distribution and transport of coal tar-derived pahs in fine-grained residuum
TRANSCRIPT
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Chemosphere 68 (2007) 554–563
Distribution and transport of coal tar-derived PAHsin fine-grained residuum
Vijay M. Vulava a,b,*, Larry D. McKay a,b, Steven G. Driese c,Fu-Min Menn b, Gary S. Sayler b
a Department of Earth and Planetary Sciences, The University of Tennessee, Knoxville, TN 37996, USAb Center for Environmental Biotechnology, The University of Tennessee, Knoxville, TN 37996, USA
c Department of Geology, Baylor University, Waco, TX 76798-7354, USA
Received 12 October 2006; received in revised form 8 December 2006; accepted 14 December 2006Available online 15 February 2007
Abstract
We investigated the distribution and transport of coal tar-derived polycyclic aromatic hydrocarbons (PAHs) in fine-grained residuumand alluvial floodplain deposits that underlie a former manufactured gas plant. All 16 USEPA priority pollutant PAHs are present at thissite and have penetrated the entire 4–5 m thickness of clayey sediments, which unconformably overly limestone bedrock. Concentrationsof less hydrophobic PAHs (e.g., naphthalene, 0.011–384 mg kg�1) were about 10 times higher than those of highly hydrophobic PAHs(e.g., benzo[g,h, i]perylene �0.002 to 56.03 mg kg�1). Microscopic examination of thin-sections of the clay-rich sediments showed thatfractures and rootholes, which can act as pathways for flow, occur throughout the profiles. Tarry residue was found coating some frac-tures and rootholes, indicating that coal tar was, in some cases, able to penetrate as an immiscible phase. However, in the vast majority ofsamples in which PAHs were detected, there was no detectable tar residue, suggesting that much of the transport occurred in the dis-solved phase. Examination of thin-sections with an epifluorescent microscope indicated that PAHs, which fluoresce brightly whenexposed to UV light, are distributed throughout the soil matrix, rather than being confined to fractures and rootholes. The widespreaddistribution of PAHs is most likely due to diffusion-controlled exchange between the fast-flow pathways in the fractures and rootholesand the relatively immobile water in the fine-grained matrix. This implies that fractures and rootholes can play a major role in controllingtransport of highly hydrophobic compounds in fine-grained sediments, which would otherwise act as barriers to contaminant migration.� 2007 Elsevier Ltd. All rights reserved.
Keywords: Former manufactured gas plant; Hydrophobic organic compounds; Dense non-aqueous liquid; Fluorescence; Polycyclic aromatic hydro-carbons; Coal tar
1. Introduction
Coal tar, a viscous byproduct of anoxic combustion ofcoal to produce coal gas and coke, is arguably one of themost complex dense non-aqueous phase liquids (DNAPLs)found in the environment. It is composed of monocyclic,polycyclic, and heterocyclic aromatic hydrocarbons exhibit-
0045-6535/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.chemosphere.2006.12.086
* Corresponding author. Present address: College of Charleston,Department of Geology and Environmental Geosciences, 66 GeorgeStreet, Charleston, SC 29424, USA. Tel.: +1 843 953 1922; fax: +1 843 9535446.
E-mail address: [email protected] (V.M. Vulava).
ing a wide range of chemical and physical properties (Mac-kay et al., 1992; Dabestani and Ivanov, 1999; Sabljic, 2001).Extensive contamination of surface water, soil, and ground-water has occurred at many former manufactured gas plant(FMGP) sites across the country (Mueller et al., 1989). Anestimated 50000 FMGP sites exist in the US alone, resultingfrom more than 150 years of gas plant operations (Hathe-way, 1997). Coal tar is generally assumed to be relativelyimmobile in fine-grained subsurface porous media becauseof its high viscosity and interfacial surface tension com-bined with the low permeability and small average pore sizeof typical fine-grained materials. However, recent field and
Fig. 1. Location and map of Chattanooga Coke Plant and the ‘‘Clean’’site in Chattanooga, TN. Exact locations of cores extracted for this studyare marked in green triangles.
