Chlorinated ethenes characterization using high-resolution rock core analysis in a weathered crystalline rock aquifer in São Paulo, Brazil

Most of the contaminant conceptual models in weathered crystalline bedrock aquifers in Brazil rely on data collected using conventional site investigation methods, at insufficient spatial resolution for associating contaminant distributions with observed heterogeneity. The DFN methodology was adapted for the characterization of a chlorinated solvent contaminated weathered crystalline bedrock aquifer, located in the region of the Jurubatuba district, Sao Paulo city, Brazil. The objective of this research is to establish a detailed 1-D conceptual model to show variable contaminant distribution within the bedrock matrix under different fracture and weathering conditions. The numerous depth-discrete rock sample analyses, assigned to specific geologic features, indicate the presence of PCE-Dense Non-Aqueous Phase Liquid (DNAPL) at discrete-depth intervals along the profile. These zones are mainly related to the lithological contacts and interfaces among the weathered bedrock. The soft and hard weathered-bedrock transition zone does not indicate a sharp decrease in the contaminant mass retention within the bedrock matrix for the studied area. This indicates that contaminant mass storage in the bedrock remains beyond this drilling method-defined interface, which can potentially sustain dissolved VOC concentrations over time and maintain the continuous vertical mass flux into deeper parts of the aquifer. There is a demonstrated need for an appropriate characterization methodology including high-resolution soil coring and sampling to delineate contaminant distribution over crystalline weathered bedrock aquifers. This is a key assumption for the definition of appropriate remedial and groundwater management strategies.