BASIC HAZARDOUS WASTE MANAGEMENT - CHAPTER 11 ppt

Site Remedial Technologies, Practices, and RegulationsOBJECTIVES At completion of this chapter, the student should: • Be familiar with the technologies that may be employed in site tion,

Site Remedial Technologies, Practices, and Regulations

OBJECTIVES

At completion of this chapter, the student should:

• Be familiar with the technologies that may be employed in site tion, e.g., on-site containment, solidification/stabilization, chemical treat-ment, bioremediation and destruction; “pump-and-treat” regimes; naturalattenuation; extraction; off-site treatment and disposal; and related RCRA1and CERCLA2 requirements and policies

remedia-• Understand the respective roles of RCRA and CERCLA in site remediation

• Be familiar with “How Clean is Clean” issues, the basis for them, someresolutions thereof, and the roles assigned to risk assessment in the reme-diation processes

• Be familiar with the National Contingency Plan, the “blueprint” role ofthe NCP in site remediation, how to find the NCP and how to maintain

or ensure currency with it

• Understand the linkages between hazardous waste site remediation, theBrownfields Initiative, and environmental justice issues

INTRODUCTION

In Chapter 10 we introduced and briefly overviewed the technologies and processesinvolved in the evaluation of contaminated or suspect sites The generic, RCRACorrective Action, and CERCLA (Superfund) approaches to site evaluation wereintroduced as the necessary precursors to site cleanup We now continue with theoverview of site cleanup procedures To the extent possible, we will continue thepattern of introduction of technologies and processes in the “generic” or established

1 Resource Conservation and Recovery Act of 1976.

2 Comprehensive Environmental Response, Compensation, and Liability Act of 1980 (and Superfund Amendment and Reauthorization Act of 1986).

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practice format We will then overview the options and/or requirements as applied

to RCRA and Superfund site remediation

The technologies for site remediation have been developed over a relatively shortperiod of time Some of the technologies were introduced in the 1970s or earlier andsome sites were remediated in the latter part of that decade However, it is arguablethat actual cleanup of Superfund sites did not begin making significant progress untilthe mid-1980s With obvious exceptions, the corporate and public cultures that even-tually gave impetus to private sector cleanups were similarly timed Thus, some ofthe technologies continue to evolve, while some have become proven and standard-ized New treatment or cleanup technologies are in plentiful supply and new man-agement philosophies are being put to the test A few of the more promising newapproaches to site remediation, as well as the time-tested ones, will also be overviewed

in this chapter References to those introduced and others will be provided

Development of treatment technologies has been given support by the EPASuperfund Innovative Technology Evaluation (SITE) program The SuperfundAmendments and Reauthorization Act (SARA) of 1986 authorized $20 million peryear, through 1991, to support development of new treatment technologies and toprovide sound engineering and cost data on selected technologies Approximatelyten new project awards were made each year to test and/or demonstrate innovative

or improved hazardous waste management technologies in laboratory and full-scaleoperations The program was extended with the Superfund reauthorization in 1991,but SITE reauthorization died with Superfund reauthorization in 1994 In followingyears, separate appropriations have enabled continuation of the SITE program (see also: EPA 1989; Payne 1998, pp 17–19)

The national programs for cleanup of uncontrolled hazardous waste sites (e.g.,RCRA corrective actions, Superfund removal, and/or remedial actions) have beenthe focus of great controversy Both programs were fought tenaciously by lobbyists,

in the courts, and by policy makers of the Reagan Administration To many inCongress and elsewhere, the Superfund program has progressed too slowly and atexcessive costs To others it has been overly aggressive, unyielding, burdened withprocess, and utopian in cleanup objectives It has been bedeviled by the “how-clean-is-clean” issue; by charges that it is “anti-business” and/or merely moves the con-taminants and creates future Superfund sites; and by the ponderousness of theSuperfund process In 1999, House3 and Senate3 Superfund reauthorization billsfailed for variations of the above issues and others At the time of this writing in

2000, neither body had produced a reauthorization bill acceptable to all parties (see also: RAND 1989; GAO 1993, 1994a,b, 1999)

Nevertheless, the program is making significant progress and is having somenotable successes Superfund, imperfections notwithstanding, is here to stay and will

be a major factor in the nation’s hazardous waste cleanup The National PrioritiesList (NPL) now includes approximately 1289 sites (65 FR 30482-8), and sites areadded to the list several times each year These sites must be cleaned up, and nopreferable program format has been suggested, although the 1994 reauthorizationbill contained significant changes to the earlier statute Moreover, the failures of the

