Evaluation of the recharge processes and impacts of irrigation on groundwater using CFCs and radiogenic isotopes in the Silao-Romita basin, Mexico

The recharge processes in the overexploited aquifer of the Silao Romita basin, central Mexico, were investigated by means of gaseous tracers (chlorofluorocarbons, CFCs) and radioactive isotopes (C-14, tritium). CFC concentrations varied between 0.06 and 12pmol/l (CFC-11), 0.03 and 1.7pmol/l (CFC-12), and <0.01 and 0.23pmol/l (CFC-113). CFC concentrations are controlled by irrigation return flow which became apparent by the comparison with tritium. Tritium activities ranged from 0 to 3.5 TU. The calculated mean residence times of 70 to more than 300years are considerably lower than the ages estimated based on the CFCs data. These data showed that CFCs were not appropriate for groundwater dating in this particular area but the CFCs were suitable as a qualitative measure of the magnitude of irrigation return flow which proved to be a significant source of recharge in the irrigated areas. Radiocarbon activities were in the range of 6-109pmC. Carbon-13 values varied between -11.9 and -7.2 parts per thousand VPDB. Modelling of carbon isotopes with NETPATH along a plausible flow path reveals considerable influences of exchange with soil CO2 and carbonate dissolution. Radiocarbon data indicate, at least in one case, the existence of groundwaters with residence times of more than 10,000 years.