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1.
Bacteria that degrade natural organic matter in groundwater contain oxygenase enzymes that can co‐oxidize trichloroethene (TCE). This degradation pathway is promising for large dilute plumes, but its evaluation is limited because the density of the bacteria with oxygenase enzymes has not been correlated to field scale rates of degradation. A 14C–TCE assay was developed to determine pseudo first‐order rate constants for the aerobic co‐oxidation of TCE in groundwater. The assay involved incubating 14C–TCE in samples of groundwater contained in 160 mL serum bottles, and monitoring the accumulation of radiolabel in degradation products. A first‐order rate constant for co‐oxidation was extracted from the rate of accumulation of 14C in products, accounting for volumetric changes in the serum bottles due to sampling and subsequent changes to the distribution of TCE between the aqueous and gaseous phases. Of the groundwater samples evaluated from 19 wells at five sites, eight samples at three sites had 14C product accumulation rates that exceeded the accumulation rate in filter‐sterilized groundwater controls. First‐order rate constants ranged from 2.65 to 0.0066 year?1, which is equivalent to half‐lives of 0.26 to 105 years. Groundwater samples from a few of the wells in which co‐oxidation occurred had volatile organic contaminants in addition to TCE; their presence may have induced the oxygenase enzymes that are needed for TCE co‐oxidation. 14CO2 represented ~37% to 97% of the 14C products that accumulated; the balance of the products was soluble and non‐volatile.  相似文献   

2.
A tracer test was conducted to characterize the flow of groundwater across a permeable reactive barrier constructed with plant mulch (a biowall) at the OU‐1 site on Altus Air Force Base, Oklahoma. This biowall is intended to intercept and treat groundwater contaminated by trichloroethylene (TCE) in a shallow aquifer. The biowall is 139‐m long, 7.3‐m deep, and 0.5‐m wide. Bromide was injected from an upgradient well into the groundwater as a conservative tracer, and was subsequently observed breaking through in monitoring wells within and downgradient of the biowall. The bromide breakthrough data demonstrate that groundwater entering the biowall migrated across it, following the slope of the local groundwater surface. The average seepage velocity of groundwater was approximately 0.06 m/d. On the basis of the Darcy velocity of groundwater and geometry of the biowall, the average residence time of groundwater in the biowall was estimated at 10 d. Assuming all TCE removal occurred in the biowall, the reduction in TCE concentrations in groundwater across the biowall corresponds to a first‐order attenuation rate constant in the range of 0.38 to 0.15 per d. As an independent estimate of the degradation rate constant, STANMOD software was used to fit curves through data on the breakthrough of bromide and TCE in selected wells downgradient of the injection wells. Best fits to the data required a first‐order degradation rate constant for TCE removal in the range of 0.13 to 0.17 per d. The approach used in this study provides an objective evaluation of the remedial performance of the biowall that can provide a basis for design of other biowalls that are intended to remediate TCE‐contaminated groundwater.  相似文献   

3.
Vapor intrusion (VI) involves migration of volatile contaminants from subsurface through unsaturated soil into overlying buildings. In 2015, the US EPA recommended an approach for screening VI risks associated with gasoline releases from underground storage tank (UST) sites. Additional assessment of the VI risk from petroleum hydrocarbons was deemed unnecessary for buildings separated from vapor sources by more than recommended vertical screening distances. However, these vertical screening distances did not apply to potential VI risks associated with releases of former leaded gasoline containing 1,2-dichloroethane (1,2-DCA), because of a lack of empirical data on the attenuation of 1,2-DCA in soil gas. This study empirically evaluated 144 paired measurements of 1,2-DCA concentrations in soil gas and groundwater collected at 47 petroleum UST sites combined with BioVapor modeling. This included (1) assessing the frequency of 1,2-DCA detections in soil gas below 10−6 risk-based screening levels at different vertical separation distances and (2) comparing the US EPA recommended vertical screening distances with those predicted by BioVapor modeling. Vertical screening distances were predicted for different soil types using aerobic biodegradation rate constants estimated from the measured soil-gas data combined with conservative estimates of source concentrations. The modeling indicates that the vertical screening distance of 6 feet (1.8 m) recommended for dissolved-phase sources is applicable for 1,2-DCA below certain threshold concentrations in groundwater, while 15 feet (4.6 m) recommended for light nonaqueous phase liquid (LNAPL) sources is applicable for sites with clay and loam soils in the vadose zone, but not sand, if 1,2-DCA concentrations in groundwater exceed 150 μg/L. This dependence of the predicted vertical screening distances on soil type places added emphasis on proper soil characterization for VI screening at sites with 1,2-DCA sources. The soil-gas data suggests that a vertical screening distance of 15 feet (4.6 m) is necessary for both dissolved-phase and LNAPL sources.  相似文献   

