HYDROLOGICAL CHANGES DURING THE HOLOCENE AND PRESENT ISOTOPICAL AND GEOCHEMICAL COMPOSITION OF A PHREATIC AQUIFER IN THE CENTRAL ANDES

The area under study covers 3500 km2 in the upstream part of the closed catchment basin of the Altiplano. The climate is semi-arid with a markedly rainy summer from December to February. The aquifer recharge consists of two types. Local temporary runoff infiltration occurs for a few hours a year at the hinge line between the relief and the plain. A river crossing the area recharges the aquifer in the downstream part of the aquifer. The outlet from the Lake Titicaca and the runoff in the Northern part of the Altiplano feed this river. These two recharge types have different chemical and isotopic signatures. The discharge of the aquifer consists of three types. The groundwater flows toward the river in the upstream part of the area. The groundwater flows toward the south through the arbitrary southern boundary. The upward evaporative outflow takes place where the piezometric head lies at less than 20 m below the soil surface. Chemical elements leave the groundwater body by these three types of discharge according to their local concentration. The amounts of these different terms of inflow and outflow have changed during the Holocene according to the climatic evolution. As a consequence, the input, transport and output of chemical elements have also changed during this period. The objective of the study is to reconstruct the time evolution of these terms of the water and chemical balances of the aquifer during the Holocene. The observed data comprise piezometric head, vertical profiles of oxygen-18 and of chloride content in the unsaturated zone, and chemical (Cl, Li) and isotopic composition (oxygen-18, deuterium, carbon-14, strontium isotopic ratio) of the groundwater. The simulations were carried on during 11 000 years taking into account the water flow, the groundwater age and the transport of soluble elements. The computed results at present are compared to the observed data. This approach of transient modelling in phreatic aquifers of arid zone area is efficient to provide and/or discuss paleoclimatic information.