Palaeoclimatic and deforestation effect on the coastal fresh groundwater resources of SE Ivory Coast from isotopic and chemical evidence

In the South-east of the Ivory Coast, two aquifer systems have been studied in the sedimentary deposits at the South and in the fractured bedrock at the North of the study area (5–6°N, 2.40–4.40°W) : (1) the Continental Terminal (CT) and (2) the Paleoproterozoïc Bedrock (PB). In the studied area, the vegetation cover has undergone significant changes since 1955 in addition to climate change. Rainforests have gradually disappeared due to natural and anthropological deforestation. The impact of deforestation on groundwater of the PB and on the CT has been studied by a geochemical approach. Stable isotopes (18O, 2H and 13C) contents, radiocarbon (14C) contents and chemical data (major ions) have been measured on a set of 25 groundwater samples. The residence time of the groundwaters is estimated with the 14C using two models: (i) the model of well-mixed reservoir (WMR model) and (ii) the piston flow model (PF model). The range of the PB groundwater residence time (15,200–8300 to ∼300–100 a BP) for both models shows that the recharge has started at the beginning of the post-glacial period whereas the CT aquifer recharge is much more recent (from 300 a BP to today). The PB groundwater provides information about paleoclimatic conditions that occurred over the studied area during the late Pleistocene. The low contents indicate cold and/or more humid conditions of recharge. During that period, the low content of 13C is consistent with a vegetation cover dominated by rainforest (C3 plants). After the 20th century, the progressive evolution of vegetation cover from forest to cultivated plants and grasses is shown by the enrichment of groundwater in 13C (C3 plants to C4 plants). The relatively high mineralization level (mean of 143.7 mg L−1) and high δ18O–δ2H values of modern PB groundwater reflect of a recharge process that is slowed by a thick layer (16.3–72.5 m) of weathered formations above the PB formations. Groundwaters of the CT aquifer are constantly supplied by the fast infiltration of rainfall towards the water-table, explaining their very low total dissolved solids (TDS) of 20.1 mg L−1, their high 14C activities and their non-evaporated characteristics.