the design of catchment wide management plans that should focus on reducing soil erosion and sediment delivery from farming areas to the river networks, ultimately supporting both food security and water quality in the Lake Victoria Basin.

Assessing the Impacts of Land Use and Climate Changes on River Discharge towards Lake Victoria

“…The catchment extends between latitude 2 • 15 and 3 • 15 South and longitudes 33 • 15 and 35 • 00 East, and its altitude ranges between 1100 and 2000 m.a.s.l. The catchment experiences semi-arid environments, with periodic variations in rainfall, most of which falls during the two rainy seasons of extended rains from March to May and short rains from October to December [29]. Between June and September there is a dry season, and January and February serve as the transitional months between the seasons [29,39].…”

“…The catchment experiences semi-arid environments, with periodic variations in rainfall, most of which falls during the two rainy seasons of extended rains from March to May and short rains from October to December [29]. Between June and September there is a dry season, and January and February serve as the transitional months between the seasons [29,39]. The region also has significant interannual variation in rainfall with drier and wetter years.…”

“…Additionally, it is crucial for maintaining the country's economy, supporting local communities, and for biodiversity conservation. The catchment's major economic activities include rainfed smallholder agriculture, fishing and pastoralism [29]. The Simiyu river provides water for both domestic and industrial uses in some towns around Mwanza Region.…”

“…The river and its tributaries are also targeted for supply of additional water for irrigation in the catchment. The water demands in the catchment are rapidly growing and are anticipated to increase even further in the future [29]. However, due to erosion, inadequate human waste management, unsustainable land use management practices, degradation of wetlands, and sand mining in the river, the catchment's water resources are declining in both quality and quantity [26,27].…”

Assessing the impact of anthropogenic influences on the sediment quality of Owalla Reservoir, Southwest, Nigeria

“…Sediment is eroded from the landscape, transported by river systems, and eventually deposited in a lakes or Reservoirs [7]. Toxic chemicals can become attached, or absorbed, to sediment particles and then transported to and deposited in other areas.…”