Influence of Precipitation and Soil on Transport of Fecal Enterococci in Fractured Limestone Aquifers

Limestone aquifers provide the main drinking water resources of southern Italy. The groundwater is often contaminated by fecal bacteria because of the interaction between rocks having high permeability and microbial pollutants introduced into the environment by grazing and/or manure spreading. The microbial contamination of springwater in picnic areas located in high mountains can cause gastrointestinal illness. This study was carried out in order to analyze the interaction between Enterococcus faecalis and the soil of a limestone aquifer and to verify the influence of this interaction on the time dependence of groundwater contamination. E. faecalis was chosen because, in the study area involved, it represents a better indicator than Escherichia coli. The research was carried out through field (springwater monitoring) and laboratory experiments (column tests with intact soil blocks). The transport of bacterial cells through soil samples was analyzed by simulating an infiltration event that was monitored in the study area. Comparison of laboratory results with data acquired in the field showed that discontinuous precipitation caused an intermittent migration of microorganisms through the soil and produced, together with dispersion in the fractured medium (unsaturated and saturated zones), an articulated breakthrough at the spring. The short distances of bacterial transport in the study area produced a significant daily variability of bacterial contamination at the field scale.     

Mobility and survival of Salmonella Typhimurium and human adenovirus from spiked sewage sludge applied to soil columns

Aims:  This study investigated the survival and transport of sewage sludge-borne pathogenic organisms in soils.
Methods and Results:  Undisturbed soil cores were treated with Salmonella enterica ssp. enterica serovar Typhimurium- lux (STM- lux ) and human adenovirus (HAdV)-spiked sewage sludge. Following an artificial rainfall event, these pathogens were analysed in the leachate and soil sampled from different depths (0–5 cm, 5–10 cm and 10–20 cm) after 24 h, 1 and 2 months. Significantly more STM- lux and HAdV leached through the soil cores when sewage sludge was present. Significantly more STM- lux were found at all soil depths, at all time periods in the sewage sludge treatments, compared to the controls. The rate of decline of STM- lux in the controls was more rapid than in the sewage sludge treatments. Survival and transport of HAdV were minimal.
Conclusions:  The presence of sewage sludge can significantly influence the transport and survival of bacterial pathogens in soils, probably because of the presence of organic matter. Environmental contamination by virus is unlikely because of strong soil adsorption.
Significance and Impact of the Study:  This study suggests that groundwater contamination from vertical movement of pathogens is a potential risk and that it highlights the importance of the treatment requirements for biosolids prior to their application to land.     

Microbial contamination of vegetable crop and soil profile in arid regions under controlled application of domestic wastewater

Increasing lack of potable water in arid countries leads to the use of treated wastewater for crop production. However, the use of inappropriate irrigation practices could result in a serious contamination risk to plants, soils, and groundwater with sewage water. This research was initiated in view to the increasing danger of vegetable crops and groundwater contamination with pathogenic bacteria due to wastewater land application. The research was designed to study: (1) the effect of treated wastewater irrigation on the yield and microbial contamination of the radish plant under field conditions; (2) contamination of the agricultural soil profile with fecal coliform bacteria. Effluent from a domestic wastewater treatment plant (100%) in Jeddah city, Saudi Arabia, was diluted to 80% and 40% with the groundwater of the experimental site constituting three different water qualities plus groundwater as control. Radish plant was grown in two consecutive seasons under two drip irrigation systems and four irrigation water qualities. Upon harvesting, plant weight per ha, total bacterial, fecal coliform, fecal streptococci were detected per 100 g of dry matter and compared with the control. The soil profile was also sampled at an equal distance of 3 cm from soil surface for fecal coliform detection. The results indicated that the yield increased significantly under the subsurface irrigation system and the control water quality compared to surface irrigation system and other water qualities. There was a considerable drop in the count of all bacteria species under the subsurface irrigation system compared to surface irrigation. The bacterial count/g of the plant shoot system increased as the percentage of wastewater in the irrigation water increased. Most of the fecal coliform bacteria were deposited in the first few centimeters below the column inlet and the profile exponentially decreased with increasing depth.     