V.M. Vulava et al. / Chemosphere 68 (2007) 554–563 555
laboratory investigations in a variety of clay-rich materials,including glacial tills (McKay and Fredericia, 1995; Shawand Hendry, 1998; McKay et al., 1999), glacial lacustrinedeposits (Rodvang and Simpkins, 2001), lacustrine deposits(Rudolph et al., 1991) and in-situ weathered rock or resid-uum (Driese et al., 2001; McKay et al., 2005) show thatthese materials can in many, but not all, cases contain frac-tures, rootholes and other macropores, which act as con-duits for flow and transport of contaminants.
Only a few of the above-mentioned studies dealt withtransport of coal tar or creosote (a lighter distillate of coaltar) and they were all carried out in the laboratory. Theselab studies show that immiscible coal tar could invade frac-tures and macropores as small as a few lm in glacial claytills (Hinsby et al., 1996) while dissolved coal tar com-pounds were easily transported by advection and matrixdiffusion mechanisms in both glacial clay tills and in resid-uum derived from weathering of shale bedrock (Broholmet al., 1999; Vulava et al., 2006). These types of fine-grainedmaterials have typically very low levels of organic carbon(<1%), so there is relatively little potential for organic com-pound sorption. Although these lab studies suggest thatcoal tar compounds are likely to be readily transported atfield-scale, there have not been any field studies to confirmthis behavior.
We hypothesize that both immiscible and dissolved coaltar can be transported to substantial depths in fracturedclay-rich materials such as residuum and floodplain depos-its. Most studies of groundwater flow and contaminanttransport (e.g., Traub-Eberhard et al., 1994; Ou et al.,1999; Kay et al., 2005; Sanchez et al., 2006) do not examinesoil features or pore structure, but the authors and a fewother researchers (Vepraskas, 1994, 2001; Driese et al.,2001, 2005; McKay et al., 2005; Lenczewski et al., 2006)have found these methods particularly useful in studies offlow and contaminant transport in fractured, fine grainedmaterials. Therefore, supplementing conventional chemicalanalyses with pedogenic and lithologic investigative tech-niques, including identification of fractures, macropores,redoximorphic features, soil texture, etc., in hand samplesand with microscopes, can be used to identify likely path-ways for transport of immiscible coal tar and dissolved coaltar compounds and to help understand contaminant trans-port processes in these materials. The primary objectives ofthis study are to (i) determine whether immiscible coal tarand coal tar compounds can penetrate fractures and mac-ropores in typical clay-rich residuum and floodplain sedi-ments and (ii) provide insights into coal tar transportprocesses in fine-grained materials using pedogenic andlithologic investigative methods.
2. Materials and methods
2.1. Site description
At the Chattanooga Coke Plant (CCP) Site (formerlyknown as the Tennessee Products Site) in Chattanooga,
Tennessee, coal gas was produced by coal carbonizationfrom 1918 until 1987. Coal tar wastes generated at this sitewere disposed of on the plant site, and in the adjacentChattanooga Creek and its floodplain (EPA, 1999). Recentanalysis at this site indicated high levels of polycyclic aro-matic hydrocarbons (PAHs), pesticides, and heavy metalcontaminants in the clay-rich surface soils (EPA, 1999).In the fractured Paleozoic limestone bedrock (�4–5 mbelow soil and residuum), coal tar-related DNAPLs werediscovered during installation of groundwater monitoringwells (EPA, 1999). Groundwater samples collected fromshallow (<10 m) and deep (>10 m) wells at the site indicatehigh levels of dissolved organic and inorganic contami-nants, as well as a few samples containing DNAPLs(EPA, 1999). The source of contamination is likely bothon-site (gas and coke manufacturing) and off-site (pesti-cide, solvents, etc.) activities.
This site is located within the Appalachian Valley andRidge Physiographic province within the ChattanoogaCreek floodplain (Fig. 1). Soils in the floodplain arecomprised mainly of moderately-to-poorly drained siltyto clayey alluvial deposits and are formed on residuum ofargillaceous limestone or on floodplain deposits derivedfrom erosion and redeposition of the residuum (Jackson,1982). Typically, there is a �20 cm-thick surface layer(Ap horizon) of yellowish-brown silty loam underlain by�120–150 cm of yellowish brown clay subsoil (Bt and Chorizons). The deeper layers are commonly firm clay-richsediments which contain grayish-brown Fe/Mn mottles,dark concretions, and pedogenic slickensides. These soils
556 V.M. Vulava et al. / Chemosphere 68 (2007) 554–563
are slightly-to-strongly acidic and are low in organic con-tent (fractional organic content, fOC < 0.05%). The clayeysubsoil restricts the movement of air and water, inhibitsthe growth of extensive root systems, and contributes toa high shrink-swell potential (Jackson, 1982).