4.
A study was conducted to evaluate monitored natural attenuation (MNA) as a remedy for arsenic in groundwater at a former phosphate mining and manufacturing facility. The mineralogy, speciation, and lability of arsenic in phosphatic wastes present in soils were characterized using sequential extraction procedures, leaching experiments, batch adsorption tests, and microchemical speciation analysis. A PHREEQC-based reactive transport model was also parameterized using these laboratory results, and it was used to evaluate the importance of identified attenuation mechanisms on arsenic concentrations along a vertical flow path from a shallow, alluvial aquifer to the underlying Floridan aquifer. Arsenic was found to occur in several chemical forms in phosphatic wastes, including unstable sulfide minerals, adsorbed surface complexes, and relatively insoluble phosphate and oxide minerals. Most arsenic was associated with stable minerals. The reactive transport model predicted that historical leaching of solid-phase waste materials in soils would not have generated enough arsenic to explain the concentrations observed in downgradient groundwater; instead, the source of arsenic to groundwater was likely acidic and saline process water that infiltrated though unlined ponds and ditches during historical manufacturing operations. A key factor affecting the long-term effectiveness of natural attenuation of arsenic in groundwater is the occurrence and stability of iron oxyhydroxides in aquifer sediments. According to laboratory and reactive transport model results, sufficient levels were found to be present at the site to effectively limit arsenic migration at concentrations exceeding drinking water standards in the future in the Floridan aquifer. This study presents the geochemical evaluations that are needed to satisfy EPA guidelines on determining whether or not MNA is an acceptable remedy for a site. It specifically details the characterization and modeling that were used to demonstrate effectiveness at a site where MNA was ultimately selected as the remedy for arsenic in groundwater.  相似文献   

5.
Site 24 was the subject of a 14-year (5110-day) study of a ground water plume created by the disposal of manufactured gas plant (MGP) tar into a shallow sandy aquifer approximately 25 years prior to the study. The ground water plume in 1988 extended from a well-defined source area to a distance of approximately 400 m down gradient. A system of monitoring wells was installed along six transects that ran perpendicular to the longitudinal axis of the plume centerline. The MGP tar source was removed from the site in 1991 and a 14-year ground water monitored natural attenuation (MNA) study commenced. The program measured the dissolved mono- and polycyclic aromatic hydrocarbons (MAHs and PAHs) periodically over time, which decreased significantly over the 14-year period. Naphthalene decreased to less than 99% of the original dissolved mass, with mass degradation rates of 0.30 per year (half-life 2.3 years). Bulk attenuation rate constants for plume centerline concentrations over time ranged from 0.33 ± 0.09 per year (half-life 2.3 ± 0.8 years) for toluene and 0.45 ± 0.06 per year (half-life 1.6 ± 0.2 years) for naphthalene. The hydrogeologic setting at Site 24, having a sandy aquifer, shallow water table, clay confining layer, and aerobic conditions, was ideal for demonstrating MNA. However, these results demonstrate that MNA is a viable remedial strategy for ground water at sites impacted by MAHs and PAHs after the original source is removed, stabilized, or contained.  相似文献   

