Land-to-lake micronutrient transfer in east Africa

The first year of my PhD has been very exciting, although not without its challenges. In November 2019, one month into my PhD, I travelled to Kenya for the first time. This fieldwork provided initial training in sample collection via the Geochemistry and health in the Kenyan Rift Valley project. With the help of our counterparts at the University of Eldoret, I had the opportunity to scope out some potential experimental soil erosion plots in the Oroba Valley, Nandi County, western Kenya (Figure 1).

The escarpment area of western Kenya is rapidly being converted from native forest into farmland, with the Oroba valley representing differing time periods for land clearance between one and more than 80 years. It was clear from our visit that the local residents were feeling the effect of soil erosion in the area, with one family having recently lost their crop due to heavy rainfall washing away the terraces they had built earlier in the year. This land degradation is not only devastating for the farmers in the area, but the soil also has the potential for loss of nutrients and transfer of metals into the river catchment, with consequences for aquatic biogeochemistry in the Winam Gulf of Lake Victoria.

The analysis of these pilot samples indicated that the valley would be an ideal area to model soil erosion processes occurring over differing timescales, within the Lake Victoria catchment. In March, I travelled back to Kenya with Dr Olivier Humphrey to revisit the Oroba valley and begin reference site sampling. During this trip I also had the opportunity to scout out some interesting areas with differing land management practices, for more intensive soil erosion plots in the future.

Unfortunately, this field trip had to be cut short by a couple of days because of the global lockdown due to COVID-19 and a need to return to the UK. However, I was fortunate enough to have collected the planned 10 soil cores to a depth of 30 cm (Figure 2) enabling me to continue work through the summer and autumn of 2020.

This summer has been somewhat mixed: the lockdown has not enabled me to undertake follow-up fieldwork in June and October, yet I have been able to progress with the laboratory analyses. These included chemical analyses of the samples, with emphasis on gamma spectroscopy to determine the radionuclide inventories of both Cs-137 and unsupported Pb-210 in the collected soil cores.

The data can then be used to determine the extent of soil redistribution processes[1]. The use of these radionuclides as soil erosion tracers have limitations in the southern hemisphere, so I am investigating the possibility of using Pu-239 and Pu-240 as alternative tracers of soil redistribution[2]. To do this, alongside members of the BGS Inorganic Geochemistry team, I will be developing a working method for the pre-concentration and rapid analysis of plutonium isotopes on the inductively coupled plasma mass spectrometer. This will then allow me to determine the best method for quantifying soil redistribution processes and relate the extent of soil erosion with the soil-micronutrient losses and ultimately soil land-to-lake transfers of metals/nutrients to the Lake Victoria catchment.

This work is a progression from previous geochemistry and health work in western Kenya and is interlinked with a Royal Society international collaboration through to 2022 to identify lake-to-land transfers resulting from land-use changes and soil erosion, subsequent impacts on fisheries and the growing aquaculture industry in Lake Victoria. Ultimately, this work will inform consequences for agricultural productivity, nutritional potential of staple foods and aquatic biogeochemistry, with a link to mitigation steps tested by colleagues in Tanzania[3].

About the author

My name is Sophia and I am a BUFI PhD student based within the BGS Inorganic Geochemistry Facility in Keyworth. My research is entitled ‘Micronutrient and pollution transfer in east African lake catchments‘. This PhD is in partnership with the University of Plymouth and is funded by the NERC ARIES doctoral training programme. Before beginning my PhD, I completed a BSc in chemistry at the University of Surrey, during which I completed a placement year within the BGS Inorganic Geochemistry team (see my previous blog) and this is what first sparked my interest in environmental geochemistry.