Denitrification in a Sand and Gravel Aquifer

Denitrification was assayed by the acetylene blockage technique in slurried core material obtained from a freshwater sand and gravel aquifer. The aquifer, which has been contaminated with treated sewage for more than 50 years, had a contaminant plume greater than 3.5-km long. Near the contaminant source, groundwater nitrate concentrations were greater than 1 mM, whereas 0.25 km downgradient the central portion of the contaminant plume was anoxic and contained no detectable nitrate. Samples were obtained along the longitudinal axis of the plume (0 to 0.25 km) at several depths from four sites. Denitrification was evident at in situ nitrate concentrations at all sites tested; rates ranged from 2.3 to 260 pmol of N₂O produced (g of wet sediment)⁻¹ h⁻¹. Rates were highest nearest the contaminant source and decreased with increasing distance downgradient. Denitrification was the predominant nitrate-reducing activity; no evidence was found for nitrate reduction to ammonium at any site. Denitrifying activity was carbon limited and not nitrate limited, except when the ambient nitrate level was less than the detection limit, in which case, even when amended with high concentrations of glucose and nitrate, the capacity to denitrify on a short-term basis was lacking. These results demonstrate that denitrification can occur in groundwater systems and, thereby, serve as a mechanism for nitrate removal from groundwater.     

Selecting Remediation Goals by Assessing the Natural Attenuation Capacity of Groundwater Systems

Remediation goals for the source areas of a chlorinated ethene-contaminated groundwater plume were identified by assessing the natural attenuation capacity of the aquifer system. The redox chemistry of the site indicates that sulfate-reducing (H₂ ∼ 2 nanomoles [nM]) per liter conditions near the contaminant source grade to Fe(III)-reducing conditions (H₂ ∼ 0.5 nM) downgradient of the source. Sulfate-reducing conditions facilitate the initial reduction of perchloroethene (PCE) to trichloroethene (TCE), cis-dichloroethene (cis-DCE), and vinyl chloride (VC). Subsequently, the Fe(III)-reducing conditions drive the oxidation of cis-DCE and VC to carbon dioxide and chloride. This sequence gives the aquifer a substantial capacity for biodegrading chlorinated ethenes. Natural attenuation capacity (the slope of the steady-state contaminant concentration profile along a groundwater flowpath) is a function of biodegradation rates, aquifer dispersive characteristics, and groundwater flow velocity. The natural attenuation capacity at the Kings Bay, Georgia site was assessed by estimating groundwater flowrates (∼0.23±0.12 m/d) and aquifer dispersivity (∼1 m) from hydrologic and scale considerations. Apparent biodegradation rate constants (PCE and TCE ∼0.01 d^-1; cis-DCE and VC ∼0.025 d^-1) were estimated from observed contaminant concentration changes along aquifer flowpaths. A boundary-value problem approach was used to estimate levels to which contaminant concentrations in the source areas must be lowered (by engineered removal), or groundwater flow velocities lowered (by pumping) for the natural attenuation capacity to achieve maximum concentration limits (MCLs) prior to reaching a predetermined regulatory point of compliance.     

Monitoring the Effect of Poplar Trees on Petroleum-Hydrocarbon and Chlorinated-Solvent Contaminated Ground Water

At contaminated groundwater sites, poplar trees can be used to affect ground-water levels, flow directions, and ultimately total groundwater and contaminant flux to areas downgradient of the trees. The magnitude of the hydrologic changes can be monitored using fundamental concepts of groundwater hydrology, in addition to plant physiology-based approaches, and can be viewed as being almost independent of the contaminant released. The affect of poplar trees on the fate of groundwater contaminants, however, is contaminant dependent. Some petroleum hydrocarbons or chlorinated solvents may be mineralized or transformed to innocuous compounds by rhizospheric bacteria associated with the tree roots, mineralized or transformed by plant tissues in the transpiration stream or leaves after uptake, or passively volatilized and rapidly dispersed or oxidized in the atmosphere. These processes also can be monitored using a combination of physiological- or geochemical-based field or laboratory approaches. When combined, such hydrologic and contaminant monitoring approaches can result in a more accurate assessment of the use of poplar trees to meet regulatory goals at contaminated groundwater sites, verify that these goals continue to be met in the future, and ultimately lead to a consensus on how the performance of plant-based remedial strategies (phytoremediation) is to be assessed.     

