Journal of Contaminant Hydrology 126 (2011) 181–194
Contents lists available at SciVerse ScienceDirect
Journal of Contaminant Hydrology
journal homepage: www.elsevier.com/locate/jconhyd
Review Article
Importance of heterocylic aromatic compounds in monitored natural
attenuation for coal tar contaminated aquifers: A review
Philipp Blum a,⁎, Anne Sagner b, Andreas Tiehm b, Peter Martus c,
Thomas Wendel d, Peter Grathwohl d
a
b
c
d
Karlsruhe Institute of Technology (KIT), Institute for Applied Geosciences (AGW), Kaiserstraße 12, 76131 Karlsruhe, Germany
Water Technology Center (TZW), Environmental Biotechnology, Karlsruher Straße 84, Karlsruhe, Germany
URS Deutschland GmbH, Sutelstraße 2, 30659 Hannover, Germany
University of Tübingen, Center for Applied Geoscience (ZAG), Sigwartstraße 10, 72076 Tübingen, Germany
a r t i c l e
i n f o
Article history:
Received 21 March 2011
Received in revised form 12 August 2011
Accepted 15 August 2011
Available online 22 August 2011
Keywords:
Natural attenuation
Coal tar creosote
Plume lengths
Biodegradation
BTEX
PAH
NSO heterocycles
a b s t r a c t
NSO heterocycles (HET) are typical constituents of coal tars. However, HET are not yet routinely monitored, although HET are relatively toxic coal tar constituents. The main objectives of the
study is therefore to review previous studies and to analyse HET at coal tar polluted sites in
order to assess the relevance of HET as part of monitored natural attenuation (MNA) or any
other long-term monitoring programme. Hence, natural attenuation of typical HET (indole,
quinoline, carbazole, acridine, methylquinolines, thiophene, benzothiophene, dibenzothiophene, benzofuran, dibenzofuran, methylbenzofurans, dimethylbenzofurans and xanthene)
were studied at three different field sites in Germany. Compound-specific plume lengths
were determined for all main contaminant groups (BTEX, PAH and HET). The results show
that the observed plume lengths are site-specific and are above 250 m, but less than 1000 m.
The latter, i.e. the upper limit, however mainly depends on the level of investigation, the considered compound, the lowest measured concentration and/or the achieved compoundspecific detection limit and therefore cannot be unequivocally defined. All downstream contaminant plumes exhibited HET concentrations above typical PAH concentrations indicating
that some HET are generally persistent towards biodegradation compared to other coal tar
constituents, which results in comparatively increased field-derived half-lives of HET. Additionally, this study provides a review on physicochemical and toxicological parameters of
HET. For three well investigated sites in Germany, the biodegradation of HET is quantified
using the centre line method (CLM) for the evaluation of bulk attenuation rate constants.
The results of the present and previous studies suggest that implementation of a comprehensive monitoring programme for heterocyclic aromatic compounds is relevant at sites, if MNA is
considered in risk assessment and for remediation.
© 2011 Elsevier B.V. All rights reserved.
Contents
1.
2.
3.
Introduction . . . . . . . . . . . . . . . . . . . . . . . .
Physicochemical properties and toxicology of NSO heterocycles
Determination of natural attenuation at 3 field sites . . . . . .
3.1.
Field sites . . . . . . . . . . . . . . . . . . . . . .
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⁎ Corresponding author at: Karlsruhe Institute of Technology (KIT), Germany. Tel.: +49 721 608 47612; fax: +49 721 606 279.
E-mail address: philipp.blum@kit.edu (P. Blum).
0169-7722/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jconhyd.2011.08.004
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182
183
185
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182
P. Blum et al. / Journal of Contaminant Hydrology 126 (2011) 181–194
3.1.1.
Hamburg (North Germany) . . . . . . . . . . . . . .
3.1.2.
‘Testfeld Süd’ (South Germany) . . . . . . . . . . . .
3.1.3.
Karlsruhe (South-West Germany) . . . . . . . . . . .
3.2.
Analytical methods . . . . . . . . . . . . . . . . . . . . . .
3.3.
Comparison of the compound-specific plume lengths . . . . . .
3.4.
Comparison of compound-specific bulk attenuation rate constants
4.
Monitored natural attenuation of NSO heterocylic aromatic compounds .
5.
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction
An enormous number of coal tar or creosote contaminated
sites exist worldwide, which are generally related to former
wood impregnation plants, manufactured-gas plants (MGP)
and waste disposal sites. In the USA, for example, up to 2000
MGP represent a significant environmental legacy (Landmeyer
et al., 1998). In Germany, more than 1400 coal tar contaminated sites were identified and only 20% have been seriously considered for remediation so far (KORA, 2008). These sites are
usually contaminated with monoaromatic hydrocarbons
(MAH such as BTEX), polycyclic aromatic hydrocarbons
(PAH), as well as phenolic and heterocyclic aromatic compounds containing nitrogen, sulfur or oxygen. The latter, NSO
heterocycles (HET), however, are currently not routinely monitored (e.g. Neuhauser et al., 2009), although up to 40% of
the water-soluble fraction of the coal tar consists of HET
(Licht et al., 1996). In addition, HET are generally among the
most toxic coal tar constituents (e.g. Eastmond et al., 1984;
Eisentraeger et al., 2008; Hartnik et al., 2007; Johansen et al.,
1998; Neuwoehner et al., 2009; Sagner et al., 2006; Seymour
et al. 1997; Tiehm and Sagner, 2008). For example, Eisentraeger et al. (2008) showed that some HET such as quinoline, 6methylquinoline and xanthene are mutagenic using the Salmonella/microsome test.
Biodegradation of HET and their metabolites was studied
on various scales from laboratory to field scale (e.g. Broholm
and Arvin, 2001; Safinowski et al., 2006; Sagner, 2009; Tiehm
et al., 2008). Several laboratory experiments were carried out
to study the biodegradability of HET and their transformation
products under different redox conditions using contaminated groundwater and soils (e.g. Althoff et al., 2001; Dyreborg
et al., 1996, 1997; Gai et al., 2008; Mundt et al., 2003). Several
studies could demonstrate that HET can have an inhibitory
effect on the biodegradation of creosote compounds such as
PAH and benzene (e.g. Dyreborg et al., 1996, 1997; Meyer
and Steinhart, 2000). Dyreborg et al. (1997), for example,
studied various HET under four redox conditions (aerobic,
denitrifying, sulphate-reducing, and methanogenic) over a
period of more than 2 years using laboratory batch microcosms. They generally concluded that anaerobic degradation
for HET was significantly slower than aerobic degradation.
