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EPA Technology News & Trends - Tacoma “Well 12A” Superfund Site

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Courtesy of TRS Group, Inc.

The U.S. EPA Region 10 office uses an adaptive site management approach that relies on high-resolution site characterization (HRSC) techniques for strategic sampling at the Well 12A project area. This approximate one-square-mile project area is one of three at the 2.5-square-mile Commencement Bay-South Tacoma Channel Superfund site in Tacoma, Washington. The vicinity of Well 12A, one of 13 wells used by the City of Tacoma to meet peak summer and emergency water demands, contains high concentrations of chlorinated volatile organic compounds (VOCs) including light and dense non-aqueous phase liquids (NAPL). Use of HRSC at Well 12A is critical to evaluating the mass discharge (flux) of contaminants associated with implementation of several treatment technologies and to determining the point at which active treatment may transition to monitored natural attenuation (MNA). The Well 12A record of decision (ROD) as amended in 2009 is the first EPA ROD specifying a mass discharge reduction as an interim goal for a remediation performance metric.

Well12A contamination is associated with past waste oil recycling operations, which resulted in release of solvent-related NAPL from drum and tank storage areas and disposal of filter cake containing NAPL. Trichloroethene (TCE) and its degradation products are the primary contaminants of concern (COCs). The well was taken out of operation when it was found to be contaminated but has operated since 1983 through use of five air-stripping systems. After the source area was identified, approximately 1,200 cubic yards of filter cake mixed with contaminated soil were excavated along a rail line and an additional 5,000 cubic yards of filter cake were removed during construction of a soil vapor extraction (SVE) system. Between 1993 and 1997, the SVE system removed an estimated 54,100 pounds of VOCs. A carbon treatment-based pump and treat (P&T) system began operating in 1988 and was expanded in 1993 with additional extraction wells. By 2011, the P&T systems had extracted and treated 860 million gallons of water and removed approximately 18,625 pounds of VOCs; however, progress towards aquifer restoration was slow, and the capture and treatment of all site-related chemicals remained a challenge.

Well12A contamination is associated with past waste oil recycling operations, which resulted in release of solvent-related NAPL from drum and tank storage areas and disposal of filter cake containing NAPL. Trichloroethene (TCE) and its degradation products are the primary contaminants of concern (COCs). The well was taken out of operation when it was found to be contaminated but has operated since 1983 through use of five air-stripping systems. After the source area was identified, approximately 1,200 cubic yards of filter cake mixed with contaminated soil were excavated along a rail line and an additional 5,000 cubic yards of filter cake were removed during construction of a soil vapor extraction (SVE) system. Between 1993 and 1997, the SVE system removed an estimated 54,100 pounds of VOCs. A carbon treatment-based pump and treat (P&T) system began operating in 1988 and was expanded in 1993 with additional extraction wells. By 2011, the P&T systems had extracted and treated 860 million gallons of water and removed approximately 18,625 pounds of VOCs; however, progress towards aquifer restoration was slow, and the capture and treatment of all site-related chemicals remained a challenge.

Remedies to address this challenge recently involved excavation and offsite disposal of remaining filter cake and contaminated shallow surface soil, in situ thermal remediation (ISTR) of deeper vadose zone and saturated zone soil and groundwater, and in situ enhanced anaerobic bioremediation (EAB) of groundwater. Continued operation of the P&T system is needed to prevent migration of contaminants until their mass is significantly reduced through excavation, ISTR and EAB. The adaptive management strategy for implementing ISTR and EAB technologies involves an overlapping operating schedule to maximize use of the ISTR applied and residual heat for advancing EAB.

The highest-priority remedial action objectives (RAOs) for Well 12A are to reduce risk from contaminated surface soil and achieve at least a 90% reduction in contaminant mass discharge from the source area (below and around the former recycling building known as the Time Oil Building) to the dissolved-phase contaminant plume. Other priorities are to achieve chemical-specific applicable or relevant and appropriate requirements (ARARs) measured at alternate points of compliance and to determine if MNA can be used to achieve ARAR requirements throughout the plume.

The conceptual site model (CSM) has been updated throughout the remedy design and remediation process. A major CSM refinement involved determining how to best measure and assess the mass discharge reduction goal while considering the significantly varying hydraulic conductivities, which range from 0.3 to 3,550 feet/day (ft/d) over relatively small vertical distances within the heterogeneous soil. Threedimensional (3-D) imaging software is used to analyze the data and effectively portray the site's complex hydrogeology and the VOC spatial distribution, masses and volumes, and mass discharge transects (Figure 1 in attached document).