2.2. Sample Collection and physical properties
Six 5.4 cm diameter boreholes (CCP-1 through CCP-6)were advanced into the fine-grained residuum and flood-plain deposits (hereafter, referred to as residuum) to depthsof 3.5–4.5 m (top of bedrock) at the northern and centralpart of the CCP site, using direct push techniques, just out-side of the main contamination zone (see Fig. 1 for loca-tions and EPA, 1999). Continuous undisturbed soil coresamples were collected from the surface to the bedrockand stored in 3.8 cm inner diameter clear acetate liners.After samples were collected, the boreholes were sealedwith cement grout and bentonite chips.
The core samples were briefly examined in the field, cutinto �67 cm lengths, capped, sealed, and stored at 4 �Cpending laboratory analyses. These cores were sliced openlength-wise, under a fume hood, with a clean stainless steelknife. One half of each cored interval was retained forphysical lithologic and PAH analysis, and the other halfwas retained for thin-section preparation and bulk densitymeasurements. Cores CCP-2 and -4 were located close tothe previous site operations, while CCP-5 was located far-ther from the coking operations. These cores were chosenfor detailed physical and chemical analyses, because theywere expected to represent a range of contaminant expo-sures and concentrations. Lithologic descriptions includedvisible features such as fractures, macropores, Fe/Mn oxideconcretions, presence of tarry residue, odor, etc. andpedogenic features of the subsoil. Bulk density was deter-mined using the wax-coated clod method (Grossman andReinsch, 2002), while bulk porosity values were calculatedby assuming a solid mineral grain density of 2.65 g cm�3.
Soil samples from a background ‘‘clean’’ site located ona floodplain near Ooltewah, TN, were utilized as controlsfor evaluating possible pre-contaminated conditions atthe CCP site (Driese et al., 2003).
2.3. Preparation of thin-sections
Split cores selected for thin-sectioning were air-dried for2–3 weeks, coated with Hillquist� 7 A and B formula thin-section epoxy resin, mounted on glass slides and polishedto optical thickness of about 30 lm, using methodsdescribed previously (Driese et al., 2001; McKay et al.,2005). The thin sections were examined with Nikon andOlympus polarized-light microscopes equipped with epi-fluorescence (UV) attachments, to determine pore structureand the extent of migration of immiscible tar into the frac-tures and macropores.
2.4. PAH extraction and analysis
Extraction for PAHs was performed on subsamplesusing a commercially available Accelerated Solvent Extrac-tor (ASE 300, Dionex Corp., Sunnyvale, CA). The extrac-tion protocols and quality control procedures closelyfollowed USEPA Method 3545A (EPA, 2004). The finalextract was concentrated to 2 ml and stored at �20 �Cpending gas chromatograph/mass spectrometer (GC/MS)analyses. Method blanks and laboratory controls were car-ried out using the same extraction protocol but with theclean soil from Ooltewah, TN.
Sample extracts were analyzed for 16 USEPA desig-nated priority PAHs using an Agilent GC (Model 6890)equipped with MS (Model 5973N). All PAH compoundswere analyzed according to USEPA Method 8270D(EPA, 2004). The peak area for each PAH was calculatedusing the Agilent Chemstation� software. Subsequently,the concentration of individual compounds were estimatedfrom their areas under the chromatographic peaks usingthe internal standard peaks as instrument references asdescribed in USEPA Method 8270D (EPA, 2004). All mea-surements were reproducible according to specific proto-cols outlined in the USEPA method.