At three creosote contaminated sites in Denmark,
Broholm and Arvin (2001) showed that some HET compounds
such as thiophene and benzothiophene may also persist
in very low concentrations throughout the contaminant
plume. Safinowski et al. (2006) investigated the anaerobic
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185
185
185
186
187
187
190
192
192
192
degradation of some PAH and HET using laboratory and field
studies at a site in South Germany (‘Testfeld Süd’), which is
also considered in this study. Both data from laboratory and
field indicate that biodegradation of PAH and HET appears to
be limited not only by the availability of electron donors, electron acceptors, and appropriate bacterial strains, but also by the
existence of potential inhibitors. The study provides an example for anaerobic cometabolic transformation of PAH and HET.
Reineke et al. (2007) studied the N-heterocycles quinoline
and isoquinoline with their hydroxylated and hydrogenated
metabolites at a field site in Germany (Castrop-Rauxel). They
showed that the ratio of hydroxylated to parent compound
can be used as potential indicator of in situ biodegradation.
However, the proposed ratio still has to be tested for other
coal tar contaminated sites.
Although detailed investigated field studies on the spatial
distribution and biodegradation of HET mainly located in
Denmark, Germany and Canada were conducted (e.g. Blum
et al., 2008; D'Affonseca et al., 2008; Fraser et al., 2008; Herold
et al. 2009; Johansen et al., 1997; Pereira and Rostad, 1986;
Reineke, 2008), only few field studies exist worldwide (e.g.
Hamburg, ‘Testfeld Süd’, Karlsruhe; D'Affonseca et al., 2011;
Griebler et al., 2004; Tiehm and Schulze, 2003), where physicochemical properties, biodegradation, toxicity, propagation
and long term risk assessment of HET are available and were
comprehensively considered in context of MNA. The latter is
part of the present review. Blotevogel et al. (2006, 2008) suggested an assessment procedure for the identification of HET
priority substances by including physicochemical properties,
biodegradability, toxicity and groundwater propagation. They
considered concentrations of HET at three different control
planes at five different sites in Germany and one in Denmark
(Johansen et al., 1997); two of these sites are also considered
here (Karlsruhe and ‘Testfeld Süd’). The main objective of
their studies was to derive a priority list of HET. However, the
suggested list of key compounds varies and is predominantly
based on substance-specific properties rather than site-specific
observations (Kern et al., 2008). A comprehensive synopsis
and review of toxicology, persistence, spreading and quantification of biodegradation of HET in the field is not yet available.
The main objective of the current study is to review existing
data on physicochemical and toxicological properties of HET
and also to include aspects on quantification of biodegradation
and long-term propagation of HET, and to discuss the importance of assessing and monitoring HET for a successful implementation of MNA in coal tar contaminated aquifers.
Furthermore, compound-specific plume lengths for all main
P. Blum et al. / Journal of Contaminant Hydrology 126 (2011) 181–194
contaminant groups (BTEX, PAH and HET) and quantification of
bulk attenuation rate constants of HET using the centre line
method (CLM) are analysed in the present study on three representative field sites in Germany (Hamburg, ‘Testfeld Süd’ and
Karlsruhe).
2. Physicochemical properties and toxicology of NSO
heterocycles
For the assessment and discussion on the implementation of
MNA for HET, the most relevant transport parameters such as
molecular structure, molecular weight (MW), aqueous solubility
(Sw) and hydrophobicity using octanol–water partition coefficients (Kow) are provided for 14 frequently analyzed HET
(Table 1; Fig. 1).
Parameters of other typical coal tar constituents such as benzene, phenol and naphthalene are also provided for comparison.
These parameters may differ significantly depending on chosen
reference or database (e.g. Blotevogel et al., 2008; Melber et al.,
2004). For example, the aqueous solubility of benzofuran varies
according to Melber et al. (2004), which refer to the Chemfinder
database, between 100 and 1000 mg/l at 18 °C. In general, aqueous solubilities of HET are much or even orders of magnitude
higher than the equivalent PAH and/or BTEX, which is mainly
caused by the increased polarity of the HET due to the heteroatoms in the molecules. For example, the aqueous solubility of
quinoline is more than two orders of magnitude higher than
the solubility of naphthalene and that of pyrrole is 33 times
higher than of benzene (Table 1).
For the fate of HET the knowledge on their sorption
behaviour is important (e.g. Bi et al., 2006, 2007, 2009;
Broholm et al. 1999, 2000; Hassett et al., 1980; Reineke
et al., 2007). Using the approach of soil column chromatography, Bi et al. (2007) studied the effects of several environmental factors such as temperature, ionic strength,
and dissolved/sorbed ion composition on the sorption of HET.
Bi et al. (2006) could demonstrate that the latter can be described by a conceptual sorption model, which includes partitioning to soil organic matter, cation exchange and an
additional sorption process (most likely surface complexation
of neutral species). For the non-linear sorption of O- and SHET partitioning to soil organic matter is the dominant sorption mechanism (Bi et al., 2007). Hence, organic carbon normalized partition coefficients (Koc), which are well correlated
with octanol–water partition coefficients (Kow) or the inverse
of their aqueous solubility (e.g. Razzaque and Grathwohl,
2008), are provided in Table 1. Koc is often also predicted
using the linear free energy relationships (LFERs). However, a
sorbate-specific linear multiple regression analysis is required
for the development of a polyparameter LFERs (e.g. Endo
et al., 2009). Thus, for the current study distribution coefficients (Kd) were determined based on organic carbon contents (foc) and listed in Table 2. Although sorption is
crucial for understanding contaminant transport, it is however not relevant under steady state conditions and thus
not considered an efficient process for NA. Thus, sorption
is not particularly evaluated for the so-called three lines of
evidence approach, which for example in the USA requires
(1) an observed decrease in contaminant concentrations
and mass, (2) geochemical indicators and (3) evidences
for microbial activity to demonstrate active biodegradation.