3 House Bill 1300; Senate Bill 1090.

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1994 and subsequent annual Superfund bills continue to point up deep unresolveddivisions in political, public, professional, and activist notions of the form that areauthorized Superfund program should take

R EMEDIAL O BJECTIVES

Programmatic Objectives

In the most general sense, hazardous waste site remedial activity is pursued to correctthe results of mismanagement and accidental releases Remedies usually involveremoval of contaminated materials and safe disposition thereof; treatment, destruc-tion, and/or containment in-place; or some variation(s) of these

Remedial actions may be taken by individuals or corporations without theinvolvement of federal and/or state regulatory agencies Indeed, privately fundedand/or executed cleanup activity preceded the advent of RCRA and Superfund, andboth statutes are structured to encourage (leverage) private cleanups

RCRA corrective actions are an essential element of the national policy tive, i.e., the minimization “… of the present and future threat to human health andthe environment.” These authorities enable the EPA to address releases to thegroundwater and other environmental media at RCRA-regulated sites The RCRAauthorities do not extend to abandoned sites or those for which responsible partiescannot be identified

objec-Superfund was originally intended to enable timely response to emergencycleanup needs and to provide resources and authorities for cleanup of abandonedsites and those for which responsible parties (1) cannot be identified or (2) refuse

or are unable to conduct the necessary cleanup Over time, government owned and/oroperated facilities have been made subject to the law, and the “innocent landowner”provision has been added in an effort to limit the reach of the strict joint and severalliability provisions Provision has been made for de minimis settlements for smallcontributors to Superfund sites (King and Amidaneau 1995, pp 68–69; see also:

U.S GAO 1993, 1994a; EPA 1998)

Technical Objectives

Whether privately funded and/or executed or carried out under statutory mandates,remedial actions must have the protection of human health and the environment astheir overall objective.4 The more applicable objectives are the prevention of furthermigration of releases that have occurred, amelioration of exposures and impactscaused by those releases, and prevention of further releases These objectives arepursued by one of two basic operations:

1 On-site treatment, destruction, or containment

2 Off-site management of hazardous wastes and contaminated materials,followed by treatment, destruction, or safe disposal

4 Studies have shown that higher than expected cancer rates may be associated with proximity to Superfund sites, e.g., the Baird and McGuire site (Environment Reporter, November 16, 1990, p 1359) L1533_frame_C11 Page 273 Tuesday, May 1, 2001 12:44 PM

While there are many variations and combinations of these two basic techniques, it

is useful to categorize remedial actions as “on-site” or “off-site” operations.RCRA and CERCLA require use of risk assessment techniques based uponsite-specific data and the circumstances of the site The technical objectives must

be stated in terms of the degree of cleanup to be achieved in order to protecthuman health and the environment (i.e.,how much residual contamination at thesite is acceptable?) This question is the crux of the “how-clean-is-clean” issue.The answer to the immediate question and the eventual resolution of the issuehave far-reaching implications for managers of public health risks and for respon-sible parties

There is no single safe level of hazardous chemical concentrations applicable

to all chemicals and all sites that, if achieved, would justify a declaration of “clean.”Epidemiologists, risk managers, and policy makers initially found it necessary torely to a great extent upon exposure criteria, such as drinking water and air qualitystandards, which were never intended for use as hazardous waste site cleanupstandards With time, rationalization of exposure criteria for some carcinogenic andnoncarcinogenic substances has been achieved Where pathways and exposure dataexist to support a risk-assessment process, EPA policy is that the level of total

individual carcinogen risk from exposures attributable to a Superfund site may be

in the range of one excess occurrence in 10,000 (10–4) to 1 in 10 million (10–7) Themost frequently proposed criteria is 10–6

Nevertheless, these standards (with a few exceptions) deal with individual ganic and organic pollutants, whereas the hazardous waste site cleanup criteria mustconsider a wide variety of inorganic and complex organic compounds and mixtures.Thus, the rigor of the risk assessment processes continue to be limited by thenecessity to incorporate a variety of assumptions for critical human health exposure,

inor-as well inor-as environmental protection Over the pinor-ast decade, the EPA hinor-as produced

an evolving and burgeoning set of risk assessment guidance documents which areintended to lend site-specificity and rigor to the cleanup goal setting (“how-clean-is-clean”) process This set entitled “Risk Assessment Guidance for Superfund”(RAGs), in three volumes, can be accessed on the Superfund Web site

