Subsurface sources contribute substantially to fine-grained suspended sediment transported in a tropical West African watershed in Burkina Faso

Increasing watershed sediment yields is an important problem in Africa, but the sources of these sediment yields have only very rarely been investigated. This study therefore aims to discriminate subsurface and surface sources of fine-grained sediments in a representative mesoscale (580 km(2)) West African savanna watershed. We used a sediment source fingerprinting approach for source apportionment including geochemical and radionuclide (Cs-137, Pb-210(ex), and Be-7) composite signatures where Be-7 was used as a tracer for the first time in the African environment. Two field campaigns were conducted collecting a total of 258 geochemical and 66 isotope samples. We found that subsurface source categories, dominantly river bank, contributed an unexpected high share of 43% (geochemistry) and 45% (radionuclides) to the sampled fine-grained sediments. Pairwise comparison of the averaged frequency distributions for predicted source proportions using five geochemical signatures with the frequency distribution generated using the single radionuclide signature suggested that the two distributions are not statistically different. Extrapolating our measured contribution of subsurface erosion to areas with similar yields in comparable environmental settings, we can assume that subsurface sources are an important component of sediment loss across large areas of West Africa. Subsurface erosion, primarily associated with bank rather than gully erosion, is likely to increase in the future with projected rises in run-off due to land use and climate change. Source tracing studies need to be undertaken more widely across Africa to help mitigation planning for sediment-related and land degradation problems.