The modeling of reactive solute transport with sorption to mobile and immobile sorbents.

This paper presents a mathematical model to describe the transport of reactive solutes with sorption to mobile and immobile sorbents. The mobile sorbent is considered to be reactive, too. The sorption processes mentioned are equilibrium and nonequilibrium processes. A transformation of the model in terms of total concentrations of solute and mobile sorbents is presented which simplifies the mathematical formulation. Effective isotherms, which describe the sorption to the immobile sorbent in the presence of a mobile sorbent and rate functions are introduced and their properties are discussed. The differences to existing approaches to model reactive solute transport are shown. Possible extensions are pointed out and the numerical approximation is sketched. The restrictions of the model as a consequence of the assumptions made on reactive solute transport are not due to mathematical reasons, but due to limitations of experimental information available

Selective successional transport of bacterial populations from rooted agricultural topsoil to deeper layers upon extreme precipitation events.

Substantial amounts of organic matter are mobilized from upper soil layers during extreme precipitation events. This results in considerable fluxes of carbon from plant-associated topsoil to deeper mineral soil and to groundwater. Microbes constitute an important part of this mobile organic matter (MOM) pool. Previous work has shown that specific bacteria associated with the rhizosphere of decaying maize roots were selectively transported with seepage water upon snowmelt in winter. However, effective mechanisms of mobilization and also possible distinctions to microbial transport for living root systems remain poorly understood. In the present study, bacteria in seepage water were sampled from lysimeters at an experimental maize field after extreme rain events in summer. We show that a distinctive subset of rhizoplane-associated bacterial populations was mobilized after summer rain, especially including abundant members of the Bacteroidetes, representing a microbial conduit for fresh plant-derived carbon inputs into deeper soil layers. Marked distinctions of seepage communities were not observed between lysimeters with a different relative contribution of preferential vs. matrix flow. Time resolved analyses of seepage water during an artificial rain event revealed temporal patterns in the mobilization of certain lineages, with members of the Chitinophagaceae, Sphingomonadaceae, and Bradyrhizobiaceae preferentially mobilized in early and late seepage fractions, and members of the candidate phyla Parcubacteria and Microgenomates mobilized mostly in intermediate fractions. While average bacterial cell counts were at similar to 10(7) ml(-1) in seepage water, the recovery of amended fluorescently labeled cells of Arthrobacter globiformis was low (0.2-0.6%) over seepage events. Still, mobilized bacteria clearly have the potential to influence bacterial activities and communities in subsoils. These findings demonstrate that dynamic hydraulic events must be considered for a better understanding of the connectivities between microbial populations and communities in soil, as well as of the links between distinct carbon pools over depth

Nanosized ferrihydrite colloids facilitate microbial iron reduction under flow conditions.

Recent studies indicated that nanoparticulate minerals in the environment exhibit a higher reactivity than their respective bulk materials. Lately, this has also been reported for microbial iron reduction using nanosized iron oxides as electron acceptors. However, these results have been obtained under small-scale, static batch conditions. The study presented here implies a flow regime within porous medium under low salt conditions providing information on the retention of nanosized iron oxide and the long-term sustainability of their reduction. Goethite, present as coating on quartz sand as a default electron acceptor, was reduced by Geobacter sulfurreducens and discharged as Fe2 + up to a maximum concentration of 0.6 mM in the column effluent, finally representing 10% of the supplied ferric iron in total. Adding ferrihydrite colloids via the influent to goethite-coated quartz sand led to partial adsorption of the colloids, but also to a high reactivity with a maximum Fe2 + discharge of 1.2 mM. In total, 1.4 mmol out of 2.6 mmol ferrihydrite colloids were reduced. Upon addition of ferrihydrite colloids to the influent of a parallel column containing quartz sand without goethite coatings, the maximum concentration of discharged Fe2 + accounted also for 1.2 mM, and 0.4 mmol out of 0.6 mmol ferrihydrite colloids were reduced in total. The column experiments demonstrated that ferrihydrite colloids are highly reactive and bioavailable for microbial reduction under approximated in situ conditions

Selective transport of plant root-associated bacterial populations in agricultural soils upon snowmelt.