3. Results
3.1. Bulk soil characteristics
The upper 70–120 cm of all three cores were mainlysand-to-gravel-sized materials that were likely anthropo-genic in origin (Fig. 2). The underlying sediment is >75%clay with 10–25% quartz silt and fine sand and minoramounts of angular to subangular, 0.5–2 cm diameter chertgrains (Fig. 2). The clay matrix fabric persisted to the deep-est cored depths of 400–500 cm. The clay-rich materials inthe cores were generally very firm, often mottled with var-ious shades of brown, red, and yellow and contained Feconcretions and rare gley bands and slickensides indicatingchanges in redox potential caused by a fluctuating watertable (cf. Autin and Aslan, 2001). The primary soil colorvaried from light olive brown (2.5Y) to yellowish brown(10YR) indicating substantial oxidation of Fe-bearing min-erals (FitzPatrick, 1983; Schwertmann, 1988, 1993).
Macropores in the form of fractures and rootholes werepresent in all residuum cores, including those at greaterdepths. In CCP-4, macropores were present for nearly theentire depth of the clay interval from 122 cm to refusal at444 cm (Fig. 2). Macropores include planar fractures (upto 2 mm aperture), as well as some smaller fractures(<0.1 mm aperture) at orientations of 45� to horizontalthat often had slickenside surfaces, indicating shear move-ment, likely due to swelling and shrinking of the clays.Some of these fractures and pores had black coatings whichpenetrated up to 1–2 mm into the adjacent fine-grainedmatrix and often had a strong aromatic tar odor. The blackcoatings are likely immiscible coal tar and/or Mn oxides,
500
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0
400
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0
400
300
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100
0
30 35 40 45Matrix ColorClay Silt Sand Gravel ChertTar/MnO Odor
Macro-pores
5R 5/3 Very Dusky Red5YR 4/6 Yellowish Red
2.5YR 4/4 Olive Brown
2.5Y 5/4 Light Olive Brown
10YR 5/4 Yellowish Brown
7.5YR 5/4 Brown
10YR 5/4 Yellowish Brown
Grain Characteristics
CCP-5
CCP-4
CCP-2
2.5Y 5/4 Light Olive Brown
2.5Y 5/6 Light Olive Brown2.5Y 5/4 Light Olive Brown2.5Y 4/3 Olive Brown
2.5Y 4/3 Olive Brown2.5Y 5/4 Light Olive Brown2.5Y 4/2 Dark Grayish Brown
Dark Gray to Black2.5Y 4/3 Olive Brown5YR 3/4 Dark Reddish BrownVery Dark Gray
10YR 5/4 Yellowish Brown5YR 4/4 Reddish Brown
2.5Y 5/4 Light Olive Brown
Black2.5Y 7/4 Pale Yellow
2.5Y 3/1 Very Dark GrayBlack
2.5Y 4/4 Olive Brown2.5Y 4/2 Dark Grayish Brown
10YR 5/4 Yellowish Brown
2.5Y 5/3 Light Olive Brown
Porosity (%)
Dep
th B
elow
Gro
und
Sur
face
(cm
)
Fig. 2. Lithology and porosity versus depth profiles from three cores sampled at CCP Site. Notes: Breaks in left column (soil fraction sizes) highlighted asgray bars refer to non-recovery of cores in those depth intervals. Marker size indicate relative amount of a particular characteristic observed at that depthwith larger size indicating a greater value.
V.M. Vulava et al. / Chemosphere 68 (2007) 554–563 557
which are very similar in appearance. The aromatic tarodor suggests that at least some of the coatings weretar, but it was not possible to confirm this because of theamount of black coating was too small for chemicalanalysis.
3.2. Hydrological properties
Saturated hydraulic conductivity (Ksat) of the fine-grained sediment measured in 20 existing monitoringwells (average depth of 2.76 m and depth range of 1.61–5.45 m) using slug tests range from 2.81 · 10�7 to 1.27 ·10�3 m s�1 with a geometric mean of 1.80 · 10�5 m s�1
(EPA, 1999). The large range of Ksat values indicate a veryhigh degree of heterogeneity in the sediment and is likelyrelated to variability in the occurrence of fractures, root-holes, and chert layers. All of the measured Ksat valuesare substantially higher than values measured in othertypes of unfractured, relatively homogeneous clay-rich sedi-ments, which typically range from 10�9 to 10�10 m s�1
(McKay and Fredericia, 1995).