183
This approach is used in several countries such as USA, UK,
Netherlands and Denmark (Rügner et al., 2006). Hence,
sorption of pollutants results in retardation, but not in a decrease in pollutant mass, and therefore is not considered an
efficient process for MNA.
More critical for the acceptance of MNA and the identification
of suitable HET key pollutants is an understanding of the toxicological effects of these heterocyclic aromatic compounds in ecosystems and towards humans. For example, Hartnik et al. (2007)
found that PAH, which comprise up to 85% of pure creosote, account for only 13% of total toxicity in the creosote-contaminated
groundwater, while methylated benzenes, phenolics (e.g. 2,6Dimethylphenol) and N-HET accounted for approximately 80%
of the total measured toxicity. Eisentraeger et al. (2008) demonstrated that S-HET are more toxic than N- and O-HET and that
quinoline, 6-methylquinoline and xanthene are even mutagenic
using the Ames fluctuation test. Recently, it was demonstrated
that the O-heterocycles benzofuran and 2,3-dimethylbenzofuran also have a dioxin-like activity in two bioassays (Hinger
et al., 2011). Neuwoehner et al. (2009) studied the ecotoxicity
of quinoline and its metabolites using genotoxicity (SOS-Chromotest) and also Ames fluctuation tests. They confirmed the previous results that quinoline shows genotoxicity (potentially
mutagenic or carcinogenic). However, the hydroxylation of
quinoline, which might be a product of biodegradation, leads
to a detoxification of the genotoxic potential (Neuwoehner
et al., 2009). Tiehm et al. (2008) demonstrated that BTEX, PAH,
and HET present in polluted groundwater could be microbiologically degraded in an aerobic bioreactor. Biodegradation of the
pollutants corresponded to the reduction of groundwater toxicity in the Vibrio fischeri test.
To provide an overview of the toxic effects of specific HET
in more detail, the results of three toxicity bioassays from
previously performed studies, using the ubiquitous soil bacterium Pseudomonas fluorescens, the marine bacterium V.
fischeri and the planktonic crustacean Daphnia (water flea)
are reviewed here (Bundy et al., 2001; Eisentraeger et al.,
2008; Hartnik et al., 2007). The outcome of these bioassays
is an effective concentration (EC), which is a rough estimate
of ecotoxicity of these compounds. For example, the median
EC50 value is the concentration of a chemical that is required
to result in a 50% effect on the studied organism. Thus, the
smaller the EC value, the higher the toxicity of the studied
compound. The results of the toxicity tests are summarised
in Table 1. Comparisons between the various studied tests
show generally very good agreement, although EC50 and
EC80 values cannot be directly compared with each other.
The highest correlation was found between the Microtox
(V. fischeri) (Hartnik et al., 2007) and Daphnia magna immobilization tests (Eisentraeger et al., 2008) with a correlation
coefficient of 0.98 for the 7 pairs (Table 1). Furthermore,
out of 14 studied HET dibenzothiophene and dibenzofuran
are the most ecotoxic HET in all three examined toxicity
tests (Fig. 2). A high ecotoxicity was also found for methyldibenzofuran using the program EPI Suite™ from the U.S. EPA
with a EC50 value of 0.45 mg/L (KORA, 2008). Although, for
naphthalene very low EC50 values were determined in comparison to their equivalent HET such as quinoline, 1benzothiophene and benzofuran, N-hetereocyclics still accounts for around 80% of the total measured toxicity in the
creosote-contaminated groundwater (Hartnik et al., 2007;
184
P. Blum et al. / Journal of Contaminant Hydrology 126 (2011) 181–194
QSAR regression developed by Bundy et al. (2001) it is however not possible to adequately predict the toxicity of HET by
log Kow (Fig. 1), although a correlation between log Kow and
EC80 values exists (Fig. 2).
Loibner et al., 2004). Quantitative structure–activity relationship (QSAR) models are also used to predict toxicity, carcinogenicity and biodegradability of HET (e.g. Bundy et al.,
2001; Philipp et al., 2007; Zhang et al., 1992). Using the
Table 1
Physicochemical and three effective concentrations (EC) based three different toxicity bioassays for the 14 studied NSO heterocyclic compounds and five other
typical organic compounds of coal tars (benzene, ethlybenzene, phenol, naphthalene and acenaphthene).
Aqueous solubility at
25 °C (mg/L)
log
Kow
EC50 (Vibrio fischeri)
15 min (mg/L)
EC50
(Daphnia)
(mg/L)c
EC80
(P. fluorescens)
(mg/L)d
67.1
58,800b
0.75b
–
–
4707
Indole
117.0
1875e
2.00e
2.39f
1.3
172
Quinoline
129.2
6718a
2.03e
31.8d
14.7
115
Carbazole
167.2
1.2g
3.72g
11.57i
3.4
–
Acridine
179.2
46.6g
3.48g
7.47i
4.6
27
6-Methylquinoline
143.2
631h
2.57h
9.03i
–
–
84.1
3600j
1.81j
–
–
753
1-Benzothiophene
134.2
130e
3.12e
–
n.d.
59
Dibenzothiophene
184.3
4.38e
0.12i
0.2
0.13
Benzofuran
118.1
2.67e
3.49f
2.1
174
Dibenzofuran
168.2
4.12e
1.09i
0.6
1.10
2-Methylbenzofuran
132.2
3.22h
–
7.4
–
2,3-Dimethylbenzofuran
146.2
62.2j
3.63j
–
6.2
–
Xanthene
182.2
1.0h
4.23h
–
2.3
–
Benzene
78.1
1780b
2.12b
102.78i
–
773
106.2
152b
3.13b
9.69i
–
–
94.1
81,300e
1.46e
25.61l
–
–
Group
Compound
N-HET
Pyrrole
S-HET
O-HET
BTEX
Thiophene
Ethylbenzene
Molecular
structure
Molecular weight
(g/mol)a,b
1.5e
678k
4.8e
160h
PKX
Phenol
PAH
Naphthalene
128.2
31.7b
3.37b
0.93i
1.89m
–
n.d.
