R EM E DI AT1 NG TA R =C0N TA M I N AT E D SOI LS AT MANUFACTURED GAS
P
rior to the widespread use of natural gas, comhustihle gas m a n u f a c t u r e d from coke, coal, and oil served as the major gaseous fuel for urban heating, cooking, and lighting in the United States for nearly 100 years ( I ) . This manufactured gas, or town gas, was produced at some 1000 to 2000 plants. Pipeline distribution of natural gas following World War I1 replaced manufactured gas as the major gaseous fuel, and as a result manufactured gas production came to an end in the 1950s ( 2 ) . Today, soil and groundwater contamination problems exist at many former manufactured gas plant (MGP) sites because of prior process operations and residuals management practices ( 3 ) . Residuals that were produced in MGP processes are summarized in Table 1 for the three primary gas production methods: coal carbonization, carbureted water gas production, and oil gas production. These process residuals are d o m i n a t e d by s i x primary classes of chemicals: polycyclic aromatic hydrocarbons (PAHs), volatile aromatic compounds, phenolics, inorganic compounds of sulfur and nitrogen, and metals. Tar residuals were produced from the volatiIe component of bituminous coals in coal carbonization, from the residue of gasifying oils in oil gas processes, and from the cracking of enriching oils used to increase gas Btu content in carbureted water gas production. MGP tars are organic liquids that typically are denser than water, with a range of physical and chemi-
1
Gnaiienges cal properties dependent on the feedstock and operating conditions of the production process (3-5). Although some MGP tar was used on site or sold, during certain periods there was insufficient demand for all the tar that was produced. Further, because of changes in tar composition owing to changes in feedstock, problems with tar-water emulsions, and other factors, the intrinsic value of MGP tars was often considered marginal. Consequently, MGP tars were sometimes managed off site or were deposited on site in tar wells, sewers, nearhy pits, or streams. Nuisances associated with the disposal of tarry gasplant wastes to streams and sewers were recognized early in this century ( 6 ) ,and a committee on waste
266 A Environ. Sci. Technot., VoI. 28, No. 6. 1994
RICHARD G. LUTHY D A V I D A. DZOMBAK CATHERINE A. PETERS SUJOY B. ROY ANURADHA RAMASWAMI Carnegie Mellon University Pittsburgh, PA 15213
DAVID V. NAKLES Remediation Technologies, Inc. Pittsburgh, PA 15238
BABU R. NOTT Electric Power Research Institute Polo Alto, CA 94303
disposal was formed in 1919 at the first annual meeting of the American Gas Association to address the resulting water pollution problems (7, 8). This paper discusses remediation of MGP sites and focuses on tarcontaminated soils at such sites. Total remediation costs for individual MGP sites are in the range of tens of millions of dollars, and the Gas Research Institute has estimated that nearly 70% of such costs may he attributed to the management of tarcontaminated soils and sediments ( 9 ) .We describe the nature and extent of the tar contamination that is often present at these sites, review existing and evolving remediation strategies for management of tar contamination, and examine current understanding of tar soluhilization phenomena that determine the risk associated with tar-contaminated soils as well as the effectiveness of water-based remediation technologies. Tar contamination at MGP sites Each MGP site has unique aspects, hut common operating and waste management practices of the past have led to similar patterns of soil contamination. Evidence of these similarities has become apparent over the past several years as the Gas Research Institute (GRI),the Electric Power Research Institute (EPRI),and others have investigated the technical aspects of MGP site management. A subsurface cross section of a
001 3-936X194/0927-266A$04.50/00 1994 American Chemical Society
typical MGP site, compiled from a review of information for 25 sites (IO),is shown in Figure 1. An MGP site usually consists of a layer of fill (a mixture of soil, ash, and demolition debris) underlain by a layer of sand and gravel and a layer of silty clay or other fine-grained material. Shallow unconfined aquifers USI ally exist at MGP sites and are h draulically connected to a watt body. The groundwater table is c ten shallow, 5 to 15 feet beneath tl surface. Today, most of the structures ar the equipment at MGP sites ha1 been removed except for some su surface portions of structures th remain in the fill and the upps layer of the unconsolidated materials. These structures (e.& tar-water separator tanks), present at depths of u p to 30 feet, often contain tars mixed with demolition debris or soil. Contamination of soil with tar also occurred at MGP sites because of leaks a n d spills from on-site vessels and piping networks, incomplete separation of tar from aqueous liquids, storage of tar in unlined pits or shallow wells, and dismantling and decommissioning activities when the plant was taken out of service. In addition, tar would sometimes be mixed with other site wastes and used as fill in the low-lying areas of a plant site. Tar released into the subsurface by the above processes migrates downward as a result of gravity until it encounters a low permeability layer that it cannot penetrate because of large capillary forces [Figure I). If present in sufficient quantity the tar may pool on the lowpermeability material or move laterally, following the geologic gradient (dip) of this material ( 1 1 ) . Contact of groundwater with the tar results in dissolution of tar constituents a n d generation of contaminated groundwater plumes. Tar contamination along river banks and in the shallow sediments of rivers and lakes has also been found near MGP sites. These observations primarily reflect the discharge of tars directly into the adjacent water bodies through site sewers or ditches. Much of the tar that escaped the plants in this manner did so as incidental carryover of hydrocarbon-water emulsions from the tar separators. Migration from tar wells and subgrade gas holder tanks also contaminated some streams.
