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1.
Detailed site investigations to assess potential inhalation exposure and risk to human health associated with the migration of petroleum hydrocarbon vapors from the subsurface to indoor air are frequently undertaken at leaking underground storage tank (UST) sites, yet documented occurrences of petroleum vapor intrusion are extremely rare. Additional assessments are largely driven by low screening‐level concentrations derived from vapor transport modeling that does not consider biodegradation. To address this issue, screening criteria were developed from soil‐gas measurements at hundreds of petroleum UST sites spanning a range of environmental conditions, geographic regions, and a 16‐year time period (1995 to 2011). The data were evaluated to define vertical separation (screening) distances from the source, beyond which, the potential for vapor intrusion can be considered negligible. The screening distances were derived explicitly from benzene data using specified soil‐gas screening levels of 30, 50, and 100 µg/m3 and nonparametric Kaplan‐Meier statistics. Results indicate that more than 95% of benzene concentrations in soil gas are ≤30 µg/m3 at any distance above a dissolved‐phase hydrocarbon source. Dissolved‐phase petroleum hydrocarbon sources are therefore unlikely to pose a risk for vapor intrusion unless groundwater (including capillary fringe) comes in contact with a building foundation. For light nonaqueous‐phase liquid (LNAPL) hydrocarbon sources, more than 95% of benzene concentrations in soil gas are ≤30 µg/m3 for vertical screening distances of 13 ft (4 m) or greater. The screening distances derived from this analysis are markedly different from 30 to 100 ft (10 to 30 m) vertical distances commonly found cited in regulatory guidance, even with specific allowances to account for uncertainty in the hydrocarbon source depth or location. Consideration of these screening distances in vapor intrusion guidance would help eliminate unnecessary site characterization at petroleum UST sites and allow more effective and sustainable use of limited resources.  相似文献   

2.
Although the ingestion of vanadium (V) in drinking water may have possible adverse health effects, there have been relatively few studies of V in groundwater. Given the importance of groundwater as a source of drinking water in many areas of the world, this study examines the potential sources and geochemical processes that control the distribution of V in groundwater on a regional scale. Potential sources of V to groundwater include dissolution of V rich rocks, and waste streams from industrial processes. Geochemical processes such as adsorption/desorption, precipitation/dissolution, and chemical transformations control V concentrations in groundwater. Based on thermodynamic data and laboratory studies, V concentrations are expected to be highest in samples collected from oxic and alkaline groundwater. However, the extent to which thermodynamic data and laboratory results apply to the actual distribution of V in groundwater is not well understood. More than 8400 groundwater samples collected in California were used in this study. Of these samples, high (≥50 µg/L) and moderate (25 to 49 µg/L) V concentrations were most frequently detected in regions where both source rock and favorable geochemical conditions occurred. The distribution of V concentrations in groundwater samples suggests that significant sources of V are mafic and andesitic rock. Anthropogenic activities do not appear to be a significant contributor of V to groundwater in this study. High V concentrations in groundwater samples analyzed in this study were almost always associated with oxic and alkaline groundwater conditions, which is consistent with predictions based on thermodynamic data.  相似文献   

3.
Quantifying the overall progress in remediation of contaminated groundwater has been a significant challenge. We utilized the GeoTracker database to evaluate the progress in groundwater remediation from 2001 to 2011 at over 12,000 sites in California with contaminated groundwater. This paper presents an analysis of analytical results from over 2.1 million groundwater samples representing at least $100 million in laboratory analytical costs. Overall, the evaluation of monitoring data shows a large decrease in groundwater concentrations of gasoline constituents. For benzene, half of the sites showed a decrease in concentration of 85% or more. For methyl tert‐butyl ether (MTBE), this decrease was 96% and for TBE, 87%. At remediation sites in California, the median source attenuation rate was 0.18/year for benzene and 0.36/year for MTBE, corresponding to half‐lives of 3.9 and 1.9 years, respectively. Attenuation rates were positive (i.e., decreasing concentration) for benzene at 76% of sites and for MTBE at 85% of sites. An evaluation of sites with active remediation technologies suggests differences in technology effectiveness. The median attenuation rates for benzene and MTBE are higher at sites with soil vapor extraction or air sparging compared with sites without these technologies. In contrast, there was little difference in attenuation rates at sites with or without soil excavation, dual phase extraction, or in situ enhanced biodegradation. The evaluation of remediation technologies, however, did not evaluate whether specific systems were well designed or implemented and did not control for potential differences in other site factors, such as soil type.  相似文献   