Field-scale bioremediation of arsenic-contaminated groundwater using sulfate-reducing bacteria and biogenic pyrite

This research demonstrates that biogenic pyrite formed by stimulation of indigenous sulfate-reducing bacteria (SRB) in a natural aquifer can remove dissolved arsenic from contaminated groundwater under strongly reducing conditions. SRB metabolism led to the precipitation of biogenic pyrite nanoparticles capable of sorbing and co-precipitating arsenic. The field site is an industrial site where shallow groundwater in an unconfined sandy aquifer is contaminated by arsenic. Therefore, biodegradable organic carbon, ferrous iron, sulfate, and fertilizer were injected into groundwater and SRB metabolism began about 1 week later. Microscopic, X-ray diffraction, X-ray fluorescence, and electron microprobe analyses confirm the bio-mineralization of pyrite and over time, pyrite nanoparticles grew to form well-formed crystals (1–10?µm in diameter) or spherical aggregates that contain 0.05–0.4?wt. % arsenic, indicative of their capacity to sequester arsenic. Consequently, dissolved arsenic decreased from its initial concentration of 0.3–0.5?mg/L to below the regulatory clean-up standard for the site of 0.05?mg/L in three downgradient wells in a matter of weeks after injection. The main sequestration stage, with total arsenic removal rates greater than 90%, lasted for at least 6 months until the arrival and mixing of untreated groundwater from upgradient. Treated groundwater with most active bacterial sulfate reduction became enriched in heavy ³⁴S (range from 2.02 to 4.00 ‰) compared to unaffected well water (0.40–0.61 ‰). One to three orders of magnitude increases in SRB cells were observed in treated wells for at least 2?months after injection. For a full-scale remediation, the injection of solution should start at positions hydrologically upgradient from the major plume and proceed downgradient. If needed, aquifers may be repeatedly amended with biodegradable organic carbon to reestablish the reducing conditions that favor arsenic sequestration.     

Spatial and temporal changes in microbial community structure associated with recharge-influenced chemical gradients in a contaminated aquifer

In a contaminated water-table aquifer, we related microbial community structure on aquifer sediments to gradients in 24 geochemical and contaminant variables at five depths, under three recharge conditions. Community amplified ribsosomal DNA restriction analysis (ARDRA) using universal 16S rDNA primers and denaturing gradient gel electrophoresis (DGGE) using bacterial 16S rDNA primers indicated: (i). communities in the anoxic, contaminated central zone were similar regardless of recharge; (ii). after recharge, communities at greatest depth were similar to those in uncontaminated zones; and (iii). after extended lack of recharge, communities at upper and lower aquifer margins differed from communities at the same depths on other dates. General aquifer geochemistry was as important as contaminant or terminal electron accepting process (TEAP) chemistry in discriminant analysis of community groups. The Shannon index of diversity (H) and the evenness index (E), based on DGGE operational taxonomic units (OTUs), were statistically different across community groups and aquifer depths. Archaea or sulphate-reducing bacteria 16S rRNA abundance was not clearly correlated with TEAP chemistry indicative of methanogenesis or sulphate reduction. Eukarya rRNA abundance varied by depth and date from 0 to 13% of the microbial community. This contaminated aquifer is a dynamic ecosystem, with complex interactions between physical, chemical and biotic components, which should be considered in the interpretation of aquifer geochemistry and in the development of conceptual or predictive models for natural attenuation or remediation.     

Effect of organic contamination upon microbial distributions and heterotrophic uptake in a Cape Cod, Mass., aquifer.

