Assessing the spatial distribution of elemental concentrations in surface sediments of Lake Victoria, Kenya: implications for ecological health and management

Abstract

Lacustrine sediment quality indicates the effects of both natural and anthropogenic activities on the ecosystem and communities. Despite its ecological importance, myriad complexities, and potential contaminant sources, the spatial distribution of surficial sediments in Lake Victoria's Winam Gulf has never been comprehensively documented. The purpose of this study was to assess the spatial distribution, pathways, and ecological risk of metal elements in the lake using a sediment matrix. Sediment samples were collected throughout the gulf in November 2022. The concentrations of Al, As, Cd, Co, Cr, Cu, Fe, K, Mn, Mo, Ni, P, Pb, Sb, Sn, Ti, Tl, U, and Zn were compared to different contamination metrics and ecological risk assessment indices. The average concentrations were in the following decreasing order: Zn > > > Cr > > Cu > Ni > Pb > Co > As > Cd with mean (± SD) of 185 ± 45 mg kg^-1, 56 ± 15 mg kg^-1, 45 ± 16 mg kg^-1, 37 ± 11 mg kg^-1, 24 ± 5 mg kg^-1, 20 ± 7 mg kg^-1, 3.9 ± 1.3 mg kg^-1, 0.30 ± 0.09 mg kg^-1, respectively, with strong indications of anthropogenic sources. Average concentrations were in the following decreasing order: Zn > > > Cr, Cu, Ni, Pb, Co, As, and Cd levels (mean ± SD) were 185 ± 45 mg kg^-1, 56 ± 15 mg kg^-1, 45 ± 16 mg kg^-1, 37 ± 11 mg kg^-1, 24 ± 5 mg kg^-1, 20 ± 7 mg kg^-1, 3.9 ± 1.3 mg kg^-1 and 0.30 ± 0.09 mg kg^-1 with strong indications of anthropogenic sources. The geo-accumulation index (I_geo) and enrichment factor categorisation schemes, respectively, classified these as uncontaminated (level 0) and depletion to minimal enrichment (level 1), while the ecological risk analysis classified them as "low risk". The mouth of the Nyando River, as well as Kisumu, Kendu, and Homa bays, were the most element-enriched and should be prioritised for focused monitoring and remediation. As a result, targeted land management of urban, industrial, transportation, and agricultural areas offers the opportunity to reduce sediment inputs into the lake ecosystem.

Keywords: Freshwater system; Lacustrine ecosystem health; Nyanza Gulf; Sediment based elements; Sustainable management.