The transport of Mycobacterium avium subsp. paratuberculosis through saturated aquifer materials

Aims:  To investigate the processes controlling the transport of Mycobacterium avium subsp. paratuberculosis ( Map ) through aquifer materials.
Methods and Results:  We measured two important surface characteristics known to affect bacterial attachment to sediment surfaces: surface charge and hydrophobicity. We then measured the transport of Map through laboratory columns packed with aquifer sand with varying ionic strength solutions and sediment surface charge. We found that Map has a strong negative charge and is highly hydrophobic and that the transport of Map through positively charged Fe-coated sands was reduced compared with transport through negatively charged clean quartz sand, although Map transport for all treatments was low compared with the transport behaviour reported in the literature for other bacteria.
Conclusions:  Our results suggest that the potential for groundwater contamination by Map is low; however, the organism may remain bound to the soil near the surface where it can be ingested by grazing animals or be released during run off events.
Significance and Impact of the Study:  This is the first study looking at the surface characteristics and transport behaviour of Map through aquifer materials and therefore provides important information for understanding the movement of Map in the environment.     

Microbes in Groundwater of a Volcanic Mountain,Mt. Fuji; 16S rDNA Phylogenetic Analysis as a Possible Indicator for the Transport Routes of Groundwater

Huge groundwater reserves are under the foot of Mt. Fuji, the tallest volcanic mountain in Japan, and the residence time of the groundwater as estimated from the ³⁶Cl/Cl isotope ratio is in the range of 20–35 years. All springwater samples contain nearly saturated concentrations of oxygen. The groundwater field was separated into four areas based on regional slopes of the groundwater table and estimated groundwater flow directions. Springwater samples showed that physicochemical characteristics of the groundwater in the region are relatively constant; Ion concentrations were characterized as the Ca-HCO₃ type, irrespective of location at the foot of the mountain. The density of prokaryotes in the springwater, which was in the range of 10²–10⁴ cells mL^?1, was low compared with any other groundwater so far reported. Although the density of prokaryotes was low, retrieved clones show that the prokaryotes belonged to eight bacterial and two archaeal phyla, and included both aerobic and obligate anaerobic prokaryotes. Some retrieved clones were related to thermophilic prokaryotes, which are optimally adapted to temperatures greater than 40°C. This finding suggests that at least some of the source of the groundwater was located at a depth of 600 m or greater, based on a temperature gradient of 4°C/100 m. This depth is far below the lava layer which was taken to be a substantial pool of groundwater. Microbiological information may provide insight into the transport route of groundwater until it emerges.     

Influence of substratum on the degradation processes in diesel polluted sub-Antarctic soils (Crozet Archipelago)

Biodegradation by naturally occurring populations of microorganisms is a major mechanism for the removal of oil hydrocarbons from the environment. Therefore, follow-up of bacterial populations and chemical indices of biodegradation are important components of contaminated site assessment studies. Over a 4-year period following an accidental diesel contamination of the sub-Antarctic Crozet Archipelago (51°51′E–46°25′S), a field study was carried out in the contaminated area that is located in a transition zone between an arid fell-field (upstream) and a wet vegetated area (downstream). This study included a monitoring of heterotrophic and hydrocarbon-degrading bacterial abundance and chemical analysis of the remaining hydrocarbons. Significant higher number of heterotrophic and hydrocarbon-degrading bacterial counts revealed a rapid acclimation of sub-Antarctic microbial soil communities to the diesel fuel contamination. A chemical survey conducted during the last 2 years (2002 and 2003) showed that the total extractable hydrocarbons (TPH) content in arid fell field was reduced to ≤50% of their value while it was reduced only to ≤65% in vegetated soil. In addition, the decrease of TPH was always higher in the presence of fertilizer in the arid contaminated area, while fertilizer addition was almost inefficient in the wet contaminated one. All these results demonstrate a serious influence of the soil properties on the degradation rate. However, all chemical indices showed a significant reduction of alkanes and light aromatics in both contaminated area confirming a regular oil degradation process.     