Acenaphthene
154.2
3.9b
3.39b
0.74i
–
–
P. Blum et al. / Journal of Contaminant Hydrology 126 (2011) 181–194
3. Determination of natural attenuation at 3 field sites
In the current study three well investigated coal tar contaminated sites were selected in Germany (Hamburg, ‘Testfeld Süd’
and Karlsruhe), which were previously investigated in great detail and where HET were analysed. In addition to these previous
studies, compound-specific plume lengths and bulk attenuation
rate constants were evaluated as part of the current study.
Table 2 summarises site-specific data such as operation time, average groundwater velocities, hydrogeological parameters,
source widths and detected redox conditions.
185
sampling campaigns over a time period of more than
2 years
(31.10.2003,
08.06.2004,
22.11.2004
and
16.06.2005) along the centre line of the plume. The measured
organic compounds demonstrate similar decreasing concentrations with increasing distance to the source. The phenol
concentrations along the central flow path decrease by a factor of 420, PAH by a factor of 330 and BTEX by a factor of 80.
Importantly, the repetitive measurements indicate that the
present contaminant plume is at steady-state and that the
present biodegradation is sufficient to compensate the continuous contaminant mass flux from the source across control planes (Blum et al. 2009; D'Affonseca et al., 2008).
3.1. Field sites
3.1.1. Hamburg (North Germany)
The Hamburg field site, which is a former wood impregnation
plant, is located 40 km south of Hamburg, Germany. The operation, which closed in 1986, resulted in a coal tar contamination
of the unsaturated and saturated zones down to 50 m depth
(Table 2). The geology is mainly comprised of glacial sands
with a local thickness of approximately 130 m separated into
an upper and a lower aquifer. The regional upper aquifer can
be subdivided in four different sediments: (1) medium to coarse
sands from 63 m up to 43 m above sea level (asl); (2) fine sands
from 43 m up to 23 m asl; (3) coarse to medium sands from
23 m up to 14 m asl and (4) silty fine sands from 14 m asl. The
groundwater level in the upper unconfined aquifer is between
44 m and 45 m asl. The lower confined aquifer, which is not impacted by the contamination, is formed mainly by silt and sand
deposits.
Based on detailed site investigation campaigns a comprehensive three-dimensional conceptual sedimentological,
hydrogeological and hydrogeochemical site model could be
developed (Blum et al. 2009; D'Affonseca et al., 2008) and a
reactive transport model was performed, which also includes
redox-dependent carbon isotope fractionation (D'Affonseca
et al., 2011). The average hydraulic conductivity of the
lower part of the upper aquifer (medium to coarse sands between 40 and 49 mbs), in which the contaminant plume is
predominantly located, was 4.2 × 10 − 4 m/s. The average
seepage velocity in this layer is approximately 90 m/a using
an average observed hydraulic gradient of 0.0015 and an average effective porosity of 21% (Blum et al., 2009). The contaminant plume of the most soluble PAH, i.e. naphthalene,
is approximately 150 m wide and more than 450 m long.
The longitudinal spreading of the organic compounds was
studied using mean concentrations of four groundwater
Note to Table 1:
n.d. = not detectable.
a
Mueller et al. (1989).
b
Melber et al. (2004).
c
Eisentraeger et al. (2008).
d
Bundy et al. (2001).
e
Broholm et al. (2000).
f
Kaiser and Palabrica (1991).
g
Johansen et al. (1997).
h
Meylan and Boethling (1996).
i
Hartnik et al. (2007).
j
Verschueren (1996).
k
Blotevogel et al. (2008).
l
Ghosh and Doctor (1992).
m
Loibner et al. (2004).
3.1.2. ‘Testfeld Süd’ (South Germany)
The second field site representing a former gaswork, which
operated for almost 100 years (Table 2), caused a substantial
subsurface contamination (e.g. Zamfirescu and Grathwohl,
2001). The shallow Quaternary aquifer with a thickness of
around 3.5 m is extremely heterogeneous and mainly consists
of sand and gravel. The aquifer is underlined by a deeper fractured aquifer system, which locally discharges to the shallow
aquifer. The average hydraulic gradient and hydraulic conductivity are 0.002 and 1.7×10− 3 m/s, respectively. Assuming an average effective porosity of 15%, the average groundwater velocity
results in 2.0 m/day (Table 2).
Herold et al. (2009) studied the contaminant mass flow
rates by a series of integral pumping tests (IPTs) and in particular
for the main contaminants of concern such methylbenzofurans,
dimethylbenzofurans and acenaphthene. They found that the
highest mass flow rates at the most distal control plane were determined for dimethylbenzofurans with 1.11 g/day and for
methylbenzofurans with 0.17 g/day. Other previous studies focused on the assessment of in situ biodegradation using compound specific isotope analysis (CSIA) for benzene, toluene, and
o-xylene (Griebler et al., 2004; Mak et al. 2006; Peter et al., 2004).
3.1.3. Karlsruhe (South-West Germany)
The third study site was a former sand pit area, located in
Karlsruhe, South-West Germany, filled with municipal and
industrial waste including slags, incineration ash and gasworks residues in the years 1925–1956. The disposal site
covers an area of about 1500 m 2. The bottom of the dump
reaches the water table in about 8 m depth. In the northern
part of the dump, tar oil was found as non-aqueous phase liquid in the fluctuation zone of the groundwater table.
The aquifer consists of porous sand and gravel representing a typical composition for the Upper Rhine Valley. The
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P. Blum et al. / Journal of Contaminant Hydrology 126 (2011) 181–194
5
different electron acceptors and analysis of the microbial
community demonstrated the impact of redox conditions
on biodegradation rates of PAH and BTEX (Schulze and
Tiehm, 2004).
