In Situ Remediation of Chlorinated Solvent Plumes

Chlorinated solvents have seen broad usage for a wide variety of purposes, from cleaning of machinery, clothes and electronic parts to use in chemical manufacturing. However, through general dispersal, during normal usage and also as a result of indiscriminate disposal, chlorinated solvents have caused a variety of environmental problems. One such problem of great concern is the contamination of soil and groundwater. This problem became most evident following the passage in the United States of the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA), or Superfund legislation, in 1980 and the subsequent evaluation of chemical contamination of groundwater. It is now recognized that there are thousands of public and private sites with chlorinated solvent related groundwater contamination problems. Remediation of such sites has been found to be especially difficult and costly. Many potential technical solutions have been developed and applied, often with limited success. Time frames for remediation tend to be long, often measured in decades. It is incumbent upon those responsible for planning, designing and overseeing the remediation of soil and groundwater contamination with chlorinated solvents to fully understand the difficulties and high costs that are likely to be involved, and to have sufficient knowledge of the uses and limitations of the many available technologies that may be applied to a solution. It is highly likely that several technical approaches will be needed together to reach a satisfactory solution that will effectively reduce risks to human health and the environment.

This chapter summarizes the principles of chlorinated solvent remediation, provides overviews of the biotic and abiotic reactions that can transform and detoxify these compounds, and discusses the remediation challenges posed by the properties and behavior of these compounds in the subsurface environment.

Biodegradation of chlorinated ethenes by naturally occurring or artificially enhanced processes is an important component of current site remediation strategies. At this writing, several microbial mechanisms for chlorinated ethene transformation and degradation have been identified. The purpose of this chapter is to briefly summarize the current understanding of those processes that lead to the biodegradation of chlorinated ethenes.

Abiotic processes, such as sorption, volatilization and chemical transformation, play an important role in the natural attenuation and treatment of chlorinated solvents. In this chapter, an overview of the principles governing these processes in the context of chlorinated solvent remediation and treatment is presented. This discussion includes a brief introduction to sorption processes and volatilization, with most attention focused on abiotic transformation pathways because of the recent advances in this area and the increasing interest in applying monitored natural attenuation (MNA) to chlorinated solvent plumes. The chapter is not meant to be a comprehensive review of the literature, but rather highlights basic information on key abiotic processes that may impact remedial technology implementability.

This chapter identifies and discusses many of the challenges of evaluating, engineering (designing) and implementing in situ remediation technologies for chlorinated solvent plumes in groundwater. Most challenges discussed are not unique to a specific remediation technology, although there are technology-specific challenges. For example, adequate delivery of reagents is the most universal challenge because so many in situ remediation technologies rely on injecting treatment reagents into a subsurface that is often highly heterogeneous. The reagents used may be liquids, solids or gases, and the challenges associated with all three of these phases are unique to some extent. While this chapter cannot cover all of the challenges faced during in situ remediation, it is hoped that the discussion presented will enable the reader to extrapolate to other challenges not covered.

Analytical and numerical modeling has emerged as a valuable tool for planning and designing groundwater remediation systems. Models have been used in a variety of settings including (1) research into the fundamental processes controlling chlorinated solvent fate and transport, (2) methods for integrating information on site hydrology, geology, contaminant distribution, transport and fate, and (3) applied aspects of plume management and remediation system design. This chapter focuses on currently available models commonly used by practitioners for simulating dissolved chlorinated solvent plumes and includes a brief summary of modeling principles, mathematical expressions useful for representing biodegradation processes, methods for representing dissolved contaminant release from source areas and case studies of models applied to sites.

Chlorinated solvent plumes, in aqueous or vapor form, are the products of contaminant source zones. So long as source zones persist, management of plumes will be an ongoing activity. The status of a plume is therefore inextricably linked to its source, and any decision regarding plume management is likely to involve a decision regarding management of the source. As bounding scenarios, sources can decay via naturally occurring processes or they can be actively addressed through engineered measures designed to contain and/or deplete the source. Typically, engineered measures are intended to reduce the magnitude and/or duration of contaminant discharge from a source to an aqueous or vapor phase plume. A typical consequential benefit of source zone treatment is a reduction in the contaminant concentrations downgradient.

The following sections summarize the major topics involved in in situ bioremediation of soluble phase (dissolved) chlorinated solvents in groundwater. This introduction is designed to provide some context for the following chapters and an overview of the development of in situ bioremediation for chlorinated solvents.

Chlorinated solvents often were released to the subsurface environment in wastewater or in the form of dense nonaqueous phase liquids (DNAPLs). As a result of their physical and chemical properties, chlorinated solvents are difficult to remediate once they have migrated into groundwater. However, enhanced in situ anaerobic bioremediation can be an effective method of degrading chlorinated solvents dissolved in groundwater, including chloroethenes, chloroethanes, and chloromethanes (collectively referred to as chlorinated aliphatic hydrocarbons, or CAHs). Advantages of enhanced in situ anaerobic bioremediation include the potential for complete detoxification of chlorinated solvents with little impact on infrastructure and relatively low cost compared to more active and aggressive engineered remedial systems.

