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 (O-18, H-2, C-13 and C-14). Water stable isotope contents (O-18, H-2) 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 (SO4-Ca, SO4-Cl-Na, 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. The C-14 activity varies between 110.4 (+/- 1.1) and 4.0 (+/- 0.2) pmc. The C-13 contents of the total dissolved inorganic C (TDIC) range between -11.0 and -3.6 parts per thousand. The calculated C-13 contents of the CO2 in equilibrium with the TDIC, varying between -18.4 parts per thousand to -10.9 parts per thousand indicate two origins of C in solution: the carbonate matrix (delta(13)C = +0.9 parts per thousand) and the soil CO2 (delta(13)C from -27.7 parts per thousand to -21.7 parts per thousand 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). (C) 1997 Elsevier Science Ltd. All rights reserved.
