Description
Project Site: 90-acre Residential Development Site, Oxnard, California
Client: LFR Inc., Remedial Engineer and Contractor
Purpose: Elimination of Potential Vapor Pathway and Remediation of VOCs
Summary: A 90 acre parcel, historically used as an oil
field waste disposal facility, has been transformed into 300 residential
units and a resource protection area for endangered plants. Volatile
organic compounds (VOCs) including 1,2 dichloroethane (1,2-DCA),
cis-1,2-dichloroethene (cis-1,2-DCE), and vinyl chloride (VC) were
historically discharged with the oil field waste materials and were
present in soil and groundwater at concentrations that posed concerns for
residential vapor intrusion and water quality protection. LFR, remedial
engineer and contractor for the site, implemented remedial measures which
included
1) excavation of three source areas,
2) excavation of the upper saturated zone over the VOC groundwater
plume, and
3) backfilling the source areas and upper saturated zone with a
reactive mixture of DARAMEND and native sand.
In addition to limited pumping and treatment, long term mitigation of
vapor pathway concerns were addressed by placing the DARAMEND-sand mixture
(intended as a "reactive blanket" as termed by Charlie
Robinson, P.E., project manager for LFR), over exposed areas containing
concentrations of dissolved VOCs. VOCs migrating into the backfilled
source areas or the reactive blanket will be degraded via in situ chemical
reduction (ISCR) processes.
In addition, dissolved organic carbon and dissolved iron produced by
the DARAMEND will stimulate the dechlorination of adjacent residual VOCs
in groundwater over the next 5-7 years. A less pervious soil was placed
above the "reactive blanket" to perch groundwater, and create an
additional soil and hydraulic barrier to augment the "reactive
barrier".
This redundant remedial strategy may eliminate the requirement for
installation of vapor barriers on the future residential buildings.
The Challenge: Soil and groundwater in three main areas were affected by numerous VOCs
(Fig. 2). The areas were named the 'left eye', 'right
eye', and 'mouth' as can be visualized on the map. The VOCs present on
site prior to soil excavation comprised chloroethenes, chloroethanes, and
chloromethanes, including up to 1,330 mg/L 1,2-DCA, 215 mg/L cis-1,2-DCE,
5.7 mg/L VC, 6.6 mg/L trichloroethene (TCE), and 15.5 mg/L methylene
chloride (DCM).
Another significant electron acceptor present at the site was sulfate
at concentrations as high as 2,690 mg/L. The site is bordered by a canal
on one side, which connects to the Pacific Ocean. Mitigation of these soil
and groundwater impacts and addressing vapor pathway concerns was required
prior to the construction of residences.
Field Work: Uncontaminated sand above the water table was removed.
DARAMEND was blended into this sand at a rate of 0.8% by mass as this
material was stockpiled (Fig 3). Each area requiring
treatment was dewatered prior to excavation. The source areas were
excavated to three feet below groundwater. The excavated affected soils
were treated on-site using ex-situ soil vapor extraction. Once the
excavation was completed, the DARAMEND- sand mixture was loaded into dump
trucks (Fig. 4) and transported to the excavation where a
bulldozer provided final placement of the material (Figs. 5 &
6). This reactive material was brought up to the natural
groundwater elevation, above which a three-foot clayey soil layer was
placed to collect infiltrating groundwater and re-establish a downward
gradient.
The backfilled 'right eye' is shown in Fig. 7. After backfill
completion, the affected areas will be pumped for approximately two months
to collect two to three pore volumes of potentially affected groundwater
for surface treatment and discharge.
The Result: It is expected that the reactive blanket will remain effective for a
period of at least five years. Numerous groundwater wells will monitor the
groundwater concentrations in and around the treated areas. Vapor
monitoring will be performed following the backfill placement for at least
one year. Should unacceptable vapor concentrations be observed within one
year, house foundations would then require sub-slab depressurization and
protective barriers may be required. It is expected that the redundant
remedial measures, including the "reactive barrier", should minimize the
need for further vapor mitigation.
The Conclusion: A multiple approach treatment process can be used for mitigating VOCs
in soil and groundwater for construction projects. The approach used,
combining excavation, on- site treatment, and in-situ treatment using a
low application of DARAMEND, provided a novel approach for this site.
The use of this approach provided greater value added in terms of
increased marketability of the >$1 million future homes due to the
potential elimination of vapor barrier perceptions compared to long term
pump-and-treat and its costs.
Figure 1: View from the site
Figure 2: Areas requiring DARAMEND treatment
Figure 3: Stockpile of sand/DARAMEND mix
Figure 4: Loading sand/DARAMEND from the stockpile
Figure 5: Backfilling excacation
Figure 6: Backfilling excavation
Figure 7: backfilled 'right eye'
Companies On This Project
ADVENTUS GROUP
