Chapter 8
Tetra Chloro Ethylene
REMOVAL OF ORGANIC POLLUTANT TETRACHLOROETHYLENE(PCE) FROM GROUNDWATER
A significant percentage of commercial properties requiring environmental cleanup have been contaminated by releases of the chlorinated solvent tetrachloroethylene(PCE) by dry cleaning operations. Dry cleaning facilities have been ubiquitous in commercial properties, particularly strip shopping centers, since the 1940s.
A significant percentage of these facilities have released PCE into the soil and groundwater beneaththese properties, and a significant percentage of these releases will display contaminant levels above applicable state regulatory standards. Therefore, some form of environmental cleanup will be required for many of these properties. Due to the physical characteristics (particularly its density being greater than water) and the health– based risks associated with PCE, this contaminant and its degradation products have proven to be difficult and expensive to remediate.The case study documented herein involves a typical strip center property contaminated by a PCE release from a small dry cleaning operation located in Arlington, Texas. In our professional experience, the release history, subsurface conditions,contaminant types and concentrations at this site are typical of these properties in Texasas well as other states. Hydrogen Release Compound (HRC) developed by Regenesis offers a viable option for the remediation of groundwater at these PCE contaminated sites. Contaminant concentration and geochemical data clearly indicates that the injection of HRC at the subject dry cleaner facility greatly accelerated the reductive dechlorination of PCE and its degradation products and has allowed the site to achieve a Conditional Certificate of Completion from the Texas Natural Resource Conservation Commission (TNRCC) Voluntary Cleanup Program.
Case Study
Site Background and History. The site is located in Arlington, Texas and is approximately 7 acres and contains two one-story multi-tenant strip shopping centers constructed in 1979. A former dry cleaner was situated in the small suite and conducted dry cleaning operations on site between approximately 1982 and 1992.
Phase I and II Environmental Site Assessments were conducted at the site in 1996,and the site was admitted into the TNRCC Voluntary Cleanup Program in November of 1996. A release of an unknown amount of chlorinated solvents (apparently PCE) had occurred at the dry cleaners and had affected soil and groundwater beneath the site.Based on the results of these investigations, PCE and degradation products [trichloroethylene(TCE), cis-1,2,-dichloroethylene (cis-DCE), trans-1,2-DCE (trans-DCE), and vinyl chloride(VC)] were identified as the chemicals of concern (COCs) for the subsequent SiteInvestigation Report (SIR) and Conceptual Environmental Assessment Model (CEAM;i.e.,health-based risk assessment) required by the Voluntary Cleanup Program. The SIRand CEAM were completed in 1996 and 1997, respectively.
The SIR has documented the installation of 11 groundwater monitoring wells and 18 soil borings at the site. The CEAM was prepared to identify potential human health risks associated with the historic release and identified potential receptors (construction workers and on site commercial workers) and exposure pathways (inhalation of vapors and dermal contact with perched groundwater). The CEAM also established site target cleanup concentrations for soil and groundwater. Site target concentrations for groundwater are,asfollows: PCE (500 ug/L), TCE (500 ug/L), cis-DCE (7,000 ug/L), VC (200 ug/L). In the contaminant source area located directly beneath the dry cleaner’s building, the COCs had affected subsurface soils in excess of site target concentrations below the building foundation to a depth of approximately 8 feet (ft) [2.4 meters (m)] bgs. This source area was excavated [excavation size approximately 10 ft (3.0 m) by 10 ft (3.0 m)by 10 ft (3.0 m) deep] in 1998, and all soils exceeding site target concentrations were removed.
Removal of this highly contaminated soil, which was located in close proximity to and in contact with the shallow aquifer, was crucial to the overall success of this project, as a continuing source of contaminants to the shallow aquifer was permanently removed by excavation.
Site investigations revealed that the perched groundwater with COC concentrations above site target concentrations occupied an area of approximately 3,500 ft 2 (330.5 m 2 ) directly beneath and downgradient of the source area. Groundwater modeling performed in 1998 indicated that unassisted natural attenuation would require approximately 20 to 25 years to reduce contaminant concentrations at the site to target concentrations.
