United States Environmental Protection Agency National Risk Management Research Laboratory Cincinnati, OH 45268 Research and Development EPA/600/SV-95/001 September 1995 4>EPA Project Summary Natural Bioattenuation of Trichloroethene at the St. Joseph, Michigan Superfund Site James W. Weaver, John T. Wilson, and Don H. Kampbell Data from the St. Joseph, Michigan, Superfund Site were used in a peer- reviewed video entitled "Natural Bioattenuation of Trichloroethene at the St. Joseph, Michigan Superfund Site." Computer visualizations of the data set show how trichloroethene, or TCE, can degrade under natural conditions. The purpose of the tape is to present sampling results from the site to a technical audience. Although the visualizations show the general distribution of chemicals at the site, it is not possible to determine the precise concentrations from the tape. Thus the data set itself is available in a companion document. The following text is an amplified version of the narration on the video. This Project Summary was developed by the National Risk Management Research Laboratory's Subsurface Protection and Remediation Division, Ada, OK, to announce key findings of the research project that is documented in a video of the same title (see video ordering information at back). Site History The site is located four miles south of St. Joseph and one-half mile east of Lake Michigan (Figure 1). Since the 1940s, the site has supported auto-parts manufacturing, including a foundry, as well as machining and painting operations. Because of past activities, ground water at the site is contaminated with industrial wastes that include trichloroethene (TCE). A plume of contamination reaches from its source near the industry, toward Lake Michigan to the west (Tiedeman and Gorelick, 1993). The aquifer is primarily composed of medium, fine, and very fine sands that are of glacial origin. The base of the aquifer is defined by a clay layer that lies between 21 and 29 meters below the ground surface. Since the ground water flows toward Lake Michigan, the contamination underlies residential and shoreline property. This water eventually discharges directly into the lake. Although the TCE contamination is moving toward the lake, evidence indicates that the contaminants are degrading naturally along the way (McCarty and Wilson, 1992). Reduction in concentration alone does not necessarily indicate bioattenuation, because concentrations can decline from the effects of advection, dispersion and sorption. Rather, bioattenuation of TCE is indicated here by the presence of daughter products and certain geochemical conditions. Degradation of TCE Chlorinated organic compounds such as TCE can be biodegraded in the subsurface, but not because the microorganisms oxidize these compounds as a food source. On the contrary, the degradation of TCE under anaerobic conditions occurs through a reductive transformation where the TCE molecule serves as an electron acceptor. In a loose analogy, we could say that the microorganisms "breathe" TCE. Forthistype of degradation to occur, another organic ------- Figure 1. Site plan showing the location of the sampling transects. compound must be present to serve as an oxidizable carbon source, or "food" for the microorganisms. Under this condition, and in the absence of oxygen, TCE can be transformed through a series of intermediate chemical compounds to ethene. The intermediates are hazardous, and therefore incomplete degradation of TCE is potentially undesirable. TCE may undergo a reductive transformation in an anaerobic environment. An enzyme or cofactor catalyzes the reduction of TCE (HC2CI3), resulting in the loss of one chlorine atom: H * + H C7 Cl = H, C7 CL + (1) Threeisomersofdicholoroethene.orDCE (H2C2CI2), can result: 1,1-DCE; cis-1,2 DCE and trans-1,2 DCE. Of these, cis-DCE is usually produced in the greatest abundance. The presence of the DCE isomers is significant, because these chemicals have rarely been used on a large scale for industrial purposes. Therefore their presence is an indication of transformations occurring in the subsurface. With the loss of another chlorine atom from a DCE isomer, vinyl chloride (H3C2CI) is produced: H2C2CI2 = H3C2CI Cl- (2) The production of vinyl chloride is undesirable because it is a known carcinogen. However, ethene, which is not a compound of regulatory concern, can result from the loss of the chlorine atom from the vinyl chloride: H * + H3 C2 Cl = H4 C2 + Cl- (3) For further information on TCE biodegradation see McCarty and Semprini, 1994, and Semprini