Bacterial abundance, distribution, and heterotrophic uptake in a freshwater aquifer contaminated by treated sewage were determined from analyses of groundwater and sediment-core samples. The number of free-living (unattached) bacteria in contaminated groundwater declined steadily with increasing distance from the source of sewage infiltration, from 1.94 (+/- 0.20) X 10(6) ml-1 at 0.21 km to 0.25 (+/- 0.02) X 10(6) ml-1 at 0.97 km. Bacterial abundance in groundwater sampled at 0.31 km correlated strongly with specific conductance and increased sharply from 4.0 (+/- 0.3) X 10(4) ml-1 at a depth of 6 m to 1.58 (+/- 0.12) X 10(6) ml-1 at 14 m, then declined at 20 and 31 m to 1.29 (+/- 0.12) X 10(6) and 0.96 (+/- 0.12) X 10(6) ml-1, respectively. A majority of the bacteria in contaminated and uncontaminated zones of the aquifer were bound to the surfaces of particulates, less than 60 micron in diameter. The glucose uptake rate, assayed at in situ and 5 microM concentrations, declined steadily in contaminated groundwater sampled along a transect. A preparative wet-sieving technique for use in processing core samples for bacterial enumeration is described and evaluated.     

Laboratory Evaluations of Factors Affecting Biodegradation of Sulfolane and Diisopropanolamine

Sulfolane and diisopropanolamine (DIPA) are used in the Sulfinol® process to remove hydrogen sulfide from sour natural gas. This process has been used in western Canada since the early 1960s, and contamination of groundwater has occurred from surface spills and from seepage from landfills and unlined process water storage ponds. Aquifer sediments from contaminated and uncontaminated areas, and muds in a wetland downgradient from the contaminated plume, were collected from a gas plant. Vigorously agitated shake-flask cultures and gently agitated 2.5-L microcosms consisting of contaminated sediment, mud and groundwater, or wetland water were used to study the biodegradation of sulfolane and DIPA. The aerobic shake-flask method showed that all five of these materials contained microbial communities that biodegraded both compounds. Microorganisms in all samples, except the uncontaminated aquifer sediment, degraded both compounds in the aerobic 2.5-L microcosms. In general, the biodegradation occurred more rapidly in the shake-flask cultures. The addition of P greatly enhanced the degradation of sulfolane and DIPA, whereas the addition of N yielded little stimulation.     

Successful treatment of an MTBE-impacted aquifer using a bioreactor self-colonized by native aquifer bacteria

A field-scale fixed bed bioreactor was used to successfully treat an MTBE-contaminated aquifer in North Hollywood, CA without requiring inoculation with introduced bacteria. Native bacteria from the MTBE-impacted aquifer rapidly colonized the bioreactor, entering the bioreactor in the contaminated groundwater pumped from the site, and biodegraded MTBE with greater than 99 % removal efficiency. DNA sequencing of the 16S rRNA gene identified MTBE-degrading bacteria Methylibium petroleiphilum in the bioreactor. Quantitative PCR showed M. petroleiphilum enriched by three orders of magnitude in the bioreactor above densities pre-existing in the groundwater. Because treatment was carried out by indigenous rather than introduced organisms, regulatory approval was obtained for implementation of a full-scale bioreactor to continue treatment of the aquifer. In addition, after confirmation of MTBE removal in the bioreactor to below maximum contaminant limit levels (MCL; MTBE = 5 μg L^?1), treated water was approved for reinjection back into the aquifer rather than requiring discharge to a water treatment system. This is the first treatment system in California to be approved for reinjection of biologically treated effluent into a drinking water aquifer. This study demonstrated the potential for using native microbial communities already present in the aquifer as an inoculum for ex-situ bioreactors, circumventing the need to establish non-native, non-acclimated and potentially costly inoculants. Understanding and harnessing the metabolic potential of native organisms circumvents some of the issues associated with introducing non-native organisms into drinking water aquifers, and can provide a low-cost and efficient remediation technology that can streamline future bioremediation approval processes.     

