Recharge sources and hydrochemical evolution of an urban karst aquifer, Sete Lagoas, MG, Brazil.

Abstract

The current water supply for the city of Sete Lagoas, Brazil, is almost entirely groundwater from the Sete Lagoas Karst Aquifer, with a small contribution from a fractured-basement rock aquifer. Characterizing the hydrogeologic processes is important for proper water resource management, avoiding contamination and other future issues. The main goal of this study is to identify active hydrogeological processes, such as possible surface– ground water interactions, sources of recharge, and the hydrochemical evolution of groundwater through the use of stable isotopes 18O and 2H and major ion data. The groundwater samples were collected from the karst aquifer (central urban area) and from the fractured aquifer (southern part of the city). Surface water samples were collected from the seven main lakes, and rainwater was sampled over a period of one year. The stable isotope results indicated the groundwater origin is directly from local precipitation, having a limited recharge period, and locally receiving surface water contributions. Groundwater quality in the central urban area may be influenced by surface water infiltration where the karst aquifer is in contact with overlying unconsolidated sediments. Some samples with nitrate concentration of 10 mg/L or higher indicates the natural composition of the groundwater has been altered by urban sewage contamination. The highest concentrations of major ions were found in the central region, where the most karstified area from the Sete Lagoas Formation is located. These data suggest a longer residence time for this water, indicating more mineralization, in accordance with the geological information.