Reduction of perchlorate and nitrate by microbial communities in vadose soil

Perchlorate contamination is a concern because of the increasing frequency of its detection in soils and groundwater and its presumed inhibitory effect on human thyroid hormone production. Although significant perchlorate contamination occurs in the vadose (unsaturated) zone, little is known about perchlorate biodegradation potential by indigenous microorganisms in these soils. We measured the effects of electron donor (acetate and hydrogen) and nitrate addition on perchlorate reduction rates and microbial community composition in microcosm incubations of vadose soil. Acetate and hydrogen addition enhanced perchlorate reduction, and a longer lag period was observed for hydrogen (41 days) than for acetate (14 days). Initially, nitrate suppressed perchlorate reduction, but once perchlorate started to be degraded, the process was stimulated by nitrate. Changes in the bacterial community composition were observed in microcosms enriched with perchlorate and either acetate or hydrogen. Denaturing gradient gel electrophoresis analysis and partial sequencing of 16S rRNA genes recovered from these microcosms indicated that formerly reported perchlorate-reducing bacteria were present in the soil and that microbial community compositions were different between acetate- and hydrogen-amended microcosms. These results indicate that there is potential for perchlorate bioremediation by native microbial communities in vadose soil.     

Reduction of Perchlorate and Nitrate by Microbial Communities in Vadose Soil

Perchlorate contamination is a concern because of the increasing frequency of its detection in soils and groundwater and its presumed inhibitory effect on human thyroid hormone production. Although significant perchlorate contamination occurs in the vadose (unsaturated) zone, little is known about perchlorate biodegradation potential by indigenous microorganisms in these soils. We measured the effects of electron donor (acetate and hydrogen) and nitrate addition on perchlorate reduction rates and microbial community composition in microcosm incubations of vadose soil. Acetate and hydrogen addition enhanced perchlorate reduction, and a longer lag period was observed for hydrogen (41 days) than for acetate (14 days). Initially, nitrate suppressed perchlorate reduction, but once perchlorate started to be degraded, the process was stimulated by nitrate. Changes in the bacterial community composition were observed in microcosms enriched with perchlorate and either acetate or hydrogen. Denaturing gradient gel electrophoresis analysis and partial sequencing of 16S rRNA genes recovered from these microcosms indicated that formerly reported perchlorate-reducing bacteria were present in the soil and that microbial community compositions were different between acetate- and hydrogen-amended microcosms. These results indicate that there is potential for perchlorate bioremediation by native microbial communities in vadose soil.     

Relationship between aquifer biofilms and unattached microbial indicators of urban groundwater contamination

Aquifers, springs and other groundwater‐dependent ecosystems are threatened by urban land use, which causes water quality deterioration through nutrient loading, sewage infiltration, groundwater extraction and, along coasts, seawater intrusion. The presence of certain microbes in groundwater can indicate that an aquifer is anthropogenically contaminated. Interpretations made from observations of indicator microbes in groundwater are limited because the relationship between the presumably allochthonous indicator microbes and relevant autochthonous microbial communities has not been characterized. This study addressed whether autochthonous aquifer biofilms can influence the presence of presumed microbial indicators in groundwater, and simultaneously used microbial indicators to trace sources of urban contamination at a karst spring of conservation concern. These questions were approached using a 17‐month time series analysis of attached biofilm and adjacent unattached bacteria in the submerged karst aquifer conduit associated with this spring. Environmental 16S rRNA gene sequencing was performed to characterize these communities, and community structure data were contextualized with groundwater geochemical and hydrogeological measurements. Linear regression models were developed to explain the relative abundance patterns of indicator microbes and other unattached microbes at this site. The results of this study suggest that dominant aquifer biofilms do not influence the presence of unattached microbial taxa that are presumed to be indicators of groundwater contamination, and generated new information about the origin of coliform bacteria at the study site. These results build confidence in the use of microbial indicators in groundwater‐dependent ecosystem conservation strategies and inform future management plans for urban aquifers and springs worldwide.     