The Administration’s 1994 Superfund reauthorization bill contained languagecalling for a numeric national cleanup goal” and a “national risk protocol.” Theprotocol would have contained standardized exposure scenarios for a range ofunrestricted and restricted land uses and standardized formulas for evaluatingexposure pathways and developing chemical concentration levels for the 100contaminants that occur most frequently at Superfund sites (Environment Reporter, April 29, 1994, p 2219) This format, of course, does little to solvethe “how-clean-is-clean” dilemma Viable exposure criteria continue to beabsent or unproven for many of the most commonly discarded chemicals andchemical compounds Without exposure criteria, a health risk assessment format

is a hollow one Failure of the 1994 Superfund reauthorization was regarded by

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The introduction of the “Superfund Accelerated Cleanup Model” (SACM) hasprovided some generalization of cleanup methods in the form of “presumptiveremedies and response strategies”5 discussed later herein Some states simply impose

a blanket requirement that all cleanups achieve background concentrations of wasteconstituents (see also: Staples and Kimerle 1986; EPA 1989; Travis and Doty 1992;Burke 1992; Sims et al 1996; Sellers 1999, Chapter 2)

O N -S ITE R EMEDIAL T ECHNIQUES

Containment Methods

As the name implies, containment methods are directed toward prevention of tion of liquid hazardous wastes or leachates containing hazardous constituents.Containment usually involves the construction of impermeable barriers to retainliquids within the site, to direct the liquids to collection points for pumping and/ortreatment, or to divert ground and surface waters away from the site Successfulapplication of these methods is usually contingent upon the presence of an imper-vious layer beneath the material to be contained and the achievement of a good seal

migra-at the vertical and horizontal interfaces Some examples follow

Slurry Walls. The slurry trench is excavated down to and, if practicable, into

an impervious layer The trench is typically 2 to 5 ft in width Early applicationsused a 4 to 7% bentonite clay suspension in water to make up the slurry The slurrymay be mixed with the excavated soil or with other suitable soils to form a verylow permeability wall More recent applications have made use of additives such aspolymers to improve the permeability or to protect the slurry from the deleteriouseffects of leachate Figure 11.1 shows a trench and soilbentonite slurry wall underconstruction The soil removed from the trench is mixed with bentonite clay andreplaced in the trench Figure 11.2 shows a cement-bentonite wall being installed

In this case, the excavated soil is not used Cement is mixed with the bentoniteslurry, which “sets” as a solid wall

Many variations of the containment wall technique have been developed Theuse of high density polyethylene (HDPE) membranes to line the excavated trench

or as a curtain in the mid-section of the slurry wall to improve effectiveness isdescribed by Cross Mitchell and van Court describe and illustrate a geomembrane

“envelope,” lining the walls of an excavated trench wherein the envelope is filled

Water at CERCLA Sites, OSWER Directive 9283.1-12.

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Monroeville Blvd., Suite 400, Monroeville, PA 15146 With permission.)

Monroe-ville Blvd., Suite 400, MonroeMonroe-ville, PA 15146 With permission.)

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with a sand and water mix to form an impermeable containment wall Suthersandescribes low permeability slurry walls as components of containment systemswhich direct contaminated groundwater to treatment gates, and permeable reactivetrenches using a variety of materials as reactants, or to collect stripped vapors (see:

EPA 1992, 1998a; Mitchell and van Court 1997; Cross 1996; Suthersan 1997;Pearlman 1999; Sellers 1999, Chapter 3)

Grout Curtains. In somewhat similar fashion, suspension grouts composed ofbentonite or Portland cement, or both, may be injected under pressure to form abarrier The method is most effective when the receiving formation is unconsolidatedand porous deposits can be filled by the injection In other situations, single, double,

or triple lines of holes are drilled in staggered positions Ideally, the grout injected

in adjacent holes should penetrate to merge and form a continuous barrier Chemicalgrouts are a more recent development and have the advantage of a range of viscos-ities Some have viscosities approaching that of water and can be used to seal veryfine rock and soil voids (see: EPA 1998a; Mitchell and van Court 1997; Cross 1996;Pearlman 1999)

