Sediment records of the 2016-2017 flood from Notwane dam, Upper Limpopo basin, Botswana

The data set contains grain size distributions, organic matter (OM) contents and trace metals distribution metal distribution (Fe, Zn, Cu, Cr and Pb) of seven (7) shallow cores of sediments from the Notwane Dam (southern Botswana). The cores have been collected with a push corer using PVC pipes of 5 cm diameter by a team from BIUST led by Dr. Franchi between November 2017 and February 2018. The data were collected as part of the PULA project, which aimed at understanding the immediate effect of heavy rainfall and floods on water resources in arid Botswana and their transitional hydrologic readjustment towards the dry period, and the role of these events in supporting either or both resources replenishment and contamination. Grain size profiles of the cores have been obtained by analyzing wet sediments with a laser diffraction particle size analyzer. The OM content of the sediments was obtained by measuring the loss on ignition (LOI) of bulk sediments after calcination in a muffle furnace. The sediments have been analyzed for trace metal distribution using a Microwave Plasma-Atomic Emission Spectrometer (MP-AES). The grain size distribution has allowed to pinpoint the presence of flood couplets, characterized by coarse lags followed by fine grained materials. The trace metal distribution profiles have been compared with the flood couplets to assess the entity of metal mobilization during floods and subsequent droughts.

Seven cores of sediments were collected at scattered locations in the Notwane dam over 2 days (22/11/2017 and 19/02/2018) for further sedimentological and geochemical analyses. Core collection was realised with 50 mm diameter PVC pipes though use of a push corer equipped with extension rods from shore or from a small boat. They were subsequently stored the same day at 4 ˚C in a refrigerator at the Botswana International University of Science and Technology (BIUST) to allow for water retention. Before analysis, core were split in half, photographed and sub-sampled at selected intervals for lab analysis. Grain size distribution for each interval was obtained through dispersing a few grams of sediment into distilled water which was analysed with a Malvern Mastersizer 3000 laser diffraction particle size analyser at BIUST. The organic matter (OM) content was measured at BIUST through measuring the loss on ignition (LOI) of bulk sediments after calcination in a muffle furnace. The porcelain crucibles were pre-heated in a muffle furnace for 1 hour at 375°C and after cooling transferred into a desiccator for further cooling and weighed. Samples were dried at 105°C for 24 hours and weighed. The dried samples were then heated for 16h at 550°C. 95 samples of sediments from selected intervals of the cores were further analysed for trace metal distribution (Fe, Zn, Cu, Cr and Pb) using an Agilent 4200 Microwave Plasma-Atomic Emission Spectrometer (MP-AES) at BIUST. About 1 g of sample was treated with 10-15 ml of 65% nitric acid at 60°C for a period of 4 hours. The samples were then filtered at <0.45 µm and brought to 50 ml adding Milli-Q distilled water. Average standard deviations were as follows: Fe %RSD = 1.81; Zn %RSD = 4.02; Cu %RSD = 1.43; Cr %RSD = 0.82; Pb %RSD = 1.72. Detection limits for the metals reported here (as weight of dry sediment) were as follows: Cr = 0.8 µg kg-1, Fe = 1.7 µg kg-1; Cu = 0.5 µg kg-1; Zn = 28 µg kg-1; Pb = 2.5 µg kg-1