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Evaluation of the recharge processes and impacts of irrigation on groundwater using CFCs and radiogenic isotopes in the Silao-Romita basin, Mexico |
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| Authors: | Axel Horst Jürgen Mahlknecht Broder J Merkel Ramon Aravena Yann R Ramos-Arroyo |
| Institution: | 1. Centro de Estudios del Agua, Tecnológico de Monterrey, Ave. Eugenio Garza Sada No. 2501, CP 64849, Monterrey, NL, México 2. Department of Geology, Technische Universit?t Bergakademie Freiberg, Gustav Zeunerstrasse 12, 09596, Freiberg, Germany 3. Department of Earth and Environmental Sciences, University of Waterloo, Waterloo, ON, N2L 3G1, Canada 4. Facultad de Ingeniería en Geomática e Hidráulica, Universidad de Guanajuato, CP 36000, Guanajuato, GTO, México |
| Abstract: | 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 12 pmol/l (CFC-11), 0.03 and 1.7 pmol/l (CFC-12), and <0.01 and 0.23 pmol/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 300 years 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 6–109 pmC. Carbon-13 values varied between –11.9 and –7.2‰ 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. |
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