4.
The artificial sweetener acesulfame (ACE) is a potentially useful tracer of waste water contamination in groundwater. In this study, ACE concentrations were measured in waste water and impacted groundwater at 12 septic system sites in Ontario, Canada. All samples of septic tank effluent (n = 37) had ACE >6 µg/L, all samples of groundwater from the proximal plume zones (n = 93) had ACE >1 µg/L and, almost all samples from the distal plume zones had ACE >2 µg/L. Mean mass ratios of total inorganic nitrogen/ACE at the 12 sites ranged from 680 to 3500 for the tank and proximal plume samples. At five sites, decreasing ratio values in the distal zones indicated nitrogen attenuation. These ratios were applied to three aquifers in Canada that are nitrate‐stressed and an urban stream where septic systems are present nearby to estimate the amount of waste water nitrate contamination. At the three aquifer locations that are agricultural, low ACE values (<0.02‐0.15 µg/L) indicated that waste water contributed <15% of the nitrate in most samples. In groundwater discharging to the urban stream, much higher ACE values (0.2‐11 µg/L) indicated that waste water was the likely source of >50% of the nitrate in most samples. This study confirms that ACE is a powerful tracer and demonstrates its use as a diagnostic tool for establishing whether waste water is a significant contributor to groundwater contamination or not.  相似文献   

5.
Propane biosparging and bioaugmentation were applied to promote in situ biodegradation of 1,4‐dioxane at Site 24, Vandenberg Air Force Base (VAFB), CA. Laboratory microcosm and enrichment culture testing demonstrated that although native propanotrophs appeared abundant in the shallow water‐bearing unit of the aquifer (8 to 23 ft below ground surface [bgs]), they were difficult to be enriched from a deeper water‐bearing unit (82 to 90 feet bgs). Bioaugmentation with the propanotroph Rhodococcus ruber ENV425, however, supported 1,4‐dioxane biodegradation in microcosms constructed with samples from the deep aquifer. For field testing, a propane‐biosparging system consisting of a single sparging well and four performance monitoring wells was constructed in the deep aquifer. 1,4‐dioxane biodegradation began immediately after bioaugmentation with R. ruber ENV425 (36 L; 4 × 109 cells/mL), and apparent first‐order decay rates for 1,4‐dioxane ranged from 0.021 day?1 to 0.036 day?1. First‐order propane consumption rates increased from 0.01 to 0.05 min?1 during treatment. 1,4‐dioxane concentrations in the sparging well and two of the performance monitoring wells were reduced from as high as 1090 µg/L to <2 µg/L, while 1,4‐dioxane concentration was reduced from 135 µg/L to 7.3 µg/L in a more distal third monitoring well. No 1,4‐dioxane degradation was observed in the intermediate aquifer control well even though propane and oxygen were present. The demonstration showed that propane biosparging and bioaugmentation can be used for in situ treatment of 1,4‐dioxane to regulatory levels.  相似文献   

6.
The use of in‐field analysis of vapor‐phase samples to provide real‐time volatile organic compound (VOC) concentrations in groundwater has the potential to streamline monitoring by simplifying the sample collection and analysis process. A field validation program was completed to (1) evaluate methods for collection of vapor samples from monitoring wells and (2) evaluate the accuracy and precision of field‐portable instruments for the analysis of vapor‐phase samples. The field program evaluated three vapor‐phase sample collection methods: (1) headspace samples from two locations within the well, (2) passive vapor diffusion (PVD) samplers placed at the screened interval of the well, and (3) field vapor headspace analysis of groundwater samples. Two types of instruments were tested: a field‐portable gas chromatograph (GC) and a photoionization detector (PID). Field GC analysis of PVD samples showed no bias and good correlation to laboratory analysis of groundwater collected by low‐flow sampling (slope = 0.96, R2 = 0.85) and laboratory analysis of passive water diffusion bag samples from the well screen (slope = 1.03; R2 = 0.96). Field GC analysis of well headspace samples, either from the upper portion of the well or at the water‐vapor interface, resulted in higher variability and much poorer correlation (consistently biased low) relative to laboratory analysis of groundwater samples collected by low‐flow sample or passive diffusion bags (PDBs) (slope = 0.69 to 0.76; R2 = 0.60 to 0.64). These results indicate that field analysis of vapor‐phase samples can be used to obtain accurate measurements of VOC concentrations in groundwater. However, vapor samples collected from the well headspace were not in equilibrium with water collected from the well screen. Instead, PVD samplers placed in the screened interval represent the most promising approach for field‐based measurement of groundwater concentrations using vapor monitoring techniques and will be the focus of further field testing.  相似文献   

7.
A pioneering investigation of semi‐volatile organic compounds (SVOCs) in shallow groundwater in China was hereby reported. Representative groundwater samples were collected from three selected regions: Eastern Hai River Plain, Yangtze River Delta, and Yunnan‐Guizhou Plateau, and analyzed for 103 SVOCs linked to agricultural and industrial practices. Analytical results showed that 70 of the 103 SVOCs were present in the groundwater samples, a detection frequency of approximately 86%. Compounds detected most frequently included P,P′‐DDT (53.49%, MDL 0.0007 µg/L), 2,4‐dinitrotoluene (51.16%, MDL 0.02 µg/L), and phenol (51.16%, MDL 0.02 µg/L). Fifteen SVOCs, such as P,P′‐DDT, 2,4‐dinitrotoluene, heptachlor, and aldrin, were detected at concentrations exceeding the USA National Recommended Water Quality Criteria‐2009 (USNRWQC‐2009). Most of these 15 SVOCs belong to polycyclic aromatic hydrocarbons and organochlorine pesticides. The detection of SVOCs in the Yunnan‐Guizhou Plateau warrants special concern, since this region has limited human activities and been assumed as an environmentally pristine area. The data from this work are expected to contribute to the database of contemporary groundwater quality in China.  相似文献   

