Story

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'