Bioaugmentation involves the introduction of microorganisms into soil or groundwater to improve biological activity. Though used for other purposes, such as improving agricultural yields or efficiency, the use of bioaugmentation to promote the degradation of contaminants in the subsurface has increased significantly in recent years (Gentry et al., 2004). Bioaugmentation has been viewed with skepticism in the past, but there has been increasing evidence in recent years that it can accelerate the bioremediation of some contaminants under some site conditions. Bioaugmentation has become particularly useful for treating groundwater contaminated with chlorinated solvents.

Over the past decade, permeable reactive barrier (PRB) technology has progressed through the conceptual, experimental and innovative stages to its current status as accepted standard practice for groundwater remediation. As represented in the schematic of Figure 16.1, a PRB can be defined as an in situ treatment zone positioned such that it passively captures a contaminant plume and removes or degrades the contaminants, discharging uncontaminated water. The recent development of PRB technology has been stimulated largely by the use of granular iron “walls” for treatment of groundwater containing chlorinated organic contaminants. While this continues to be the most common application of PRBs and is the subject of this chapter, it should be noted that many other reactive materials have been proposed and tested for removal of a wide range of groundwater contaminants (ITRC, 2005).

The e-barrier is an emerging technology that applies fundamental electrochemical principles to a permeable reactive barrier (PRB). The e-barrier consists of closely spaced (e.g., 1 centimeter [cm]) permeable electrodes installed in a trench that intercepts a plume of contaminated groundwater (Figure 17.1). Low-voltage direct current (DC) sufficient to drive the degradation reactions of interest is applied to the electrodes. If sufficient electrical potential is applied, oxidizing conditions develop at the anode (positive electrode) and reducing conditions develop at the cathode (negative electrode). Since a complete electrical circuit is present, the dissolved contaminants are subject to sequential oxidation-reduction or reduction-oxidation, depending on the sequence of charges applied to the electrode set. This sequence can be altered depending on the contaminant of interest and the chemistry of the local groundwater. Through sequential oxidation-reduction (or reduction-oxidation), an aqueous phase chlorinated compound is degraded into thermodynamically favored carbon dioxide or methane and chloride.

The goal of this chapter is to provide an introduction to in-well technologies including the history of development, the basic principles of operation of groundwater circulation and in-well treatment processes, and the applicability and limitations of this class of technologies. In-well treatment technologies include a number of well configurations in which contaminants are removed from groundwater within the confines of the well as the water is pumped through the well. For most configurations, this is accomplished without pumping the water above ground for treatment. These systems mostly fall into the grouping of technologies termed groundwater circulation wells (GCWs) and most share a common design characteristic that includes at least two screened sections separated by a blank riser section. Groundwater enters the well through one or more screened sections, is treated, and then discharged back into the formation through the other screen(s), creating a circulation pattern within the aquifer that facilitates transport of contaminant to the well.

Increasing knowledge of bioremediation has demonstrated the self-remediation potential of water, sediments and soil in natural ecosystems. While abiotic processes can be involved to some extent, biological systems play the major role in natural remediation of organic pollutants. Living organisms are commonly exposed to natural and xenobiotic toxic compounds. As a consequence, these organisms have developed multiple detoxification mechanisms to prevent harmful effects from exposure to these compounds. Environmental biotechnologies exploit the natural potential of living organisms, such as bacteria, plants and fungi, to detoxify human-made pollutants discharged into the environment. Bacteria are extremely versatile organisms, more so than higher life forms. As a consequence, bacteria constantly develop new metabolic pathways for the degradation of a large range of xenobiotic pollutants (Limbert and Betts, 1996). While less versatile and adaptive, higher organisms such as plants also possess detoxification mechanisms to counteract the harmful effects of exposure to toxic contaminants (Sandermann, 1994). Often, symbiotic bacteria living in association with plants also play an important role in the degradation process (Chaudhry et al., 2005). Phytoremediation seeks to use the natural and adaptive capabilities of plants, and their associated organisms, to treat a wide range of pollutants including chlorinated solvents.

The economic analysis of remedial options is a key activity in any remedial selection process. It is typically employed in the feasibility study phase of remedy selection to aid in selecting the optimal remedial option for a site, in concert with a number of other criteria. In this case, for dissolved phase chlorinated solvent contaminated groundwater, the capital and recurring costs of various alternatives are compared, typically using a net present value (NPV) calculation. In addition to the ability to compare remedial options, detailed economic analysis also allows the design engineer to determine which cost elements of a specific remedy drive the cost of that remedy. Understanding the primary cost drivers improves the potential for cost optimization and therefore can lead to more cost effective remedial designs.

Remediation of chlorinated solvent plumes has improved tremendously over the past 25 years. Several treatment and containment technologies have been developed and successfully deployed. These technologies have allowed management of plumes for far less cost than the earlier presumptive remedy of pump-and-treat. The ability to restore contaminated sites has increased as well, reducing health and environmental risks and often allowing for higher value uses of redeveloped lands.