L Field workers at pile of coal lor
Process residuals f r o m the manufacture o f gas from coal, cok and oil PhystcsI farm and principal
chemical cowen1
Ash
Carburetec gas tar Coal tar Coke and L~~~ breeze Lampblack Light oils Oil tar
Spent oxide or lime, wood chips (support media) Tar sludges Tar-oil-water emulsions Wastewater treatment sludges
Aqueous liquid: Inorganics, phenolics Solid: metals (and unburned coke or coal) Organic liquid: PAHs,
Gas manuiactUrinQprocess Carbumled carbonization water gas Oil gas
Cqal
X' V
-
-
II
V
BEX4
Organic liyid: PAHs. BTEX, an phenolics Solid: pyrolyzed coal Sludge: elemental carbon and oil t a r Organic liquid: BTEX Or anic liquid: PAHs. BAX
Solid: metals. cyanide, sulfur, tar
Solid-liquid: PAHs, BTEX Aqueous and or anic liquids: PAHs, BQEX
Solids, aqueous, and organic liquids: inor anics, phenolics, PAHs. BYEX
="X" indicates that residual was produced;
X
X
indicates that residual was not produced in su
hurlmrArhnns' RTFX = henlane toluene.
ethylbenzene, and XYI
Environ. Sci. Technol.. Vol. 28. No. 6, 1994
267 A
,
."-..L
,
Generalized cross section of a typical manufactured gas plant site 300 - 500
Coal tar
disposal site
J-
Management of MGP sites While characterization of tar contamination problems at MGP sites has progressed, there have been only limited attempts at remediation. At the same time, there has been a significant amount of investigation of remediation approaches. Comprehensive reviews of potential remediation technologies for use at MGP sites have been prepared ( 1 2 141. Most of these technologies have been proposed based on their performance on comparable nonaqueous-phase liquid (NAPL) wastes from industries such as petroleum refining, wood treating, byproduct coke manufacture, and synthetic fuel production. There has been substantial laboratory a n d pilotscale research on the effectiveness of the most promising technologies for application at MGP sites. Some large-scale field trials and demonstration projects are in progress as well (see box). Such efforts are trying to address the now widely recognized technological difficulties with remediation of dense NAPLs in the subsurface. To date, full-scale site remediation activities conducted at MGP sites have been limited in scope and have emphasized the isolation and removal of source materials with management of off-site migration via groundwater pump-and-treat systems. Source materials include free-phase liquid hydrocarbons or tars, soils contaminated with tars, 268 A
c
a n d iron oxidelcyanide wastes. Source material has been isolated largely through the use of slurry walls and, in one instance, using in situ stabilization ( 1 4 ) . Source removal has included emptying of subsurface structures, excavation of heavily contaminated soils, and direct pumping of liquid tars. Removed source materials have been managed in land disposal facilities or, occasionally, as raw materials or fuel in the production of aggregate, hot-mix asphalt, or cement. Several utilities have examined limited applications of the management of source materials in utility boilers ( 1 6 ) . The groundwater treatment systems that have been installed consist primarily of recovery wells followed by treatment using hydrocarbon-water separation, air stripping, andlor carbon adsorption. Research efforts have focused on t h e development of lower cost methods for site remediation, especially alternatives to excavation and incineration, and to land disposal for treatment of tar-contaminated soils. Various ex situ treatment techniques have been investigated, including thermal desorption (17201, biological treatment in slurry reactors ( 2 1 ) , and water-based soil washing (22). In situ treatment approaches also have been investigated, not only because of potential cost effectiveness but also because contaminated soils cannot always he accessed easily due to the pres-
Environ. Sci. Technol.. Vol. 28,No. 6 , 1994