Geology and Hydrogeology. Topography in the site vicinity slopes very gently toward the north; however, the surface relief is probably not sufficiently pronounced to definitively affect shallow groundwater gradient. The site is located on the outcrop of the Eagleford Group (Bureau of Economic Geology, 1972). This formation is generallycharacterized as predominantly medium to dark gray shale which readily weathers to a thick, clayey soil horizon.Two TNRCC- designated aquifers exist beneath the Eagleford Group at the site; the Woodbine and Trinity Aquifers (Texas Water Development Board, 1994). Thehydrogeologic setting is not considered sensitive based on the depth and distance to the recharge zones for these beneficial use aquifers. Shallow, perched groudwater in the site vicinity is definitely hydrologically separated from these deeper aquifers by aquicludes formed by unweathered shales of the Eagleford Group.
There is no known beneficial use of shallow, perched groundwater within a 0.5 mile (0.8 Km) radius of the site, and the total dissolved solids concentration is 3,170milligrams/liter (mg/l) in the shallow aquifer. encountered; commonly causing auger refusal at or near the top. Cores taken from this shale aquiclude had a vertical intrinsic permeability of approximately 7.72 X 10 -16 centimeters (cm 2 ). This layer effectively limits the vertical migration of contaminants. Water levels in the aquifer perched in the weathered clays above this aquiclude occur at approximately 4 to 7 ft (1.2 to 2.1 m) bgs at the site. Local groundwater gradient in this perched aquifer has consistently been toward the southeast in the vicinity of the on site source area . Slug tests have demonstrated conclusively that this shallow aquiferhas a very low permeability and is not capable of producing 150 gallons/day [567.8 liters (l)/day] (TNRCC benchmark for consideration as a potentially beneficial use aquifer). This fact has allowed the application of the site specific target contaminant concentrations.
HRC Application
As an alternative to natural attenuation alone as a response action, injection of HRC ]hydrogen elease release compound] material into the saturated zone was chosen as the means of enhancing and expediting the biodegradation process, thus reducing dissolved-phase COC concentrations more rapidly. Utilizing site specific geological, hydrogeological, and geochemical data in conjunction with software provided by Regenesis, it was estimated that approximately 7,000 pounds (3,171 kg) of HRC should be sufficient for achieving target concentrations at the site, and the time required to achieve target concentrations was estimated at two years or less.
The HRC was injected into the shallow aquifer near the source area at the site from May 1 through May 12, 2000, and a total of approximately 7,000 pounds (3,171 kg) of HRC was injected. The injection area [approximately 3,000 ft 2 (283 m 2 )] encompassed the majority of the area with dissolved-phase COC concentrations greater than established target concentrations, which was estimated to be approximately 3,500 ft 2 (330.5 m 2 )located beneath and southeast of the former dry cleaners suite. The HRC was injectedinto 45 injection borings within this area, and each boring was injected with HRC from approximately 7 ft to 22 ft in (2.1 to 6.7 m) depth. Twenty nine injection borings were advanced perpendicular to the ground surface, and 16 additional borings were advanced at angles of 15 and 30 degrees from vertical to extend beneath the building’s foundation. Each of the 45 injection borings were advanced using a Stratoprobe boring unit. A “top-down”injection system was utilized by advancing two ft (0.6 m) into the target interval and pumping the HRC material into the formation through a 2-ft (0.6 m) by 1.5-inch(3.81 cm) perforated injection body. The overlying interval was sealed off from the injection zone by a 2-ft (0.6 m) by 2-inch (5.1 cm) sealing packer, and the section below the perforated target interval was sealed by a 6-inch (15.2 cm) by 1.75-inch (4.4 cm) pre-probe.
After pumping the proportionate amount of HRC material [approximately 10 pounds (4.5 kg) per vertical, saturated foot] into the formation, the probe was driven another two feet (0.6 m) and the process repeated. The injection was accomplished by use of a hydraulically-powered, progressive cavity, Moyno pump with a flowrate of 1 to 2gallons per minute (3.8 to 7.8 liters per minute) at a pressure ranging from 200 to 300 lbs.per square in. (14.1 to 21.2 Kg per square cm). This method of injection, while a time consuming process, assures an even injection of the HRC material over the entire thickness of the target subsurface interval. We believe that this even HRC distribution is crucial to the success of the overall HRC injection project, particularly in low permeability aquifers.HRC material costs were approximately $31,200. Direct push boring costs,concrete cutting and coring, site cleanup, field supervision, consulting oversight and design, and report writing costs were approximately $28,500. No future operating expenses, other than the expense of field collection and field and laboratory analysis of geochemical parameters, were anticipated as a result of the HRC injection.