Dynamics of an oligotrophic bacterial aquifer community during contact with a groundwater plume contaminated with benzene, toluene, ethylbenzene, and xylenes: an in situ mesocosm study

An in situ mesocosm system was designed to monitor the in situ dynamics of the microbial community in polluted aquifers. The mesocosm system consists of a permeable membrane pocket filled with aquifer material and placed within a polypropylene holder, which is inserted below groundwater level in a monitoring well. After a specific time period, the microcosm is recovered from the well and its bacterial community is analyzed. Using this system, we examined the effect of benzene, toluene, ethylbenzene, and xylene (BTEX) contamination on the response of an aquifer bacterial community by denaturing gradient gel electrophoresis analysis of PCR-amplified 16S rRNA genes and PCR detection of BTEX degradation genes. Mesocosms were filled with nonsterile or sterile aquifer material derived from an uncontaminated area and positioned in a well located in either the uncontaminated area or a nearby contaminated area. In the contaminated area, the bacterial community in the microcosms rapidly evolved into a stable community identical to that in the adjacent aquifer but different from that in the uncontaminated area. At the contaminated location, bacteria with tmoA- and xylM/xylE1-like BTEX catabolic genotypes colonized the aquifer, while at the uncontaminated location only tmoA-like genotypes were detected. The communities in the mesocosms and in the aquifer adjacent to the wells in the contaminated area consisted mainly of Proteobacteria. At the uncontaminated location, Actinobacteria and Proteobacteria were found. Our results indicate that communities with long-term stability in their structures follow the contamination plume and rapidly colonize downstream areas upon contamination.     

Determination of major pollutant and biogeochemical processes in an oil-contaminated aquifer using human health risk assessment and multivariate statistical analysis

Techniques for monitored natural attenuation usually produce large complex datasets that are difficult to interpret. Here, human health risk assessments and multivariate statistical analyses are combined to extract and analyze useful information from large monitoring datasets to identify the main pollutants in a petroleum-contaminated aquifer in northeast China and the main biogeochemical processes affecting the pollutants. The data included organic and inorganic geochemical species concentrations, physicochemical indicators, C and S stable isotope data collected for four years of more than 10 monitoring. The health risk assessment indicated that benzene was a representative pollutant. Cluster analysis classified the groundwater samples into two groups and indicated strong biodegradation occurred near the core and upgradient of the petroleum hydrocarbon plume. The factors explaining most variability were extracted by principal component analysis, which correlated with biodegradation and mineral dissolution processes. The factor scores and spatial distributions of hydrogeochemical and isotope indicators confirmed that biodegradation effects weakened and mineral dissolution strengthened upgradient to downgradient of the contaminated plume. The analysis method could be useful for rapidly studying pollution characteristics and identifying biodegradation processes in contaminated aquifersfrom large complex datasets. The results will provide a basis for developing an enhanced bioremediation scheme for the study site.     

Organic carbon and nutrients drive prokaryote and metazoan communities in a floodplain aquifer

In an alluvial aquifer in the River Fulda Valley (Germany) the influence of agricultural inputs on the subterranean physical, chemical and biological relationships was examined. A 40-year-old (1977–1981) comprehensive data set on the groundwater microbiome plus metazoa was now analysed for the first time in full (measurements for up to 4 years: hydrological, chemical, physical, prokaryote, and metazoa characteristics). Four hydrogeochemically different groundwater zones were identified across the floodplain. In addition, the prokaryote (Archaea and Bacteria) and metazoan communities differed among the four zones. The hydraulic exchange between the alluvial aquifer and the River Fulda influenced the sites closest to the river, leading to the highest prokaryote and metazoan biomasses at these locations. An organic carbon plume zone of anthropogenic origin exhibited high prokaryote abundances and production, which were higher than in the surrounding mixing zone. This mixing zone represented a transition area to the river-influenced sites as well as to the fourth zone, which was characterized by high nutrient levels from intense agriculture and which exhibited low prokaryote abundance and activity and intermediate metazoan abundance. Despite high prokaryote productivity, metazoa did not favor the organic carbon plume, due probably to low oxygen concentrations. At the sites, where metazoa occurred, their biomass corresponded mostly to about one hundredth of the prokaryote biomass. The main implication from this new analysis of an old data set is that even on a coarse taxonomical resolution, patterns emerge that show – in a geologically homogeneous area – an unprecedented complexity among different groundwater zones resulting from different external influences of natural as well as anthropogenic origin. Future studies need to ascertain an adequate temporal and spatial resolution.     

