Nov2019
Since the beginning of 2019, Ellingson-DTD has completed seven projects that have included installing horizontal soil vapor extraction wells under occupied businesses or residences. A total of 24 sub slab wells have been completed for those seven projects. Several of the wells serve the crucial role of sub slab depressurization along with removing chlorinated solvent vapors from below the buildings.
Vapor intrusion mitigation is a hot topic in the environmental industry. Our understanding of how the vapor phase concentration of contaminants affect indoor air quality continues to evolve. As a result of recent US EPA assessments, many state and local agencies have tightened regulations in this area, particularly at sites impacted by chlorinated solvents. Combined with the relatively strong real estate market across the US and the emphasis on brownfield revitalization, many environmental consultants have found themselves designing vapor intrusion mitigation systems.
Do Horizontal Wells Make Sense for Vapor Intrusion Mitigation?
Of course, for new construction there are many engineering controls that can be incorporated into the building design to prevent vapor intrusion. However, existing buildings require modifications that can be quite the logistical puzzle. If the building is vacant or between tenants many system designs involve extensive concrete drilling, cutting and trenching that may be economically infeasible. In cases of occupied facilities, landlords don’t want to kick out paying tenants, even temporarily, for system construction. Using horizontal directional drilling (HDD) to install soil vapor extraction wells under a building is a minimally intrusive method to achieve vapor intrusion mitigation goals without massive site disruption. However, horizontal wells have some limitations and it’s important for consultants to understand the applicability of method for their specific project.
Is Remediation Also an Objective?
The goal of a traditional sub slab depressurization system (SSD) is to create a negative pressure barrier beneath the slab of the building. This barrier interrupts intrusion of soil vapors into the building’s air space. A small sump tapped into the gravel base below the slab will usually suffice to create a negative pressure barrier. A low CFM fan or “whirly bird” vented to the building roof provides just enough air flow from the sump.
While this type of system does mitigate the intrusion of vapors from the impacted subsurface, it doesn’t address the source of the problem. The vapors still transport volatile contaminants. So, while a low flow SSD might meet minimal indoor air regulations and permit the continued occupation of the building, the site is unlikely to get regulatory closure for the underlying cause of the problem. The burden of the contaminated subsurface will continue to weigh on the responsible party until they address the source of the impacted vapors.
A properly installed horizontal sub slab well(s) add value by providing a negative pressure barrier and removing contaminant mass.
Horizontal Wells Can Serve Multiple Roles
Horizontal SVE wells have a proven track record for rapid remediation of impacted soil vapors at contaminated sites. Long screen lengths maximize the contact between well screen and the affected formation. This accelerates mass removal compared to vertical wells. A horizontal well creates an oblong zone of vacuum influence centered around the screen interval. This influence extends both horizontally and vertically from the well screen.
If the well is positioned correctly, this zone of influence can intersect the gravel base beneath the slab. This allows the well to provide the negative pressure barrier of an SSD while also driving remediation through mass removal. If you’re concerned with both remediation and vapor intrusion mitigation as concurrent objectives, horizontal SVE wells installed using HDD are an excellent solution. If you have a project that might be a good fit for directionally drilled horizontal wells, please contact us to review the project specifics.