Bacteria and carboxylate-modified microsphere transport experiments were performed in glass bead packed columns in order to examine the distribution of retained colloids on the sediment. Solution pH was allowed to vary from 6.0 to 9.4 across the length of the column (20 cm) in order to examine potential effects of solution chemistry on the retained profiles. Both the microspheres and the bacteria showed retained profiles that deviated strongly from log-linear behavior expected from a spatially invariant colloid deposition rate coefficient. Deviation for the microspheres was in the form of steeper-than-expected decreases in retained concentrations with distance from source. Deviation for the bacteria was in the form of maximum retained concentrations that were located down-gradient from the column inlet. Subsidiary experiments with varying elution times showed that detachment during elution moved the peak of mass of retained bacteria down-gradient of the column inlet; however, the disproportional translation of the peak of mass relative to elution time indicated that processes operating during injection produced the initial down-gradient translation of peak concentrations of retained cells.

No abstract

No abstract

Benzylsuccinate synthase (bssA) genes associated with toluene degradation were profiled across a groundwater contaminant plume under nitrate-reducing conditions and were detected in significant numbers throughout the plume. However, differences between groundwater and core sediment samples suggested that microbial transport, rather than local activity, was the underlying cause of the high copy numbers within the downgradient plume. Both gene transcript and reactant concentrations were consistent with this hypothesis. Expression of bssA genes from denitrifying toluene degraders was induced by toluene but only in the presence of nitrate, and transcript abundance dropped rapidly following the removal of either toluene or nitrate. The drop in bssA transcripts following the removal of toluene could be described by an exponential decay function with a half-life on the order of 1 h. Interestingly, bssA transcripts never disappeared completely but were always detected at some level if either inducer was present. Therefore, the detection of transcripts alone may not be sufficient evidence for contaminant degradation. To avoid mistakenly associating basal-level gene expression with actively degrading microbial populations, an integrated approach using the ratio of functional gene transcripts to gene copies is recommended. This approach minimizes the impact of microbial transport on activity assessment and allows reliable assessments of microbial activity to be obtained from water samples.T he use of molecular biological tools has led to significant advancements in the area of in situ bioremediation. Of particulate note is the use of quantitative PCR (qPCR) for the detection and quantification of functional genes associated with the degradation of contaminants. However, as has been repeatedly demonstrated in both column and field studies, functional gene abundance is not always predictive of contaminant degradation, nor does it consistently correlate with gene expression or contaminant concentration (1, 2). Furthermore, high gene copy numbers are often sustained when there is little or no active degradation (3, 4), making such measurements difficult to interpret.In some cases, microbial transport may provide an explanation for elevated gene copies in the absence of degradation. It has been suggested that genes recovered from groundwater samples may represent conditions upgradient of where they were sampled, because planktonic microorganisms are subjected to transport (5). As a result, organisms observed in groundwater samples may not reflect local chemical conditions; rather, their presence may be the result of transport from upstream locations where chemical conditions favored their activity and growth. The impact of microbial transport can be minimized by assessing activity with sediment sampling (6); however, the relative simplicity of groundwater sampling makes it more attractive from a practical standpoint.Functional gene transcripts have been suggested as a more reliable indicator of local contaminant biodegradation,...

A cryogenic coring system for the collection and preservation of biomolecules in unconsolidated subsurface solid samples is presented here. The sampler is based on existing direct‐push coring technology, with the addition of a cryogenic step to freeze the sample in situ. Once brought to the surface, the frozen cores can be packed in dry ice and shipped to the laboratory for further processing and analysis. The approach prevents redistribution of fluids during sample recovery and shipping, and because the cores are frozen in situ there is little loss of solid material during retrieval to ground surface. To evaluate the performance of the approach, DNA analyses of samples collected by cryogenic coring in a very large physical model are compared with results from water samples and horizontal core samples taken in close proximity. The data indicate that the vertical distribution of DNA within the cryogenic core can be measured at the centimeter scale, providing unprecedented characterization of subsurface biogeochemical interfaces.