Rapid intrinsic biodegradation of benzene, toluene, and xylenes at the boundary of a gasoline-contaminated plume under natural attenuation

A groundwater plume contaminated with gasoline constituents [mainly benzene, toluene, and xylenes (BTX)] had been treated by pumping and aeration for approximately 10 years, and the treatment strategy was recently changed to monitored natural attenuation (MNA). To gain information on the feasibility of using MNA to control the spread of BTX, chemical and microbiological parameters in groundwater samples obtained inside and outside the contaminated plume were measured over the course of 73 weeks. The depletion of electron acceptors (i.e., dissolved oxygen, nitrate, and sulfate) and increase of soluble iron were observed in the contaminated zone. Laboratory incubation tests revealed that groundwater obtained immediately outside the contaminated zone (the boundary zone) exhibited much higher potential for BTX degradation than those in the contaminated zone and in uncontaminated background zones. The boundary zone was a former contaminated area where BTX were no longer detected. Denaturing gradient gel electrophoresis (DGGE) analysis of polymerase chain reaction (PCR)-amplified bacterial 16S rRNA gene fragments revealed that DGGE profiles for groundwater samples obtained from the contaminated zone were clustered together and distinct from those from uncontaminated zones. In addition, unique bacterial rRNA types were observed in the boundary zone. These results indicate that the boundary zone in the contaminant plumes served as a natural barrier for preventing the BTX contamination from spreading out.     

Relationships between microbial community structure and hydrochemistry in a landfill leachate-polluted aquifer

Knowledge about the relationship between microbial community structure and hydrogeochemistry (e.g., pollution, redox and degradation processes) in landfill leachate-polluted aquifers is required to develop tools for predicting and monitoring natural attenuation. In this study analyses of pollutant and redox chemistry were conducted in parallel with culture-independent profiling of microbial communities present in a well-defined aquifer (Banisveld, The Netherlands). Degradation of organic contaminants occurred under iron-reducing conditions in the plume of pollution, while upstream of the landfill and above the plume denitrification was the dominant redox process. Beneath the plume iron reduction occurred. Numerical comparison of 16S ribosomal DNA (rDNA)-based denaturing gradient gel electrophoresis (DGGE) profiles of Bacteria and Archaea in 29 groundwater samples revealed a clear difference between the microbial community structures inside and outside the contaminant plume. A similar relationship was not evident in sediment samples. DGGE data were supported by sequencing cloned 16S rDNA. Upstream of the landfill members of the beta subclass of the class Proteobacteria (beta-proteobacteria) dominated. This group was not encountered beneath the landfill, where gram-positive bacteria dominated. Further downstream the contribution of gram-positive bacteria to the clone library decreased, while the contribution of delta-proteobacteria strongly increased and beta-proteobacteria reappeared. The beta-proteobacteria (Acidovorax, Rhodoferax) differed considerably from those found upstream (Gallionella, Azoarcus). Direct comparisons of cloned 16S rDNA with bands in DGGE profiles revealed that the data from each analysis were comparable. A relationship was observed between the dominant redox processes and the bacteria identified. In the iron-reducing plume members of the family Geobacteraceae made a strong contribution to the microbial communities. Because the only known aromatic hydrocarbon-degrading, iron-reducing bacteria are Geobacter spp., their occurrence in landfill leachate-contaminated aquifers deserves more detailed consideration.     

