Hydro-chemistry and isotopic geochemistry of the Comarca Lagunera ground water (Northern Mexico)

A chemical ad isotopic study has been conducted on groundwaters from the endoreic basin of the Comarca Lagunera, one of the most important agricultural regions of Northern Mexico. This region experiences semi-arid climatic conditions and is affected by an important hydric deficit (precipitation = 200 to 300 mn per year, potential evapotranspiration = 2500 mn per year). Besides, groundwater exploitation for agricultural use causes the lowering of the piezometric level of about 1.5 m per year. Groundwaters show a wide range of total dissolved salt contents, from 200 to 5000 mg per liter and different geochemical facies related to (1) evaporation, (2) to calcite and gypsum dissolution, (3) to calcite reprecipitation and (4) to cation exchange with the clay from the aquifer. The 14C activities indicate, that the recharge occurs from the surface network (river Nazas) and from the ''cerros'' located in the western part of the basin, and, show, the presence of three types of waters according to their residence times ranging from 30 000 B.P. to present. The 18O and 2H values show that groundwaters are affected by evaporation. The increase of the evaporative signal in the water, as a function of their residence time, shows that the region is progressively becoming more arid since 30 000 B.P.. Besides, groundwater pumping for agricultural use highly affects the hydraulic gradient and causes a present day circulation of old groundwaters towards recent waters. The over-exploitation of this aquifer will most probably lead to a progressive aging of pumped groundwaters and a combined increase of their mineralisation. (author)