Sheet Piling Cut-Off Walls. Pilings of wood, precast concrete, or steel can beused to form a cut-off wall Sheet piling of steel is the most effective and has theadvantages of great structural strength, it can be driven to depths as great as 100 ft,and it can accommodate irregularly shaped and/or confined areas It has the disad-vantages that it cannot be used effectively in rocky soil, the interlocking jointsbetween the sheet piles must be sealed to prevent leakage,6 and the steel is subject

to attack by the contained corrosive liquids (see: EPA 1998a; Sims et al 1996;Mitchell and van Court 1997; Suthersan 1997, pp 196–197; Pearlman 1999; Sellers

1999, Chapter 3)

Less frequently used containment techniques include the use of frozen soilbarriers and hydraulic barriers (Mitchell and van Court 1997; EPA 1998) Othercontainment methods make use of surface diversions to route run-off away from thewaste deposit and impervious caps to carry rainfall and snowmelt beyond the perim-eter of the deposit

Extraction Methods

Two basic approaches to on-site extraction have gained general acceptance and areeffective when properly designed and operated The methods are pumping of con-taminated groundwater to the surface for treatment and discharge or reinjection andactive or passive extraction and treatment of soil gases produced in a waste deposit.Uncontaminated groundwater may also be pumped to deny it contact with a wastedeposit In addition, a recognized scientific phenomenon is being employed, inseveral variations, as the technology phytoremediation, with encouraging results.These methods will be briefly overviewed

Groundwater Pumping. At least three different applications of groundwaterpumping are used to control contaminated water beneath a disposal site Theseapplications are

6 A variety of patented sealant technologies have been developed to seal the joints.

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• Pumping to lower a water table

• Pumping to contain a plume

• Groundwater treatment systems

The effect of lowering a water table may be to prevent contaminated water fromreaching a surface stream as base flow; to prevent contact with a contaminationsource; or to prevent migration to another aquifer (Figures 11.3 through 11.6)

pump-ing) (From U.S Environmental Protection Agency.)

(From U.S Environmental Protection Agency.)

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Extraction wells or combinations of extraction and injection wells may be used

to contain a plume and/or alter plume movement to force contaminated groundwatertoward collection wells (Figures 11.7 and 11.8) One of the most frequentlyemployed remediation procedures for large plumes of contaminated groundwater isthe “pump and treat” (P & T) approach, wherein extraction wells are placed to drawfrom the plume and prevent or reverse downgradient movement of the plume Theextracted water is treated to remove the pollutant(s) and is then discharged or used

on the surface The treated water may be reinjected at the perimeter of the inant plume to create an artificial groundwater mound, thereby assisting in moving

(before pumping) (From U.S Environmental Protection Agency.)

(after pumping) (From U.S Environmental Protection Agency.)

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the contaminants toward the extraction well The system illustrated in Figure 11.9employs the ion exchange process for removal of chromium; air stripping of chlo-rinated solvents; and carbon adsorption to remove stripped organics from the exhauststream For treating organic contaminants in groundwater produced by P & Tsystems, Suthersan lists air stripping, carbon adsorption, steam stripping, chemicaloxidation, biodegradation, and membrane filtration For treatment of inorganic con-taminants, he lists precipitation, ion exchange, adsorption, reverse osmosis, steamstripping, and chemical oxidation (Suthersan 1997)

Recent evaluations of P & T projects at 28 groundwater contamination sitesreveals that the technique does not always attain expectations, with respect to costand/or cleanup times Cost increases of 80% over original estimates were found to

be typical Cleanup times are projected to be as much as three times longer thanoriginally estimated The studies determined that P & T systems effectively containedthe dissolved phase contaminant plume at most sites Contaminant concentrationsdropped rapidly as treatment progressed, but leveled off at concentrations greaterthan the Maximum Concentration Limits (MCLs) The concentrations slowlydecreased once they reached this plateau, resulting in long cleanup times Theobserved phenomena are attributed to preferential flow in areas of high permeability;low or differential desorption rates; immobile water zones within soil grains; and/orcontinuing sources of groundwater contamination Other referenced material men-tions concentrations in remaining groundwater actually rebounding when pumps areshut off Practitioners are using other aquifer restoration techniques in tandem with

