Cleaning up chlorinated hydrocarbons
Chlorinated hydrocarbons (CHs) such as perchloroethylene (PCE), trichloroethylene (TCE) and trichloroethane (TCA) have been a central concern in the inventory of contaminated sites. Such concern can be attributed to the toxic and potentially carcinogenic properties of these compounds. CHs are hard to locate and treat, making traditional remedial approaches difficult and inefficient. Site owners with CH contamination in groundwater have been searching for a compromise between capital- and operation-intensive treatment systems such as pump and treat or soil vapor extraction, and the risk associated with monitored natural attenuation (MNA).
The recent introduction of Hydrogen Release Compound (HRCTM), an innovative technology for in situ management of CH plumes, provides property owners with a new approach. The technology accelerates anaerobic bioremediation by providing a time-released hydrogen source essential to the anaerobic dechlorination of CHs.
When it comes to CH remediation, it seems that few aspects of the problem have much in common with petroleum hydrocarbon (PH) remediation. With PH contamination, the majority of property owners are concentrated in gas and petroleum product manufacturers and retailers; CH contamination covers a multi-industry market that includes small- to medium-sized businesses as well as large corporations. While a large underground storage tank (UST) contamination plume can reach a distance of hundreds of feet, a CH plume can reach a distance of thousands of feet. While time frames for site closure of PH contamination are discussed in terms of years, CH site closures are discussed in terms of decades.
Though complete plume remediation is a realistic expectation for many petroleum hydrocarbon sites, for the majority of CH sites with large source areas and large plume sizes, the operative economic pursuit seems to be plume management. Selecting a plume-wide strategy for CH remediation may not be as time- or cost-efficient as implementing localized plume treatment strategies.
Clearly, project managers are met with a new challenge with CHs. Within the scope of existing remedial approaches, these challenges will require innovative and creative solutions.
Limitations of natural attenuation and active remediation systems
Property owners whose sites may qualify for the use of MNA are faced with the risk of future liability associated with a migrating plume. With the cost of remediation reaching up to the millions of dollars on some sites, property owners have accepted the risk of MNA in the absence of low-cost remedial alternatives.
The introduction of risk-based corrective action (RBCA) for PH plume management generated a wave of natural attenuation enthusiasts; however, this natural attenuation paradigm does not readily carry over to the management of chlorinated sites. It is commonly understood that chlorinated compounds do not naturally attenuate the way petroleum hydrocarbons do. The density and low aqueous solubility of dense non-aqueous phase liquids (DNAPLs) such as PCE causes them to migrate rapidly toward the bottom of the aquifer, making them difficult to locate and remediate. Furthermore, the options for having a system naturally attenuate at an accelerated rate are limited because anaerobic processes are very slow in comparison to the more familiar aerobic processes that mediate PH degradation. Thus, given the nature of CHs, the assertion that natural attenuation will be protective of health and the environment at a given site must be substantiated with thorough and accurate evaluation.
Active remediation systems such as pump and treat and soil vapor extraction have traditionally been the method of choice in remediating CHs. However, running such systems on large plumes with long remediation time frames can make these approaches unacceptably expensive and inefficient. Although they may represent the only alternative for heavily contaminated CH source areas, they may not be appropriate after a certain time or in certain areas of a plume when concentrations reach a level of diminishing returns on such operation-intensive systems.
Accelerated natural attenuation as a new approach
In situ, passive approaches that aim to accelerate the rate of biodegradation of contaminants represent a sensible alternative for CH remediation. Such an approach avoids the risks of unassisted natural attenuation by controlling plume migration and reducing the risk of off-site liability. Simultaneously, such treatment avoids the high costs and other disadvantages associated with remedial systems like pump and treat.
The new technology is a proprietary polylactate ester specially formulated for the slow release of lactic acid upon hydration. When it is introduced to the subsurface, various indigenous organisms help separate the lactic acid from HRC. Then, fermentative anaerobic microbes metabolize the lactic acid to a series of other organic acids; this activity produces hydrogen. The hydrogen thus generated can be used by microbes called reductive dechlorinators capable of biological dechlorination of the contaminants. HRC is a moderately flowable substance that can be pressure-injected using various direct-push technologies. Its use can maintain dechlorinating conditions in the aquifer for about six months to one year or more, depending on site conditions.
While other options are available in choosing substrate enhancements - e.g., a variety of organic acids, alcohols and complex natural products - there are some inherent advantages in using time-release strategies. Implementation of the appropriate time-release system can eliminate major design capital and operational costs, is minimally invasive and is invisible during the treatment. As a function of the controllable viscosity, the new technology will remain where injected and slowly generate highly diffusible organic acids and hydrogen. Since CH plumes are difficult to locate, a continuous, highly diffusible substrate may increase the effectiveness of contact, containment and remediation. The metabolic activity enhances the concentration gradient for the contaminants, which in turn may facilitate desorption of hydrophobically sorbed CHs from the soil matrix.
