Vulnerabilidade à intrusão salina na água subterrânea da Bacia do Rio Tibiri, São Luís – MA

Abstract

During the 1970s and 1980s, the city of São Luís began, rather late, a process of industrialization, marked by the birth of the Industrial District in 1980. This was followed by a rapid increase in population density due to intense migration to the capital, resulting in a noticeable change in the demographics inside the Tibiri River basin and its neighboring areas. As a direct consequence of this, we have seen in the last few decades a significant deterioration of groundwater resources due to the occupation of recharging areas of aquifers, as well as the increased pressure on these resources by the city of São Luís. This indicates the city’s need for careful planning and strategizing, in order to protect its aquifers. The aquifers of insular environments become even more prone to saline intrusion, which can occur horizontally, through water that trickles in from the sea, or also vertically, due to the influence of mangroves and estuaries. The presence of mangrove vegetation indicates that saline waters and an estuarine environment are present in all coastal basins of the city of São Luís, such as Tibiri, Anil, Paciência, Bacanga, Rio dos Cachorros, Santo Antônio, Igaraú and more. In the Tibiri River basin, groundwater is used for different purposes in rural communities, for public and private supply in residential areas, and to meet the demand of businesses located in the Industrial District. Given the importance of this watershed, and of the saline wedge problem, the GALDIT method proposed by Chachadi and Lobo Ferreira (2001) is one of the tools that can be used to investigate the vulnerability of the aquifers to saline intrusion, splitting up the area of focus into high vulnerability (>7.5), medium (5 -7.5) or low (<5) classes. As such, this research study will thus apply the Aquifer Vulnerability Chart to Saline Intrusion to the areas of the Tibiri River Basin. This analysis identified the vulnerability classes inside the aforementioned regions, which could then be applied as a decision-making instrument regarding groundwater management in the city of São Luís. The results achieved show a high vulnerability only to the rural area of Mato Grosso, on the left bank of the river, which is heightened due to its location near the mouth of the Tibiri River, where the flow of the tide begins. The rural region of Quebra Pote, Santa Helena, and Itapera do Quebra Pote, located on the right bank of the river, on the lower part of the watershed, was classified as a region of moderate vulnerability. Likewise, the Tibirizinho and Tajaçuaba regions, in the upper part of the basin, also indicated moderate vulnerability. On the left bank of the river, the Andiroba extension to the São Raimundo neighborhood showed low vulnerability to saline intrusion. On the right bank, the communities of Cinturão Verde and Tinaí also showed low vulnerability, despite their proximity to the mangrove area. In addition to the natural processes of estuarine environments, the characteristics of land use in the basin involving residential, industrial, and rural uses, also strain the groundwater resources. The high and moderate vulnerability classes identified in the basin attests the occurrence of the saline wedge phenomenon inside the Tibiri River basin, which, despite being mostly rural, has an insular condition. This research study demonstrates the importance of using knowledge of hydrodynamics data of pipe wells and qualitative groundwater data analysis for the purposes of monitoring the saline wedge behavior and subsidizing measures that will mitigate the salinization of groundwater in the basin.