6.
Large laboratory columns (15.2 cm diameter, 183 cm long) were fed with groundwater containing trichloroethylene (TCE), were biostimulated and bioaugmented, and were monitored for over 7.5 years. The objective of the study was to observe how the selection of the carbon and energy source, i.e., whey, Newman Zone® standard surfactant emulsified oil and Newman Zone nonionic surfactant emulsified oil, affected the rate and extent of dechlorination. Column effluent was monitored for TCE and its degradation products, redox indicators (nitrate‐N, Fe(II), sulfate), and changes in iron mineralogy. Total bacteria and Dehalococcoides mccartyi strains were quantified using q‐PCR. Complete dechlorination was only observed in the whey treated columns, occurring 1 year after bioaugmentation with addition of a culture known to dechlorinate TCE to ethene, and 3 years later in the non‐bioaugmented column. The addition of the emulsified oils with or without bioaugmentation resulted in dechlorination only through cis‐DCE and vinyl chloride. While Dehalococcoides mccartyi strains are the only known bacteria that can fully dechlorinate TCE, their presence, either natural or augmented, was not the sole determiner of complete dechlorination. The establishment of a supporting microbial community and biogeochemistry that developed with continuous feeding of whey, in addition to the presence of D. mccartyi, were necessary to support complete reductive dechlorination. Results confirm that careful selection of a biostimulant is critical to the success of TCE dechlorination in complex soil environments.  相似文献   

7.
In situ bioremediation is being considered to optimize an existing pump‐and‐treat remedy for treatment of explosives‐contaminated groundwater at the Umatilla Chemical Depot. Push‐pull tests were conducted using a phased approach to measure in situ hexahydro‐1,3,5‐trinitro‐1,3,5‐triazine (RDX) and 2,4,6‐trinitrotoluene (TNT) degradation rates associated with various carbon substrates. Phase I included short‐duration transport tests conducted in each well to determine dilution rates and retardation factors for RDX and TNT. Phase II included aquifer “feedings” conducted by injecting 150 gallons of treated site groundwater amended with ethanol, corn syrup, lactose or emulsified oil (concentrations 10, 25 and 27 mM, respectively; 12% by volume for emulsified oil). Wells received up to 6 substrate “feedings” over the course of 3 months followed by monitoring dissolved oxygen, nitrate, Fe(II), and sulfate to gauge in situ redox conditions as indicators of anaerobic microbial activity. Phase III included push‐pull tests conducted by injecting 150 gallons of site groundwater amended with approximately 1000 µg/L RDX, 350 µg/L TNT, carbon substrate and a conservative tracer, followed by sampling over 8 d. Corn syrup resulted in the best RDX removal (82% on average) and the largest RDX degradation rate coefficient (1.4 ± 1.1 d?1). Emulsified oil resulted in the best TNT removal (99%) and largest TNT degradation rate coefficient (5.7 × 10?2 d?1). These results will be used to simulate full‐scale in situ bioremediation scenarios at Umatilla and will support a go/no‐go decision to initiate full‐scale bioremediation remedy optimization.  相似文献   

8.
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9.
Thermal methods are promising for remediating fractured geologic media contaminated with volatile organic compounds, and the success of this process depends on the coupled heat transfer, multiphase flow, and thermodynamics. This study analyzed field‐scale removal of trichloroethylene (TCE) and heat transfer behavior in boiling fractured geologic media using the multiple interacting continua method. This method can resolve local gradients in the matrix and is less computationally demanding than alternative methods like discrete fracture‐matrix models. A 2D axisymmetric model was used to simulate a single element of symmetry in a repeated pattern of extraction wells inside a large heated zone and evaluate effects of parameter sensitivity on contaminant recovery. The results showed that the removal of TCE increased with matrix permeability, and the removal rate was more sensitive to matrix permeability than any other parameter. Increasing fracture density promoted TCE removal, especially when the matrix permeability was low (e.g., <10?17 m2). A 3D model was used to simulate an entire treatment zone and the surrounding groundwater in fractured material, with the interaction between them being considered. Boiling was initiated in the center of the upper part of the heated region and expanded toward the boundaries. This boiling process resulted in a large increase in the TCE removal rate and spread of TCE to the vadose zone and the peripheries of the heated zone. The incorporation of extraction wells helped control the contaminant from migrating to far regions. After 22 d, more than 99.3% of TCE mass was recovered in the simulation.  相似文献   

