Survey of geomorphological and hydrogeological data for mapping groundwater vulnerability of the Guarani Aquifer in Portão and Estância Velha/RS using the DRASTIC method

Abstract

The vulnerability mapping of groundwater contamination has been widely developed and included in municipal plans to guide environmental management policies and it is recommended in CONAMA 396 Resolution (Brasil, 2008). Portão and Estância Velha, RS regions present potencial risk of contamination due to industrial activities. The Guarani Aquifer System (SAG) in the study area is composed by the Pirambóia and Botucatu Formations. Both formations are formed by sandstones and they are partially confined by the basalts of the Serra Geral Aquifer. The method DRASTIC was chosen to build the vulnerability map. The shallow water table and the sedimentary lithology are indicative factors of intermediate to high vulnerability. The soils were divided in domains according to their texture. The first domain of claysoil has an impermeable behavior. The infiltration tests and thick layers of clay confirm a low hydraulic conductivity of this pedological unit, providing a natural protection of the aquifer. However, in the study area, the second claysoil domain is dominant and has more diffuse sand in the profile. The restricted presence of expansive clay minerals increases the vulnerability. Occupancy of areas of high vulnerability is directly related to the contamination of the aquifer. The efficiency of the map is related to the interdependence of parameters involving hydrogeological and geomorphological aspects. Thus, it has been observed a high degree of vulnerability to groundwater contamination in 25% of the area, an intermediate vulnerability in 72.4%, and a low degree in only 2.6% of the area.