Results from several university studies, including the Gossett laboratory at Cornell and the McCarty laboratory at Stanford, suggest that there is competition for hydrogen between the reductive dechlorinators and another group of anaerobes called methanogens. (Fennell, D.E.., J.M. Gossett and S.H. Zinder (1997, ES&T 31:918-926) and Y. Yang and P. L. McCarty (1998, ES&T 32:3591-3597.)) While methanogen survival is favored under elevated hydrogen conditions, reductive dechlorinators may best be supported in conditions of more moderate hydrogen concentration; thus a slow release process may optimize this process. The McCarty group reports that hydrogen concentrations between 2 and 10 nanomolars (nM) are optimal for reductive dechlorination over competing methanogen activity. The theory postulates that when hydrogen concentrations get too high, the hydrogen is monopolized by methane-producing microbes (methanogens) at the expense of the reductive dechlorinators. Therefore, moderate hydrogen concentrati
ons facilitated by the new technology's slow-release mechanism may optimize reductive dechlorination.
Results from field applications of the new technology have shown significant CH degradation in a fairly rapid timeframe. In the cases presented, various daughter products of the primary contaminant were shown to increase - an indication that the contaminants are being degraded biologically and not simply becoming attenuated via dilution or migrating away as a function of groundwater flow. In many cases, the expected sequential degradation of the daughter products was documented. In other cases, more time will be necessary to document those changes.
PCE remediation in Wisconsin. A dry-cleaning site in Wisconsin was contaminated with levels of PCE as high as 25 parts per million (ppm) in the dissolved phase. Two-hundred and forty pounds of HRC were injected in 12 injection points in a 60 square foot (ft2) area. Groundwater velocity was on the order of 0.1 feet per day (ft/day) in a sandy aquifer. PCE mass was reduced 70 percent within 190 days following the injection. The effects of this single application were demonstrated to continue at least 253 days, at the end of which 80 percent of the contaminant had been removed. Based on these results, a full-scale treatment is planned for plume control.
Other indicators that the new techology had established conditions that facilitate reductive dechlorination include 1) highly negative oxidation reduction (redox) conditions, 2) significant reductions in sulfate, 3) substantial increase in anaerobic bacterial counts and 4) elevated dissolved hydrogen levels. In fact, the dissolved hydrogen levels were within the critical 2 to 10 nM range as illustrated in Figure 1. As mentioned earlier, these levels are considered by some to be optimal for reductive dechlorination.
PCE remediation in Florida. A commercial dry-cleaning facility in Orlando released PCE into groundwater underlying a shopping center. PCE concentrations exceeded 8 parts per million (ppm) in one well and approached 2 ppm in another. Approximately 6,810 pounds of HRC were injected into a 14,600 ft2 area via 145 direct-push points spaced 10 feet on center. In 152 days, there was a 96 percent reduction of total dissolved phase PCE mass. The aquifer consists of fine-grained sand with areas of clay, and is divided into an upper and lower zone by a clay unit. Groundwater flows at a velocity of between 0.04 ft/day and 0.07 ft/day. Based on the success in the upper aquifer, the consultant is considering extending treatment to the lower aquifer.
TCA and TCE remediation at Hurlburt Air Force Base, Fla. TCA and TCE were the major contaminants released over a long period of time at Hurlburt Field. The concentrations were as high as 1.5 ppm for TCA and as high as 9.5 ppm for TCE. Approximately 6,000 pounds of HRC were injected into the contaminated saturated zone via 25 direct-push points spaced 5 feet on center within a 500 ft2 area. The aquifer consists of layers of sand, silty sand and clay lenses and supports a groundwater flow of approximately 0.35 ft/day. After only 103 days, there was a 59 percent decrease in TCA and a 71 percent decrease in TCE.
The consultant filed a report with the client, the U.S. Department of Defense, offering the preliminary conclusion that this interim measure appeared to be meeting or exceeding expectations. Upon completion of this phase, the objective is to allow any remaining, significantly reduced concentrations to be handled with natural attenuation.
The use of bioremediation to address chlorinated hydrocarbon contamination is receiving significant attention. Based on the initial findings reported here, it appears that HRC has a future in shaping new dimensions in chlorinated hydrocarbons plume management within this venue. This new technology presents a tool for consultants to limit the application of excessive remedies while avoiding the risk and high long-term costs of simply monitoring a site prematurely.
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This article originally appeared in the 09/01/1999 issue of Environmental Protection.