Hazardous Waste Site Remediation
Hazardous waste site remediation
The overall objective in remediating hazardous waste sites is the protection of human health and the environment by reducing risk. There are three primary approaches which can be used in site remediation to achieve acceptable levels of risk:
- the hazardous waste at a site can be contained to preclude additional migration and exposure
- the hazardous constituents can be removed from the site to make them more amenable to subsequent ex situ treatment, whether in the form of detoxification or destruction
- the hazardous waste can be treated in situ (in place) to destroy or otherwise detoxify the hazardous constituents
Each of these approaches has positive and negative ramifications. Combinations of the three principal approaches may be used to address the various problems at a site. There is a growing menu of technologies available to implement each of these remedial approaches. Given the complexity of many of the sites, it is not uncommon to have treatment trains with a sequential implementation of various in situ and/or ex situ technologies to remediate a site.
Hazardous waste site remediation usually addresses soils and groundwater . However, it can also include wastes, surface water, sediment , sludges, bedrock, buildings, and other man-made items. The hazardous constituents may be organic, inorganic and, occasionally, radioactive. They may be elemental ionic, dissolved, sorbed, liquid, gaseous, vaporous, solid, or any combination of these.
Hazardous waste sites may be identified, evaluated, and if necessary, remediated by their owners on a voluntary basis to reduce environmental and health effects or to limit prospective liability. However, in the United States, there are two far-reaching federal laws which may mandate entry into the remediation process: the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA, also called the Superfund law), and the Resource Conservation and Recovery Act (RCRA). In addition, many of the states have their own programs concerning abandoned and uncontrolled sites, and there are other laws that involve hazardous site remediation, such as the cleanup of polychlorinated biphenyl (PCB) under the auspices of the federal Toxic Substances Control Act (TSCA).
Potential sites may be identified by their owners, by regulatory agencies, or by the public in some cases. Site evaluation is usually a complicated and lengthy process. In the federal Superfund program, sites at which there has been a release of one or more hazardous substances that might result in a present or potential future threat to human health and/or the environment are first evaluated by a Preliminary Assessment/Site Inspection (PA/SI). The data collected at this stage is evaluated, and a recommendation for further action may be formulated. The Hazard Ranking System (HRS) of the U.S. Environmental Protection Agency (EPA) may be employed to score the site with respect to the potential hazards it may pose and to see if it is worthy of inclusion on the National Priorities List (NPL) of sites most deserving the attention of resources.
Regardless of the HRS score or NPL status, the EPA may require the parties responsible for the release (including the present property owner) to conduct a further assessment, in the form of the two-phase Remedial Investigation/Feasibility Study (RI/FS). The objective of the RI is to determine the nature and extent of contamination at and near the site. The RI data is next considered in a baseline risk assessment . The risk assessment evaluates the potential threats to human health and the environment in the absence of any remedial action, considering both present and future conditions. Both exposure and toxicity are considered at this stage.
The baseline risk assessment may support a decision of no action at a site. If remedial actions are warranted, the second phase of the RI/FS, an engineering Feasibility Study, is performed to allow for educated selection of an appropriate remedy. The final alternatives are evaluated on the basis of nine criteria in the federal Superfund program. The EPA selects the remedial action it deems to be most appropriate and describes it and the process which led to its selection in the Record of Decision (ROD). Public comments are solicited on the proposed clean-up plan before the ROD is issued. There are also other public comment opportunities during the RI/FS process. Once the ROD is issued, the project moves to the Remedial Design/Remedial Action (RD/RA) phase unless there is a decision of no action. Upon design approval, construction commences. Then, after construction is complete, long-term operation, maintenance, and monitoring activities begin.
For Superfund sites, the EPA may allow one or more of the responsible parties to conduct the RI/FS and RD/RA under its oversight. If possibly responsible parties are not willing to participate or are unable to be involved for technical, legal, or financial reasons, the EPA may choose to conduct the project with government funding and then later seek to recover costs in lawsuits against the parties. Other types of site remediation programs often replicate or approximate the approaches described above. Some states, such as Massachusetts, have very definite programs, while others are less structured.