3.3. PAH distribution
PAHs were detected in all subsamples regardless ofdepth or lithology (Fig. 3 and Table 1). In all cases, totalPAH (tPAH) concentrations were highest in the upper200 cm of the cores and decreased as depth increased(Fig. 3). The highest measured tPAH concentrations (53,2690, and 16 mg kg�1 soil) were present at 45 cm, 95 cm,and 160 cm below ground surface (bgs) for CCP-2, -4,and -5, respectively. The highest PAH concentrationsoccurred in CCP-4, with lower values in CCP-2 andCCP-5 in that order. The higher concentrations measuredin CCP-4 may be due to differences in Coke Plant activitiesor spills in the general vicinity of this core sample.
Naphthalene was usually present at the highest concen-tration relative to other PAHs in all cores (Table 1). How-ever, the heavier PAHs (e.g., benzopyrene) were alsopresent in significant concentrations in all cores and at alldepths (Table 1). The concentrations of the 2–3 benzenering PAHs (2–3rPAH) were usually higher than those ofthe 4–6 benzene ring PAHs (4–6rPAH) except in CCP-2.
500
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100
0
500
400
300
200
100
0-2 0 2 4 -2 0 2
500
400
300
200
100
0
CCP-5
CCP-4
log[tPAH
Concentration, mg kg-1]
CCP-2
log[Ratio])
mc(ec
afru
Sd
nu
orG
wol
eB
htp
eD
Fig. 3. Total PAH concentration and ratio of 2-3rPAH and 4-6rPAHconcentrations versus depth profiles from three cores sampled at CCP Site.‘‘Ratio’’ in the right column refers to the ratio of concentrations of 2-3rPAH versus 4-6rPAH.
558 V.M. Vulava et al. / Chemosphere 68 (2007) 554–563
The ratio of 2–3rPAH and 4–6rPAH concentrations (hereonwards referred to as ‘‘Ratio’’) was usually greater thanone (Fig. 3). These Ratios range between 0.04–18, 0.77–26, and 0.08–41 for CCP-2, -4, and -5, respectively.
3.4. Thin-section analysis
Thin-sections confirm assessment of soil fabric fromresiduum cores and provide additional detail. The fill mate-rials included clinker and cinder grains, pieces of concrete,and unweathered limestone grains (Figs. 4A and B). Thesematerials did not exhibit well-developed soil microstruc-tures, but contained some relict soil features inherited fromthe original undisturbed fine-grained soil material, whichwas mixed in with the coarser grained ‘‘fill.’’ The claymatrix fabrics include fine subangular blocky peds, sepic-plasmic fabrics, and pedogenic slickensides (Fig. 4C).Redox concentration or enrichment of Fe was evidencedby reddish brown, spherical Fe-oxide (or oxyhydroxide)nodules and concretions, 0.2–2 mm diameter, which occurthroughout the soil but are especially abundant in theupper 200–300 cm, where they mainly occur embeddedwithin the clay matrix (Fig. 4D). In the upper 100–300 cm, soil macropores are stripped of Fe and Mn, withthe Fe and Mn concentrated as a hypocoating that impreg-nated the soil matrix adjacent to the macropore (Fig. 4E).Fine 10–20 lm diameter siderite and 10–100 lm sphaero-
siderite (FeCO3) crystals also occurred in some macropores(Fig. 4F).
Thin-sections were also examined under an epi-fluores-cence microscope (Fig. 5). Bright regions of yellow-greenfluorescence can be found in thin-sections up to a depthof over 4 m (Fig. 5A–D). Thin-sections prepared from soilssampled at a ‘‘clean’’ floodplain site in Ooltewah, TN werealso examined under an epi-fluorescence microscope. Plantorganic matter (roots, seeds, etc.) fluoresced brightly fol-lowing exposure to UV light in the upper soil horizon(Fig. 5E). However, in deeper B horizon soils (shallowerthan cores at CCP Site) only minimal background fluores-cence was observed due to lack of significant organic mat-ter (Fig. 5F).