ence of surface structures, utility lines, or other physical barriers. These bench and pilot studies have emphasized in situ flushing with aqueous solvent or surfactant solutions to enhance the rate and extent of tar solubilization (10, 231 and in s i t u h i o r e m e d i a t i o n for watersoluble tar contaminants ( 2 4 ) .Techniques aimed at volatilization of organic contaminants from soils (e.g., steam stripping or vapor extraction) are not expected to be effective for removal of t h e high molecular weight, low-volatility tars encountered at MGP sites. Research on in situ methods is largely focused on removal or destruction of tar at residual saturation. Although free tar may be directly p u m p e d from s e l e c t e d locations in the subsurface (2.51, often the greatest mass of tar is that held at residual saturation by capillary forces. Residual saturations of dense NAPLs (DNAPLs) are typically 5-25% of the pore volume ( 1 1 ) . Such tar is effectively immobile and fairly insoluble, but its dissolution is sufficient to contaminate large amounts of groundwater for decades and longer. Mobilization of the residual tar phase by interfacial tension lowering (e.g., through surfactant addition] or by viscosity reduction [e.g., through heating) is a possible remediation approach, but the difficulty in controlling the downward movement of a mobilized dense organic liquid may pose an unacceptable risk (26).
tamination does not pose an unacceptable ecological or public health risk. Although contaminated but nonleachable residual may result from ex situ water-based treatment of tarcontaminated soils, tars and other DNAPLs at residual saturation may continue to leach contaminants after treatment with water-based remediation technologies. Restoration goals may be established at contaminant concentrations that are unachievable using these technologies, necessitating alternative management strategies based on source control (27). To better understand the limits of the water-based remediation technologies, researchers have studied the extent and rate of tar solubilization from contaminated soil in contact with water, and have examined the enhancement of tar solubilization that can be achieved using solvent-water and surfactant-water solutions. Collectively, the effort has provided insight into the fundamental processes governing tar dissolution, mobility, and lability in soil-water systems and bas permitted an assessment of the effects of these processes on the remediation of tar-contaminated MGP sites. In the remainder of this paper, we address dissolution phenomena in the context of in situ and ex situ remediation technologies involving contaminant extraction and biodegradation. We d o n o t attempt to address the full range of remediation approaches for such sites.
ne of the more significa yracuse, NY, with the support RI and EPRI, the utility trade late Energy Electric Research id EPA.
asphalt that was produced
I---
Limitations of water-based remediation techniques It is generally recognized that complete groundwater restoration may be technically impracticable at DNAPL-contaminated sites. In fact, this observation was a primary factor that led to the issuance of a recent guidance document by EPA for the evaluation of the technical impracticability of groundwater restoration (271. EPA acknowledges in this document that DNAPLs “often are particularly difficult to locate and remove from the subsurface” and “very long restoration time frames (e.g., longer than 100 years) may be indicative of hydrogeologic or contaminant-related constraints to remediation.” EPA notes that restoration to stringent levels mav not always he achi&able at some of these sites because of limitations of
available remediation technologies. The research efforts of EPRI, GRI, and others have confirmed that the performance of water-based remediation technologies such as extraction and biodegradation with tarc o n t a m i n a t e d soils is q u i t e dependent on the specific physical and chemical characteristics of the soil-contaminant matrix. The primary factors that limit the performance of these technologies are the extent and rate of contaminant dissolution from the DNAPL into the aqueous phase (10, 21, 28). At the same time, however, if the most water-soluble contaminants in a multicomponent DNAPL are substantially removed by contact with water, leaving only the higher molecular weight, essentially insoluble comuonents, then treatment with watM"Monogement of MonuJocturrd Gas Plant Sites: Gas Research Institute: Chicago. IL. 1987:Vol. 1. (fi) Hansm. P. American Gos Light lournal 1916.April IO. 228-29. (7) Willien. L. J. In Proceedings of the Amrricnn Gos Association; First AnI51
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