P & T technology, or as alternatives, in attempts to achieve more timely cleanupgoals (Olsen and Kavanaugh 1993, pp 42ff; Sellers 1999, Chapter 3; see also: EPA

1995, 1999; Keely 1996; Palmer and Fish 1996; Wilson 1997)

Soil Vapor Extraction (SVE). Anaerobic decomposition of organics producesmethane gas, which is flammable, can accumulate to explosive concentrations, and

is toxic Deposits of hazardous waste may generate other toxic, flammable, ormalodorous vapors Prevention of dangerous buildups of such vapors is an importantaspect of hazardous waste management, in general, and site remediation, in partic-ular In earlier times, simple venting of such vapors to the atmosphere was widely

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FIGURE 11.8 Combinations of extraction and injection wells to contain a contaminant plume.

© 2001 by CRC Press LLC

FIGURE 11.9 Groundwater treatment plant layout (From U.S Air Force.)

© 2001 by CRC Press LLC

practiced These primitive practices are now prohibited by most jurisdictions andare generally unacceptable.7 Elaborate soil vapor collection and treatment systemshave been developed to meet site-specific needs, but are not always necessary.The objective of soil vapor collection and treatment systems is, of course, toprevent hazardous buildups of the gases and to render the collected gases harmless

to human health and the environment Vapors may be vented by passive collectionsystems, but forced ventilation or vacuum systems are necessary to maintain steadyflow to treatment systems The vapors are collected in pipe wells or trenches by 4-

or 6-in PVC perforated pipe If a trench or more than one well is necessary, a manifoldjoins the individual collectors and conveys the vapors to a blower The blowerdischarges to a treatment system Figure 11.10 illustrates some basic configurations.On-site treatment of extracted vapors is frequently accomplished by granularactivated carbon (GAC) adsorption of the organics contained in the removed vapors.The GAC system has the advantages and disadvantages discussed in earlier chapters.The most serious disadvantage is the declining efficiency of carbon adsorption asthe adsorptive capacity is approached Frequent or continuous regeneration orreplacement of carbon is necessary to ensure consistent high efficiency

SVE systems may also be configured to add oxygen to stimulate subsurfaceaerobic biodegradation processes thereby enhancing removal of subsurface organiccontaminants Effectiveness of SVE systems may also be enhanced with hot air or

in situ steam extraction Steam extraction facilitates the removal of moderatelyvolatile residual organics from the vadose zone (Suthersan 1997; Mercer et al 1997).On-site destruction of some vapors can be accomplished by flares or afterburners.Supplemental fuel may be necessary to achieve the desired combustion efficiencyand/or to sustain combustion (Corbitt 1990, pp 4.66ff)

Phytoremediation. The ever-intensifying search by legislators, public officials,environmentalists, scientists, regulators, industrial leaders, financiers, and many othersfor a less costly, less disruptive, less time-consuming means of remediating contam-inated sites has spawned or given new life to a variety of technologies Phytoremedi-ation appears to be a promising means of in situ treatment of contaminated soils,sediments, and surface and/or groundwater by direct use of living green plants onsites wherein immediate cleanup is not imperative The term phytoremediation encom-passes five subtechnologies, which together or singly perform the following:

• Phytotransformation is the uptake of organic and nutrient contaminantsfrom soil and groundwater and the accumulation of metabolites in planttissue In site remediation applications, it is important that the metabolitesthat are accumulated in vegetation be nontoxic or significantly less toxicthan the parent compound

• Rhizosphere bioremediation increases soil organic carbon, bacteria, andmycorrhizal fungi, which encourages degradation of organic chemicals insoil Plants may also release exudates to the soil environment, helping tostimulate the degradation of organic chemicals by inducing enzyme sys-

7 In most situations, vapor releases from RCRA facilities are subject to MACT and/or other standards L1533_frame_C11 Page 283 Tuesday, May 1, 2001 12:44 PM

FIGURE 11.10 Soil vapor extraction system (From U.S Environmental Protection Agency.)