Leaching of Cryptosporidium parvum oocysts, Escherichia coli, and a Salmonella enterica serovar Typhimurium bacteriophage through intact soil cores following surface application and injection of slurry

Increasing amounts of livestock manure are being applied to agricultural soil, but it is unknown to what extent this may be associated with contamination of aquatic recipients and groundwater if microorganisms are transported through the soil under natural weather conditions. The objective of this study was therefore to evaluate how injection and surface application of pig slurry on intact sandy clay loam soil cores influenced the leaching of Salmonella enterica serovar Typhimurium bacteriophage 28B, Escherichia coli, and Cryptosporidium parvum oocysts. All three microbial tracers were detected in the leachate on day 1, and the highest relative concentration was detected on the fourth day (0.1 pore volume). Although the concentration of the phage 28B declined over time, the phage was still found in leachate at day 148. C. parvum oocysts and chloride had an additional rise in the relative concentration at a 0.5 pore volume, corresponding to the exchange of the total pore volume. The leaching of E. coli was delayed compared with that of the added microbial tracers, indicating a stronger attachment to slurry particles, but E. coli could be detected up to 3 months. Significantly enhanced leaching of phage 28B and oocysts by the injection method was seen, whereas leaching of the indigenous E. coli was not affected by the application method. Preferential flow was the primary transport vehicle, and the diameter of the fractures in the intact soil cores facilitated transport of all sizes of microbial tracers under natural weather conditions.     

Effect of fluorochromes on bacterial surface properties and interaction with granular media.

Simple, efficient, and safe tagging methods are desired in short-term microbial transport studies such as in the study of filtration systems for water and wastewater treatment. Suitability of selected fluorochromes as bacterial tagging agents in transport studies was evaluated on the basis of stability of stained cells and the effect of staining on bacterial surface characteristics and interaction with granular media. Surface properties were characterized by zeta potential and microbial adhesion to hydrocarbons. The effect of staining on interactions between bacteria and porous media was evaluated in terms of removal of bacteria in batch adsorption tests using sand coated with aluminum hydroxide to enhance adsorption. The DNA-specific fluorochrome 4',6-diamidino-2-phenylindole (DAPI) had generally negligible effects on bacterial surface properties and interaction with sand, as indicated in batch adsorption tests using pure cultures (Escherichia coli or Acinetobacter sp.) and wastewater bacteria. Cells stained with DAPI were stable for 48 h at 4 or 20 degrees C. Other nucleic acid fluorochromes tested had different but significant effects on bacterial cells and produced less stable fluorescence. Since transport through porous media is modulated by surface properties, it may be concluded based on these results that the choice of fluorochromes is critical in microbial transport studies. DAPI appeared to be a promising tagging agent. Time dependence of fluorescence of stained cells may limit the use of fluorochrome-tagged cells in long-term transport studies.     

VIRTUS, a model of virus transport in unsaturated soils.

As a result of the recently proposed mandatory groundwater disinfection requirements to inactivate viruses in potable water supplies, there has been increasing interest in virus fate and transport in the subsurface. Several models have been developed to predict the fate of viruses in groundwater, but few include transport in the unsaturated zone and all require a constant virus inactivation rate. These are serious limitations in the models, as it has been well documented that considerable virus removal occurs in the unsaturated zone and that the inactivation rate of viruses is dependent on environmental conditions. The purpose of this research was to develop a predictive model of virus fate and transport in unsaturated soils that allows the virus inactivation rate to vary on the basis of changes in soil temperature. The model was developed on the basis of the law of mass conservation of a contaminant in porous media and couples the flows of water, viruses, and heat through the soil. Model predictions were compared with measured data of virus transport in laboratory column studies and, with the exception of one point, were within the 95% confidence limits of the measured concentrations. The model should be a useful tool for anyone wishing to estimate the number of viruses entering groundwater after traveling through the soil from a contamination source. In addition, model simulations were performed to identify parameters that have a large effect on the results. This information can be used to help design experiments so that important variables are measured accurately.     

