Formatted Title
Challenges Implementing In Situ Solidification/Stabilization of DNAPL (Chlorinated/Brominated VOCs) at a Park Development Site
Background/Objectives
A former chemical manufacturing site in California, where EDB was manufactured from 1929 to 1968, contains COCs (including EDB and 1,2-DCA) in soil and groundwater which had migrated to a deeper aquifer and had to be remediated. Impacts extended across ~1 acre and to a depth of ~30 feet. The final remedy included excavation and in situ solidification/stabilization (ISS) with 6- and 10-foot-diameter augers. The portion of the site requiring ISS will be developed into a park. Other portions of the site with differing chemical impacts were defined as separate management areas, allowing for remedy completion and regulatory approval in time to meet redevelopment schedules.
Approach/Activities
The project implementation team faced many challenges and constraints including drilling refusal due to soil conditions, worker safety, access limitations, and community protection measures and monitoring. Drilling depths in stiff clay combined with overhead active electrical line limitations necessitated a varied approach. Worker safety was a challenge due to the volatile nature of the COCs and restrictive PPE. The proximity of a housing development required strict fenceline air emissions limits and perimeter monitoring. The window to complete the project was also restricted by seasonal fluctuations in the groundwater level and potential for stormwater ponding.
Results/Lessons Learned
The drill rig utilized a custom configuration to reach full remedial depths with sufficient torque, maintain required clearances below active power lines, and allow for a source vapor collection shroud. The platform elevation for the drill rig was frequently adjusted based on height of overhead power lines, required treatment depths, groundwater level, and maintaining a stable surface for the drill rig (crane mats and solidified slurry were also used to fortify rig stability). The short timeframe to complete the work necessitated strategic sequencing to manage multiple crews and large equipment in a small area. Worker safety required planning and hazard analysis due to physical hazards and volatilization of EDB during ISS. Mitigation measures included personnel in Level-B supplied-air PPE and chemical protective coveralls, clearly defined communication protocols, and detailed mobile construction equipment procedures. Heat stress risk required a heat illness prevention program that included a supervised rotation schedule, physiological monitoring, an apparatus designed to chill supplied breathing air, and a shaded rest area with a large evaporative cooler.
The remedial work posed an airborne health risk to nearby residents if emissions were not controlled, and as such, a robust perimeter air sampling and monitoring program was implemented. Air samples were collected to assess the overall effectiveness of VOC management techniques and verify fenceline emissions did not exceed acceptable levels. Real-time monitoring (including PIDs with telemetry-supported data management, and a portable GC/MS device) was performed to guide proactive decisions on work modifications and increasing VOC mitigation measures. These included a vapor collection shroud to filter air through multiple treatment medias, surface application of vapor suppression foam and cellulose fiber mulch, a work zone perimeter misting system with odor neutralizer, and work adjustments based on wind velocity. A social risk assessment of the surrounding communities was performed and used to create a stakeholder engagement plan. This included producing informative flyers, setting up a public facing project website, and posting informative signage with contact info. The project was completed under budget, on schedule, with no public complaints, and no recordable safety incidents.