Isotopic tracers revealing the water movement in watersheds of the São Paulo State

Abstract

In a global scenario of climate change, the sustainable use of water resources requires an understanding about the mechanisms involved in the water movement and storage within the hydrological cycle compartments, since of water scarcity and increased competition for more water supply have been occurring. Future management solutions should consider the climatic factors involved in rainfall formation, the resilience of water in watersheds and their connection to groundwater, leading to formulate policies with the objective of increasing the water supply. In this context, several tracers have been used to understand the water movement and determine their subsurface residence times. Water molecule stable isotopes (2H/1H and 18O/16O) are excellent tracers of water movement along the hydrological cycle, and they are used as complementary tools to interpret discharge origins within watersheds, and climatic controls over precipitation, allowing the use in paleoclimatic reconstitution. In tropical areas climatic factors that control variations in isotopic composition of precipitation remain a controversial issue. Some authors propose that the isotopic composition of precipitation is influenced by aspects of local climate dynamics, others advocate the importance of processes of global scale, such as the Rayleigh distillation type. Environmental isotopes have also been used as a complementary tool in the understanding of hydrological processes in watersheds, helping to understand the mechanisms and processes involved in infiltration and aquifer recharge, as well as the origin of water flows in river discharge, estimation of the water mean transit times (MTT), identification of the flow components origin, as well as in the calibration and/or validation of hydrological models through the estimation of hydrological parameters. Several isotopes can be used to estimate groundwater ages ranging from decades to thousands of years. Two examples where isotopes may be useful for water management are: (i) determining aquifer recharge rates and (2) estimating of mean transit time (MTT) of groundwater in catchments (where transit time is the length of time between recharge and discharge from the aquifer). In both cases, these tracers can be used to formulate hypotheses about groundwater flow and interaction with surface water, and to infer how water resources management and/or changes in climate may impact aquifer systems. Considering these premises and tracers application, the present research project has three main thematic axes, whose scientific challenge is to elucidate the water movement in watersheds of São Paulo state, through the use of a series of isotopic tracers, seeking to understand the variability of the isotopic composition of precipitation and its relation with atmospheric processes in diverse temporal and spatial scales; to evaluate the spatial and temporal variability of the isotopic composition of surface waters in large basins and to study the processes of water infiltration and determine the average times of groundwater transit in small watersheds draining recharge areas of the Guarani Aquifer System. (AU)