4. Discussion
The high Ksat values indicated high potential for rapidmigration of various chemicals in the residuum and wereconfirmed by the presence of high concentrations of PAHat all depths and in all cores. The black coatings on macro-pore surfaces and accompanying strong tar odor indicatethat at least some of the fractures and rootholes wereinvaded by immiscible coal tar. However, these coatingswere relatively rare and immiscible transport of coal tardoes not explain the observed widespread distribution ofall 16 PAHs. This suggests that there was extensive advec-tive transport of dissolved PAHs, with possibly some trans-port of colloidal PAHs either as tar droplets, clusters of tarmacromolecules, or sorbed to other particles. However, thedissolved phase transport was likely dominant because itbest explains the observed spreading of PAHs into the finematrix pore structure, where most colloids other than thosethat are very small are excluded. The spreading of coal tarcompounds throughout the fine-grained matrix is likelydue to diffusion of solute-phase PAHs from the fast flowpathways in the fractures and macropores. This process,which is often referred to as matrix diffusion, has beenobserved in experimental studies of transport of other typesof dissolved contaminants in fractured, fine-grained materi-als (McKay et al., 1993, 1997; Jørgensen et al., 1998; Lenc-zewski et al., 2006). Small tar colloids can also potentiallydiffuse into the fine-grained matrix, facilitated by presenceof dissolved humic substances in groundwater. The generaldecreasing concentration trend with depth is indicative ofattenuation of PAHs as they were vertically transporteddue to sorption, matrix diffusion, or degradation processes.
In two cores (CCP-4 and -5), the Ratios also increasedslightly with depth indicating enrichment of 2-3rPAHsalong the flow path, while no clear trends were observedfor CCP-2. Potentially several mechanisms influence varia-tion in these Ratios with depth. For instance, (i) variabilityof the subsurface coal tar source as a function of time – ascoal tar ages, it is expected to lose the more volatile and sol-uble compounds which are then preferentially transported(Mackay et al., 1992; Lide, 2003), (ii) chromatographic sep-aration of coal tar compounds as determined by sorption
Table 1Concentration profiles of 16 PAHs as a function of depth in soils subsampled from three cores
Depth Naph Acen1 Acen2 Fluor1 Phen Anthr Fluor2 Pyr B[a]An Chrys B[b]Fl B[k]Fl B[a]Py Ind[]Py D[]An B[]Pe
CCP-245 18.35 1.338 0.979 1.026 1.912 1.275 3.234 3.255 3.127 2.512 2.304 0.811 2.933 3.388 0.837 3.08875 3.271 0.118 0.184 0.128 0.511 0.185 0.64 0.638 0.388 0.503 0.462 0.162 0.572 0.628 0.133 0.56881 8.317 0.185 0.444 0.304 1.304 0.382 1.516 1.647 2.417 1.099 1.022 0.342 0.962 0.89 0.18 0.742
134 0.305 0.176 0.35 0.275 0.045 0.315 0.992 1.011 3.237 1.251 1.164 0.389 0.332 0.168 0.041 0.135148 2.58 1.044 2.156 1.357 0.199 1.758 2.509 3.837 2.534 0.624 2.519 0.932 2 1.38 0.293 0.804168 1.573 0.61 0.96 0.613 0 0.804 2.882 3.412 2.686 1.361 0 0 0.909 0.522 0.115 0.342254 0.329 0.355 0.537 0.402 0 0.473 2.687 2.983 4.217 1.074 0 0 0.684 0.306 0.078 0.254280 0.076 0.014 0.016 0.019 0.037 0.008 0.033 0.036 1.91 1.241 0.983 0.549 0.013 0.011 0.007 0.007301 0.46 0.008 0.014 0.015 0.036 0.004 0.004 0.002 0.008 0 0.004 0.002 0.002 0.005 0.005 0.002325 0.011 0.002 0.009 0.004 0.01 0 0.001 0.002 0.005 0 0.002 0.001 0.001 0.003 0.003 0.002345 0.035 0.009 0.01 0.014 0.047 0 0.004 0.002 0.532 0.335 0.005 0.008 0.003 0.017 0.018 0.008365 0.096 0.023 0.015 0.037 0.111 0 0.008 0.004 0.602 0.337 0.005 0.004 0.003 0.005 0.01 0.003