Field and microcosm experiments to evaluate the effects of agricultural Cu treatment on the density and genetic structure of microbial communities in two different soils

The effects of Cu amendment on indigenous soil microorganisms were investigated in two soils, a calcareous silty clay (Ep) and a sandy soil (Au), by means of a 1-year field experiment and a two-month microcosm incubation. Cu was added as 'Bordeaux mixture' [CuSO(4), Ca(OH)(2)] at the standard rate used in viticulture (B1=16 kg Cu kg(-1) soil) and at a higher level of contamination (B3=48 kg Cu ha(-1) soil). More extractable Cu was observed in sandy soil (Au) than in silty soil (Ep). Furthermore, total Cu and Cu-EDTA declined with time in Au soil, whereas they remained stable in Ep soil. Quantitative modifications of the microflora were assessed by C-biomass measurements and qualitative modifications were assessed by the characterization of the genetic structure of bacterial and fungal communities from DNA directly extracted from the soil, using B- and F-ARISA (bacterial and fungal automated ribosomal intergenic spacer analysis). In the field study, no significant modifications were observed in C-biomass whereas microcosm incubation showed a decrease in B3 contamination only. ARISA fingerprinting showed slight but significant modifications of bacterial and fungal communities in field and microcosm incubation. These modifications were transient in all cases, suggesting a short-term effect of Cu stress. Microcosm experiments detected the microbial community modifications with greater precision in the short-term, while field experiments showed that the biological effects of Cu contamination may be overcome or hidden by pedo-climatic variations.     

VIRTUS, a model of virus transport in unsaturated soils.

As a result of the recently proposed mandatory groundwater disinfection requirements to inactivate viruses in potable water supplies, there has been increasing interest in virus fate and transport in the subsurface. Several models have been developed to predict the fate of viruses in groundwater, but few include transport in the unsaturated zone and all require a constant virus inactivation rate. These are serious limitations in the models, as it has been well documented that considerable virus removal occurs in the unsaturated zone and that the inactivation rate of viruses is dependent on environmental conditions. The purpose of this research was to develop a predictive model of virus fate and transport in unsaturated soils that allows the virus inactivation rate to vary on the basis of changes in soil temperature. The model was developed on the basis of the law of mass conservation of a contaminant in porous media and couples the flows of water, viruses, and heat through the soil. Model predictions were compared with measured data of virus transport in laboratory column studies and, with the exception of one point, were within the 95% confidence limits of the measured concentrations. The model should be a useful tool for anyone wishing to estimate the number of viruses entering groundwater after traveling through the soil from a contamination source. In addition, model simulations were performed to identify parameters that have a large effect on the results. This information can be used to help design experiments so that important variables are measured accurately.     

Transport of ammonium and nitrate in saturated porous media incorporating physiobiotransformations and bioclogging

