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1.
A field study of oxygen-enhanced biodegradation was carried out in a sandy iron-rich ground water system contaminated with gasoline hydrocarbons. Prior to the oxygen study, intrinsic microbial biodegradation in the contaminant plume had depleted dissolved oxygen and created anaerobic conditions. An oxygen diffusion system made of silicone polymer tubing was installed in an injection well within an oxygen delivery zone containing coarse highly permeable sand. During the study, this system delivered high dissolved oxygen (DO) levels (39 mg/L) to the ground water within a part of the plume. The ground water was sampled at a series of monitors in the test zone downgradient of the delivery well to determine the effect of oxygen on dissolved BTEX, ground water geochemistry, and microbially mediated biodegradation processes. The DO levels and Eh increased markedly at distances up to 2.3 m (7.5 feet) downgradient. Potential biofouling and iron precipitation effects did not clog the well screens or porous medium. The increased dissolved oxygen enhanced the population of aerobes while the activity of anaerobic sulfate-reducing bacteria and methanogens decreased. Based on concentration changes, the estimated total rate of BTEX biodegradation rose from 872 mg/day before enhancement to 2530 mg/day after 60 days of oxygen delivery. Increased oxygen flux to the test area could account for aerobic biodegradation of 1835 mg/day of the BTEX. The estimated rates of anaerobic biodegradation processes decreased based on the flux of sulfate, iron (II), and methane. Two contaminants in the plume, benzene and ethylbenzene, are not biodegraded as readily as toluene or xylenes under anaerobic conditions. Following oxygen enhancement, however, the benzene and ethylbenzene concentrations decreased about 98%, as did toluene and total xylenes.  相似文献   

2.
The natural attenuation behavior of a ground water contaminant plume containing chromium and chlorinated ethenes in glaciated sediments was assessed using traditional and nontraditional methods. The mixed waste is transported through and attenuated within an estuarine influenced ground water aquifer of spatially varying redox character and organic carbon content. Contaminant fate and speciation were assessed as a function of geochemical conditions. Total, speciation-based, and sequential chemical extraction analyses were performed to determine contaminant partitioning and the redox capacity of the aquifer. Chromium speciation and partitioning were correlated with the reductive capacity and redox conditions of the aquifer sediments spatially distributed within the aquifer. Reductive dechlorination and partitioning of chlorinated ethenes were correlated with the organic carbon content and redox conditions of the aquifer sediments. The data showed that sharp redox gradients existed within the aquifer. Active reduction and retardation of both chromium and chlorinated ethenes was exhibited. The aqueous hexavalent chromium concentrations decreased to near nondetect levels in the vicinity of the receptor, whereas degradation products of higher-order chlorinated ethenes increased as a fraction of the total chlorinated ethene concentrations along the length of the plume. The potential for competition for reducing power under specific cases within the aquifer was suggested by the data, highlighting the need to include contaminant interactions in natural attenuation assessments.  相似文献   

3.
Geochemical Indicators of Intrinsic Bioremediation   总被引：19，自引：0，他引：19  
A detailed field investigation has been completed at a gasoline-contaminated aquifer near Rocky Point, NC, to examine possible indicators of intrinsic bioremediation and identify factors that may significantly influence the rate and extent of bioremediation. The dissolved plume of benzene, toluene, ethylbenzene, and xylene (BTEX) in ground water is naturally degrading. Toluene and o-xylene are most rapidly degraded followed by m-, p-xylene, and benzene. Ethylbenzene appears to degrade very slowly under anaerobic conditions present in the center of the plume. The rate and extent of biodegradation appears to be strongly influenced by the type and quantity of electron acceptors present in the aquifer. At the upgradient edge of the plume, nitrate, ferric iron, and oxygen are used as terminal electron acceptors during hydrocarbon biodegradation. The equivalent of 40 to 50 mg/I of hydrocarbon is degraded based on the increase in dissolved CO2 relative to background ground water. Immediately downgradient of the source area, sulfate and iron are the dominant electron acceptors. Toluene and o-xylene are rapidly removed in this region. Once the available oxygen, nitrate, and sulfate are consumed, biodegradation is limited and appears to be controlled by mixing and aerobic biodegradation at the plume fringes.  相似文献   

