Importance of heterocylic aromatic compounds in monitored natural attenuation for coal tar contaminated aquifers: A review

Abstract

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 compound-specific 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.

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, 6-methylquinoline 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, Dyreborg et al., 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, Dyreborg et al., 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 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, Blotevogel et al., 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 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).