N heterocycles
S heterocycles
O heterocycles
Benzene
Phenol
Naphthalene
4
3.2. Analytical methods
log Kow
3
2
1
0
0
1
2
3
4
5
log Sw (mg/L)
Fig. 1. Relationship between log aqueous solubility (Sw) and log octanol–
water partition coefficient (Kow) for the 14 studied NSO heterocyclic compounds and three other typical organic compounds of coal tars (benzene,
phenol and naphthalene). Values are taken from Table 1.
average groundwater velocity is 0.8 m/day with an average
hydraulic conductivity of about 2 × 10 − 3 m/s (Table 2)
(Schulze and Tiehm 2004; Tiehm and Schulze, 2003).
Schulze and Tiehm (2004) could demonstrate different
dominating redox zones in the plume by analysing the electron acceptors and respiration products in the field. They observed a succession of redox processes downgradient of the
pit as well as a significant decrease in pollutant concentration
with increasing distance to the disposal site. The changing
ratio of naphthalene and acenaphthene along the plume
pointed to active biodegradation processes, because naphthalene has a higher solubility than acenaphtene, but decreases more rapidly in the aqueous phase than the more
hydrophobic and more sorptive acenaphthene (Tiehm and
Schulze, 2003). Microcosm studies in the presence of
Coal tar or creosotes are complex organic mixtures — creosote is obtained by the fractional distillation of crude coal
tars. Coal tar is a mixture of several hundred, probably thousands of chemicals and by far not all of them have been identified. Six major classes of compounds can be distinguished:
(1) aromatic hydrocarbons, including polycyclic aromatic hydrocarbons (PAHs) and alkylated PAHs, (2) tar acids/phenolics, (3) tar bases/nitrogen-containing heterocycles, (4)
aromatic amines, (5) sulfur-containing heterocycles and (6)
oxygen-containing heterocycles. HET and their metabolites
are typically analysed by gas chromatography mass spectrometry (GC–MS) and high pressure liquid chromatography
(HPLC) with diode array detector (DAD) or MS(–MS)-detectors (e.g. Johansen et al., 1996; Mundt and Hollender, 2005).
In the current study 13 typical HET (indole, quinoline, carbazole, acridine, 6-methylquinoline, thiophene, 1-benzothiophene,
dibenzothiophene, benzofuran, dibenzofuran, methylbenzofurans, 2,3-dimethylbenzofuran and xanthene) were enriched
from specific groundwater samples (Hamburg, 15.06.2005;
‘Testfeld Süd’, 19.01.2006) by solid-phase-extraction (SPE) and
liquid–liquid-extraction (LLE), respectively. The SPE-cartridges
were extracted with ethyl acetate. PAHs were extracted from
groundwater samples using cyclohexane as a solvent. Ethyl acetate extracts and cyclohexane extracts were analysed for NSOheterocyclic aromatic compounds and 16 EPA-PAHs by GC–MS.
NSO-heterocyclic aromatic compounds were analysed by GC–
MS in the full-scan mode, the PAHs were analysed in the selected
ion monitoring (SIM) mode. BTEX analyses of the groundwater
samples were performed using Purge and Trap Concentrator
connected to GC–MS.
For the LLE 45 mL of groundwater were extracted with 5 mL
MTBE for 20 min. After separation and adding the internal standard, the organic phase was analysed with GC–MS. The latter
Table 2
Summary of site-specific data.
Site name
Production/
operation time
Average
groundwater
velocity, va
[m/day]
Transverse
dispersivity,
αT [m]
Hamburga
1904–1986
0.25a
0.84
‘Testfeld Süd’
Karlsruhe
g
d
1875–1970
1925–1956
2.0
d
0.80
n.a. = not analysed.
a
D'Affonseca et al. (2008).
b
Blum et al. (2009).
c
D'Affonseca et al. (2011).
d
Bösel et al. (2000).
e
Griebler et al. (2004).
f
Zamfirescu and Grathwohl (2001).
g
Tiehm and Schulze (2003).
30
0.30
g
0.03
Source
width,
Ws [m]
160
g
40
Average
organic
carbon
content, foc [−]
Detected redox conditions
0.0010 ± 0.0020
Aerobic, methanogenic, nitrate-, iron-, manganese and
sulfate-reducingb,c
Strongly reducing conditions, but no detailed delineation
of the redox zonese,f
Aerobic, methanogenic, nitrate-, iron-, manganese and
sulfate-reducingg
n.a.
0.0011 ± 0.0005
P. Blum et al. / Journal of Contaminant Hydrology 126 (2011) 181–194
187
was performed with Agilent 6890N gas chromatograph
equipped with an Agilent 5973 mass spectrometer. An Optima
5 capillary column (25 m×0.25 mm; 0.25 μm film thickness,
Macherey & Nagel, Düren, Germany) was used for compound
separation and helium was used as carrier gas. The temperature
program was as follows: 40 °C (4 min isothermal), 40°–300 °C
increasing with a rate of 5 °C/min, and 300 °C (12 min isothermal). The MS was operated in SIM mode. This method was
used for the groundwater samples at the site in Karlsruhe.
26.1 mg/L close to the source (5 m). In a distance of 260 m
from the source only methylbenzofurans could be detected
with 0.5 μg/L and all other 10 HET were below the detection
limit of 0.1 μg/L.
Hence, the study of the compound-specific plume lengths
for all three sites indicates that HET concentrations at the tip
of contaminant plumes are generally high and frequently
above PAH concentrations indicating that some HET are generally more persistent than other coal tar constituents.
3.3. Comparison of the compound-specific plume lengths
3.4. Comparison of compound-specific bulk attenuation rate
constants
In the current study plume lengths were determined for
BTEX, PAH and NSO heterocycles along the centre line of the contaminant plumes using the maximum observed concentration
with increasing distance from the source (Figs. 3 and 4).
At the Hamburg site the total concentration of 11 measured
HET at the edge of the contaminant plume, which include all
HET in Table 1 apart from pyrrole, thiophene and dibenzothiophene, amounts to 56 μg/L at a distance of around 450 m from
the source showing the highest total concentrations of all considered contaminant groups (Fig. 3). 1-Benzothiophene showed the
highest concentration with 52 μg/L of all analyzed HET.