4.
In situ air sparging is used to remediate petroleum fuels and chlorinated solvents present as submerged contaminant source /ones and dissolved contaminant plumes, or to provide barriers to dissolved contaminant plume migration. Contaminant removal occurs through a combination of volatilization and aerobic biodegradation: thus, the performance at any given site depends on the contaminant and oxygen mass transfer rates induced by the air injection. It has been hypothesized that these rates are sensitive to changes in process flow conditions and site lithology, but no data is available to identify trends or the magnitude of the changes. In this work, oxygenation rates were measured for a range of air injection rates, ground water flow rates, and pulsing frequencies using a laboratory-scale two-dimensional physical model constructed to simulate a homogeneous hydrogeologic setting. Experiments were conducted with water having low chemical and biochemical oxygen demand. Results suggest the following: that there is an optimum air injection rate: advective How of ground water can be a significant factor when ground water velocities are > 0.3 m/d: and pulsing the air injection had little effect on the oxygenation rate relative lo the continuous air injection case.  相似文献   

5.
Methodology for comparing source and plume remediation alternatives   总被引：2，自引：0，他引：2  
Falta RW 《Ground water》2008,46(2):272-285
It is often difficult at contaminated sites to decide whether remediation effort should be focused on the contaminant source, the dissolved plume, or on both zones. The decision process at these sites is hampered by a lack of quantitative tools for comparing remediation alternatives. A new screening-level mass balance approach is developed for simulating the transient effects of simultaneous ground water source and plume remediation. The contaminant source model is based on a power function relationship between source mass and source discharge, and it can consider partial source remediation at any time after the initial release. The source model serves as a time-dependent mass flux boundary condition to a new analytical plume model, where flow is assumed to be one dimensional, with three-dimensional dispersion. The plume model simulates first-order sequential decay and production of several species, and the decay rates and parent/daughter yield coefficients are variable functions of time and distance. This new method allows for flexible simulation of natural attenuation or remediation efforts that enhance plume degradation. The plume remediation effort may be temporary or delayed in time, limited in space, and it may have different chemical effects on different contaminant species in the decay chain.  相似文献   

6.
Evaluation of BTEX Remediation by Natural Attenuation at a Coastal Facility   总被引：1，自引：0，他引：1  
Natural attenuation has emerged as a potential alternative for remediating sites contaminated with fuel hydrocarbons. This paper examines the viability of using attenuation through natural processes to remediate ground water at an industrial facility. The research combined field assessments with data analysis and modeling to evaluate plume stability and predict remediation times. Field data on natural attenuation indicate that BTEX contamination at the site is being attenuated at rates that vary within the range of 0.0001 to 0.0073/day. Stability analyses confirm that the BTEX plume has reached steady state. An analysis on mass flux showed that between 1979 and 1996, 95,000 pounds of BTEX were lost via biodegradation, while 8000 pounds were lost through other mechanisms. A first-order biodegradation rate of 0.0002/day for BTEX was obtained from the change in the total mass of dissolved BTEX with time. Cleanup times in excess of 200 years for the site were estimated using analytical modeling of natural attenuation, which agreed well with the remediation times estimated using the attenuation rates calculated for the site.  相似文献   

7.
More than 70 individual VOCs were identified in the leachate plume of a closed municipal landfill. Concentrations were low when compared with data published for other landfills, and total VOCs accounted for less than 0.1% of the total dissolved organic carbon. The VOC concentrations in the core of the anoxic leachate plume are variable, but in all cases they were found to be near or below detection limits within 200 m of the landfill. In contrast to the VOCs, the distributions of chloride ion, a conservative tracer, and nonvolatile dissolved organic carbon, indicate little dilution over the same distance. Thus, natural attenuation processes are effectively limiting migration of the VOC plume. The distribution of C2-3-benzenes, paired on the basis of their octanol-water partition coefficients and Henry's law constants, were systematically evaluated to assess the relative importance of volatilization, sorption, and biodegradation as attenuation mechanisms. Based on our data, biodegradation appears to be the process primarily responsible for the observed attenuation of VOCs at this site. We believe that the alkylbenzenes are powerful process probes that can and should be exploited in studies of natural attenuation in contaminated ground water systems.  相似文献   

8.
A natural gradient tracer test using perdeuterated MTBE was conducted in an anaerobic aquifer to determine the relative importance of dispersion and degradation in reducing MTBE concentrations in ground water. Preliminary ground water chemistry and hydraulic conductivity data were used to place the tracer within an existing dissolved MTBE plume at Port Hueneme, California. Following one year of transport, the tracer plume was characterized in detail.
Longitudinal dispersion was identified as the dominant mechanism for lowering the perdeuterated MTBE concentrations. The method of moments was used to determine the longitudinal and lateral dispersion coefficients (0.85 m2/day and 0.08 m2/day, respectively). A mass-balance analysis, carried out after one year of transport, accounted for 110% of the injected mass and indicated that no significant mass loss occurred. The plume structure created by zones of higher and lower hydraulic conductivity at the site was complex, consisting of several localized areas of high tracer concentration in a lower concentration plume. This is important because the aquifer has generally been characterized as exhibiting fairly minor heterogeneity. In addition, the tracer plume followed a curved flowpath that deviated from the more macroscopic direction of ground water flow inferred from local ground water elevation measurements and the behavior of the existing plume. Understanding the mass balance, plume structure, curvature of the tracer plume, and consequently natural attenuation behavior required the detailed sampling approach employed in this study. These data imply that a detailed understanding of site hydrogeology and an extensive sampling network may be critical for the correct interpretation of monitored natural attenuation of MTBE.  相似文献   

