The aim of this study was to perform a phytoscreening of per- and polyfluoroalkyl substances (PFAS) at a contaminated site in Germany, to investigate the applicability of this technique for PFAS contaminations. Foliage of three species, namely, white willow (Salix alba L.), black poplar (Populus nigra L.), and black alder (Alnus glutinosa L.), were sampled to evaluate seasonal and annual variations in PFAS concentrations. The results of the phytoscreening clearly indicated species and specific differences, with the highest PFAS sum concentrations ∑23 observed in October for white willow (0-1800 μg kg-1), followed by black poplar (6.7-32 μg kg-1) and black alder (0-13 μg kg-1). The bulk substances in leaves were highly mobile short-chain perfluoroalkyl carboxylic acids (PFCAs). In contrast, the PFAS composition in soil was dominated by long-chain PFCAs, perfluorooctanoic acid (PFOA) and perfluorodecanoic acid (PFDA), as a result of the lower mobility with ∑23PFAS ranging between 0.18 and 26 μg L-1 (eluate) and between 66 and 420 μg kg-1 (solid). However, the PFAS composition in groundwater was comparable to the spectrum observed in leaves. Spatial interpolations of PFAS in groundwater and foliage correspond well and demonstrate the successful application of phytoscreening to detect and delineate the impact of the studied PFAS on groundwater.

The objective of this study is to estimate hydraulic conductivities and biodegradation rate constants in a coal-tar contaminated aquifer by compound-specific isotope analysis (CSIA) and tracer-based (3H?3He) groundwater dating (TGD). In two observation wells downgradient from the contaminant source in situ biodegradation of oxylene, toluene and naphthalene under sulfate-reducing redox conditions could be demonstrated using CSIA. Median biodegradation rate constants for o-xylene ranging between 0.08 and 0.22 a-1 were estimated. By using tracer-based groundwater dating in these two wells, hydraulic conductivities could be also estimated, which are in a similar range as k-values derived from sieve analysis, a pumping test and a calibrated groundwater flow model. These results clearly demonstrate the applicability of tracer-based groundwater dating for the determination of in situ hydraulic conductivities in aquifers without pumping contaminated groundwater. Finally, a sensitivity analysis is performed using a Monte Carlo simulation. These results indicate high sensitivities of the assumed effective porosity for the estimation of the hydraulic conductivity and the selected isotope enrichment factor for the biodegradation rate constant, respectively. Conversely, the outcome also evidently demonstrates the main limitations of the novel combined isotope approach for a successful implementation of monitored natural attenuation (MNA) at such field sites.

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 250m, but less than 1000m. 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.

Signature metabolites provide direct geochemical indication that in-situ biodegradation of released organic compounds (e.g., oil and its refined products) is occurring. Experience shows that monitored natural attenuation site conditions are often more complex than in theory and often require a more profound comprehension of the governing natural attenuation processes. Frequently, there is lack of direct proof that contaminant degradation (mainly through biodegradation) is occurring. Advanced tools are emerging that aim to provide answers about whether contaminants of concern are actually (bio)degraded and to the extent. Signature metabolite analysis provides direct proof of mineral oil hydrocarbon biodegradation and is among these advanced tools. Yet, during the previous 15 years, metabolite analysis has only been used sporadically in research projects. The target metabolites consist of aromatic acids such as benzoates and benzylsuccinates and uniquely indicate in-situ biodegradation of individual contaminants of concern. Three case studies have been summarized to share practical experience with signature metabolite analysis for contaminants that include benzene, toluene, ethyl benzene, and xylenes; trimethylbenzenes; and polycyclic aromatic hydrocarbons. The summarized case study involving jet fuel-contamination was the first reported field study in which aromatic acid homologs were formed by microbial metabolism of C4 through C7 benzene. Signature metabolite analysis can be used to improve understanding of natural attenuation processes to close data gaps with respect to the general degradation mechanisms. Direct evidence for biodegradation (e.g., metabolite identity and concentration; microbial identity and quantity; daughter product degradation ratios, stable carbon isotope ratios) facilitates remediation planning and management; provides information useful to scientists and engineers that must determine the mechanisms that produced observed environmental conditions; and provides information for stakeholders involved in environmental cleanup litigation and cost appropriation.

