1 United States Office of Office of Solid Waste EPA/540/S-92/002. Environmental Protection Research and and Emergency January 1992. Agency Development Response EPA Ground Water Issue TCE Removal from Contaminated Soil and Ground Water Hugh H. Russell*, John E. Matthews**, and Guy W. Sewell*. Background production of vinyl chloride (VC) which, in contrast to TCE, is a known carcinogen [Fed. Regist. 1984, 49:114, 24334(11)]. The Regional Superfund Ground Water Forum is a group of EPA professionals, representing EPA's Regional Superfund Wastewater or municipal Water supply treatment systems Offices, committed to the identification and the resolution of which utilize coagulation, sedimentation, precipitative Ground Water issues impacting the remediation of Superfund softening, filtration and chlorination have been found sites. The Forum is supported by and advises the Superfund ineffective for reducing concentrations of TCE to non- Technical Support Project.
2 Remediation of trichloroethylene- hazardous levels (Robeck and Love, 1983). Other methods contaminated soils and Ground waters is an Issue identified are required for remediation of Water contaminated with TCE. by the Forum as a concern of Superfund decision-makers. if such Water is to be used for human consumption. The For further information contact Hugh H. Russell (FTS-743- purpose of this paper is to: 1) present a synopsis of physico- 2444), John E. Matthews (FTS-743-2408), or Guy W. Sewell chemical properties and reactive mechanisms of TCE, and 2). (FTS-743-2232). delineate and discuss promising remediation technologies that have been proposed and/or demonstrated for restoring TCE-contaminated subsurface environmental media. Introduction Trichloroethylene (TCE) is a halogenated aliphatic organic compound which, due to its unique properties and solvent Physical and Chemical Properties of effects, has been widely used as an ingredient in industrial Trichloroethylene cleaning solutions and as a universal degreasing agent.
3 TCE, perchloroethylene (PCE), and trichloroethane (TCA) are Preliminary assessment of remediation technologies feasible the most frequently detected volatile organic chemicals for reclamation of subsurface environmental media (VOCs) in Ground Water in the United States (Fischer et al., contaminated with TCE must involve consideration of the 1987). Approximately 20% of 315 wells sampled in a New compound's physical and chemical properties ( , dis- Jersey study contained TCE and/or other VOCs above the 1 tribution coefficients, reactivity, solubility, etc.). These ppb detection limit (Fusillo , 1985). The presence of TCE properties are directly responsible for behavior, transport and has led to the closure of Water supply wells on Long Island, fate of the chemical in the subsurface environment. and in Massachusetts (Josephson, 1983). Detectable Knowledge of a compound's physico-chemical tendencies levels of at least one of 18 VOCs, including TCE, were can be used to alter behavior and fate of that compound in reported in of 63 Water wells sampled in Nebraska, a the environment.
4 Important considerations derived from State having a low population density and industrial base physical and chemical properties of TCE presented in Table 1. (Goodenkauf and Atkinson, 1986). are: Trichloroethylene per se is not carcinogenic; it is thought to Density ( g/ml) - Density can be defined as the become a human health hazard only after processing in the concentration of matter, and is measured by the mass per unit human liver (Bartseh et al., 1979). Epoxidation by liver oxidase enzymes confers a suspected carcinogenic nature (Apfeldorf and Infante, 1981; Tu et al., 1985). However, processing in the human liver is not the only way in which *Research Microbiologist, **Research Biologist, Robert S. TCE may become a health hazard. Reductive dehalogenation Kerr Environmental Research Laboratory, EPA, Ada, of TCE through natural or induced mechanisms may result in OK. Superfund Technology Support Center for Technology Innovation Office Ground Water Office of Solid Waste and Emergency Response, US EPA, Washington, Robert S.
5 Kerr Environmental Walter W. Kovalick, Jr., Research Laboratory Director Ada, Oklahoma 1. volume. In relation to liquids, these units are grams per translates into little retardation by soil or aquifer organic milliliter. The density of a substance is usually referenced to materials. Since the retardation is so low, pump-and-treat pure Water which is taken to be 1 gram per milliliter. TCE is technologies appear attractive as remediation alternatives. heavier than Water ; therefore, a spill of sufficient magnitude is Mehran et al. (1987) through field and laboratory likely to move downward through the subsurface until lower investigations determined a retardation factor of permeability features impedes its progress. This often results approximately for TCE which agrees with the data of in formation of a plume or pool(s) of dense nonaqueous Wilson et al. (1981) and falls within the range of 1-10 given by phase liquid (DNAPL) in the aquifer plus a trail of residual Mackay et al.
