Theme: 10. Characterization, Fate and Transport

Topic Area: 10d. Conceptual Site Models: Improvements in Development and Application

Formatted Title:

Challenges of Conducting an Environmental Site Investigation in Aquifers with Low-Permeability Zones

Background/Objectives:

The site is in a former industrial area located in the city of Jacareí/SP, State of São Paulo, Brazil. Historically, the local aquifer presented concentrations up to 95,000 µg/L of perchloroethylene (PCE) associated with concentrations up to 5,400 µg/L of trichloroethylene (TCE) and 26,000 µg/L of cis-1,2-dichloroethylene (cis-1,2-DCE), in addition to other organochlorine compounds. The average water level is 2.4 mbgl and the presence of clayey soil interspersed with sandy layers results in a low effective porosity (1.1 to 5.9%). Considering the abovementioned COCs and challenging lithological characteristics, a complementary site investigation was performed, making possible detailed planning of the products and dosages to be used in the full-scale remediation step and their horizontal and vertical distribution in the target area.

Approach/Activities:

The monitoring network of the area of interest includes 36 groundwater monitoring wells within three aquifer levels, 14 sub-slab vapor monitoring wells installed in sensitive areas and 04 ambient air monitoring points. Aiming to detail the existing contamination in the warehouse from the perspective of identifying active residual sources (secondary sources) in the saturated and unsaturated zones, 20 boreholes were carried out in a grid of approximately 5x5 meters, using the direct push method with readings of volatile gases with a portable photoionization detector (PID) every 20 centimeters drilled. In this stage, 85 soil samples were collected for chemical analysis of VOCs and 25 undisturbed soil samples for geotechnical tests and analyses of total and effective porosity (macroporosity), vertical hydraulic conductivity, granulometry and fraction of organic carbon (f.o.c) aiming to delineate the sources in the aquifer and estimate low-permeability zones (retention zones).

Results/Lessons Learned:

Once all investigation data were collected and integrated, it was possible to detail the secondary sources existing in the local aquifer, as well as define the aquifer low-permeability zones (retention zones) by correlating the soil samples and the geotechnical analytical results of the undisturbed samples (granulometric/lithological profile, vertical hydraulic conductivity, fraction of organic carbon and effective porosity). Through the soil samples collected in the boreholes, 04 main low-permeability zones were identified. Together, these 04 boreholes represent 98.8% of the mass of PCE detected in the soil. In these locations, the analytical results showed concentrations of PCE above 100 mg/kg, with emphasis on the soil sample that presented concentration of 9,810.2 mg/kg (~81% of the PCE mass). In general, a predominantly clayey soil profile is observed, varying between silty clay and organic clay, with vertical hydraulic conductivity between 10^-6 and 10^-5 cm/s, effective porosity between 1.9% and 2.9% and f.o.c between 2.32% and 9.98%. It can be concluded that the results obtained corroborate the initial hypothesis of the existence of low-permeability zones containing contaminant mass stored in accordance with the characteristics described and identified in the soil samples collected in the 20 boreholes drilled.

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Claudia Pinto