A importância das depressões cársticas para a gestão das águas subterrâneas na bacia do rio sobrado, estado do Tocantins

Abstract

Having accurate knowledge of exploitable reserves is essential for groundwater management and understanding its regulatory mechanisms. Karst aquifers, in turn, exhibit heterogeneity and anisotropy that prevent the application of traditional methods for calculating recharge. For karst aquifer management to be effective, it is necessary to know when and where recharge occurs, as land use can impact recharge pathways and thus influence the quality and quantity of groundwater in the aquifer. In addition, in karst, the dissolution process creates a landscape characterized by surface depressions, which become important sources of localized recharge, such as caves, sinkholes, and swallow holes, which must be considered in recharge estimates. This study proposes an integrated approach to calculate recharge rates, combining indirect methods (water balance in karst depressions, separation of base flow, and the APLIS method) with validation using the direct Chloride Mass Balance (CMB) method. The study area is the Sobrado River basin, in the southeast of the state of Tocantins, part of the Bambuí Aquifer System (BAS), characterized by carbonate rocks and karst features such as sinkholes. To map the recharge zones, global Digital Elevation Models (DEMs) were used: AW3D30, NASA DEM, Copernicus GLO-30, and a hybrid DEM derived from the three DEMs. The depressions were delimited using a DEM generated by Sentinel-1 image interferometry (Res. 15 m), identifying 299 depressions, of which 270 (803.42 ha) are of karst origin. The calculation of localized recharge was based on the water balance of these depressions, with actual evapotranspiration estimated using the SAFER method, resulting in an average annual rate of 36.89% of precipitation. At the same time, the separation of the base flow through the analysis of hydrographs with digital filters and an equation derived from the Q90/Q50 ratio (flow-duration values) provided a rate of 39.12%, confirming the correlation between hydrological parameters and recharge. The APLIS method, applied as a complement, allowed for the spatial evaluation of infiltration based on factors such as altitude, slope, lithology, infiltration, and soil, but with limitations due to the subjectivity in the weighting of parameters. The original APLIS presented an average annual recharge rate of 31.44%, similar to the compared methods. The consistency of the results was confirmed by CMB, which indicated 34.55% recharge. It was concluded that the integration of indirect methods, water balance in depressions, and baseflow separation can be used effectively to assess and estimate the recharge rate in karst regions. In addition, the fusion of DEMs from multiple sources proved effective in detecting features that could go unnoticed. This work is the first integrated application of these methods in the Bambuí Aquifer System in southeastern Tocantins, contributing to a replicable methodology for regions lacking hydrogeological data and highlighting the need for an integration of methods to overcome uncertainties inherent to the complexity of karst. The results reinforce the importance of karst depressions as sources of recharge and preservation of groundwater, serving as a water management tool in vulnerable areas, such as the Sobrado watershed.