Theme: 04. Addressing Challenging Site Conditions

Topic Area: 4b. Landfill Assessment and Remediation

Formatted Title:

Implementation of an Inclined Soil Vapor Extraction System (SVE) under Residential Buildings and Air Flow Modeling in the Subsoil

Background/Objectives:

From 1970s to 1990s, waste from industrial activities was deposited on a site located in the metropolitan region of the city of São Paulo, in the state of São Paulo, Brazil. In the mid-1990s the site was landfilled with approximately 1.0 m of clean soil, and a residential condominium was designed for the construction of 72 8-storey buildings, each with four apartments. By the early 2000s, 62 buildings had been built and 53 were already inhabited, totaling more than 5,000 residents.

In the 2000s, an accident in one of the condominium's underground water tank triggered CETESB envolviment, the São Paulo state environmental agency, which detected the presence of methane gas (CH₄) inside. This event demanded a series of investigations into different matrices (soil, groundwater, soil vapor), leading to the entire condominium to be classified as a critical contaminated area in the state of São Paulo. As a result, engineering measures had to be implemented aiming risk mitigation associated with the soil vapor intrusion into the buildings.

The results of soil vapors investigations identified an extensive area of methane gas (CH₄), consistent with the area where waste is deposited in the landfill, in addition to occurrences of aromatic and organochlorine compounds in the northern and central regions of the area with concentrations above the reference values adopted from the U.S. Environmental Agency (USEPA).

Approach/Activities:

In view of these results, an SVE system with 120 extraction wells was installed throughout the open areas and around the buildings located over the methane (CH₄) plume. In four of these buildings, especially those located under the areas where VOCs occur in the soil vapors, EBP BRASIL opted to install inclined extraction wells (SVEI) under each apartment, to reduce the potential risks associated with soil vapor intrusion. At the same time, inclined monitoring wells (PMGI) were installed to evaluate the efficiency of the extraction and monitor the reduction in methane (CH₄) and VOC levels under these apartments.

After one (1) year of system operating in the open areas, there was a reduction in methane levels to zero (0%) in practically the entire length of the condominium, remaining only punctual occurrences. Under the buildings, the implementation of the Inclined SVE indicated a reduction in VOC levels of up to two orders of magnitude, reaching levels below the USEPA reference, plus the reduction in methane (CH₄) levels to zero (0%).

In order to verify the distribution of vacuum under the buildings with inclined SVE, a three-dimensional modeling of air flow in the soil was performed using Bentley® SOILVISION and FlexPDE® software. The results indicated the continuous presence of a vacuum under the entire length of the residential buildings, with minimum values between -0.5 and -2.4 mbar starting from the base of the capping soil (clean soil) and increasing in depth.

Results/Lessons Learned:

The installation of the Inclined SVE system proved to be an innovative and efficient solution, specifically because these are residential buildings with restricted forms of operation, contributing to a significant improvement in the environmental scenario, reducing methane gas (CH₄) levels to zero (0%) and VOCs in the soil vapor to levels below the USEPA reference. The continuous presence of a vacuum under the entire length of the buildings, proven by the results of the three-dimensional soil air flow model, indicated a strong line of evidence as to the absence of soil vapor intrusion through the first-floor apartments, supporting the decision-making process regarding future intervention and/or institutional measures.

Rafael Franklin

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(Group 1, Poster Board #76) Implementation of an Inclined Soil Vapor Extraction System (SVE) under Residential Buildings and Air Flow Modeling in the Subsoil