Plants introduce abundant carbon into soils, where it is mineralised and sequestered. Proportions of this fresh organic carbon introduced to top soils can be relocated to deeper soil layers and even to groundwater by event-driven transport upon heavy rainfalls or after snowmelt. It is assumed that a significant fraction of this flux involves biocolloids and possibly microbial biomass itself. However, the nature of such transported microbes, their origin and the mechanisms of their mobilisation are still poorly understood. Here, we provide primary evidence that specific microbial populations are exported from top soils upon seepage events. At an experimental maize field, we have analysed the composition of mobilised bacterial communities collected in seepage water directly after snowmelt in winter at different depths (35 and 65 cm), and compared them to the corresponding bulk soil microbiota. Using T-RFLP fingerprinting and pyrotag sequencing, we reveal that mostly members of the Betaproteobacteria (Methylophilaceae, Oxalobacteraceae, Comamonadaceae), the Alphaproteobacteria (Sphingomonadaceae, Bradyrhizobiaceae), the Gammaproteobacteria (Legionellaceae) and the Bacteroidetes (Sphingobacteriaceae) were mobilised, all characteristic taxa for the rhizoplane. This highlights the importance of preferential flow along root channels for the vertical mobilisation and transport of microbes. Although the estimated quantitative fluxes of bacterial biomass carbon appeared low, our study allows for an improved understanding of the links between top soil, subsoil, and groundwater microbiota, as well as carbon fluxes between soil compartments

Size- and composition-dependent toxicity of synthetic and soil-derived Fe oxide colloids for the nematode <em>Caenorhabditis elegans</em>.

Colloidal iron oxides (FeOx) are increasingly released to the environment due to their use in environmental remediation and biomedical applications, potentially harming living organisms. Size and composition could affect the bioavailability and toxicity of such colloids. Therefore, we investigated the toxicity of selected FeOx with variable aggregate size and variably composed FeOx-associated organic matter (OM) toward the nematode Caenorhabditis elegans. Ferrihydrite colloids containing citrate were taken up by C. elegans with the food and accumulated inside their body. The toxicity of ferrihydrite, goethite, and akaganeite was dependent on aggregate size and specific surface area, with EC50 values for reproduction ranging from 4 to 29 mg Fe L(-1). Experiments with mutant strains lacking mitochondrial superoxide dismutase (sod-2) showed oxidative stress for two FeOx and Fe(3+)-ions, however, revealed that it was not the predominant mechanism of toxicity. The OM composition determined the toxicity of mixed OM-FeOx phases on C. elegans. FeOx associated with humic acids or citrate were less toxic than OM-free FeOx. In contrast, soil-derived ferrihydrite, containing proteins and polysaccharides from mobile OM, was even more toxic than OM-free Fh of similar aggregate size. Consequently, the careful choice of the type of FeOx and the type of associated OM may help in reducing the ecological risks if actively applied to the subsurface

Fast microbial reduction of ferrihydrite colloids from a soil effluent.

