RESUMO
Overexploitation of groundwater in urban karst aquifers can lead to negative consequences such as land subsidence, depletion of springs and lakes, and water pollution. It can also have indirect effects such as environmental, socio-economic, and political instability. In the municipality of Sete Lagoas, Brazil, the long-term effects of extensive groundwater extraction have been observed and studied over the years. This paper analyzes the response of the karst aquifer to urban, industrial, and climatological changes that may have contributed to changes in the aquifer over the last four decades. The results show that groundwater extraction has exceeded the average aquifer recharge since the year 2000. From the 1980s, the number of wells has steadily increased due to unplanned urban development, resulting in higher demand for groundwater. In the 2010s, pumping from tubular wells (7.39 × 107 m3/yr) exceeded the maximum recharge capacity of the aquifer (7.33 × 107 m3/yr). These wells are mainly concentrated in two areas: the central urban zone and the industrial sector. As a result, kilometer-long cones of depression have formed, changing the aquifer from confined to unconfined within these regions. According to the census data, about 67% of the wells remain unidentified. The average annual recharge to the aquifer is estimated to be 5.68 × 107 m3/yr, which accounts for 12% of the average annual rainfall in the region. Climatological trends indicate an incipient decrease in precipitation and an increase in temperature, suggesting a potential decrease in future aquifer recharge. In addition, only 17% of the area has high infiltration rates ranging from 35% to 75%. The current situation in Sete Lagoas is one of overexploitation of groundwater resources, which could be mitigated by localized reduction of groundwater consumption and implementation of effective management strategies to increase aquifer recharge.
RESUMO
Karst environments have an inherent complexity that interferes with their hydrogeology comprehension. Hence, isotope hydrology can be a valuable tool to assess trajectory of subsurface flows in an unexplored setting. The study area is located in the Lagoa Santa Karst, an environmental protection area of great economic, cultural and ecological importance, where Neoproterozoic metalimestones accommodate karst-fractured aquifers, characterized by complex water dynamics, essential vulnerability and high productivity. The purpose of this study was to investigate groundwater flow origins of springs using principally environmental stable isotopes 2H and 18O. Rainwater and spring water were sampled and analysed. The LMWL presents angular and linear coefficients strongly similar to those of the GMWL. Spring isotopic signatures, which represent the base flow and present wide-ranging of 2H and 18O, were separated into two groups. The first group can be associated with recent rainwater major contributions, while the second group shows significant evaporated water contributions, largely represented by resurgences. Tritium concentration and physico-chemical parameter data supported this interpretation, pointing that waters of the second group remained more time on the surface and subsurface. Therefore, using isotope tracers to evaluate upper groundwater zone in this tropical karst system is a powerful instrument for water resources management.