The use of carbon isotopes in the study of groundwater of the Bambui calcareous-central region of Bahia (Brazil)

Groundwater of 34 wells and of a spring of the Bambui limestone aquifer, in central Bahia, Brazil, were analized for the 14C and 13C content. One sample of soil CO2 and four of soil organic matter were analized for 13C. From these data were calculated the 14C ages of these waters. A major difficulty in the use of radiocarbon in groundwater hydrology is the estimation of the initial 14C concentration. In many cases, this can be simply determined by the fraction of carbon derived from soil gas, relative to the total carbon dissolved, by the use of Δ13C of the soil organic matter, limestone and dissolved carbon in water. This approach does not seem to be completely valid in arid ou semi-arid regions, specially where the pH of the soil is relatively high. In this case, the isotopic composition of the soil water can be determined if the pCO2 and pH of the soil can be estimated and if the isotopic composition of the soil CO2 can be known. The final isotopic composition of the groundwater is a combination of the isotopic composition of the soil water and any limestone thereafter dissolved. The 14C ages of the water samples analized ranged from modern to about 13000 years. The recharge areas of the aquifer are clearly indicated, as the probable underground flow directions. The interpretation of the radiocarbon data is in accord with the hydrologic data. (Author)