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Nov 2019 – Sub Slab Horizontal Wells

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Slab Horizontal Wells – Vapor Mitigation and Mass Removal

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



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

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 his 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

A properly installed horizontal sub slab well(s) add
value by providing a negative pressure barrier and
removing contaminant mass.

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.