Formatted Title

Dispersion Controls of a Chlorinated Solvent’s Plume at the Border of a Sedimentary Basin in Contact with Crystalline Basement

Background/Objectives

Understanding the distribution of contaminant plumes and their transport control mechanisms is one of the biggest challenges in contaminated site management. This study presents the measures taken during investigation of a contaminated area carried out in south Brazil. The area of interest is a medium-sized manufacturing industry established at the site for almost 50 years, where tetrachloroethylene was used until 2006 as an industrial process, particularly for degreasing parts. The main goal of this paper is to display the results of the combination of different environmental investigation methods as a three-dimensional model of the industrial site. The location exhibits widespread primary contaminant sources over the contact of igneous and sedimentary rocks with distinct topographic levels, resulting in vertical and horizontal hydrogeological variations, which impact the distribution of the contaminant plume in the aquifer units. The exact location of the project is confidential.

Approach/Activities

The project methodology features a regional geological characterization based on field mapping and interpretation of secondary data from well logs of the Brazilian Groundwater Information System (SIAGAS). After this evaluation, an investigation plan was carried out, according to EPA standards, which resulted in 35 soil boreholes, 80 passive soil gas samplings, 11 high-resolution investigation points using the Membrane Interface Probe/ Hydraulic Profiling Tool (MiHpt), and 35 monitoring wells installed at the shallow and lower portion of the aquifer. The investigation also included the collection of soil samples from sampling cores to analyze volatile organic compounds (VOC) and geotechnical characterization, as well as collection of groundwater samples. As a way of combining all the information gathered during the environmental investigation phases and to consolidate a three-dimensional (3-D) representative model of the area, the software Leapfrog (Seequent) was applied to generate the 3-D model.

Results/Lessons Learned

The results suggest that the local geology is associated to the border context of the Paraná Sedimentary Basin, identifying two sedimentary facies in contact with crystalline basement rocks, understood as the lower contact of the Rio Bonito Formation with the Pinheiro Machado Gneiss Complex (CPRM, 2008). The development of two distinct types of residual soil was observed, one over the sedimentary formation and the other over the crystalline basement. From a hydrogeological point of view, four hydrostratigraphical units were identified: i) Unit A, unconfined aquifer associated with the residual soil of the sedimentary formation; ii) Unit B, semi-confined aquifer associated with the siltstone facies of the Rio Bonito Formation; iii) Unit C, semi-confined aquifer associated with the shale facies of the Rio Bonito Formation; and iv) Unit D, unconfined aquifer associated with the residual soil of the crystalline basement. In such manner, each unit exhibits its own unique characteristics of contaminant transport, with distinct rates of hydraulic conductivity, transmissivity, flow speed and porosity. The results obtained from passive soil gas samples indicate a potential impact of chlorinated solvents in the area of interest, which was confirmed by the MiHpt investigation for both saturated and unsaturated zones. Meanwhile, the chemical analysis of soil and groundwater have quantified the amount of contamination by tetrachloroethylene and its degradation products trichloroethylene, 1,2-dichloroethylene, and vinyl chloride in concentrations higher than those established by the Human Health Risk Assessment. The use of distinct techniques in the environmental investigation of this intricate site allowed the understanding of local geology and hydrogeology, as well as the differentiation of  contaminant’s plume transport mechanism in each of the established units, since in the sedimentary units, hydrogeology is the most important factor concerning the plume’s distribution, while in the lower portion of the limit between basement and soil geology plays the biggest part in the geometry of the organochloride plume’s dispersion.