Different degrees of isolation of groundwater as cause of its variability: evidences from isotopic and hydrochemical data and water level seasonal variation in Pantanal of Nhecolândia

Abstract

This study aims to analyze the degree of isolation of natural lakes with unusual values of salinity and pH in the area of Pantanal da Nhecolândia, the largest field of lakes in the tropical zone of the planet. This process is still poorly understood and is essential for understanding the entire environment. The salinity was measured by electrical conductivity and shows great variations. The possibility of important variability of evaporation or precipitation was dismissed because the lakes are in the same area and the region is flat. Therefore, salinity differences may be associated with a present processes, provided by different degrees of isolation between the lakes and the groundwater, or originated from salt deposits formed in the past, under arid climate. To test the first hypothesis, we analyzed the ratios of stable isotopes of oxygen and deuterium, since the isotopic fractionation that occurs during evaporation enriches the liquid phase in 18O and D. The correlation between the ratios and electrical conductivity is exponential, indicating that the salinity increases with the isolation of the lakes from the groundwater and that a significant isolation of the lakes results in salinities ranging from intermediate to high. The study presents data on seasonal variation in water level (July 2008, end of rainy season and October 2008, end of dry season) of 14 lakes. The data show erratic decrease in the water level of fresh water lakes, which was interpreted as evidence of greater exchange of water between these lakes and the groundwater.