6 (1985) for sand and gravel aquifers with low saturation within the downward path. Implicit in this statement organic content. Measured partition coefficients, however, is the fact that residual saturation also will serve as source may be considerably higher than calculated values, especially areas for contamination and migration of TCE within an at lower aqueous concentrations. Johnson et al. (1989) found aquifer system. There is an inherent difficulty associated with that at equilibrium concentrations of approximately 1 ppm, the location of DNAPL in the subsurface and subsequent removal measured partition coefficient of TCE was significantly higher of DNAPL pools or plumes using a standard pump and treat than calculated values. These authors also report that other regime. A major reason for this difficulty of removal is that workers who have observed the same effect theorize that this Water coning using conventional extraction wells results in higher value may be related to clay interactions.
7 Poor DNAPL/ Water ratios. If the retardation factor is considered to be , TCE should A density difference of about 1% above or below that of Water migrate at half the speed of Water through soil and aquifer ( gm/ml) can significantly influence movement of materials low in organic carbon content, and theoretically contaminants in saturated and unsaturated zones somewhat slower in material with a high organic carbon (Josephson, 1983). This does not mean that density content. differences of less than 1% do not influence movement of contaminants. These density differences may even be Henry's Law Constant - Henry's Law states that the amount of apparent at low solute concentrations (Short, 1991, personal gas that dissolves in a given quantity of liquid at constant communication). At a site in New Jersey, it was determined temperature and pressure, is directly proportional to partial that although soil and shallow Ground Water at the source pressure of the gas above the solution.
8 Henry's coefficients, area were contaminated with benzene, toluene and other as a result, describe the relative tendency of a compound to volatile organic compounds with relatively low specific volatilize from liquid to air. The Henry's Law Constant for gravities, the compounds did not migrate downward into TCE is which is high enough, when combined with its deeper municipal supply wells (Spayd, 1985). This is in low solubility in Water and high vapor pressure, for efficient contrast with what could happen if the source area were transfer of TCE to the atmosphere. The evaporation half-life contaminated with chlorinated aliphatics, including TCE. of TCE in Water is on the order of 20 minutes at room Areas containing insoluble TCE, , DNAPL pools, can serve temperature in both static and stirred vessels (Dilling, 1975;. as source areas for spreading of contamination. As Ground Dilling et al., 1975). Water moves through and/or around these source areas, equilibrium concentrations partition into the aqueous phase.
9 Aqueous phase TCE is then spread through the aquifer by advection and dispersion. As a result small source areas can Table 1. Physico-chemical Properties of Trichloroethylene serve to contaminate large portions of an aquifer to levels exceeding drinking Water standards. At a site in Texas, for Density g/ml example, Freeberg et al. (1987) determined that 8 kg of non- Water Solubility 1000 mg/L. aqueous phase TCE was responsible for contaminating Henry's Law Constant x 106 gallons of Water at an average concentration of 176 (atm-m3/mol @ 20o C) ppb. Molecular Weight Boiling Point C. Water solubility - Water solubility can be defined as the Log Octanol- Water Partition maximum concentration of a solute which can be carried in Coefficient Water under equilibrium conditions and is generally given as ppm (parts per million) or mg/l (milligrams per liter). The Water solubility limit of TCE is 1000 mg/l, the maximum concentration of TCE that can be in aqueous solution at 20o C.
10 Water solubility of a compound has a direct relation on The chemical structure of TCE bestows chemical reactivity. distribution coefficients and biodegradability of a particular Three chlorine atoms attached to the carbon-carbon double compound. A compound that is relatively insoluble in Water bond make TCE a highly oxidized compound. Oxidized will prefer to partition into another phase; , volatilize into molecules readily accept electrons (reduction) under soil gas or sorb to organic material. Compounds that are appropriate conditions, but resist further oxidation. As a relatively insoluble also are not as readily available for result, chemical reactivity of TCE is greatest under a reducing transport across the bacterial membrane, and thus less atmosphere, conditions that favor transfer of electrons to subject to biological action. TCE. Koc value - Koc is defined as the amount of sorption on a unit Due to their size, the three carbon atoms surrounding the carbon basis.