Remediation

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Remediation

The treatment of hazardous wastes in most industrialized countries is now controlled through regulations, policies, incentive programs, and voluntary efforts. However, until the 1980s, society knew little of the harmful effects of inadequate management of hazardous wastes, which resulted in many contaminated sites and polluted natural resources . Even in the 1970s, when environmental legislation required disposal of hazardous wastes to landfills and other systems, contamination of soils and groundwater continued, because these systems were often not leak proof. To meet the environmental and public health standards of the twenty-first century, these contaminated sites must continue to be rehabilitated, or remediated, so they no longer pose a threat to the public or the ecosystem . However, the price will be high; total costs for cleaning up the U.S. Department of Energy's (DOE) nuclear weapons sites alone will cost $147 billion from 1997 to 2070. Cleaning America's contaminated groundwater will cost even more: $750 billion over the next 30 years.

The Superfund program for remediation of contaminated sites was created in 1980 by the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) and was amended by the Superfund Amendments and Reauthorization Act (SARA) of 1984. The Superfund program is designed to provide an immediate response to emergency situations that pose an imminent hazard, known as removal or emergency response-actions, and to provide permanent remedies for environmental problems resulting from past practices (i.e., abandoned or inactive waste sites), know as remedial-response actions. Under the Superfund program, the remedial action process is implemented in structured stages: (1) Preliminary assessment; (2) Site inspection; (3) Listing on the National Priorities List (NPL), a rank ordering of sites representing the greatest threats; (4) Remedial investigation/feasibility study (RI/FS); (5) U.S. Environmental Protection Agency's Record of Decision (ROD); (6) Negotiation of a consent decree and remedial action plan between the U.S. Environmental Protection Agency and the responsible parties; and (7) Final design and construction.

Money for the Superfund came from special taxes on chemical and petroleum companies, which expired in 1995 and were not renewed. These taxes contributed about $1.3 billion a year prior to 1996. Since 1995, the fund has dwindled from a high of $3.6 billion to a projected $28 million in 2003. Currently, debate continues on where additional funds will come from, which may eventually be the taxpayer if special taxes are not reestablished. The Bush administration had proposed that the Superfund pay $593 million of 2002's projected $1.3 billion in cleanup costs, with the remaining $700 million coming from the Treasury. After this, the Superfund will be depleted. Money will have to come from somewhere because according to Resources for the Future , Superfund programs will cost $14–$16.4 billion between 2000 and 2009, with annual costs of between $1.3 billion and $1.7 billion.

In addition to the Superfund program, site remediation projects may be required for: the Resource Conservation and Recovery Act (RCRA) Corrective Action Program for operating treatment, storage, and disposal facilities; the underground storage tanks program established by RCRA; Federal Facility cleanup of sites operated primarily by DOE and the Department of Defense (regulated under both CERCLA and RCRA); and state-regulated, private, and voluntary programs. The cleanup of federal facilities is expected to be the most expensive, followed by the RCRA Corrective Action Program, and then Superfund. DOE must characterize, treat, and dispose of hazardous and radioactive waste at more than 120 sites in 36 states and territories, which is expected to cost more than $60 billion over the next 10 years.

The costs of remediation are determined by the degree of remediation required. Superfund cleanup standards require that contaminated waters be remediated to concentrations at least as clean at the Maximum Contaminant Levels (MCLs) of the Safe Drinking Water Act and the water quality criteria of the Clean Water Act . Carcinogen risks from exposures to a Superfund site should be less than one excess cancer in a population of one million people. Some states require that remediation projects restore the site to an equivalent of its original condition.

Remedial techniques are divided into two basic types: on-site methods and removal methods. Most remedial techniques are used in combination (e.g., pump and treat systems) rather than singly. Generally remediation techniques that significantly reduce the volume, toxicity, or mobility of hazardous wastes are preferred for use. Off-site transport and disposal is usually the least favored option. On-site methods include containment, extraction, treatment, and destruction, while removal methods consist of excavation and dredging . Containment is used to contain liquid wastes or contaminated ground waters within a site; or to divert ground or surface waters away from the site. Containment is achieved by the construction of low-permeability or impermeable cutoff walls or diversions. On-site groundwater extraction is accomplished by pumping, which is usually followed by aboveground treatment (referred to as pump and treat). Pumping is used to control water tables so as to contain or remove contaminated groundwater plumes or to prevent plume formation. Soil vapor extraction, by either passive or forced ventilation collection systems, is used to remove hazardous gases from the soil. Gases are collected in PVC perforated pipe wells or trenches and transported above ground to treatment or combustion units.

On-site treatment of hazardous wastes can be conducted above ground, following extraction or excavation, or below ground (in situ ). Above ground treatment methods include the use of conventional environmental engineering unit processes. Below ground treatment can be nonbiological or biological. Non-biological in situ treatment involves the delivery of a treatment fluid through injection wells into the contaminated zone to achieve immobilization and/or detoxification of the contaminants. This treatment is accomplished by processes such as oxidation, reduction, neutralization, hydrolysis, precipitation, chelation, and stabilization/solidification. Biological in situ treatment (bioremediation ) is accomplished by modifying site conditions to promote microbial degradation of organic compounds by naturally occurring microorganisms . Exogenous microorganisms that have been adapted to degrade specific compounds or classes of compounds are also sometimes pumped into ground water or applied to contaminated soil . Certain types of bacteria and plants containing peptides are also being developed for treating radioactive materials and heavy metals .

Finally, on-site destruction of organic hazardous wastes can be accomplished by the use of high-temperature incineration . Inorganic materials are not destroyed, so residues from the incineration process must be disposed of in a secure landfill . In situ vitrification (ISV), or thermal destruction, involves the heating of buried wastes to melting temperatures (by applying electric current to the soil) so that a chemically inert, stable glass block is produced. Nonvolatile materials are immobilized in the vitrified mass, and organic materials are pyrolyzed. Combustion off-gases are collected and treated.

Removal methods of remediation include extraction and dredging. Excavation of contaminated solid or semisolid hazardous wastes may be required if the wastes are not amenable to in situ treatment. Excavated materials may be treated and replaced or transported off-site. Dredging is used to remove contaminated sediments from streams, estuaries, or surface impoundments. Removal of well-consolidated sediments in shallow waters requires the use of a clamshell, dragline, or backhoe. Sediments with a high liquid content or that are located in deeper waters are removed by hydraulic dredging. Dredged materials are pumped or barged to treatment facilities.

Although not every remediation project has been a complete success, many Superfund sites have been cleaned up significantly. Of the 1,310 sites on the list, 773 sites had been cleaned up by 2001, with the remainder in various stages of cleanup. However, the rate of cleanups is slowing; about half as many will be completed during the current administration compared to the previous one. As better procedures for remediation continue to be developed and refined, perhaps this rate will increase. Nanoparticle technology is one method that shows promise for cleaning groundwater sites, since it can reduce 96% of contaminants compared to 25% with conventional methods.

[Laurel M. Sheppard ]