As an alternative to conventional remediation technologies, the engineered use of natural attenuation (NA) processes for site remediation coined Monitored Natural Attenuation (MNA) is becoming more and more accepted in a growing number of industrialized countries. The analytical “cookbook” of methods to investigate NA processes matured during the 1990’s and has amalgamated in a wealth of guidance documents. Practical experience with MNA has shown site conditions to be more complex than in theory requiring a more profound understanding of the governing NA processes. Case studies are presented focusing on the application of novel tools such as compound-specific isotope analysis on C/C (CSIA), signature metabolites analysis (SMA) and redox-sensitive tapes (RST), which are aimed at providing answers as to whether the contaminants of concern are actually (bio)degraded and to what extent. These tools have the potential to provide more reliable field quantification of degradation, and improved investigation and assessment of small-scale biogeochemical processes. RÉSUMÉ Les phénomènes d'atténuation naturelle (“Natural Attenuation” – NA), communément appelés "Monitored Natural Attenuation" (MNA), utilisé pour réhabiliter des sites pollués, propose une alternative aux techniques conventionnelles de dépollution, qui est de mieux en mieux acceptée dans un nombre croissant de pays industrialisés. Les méthodes analytique utilisées pour comprendre les procédés de NA a mûri aux cours des années 90 de manière à constituer une somme de documents de référence. L'application des techniques de MNA a montré que les conditions des sites traités étaient plus complexes que la théorie ne le supposait, ce qui exige une bien meilleure compréhension des principaux phénomènes de NA. Les cas pratiques qui sont présentés mettent l'accent sur l'utilisation de nouveaux outils tels que l'analyse du composé spécifique des isotopes stables C/C (CSIA), l'analyse de la signature des métabolites et les bandes sensibles au redox, dont le but est montrer si les agents polluants recherchés sont réellement biodégradables et jusqu'à quel point. Ces outils offrent la possibilité de fournir des données plus fiables pour quantifier le degré de dégradation, et proposent des moyens améliorés d'investigation et d'évaluation des processus bio géochimiques intervenant à petite échelle.

Evidence for the presence of previously unreported alkylated aromatic acids in anaerobic groundwater samples from a jet fuel-contaminated site is presented. The redox conditions are dominated by sulfate reduction. A gas chromatography/mass spectrometry screening showed a composition of aromatic hydrocarbons ranging from BTEX (C(0)-C(2)-substituted benzenes) up to C(7)-benzenes. Known metabolites like methylbenzoates (C(1)-BA), C(2)-BA, C(3)-BA as well as methylbenzylsuccinates (C(1)-BSA) were detected. Additionally, previously unreported metabolites of higher alkylated benzenes have been identified in the extracts: those are C(2)-BSA to C(5)-BSA as well as C(4)-BA to C(6)-BA, which were detected in the microg/l-concentration range. The formation of C(1)- and C(2)-BSA as well as the C(1)- and C(2)-BA can be linked to structurally corresponding parent contaminants like the xylene isomers and the trimethylbenzene isomers. The C(3)-BSA and C(3)-BA represent metabolites of C(4)-benzenes. The occurrence of C(4)-C(5)-BSA and C(4)-C(6)-BA is attributed to C(5)-C(7)-benzenes acting as parent compounds. The concentration of total aromatic contaminants decreased along the plume centre line. In contrast, benzoates and benzylsuccinates (BA and BSA) showed constant concentrations over a distance of 84 m. BA concentrations were up to 162 times higher compared to BSA, as indicated by the ratio f(BA/BSA). A pronounced transient behaviour of BSA or an overall persistent behaviour of BA can explain this. Hence, along the plume centre line, f(BA/BSA) was constant over a distance of 128 m. The degradation products detected in a monitoring well are not necessarily tied to contaminants detected in that well. Therefore, the metabolite plume is partly located in front of the contaminant plume, but does not significantly migrate downstream the plume toe.

Bei “Monitored Natural Attenuation” (MNA)-Maßnahmen werden anhand von Felddaten Prognosen zur Schadensentwicklung erstellt. Zur Überprüfung der Aussagefähigkeit von Felddaten hinsichtlich MNA wurden an einer Grundwasserkontamination durch BTEX hydrogeologische Untersuchungen durchgeführt. Hierzu wurden Pumpversuchsdaten, geologische Bohrdaten und Niederschlagsdaten berücksichtigt, daneben die zeitliche Entwicklung der Grundwasserstände, der Schadstoffgehalte sowie der hydraulischen Gradienten des Grundwassers. Es zeigte sich eine signifikante Abhängigkeit der Schadstoffgehalte für BTEX maximal |r|=0,89 von einem bis zu ±1,4m schwankenden Grundwasserstand, der sich unterschiedlich auswirkt: bei hohem Grundwasserstand sind die Schadstoffgehalte in Herdnähe verringert, gleichzeitig erfolgt eine Ausdehnung der Fahne in Abstromrichtung. Die hydraulischen Gradienten schwanken zwischen 1,5‰ und 15,1‰. Im herdnahen Bereich ist bei hohem Grundwasserstand eine Gradientenverflachung, im Abstrom eine Gradientenversteilung festzustellen. Die Richtungen der horizontalen hydraulischen Gradienten variieren von 12° bis 67°. Unter Berücksichtigung dieser Abhängigkeiten ist eine differenzierte Betrachtung der Felddaten für eine nachfolgende Auswertung hinsichtlich MNA erforderlich. Die hydrogeologische und die hydraulische Erkundung ist eine wichtige Voraussetzung zum Verständnis der hydrochemischen Prozesse bei NA in kontaminierten Grundwasserleitern. Die Ergebnisse zeigen, dass hydraulische und hydrogeologische Aspekte bei den in den USA gebräuchlichen Protokollen zur Anwendung von NA eine zu geringe Beachtung finden.