Diffusion and Sorption of Hexachlorobenzene in Sediments and Saturated Soils

In order to understand the transport and fate of hydrophobic organic chemicals (HOCs) in consolidated sediments and saturated soils, long-term (up to 512 d), one-dimensional, time-dependent diffusion experiments were done with a representative HOC, hexachlorobenzene (HCB), in three different sediments and soils. In addition, long-term HCB partitioning experiments in suspended sediments at high concentrations and tritiated water diffusion experiments in consolidated sediments were done in order to more accurately interpret the HCB diffusion experiments. Numerical models were used to analyze the results. Good agreement between the experimental data and calculated results was obtained. Despite the length of the experiments, the experimental results demonstrate that the HCB in the consolidated sediments was far from a steady-state chemical equilibrium. Experiments and theoretical results also indicate that the assumption of local chemical equilibrium was not valid, and sorption rates must therefore be considered for a valid analysis of transport.     

Konventionelle PCR- und Real-Time PCR- Verfahren zum Nachweis von thermophilen Campylobacter jejuni, C. coli und C. lari : ein Überblick

Many PCR-techniques have already been published for the detection of thermophilic Campylobacters. In order to provide a practical overview for the user – without laying claim to completeness – we present a collection of actual PCR detection methods, some of them were modified and some are still unpublished. They are listed along with their target genes and specificities. Four conventional and seven real-time PCR methods were tested and compared using a panel of 65 Campylobacter strains belonging to 12 different more or less related species. The regular PCR methodes tested include the 16S rRNA gene based technique that specifically detects C. jejuni, C. coli and C. lari without differenciating to species level, two flaA gene based typing-methods, one for C. jejuni and C. coli in combination, which was slightly modified according to Nachamkin et al. (1993, 1996), and additionally for C. lari. Another duplex PCR was carried out which allows identification of the two species C. jejuni and C. coli simultaneously. Among the real-time methods tested there were three techniques allowing identification of C. jejuni alone, two assays based on the flagellin A gene capable to identify C. jejuni and C. coli in combination, a duplex PCR performed as triplex PCR combined with an internal amplification control that detected C. jejuni and C. coli separately, and a Lightcycler method that detected C. jejuni, C. coli and C. lari simultaneously. Inclusivity and exclusivity were shown for each of the methods. In most cases inclusivity was good, but some techniques raised problems concerning exclusivity. Flagellin based real-time PCR techniques partially crossreacted either with C. lari or C. upsaliensis. C. hyoilei which is very closely related to C. coli reacted in almost all methods applied like C. coli.

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Zusammenfassung: Eine Vielzahl von PCR-Nachweismethoden für Campylobacter jejuni und C. coli wurden bereits publiziert. Hier wird eine Zusammenfassung von aktuellen PCR-Nachweismethoden für thermophile Campylobacter spp. pr?sentiert, die dem Anwender exemplarisch eine schnelle tabellarische übersicht über aktuelle Methoden bietet sowie über deren Zielgene und Spezifit?ten. Es wurden vier konventionelle und sieben Real-Time PCR-Methoden zur Identifizierung von Campylobacter spp. verglichen. Bei den konventionellen Methoden handelt es sich um eine auf dem 16S rRNA-Gen basierende PCR, die spezifisch mit C. jejuni, C. coli und C. lari reagiert, zwei auf dem Flagellin-Gen beruhende PCR-Typisierungsmethoden, die in modifizierter Form für C. jejuni, C. coli und zus?tzlich für C. lari durchgeführt wurden. Als vierte Methode wurde eine Duplex-PCR zur Identifzierung von C. jejuni und C. coli mit einbezogen. Unter den getesteten Real-Time PCR-Methoden befanden sich drei 5′-Nuklease Assays, die spezifisch C. jejuni detektieren, zwei Assays, die auf dem Flagellin-Gen beruhen und C. jejuni und C. coli gemeinsam erfassen, eine Duplex-PCR, die C. jejuni (mapA-Gen) und C. coli (ceu-EGen) identifizieren kann und noch mit einer unver?ffentlichten internen Kontrolle kombiniert wurde, sowie eine Lightcycler-Methode, die auf dem 16S rRNA-Gen basiert und C. jejuni, C. coli und C. lari gemeinsam detektiert, aber nicht differenziert. Die Methoden sind entweder unver?ndert aus Publikationen übernommen oder auch nach Datenbankanalysen modifziert worden. Zum Testen wurden 65 Campylobacter-St?mme aus 12 verschiedenen Spezies eingesetzt. Die Inklusivit?t war insgesamt bei allen Methoden sehr gut, bei der Exklusivit?t gab es Unsicherheiten. Real-Time PCR-Methoden, die auf dem Flagellin-Gen beruhen, k?nnen in manchen F?llen die verwandten Arten C. lari oder C. upsaliensis mit erfassen. Der enge Verwandte C. hyoilei wird in fast allen F?llen gleichzeitig mit C. coli detektiert.

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Eingegangen: 31. Januar 2007     

The sediment-water flux of HOCs due to "diffusion" or is there a well-mixed layer? If there is, does it matter?

In most present modeling of the sediment-water flux of hydrophobic organic chemicals (HOCs), the flux due to sediment erosion/deposition is described explicitly by means of a sediment transport model, while the remaining components of the flux (molecular diffusion, bioturbation, and groundwater flow) are lumped together and modeled as a "diffusive" flux. This diffusive flux is usually described by means of a mass transfer approximation with the implicit assumption of a well-mixed contaminant layer of thickness h in the sediments. On the basis of recent experiments and theoretical modeling, the justification forthis assumption and the quantification of this diffusive flux are discussed here. In particular, for HOCs with large partition coefficients, it is argued that a well-mixed layer often may not exist, and, when it does, it is slow to form; that h is difficult to define and even harder to quantify; and that, as far as long-term predictions (up to 100 years) of this diffusive flux are concerned, the exact value for h probably does not matter. What does matter are the magnitudes and time dependencies of each of the components of the flux and the interactions between the diffusive flux and the flux due to erosion/deposition.