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 Sr-87/Sr-86 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). delta C-13 values (-11.91--6.87%o VPDB) of groundwaters confirmed the solution of carbonates but indicated forthermore influences of soil-CO2. Deuterium and O-18 contents showed a relatively narrow range of -80.1--70.0%. VSMOW and -10.2--8.8 parts per thousand, VSMOW, respectively but are affected by evaporation and mixing processes. The use of delta C-13 together with 87Sr/86Sr revealed three possible sources: (i) carbonate-controlled waters showing generally higher Sr-concentrat ions, (ii) fissure waters with low-strontium contents and (iii) infiltrating water which is characterized by low delta C-13 and Sr-87/Sr-86 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).