Avaliação da vulnerabilidade à intrusão salina pelo método Galdit no aquífero costeiro da cidade de Raposa – Maranhão

Abstract

Groundwater is an important source of public water supply on São Luís Island and is generally considered safer because it is stored underground and usually does not require prior treatment. The municipality of Raposa, located in the northern portion of Maranhão State, is part of São Luís Island and is predominantly supplied by tubular wells. Because it is situated in an insular area, the city is directly influenced by tidal variations, which may affect the dynamics and quality of its groundwater. Tidal dynamics, associated with the advance of soil impermeabilization, which reduces aquifer recharge, and the increase in groundwater extraction from tubular wells, contribute to the reduction of hydraulic pressure exerted by freshwater in relation to saltwater. This pressure reduction favors the penetration of the saline wedge toward the continent. This movement is known as saltwater intrusion and is considered a natural phenomenon; however, it can be intensified by anthropogenic activities. The objective of this research is to identify and evaluate areas vulnerable to saltwater intrusion in the municipality of Raposa (MA). For this purpose, the GALDIT method, developed by Chachadi and Ferreira (2001), was applied. This method is designed to map and assess the vulnerability of coastal aquifers to seawater intrusion, especially considering the intense groundwater exploitation for water supply and other anthropogenic activities. The municipality of Raposa presents concerning gaps in hydrodynamic data, which compromises the management of tubular wells, since out of the 37 existing wells, only 25 provided usable information for this research. The application of the GALDIT index identified areas with moderate to high vulnerability, particularly in regions with higher population concentration and located close to the coastline or under tidal influence. The parameters that exerted the greatest influence on vulnerability were parameter A (hydraulic conductivity), parameter L (piezometric level), and parameter D (distance from the coastline), due to the higher weights assigned to these factors. Given these findings, it is concluded that the municipality presents weaknesses in hydrodynamic data related to tubular wells, which are essential for understanding groundwater behavior. The absence of this information mainly compromises the quality and management of the municipality’s water resources. The GALDIT index allows the identification of areas most sensitive to saltwater intrusion, which require greater attention from public authorities. In this context, the adoption of mitigation measures is recommended, including updating hydrodynamic and hydrogeological data, conducting physicochemical analyses of groundwater, rationalizing pumping from tubular wells, and encouraging sustainable use of groundwater resources