9.
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   

10.
The screened auger is a laser-slotted, hollow-stem auger through which a representative sample of ground water is pumped from an aquifer and tested for water-quality parameters by appropriate field-screening methods. Screened auger sampling can be applied to ground water quality remedial investigations, providing:(1) a mechanism for determining a monitoring well's optimal screen placement in a contaminant plume; and (2) data to define the three-dimensional configuration of the contaminant plume.
Screened auger sampling has provided an efficient method for investigating hexavalent chromium and volatile organic compound contamination in two sandy aquifers in Cadillac, Michigan. The aquifers approach 200 feet in thickness and more than 1 square mile in area. A series of screened auger borings and monitoring wells was installed, and ground water was collected at 10-foot intervals as the boreholes were advanced to define the horizontal and vertical distribution of the contaminant plumes. The ability of the screened auger to obtain representative ground water samples was supported by the statistical comparison of field screening results and subsequent laboratory analysis of ground water from installed monitoring wells.  相似文献   

11.
At three industrial sites in Ontario, New Hampshire, and Florida, tetrachloroethylene (PCE) and trichloroethylene (TCE), released decades ago as dense nonaqueous phase liquids (DNAPLs), now form persistent source zones for dissolved contaminant plumes. These zones are suspended below the water table and above the bottoms of their respective, moderately homogeneous, unconfined sandy aquifers. Exceptionally detailed, depth-discrete, ground water sampling was performed using a direct-push sampler along cross sections of the dissolved-phase plumes, immediately downgradient of these DNAPL source zones. The total plume PCE or TCE mass-discharge through each cross section ranged between 15 and 31 kg/year. Vertical ground water sample spacing as small as 15 cm and lateral spacing typically between 1 and 3 m revealed small zones where maximum concentrations were between 1% and 61% of solubility. These local maxima are surrounded by much lower concentration zones. A spacing no larger than 15 to 30 cm was needed at some locations to identify high concentration zones, and aqueous VOC concentrations varied as much as four orders of magnitude across 30 cm vertical intervals. High-resolution sampling at these sites showed that three-quarters of the mass-discharge occurs within 5% to 10% of the plume cross sectional areas. The extreme spatial variability of the mass-discharge occurs even though the sand aquifers are nearly hydraulically homogeneous. Depth-discrete field techniques such as those used in this study are essential for finding the small zones producing most of the mass-discharge, which is important for assessing natural attenuation and designing remedial options.  相似文献   

12.
The mobility of phosphorus (P) in septic system plumes remains a topic of debate because of the considerable reactivity of this constituent. In this study, a septic system plume in Ontario was monitored over a 16-year period with detail that clearly shows the advancing frontal portion of the P plume. This monitoring record provides insight into the extent of secondary P attenuation in the ground water zone beyond that available from previous studies. A P plume 16 m in length developed over the monitoring period with PO(4)-P concentrations (3 to 6 mg/L) that approached the concentrations present under the tile bed. Simulations using an analytical model showed that when first-order solute decay was considered to account for the possibility of secondary P attenuation in the ground water zone, field values could only be matched when decay was absent or occurred at an exceedingly slow rate (half-life greater than 30 years). Thus, hypothesized secondary P attenuation mechanisms such as slow recystallization of sorbed P into insoluble metal phosphate minerals, diffusion into microsites, or kinetically slow direct precipitation of P minerals such as hydroxyapatite were inactive in the ground water zone at this site or occurred at rates that were too slow to be observed in the context of the current 16-year study. Desorption tests on sediment samples from below the tile bed indicated a PO(4) distribution coefficient (K(d)) of 4.8, which implies a P retardation factor of 25, similar to the field apparent value of 37 determined from model calibrations. This example of inactive secondary P attenuation in the ground water zone shows that phosphorus in some ground water plumes can remain mobile and conservative for decades. This has important implications for septic systems located in lakeshore environments when long-term usage scenarios are considered.  相似文献   

