Shallow groundwater quality evolution after 20years of exploitation in the southern Lake Chad: hydrochemistry and stable isotopes survey in the far north of Cameroon

Two field hydrogeological surveys were conducted in 1992 and 2013 using hydrochemistry and isotope techniques in the far north region of Cameroon which corresponds to the southern part of Lake Chad basin. The dataset relates to shallow groundwaters of the Quaternary and substratum aquifers that were sampled at the same locations to potentially reveal any temporal variation in the chemical and isotopic characteristics of the groundwater resource. Groundwater shows mainly a CaMg-HCO3 water type but CaMg-Cl-NO3 and NaK-HCO3 water types can also be found. The mineralization is governed essentially by cation exchange processes. However, we also observe dissolution processes of carbonates. Strong anthropogenic contamination of groundwater with nitrates, chlorides, and sulfates is highlighted. It is found that the nitrate content after 20years has increased by an average factor of 6. These high concentrations in nitrate (up to 600mg/l) are related to local anthropogenic activities and to the very bad conditions of maintaining of wells and boreholes. The isotopic content is ranging from -6.46 parts per thousand to +0.46 parts per thousand for O-18 in 2013 and from -5.57 parts per thousand to +0.37 parts per thousand in 1992 without noticeable evolution. The stable isotope approach underlined the hydraulic connection between basement and quaternary aquifers. Stable isotopes also demonstrated that groundwater in the region is recharged by modern precipitation, but also by evaporated surface waters from rivers during flood episodes. After 20 years, the evolution in Cl-, SO42-, and NO3- concentrations in groundwater is worrying in terms of public health and the government is encouraged to develop a monitoring of this transboundary resource.