Formatted Title

Horizontal Electrode Installation for Thermal Remediation Operations while Minimizing Business Impacts: Installation Strategies and Implementation

Background/Objectives

Industrial operations took place at the subject property for 34 years until the business closed in 1999. Chlorinated volatile organic compounds (CVOCs), namely perchloroethene (PCE), were released as part of component degreasing during the facilities operation. 1,4-Dioxane, which was used as a stabilizing agent in the degreasing tank, was also released with the PCE. Since the closure of the industrial facility, the property has been redeveloped and now various retail and commercial units have been established on this valuable real estate located in Santa Cruz, California. A CVOC plume was identified in the 1990s and various investigations and remedial actions have taken place in various portions of the plume. Previous remediation activities have included soil gas extraction and many subsurface injections including in situ oxidation and bioremediation enhancements. Despite these efforts, 1,4-dioxane and PCE contamination in groundwater remained at concentrations indicative of a dense, non-aqueous phase liquid (DNAPL).

Approach/Activities

Trinity Source Group, Inc. (TSG) performed additional investigations as recently as 2018 to further define remaining 1,4-dioxane and CVOCs onsite and to ascertain why other remedial techniques were unsuccessful. Following the investigations, in situ thermal remediation (ISTR) was considered to ensure remediation goals would be achieved regardless of the subsurface geologic complexities existing onsite.  The presence of 1,4-dioxane further encouraged the use of ISTR as the selected remedial technology due to enhanced reductive dechlorination (ERD) inability to treat 1,4-dioxane (soil excavation was not feasible due to the overlying building and the cost for building demolition and re-building). Subsequently, electrical resistance heating (ERH), coupled with soil vapor extraction and treatment (SVET), was the ISTR technique ultimately selected, and was implemented by TRS Group Inc. (TRS). The treatment area covered about 32,500 ft², with about 60% below active business’ and the remaining 40% below an active parking lot.  An innovative approach was required to minimize impacts on the current tenants during the ERH installation and operations. A combination of 31 horizontal and 80 vertical electrodes which were completed below grade were designed to treat the 10,700 yd³ source zone treatment volume.

Results/Lessons Learned

The ERH system was installed and operated with minimal impact to the facilities’ business operations. Our presentation will cover the major challenges and the solutions developed for them including:

• Horizontal drilling was successfully used to install single entry point electrodes up to 200 ft long.
• Drilling fluid recovery generated during horizontal electrode installation was recycled using a driller-provided “shaker” to minimize fluid disposal transport and disposal.
• Energy was delivered to the soil through the horizontal electrodes at a rate that ensured efficient remediation operations.
• The electrodes were successfully phased to ensure even heating between the horizontal electrodes.
• SVET was performed during ERH using dual-purpose horizontal electrodes and vapor recovery wells, shallow vapor extraction piping, and sub-slab vapor recovery trenches.
• ERH horizontal electrode dry-out was mitigated with an innovative approach of introducing water to the dual-purpose horizontal electrodes/vapor recovery wells.
• A maximum underground temperature of 103°C and an average maximum temperature of 93°C was achieved within the horizontal electrode zone.
• A maximum underground temperature of 103°C and an average maximum temperature of 89°C was achieved within the vertical electrodes.
• Indoor air was sampled to ensure tenants would not be impacted by the remedial effort.
• Tenant and worker safety were maintained throughout installation and operational activities.
• Grant funding collapse resulting in abrupt ERH system shutdown and removal prior to reaching the estimated energy application target.
• SVET operation was maintained after ERH shutdown for building tenant safety. 

Post ERH system shut down challenges:  7 to 8 months of continued sub-slab vapor extraction trench SVET to remove heat and maintain vacuum influence for vapor intrusion safety; ongoing SVET system O&M and permit compliance costs; post thermal shutdown soil, groundwater, soil vapor, and indoor air sampling to confirm cleanup level achievement and ensure commercial tenants are safe to occupy building post thermal treatment.