CCP-44 15.56 1.305 0.168 0.276 3.117 1.185 3.184 2.604 2.203 3.324 2.352 0.77 2.601 2.91 0.505 2.493
33 120.2 11.8 2.692 4.166 12.52 9.257 7.292 9.713 5.469 8.434 6.051 1.881 8.742 10.93 2.271 9.00241 359 58.6 11.16 12.05 36.2 27.79 37.08 73.48 50.32 22.2 14.63 6.094 47.75 37.27 10.36 25.4870 13.38 0.665 0.722 1.066 2.79 0 1.842 1.606 0.639 0.938 0.682 0.201 0.638 0.511 0.126 0.49780 208 4.882 11.58 19.39 40.69 15.34 42.43 24.14 13.18 15.77 11.16 3.654 11.99 10.34 1.819 8.49895 384.1 44.47 24.49 98.09 413 133.9 259.1 274.8 209 186.7 132.1 43.24 206.2 91.41 18.34 54.92
125 131.3 24.82 10 50.19 314.1 0 224.8 155.8 148.6 109.2 79.41 23.42 123.9 62.35 13.04 56.03149 143.5 2.21 2.34 15.23 21.4 0 10.77 6.902 4.442 4.055 2.948 0.87 3.424 1.993 0.365 1.702168 45.41 0.906 0.904 6.278 3.855 0 1.384 1.436 0.579 0.826 0.544 0.227 0.429 0.242 0.048 0.222199 18.6 0.902 0.718 6.588 3.26 0 1.307 1.503 0.519 0.741 0.496 0.197 0.376 0.239 0.044 0.203248 19.47 0.519 0.44 3.435 1.873 0.594 0 0.843 0.288 0.425 0.284 0.113 0.19 0.095 0.054 0.102268 3.136 0.226 0.198 1.751 1.025 0 0.618 0.383 0.221 0.229 0.161 0.074 0.132 0.066 0.014 0.063298 4.134 0.084 0.108 0.505 0.388 0.137 0.151 0.184 0.083 0.104 0.077 0.031 0.058 0.036 0.013 0.03318 1.569 0.064 0.107 0.755 0.396 0.213 0.171 0.157 0.115 0.174 0.126 0.053 0.062 0.033 0.008 0.029343 0.281 0.026 0.044 0.134 0.096 0.036 0.047 0.047 0.026 0.033 0.024 0.01 0.019 0.009 0.004 0.011361 0.141 0.005 0.007 0.004 0.016 0.007 0.019 0.017 0.015 0.022 0.014 0.008 0.013 0.01 0.001 0.01391 0.219 0.005 0.009 0.004 0.011 0.003 0.017 0.02 0.014 0.017 0.013 0.005 0.008 0.007 0.002 0.006437 0.224 0.005 0.013 0.013 0.05 0.006 0.008 0.006 0.007 0 0.002 0.001 0.001 0.001 0 0.001
(continued on next page)
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Dep
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560 V.M. Vulava et al. / Chemosphere 68 (2007) 554–563
rates for different PAHs within the porous media – high 4–6rPAHs compounds are expected to sorb more strongly tosoil minerals than 2–3rPAHs and, hence, are more stronglyretarded during transport (Allen-King et al., 1996), (iii) dif-ferences in matrix diffusion for different compounds – smal-ler compounds are expected to diffuse faster than largercompounds (Parker et al., 1996), and (iv) preferential micro-bial degradation of some 2–3rPAHs (Cerniglia, 1993). Thefirst three mechanisms would result in an increase in Ratios
with depth while the fourth mechanism would result in adecrease. All four mechanisms are expected to occursimultaneously in natural environments and it is difficult
Fig. 4. Thin-section photomicrographs showing anthropogenic and ped-ogenic features of soil cores from Chattanooga Coke plant site. (A) CCP-2(92–96 cm), coal cinder grains embedded in remolded ‘‘fill’’ material;Plane-polarized light (PPL). (B) CCP-2 (92–96 cm), clinker/slag grains andFe oxide concretions embedded in remolded ‘‘fill’’ material; PPL. (C)CCP-4 (427–437 cm), soil ped bounded by slickensides and clay matrixwith birefringence fabric. Crossed-polarized light (x-nicols). (D) CCP-4(200–206 cm), zoned Fe oxide concretion in ‘‘native’’ soil matrix; PPL (E)CCP-4 (413–424 cm), root macropore with Mn oxide or coal tar coating/hypocoating, and Fe concretions in soil matrix; PPL and (F) CCP-4 (135–144 cm), very fine siderite crystals concentrated within soil macropore;PPL.
