Hydrogeochemical and nitrate isotopic evolution of a semiarid mountainous basin aquifer of glacial-fluvial and paleolacustrine origin (Lake Titicaca, Bolivia): the effects of natural processes and anthropogenic activities

hydrogeochemical and stable isotopic ( ^15N_NO_3 and ^18O_NO_3 ) multitracer approach was combined with previous geological and hydrogeological knowledge in a groundwater-dominated basin, located within the semiarid region of the Bolivian Altiplano (SE of Lake Titicaca). Major natural processes and anthropogenic impacts controlling water chemistry and isotopic compositions of groundwater were identified and corresponding aquifer impacted zones determined. The main natural processes are, by following water flowlines, (1) silicate weathering in the piedmont subsystem ( 4,600–3,910 m asl, Ca(Mg)-HCO 3 facies), (2) Na-Ca exchange within glacial-fluvial deposits overlying paleolacustrine deposits ( 3,910 to 3,860 m asl, Na-HCO 3 facies), and (3) evaporite dissolution in the confined zone of the lacustrine plain ( 3,860–3,810 m asl, Na-Cl-SO 4 facies). The highest contributions of anthropogenic nitrate in groundwater have been observed at 3,960–3,860 m asl in the piedmont subsystem and were isotopically associated with leaching from areas influenced by manure piles, synthetic N fertilizers, and sewage collector pipes. In this subsystem, natural water–rock interactions could be deciphered with minimal anthropogenic impact, allowing nitrate sources to be clearly identified. Denitrification, occurring in the topographic lows of the piedmont subsystem, was identified as the main natural attenuation process. The multitracer approach provided a consistent understanding of the major processes that take place along the groundwater flow system and confirmed the significant role of anthropogenic nitrate. This aquifer system thus represents an ideal model of the region’s hydrochemical evolution along the gravity-driven flow caused by natural water–rock interaction processes and the influence of anthropogenic contamination.