apartment sites which
included two, 93-year-
located in Minneapolis, Minn. As required
by the buyer and the
lender, a Phase I ESA
was performed at the
site resulting in the
identification of RECs.
The RECs included
historical use of under-
ground storage tanks
containing fuel oil for
heating at the site as
well as the potential for
impacts from up-gradient area-wide sources including dry cleaning and
the associated solvent use, which led to a Limited Site Investigation
During the LSI, Terracon collected soil gas samples from the build-
ing footprints outside and from beneath the building slabs to evaluate
whether impacts were present in soil vapor and if an exposure potential
existed from vapor intrusion. The identified concentrations of vola-
tile organic compounds (VOCs) in soil gas exceeded regulatory risk
thresholds, thus requiring immediate action to prevent exposure to the
included: lack of
footing design and
layout, variability in
floor slab condition
and thicknesses, lim-
ited access to tenant
abatement of asbestos
presence of lead-
based paint, and the
ongoing use of the
site as an apartment complex.
With these challenges in mind, a Response Action Plan (RAP) was prepared for approval by the regulatory agency, which included an initial
design and a phased pilot testing plan for a sub-slab depressurization
(mitigation) system. A phased approach was necessary given the buildings’ ongoing occupancy and variable sub-surface conditions.
Pilot testing allowed for an assessment of soil conditions beneath the
slabs and identification of the radius of influence of vapor removal to
identify areas where vapor collection sumps could be installed. The
pilot test data was used to begin a mitigation system design that considered the site limitations noted above.
Our phased approach to the pilot testing revealed the need for additional suction points in occupied spaces to effectively cover the entire
buildings’ footprints. The final mitigation system design included 11
suction points in the lower level of each building connected through
a network of piping leading to exhaust fans installed in each of the
buildings’ unused coal rooms. Vented exhaust was discharged above
the third story roofline.
Post-mitigation diagnostic testing and sampling confirmed effective
sub-slab air flow, decreased concentrations of VOCs beneath the building slabs, and indoor air concentrations meeting regulatory standards.
Based upon these results, the site was issued a No Further Action
(NFA) letter from the Minnesota Pollution Control Agency. The system
design and performance allowed our client to meet post-closing obligations as well as provide a protective environment for the occupants.
Jason Gelling is a senior geologist in
Terracon’s Minneapolis office. He has
20+ years of environmental consulting
experience related to environmental
assessment, tanks, spills, investiga-
tions of petroleum and non-petroleum
impacted sites, and management of
hazardous materials in various regions
of the U.S. For more information on
due diligence or vapor intrusion you
Jason at email@example.com
Photo showing subject Site.
Photo showing active system primary blower,
auxiliary room fan, and condensate drain.
Active system collection sump with valve, riser pipe, manometer, and
vent line. Note area of abated asbestos floor tile around the sump pen-