A multi-tracer approach for assessing the origin, apparent age and recharge mechanism of shallow groundwater in the Lake Nyos catchment, Northwest, Cameroon

The shallow aquifer in the vicinity of Lake Nyos (Northwest, Cameroon) is one of the main water supply sources to meet the water needs of the inhabitants to be resettled after 1986s tragedy. Unfortunately, there is a lack of knowledge on the groundwater recharge and flow systems in the area. Multiple environmental tracers (δ18O, δD, Cl-, CFCs and SF6) and a yearly record of rainfall, surface waters and groundwater were employed to characterize the recharge mechanism of the shallow groundwater seeping in the fractured rock of the Lake Nyos catchment (LNC). The δ18O-δD relationship of the rainfall events gave the Nyos Meteoric Water Line: δD=8.28 δ18O+11.87. Inland moisture vapor may have impacted the isotopic composition of original vapor masses from Gulf of Guinea prior to precipitation. Shallow groundwater in the LNC shows a similar trend of enrichment in 18O and D as surface waters indicating a well-mixed aquifer. The proportions of surface waters and rainfall in the groundwater reservoir were 87% and 13%, respectively. The high annual recharge rate (941mm/yr) and the seasonal variability in the isotopic signatures of groundwater indicate a renewable aquifer system. CFCs apparent ages-based piston flow model revealed a young age (average of 24 a.) of the groundwater in the LNC. SF6-based ages were biased young as compared to relatively younger than CFCs-based ages, implying an additional terrigenic production of SF6. The conceptual model for groundwater flow suggests that three main flow regimes, mainly controlled by the physical properties of the rock heterogeneities govern the movement of water in the aquifer. The piston flow model appears, however, to be the better model to explain the flow regime in the highly faulted and fissured area where recharge occurs (~1200-1600masl). The rapid circulation and the low solubility lead to low mineralization. In the middle-lower area where waters circulate more in the weathered layer, exponential mixing model dominates, yielding more mineralized water. Downward to the valley, with prevailing gentle slope, interaction between surface and recently infiltrated groundwater better explain the observed similarity in the isotopic compositions and the mineralization of water.