et al. 1995. Field Evidence for TCE Bioattenuation At a field site, natural bioattenuation of TCE is indicated • by the presence and degradation of an oxidizable substrate; • by the absence of oxygen and the presence of strongly reducing conditions (i.e., the abundance of methane); • by the presence of the intermediate products (the DCE isomers and vinyl chloride); and, • by the presence of ethene, the end product. Specific site conditions determine the rate at which the transformations occur and the likelihood of producing a harmless end product. Each site must be evaluated individually for its potential to degrade TCE. There are sites where TCE either does not degrade or is only partially degraded. Thus the results from St. Joseph show the possilbilty of degradation of TCE, but do not indicate that degradation will occur at all sites. Representation of the Data In the visualizations, each data set is displayed as a set of colored cubes that ------- surround the borings. Each boring appears as an elongated, colored stack of cubes. This approach was taken so that the data was not smoothed, interpolated, nor extrapolated. The representations of the data, therefore, show the variation in concentration that occurs over small intervals at the site, and the irregularity of the distributions. The top of each set of cubes roughly corresponds to the water table; and the bottom corresponds to the clay layer that forms the base of the aquifer. Narration on the tape makes it clear that the views have been exaggerated in the vertical direction in order to better illustrate the distribution of the chemicals over the thickness of the aquifer. The lengths of the borings were indicated by noting that each cube in the on-shore borings is 1.5 meters tall, and that the borings contain from five to eleven cubes. Thus the borings represent aquifer thicknesses varying from 7.5 to 16.5 meters. This scale is also noted by the distance (16.5 meters) between the top of the bluff and the shore line. The exaggerated veritcal distances contrast with the distance across the site from the industry's parking lot to the shore of Lake Michigan (730 meters); and the width of the contaminant plume (110 meters). These features of the visualizations indicate that the views emphasize vertical variations in the contaminant distribution. In actuality the contaminant plume is a long and thin object. The color scale that is used to indicate concentration ranges from blue to red, indicating low to high concentrations, respectively. A logarithmic scale was used to discriminate between concentrations that range over six orders of magnitude. The St. Joseph Data Set Data were collected at St. Joseph in sets of borings that form transects across the site. The borings were made with a 1.5 meter long slotted auger from which water samples were taken. A gas chromatograph was used to detect the pollutants as the borings were made. These procedures assured thatthe transects crossed the entire width of the contaminant plume. Data were collected from the site in 1991 along transects nearthe source region, and in 1992 along two transects lying between the source and the lake (Semprini et al., 1994). In August 1994, a set of samples were taken from a barge anchored in Lake Michigan. These samples determined the contaminant concentrations in the ground water immediately before it discharges into the lake. Features of the St. Joseph Data Set In the vicinity of the plume, dissolved oxygen is depleted from the ground water, even though the ground water is oxygen- rich outside the contaminated zone. The ground water is depleted of oxygen nearthe bottom of the aquifer. Oxygen at intermediate and high concentrations, from two to ten milligrams per liter, is found in some locations near the water table. The methane data show a pattern that is almost exactly opposite that of the dissolved oxygen. Methane concentrations are highest near the bottom of the aquifer and are lowest nearthe watertable. This distribution shows that the aerobic and anaerobic zones in the aquifer are clearly separated. At St. Joseph, there is a decrease of COD from the source area to the lake. This is indicative of anaerobic degradation of the oxidizable carbon source, which remains to be specifically identified. The highest TCE concentration at the site is 89,000 micrograms per liter and is found near the source area. The contaminants tend to move toward the lake in the deeper part of the aquifer, as noted by the absence of TCE near the water table. By the time TCE reaches the lake, however, the