13.
Ground water scientists have made significant advances in understanding the soil interactions, hydrogeology, fate and transport, and subsurface microbiology of aromatic hydrocarbons (BTEX) in aquifer systems. It is now generally recognized that a major factor responsible for the attenuation and mass reduction of BTEX in plumes is the widespread occurrence of hydrocarbon biodegradation by indigenous soil microorganisms in aquifer material. Most well-studied BTEX plumes that develop from the accidental release of gasoline fuels contain low levels of soluble hydrocarbons (< 1 to 5000 ppb) and have been shown to be spatially confined because of natural biotransformation mechanisms. These in situ processes are controlled by source and aquifer characteristics, permeability, sorption, and geochemical properties of the aquifer. Many laboratory subsoil-ground water microcosms and field studies (10 to 20 C) have demonstrated the rapid biodecay (1 to SO percent/day for microcosms and 0.5 to 1.5 percent/day for plumes) of these aromatic compounds under primarily aerobic conditions (i.e., those with sufficient dissolved oxygen). The ability to implement ground water bioremediation will depend upon our understanding of source control and aquifer recharge effects on the spatial distribution of plumes. In addition, estimating the biodegradation of sorbed BTEX, determining limits and potential for in situ biostimulation of soluble plumes, and establishing data requirements for predictive modeling of natural attenuation will be useful for this remediation technology. The use of these tools to manage ground water quality appears to represent the most practical alternative, particularly for low-risk ground water supplies.  相似文献   

14.
Investigations of geology, hydrogeology, and ground water chemistry in the aquifer downgradient from Sjoelund Landfill, Denmark, formed the basis for an evaluation of natural attenuation as a remediation technology for phenoxy acid herbicides at the site. Concentrations of phenoxy acids were up to 65 μg/L in the ground water, primarily 4-chlor-2-methylphenoxypropionic acid (MCPP) and 2,4-dichlorophenoxypropionic acid (dichlorprop). Mass removal of the phenoxy acids was shown within 50 to 100 m of the landfill by calculation of contaminant fluxes passing transects at three distances. There was accordance between increasing oxygen concentrations and decreasing phenoxy acid concentrations with distance from the landfill, indicating that aerobic degradation was a major mass removal process. Presence of high concentrations of putative anaerobic phenoxy acid metabolites suggested that anaerobic degradation was also occurring. Laboratory degradation experiments using sediment and ground water from the aquifer supported aerobic and anaerobic degradability of MCPP at the site. It was concluded that natural attenuation may be applicable as a remedy for the phenoxy acids at the Sjoelund Landfill site, although uncertainties related to calculations of chloride and phenoxy acid fluxes at a complex site and identification of specific in situ indicators were encountered. Thus, there is a pronounced need for development and broader experience with evaluation tools for natural attenuation of phenoxy acids, such as specific metabolites, changes in enantiomeric fractions, compound-specific stable carbon isotope ratios, or microbial fingerprints.  相似文献   

15.
Consumption of aquifer Fe(III) during biodegradation of ground water contaminants may result in expansion of a contaminant plume, changing the outlook for monitored natural attenuation. Data from two research sites contaminated with petroleum hydrocarbons show that toluene and xylenes degrade under methanogenic conditions, but the benzene and ethylbenzene plumes grow as aquifer Fe(III) supplies are depleted. By considering a one-dimensional reaction front in a constant unidirectional flow field, it is possible to derive a simple expression for the growth rate of a benzene plume. The method balances the mass flux of benzene with the Fe(III) content of the aquifer, assuming that the biodegradation reaction is instantaneous. The resulting expression shows that the benzene front migration is retarded relative to the ground water velocity by a factor that depends on the concentrations of hydrocarbon and bioavailable Fe(III). The method provides good agreement with benzene plumes at a crude oil study site in Minnesota and a gasoline site in South Carolina. Compared to the South Carolina site, the Minnesota site has 25% higher benzene flux but eight times the Fe(III), leading to about one-sixth the expansion rate. Although it was developed for benzene, toluene, ethylbenzene, and xylenes, the growth-rate estimation method may have applications to contaminant plumes from other persistent contaminant sources.  相似文献   

16.
Data from an existing network of ground water monitoring wells at the U.S. Department of Energy (DOE) Hoe Creek Underground Coal Gasification (UCG) Experimental Site indicated that organic contaminants, particularly phenols produced during gasification experiments, were threatening neighboring ground water resources. The existing monitoring well network was sparse and further definition of the extent and direction of contaminant migration was needed. Additionally, water level data, important in determining flow directions, was incomplete. A field program was designed and implemented to locate and define the organic contamination and expand the existing ground water monitoring program. The program utilized field analysis of phenol for contaminant detection and well location, followed by completion using gas-drive ground water samplers/piezometers. Geophysical logging was used to permit optimum placement of the samplers. The geologic aspects of the site posed some interesting problems to the installation of the samplers. The contaminant plume edge was defined in the east, west and south directions during the field program. Further work is needed in the north direction.  相似文献   