© 2001 by CRC Press LLC

tems of existing bacterial populations, stimulating growth of new species

that are able to degrade the wastes, and/or increasing soluble substrate

concentrations for all microorganisms

• Phytostabilization is the holding of contaminated soils and sediments in

place by vegetation and immobilization of toxic contaminants in soils

Rooted vegetation prevents or inhibits windblown dust, which is an

impor-tant source of human exposure from hazardous waste sites Hydraulic

control may be achieved by the transpiration of large volumes of water,

thereby preventing migration of leachate toward ground or surface water

• Phytoextraction uses metal-accumulating plants to translocate and

con-centrate metals from the soil in roots and above-ground shoots or leaves

An important issue is whether the metals can be economically recovered

from the plant tissue or whether disposal of the waste is required

• Rhizofiltration uses plant roots to sorb, concentrate, and precipitate metal

contaminants from surface or groundwater Roots of plants are capable

of sorbing large quantities of lead and chromium from soil water or from

water that has passed through the root zone of densely growing vegetation

The potential for treatment of radionuclide contaminants is being

inves-tigated in a Department of Energy pilot project involving uranium wastes

and on water from a pond near the Chernobyl nuclear generating plant

disaster site (Schnoor 1997)

The advantages of phytoremediation are the low capital costs, aesthetic benefits,

minimization of leaching of contaminants, and soil stabilization The operational

cost of phytoremediation is also substantially less and involves mainly fertilization

and watering for maintaining plant growth In the case of heavy metals remediation,

operational costs will also include harvesting, disposal of contaminated plant mass,

and repeating the plant growth cycle

The limitations of phytoremediation are that the contaminants below rooting

depth will not be extracted and that the plant or tree may not be able to grow in the

soil at every contaminated site due to toxicity In addition, the remediation process

can take years for contaminant concentration to reach regulatory levels and thus

requires a long-term commitment to maintain the system (Suthersan 1997)

The Interstate Technology and Regulatory Cooperation Workgroup8 (ITRC)

Phytoremediation Work Team has produced a useful decision tree document for

determining suitability and effectiveness of phytoremedation at a given site (ITRC

1999) The document can be accessed browsing the EPA Office of Solid Waste and

Emergency Response (OSWER) Web site, searching the Phytoremediation Decision

Tree Appendix A provides a summary table showing applications of the five

phy-toremediation technologies to appropriate media, target contaminants, and suitable

plant species (see also: EPA 1998b: Sajwan and Ornes 1997; Sellers 1999)

8 The ITRC is a state-led, national coalition of personnel from regulatory and technology programs of

states, federal agencies, and tribal, public, and industry stakeholders.

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Treatment Methods

On-site treatment of hazardous wastes may be accomplished in situ or by excavation,

treatment, and replacement (ex situ) The EPA recently released the Ninth Edition,

Treatment Technologies for Site Cleanup — Annual Status Report, documenting the

use of the increasingly numerous treatment technologies to remediate more than 900

contaminated waste sites In remediating these sites, 32 million cubic yards of soil

were treated using in situ technology, while 10 million cubic yards were treated ex

situ (EPA 1999a) In situ methods will now be described

Low Temperature Thermal Desorption. The process uses ambient air, heat,

or mechanical agitation to increase the rate of mass transfer of contaminants to the

vapor phase Once in the vapor phase, the contaminants can be further treated by

thermal or physical methods The process can effectively remove halogenated

aro-matic and aliphatic compounds, volatile nonhalogenated compounds, and

semi-volatile nonhalogenated organics (to a limited extent) from the soil matrix (Grasso

1993) Removal efficiencies for this treatment method range to more than 90% and

primarily depend on the volatility of the contaminant (Udell 1997) In situ desorption

of organics may be accomplished by radio frequency or electrical resistance (AC)

heating, even in low permeability, clay-rich soils In sandy, more permeable

forma-tions, steam can be injected to create an advancing vapor front which displaces soil,

water, and contaminants by vaporization The organics are transported in

vapor-phase to the condensation front, where they can be pumped to the surface Injection

of moderately hot (50EC) water may serve the same purpose, provide easier

pump-ing, and has the added benefit of creating a less harsh environment for beneficial

biomass that may enhance removal of residuals (EPA 1994; see also: EPA 1995a,

1997; Cook 1996; Udell 1997; Sellers 1999)

Chemical Treatment. Liquid, gaseous, or colloidal reactive chemicals may be

applied to, or injected into, a subsurface hazardous waste deposit or a contaminated

aquifer by conventional injection wells, by permeable chemical treatment walls, or

by deep soil mixing (DSM, discussed later herein) Treatment by these techniques

can be oxidative, reductive/precipitative, or desorptive/dissolvable depending upon

the character of the wastes to be treated (Yin and Allen 1999) If treatment is to be

accomplished by injection or infiltration of an aqueous solution into a contaminated

soil or groundwater zone, it must be followed by downgradient extraction of

ground-water and elutriate and above-ground treatment and discharge or reinjection