10.
Like tree rings, high‐resolution soil sampling of low‐permeability (low‐k) zones can be used to evaluate the style of source history at contaminated sites (i.e., historical pattern of concentration and composition vs. time since releases occurred at the interface with the low‐k zone). This is valuable for the development of conceptual site model (CSM) and can serve as an important line of evidence supporting monitored natural attenuation (MNA) as a long‐term remedy. Source histories were successfully reconstructed at two sites at Naval Air Station Jacksonville using a simple one‐dimensional (1D) model. The plume arrival time and historical composition were reconstructed from the time initial releases that were suspected to occur decades earlier. At the first site (Building 106), the source reconstructions showed relatively constant source concentrations, but significant attenuation over time in the downgradient plume in the transmissive zone, suggesting MNA may not be an appropriate remedy if source control is a requirement, but attenuation processes are clearly helping to maintain plume stability and reduce risk. At the second site (Building 780), source concentrations in the transmissive zone showed an approximately a one order of magnitude over time, but apparently less attenuation in the downgradient plume. The source reconstruction method appeared to reflect site remediation efforts (excavation, soil vapor extraction) implemented in the 1990s. Finally, a detailed analysis using molecular biological tools, carbon isotopes, and by‐products suggests that most degradation activity is associated with high‐k zones but not with low‐k zones at these source areas. Overall, the source reconstruction methodology provided insight into historical concentration trends not obtainable otherwise given the limited long‐term monitoring data.  相似文献   

11.
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12.
The potential application of monitored natural attenuation (MNA) as a remedy for ground water contaminated with arsenic (As) is examined for a subset of contaminated sites, specifically those where naturally occurring As has been mobilized due to localized anthropogenic organic carbon (OC) releases. This includes sites subject to petroleum releases, exposure to landfill leachates, and OC additions for biostimulation of reductive dechlorination of chlorinated solvents. The key characteristic of these sites is that, under conditions prevailing before the anthropogenic OC introduction, the naturally occurring As in the subsurface was not mobile and did not adversely affect ground water quality. This suggests that, in the far-field (where background conditions are (re) established), As may be sequestered upon contact of the contaminated ground water with either or both the (uncontaminated) ambient ground water and the background aquifer minerals. The observed extents of elevated concentrations (or "footprints") of As and other chemical species, such as dissolved OC and iron (Fe), and related parameters, such as redox potential ( E h) and dissolved oxygen, and their evolution over time can be used to assess the mobilization and sequestration of As and the potential feasibility of MNA as a remedial option. Ultimately, the capacity for As sequestration must be assessed in the context of the OC loading to the site, which may require "active" measures for source control. Monitoring is needed to confirm the continuing effectiveness of the MNA remedy or to indicate if contingency measures must be implemented.  相似文献   

13.
There has been a great deal of research interest regarding changes in flow path/runoff source with increases in catchment area. However, there have been very few quantitative studies taking subscale variability and convergence of flow path/runoff source into account, especially in relation to headwater catchments. This study was performed to elucidate how the contributions and discharge rates of subsurface water (water in the soil layer) and groundwater (water in fractured bedrock) aggregate and change with catchment area increase, and to elucidate whether the spatial variability of the discharge rate of groundwater determines the spatial variability of stream discharge or groundwater contribution. The study area was a 5‐km2 forested headwater catchment in Japan. We measured stream discharge at 113 points and water chemistry at 159 points under base flow conditions. End‐member mixing analysis was used to separate stream water into subsurface water and groundwater. The contributions of both subsurface water and groundwater had large variability below 1 km2. The contribution of subsurface water decreased markedly, while that of groundwater increased markedly, with increases in catchment area. The specific discharge of subsurface water showed a large degree of variability and decreased with catchment area below 0.1 km2, becoming almost constant above 0.1 km2. The specific discharge of groundwater showed large variability below 1 km2 and increased with catchment area. These results indicated that the variabilities of stream discharge and groundwater contribution corresponded well with the variability of the discharge rate of groundwater. However, below 0.1 km2, it was necessary to consider variations in the discharge rates of both subsurface water and groundwater. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