Fig. 5. Photomicrographs of thin-sections from various depths in soil core CCP-4 and from background ‘‘clean’’ site in Ooltewah, TN. On the left paneltotal PAH concentrations are plotted. On the right, thin-section photomicrographs are viewed under blue-green UV light and under plane-polarized light.Bright yellow areas under blue-green UV light indicate presence of aromatic hydrocarbons (A–D). Sphaerosiderite crystals (small black spheres) are visiblein (A), (C), and (D). Brown patches of Fe–Mn stains are present in B. A small contaminant ‘‘bubble’’ is visible in center of (C) and black mass is visible onleft side in (D). Organic root tissues fluoresce in the A horizon soil (E) while no significant fluorescence occurs in the deeper Btg horizon soil (F). Note:Yellow bars in the thin-section photomicrographs indicate a scale of 1 mm.
V.M. Vulava et al. / Chemosphere 68 (2007) 554–563 561
to attribute one specific mechanism as being dominant with-out further controlled experimentation.
Epi-fluorescence in thin-sections helped visually inter-pret contaminant distribution which helped confirm theimportance of diffusion as a transport process. Naturalfluorescence was observed in the presence of organic matterin background soils (Fig. 5E), but the contaminated soils
from CCP site are inherently very low in organic matter(fOC < 0.05%), especially at depths greater than 100 cm(Jackson, 1982; McCarthy et al., 2000). Low molecularweight (LMW) PAHs present in coal tar are known to fluo-resce brightly and hence this property is often exploited intheir detection (Fetzer and Kershaw, 1995; Miller, 1999;Song et al., 2006). Hence, the bright fluorescence in the
562 V.M. Vulava et al. / Chemosphere 68 (2007) 554–563
matrix is most likely caused by the presence of LMWPAHs and occurs within the matrix and along fracturesand macropores (Fig. 5). These compounds also fluorescedin a previous lab-scale study of coal tar transport in clay-rich residual soil (Vulava et al., 2006). Some dark non-fluorescent regions were also present in the thin-sectionsimmediately adjacent to fluorescent matrix (Fig. 5D). Someconstituents of coal tar (e.g., pitch) did not fluoresce in pre-vious studies (Vulava et al., 2006). However, there was noapparent correlation between PAH concentration andbrightness (Fig. 5). As thin-sections are prepared from avery small section of the core sample, they may not repre-sent the bulk sample from which PAHs were extracted.
5. Conclusions
Coal tar derived-PAHs were present throughout theentire 4–5 m thick deposit of fine-grained residuum under-lying the FMGP site in Chattanooga, TN implying that thepresence of fine-grained materials did not inhibit down-ward migration of coal tar solutes. The PAH data suggeststhat the presence of high PAH concentrations in the matrixindicates dissolution and subsequent transport throughboth macropores and the matrix. Visual and lithologicalevidence suggests that coal tar may have penetrated somemacropores as an immiscible phase, but this may not havebeen the primary pathway for coal tar transport. It is likelythat constituents of coal tar may have dissolved and weretransported advectively through the residuum. This studyclearly indicates that fine-grained subsurface materials can-not be assumed, a priori, to provide an effective barrier todownward migration of coal tar compounds at FMGPsites. High concentrations of these compounds are expectedto slowly leach out of the fine-grained sediments and couldact as a long-term source of secondary contamination tothe soils and the underlying bedrock.
Acknowledgements
We thank Jeremy Bennett, Jim Easter, and SyreetaDickerson for their laboratory assistance. Funding for thisresearch was provided by the Waste Management Researchand Education Institute at the University of Tennessee,Knoxville (UTK). We thank Tennessee Department ofEnvironment and Conservation (TDEC) for providing ac-cess to this site.
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