Spatial Electron Acceptor Variability: Implications for Assessing Bioremediation Potential

An extensive network of multilevel samplers was established in a hydrocarbon-contaminated wetland aquifer. Results of groundwater sampling for benzene, toluene, ethylbenzene, and xylenes (BTEX), and electron acceptors show that both pristine and contaminated groundwater have spatially variable chemical signatures, owing primarily to microbially mediated oxidation-reduction reactions. Due to these spatial variations, estimates of the efficiency of intrinsic bioremediation can vary significantly depending on how geochemical data are collected. Use of data collected from monitoring wells with screens longer than the vertical extent of the plume will generally underestimate the potential for intrinsic bioremediation for the most chemically active horizon of the plume. A comparison of pristine and contaminated redox patterns demonstrates that, although BTEX exerts the highest demand for electron acceptors, oxidation of natural organic matter also contributes to electron acceptor utilization. If natural and other non-BTEX losses of electron acceptors are ignored, the assimilative capacity, defined as the amount of a contaminant that can potentially be degraded with known amounts of electron acceptors, will be overestimated. Many numerical and analytical models designed to simulate biodegradation are directly or indirectly based on assimilative capacity estimates. Proper estimation of assimilative capacity is crucial if models are to accurately quantify solute concentrations over time and space.     

Dynamics of an Oligotrophic Bacterial Aquifer Community during Contact with a Groundwater Plume Contaminated with Benzene, Toluene, Ethylbenzene, and Xylenes: an In Situ Mesocosm Study

An in situ mesocosm system was designed to monitor the in situ dynamics of the microbial community in polluted aquifers. The mesocosm system consists of a permeable membrane pocket filled with aquifer material and placed within a polypropylene holder, which is inserted below groundwater level in a monitoring well. After a specific time period, the microcosm is recovered from the well and its bacterial community is analyzed. Using this system, we examined the effect of benzene, toluene, ethylbenzene, and xylene (BTEX) contamination on the response of an aquifer bacterial community by denaturing gradient gel electrophoresis analysis of PCR-amplified 16S rRNA genes and PCR detection of BTEX degradation genes. Mesocosms were filled with nonsterile or sterile aquifer material derived from an uncontaminated area and positioned in a well located in either the uncontaminated area or a nearby contaminated area. In the contaminated area, the bacterial community in the microcosms rapidly evolved into a stable community identical to that in the adjacent aquifer but different from that in the uncontaminated area. At the contaminated location, bacteria with tmoA- and xylM/xylE1-like BTEX catabolic genotypes colonized the aquifer, while at the uncontaminated location only tmoA-like genotypes were detected. The communities in the mesocosms and in the aquifer adjacent to the wells in the contaminated area consisted mainly of Proteobacteria. At the uncontaminated location, Actinobacteria and Proteobacteria were found. Our results indicate that communities with long-term stability in their structures follow the contamination plume and rapidly colonize downstream areas upon contamination.     

Effect of growth conditions and staining procedure upon the subsurface transport and attachment behaviors of a groundwater protist

The transport and attachment behaviors of Spumella guttula (Kent), a nanoflagellate (protist) found in contaminated and uncontaminated aquifer sediments in Cape Cod, Mass., were assessed in flowthrough and static columns and in a field injection-and-recovery transport experiment involving an array of multilevel samplers. Transport of S. guttula harvested from low-nutrient (10 mg of dissolved organic carbon per liter), slightly acidic, granular (porous) growth media was compared to earlier observations involving nanoflagellates grown in a traditional high-nutrient liquid broth. In contrast to the highly retarded (retardation factor of approximately 3) subsurface transport previously reported for S. guttula, the peak concentration of porous-medium-grown S. guttula traveled concomitantly with that of a conservative (bromide) tracer. About one-third of the porous-medium-grown nanoflagellates added to the aquifer were transported at least 2.8 m downgradient, compared to only approximately 2% of the broth-grown nanoflagellates. Flowthrough column studies revealed that a vital (hydroethidine [HE]) staining procedure resulted in considerably less attachment (more transport) of S. guttula in aquifer sediments than did a staining-and-fixation procedure involving 4',6'-diamidino-2-phenylindole (DAPI) and glutaraldehyde. The calculated collision efficiency (approximately 10(-2) for porous-medium-grown, DAPI-stained nanoflagellates) was comparable to that observed earlier for the indigenous community of unattached groundwater bacteria that serve as prey. The attachment of HE-labeled S. guttula onto aquifer sediment grains was independent of pH (over the range from pH 3 to 9) suggesting a primary attachment mechanism that may be fundamentally different from that of their prey bacteria, which exhibit sharp decreases in fractional attachment with increasing pH. The high degree of mobility of S. guttula in the aquifer sediments has important ecological implications for the protistan community within the temporally changing plume of organic contaminants in the Cape Cod aquifer.     