The vertical transport of nitrates from fertilizer application and wastewater irrigation through the subsurface and saturated zone is of major concern to assess the vulnerability of groundwater contamination. The present study addresses the transport of nitrogenous fertilizers such as ammonium and nitrate in the presence of organic carbon (acetate) in a one-dimensional soil column under saturated conditions, considering the effect of adsorption and biotransformation. The soil had a neutral pH range and was classified as loamy sand, with a 0.89% organic carbon content. Batch studies revealed that sorption occurred in the order of ammonium > acetate > nitrate following a Freundlich isotherm model. Mixed heterotrophic native soil bacteria for aerobic nitrification and anoxic denitrification were developed, and the growth kinetic parameters were simulated using a Haldane inhibition model for nitrification and a Monod inhibition model for denitrification. Results from biotransformation studies suggested that denitrification was the predominant process, with significant bacterial growth and clogging of pores occurring monotonously reaching a stationary phase by 12 days. Pore-clogging phenomenon not only reduces the permeability of the soil by 5 orders of magnitude but also increases the contact time of the contaminant with the soil microbe and thereby delays the transport process and decreases the effluent ammonium and nitrate concentrations. A tailing breakthrough in a leaching study illustrates that water flux variation (0.153 and 0.509 cm/min) did not influence the transport of solutes, rather irreversible chemical bonding retains more ammonium than nitrate in the soil matrix.     

Microbial diversity in an in situ reactor system treating monochlorobenzene contaminated groundwater as revealed by 16S ribosomal DNA analysis

A molecular approach based on the construction of 16S ribosomal DNA clone libraries was used to investigate the microbial diversity of an underground in situ reactor system filled with the original aquifer sediments. After chemical steady state was reached in the monochlorobenzene concentration between the original inflowing groundwater and the reactor outflow, samples from different reactor locations and from inflowing and outflowing groundwater were taken for DNA extraction. Small-subunit rRNA genes were PCR-amplified with primers specific for Bacteria, subsequently cloned and screened for variation by restriction fragment length polymorphism (RFLP). A total of 87 bacterial 16S rDNA genes were sequenced and subjected to phylogenetic analysis. The original groundwater was found to be dominated by a bacterial consortium affiliated with various members of the class of Proteobacteria, by phylotypes not affiliated with currently recognized bacterial phyla, and also by sporulating and non-sporulating sulfate-reducing bacteria. The most occurring clone types obtained from the sediment samples of the reactor were related to the beta-Proteobacteria, dominated by sequences almost identical to the widespread bacterium Alcaligenes faecalis, to low G+C gram-positive bacteria and to Acidithiobacillus ferrooxidans (formerly Thiobacillus ferrooxidans) within the gamma subclass of Proteobacteria in the upper reactor sector. Although bacterial phylotypes originating from the groundwater outflow of the reactors also grouped within different subdivisions of Proteobacteria and low G+C gram-positive bacteria, most of the 16S rDNA sequences were not associated with the sequence types observed in the reactor samples. Our results suggest that the different environments were inhabited by distinct microbial communities in respect to their taxonomic diversity, particular pronounced between sediment attached microbial communities from the reactor samples and free-living bacteria from the groundwater in- and outflow.     

Intrinsic potential for immediate biodegradation of toluene in a pristine,energy-limited aquifer

Pristine and energy-limited aquifers are considered to have a low resistance and resilience towards organic pollution. An experiment in an indoor aquifer system revealed an unexpected high intrinsic potential for the attenuation of a short-term toluene contamination. A 30 h pulse of 486 mg of toluene, used as a model contaminant, and deuterated water (D₂O) through an initially pristine, oxic, and organic carbon poor sandy aquifer revealed an immediate aerobic toluene degradation potential. Based on contaminant and tracer break-through curves, as well as mass balance analyses and reactive transport modelling, a contaminant removal of 40 % over a transport distance of only 4.2 m in less than one week of travel time was obtained. The mean first-order degradation rate constant was λ = 0.178 day^?1, corresponding to a half-life time constant T_1/2 of 3.87 days. Toluene-specific stable carbon isotope analysis independently proved that the contaminant mass removal can be attributed to microbial biodegradation. Since average doubling times of indigenous bacterial communities were in the range of months to years, the aerobic biodegradation potential observed is assumed to be present and active in the pristine, energy-limited groundwater ecosystems at any time. Follow-up experiments and field studies will help to quantify the immediate natural attenuation potential of aquifers for selected priority contaminants and will try to identify the key-degraders within the autochthonous microbial communities.     