At the ‘Testfeld Süd’ site the PAH concentrations along the
centre line were mainly higher than the HET concentration towards the edge of the contaminant plume at a distance of
around 420 m from the source, the total HET concentrations
were 0.25 μg/L (only methylbenzofurans) and thus slightly
above the PAH concentration (only acenaphthene and fluorene)
at 0.17 μg/L. The comparatively high concentrations of methylbenzofurans, acenaphthene and fluorene demonstrate the persistence of these specific compounds. At this site the detection
limit was 0.01 μg/L (PAH) and 0.1 μg/L (HET).
At the Karlsruhe site the same 11 HET as at the site in
Hamburg were analysed. However, here BTEX showed the
highest observed concentrations followed by HET, which is
probably due to the initial high concentrations of BTEX with
Biodegradation was found to be effective at all three sites
using various assessment methods such as hydrochemical analysis, microcosm biodegradation studies, microbial community
analysis, signature metabolite analysis (SMA) and/or CSIA (e.g.
Martus and Schaal, 2010; Peter et al. 2004; Tiehm and Schmidt,
2007; Tiehm and Schulze, 2003). For example, using CSIA in situ
biodegradation could be qualitatively demonstrated for benzene, toluene, o-xylene, m/p-xylene, naphthalene and 1methylnaphthalene at the ‘Testfeld Süd’ (Griebler et al., 2004)
and at the site in Hamburg for o-xylene and naphthalene
(Blum et al., 2009). However, CSIA was not yet performed for
HET. Hence, to quantitatively assess the in situ attenuation of
HET a classical first order decay model was applied along the
centre line of the plumes to determine bulk attenuation rate
constants (e.g. Beyer et al., 2006; Blum et al., 2009; Zamfirescu
and Grathwohl, 2001):
λ¼−
N heterocycles
EC 50 (Vibrio fischeri )
S heterocycles
O heterocycles
N heterocycles
EC 50 (Daphnia )
S heterocycles
O heterocycles
N heterocycles
S heterocycles EC 80 (P. fluorescens )
O heterocycles
Benzene
Phenol
Naphthalene
QSAR (Bundy et al., 2001)
2
log EC 50,80 (mg/L)
ð1Þ
The bulk attenuation rate constant λ is obtained by linear
regression plotting the logarithmic concentrations change
C(x)/C0 over travel time. The travel time was derived from
travel distance Δx divided by the average groundwater
3
1
0
-1
va
C ðxÞ
ln
Δx
C0
1
2
3
4
5
log Kow (mg/L)
Fig. 2. Relationship between log octanol–water partition coefficient (Kow) and the effective concentration (EC50 and EC80) using three different toxicity bioassays
(Vibrio fischeri, Daphnia, Pseudomonas fluorescens) for the 14 studied NSO heterocyclic compounds and three other typical organic compounds of coal tars (benzene, phenol and naphthalene) including the quantitative structure–activity relationship (QSAR) regression by Bundy et al. (2001).
188
P. Blum et al. / Journal of Contaminant Hydrology 126 (2011) 181–194
a)
100,000
Hamburg (Germany)
PAH (EPA)
10,000
BTEX
HET
1,000
100
Groundwater flow
10
1
0
100
200
300
400
500
Distance from the source [m]
b) 100,000.0
Testfeld Süd (Germany)
PAH (EPA)
10,000.0
BTEX
1,000.0
HET
100.0
10.0
1.0
0.1
Groundwater flow
0.0
0
100
200
300
400
500
600
Distance from the source [m]
c)
100,000
Karlsruhe (Germany)
PAH (EPA)
10,000
BTEX
HET
1,000
100
10
Groundwater flow
1
0
0
100
200
300
Distance from the source [m]
Fig. 3. Concentrations along the centre line of the plumes for the a) Hamburg, b) ‘Testfeld Süd’ (please note that at the final well all measured HET concentrations
are below the detection limit of 0.1 μg/l and therefore no data point is shown) and c) Karlsruhe sites in Germany.
velocity va (Table 2). Half-lives (t1/2 = 0.693 / λ) for all three
sites are summarised in Table 3 and illustrated in Fig. 5.
Due to very low concentrations close or even below the detection limit it was not always possible to evaluate the bulk attenuation rate constants or half-lives, respectively (Table 3).
The overall lowest bulk rate attenuation constant was determined for 2,3-dimethylbenzofuran with 0.40 a− 1 at the Hamburg site, which in comparison to the other two sites also has
the lowest bulk rate attenuation constants. A previously performed sensitivity analysis at the Hamburg site for the evaluation
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P. Blum et al. / Journal of Contaminant Hydrology 126 (2011) 181–194
a)
1,000
Concentration [µg/l]
Hamburg (Germany)
100
10
Acenaphthene
Ethylbenzene
Groundwater flow
Benzothiophene
1
0
100
200
300
400
500
400
500
Distance from the source [m]
b)
1,000.0
Concentration [µg/l]
Testfeld Süd (Germany)
100.0
10.0
Acenaphthene
Ethylbenzene
1.0
Methylbenzofurans
Groundwater flow
0.1
0
100
200
300
Distance from the source [m]
c)
100,000.0
Karlsruhe (Germany)
Concentration [µg/l]
10,000.0
1,000.0
Groundwater flow
100.0
10.0
Acenaphthene
Benzene
1.0
Methylbenzofurans
0.1
0
100
200
300
Distance from the source [m]
Fig. 4. Highest compound-specific concentrations along the centre line of the plumes for the a) Hamburg, b) ‘Testfeld Süd’ and c) Karlsruhe sites in Germany.
of the bulk rate attenuation constants showed that the groundwater velocity (hydraulic conductivity and porosity) has the
greatest influence followed by other input parameters such as
source concentration, source width, longitudinal and transversal
dispersions (Blum et al., 2007; 2009). Although first-order decay
rates or half-lives do not reflect the relevant biodegradation
processes, this approach is suitable to compare the overall elimination rates for different pollutants or different sites.
Very high half-lives were determined for 1-benzothiophene
and methylbenzofurans, which also showed high concentrations
downgradient of the source indicating the persistence of these
two compounds (Table 3, Fig. 4). The bulk rate attenuation
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P. Blum et al. / Journal of Contaminant Hydrology 126 (2011) 181–194
plume and therefore lower measured concentrations at the tip
of the plume. The latter indicates that with ongoing knowledge
of the delineation of the contaminant plume, the evaluation of
bulk rate attenuation constants might also change in time and
space.
