Geochemistry and residence time estimation of groundwater from the upper aquifer of the Chihuahua desert (Comarca Lagunera, Northern Mexico)

178 groundwater and surface waters have been sampled from April to September 1994 in an endoreic basin located in the N of Mexico (Comarca Lagunera). In this area, groundwater has been exploited over the past century mainly for irrigation and cattle supply. Recent intensive pumping has caused the lowering of the water table at a rate of 1 m a⁻¹ Chemical analyses have been performed on all collected samples and 37 of them have been selected for isotopic measurements (¹⁸O,²H,¹³C and¹⁴C). Water stable isotope contents (¹⁸O,²H) show an increasing evaporation of the groundwater towards the Nazas river. They also indicate that the recharge occurs from the Nazas river and from the mountains surrounding the depression (Sierra Madre Occidental). Water presents a large spatial variability of the chemical facies (SO₄Ca, SO₄ClNa, HCO₃-Ca and HCO₃-Na) which is in relation with (i) their interaction with the geological formations of the basin (carbonates, gypsum and various silicates) and (ii) evaporation. This evaporation occurs in the upper part of the unsaturated zone during infiltration especially for the groundwater sampled near the Nazas river. The¹⁴C activity varies between 110.4 (± 1.1) and 4.0 (±0.2) pmc. The¹³C contents of the total dissolved inorganic C (TDIC) range between −11.0 and −3.6‰. The calculated¹³C contents of the CO₂ in equilibrium with the TDIC, varying between −18.4%0 to −10.9% indicate two origins of C in solution: the carbonate matrix (δ¹³C= +0.9‰) and the soil CO₂ (δ¹³C from −27.7‰ to −21.7‰ for the cultivated areas). Mean residence times have been determined after correction of the initial activities for dead C from the rock matrix. The mean residence times confirm a modern recharge of the groundwater from the Nazas and indicate the presence of palaeowaters in the northern and southern parts of the basin (up to 30 ka BP).