Theme: 09. Vapor Intrusion

Topic Area: 9b. Vapor Intrusion Mitigation and Effectiveness

Formatted Title:

Mitigation of Vapor Intrusion through Dynamic Control of Subslab Depressurization

Background/Objectives:

The contamination of sites by volatile organic compounds can lead to a number of problems, including imminent or chronic risks to human health. Due to the volatilization processes of these substances present in soil and groundwater, they are partitioned from the liquid phase (NAPL or dissolved) into the vapour phase and, under the influence of advective flow or diffusion, end up migrating into the atmosphere or inside buildings (USEPA, 2008) . Depends on the concentrations identified, intervention measures must be adopted, either by restricting use or by installing remediation and risk mitigation systems.

Soil and groundwater remediation processes are often time-consuming, expensive and often requires restricted access to contaminated sites, due to the potential for vapor intrusion from volatile contaminants. This, in addition to the cost of remediation, restricting productive or administrative areas to avoid exposing people to the risk also increases the costs of possible relocations, personnel and infrastructure in productive areas. In this context, the subslab depressurization system (SSDS) has been used as a quick and effective alternative to mitigate the risk of vapor intrusion, proving to be a viable strategy for restricting the use of environments and buildings in hazardous conditions. However, it is necessary to maintain control of the negative pressure in the subfloor quickly and dynamically in order to prevent the potential for vapor intrusion and ensure the mitigation of existing risks.

The aim of this study is therefore to present the results obtained through the installation and operation of a SSDS as an alternative to restricting access to rooms during the remediation process of sites contaminated by volatile compounds.

Approach/Activities:

This study used data obtained during the operation of a SSDS to mitigate the risk of vapor intrusion by petroleum hydrocarbons in an indoor air. Four vapor extraction wells (PEV) and three vapor observation wells with an average depth of 0.60 meters were installed. All of them are located around Room 1, as it was not possible to perform any work inside the room. Inside Room 1 there are 02 vapor monitoring wells, 01 installed at a depth of 0.60 cm (PIV-01) and 01 subfloor well (SS-01). The SS-01 well defined as the compliance point for monitoring the system's effectiveness. The 03 POVs were monitored for negative pressure to ensure that the 04 PEVs were working properly. Tests were performed to obtain an adequate vacuum pressure, within the range indicated to mitigate the potential for intrusion (EPA, 2008).

Results/Lessons Learned:

Real-time monitoring of the system's operating parameters via telemetry enabled quick, dynamic and precise adjustments whenever low values of negative pressure were identified in the POV. Dynamic control and the appropriate adjustments were also made in order to mitigate the risk of intrusion (by maintaining the negative pressures defined in the test) without promoting excessive dragging of the contaminant mass and thus increasing the useful life of the activated carbon column for treating gaseous effluents.

When adjustments were made to the rotation of the vacuum pump, the negative pressure was measured manually in the underground well (SS-01) and confirmed that the vacuum ranges defined as ideal for mitigating the potential for intrusion were being maintained. Due to the impossibility of intervention inside Room 1, it was not possible to automate pressure monitoring in SS-01, but the results obtained manually, when compared with the pressures observed in the POVs (interconnected to the system) corroborated the results obtained during the initial tests and validated the efficiency and effectiveness of the system.

Based on the results obtained, it can be concluded that the mitigation of vapor intrusion using dynamic control of subfloor depressurization is a reliable and viable technology as an alternative to restricting the use of environments or buildings at risk of intrusion, allowing greater flexibility in decision-making during the investigation and remediation processes. It is also concluded that it is possible to apply the technology without the need to intervention inside buildings, depending on the specific characteristics of each location, also leading to a reduction in costs and the additional inconvenience generated.

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Patricia Lupi Guimarães