14.
Lack of filtration and rapid transport of groundwater and particulate matter make karst aquifers susceptible to bacterial contamination. This study utilized quantitative polymerase chain reaction (qPCR) to examine the transport and attenuation of two nonvirulent isolates of Escherichia coli (E. coli) in relation to traditional groundwater tracers (rhodamine WT dye and 1-µm diameter latex microspheres) in a karst-conduit aquifer in central Kentucky. Bacterial isolates were labeled with stable isotopes (15N and 13C). All tracers were detected more than 6 km downstream from the injection site and demonstrated overlapping breakthrough curves, with differential transport observed between the two bacterial strains. The E. coli isolate containing the kps gene (low attachment) arrived at sampling sites 1.25 to 36 h prior to the bacterial isolate containing the iha gene (high attachment) and was detected in samples collected following storm events in which the iha isolate was not detected. The storage potential of contaminants within karst systems was demonstrated by the remobilization of all tracers during storm events more than 1 month after injection. Bacteria-sized microspheres were more easily remobilized during periods of increased discharge compared to other tracers. The study demonstrated that molecular biology techniques such as qPCR can be utilized as a sensitive analysis of bacterial tracers in karst aquifers and may prove to be a more sensitive analytical technique than stable isotope analysis for field-scale traces.  相似文献   

15.
Kim Y  Istok JD  Semprini L 《Ground water》2004,42(3):329-337
Three types of single-well push-pull tests were developed for use in assessing the feasibility of in situ aerobic cometabolism of chlorinated aliphatic hydrocarbons (CAHs). These included transport tests, biostimulation tests, and activity tests. Transport tests are conducted to evaluate the mobility of solutes used in subsequent tests. These included bromide or chloride (conservative tracers), propane (growth substrate), ethylene, propylene (CAH surrogates), dissolved oxygen (electron acceptor), and nitrate (a minor nutrient). Tests were conducted at an experimental wellfield of Oregon State University. At this site, extraction phase breakthrough curves for all solutes were similar, indicating apparent conservative transport of the dissolved gases and nitrate prior to biostimulation. Biostimulation tests were conducted to stimulate propane-utilizing activity of indigenous microorganisms and consisted of sequential injections of site ground water containing dissolved propane and oxygen. Biostimulation was detected by the increase in rates of propane and oxygen utilization after each injection. Activity tests were conducted to quantify rates of substrate utilization and to confirm that CAH-transforming activity had likely been stimulated. In particular, the transformation of injected CAH surrogates ethylene and propylene to the cometabolic byproducts ethylene oxide and propylene oxide provided evidence that activity of the monooxygenase enzyme system, responsible for aerobic cometabolic transformations of CAHs, had likely also been stimulated. Estimated zero-order transformation rates decreased in the order propane > ethylene > propylene. The series of push-pull tests developed and field tested in this study should prove useful for conducting rapid, low-cost feasibility assessments for in situ aerobic cometabolism of CAHs.  相似文献   

16.
Comparative studies of the use of chlorine/ultraviolet (Cl2/UV) and hydrogen peroxide/ultraviolet (H2O2/UV) Advanced oxidation processes (AOPs) to remove trichloroethylene (TCE) from groundwater in a pump‐and‐treat application were conducted for the first time at the full‐scale operational level at two water treatment facilities in Northern California. In these studies, aqueous chlorine replaced hydrogen peroxide in the AOP treatment step, where the oxidant is exposed to UV light to produce highly reactive radical species that degrade groundwater contaminants. TCE removal rates as a function of initial chlorine dose and pH were then determined. At the site where the natural pH of the water was 7.1, TCE was removed (to a concentration of less than 0.5 µg/L) for nearly every chlorine dose point tested, and pH adjustment slightly enhanced the treatment process at this facility. The second site had a high natural pH of 7.7, and here, TCE was not completely removed for any chlorine dose up to 5.7 mg/L, although TCE removal did increase when the chlorine dose increased between 0.9 and 3.6 mg/L. Residual TCE remaining in the water post‐Cl2/UV was readily removed using active carbon filtration, which is part of the overall treatment train at this facility. These studies also verified that Cl2/UV AOP did not interfere with the photolysis of N‐nitrosodimethylamine or result in an effluent acutely toxic toward Ceriodaphnia dubia. Comparative economic analysis revealed that the chemical costs associated with Cl2/UV AOP were 25 to 50% of the costs associated with in place H2O2/UV AOP treatment.  相似文献   