The refined CSM reflects two mass flux transects suggesting that most of the mass discharge occurs in three distinct hydrostatrographic units. Close to the source area in transect 1, 95% of the mass discharge occurs in two units (Figure 1) designated as Qpfc1/Qpf (accounting for 31% of the discharge) and Qpfc2 (64% of the discharge). Both units consist of primarily coarse-grained soil with a hydraulic conductivity ranging from 35 to 782 ft/d. However, the Qpf sub-unit consists of fine-grained material (conductivity below 1 ft/d) that stores contaminant mass and acts as a secondary diffusional source feeding units above and below it. Farther downgradient in transect 2, 90% of the mass discharge occurs in the Qpfc2 unit (57% of the discharge) plus a unit designated as Qpogc (33% of the discharge).This portion of the Qpfc2 unit is coarse grained (conductivity of 35-782 ft/d) while the Qpogc unit represents a mass storage area in a relatively thin finer-grained unit (hydraulic conductivity of approximately 1 ft/d) that transitions to the aquitard. The hydraulic conductivity ranges and mass discharge estimates of these hydrostratigraphic units are key elements of the lifecycle CSM as it is used throughout the adaptive remedy implementation process.

Other tools for analyzing and facilitating decision-making on the VOC mass discharge include analytical mapping software that highlights the estimated discharges to the P&T system from the thermal treatment zone and the bioremediation zone (Figure 2). Based on this information, the P&T operations were slightly modified and will be used as the method for RAO compliance. 3- D imagery and other HRSC techniques also were used to develop vertical profiles to help visualize subsurface contaminant distribution, calculate contaminant mass, and delineate treatment zones for excavation, ISTR and EAB (Figure 3). One significant benefit of using vertical profiling in this adaptive management approach is the capability to prioritize treatment based on the mass expected to be discharging to more transmissive zones. For EAB, this capability also helped reduce the target vertical interval from approximately 60 ft to 15 ft, which substantially reduced the EAB treatment volume, saved approximately $1 million in EAB amendment purchasing costs, and significantly reduced the environmental footprint of EAB implementation.

Excavation was completed in 2012, and a portion of the Time Oil Building was demolished in 2013. In early 2014, remediation focused on installing and operating the ISTR system and performing two rounds of EAB injections. The first round, which was conducted as a pilot test, involved injecting waste vegetable oil in two wells and a commercial, emulsified vegetable oil in one additional well. These oils were used to promote microbial activity and improve vertical distribution of other amendments to be injected a few months later.

The second injection round, completed in November 2014, entailed injecting over a million gallons of amendment including bioaugmentation culture in 47 wells at select locations to treat an area covering 3.7 acres. The injection strategy was adaptive, using different design specifications to adjust for variability in the soil permeabilities, observed contaminant mass levels, geochemical conditions, and presence of important contaminant-degrading microbes throughout the treatment zone.

Active ISTR heating of the subsurface began in April 2014 and continued for 117 days at an average temperature of 96.8°C, using a total of approximately 4,026,000 kWh of electricity. The operation involved use of three isolation transformers inside a power control unit to generate electrical resistance heating, 70 electrodes, 35 independent recovery wells, 299 temperature monitoring points and 23 pressure monitoring points (Figure 4).

Based on the evaluated data, it is estimated that a minimum of 22,300 pounds of contaminant mass was removed from the target treatment zone during ISTR. This total mass estimate includes approximately 9,600 pounds of VOC mass removed in the vapor phase, 7 pounds of VOC mass removed in water and 12,700 pounds of NAPL. This estimate does not include aliphatic hydrocarbons removed from the treatment volume via vapor recovery due to the difficulty in quantifying aliphatic carbons. However, based on the carbon consumption rates and other available data, it is assumed that the total mass of aliphatic hydrocarbons removed in the vapor phase is equivalent to or greater than the total mass of VOCs (9,600 pounds) removed in the vapor phase. Accounting for this additional recovery, it is estimated that a minimum of 31,900 pounds of total contaminant mass was recovered from the treatment zone.

By implementing ISTR and EAB in parallel (Figure 5), excess or residual heat from active heating have increased temperatures in the EAB zone, fostering additional activity of anaerobic microorganisms capable of degrading the contaminants.

Performance monitoring of the EAB injections indicates that high levels of carbon may persist in the target zone for at least one year, with an overall longevity of approximately two years.

EPA is currently monitoring remedial performance and progress towards reducing mass discharge. In addition, EPA is considering strategies to address previously unidentified hotspots of dense non-aqueous phase liquids that were encountered during installation of the EAB wells. 

 

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