Containment is one of the available treatment options. There are several reasons for using containment techniques. A primary reason is difficulty in excavating the waste or treating the hazardous constituents in place. This may be caused by construction and other man-made objects located over and in the site. Excavation could also result in uncontrollable releases at concentrations potentially detrimental to the surrounding area. At many sites, the low levels of risks posed, in conjunction with the relative costs of treatment technologies, may result in the selection of a containment remedy.
One means of site containment is the use of an impermeable cap to reduce rainfall infiltration and to prevent exposure of the waste through erosion . Another means of containment is the use of cut-off walls to restrict or direct the movement of groundwater. In situ solidification can also be used to limit the mobility of contaminants. Selection among alternatives is very site specific and reflects such things as the site hydrogeology , the chemical and physical nature of the contamination, proposed land use , and so on. Of course, the resultant risk must be acceptable.
As with any in situ approach, there is less control and knowledge of the performance and behavior of the technology than is possible with off-site treatment. Since the use of containment techniques leaves the waste in place, it usually results in long-term monitoring programs to determine if the remediation remains effective. If a containment remedy were to fail, the site could require implementation of another type of technology.
The ex situ treatment of hazardous waste provides the most control over the process and permits the most detailed assessments of its efficacy. Ex situ treatment technologies offer the biggest selection of options, but include an additional risk factor during transport. Examples of treatment options include incineration ; innovative thermal destruction, such as infrared incineration; bioremediation ; stabilization/solidification; soil washing; chemical extraction; chemical destruction; and thermal desorption. Another approach to categorizing the technologies available for hazardous waste site remediation is based upon their respective levels of demonstration. There are existing technologies, which are fully demonstrated and in routine commercial use. Performance and cost information is available. Examples of existing technologies include slurry walls, caps, incineration, and conventional solidification/stabilization.
The next level of technology is innovative and has grown rapidly as the number of sites requiring remediation grew. Innovative technologies are characterized by limited availability of cost and performance data. More site-specific testing is required before an innovative technology can be considered ready for use at a site. Examples of innovative technologies are vacuum extraction, bioremediation, soil washing/flushing, chemical extraction, chemical destruction, and thermal desorption. Vapor extraction and in situ bioremediation are expected to be the next innovative technologies to reach "existing" status as a result of the growing base of cost and performance information generated by their use at many hazardous waste sites.
The last category is that of emerging technologies. These technologies are at a very early stage of development and therefore require additional laboratory and pilot scale testing to demonstrate their technical viability. No cost or performance information is available. An example of an emerging technology is electrokinetic treatment of soils for metals removal.
Groundwater contaminated by hazardous materials is a widespread concern. Most hazardous waste site remediations use a pump and treat approach as a first step. Once the groundwater has been brought to the surface, various treatment alternatives exist, depending upon the constituents present. In situ air sparging of the groundwater using pipes, wells , or curtains is also being developed for removal of volatile constituents. The vapor is either treated above ground with technologies for off-gas emissions, or biologically in the unsaturated or vadose zone above the aquifer . While this approach eliminates the costs and difficulties in treating the relatively large volumes of water (with relatively low contaminant concentrations) generated during pump-and-treat, it does not necessarily speed up remediation.
Contaminated bedrock frequently serves as a source of groundwater or soil recontamination. Constituents with densities greater than water enter the bedrock at fractures, joints or bedding planes. From these locations, the contamination tends to diffuse in all directions. After many years of accumulation, bedrock contamination may account for the majority of the contamination at a site. Currently, little can be done to remediate contaminated bedrock. Specially designed vapor stripping applications have been proposed when the constituents of concern are volatile. Efforts are on-going in developing means to enhance the fractures of the bedrock and thereby promote removal. In all cases, the ultimate remediation will be driven by the diffusion of contaminants back out of the rock, a very slow process.
The remediation of buildings contaminated with hazardous waste offers several alternatives. Given the cost of disposal of hazardous wastes, the limited disposal space available, and the volume of demolition debris, it is beneficial to determine the extent of contamination of construction materials. This contamination can then be removed through traditional engineering approaches, such as scraping or sand blasting. It is then only this reduced volume of material that requires treatment or disposal as hazardous waste. The remaining building can be reoccupied or disposed of as nonhazardous waste.
[Ann N. Clarke and Jeffrey L. Pintenich ]
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