Hydrogeochemical and Isotopic (δ2H-δ18O) Investigations of Hydrologic Dynamics of the Southern Urucuia Aquifer System, Brazil

The integration of hydrogeochemical and isotopic data (δ 2 H-δ 18 O) allowed the reconstruction of the hydrological dynamics of Southern Urucuia Aquifer System. This system is a combination of aquifers that are responsible for the perennity of the rivers during the rainy recession. In this study, water samples were collected from three sources: rainfall, rivers and pumping wells. The hydrogeochemical data lead to subdivide the aquifers in three main types: regional unconfined, confined, and unconfined with deep water level. The unconfined aquifer with deep water level originates from an groundwater flow deflection in the westernmost portion of the study area. The occurrence of silicified and fractured levels divide the aquifer types and represents a mixing and interfaced zone, allowing a leakage between aquifers and a great variability of hydrogeochemical facies. The hydrogeochemical evolution occurs by local, intermediate and regional flow systems. The buildup of dissolved solids is the major controlling mechanism of the groundwater composition represented by the systematic changes of anion species from HCO 3 to SO 4 to Cl, and cationic exchange between Ca and Na. The discharge occurs through the baseflow of rivers, which have hybrid composition between regional unconfined and confined aquifer from Na-Ca-Cl to Na-Cl. The analysis of stable isotopes shows that the surface water and groundwater are located in the same range of values, which indicates a connection between the reservoirs. During the rainy season, the regional unconfined isotopic composition becomes similar to the precipitation isotopic composition, with the main recharge occurring mostly by direct infiltration of rainwater. However, there is a modification of this composition in the dry season due to strong isotopic enrichment caused by the evaporation process. The seasonal variation in the isotopic composition represents a continuous cycle. In other words, as the rainy season approaches, the atmospheric air column becomes increasingly saturated with water vapor, what results in a considerably diminishment of evaporation.