Meth-ods for in situ treatment of organics include soil flushing, oxidation, hydrolysis, and

polymerization; methods for inorganics include precipitation, soil flushing,

oxida-tion, and reduction (Corbitt 1990, pp 9.27, 9.28; see also: Grasso 1993; Suthersan

1997, pp 222–224; Rawe 1996; Fountain 1997; Palmer and Fish 1997; Sellers 1999,

Chapters 3 and 4; Strbak 2000)

Bioremediation. Bioremediation is a managed or spontaneous process in which

microbiological processes are used to degrade or transform contaminants to less

toxic or nontoxic forms, thereby mitigating or eliminating environmental

contami-nation Microorganisms depend on nutrients and carbon to provide the energy needed

for their growth and survival Degradation of natural substances in soils and

sedi-ments provides the necessary food for the development of microbial populations in

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these media Bioremediation harnesses the natural processes by selecting or

promot-ing the enzymatic products and microbial growth necessary to convert the target

contamination to nontoxic end products (van Cawenberghe and Roote 1998)

Organic waste deposits may be seeded with soil microorganisms from other

locations or laboratories (exogenous microorganisms) to alter or destroy the wastes

Alternatively, nutrients may be added to an organic waste deposit to enhance

natu-rally occurring (or indigenous) microorganisms and cause them to more actively

consume or break down the pollutants Bioremediation has been widely acclaimed

as the hazardous waste treatment technology of the future Limitations to the

feasi-bility of bioremediation are plentiful, and its successful use requires a thorough

understanding of the on-site hydrology, microbiology, and chemical characteristics

Aerobic biodegradation processes take place in the presence of oxygen and

nutrients and result in the formation of carbon dioxide, water, and microbial cell

mass Bioventing9 may be used to provide subsurface oxygen, in the vadose or

unconsolidated zones, by circulating air with or without pumping In the saturated

zone, air sparging10 may be used to aerate the groundwater Liquid oxygen, peroxide,

or ozone injection can also be used to ensure that aerobic conditions are maintained

The literature reports that aerobic biodegradation has been successfully used to

degrade gasoline and other petroleum hydrocarbons, some VOCs, and pesticides

Aerobic treatment schemes for contaminated soils are diagrammed in Figures 11.11

Anaerobic biodegradation processes take place in the absence of oxygen and

result in the formation of methane, carbon dioxide, and cell protein Experimental

work with anaerobes continues, but the practical application thereof is limited In

most cases involving remediation of waste deposits having anaerobic conditions, the

approach has been to attempt oxygenation and conversion to aerobic conditions

Alternate electron acceptors such as nitrate or sulfate make use of existing bacterial

populations, but in both cases the end products are toxic to humans (van

Cauwen-berghe and Roote 1998; see also: Grasso 1993; Rawe and Meagher-Hartzell 1996;

Sims et al.1996; Sims, Suflita, and Russell 1996; Suthersan 1997, Chapter 5; Ward

et al 1997, pp 94–95; Sellers 1999, Chapter 3)

Natural Attenuation. As was noted in Chapter 3, chemical transformations of

TCA, TCE, and other aliphatics were shown in the early 1980s to occur in

ground-water where anaerobic bacteria were present (Vincent 1984) Perhaps the most

plentiful example of the viability of natural attenuation can be found in the thousands

of leaking underground fuel storage tank sites It has been clear for a number of

years that natural processes, in an obviously anaerobic environment, will achieve

remediation of the groundwater and unsaturated zone beneath the tank, once the

supply of leakage has stopped Knowledge of these naturally occurring chemical,

biological, and physical processes has continued to grow, giving rise to a passive

9 Bioventing uses extraction wells to circulate air with or without pumping.

10 Air sparging uses injection of air or oxygen under pressure into the saturated zone to transfer volatiles

to the unsaturated zone for biodegradation and/or to aerate and oxygenate groundwater to enhance the

rate of biological degradation.

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FIGURE 11.12 In situ bioreclamation using recharge wells or trenches (Adapted from Al W Bourquin, Bioremediation of hazardous waste, Hazardous

© 2001 by CRC Press LLC