Table 3
Half-lives based on the determined bulk attenuation rate constants for all
three studied sites including the Pearson product–moment correlation coefficients (PMCC) in brackets between the measured and simulated concentrations using a first order decay model (Eq. (1)).
Compound
Half-life t1/2 [days] and PMCC in
brackets
4. Monitored natural attenuation of NSO heterocylic
aromatic compounds
Hamburg ‘Testfeld Süd’ Karlsruhe
N-HET Pyrrole
Indole
Quinoline
Carbazole
Acridine
6-Methylquinoline
S-HET Thiophene
1-Benzothiophene
Dibenzothiophene
O-HET Benzofuran
Dibenzofuran
Methylbenzofurans
2,3-Dimethylbenzofuran
Xanthene
BTEX
Benzene
Ethylbenzene
PAH
Naphthalene
Acenaphthene
n.a.
154 (1.0)
n.p.b
204 (0.8)
329 (1.0)
n.p.
n.a.
538 (0.8)
230 (0.5)
460 (1.0)
198 (0.9)
538 (0.9)
632 (0.6)
n.p.
347 (0.6)
329 (0.7)
182 (1.0)
316 (0.9)
n.a.
n.a.
n.p.c
13 (1.0)
n.p.c
n.a.
n.p.d
11 (1.0)
24 (0.8)
n.p.e
n.a.
35 (0.9)
n.p.f
n.p.e
7 (1.0)
9 (1.0)
1 (1.0)
21 (0.9)
n.a.
63 (–)a
n.p.d
25 (0.9)
8 (–)a
n.p.d
n.a.
22 (1.0)
67 (0.9)
13 (1.0)
33 (0.8)
44 (1.0)
60 (0.9)
13 (–)a
26 (0.8)
81 (0.7)
16 (1.0)
90 (0.9)
For the successful implementation of MNA two main different concepts are currently applied worldwide: (1) risk-based
MNA concepts as used, for example, in the USA, and (2) MNA
concepts that rely on a precautionary principle of soil and
groundwater protection as developed, for example, in Germany
(Rügner et al., 2006). To comprehensively assess MNA at a site,
several properties such as toxicology, persistence, spreading
and biodegradation of the contaminants are usually considered
(Tiehm and Schmidt, 2007). Hence, based on the present review
Table 4 summarises the top three HET regarding physicochemical, toxicological properties, spreading and natural attenuation.
In addition, 9 minimal recommended priority HET are provided
for the assessment of MNA at coal tar contaminated sites, which
are based on the current review and two previously performed
studies (Table 4).
The study by Johansen et al. (1997) at three creosote contaminated sites in Denmark showed that pyrrole was only
detected once with a very low concentration of 0.22 μg/L. Quinoline, also with a high aqueous solubility, was measured at several sites in Germany and Denmark (Blotevogel et al., 2008;
Johansen et al., 1997). At the three sites of the present study
the highest concentration was measured at the ‘Testfeld Süd’
with 1.6 μg/L, though only close to the source. In all other samples quinoline was always below the detection limit. Thiophene,
the third most soluble HET (Table 4), was measured at the Danish site by Johansen et al. (1997), could be detected only close to
the contaminant sources (b55 m) with a maximum concentration of 9.2 μg/L. Although, pyrrole, quinoline and thiophene
have rather high aqueous solubilities, they are hardly present
n.a. = not analysed; n.p. = no best fit is possible.
a
Only two concentrations.
b
All concentrations below the detection limit (b 0.8 μg/L).
c
Only two concentrations above the detection limit (b 0.5 μg/L).
d
Concentrations below the detection limit (b0.1 μg/L).
e
Only one concentrations above the detection limit (b 0.5 μg/L).
f
Initial concentration lower than first concentration downgradient.
constants for the ‘Testfeld Süd’ determined in this study
are generally higher in comparison with previous studies by
Zamfirescu and Grathwohl (2001), Bockelmann et al. (2003)
and Piepenbrink et al. (2005), who applied an integral
approach, mainly due to ongoing site investigations at the tip of
the plume resulting in a better delineation of the contaminant
Hamburg Karlsruhe Testfeld Süd
Half-life [days]
1000
100
10
en
e
e
N
ap
ht
h
al
en
Be
nz
en
e
nz
hy
ot
h
nz
1-
Be
lb
e
io
ba
ph
en
e
zo
le
e
ar
C
en
ap
ht
he
n
en
e
Ac
ot
en
z
ib
D
M
et
hy
lb
en
zo
hi
op
h
fu
r
an
s
1
Et
Group
Compound
Fig. 5. Comparison of selected field-derived half-lives for all three studied sites.
191
P. Blum et al. / Journal of Contaminant Hydrology 126 (2011) 181–194
Table 4
Important parameters for the implementation of monitored natural attenuation (MNA) and the top three NSO heterocyclic compounds.
Parameter
NSO heterocyclic compounds
Top 1
Top 2
Top 3
Physicochemical properties
Highest aqueous solubility
Pyrrole
Quinoline
Thiophene
Toxicological properties
Lowest EC50 (Daphnia)
Dibenzothiophenea
Dibenzofurana
Acridineb
Spreadingc
Highest concentration at the tip of the plume
Benzothiophene
Methylbenzofurans
Natural attenuationc
Highest half-life
2,3-Dimethylbenzofuran
Methylbenzofurans/benzothiophene
a
b
c
Dibenzothiophene
Using EC50 by Hartnik et al. (2007).
using EC80 by Bundy et al. (2001).
Site-specific data from the three studied sites.
in the studied aquifers and if present only close to the source and
in low concentrations. Thus, we do not recommend them as priority HET (Table 5).
The review of several toxicity studies shows that dibenzothiophene and dibenzofuran are the most ecotoxic HET.
In addition, both HET are also present in high concentrations
in contaminant plumes. Furthermore, dibenzothiophene
shows one of the lowest bulk attenuation rate constants in
all three studied sites. Thus, both HET should be carefully
considered for a successful implementation of MNA at a
given site.
