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 Sep tember 1994 in an endoreic basin located in the N of Mexico (Comarca L agunera). In this area, groundwater has been exploited over the past c entury mainly for irrigation and cattle supply. Recent intensive pumpi ng 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 increasin g evaporation of the groundwater towards the Nazas river. They also in dicate that the recharge occurs from the Nazas river and from the moun tains surrounding the depression (Sierra Madre Occidental). Water pres ents a large spatial variability of the chemical facies (SO4-Ca, SO4-C l-Na, HCO3-Ca and HCO3-Na) which is in relation with (i) their interac tion with the geological formations of the basin (carbonates, gypsum a nd 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 v aries 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 equil ibrium with the TDIC, varying between -18.4 parts per thousand to -10. 9 parts per thousand indicate two origins of C in solution: the carbon ate matrix (delta(13)C = +0.9 parts per thousand) and the soil CO2 (de lta(13)C from -27.7 parts per thousand to -21.7 parts per thousand for the cultivated areas). Mean residence times have been determined afte r correction of the initial activities for dead C from the rock matrix . The mean residence times confirm a modern recharge of the groundwate r from the Nazas and indicate the presence of palaeowaters in the nort hern and southern parts of the basin (up to 30 ka BP). (C) 1997 Elsevi er Science Ltd. All rights reserved.