17.
Lu C  Chen Y  Luo J 《Ground water》2012,50(3):386-393
Prevention of sea water intrusion in coastal aquifers subject to groundwater withdrawal requires optimization of well pumping rates to maximize the water supply while avoiding sea water intrusion. Boundary conditions and the aquifer domain size have significant influences on simulating flow and concentration fields and estimating maximum pumping rates. In this study, an analytical solution is derived based on the potential-flow theory for evaluating maximum groundwater pumping rates in a domain with a constant hydraulic head landward boundary. An empirical correction factor, which was introduced by Pool and Carrera (2011) to account for mixing in the case with a constant recharge rate boundary condition, is found also applicable for the case with a constant hydraulic head boundary condition, and therefore greatly improves the usefulness of the sharp-interface analytical solution. Comparing with the solution for a constant recharge rate boundary, we find that a constant hydraulic head boundary often yields larger estimations of the maximum pumping rate and when the domain size is five times greater than the distance between the well and the coastline, the effect of setting different landward boundary conditions becomes insignificant with a relative difference between two solutions less than 2.5%. These findings can serve as a preliminary guidance for conducting numerical simulations and designing tank-scale laboratory experiments for studying groundwater withdrawal problems in coastal aquifers with minimized boundary condition effects.  相似文献   

18.
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19.
Groundwater discharge flux into rivers (riverine groundwater discharge or RGD) is essential information for the conservation and management of aquatic ecosystems and resources. One way to estimate area-integrated groundwater discharge into surface water bodies is to measure the concentration of a groundwater tracer within the water body. We assessed groundwater discharge using 222Rn, a tracer common in many surface water studies, through field measurements, surface water 222Rn mass balance model, and groundwater flow simulation, for the seldom studied but ubiquitous setting of a flooding river corridor. The investigation was conducted at the dam-regulated Lower Colorado River (LCR) in Austin, Texas, USA. We found that 222Rn in both the river water and groundwater in the river bank changed synchronously over a 12-hour flood cycle. A 222Rn mass balance model allowed for estimation of groundwater discharge into a 500-m long reach of the LCR over the flood. The groundwater discharge ranged between negative values (indicating recharge) to 1570 m3/h; groundwater discharge from groundwater flow simulations corroborated these estimates. However, for the dynamic groundwater discharge estimated by the 222Rn box model, assuming whether the groundwater 222Rn endmember was constant or dynamic led to notably different results. The resultant groundwater discharge estimates are also highly sensitive to river 222Rn values. We thus recommend that when using this approach to accurately characterize dynamic groundwater discharge, the 222Rn in near-stream groundwater should be monitored at the same frequency as river 222Rn. If this is not possible, the 222Rn method can still provide reasonable but approximate groundwater discharge given background information on surface water-groundwater exchange time scales.  相似文献   

20.
A field-scale demonstration project was conducted to evaluate the capability of eastern cottonwood trees (Populus deltoides) to attenuate trichloroethene (TCE) contamination of ground water. By the middle of the sixth growing season, trees planted where depth to water was <3 m delivered enough dissolved organic carbon to the underlying aquifer to lower dissolved oxygen concentrations, to create iron-reducing conditions along the plume centerline and sulfate-reducing or methanogenic conditions in localized areas, and to initiate in situ reductive dechlorination of TCE. Apparent biodegradation rate constants for TCE along the centerline of the plume beneath the phytoremediation system increased from 0.0002/d to 0.02/d during the first six growing seasons. The corresponding increase in natural attenuation capacity of the aquifer along the plume centerline, from 0.0004/m to 0.024/m, is associated with a potential decrease in plume-stabilization distance from 9680 to 160 m. Demonstration results provide insight into the amount of vegetation and time that may be needed to achieve cleanup objectives at the field scale.  相似文献   

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