Avaliação da vulnerabilidade e perigo à contaminação do Aquífero São Sebastião / Marizal, Recôncavo baiano

Abstract

The Marizal/São Sebastião aquifer system is the main water supply source in the municipality of Alagoinhas and surrounding regions (state of Bahia). However, the human interventions in the region contribute to soil and underground pollution. Thus, the present study had the objective of investigating the vulnerability and danger of contamination in this aquifer system, associated with a study on water quality based on statistical tools. In bibliographic studies, the main methods for investigating aquifer vulnerability in different geological environments were analyzed, selecting GOD, DRASTIC, AVI and COP methods as the most relevant today. In view of conditional comparisons, it is concluded that the GOD and DRASTIC methods presented more appropriate responses for sedimentary environments, such as the study area, and were therefore selected for the present study. To carry out this work, 20 groundwater samples were taken in situ in the year 2021 and 34 samples from wells in the SIAGAS (Groundwater Information System) and SAAE (Alagoinhas Autonomous Water and Sewage System) databases were considered. Data was assessed using factorial analysis, multivariate cluster analysis, and evaluations on vulnerability (GOD and DRASTIC) and risk of contamination (POSH). The results indicated that the characteristics that compose the Marizal/São Sebastião aquifer system are the main factors responsible for the vulnerability indices found in the area. The regions that are preferentially composed by the Marizal Formation (central region of the area) were classified with intermediate to high vulnerability in response to the sandstones that form this geological Formation. In turn, the regions delimited by the São Sebastião Formation presented intermediate to low vulnerability (peripheral regions), which is in response to the presence of interleaved sandstones and shales that promoted greater protection to the underground system. The risk of contamination followed the same pattern, with the urban zone presenting the highest contamination hazard and the rural zone had the lowest hazard level. Moreover, the statistical results indicated that groundwater characteristics presented higher influence over the geological formations than human contamination or pollution, indicating that despite the high level of vulnerability and contamination hazard of some areas in the municipality, the aquifer system in general can still be considered preserved. The most relevant parameters to characterize groundwater were those related to water salinity (pH, EC, salinity, calcium, chloride, bicarbonate, sulfate, phosphate, manganese, and iron), and indicated that 70% of the groundwater is within the drinking water standards established by the Ministry of Health Ordinance No. 888/2021 and CONAMA Resolution 396/2008. The secondary relevant parameters were those related to the alkalinity/acidity of the aqueous medium (pH, bicarbonate, and phosphate) and indicated that most of the groundwater in this system is acidic (95%).