Análise de parâmetro para mapeamento de vulnerabilidade de contaminação das águas subterrâneas do aquífero Guarani nos municípios de Portão e Estância Velha/RS

Abstract

The vulnerability mapping and risk analysis of groundwater contamination are essential to guide municipal plans of environmental management. Nowadays, new criteria to classify the groundwater quality are required by the CONAMA Resolution Nº 396/2008 which justify this work. The cities chosen for this study were Portão and Estância Velha/RS due to industrial activities like leather-footwear, plastics, paints and solvents, and domestic effluent in significant volume. The DRASTIC method was chosen to build the vulnerability map, which is a acronyms of the parameters to be analyzed: Depth water table, Recharge, Aquifer media, Soil type, Topography, Impact of vadose zone, Hydraulic Conductivity. Each parameter was built dynamically as an information layer in the GIS from characteristics of the environment that were spacialized and, in the final stage, integrated to generate vulnerability classes as low, intermediate, high and very high class. A preliminary risk analysis was done by overlapping of this map with the map of landuse. The Guarani Aquifer System (GAS) in the study area is composed by the sandstones from the Pirambóia and Botucatu Formations. The SAG is partially confined by the Serra Geral Aquifer when the recharge is favored by the densely fractured basalt. The shallow groundwater levels and sedimentary lithology are decisive factors for an intermediate to high vulnerability. However, due to human action, groundwater levels are depleted in urban areas. The soils were divided in domains according to their texture. The domain claysoil 1 shows thick layers with high content of clays in the B horizon, indicating an impermeable behavior. The infiltration tests and the thick layers of clay confirm the low hydraulic conductivity of this pedological unit, providing natural protection of the aquifer. However, in the study area dominates the claysoil 2, which is more sandy and whose low levels of expansive clay minerals in the surface decrease the vulnerability. The study of risk is directly related to the occupation of the area, which was classified as urban, rural, forestry or native forest. These classes occupy areas of high vulnerability to contamination of the aquifer. As the quality of groundwater, these cities have limited data available, not showing the history of environmental degradation in the area. This is an alert to the need of installation of properly distributed monitoring wells. The efficiency of the map is linked to the interdependence of parameters involving hydrogeological and geomorphological aspects, having been obtained a high degree of vulnerability to groundwater contamination in 25% of the area. An intermediate vulnerability was found in 72,4% of the area and a low degree in 2,6% of the area. In the recommendations, the qualitative and quantitative control of the groundwater in the east area was included because it constitutes high vulnerability with over urban use and industrial activities.