Estimating groundwater mixing and origin in an overexploited aquifer in Guanajuato, Mexico, using stable isotopes (strontium-87, carbon-13, deuterium and oxygen-18).

Stable Isotopes (strontium-87, deuterium and oxygen-18, carbon-13) have been used to reveal different sources of groundwater and mixing processes in the aquifer of the Silao-Romita Valley in the state of Guanajuato, Mexico. Calcite dissolution appeared to be the main process of strontium release leading to relatively equal 87Sr/86Sr ratios of 0.7042-0.7062 throughout the study area which could be confirmed by samples of carbonate rocks having similar Sr ratios (0.7041-0.7073). δ13C values (-11.91- -6.87‰ VPDB) of groundwaters confirmed the solution of carbonates but indicated furthermore influences of soil-CO2. Deuterium and 18O contents showed a relatively narrow range of-80.1- -70.0 ‰ VSMOW and-10.2- -8.8 ‰, VSMOW, respectively but are affected by evaporation and mixing processes. The use of δ13C together with 87Sr/86Sr revealed three possible sources: (i) carbonate-controlled waters showing generally higher Sr-concentrations, (ii) fissure waters with low-strontium contents and (iii) infiltrating water which is characterized by low δ13C and 87Sr/86Sr ratios. The third component is affected by evaporation processes taking place before and during infiltration which might be increased by extraction and reinfiltration (irrigation return flow). [ABSTRACT FROM AUTHOR]