Cost effective remediation of chlorinated solvents in groundwater is a significant challenge for retail dry cleaner facilities across the United States. The toxicity and distribution of the contaminants coupled with the proximity of nearby businesses and residential areas limits the applicability of many conventional groundwater remediation technologies. This paper presents results from a full scale, in situ groundwater remediation application at a Texas dry cleaner facility. Hydrogen Release Compound (HRC ® ) was injected into groundwater to serve as a slow release electron donor in order to enhance the reductive dechlorination of the chlorinated solvents. Primary groundwater contaminants at the site include tetrachloroethylene (PCE),trichloroethylene (TCE), cis-1,2-dichloroethylene (cis-DCE), and vinyl chloride (VC).
Near the source area, groundwater contaminant concentrations for PCE (4,500micrograms/liter, ug/l), TCE (1,000 ug/l), cis-DCE (7,300 ug/l), and VC (870 ug/l) wererecorded in MW-3 immediately prior to HRC injection. The treatment area wasapproximately 3,000 square feet (ft 2 ) [283 square meters (m 2 )] - a portion of which underlies the former dry cleaner’s building foundation. In May of 2000, HRC was injected into 45 borings within the contaminated area. All 45 direct push borings were installed to approximately 22 ft (6.7 m) below ground surface (bgs), and the HRC was injected evenly over the interval from approximately 7 to 22 ft (2.1 to 6.7 m) bgs.Sixteen of the borings were installed at angles of 15 and 30 degrees from vertical in order to access contaminated groundwater beneath the building’s foundation. A total of approximately 7,000 pounds [3,171 kilograms (Kg)] of HRC were injected at the site. HRC material costs were approximately $31,200. Direct push boring costs, concrete coring and cutting, site cleanup, field supervision, consulting oversight and design, and report writing costs were approximately $28,500. These one-time expenditures compare very favorably to the initial and recurrent operating costs anticipated for other remedial options. Approximately 18 months after HRC injection, PCE, TCE, cis-DCE, and VC levels had decreased to 408, 87, 438, and 132 ug/l, respectively. As of May 2001, all contaminant levels at the site were below site target concentrations established within the Texas Voluntary Cleanup Program.Injection of HRC into shallow aquifers utilizing direct push technology can be a cost effective remedial technology for dry cleaner facilities contaminated by PCE and its degradation products.
Contaminant and geochemical data presented from this case study of a dry cleaner facility in Arlington, Texas clearly demonstrate that the HRC injected into this shallow, perched aquifer aided in creating the requisite anaerobic subsurface environment and provided the hydrogen required for efficient reductive dechlorination which has significantly reduced contaminant concentrations within a year of application. As a direct result of HRC application, this site has received a Conditional Certificate of Completion from the TNRCC Voluntary Cleanup Program, as all contaminant concentrations are below site target concentrations.
The successful use of HRC is dependent upon its effective injection over the affected zone in the shallow aquifer. The “top-down” injection methods employed in this project achieved even subsurface distribution of the HRC over a 15 ft (4.6 m) interval in a very low permeability aquifer. Other, less stringent, methods will undoubtedly decrease required field time; however, poor treatment distribution can compromise the overall success of the injection project. Furthermore, removal of the source area contaminated soils (completed in 1998 by excavation under the dry cleaner’s building) has also been important in the subsequent remedial success of this project. If allowed to remain in place, these highly contaminated source area soils provide a continuing source of contaminants leaching into the shallow aquifer.
REFERENCES
Rick Railsback G. Shawn Hardy Rick Gillespie Paper 2B-57, in: A.R. Gavaskar and A.S.C. Chen (Eds.), Remediation of Chlorinated and Recalcitrant Compounds—2002. Proceedings of the Third International Conference on Remediation of Chlorinated and Recalcitrant Compounds (Monterey, CA; May 2002). ISBN 1-57477-132-9, published by Battelle Press, Columbus, OH,Bureau of Economic Geology, University of Texas. 1972. Geologic Atlas of Texas, Dallas Sheet.Puls, R.W. and M.J. Barcelona. 1996. Low Flow (Minimal Drawdown) Ground-Water Sampling Procedures. EPA Publication EPA/540/S-95/504.
Texas Water Development Board. 1994. Major and Minor Aquifers of Texas.
Wiedemeier, T.H., J.T. Wilson, J.E. Hansen, and F.H. Chapelle. 1996. Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Groundwater. Prepared for the AirForce Center for Environmental Excellence.