δ~(18)O and δ~2H characteristics of rainwater, groundwater and springs in a mountainous region of Ghana: implication with respect to groundwater recharge and circulation

摘要

In this study, the characteristics of δ~2H and δ~(18)O in rainwater, groundwater and spring water have been used to understand the groundwater recharge dynamics in the Amedzofe area (one of the highest human habitation in Ghana). The local Meteoric Water Line (LMWL) using all the rainfall events gave an equation with the best fit line of δ~2H =7.32 δ~(18)O + 8.20 (n=113; r~2 = 0.94). Heavier rainfall events greater than 10, 20 and 30 mm further orient the slope closer to the Global Meteoric Water Line (GMWL). The data showed that the isotopic compositions of the rainwater in the area are mainly influenced by the rainfall amount with temperature having minimal effect. All the spring water and most of the groundwater plot around the GMWL, implying they are of meteoric origin. The Chloride Mass Balance (CMB) and δ~(18)O shift methods suggest annual mean recharge rate of 160.43 and 125.79 mm/year representing 10.70 and 8.39% of annual rainfall, respectively. The estimated mean recharge rate for the spring was 317.51 mm/year and 300.68 mm/year representing 26.46 and 25.06% of annual rainfall for CMB and δ~(18)O shift methods, respectively. Rainfall stable isotopes from Kpando and Amedzofe gave an altitude effect of 0.17/100 m and altitude of recharge of the springs from +724.00 to +788.61 m amsl. The stable isotopes data identified two distinct moisture sources, isotopically enriched local moisture (evaporated surface water bodies and evapo-transpiration water vapour flux from vegetation) and isotopically depleted moisture source from the Atlantic Ocean. The results further showed that significant groundwater recharge in the study area takes place in the months of March, June and August.