Geochemical processes in the groundwater of Soubré, Ivory Coast, West Africa: Evidence from 14C, 13C, 18O, 2H and major ions

Abstract

Characteristics of the bedrock groundwater in the southwestern (SW) Ivory Coast (IC) (5–6°N, 4–7°W) are geochemically less known compared to those in the eastern part of the country. Several main cash crops, such as cocoa, coffee, rubber and palm oil, started being grown in this area twenty years ago because of the migration of agricultural activities from the East to the West of IC; this migration of agricultural activites was tied to climate variability. Within these large land use areas, food crops such as plantain bananas, yams, rice, maize, and vegetables are also grown. In this work, the mineralisation and groundwater pollution from East IC to West IC are studied within groundwater of the Archean and Paleoproterozoïce bedrock aquifers located in the SW IC. Stable isotopes ratios (δ18O, δ2H and δ13C), radiocarbon activity (A14C) and chemical contents (major ions) were measured on a set of 23 groundwater samples. In SW IC, the residence times calculated by an exponential mixing model (EMM) and a dispersion model (DM) with Dispersion Parameter values of 0.05 and 0.5 reveal that the oldest groundwater is located in the aquifer of migmatitic and granulitic gneiss (MiGn aquifer). Recent groundwater is located in the metagranitoid (MetGr) and the metasedimentary rocks (MetSe) aquifers. The most appropriate model to compute residence times is EMM because of the highest proportion of recharge in accordance with the nitrate content in groundwater after 1958, the beginning of Ivorian deforestation. However, this model lead to an overestimate of the residence time of groundwater particularly high in NO3. The oldest age of MiGn groundwater according to EMM calculation is ∼5 ka BP. The MiGn aquifer is less renewed (mean annual renewal rate of 0.03%) followed by those of MetSe (∼2.5 ka BP) and MetGr (∼2 ka BP), with mean annual renewal rates of 0.09% and 0.012%, respectively. Groundwater mineralisation is mainly governed by the hydrolysis of Al-silicates minerals and cultivation practice linked to K and N-fertilisers use, which leads to an increase of Ca, Mg and Na content. The values of TDS ranged from 49.4 to 720.4 mg L−1, which indicates the chemical characteristics of fresh water in SW IC. Most of the studied area groundwater are evaporated, likely due to both climate variability since the last 5 ka BP, delayed infiltration of rainwater which would be caused by impervious sediments covering the hard rock aquifers and agricultural practices affecting the most recently recharged groundwater. River water recharge from irrigation in cultivated areas may be possible. The groundwater with low 14C activities (64.33 pMC) has high residence times and is depleted in 13C content, while that with high 14C activities (99.58 pMC) is enriched in 13C content. This demonstrates that vegetation cover changed from C3 plants or forest (−23.9‰) to C4 plants, savannah or cultivation plants (−12.5‰) similar to those found in the southeastern IC.