For the studied sites the highest concentrations of HET at
the tip of the plumes were observed for benzothiophene and
methylbenzofurans. Similar observations were reported for
the Danish sites (Johansen et al., 1997) and four other German sites (Blotevogel et al., 2008), where both contaminants
also showed generally high concentrations close to the
source, and also downgradient of the source. In addition to
the previous observations, 2,3-dimethylbenzofuran shows
very low bulk attenuation rate constants indicating a high
persistence. Based on physicochemical and toxicological
properties, microbial degradation and site-specific concentrations of HET at 6 coal tar contaminated sites Blotevogel
et al. (2008) derived a priority list of 20 HET for the investigation of the contaminant plumes (Table 5), which also contains substituted derivates and transformation products.
The assessment, which is based on substance-specific properties and site-specific observations, is biased towards
substance-specific properties with an average ratio of 2.3 between substance- and site-specific properties. In the current
study 9 key HET were identified (Table 4), 7 of them are
also part of the previously derived priority list of 20 HET.
Dibenzothiophene with the lowest EC50 value and a low
bulk attenuation rate constant and two other HET (pyrrole
Table 5
Derived priority NSO heterocycles based on three different studies by Johansen et al. (1997), Blotevogel et al. (2008), this study and minimal recommended priority NSO heterocycles for the assessment of MNA for coal tar contaminated aquifers.
Group
Priority NSO heterocycles
Johansen et al. (1997)
Blotevogel et al. (2008)
This study
Recommendation
N-HET
–
–
–
–
Quinoline
Isoquinoline
Isoquinoline-1-on
Quinoline-2-on
4-Methylquinoline-2-on
2,4-Dimethylquinoline
1-Methylisoquinoline
2-Methylquinoline
Carbazole
Acridine
–
–
Benzothiophen
–
3-Methylbenzothiophen
2-Hydroxybiphenyl
Benzofuran
Dibenzofuran
Methylbenzofurans
Dimethylbenzofurans
Xanthenon
Pyrrole
Quinoline
–
–
–
–
–
–
–
–
Acridine
–
Thiophene
Benzothiophene
Dibenzothiophene
–
–
–
Dibenzofuran
Methylbenzofurans
Dimethylbenzofurans
–
–
–
–
–
–
–
–
–
2-Methylquinoline
Carbazole
Acridine
–
–
Benzothiophene
Dibenzothiophene
–
–
Benzofuran
Dibenzofuran
Methylbenzofurans
Dimethylbenzofurans
–
–
S-HET
O-HET
–
–
–
Carbazole
–
Hydroxylquinolines
Thiophene
Benzothiophene
–
–
–
Benzofuran
Dibenzofuran
–
–
–
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P. Blum et al. / Journal of Contaminant Hydrology 126 (2011) 181–194
and thiophene) however are not included in this priority list
and in the 6 recommended HET (carbozole, thiophene, benzothiophene, benzofuran, dibenzofuran and hydroxylquinolines) by Johansen et al. (1997). Several studies including the present study (e.g. Blotevogel et al., 2008; Blum et al.,
2008; Johansen et al., 1997, Kern et al. 2008), wherein physicochemical properties, biodegradation, toxicity and spreading of HET were comprehensively considered in the field,
could evidently demonstrate the importance of HET for
MNA. Since 1997 the 16 EPA priority pollutants for PAH are
widely accepted to assess their environmental impact. How
many HET should be considered and regularly monitored
for the long term risk management of coal tar or creosote
contaminated aquifers, however is still under debate and
might also depend on the country-specific MNA approach.
Nevertheless, based on the current knowledge we recommend a minimum of 9 HET that should be regularly monitored for a successful implementation of MNA (Table 5).
5. Conclusions
In the present study substance-specific properties such as
physicochemical and toxicological properties and sitespecific observations such as compound-specific plume
lengths and half-lives of HET were investigated. The major
findings of the study can be summarised as follows:
• The aqueous solubilities of HET are generally several times
higher than those of equivalent PAH and/or BTEX, which is
mainly caused by the increased polarity of the HET due to
the hetero-atoms in the molecules;
• The few currently available toxicity studies indicate the
high toxicity of NSO heterocycles — out of 14 studied HET
dibenzothiophene and dibenzofuran are the most ecotoxic
HET in all three examined toxicity assays;
• Among the studied HET, 1-benzothiophene and methylbenzofurans show the highest concentrations relative to
other compounds at the tip of the plumes;
• The relative concentrations and transport distances of some
HET in comparison to other typical coal tar constituents
(PAH and BTEX) clearly demonstrate the persistence of
such compounds in contaminant plumes;
• Estimated field-derived half-lives of HET are usually higher
than their equivalent PAH and/or BTEX.
• A minimum of 9 HET (methylquinoline, carbazole, acridine,
benzothiophene, dibenzothiophene, benzofuran, dibenzofuran, methylbenzofurans and dimethylbenzofurans) are
recommended for the assessment of MNA in coal tar contaminated aquifers.
Our analysis of the literature data and our results on
the plume length and bulk attenuation rates clearly indicate
that – due to their comparatively high aqueous solubility,
low octanol–water partition coefficients, high toxicity, relative long and persistence plumes – some NSO heterocycles
form relatively long and persistence plumes that are comparable to other typical coal tar constituents such as PAH and
BTEX, in relation to their size. Thus, at least a number of key
NSO heterocycles should be regularly assessed and monitored
for a successful implementation of MNA in coal tar or creosote
contaminated aquifers. Hence, there is an urgent need to
derive a priority list of HET, which is generally accepted, applicable and appropriate for any chosen MNA concept.
Acknowledgments
We would like to thank Christoph Olk and Rainer Domalski
from the Rütgers GmbH for their support. The financial support
by the Rütgers GmbH for the field studies at the Hamburg site is
highly appreciated. Financial support of the German Ministry of
Education and Research (BMBF, grants no. 02WT9957/9,
02WN0361 and 02WN0362) for studying the Karlsruhe and
‘Testfeld Süd’ sites is also gratefully acknowledged. Finally, we
also thank three anonymous reviewers for their helpful comments and suggestions that improved this manuscript.
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