concentrations are reduced to levels that are mostly undetectable. There are a few TCE concentrations of one to two micrograms per liter in the lake transect. These concentrations are below the EPA drinking water standard of 5 micrograms per liter. The pattern of declining concentration as the chemicals flow toward the lake is repeated in both the DCE and vinyl chloride data sets. Dechlorination of TCE usually produces the cis-DCEisomer in the greatest abundance. At St. Joseph, for example, the trans-DCE andthe 1,1-DCE concentrations are generally lower than the cis-DCE concentrations by at least a factor of 10. The transformation of TCE to DCE may occur undersulfate reducing conditions and sulfate concentrations should be measured. The maximum cis-DCE concentration is 128,000 micrograms perliter, occurring near the bottom of the aquifer in the source region. cis-DCE concentrations decline toward the lake and the compound is undetectable in the lake transect. Because the cis-DCE is the dominant isomer at St. Joseph, the trans- DCE and 1,1-DCE data sets are not shown in the video tape. The vinyl chloride distribution follows the general pattern of highest concentrations nearthe bottom of the aquifer and declining concentrations toward the lake. No vinyl chloride concentrations above the drinking water standard of two micrograms per liter were detected from samples taken in the lake. The presence of ethene is evidence for the complete dechlorination of the TCE. Ethene is present throughout the contaminant plume; its distribution follows the pattern oftheotherdegradation products. Summary In summary, the intermediate products of TCE bioattenuation are found in oxygen depleted portions of the aquifer that are also rich in methane. Ethene is found in significant concentration, indicating some of the TCE is degraded to a compound that is not of regulatory concern. The concentrations of TCE and the degradation products significantly decline toward the lake. The off-shore data show that only minute concentrations of these chemicals exist in the ground water that discharges into the lake. Analysis of data from the St. Joseph, Michigan Superfund site indicates that natural bioattenuation of TCE is occurring as the contaminants flow toward Lake Michigan. Depletion of oxygen, the presence of methane and the appearance of degradation products indicate that the reduction in TCE concentrations is not solely due to volatilization or dilution. Rather, they are indicative of microbial processes helping to reduce the contaminant concentrations below EPA drinking water standards before the water is discharged into Lake Michigan. Continued monitoring of the site is necessary to demonstrate that contaminant levels remain below accepted standards and that the flux of chemicals into the lake remains low. REFERENCES McCarty, P.L. and L. Semprini, Ground- water treatment for chlorinated solvents, Handbook of Bioremediation, Norris et al., pp5-1 to 5-30, 1994. McCarty, P.L. and J.T. Wilson, Natural anaerobic treatment of a TCE plume St. Joseph, Michigan NPLsite, Bioremediation of Hazardous Wastes, US EPA, EPA/600/ R-92/126, 47-50, 1992. ------- Semprini, L, P.K. Kitanidis, D. Kampbell and J.T. Wilson, Anaerobic transformation of chlorinated aliphatic hydrocarbons in a sand aquifer based on spatial chemical distributions, Water Resources Research, 31(4), 1051-1062, 1995. Tiedeman, C., and S. Gorelick, Analysis of uncertainty in optimal groundwater contaminant capture design, Water Resources Research, 29(7), 2139-2153, 1993. The EPA authors, James W. Weaver (also the EPA Project Officer, see below), John T. Wilson, and Don H. Kampbell, are with the National Risk Management Research Laboratory's Subsurface Protection and Remediation Division, Ada, OK 74820. The video, entitled "Natural Bioattenuation ofTrichloroethene at the St. Joseph, Michigan Superfund Site" (EPA/600A/-95/001), will be available upon request from: Subsurface Remediation Information Center National Risk Management Research Laboratory U. S. Environmental Protection Agency P.O. Box1198 Ada, Oklahoma 74820 Telephone: 405-436-8651 FAX: 405-436-8503 The EPA Project Officer can be contacted at: Subsurface Protection and Remediation Division National Risk Management Research Laboratory U.S. Environmental Protection Agency Ada, OK 74820 United States Environmental Protection Agency National Risk Management Research Laboratory (G-72) Cincinnati, OH 45268 Official Business Penalty for Private Use $300 BULK RATE POSTAGE & FEES PAID EPA PERMIT No. G-35 EPA/600/SV-95/001 ------- |