17.
Abstract

Field and laboratory evidence show natural degradation of toluene, ethylbenzene, m-, p- and o-xylenes, 1,3,5-trimethylbenzene and naphthalene in sulphate reducing groundwater conditions of the Bassendean Sands in the Perth basin, Western Australia. Natural degradation rates were obtained from a groundwater tracer test with deuterated organic compounds injected into a dissolved hydrocarbon plume, down-gradient of a leaking underground storage tank at an urban service station. These were compared with similar data obtained from modelling of the whole contaminant plume itself and also with data obtained from large-scale laboratory column experiments with groundwater spiked with BTEX compounds. Toluene degradation rate was 200 to 500 times higher in the anaerobic laboratory columns than in the field. Degradation rates in the tracer test compared well with model-derived field estimates.  相似文献   

18.
Matrix diffusion can attenuate the rate of plume migration in fractured bedrock relative to the rate of ground water flow for both conservative and nonconservative solutes of interest. In a system of parallel, equally spaced constant aperture fractures subject to steady-state ground water flow and an infinite source width, the degree of plume attenuation increases with time and travel distance, eventually reaching an asymptotic level. The asymptotic degree of plume attenuation in the absence of degradation can be predicted by a plume attenuation factor, beta, which is readily estimated as R' (phi(m)/phi(f)), where R' is the retardation factor in the matrix, phi(m) is the matrix porosity, and phi(f) is the fracture porosity. This dual-porosity relationship can also be thought of as the ratio of primary to secondary porosity. Beta represents the rate of ground water flow in fractures relative to the rate of plume advance. For the conditions examined in this study, beta increases with greater matrix porosity, greater matrix fraction organic carbon, larger fracture spacing, and smaller fracture aperture. These concepts are illustrated using a case study where dense nonaqueous phase liquid in fractured sandstone produced a dissolved-phase trichloroethylene (TCE) plume approximately 300 m in length. Transport parameters such as matrix porosity, fracture porosity, hydraulic gradient, and the matrix retardation factor were characterized at the site through field investigations. In the fractured sandstone bedrock examined in this study, the asymptotic plume attenuation factors (beta values) for conservative and nonconservative solutes (i.e., chloride and TCE) were predicted to be approximately 800 and 12,210, respectively. Quantitative analyses demonstrate that a porous media (single-porosity) solute transport model is not appropriate for simulating contaminant transport in fractured sandstone where matrix diffusion occurs. Rather, simulations need to be conducted with either a discrete fracture model that explicitly incorporates matrix diffusion, or a dual-continuum model that accounts for mass transfer between mobile and immobile zones. Simulations also demonstrate that back diffusion from the matrix to fractures will likely be the time-limiting factor in reaching ground water cleanup goals in some fractured bedrock environments.  相似文献   

19.
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   

20.
Alight nonaqueous phase liquid (LNAPL) ground water contaminant plume has been discovered by purely geophysical means at the former Wurtsmith Air Force Base (AFB) near Oscoda, Michigan. It is located near another plume called FT-02, which is a well-studied area undergoing natural bioremediation. The plume was discovered by ground penetrating radar (GPR) profiling while extending a long line from FT-02 to establish background variability around that plume. The new plume was apparent because of a high-conductivity "shadow' or GPR reflection attenuation observed below the conductive zone at the top of the aquifer, identical to the pattern observed at the FT-02 plume. Further GPR surveys were conducted by students of a Western Michigan University geophysics field course to outline the proximal part of the plume. The GPR survey was supplemented by an electromagnetic induction (EM) survey which showed a group of four cables crossing the area. Finally, a magnetometer survey was conducted to search for any buried steel objects which might have been missed by the EM survey. The results of the three geophysical surveys were then used by students of a University of Michigan field course to guide subsurface soil and fluid sampling, which verified the presence of residual LNAPL product and ground water with conductivities 2.5 to 3.3 times above background. The plume source is in the vicinity of a vaulted underground storage tank (UST) formerly used for the collection of waste solvents and fuels for subsequent use in the fire training exercises at FT-02. This newly discovered LNAPL plume, along with other "mature' plumes, fits the electrical model which predicts conductive ground water below the decomposing but electrically resistive LNAPLs. Finally, this is a fine example of the cooperative use of a dedicated research site for training by students of two different universities.  相似文献   

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