Recent studies on the microbial reduction of synthetic iron oxide colloids showed their superior electron accepting property in comparison to bulk iron oxides. However, natural colloidal iron oxides differ in composition from their synthetic counterparts. Besides a potential effect of colloid size, microbial iron reduction may be accelerated by electron-shuttling dissolved organic matter (DOM) as well as slowed down by inhibitors such as arsenic. We examined the microbial reduction of OM- and arsenic-containing ferrihydrite colloids. Four effluent fractions were collected from a soil column experiment run under water-saturated conditions. Ferrihydrite colloids precipitated from the soil effluent and exhibited stable hydrodynamic diameters ranging from 281 (&plusmn;146) nm in the effluent fraction that was collected first and 100 (&plusmn;43) nm in a subsequently obtained effluent fraction. Aliquots of these oxic effluent fractions were added to anoxic low salt medium containing diluted suspensions of Geobacter sulfurreducens. Independent of the initial colloid size, the soil effluent ferrihydrite colloids were quickly and completely reduced. The rates of Fe2+ formation ranged between 1.9 and 3.3 fmol h&minus;1 cell&minus;1, and are in the range of or slightly exceeding previously reported rates of synthetic ferrihydrite colloids (1.3 fmol h&minus;1 cell&minus;1), but greatly exceeding previously known rates of macroaggregate-ferrihydrite reduction (0.07 fmol h&minus;1 cell&minus;1). The inhibition of microbial Fe(III) reduction by arsenic is unlikely or overridden by the concurrent enhancement induced by soil effluent DOM. These organic species may have increased the already high intrinsic reducibility of colloidal ferrihydrite owing to quinone-mediated electron shuttling. Additionally, OM, which is structurally associated with the soil effluent ferrihydrite colloids, may also contribute to the higher reactivity due to increasing solubility and specific surface area of ferrihydrite. In conclusion, ferrihydrite colloids from soil effluents can be considered as highly reactive electron acceptors in anoxic environments

Transport and degradation of propylene glycol in the vadose zone: model development and sensitivity analysis

Transport and degradation of de-icing chemical (containing propylene glycol, PG) in the vadose zone were studied with a lysimeter experiment and a model, in which transient water flow, kinetic degradation of PG and soil chemistry were combined. The lysimeter experiment indicated that aerobic as well as anaerobic degradation occurs in the vadose zone. Therefore, the model included both types of degradation, which was made possible by assuming advection-controlled (mobile) and diffusion-controlled (immobile) zones. In the mobile zone, oxygen can be transported by diffusion in the gas phase. The immobile zone is always water-saturated, and oxygen only diffuses slowly in the water phase. Therefore, the model is designed in a way that the redox potential can decrease when PG is degraded, and thus, anaerobic degradation can occur. In our model, manganese oxide (MnO2, which is present in the soil) and NO 3 - - 3 (applied to enhance biodegradation) can be used as electron acceptors for anaerobic degradation. The application of NO 3 - - 3 does not result in a lower leaching of PG nor in a slower depletion of MnO2. The thickness of the snowcover influences the leached fraction of PG, as with a high infiltration rate, transport is fast, there is less time for degradation and thus more PG will leach. The model showed that, in this soil, the effect of the water flow dominates over the effect of the degradation parameters on the leaching at a 1-m depth

The modeling of reactive solute transport with sorption to mobile and immobile sorbents

This paper presents a mathematical model to describe the transport of reactive solutes with sorption to mobile and immobile sorbents. The mobile sorbent is considered to be reactive, too. The sorption processes mentioned are equilibrium and nonequilibrium processes. A transformation of the model in terms of total concentrations of solute and mobile sorbents is presented which simplifies the mathematical formulation. Effective isotherms, which describe the sorption to the immobile sorbent in the presence of a mobile sorbent and rate functions are introduced and their properties are discussed. The differences to existing approaches to model reactive solute transport are shown. Possible extensions are pointed out and the numerical approximation is sketched. The restrictions of the model as a consequence of the assumptions made on reactive solute transport are not due to mathematical reasons, but due to limitations of experimental information available. (orig.)Available from TIB Hannover: RR 5549(24)+a / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

How do long-term development and periodical changes of river-floodplain systems affect the fate of contaminants?

In many densely populated areas, riverine floodplains have been strongly impacted and degraded by river channelization and flood protection dikes. Floodplains act as buffers for flood water and as filters for nutrients and pollutants carried with river water and sediment from upstream source areas. Based on results of the EU-funded "AquaTerra" project (2004-2009), we analyze changes in the dynamics of European river-floodplain systems over different temporal scales and assess their effects on contaminant behaviour and ecosystem functioning. We find that human-induced changes in the hydrologic regime of rivers have direct and severe consequences on nutrient cycling and contaminant retention in adjacent floodplains. We point out the complex interactions of contaminants with nutrient availability and other physico-chemical characteristics (pH, organic matter) in determining ecotoxicity and habitat quality, and draw conclusions for improved floodplain managemen