Flow dynamics and age of groundwater within a humid equatorial active volcano (Mount Cameroon) deduced by δD, δ~(18)O, 3H and chlorofluorocarbons (CFCs)

摘要

We used environmental tracers (δD, δ~(18)O, 3H and chlorofluorocarbons (CFCs)) to establish the groundwater flow system within the Mount Cameroon area: a 4090m high active entirely alkaline volcano located within a humid equatorial region. Seventy-five ground and surface water samples (68 spring samples, 6 borehole samples and 1 stream sample) were collected around the volcano during the wet (October-November 2010) and dry seasons (January-February 2010) as well as 10 rain samples in June-September 2010 in order to establish recharge elevations, apparent ages (residence times) and groundwater flow system in the fractured volcanic (basaltic) aquifers. The δ~(18)O and δD composition of rainwater and groundwater in the Mount Cameroon area fit the Douala Local Meteoric Water Line (DLMWL). The recharge elevations of the individual springs were calculated from their δ~(18)O-values and the equation of the altitude gradient (-0.16‰ δ~(18)O/100m) of precipitation on the mountain. For springs sampled during the two seasons, dry season samples have higher recharge elevations than rainy season samples. Seasonal variation exists in the tritium values as rainy season samples systemically are higher than those of the dry season samples This implies the input of new water from the rain causing the groundwater system to rejuvenate. Apparent ages for the Mount Cameroon springs calculated from CFCs based on the Piston Flow model range from 10 to approximately 57years. CFC apparent ages for rainy season samples are systemically younger than those of dry season samples. There is seasonal variation in groundwater flow system in the Mount Cameroon area and also along the different flanks of the volcano. The groundwater flow system is conceptualized by 2 models; a Bypass/piston flow model for the flow system during the rainy season and a binary mixing/piston flow model characterizes the flow system during the dry season.