Application of the Discrete Fracture Network Approach for defining the conceptual model of the fractured aquifer in the Jurubatuba region, São Paulo, Brazil

Abstract

The Jurubatuba Channel region, Sao Paulo-SP, presents a complex environmental context. Since the 1950s, this area has been mainly of industrial use, with intense groundwater exploitation through deep wells; in the last few years, the soil use has been gradually changing to commercial and residential. Many of the industries installed back then have used organochlorine solvents in their processes; the incorrect handling of such substances has induced water and soil contamination in different areas. Thus, contaminants were detected at dozens of supply wells registered at DAEE, and these were interdicted during the past decade by the government, when a pumping restriction area was established in the region. The association of the possible free dense non-aqueous phase liquid in the contaminated areas with strong vertical hydraulic potentials, caused by the intense pumping, has enabled the transport of these substances to depths of dozens of meters into the fractured aquifer. Nonetheless, the investigation on the intensity and extent of the impacts on this aquifer unit was not yet started by the environmental agencies, as there is no understanding regarding the responsibilities to the contamination at such depths, given it may have been caused by the superposition of plumes from several source areas. Due to this complex situation, CETESB has classified the Jurubatuba region as one of the few critical contaminated areas in Sao Paulo State, creating a demand for special management procedures, as well as strategies of intervention, risk communication and information management. Given the complexity of studies in fractured aquifers and its scarce knowledge in the Jurubatuba region, the present Ph.D. project aims to establish a conceptual model of the contamination in the area of interest, applying the Discrete Fracture Network (DFN) method. The studies shall be carried through a new borehole in the bedrock at a strategic place, which will allow the installation of a multilevel system for groundwater monitoring as well as the execution hydraulic tests in the fractured aquifer.The first step to characterize the fractured aquifer according to the DFN method is the geological description of rock cores. Secondly, geophysical loggings are performed: caliper, natural gamma, optical and acoustic televiewers, and heatpulse flowmeter, after which the borehole is temporarily sealed with FLUTe blank liner, in order to avoid cross-contamination. Once the conditions in the sealed borehole are stabilized, temperature loggings are performed without removing the seal, contributing with the identification of brittle structures responsible for groundwater flow in the borehole. The information collected so far is used to the determination of the interest intervals for the hydraulic tests using straddle packers, to calculate the hydraulic properties of the analyzed intervals. The evaluation of the data acquired through the several fieldwork phases corroborates the elaboration of a hydrogeological conceptual model of the studied fractured aquifer. Thus, the specific objectives of the present Ph.D. project are: the geological characterization of the local aquifers, including their brittle structures; the definition of hydraulic connections between the shallow and deep aquifers; the identification of the roles the supply wells have in the hydraulic communication between the aquifer units; the identification of the role of open fractures in the hydraulic communication between the aquifer units. (AU)