Detection and quantification of methyl tert-butyl ether-degrading strain PM1 by real-time TaqMan PCR.

The fuel oxygenate methyl tert-butyl ether (MTBE), a widely distributed groundwater contaminant, shows potential for treatment by in situ bioremediation. The bacterial strain PM1 rapidly mineralizes and grows on MTBE in laboratory cultures and can degrade the contaminant when inoculated into groundwater or soil microcosms. We applied the TaqMan quantitative PCR method to detect and quantify strain PM1 in laboratory and field samples. Specific primers and probes were designed for the 16S ribosomal DNA region, and specificity of the primers was confirmed with DNA from 15 related bacterial strains. A linear relationship was measured between the threshold fluorescence (C(T)) value and the quantity of PM1 DNA or PM1 cell density. The detection limit for PM1 TaqMan assay was 2 PM1 cells/ml in pure culture or 180 PM1 cells/ml in a mixture of PM1 with Escherichia coli cells. We could measure PM1 densities in solution culture, groundwater, and sediment samples spiked with PM1 as well as in groundwater collected from an MTBE bioaugmentation field study. In a microcosm biodegradation study, increases in the population density of PM1 corresponded to the rate of removal of MTBE.     

PHYTOREMEDIATION OF BTEX HYDROCARBONS: POTENTIAL IMPACTS OF DIURNAL GROUNDWATER FLUCTUATION ON MICROBIAL DEGRADATION

Volatile hydrocarbons have multiple potential fates in phytoremediation. This research investigated the relationship between biodegradation and plant uptake of BTEX compounds in laboratory and field settings. At a phytoremediation site, preliminary studies revealed minimal uptake into trees and enhanced degradation potential in the rhizosphere and in the bulk soil. Increased oxygen transport to the vadose zone caused by diurnal rise and fall of the water table was hypothesized to enhance degradation in the bulk soil. A detailed greenhouse study was then conducted to investigate potential bioremediation impacts using field-site soil and DN34 hybrid poplar trees.

In rhizosphere soils, the contaminated-planted reactor had significantly higher BTEX degrader populations versus the uncontaminated-planted reactor, as was anticipated. The bulk soil in the planted-contaminated reactor had increased degrader populations than the unplanted-contaminated soil or planted-uncontaminated soil, and planting increased degradation throughout the soil profile, not just in the limited volume of rhizosphere soils. Oxygen diffusive and advective transport into reactors was modeled and calculated. Oxygen input in planted reactors was at least 3 to 5 times higher than in unplanted reactors, and increasing oxygen input lead to increased degrader populations in a linear manner. These results combined with the knowledge that high-transpiration trees draw the contaminated groundwater to the capillary fringe and the rhizosphere indicate that phytoremediation can aid microbial degradation via multiple mechanisms: increasing degrader populations, increasing oxygen input via groundwater diurnal fluctuations, and transporting contaminants to the biologically-enriched soil profile.     

Natural attenuation and enhanced bioremediation of organic contaminants in groundwater

An area of intense scientific and practical interest is the biogeochemical and microbial processes determining the success of natural attenuation, biostimulation and/or bioaugmentation treatments for organic contaminants in groundwater. Recent studies in this area have focused on the reductive dechlorination of chlorinated solvents, the degradation of the fuel additive methyl tert-butyl ether, and the removal of long-term hydrocarbon contamination. These studies have been facilitated by the use of stable isotope analysis to demonstrate in situ bioremediation and push-pull tests, in which isotopes are injected into aquifers and then quickly retrieved and analyzed, to measure in situ activity. Molecular tools such as quantitative PCR, the detection of mRNA expression, and numerous DNA fingerprinting methods have also proved valuable, being employed to identify and sometimes quantify environmentally important organisms or changes in communities. Methods to track bacteria and tools to characterize bacterial attachment properties have also offered insight into bacterial transport in situ.