Kinetic and microbial community analysis of methyl ethyl ketone biodegradation in aquifer sediments

Methyl ethyl ketone (MEK) is a common groundwater contaminant often present with more toxic compounds of primary interest. Because of this, few studies have been performed to determine the effect of microbial community structure on MEK biodegradation rates in aquifer sediments. Here, microcosms were prepared with aquifer sediments containing MEK following a massive spill event and compared to laboratory-spiked sediments, with MEK biodegradation rates quantified under mixed aerobic/anaerobic conditions. Biodegradation was achieved in MEK-contaminated site sediment microcosms at about half of the solubility (356 mg/L) with largely Firmicutes population under iron-reducing conditions. MEK was biodegraded at a higher rate [4.0 ± 0.74 mg/(L days)] in previously exposed site samples compared to previously uncontaminated sediments [0.51 ± 0.14 mg/(L days)]. Amplicon sequencing and denaturing gradient gel electrophoresis of 16S rRNA genes were combined to understand the relationship between contamination levels, biodegradation, and community structure across the plume. More heavily contaminated sediments collected from an MEK-contaminated field site had the most similar communities than less contaminated sediments from the same site despite differences in sediment texture. The more diverse microbial community observed in the laboratory-spiked sediments reduced MEK concentration 47 % over 92 days. Results of this study suggest lower rates of MEK biodegradation in iron-reducing aquifer sediments than previously reported for methanogenic conditions and biodegradation rates comparable to previously reported nitrate- and sulfate-reducing conditions.     

Growth determinations for unattached bacteria in a contaminated aquifer.

Growth rates of unattached bacteria in groundwater contaminated with treated sewage and collected at various distances from the source of contamination were estimated by using frequency of dividing cells and tritiated-thymidine uptake and compared with growth rates obtained with unsupplemented, closed-bottle incubations. Estimates of bacterial generation times [(In 2)/mu] along a 3-km-long transect in oxygen-depleted (0.1 to 0.7 mg of dissolved oxygen liter-1) groundwater ranged from 16 h at 0.26 km downgradient from an on-land, treated-sewage outfall to 139 h at 1.6 km and correlated with bacterial abundance (r2 = 0.88 at P less than 0.001). Partitioning of assimilated thymidine into nucleic acid generally decreased with distance from the contaminant source, and one population in heavily contaminated groundwater assimilated little thymidine during a 20-h incubation. Several assumptions commonly made when frequency of dividing cells and tritiated-thymidine uptake are used were not applicable to the groundwater samples.     

FIELD NOTE: HYDRAULIC CONTAINMENT OF A BTEX PLUME USING POPLAR TREES

In 1999, 275 poplar trees were planted on a field site near a car factory in order to install a bioscreen. The aim was to combine the biodegradation activities of poplar and its associated rhizosphere and endophytic microorganisms for containing a BTEX contaminated groundwater plume. This BTEX plume occurred as the result of leaking solvents and fuel storage tanks. Monitoring, conducted over a 6-year period (1999–2005) after the planting of the trees suggested that the poplar trees and their associated microorganisms had, once the tree roots reached the contaminated groundwater zone, an active role in the remediation of the BTEX plume, resulting in full containment of the contamination. Analysis of the microbial communities associated with poplar demonstrated that, once the poplar roots got in contact with the BTEX contaminated groundwater, enrichment occurred of both rhizosphere and endophytic bacteria that were able to degrade toluene. Interestingly, once the BTEX plume was remediated, the numbers of toluene degrading rhizosphere and endophytic bacteria decreased below the detection limit, indicating that their population resulted from selective enrichment by the presence of the contaminants.