Characteristics of chemical weathering and water-rock interaction in Lake Nyos dam (Cameroon): Implications for vulnerability to failure and re-enforcement

For the first time, comprehensive study of hydrogeochemistry of water seeps, role of chemical weathering on dam failure, estimation of minimum width of dam to resist failure and simulation of changes in dissolved ions and secondary mineral was conducted on the Lake Nyos dam. The salient results and conclusions were; the dam spring water represented a mixture of 60-70% rainwater and 30-40% Lake water (from 0 to -40m). The chemistry of the observed waters was Ca-HCO₃ for rainwater, Ca-Mg-HCO₃ in boreholes, and Mg-Ca-HCO₃^- for spring water. The relative rate at which ions dissolved in water was HCO₃^->Mg²⁺>Ca²⁺>Na⁺>SiO₂>K⁺>NO₃^->SO₄^2->Cl^-. Weathering of rocks resulted in the formation of clay minerals such as kaolinite and smectite. Relative mobility of elements compared to Alumina (Al₂O₃) indicated that in monzonites there was a loss of CaO, Na₂O, K₂O, P₂O₅ and gain of SiO₂, Fe₂O₃, TiO₂, MnO and MgO, while in basalts there was a loss of SiO₂, Fe₂O₃, Ca₂O, NaO, MgO and gain of TiO₂, K₂O and P₂O₅. Values of chemical alteration index that ranged from 49 to 82 suggest a weak to intermediate categories of chemical weathering that occurred at a rate of 5.7mm/year. Paired to that rate, which suggests that the dam is not vulnerable to failure at the previously thought time scale, some other processes (physical weathering, secondary mineral formation and lake overflow) can cause instant failure. Hydrostatic pressure of 1.6GN generated by Lake water can be supported only when the width of the dam is greater than 19m. PHREEQC-based simulation for 10years indicates decoupling of Ca and Mg, and Na and Mg. Multidisciplinary monitoring of the dam is advocated.