Isotopic characterization of precipitation and its relationship with groundwater in the Central Andes

Abstract

The availability of water resources is increasingly limited worldwide. Therefore, optimal and sustainable management is necessary to ensure a balance between ecosystems and human rights and activities. In the arid Central Andes, there is a great demand for water for different uses, including domestic consumption, intensive mining and, in some areas, livestock and emerging ecotourism. These human activities must respect and conserve the lagoon systems of the salt flats, on which the fauna depend. Understanding the hydrological and hydrogeological functioning of basins is essential for the optimal and sustainable management of water resources. To address this issue, the objective of this thesis is to advance the knowledge on the isotopic composition of precipitation and its relationship with groundwater in the Central Andes, particularly in the Salar de Atacama basin (Chile).

This thesis contributes to the (1) understanding of the origin of precipitation in the Central Andes and reviews how it varies isotopically (δ18O and δ2H) throughout the Amazon basin and in the Central Andes between 14°S and 28°S, (2) identification of the factors that control the isotopic composition of precipitation in areas with scarce isotopic and meteorological data and estimation of the meteorological and isotopic variables of precipitation, (3) characterisation of precipitation and identification of the recharge zone of the Salar de Atacama basin using stable isotopes, and (4) quantification of the contribution of snowmelt to the recharge of the aquifer of the basin.

Through an exhaustive review of the literature, precipitation in the Central Andes has been characterised. Precipitation occurs mainly during the austral summer. The masses of humidity have different origin: (1) from the North Atlantic Ocean through the Amazon basin, (2) from the South Atlantic Ocean through the Plate River basin and the Gran Chaco in summer, and (3) the Pacific Ocean in winter. The processes involved in the isotopic evolution of summer convective precipitation in the Eastern Cordillera are known. However, there is no consensus on the factors that control the isotopic variation in the Western Cordillera.

To identify the processes that determine the precipitation isotopy in the Cordillera Occidental, a statistical analysis is carried out between geospatial, meteorological and isotopic variables. The result indicates that temperature is controlled by altitude and latitude, while relative humidity is controlled by latitude, and summer precipitation is controlled by altitude and latitude. The stable isotopes of precipitation are controlled by temperature, altitude, latitude, longitude, and precipitation. Finally, based on these relationships, these variables are estimated by independent models throughout the study area and three isotopic models (summer, winter and annual).

The isotopic compositions of the surface water and groundwater indicate the presence of evaporation and that they are similar to each other in each subbasin, both in the Salar de Atacama and in the Altiplano-Puna. Finally, the meteoric source of the waters of the Altiplano-Puna basins is more depleted of heavy isotopes than are the waters of the peripheral subbasins of the salar. Therefore, these results show that there is no hydraulic connection between basins and that there is no significant transfer of water from the Altiplano-Puna to the peripheral aquifer of the Salar de Atacama.

This thesis presents a first view of the behavior of snow in the Toconao subbasin (subbasin east of the Salar de Atacama) through physical models. The total average precipitation was 139 mm/year in 2017, of which a third was in the form of snow. In summer the snow was above 4500 m asl and it melted in 10 days. In winter, when the temperature is lower, the snow cover covered much of the basin and slowly melted for a month.

La disponibilidad de los recursos hídricos cada vez es más limitada en el mundo. Por ello, es necesaria su gestión óptima y sustentable, asegurando el equilibrio entre los ecosistemas y la actividad humana. En los Andes centrales existe una gran demanda de agua para diferentes usos, el consumo doméstico, la explotación intensiva de la minería y, a nivel local, la ganadería y el turismo de naturaleza emergente. Estas actividades antrópicas deben de respetar y conservar los sistemas lagunares de lo s salares, de los cuales depende la fauna. Comprender el funcionamiento hidrológico e hidrogeológico de las cuencas es fundamental para una gestión óptima y sustentable de los recursos hídricos. Para solucionar esta situación, el objetivo de esta tesis ha sido avanzar en el conocimiento de la composición isotópica de la precipitación y su relación con las aguas subterráneas en los Andes Centrales, particularmente en la cuenca del Salar de Atacama (Chile).

Esta tesis contribuye a (1) comprender el origen de la precipitación en los Andes Centrales y revisar cómo varía isotópicamente (δ18O y δ2H) a lo largo de la cuenca del Amazonas y en los Andes Centrales entre 14°S y 28°S, (2) a la identificación de los factores que controlan la composición isotópica de la precipitación en áreas con escasos datos isotópicos y meteorológicos y a la estimación de las variables meteorológicas e isotópicas de la precipitación, (3) a la caracterización de la precipitación e identificación de la zona de recarga mediante isótopos estables en la cuenca del Salar de Atacama y (4) a cuantificar el aporte del deshielo en la recarga del acuífero de la cuenca.

En esta tesis se concluye que (i) la incertidumbre que existía en la literatura sobre los procesos involucrados en los isótopos estables de la precipitación en la Cordillera Occidental se ha resuelto, que (ii) lo descrito en la literatura a gran escala (Andes Centrales) es coherente con los resultados de las muestras recolectadas y analizadas en la cuenca del Salar de Atacama y (iii) que la nieve es una componente importante en el ciclo hidrológico del Salar de Atacama.