8.
Quantitative information regarding the length and stability condition of groundwater plumes of benzene, methyl tert‐butyl ether (MTBE), and tert‐butyl alcohol (TBA) has been compiled from thousands of underground storage tank (UST) sites in the United States where gasoline fuel releases have occurred. This paper presents a review and summary of 13 published scientific surveys, of which 10 address benzene and/or MTBE plumes only, and 3 address benzene, MTBE, and TBA plumes. These data show the observed lengths of benzene and MTBE plumes to be relatively consistent among various regions and hydrogeologic settings, with median lengths at a delineation limit of 10 µg/L falling into relatively narrow ranges from 101 to 185 feet for benzene and 110 to 178 feet for MTBE. The observed statistical distributions of MTBE and benzene plumes show the two plume types to be of comparable lengths, with 90th percentile MTBE plume lengths moderately exceeding benzene plume lengths by 16% at a 10‐µg/L delineation limit (400 feet vs. 345 feet) and 25% at a 5‐µg/L delineation limit (530 feet vs. 425 feet). Stability analyses for benzene and MTBE plumes found 94 and 93% of these plumes, respectively, to be in a nonexpanding condition, and over 91% of individual monitoring wells to exhibit nonincreasing concentration trends. Three published studies addressing TBA found TBA plumes to be of comparable length to MTBE and benzene plumes, with 86% of wells in one study showing nonincreasing concentration trends.  相似文献   

9.
The influence of large‐scale mining operations on groundwater quality was investigated in this study. Trace element concentrations in groundwater samples from the North Mara mining area of northern Tanzania were analyzed. Statistical analyses for relationships between elemental concentrations in the samples and distance of a sampling site from the mine tailings dam were also conducted. Eleven trace elements (Al, As, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, and Zn) were determined, and averages of Fe and Al concentrations were higher than levels accepted by the Tanzanian drinking water guideline. Levels of Pb in three samples were higher than the World Health Organization (WHO) and United States Environmental Protection Agency (USEPA) drinking water guidelines of 10 and 15 µg/L, respectively. One sample contained a higher As level than the WHO and USEPA guideline of 10 µg/L. The correlation between element concentrations and distance from the mine tailings dam was examined using the hierarchical agglomeration cluster analysis method. A significant difference in the elemental concentration existed depending on the distance from the mine tailings dam. Mann–Whitney U‐test post hoc analysis confirmed a relationship between element concentration and distance of a sampling site from the mine tailings dam. This relationship raises concerns about the increased risks of trace elements to people and ecosystem health. A metal pollution index also suggested a relationship between elemental concentrations in the groundwater and the sampling sites’ proximity from the mine tailings dam.  相似文献   

10.
This study investigates potential occurrence, distribution, and sources of the newly added gasoline oxygenate, methyl‐tert‐butyl ether (MTBE) and the petroleum derivatives benzene, toluene, ethylbenzene, and xylenes called collectively, BTEX, in Jordan's heavily populated Amman–Zarqa Basin (AZB). It presents the first data on the levels of MTBE and BTEX in the aquifers of this basin. One hundred and seventy‐nine (179) groundwater wells were sampled near petrol service stations, oil refinery storage tanks, car wrecks, bus stations, and chemical industries at different locations in the basin. Headspace GC and purge and trap GC–MS were utilized to determine the target substances in the samples. Concentrations of BTEX varied between no‐detection (minimum) for all of them to 6.6 µg/L (maximum) for ethylbenzene. MTBE was found in few samples but none has exceeded the regulated levels; its concentrations ranged between no‐detection to 4.1 µg/L. However, though the contamination levels are very low they should be considered alarming.  相似文献   

11.
Arsenic Removal from Natural Groundwater Using Cupric Oxide   总被引：1，自引：0，他引：1  
Groundwater is a main source of drinking water for some rural areas. People in these rural areas are potentially at risk from elevated levels of arsenic (As) due to a lack of water treatment facilities. The objectives of this study were to (1) measure As concentrations in approximately 50 groundwater samples from rural domestic wells in the western United States, (2) explore the potential of cupric oxide (CuO) particles in removal of As from groundwater samples under natural conditions (i.e., without adding competing anions and adjusting the pH or oxidation state), and (3) determine the effects of As removal on the chemistry of groundwater samples. Forty‐six groundwater well samples from rural domestic areas were tested in this study. More than 50% of these samples exceeded the U.S. Environmental Protection Agency Maximum Contaminant Limit (US EPA MCL) of 10 µg/L for As. CuO particles effectively removed As from groundwater samples across a wide range of pH (7.11 and 8.95) and concentrations of competing anions including phosphate (<0.05 to 3.06 mg/L), silica (<1 to 54.5 mg/L), and sulfate (1.3 to 735 mg/L). Removal of As showed minor effects on the chemistry of groundwater samples, therefore most of the water quality parameters remained within the US EPA MCLs. Overall, results of this study could help develop a simple one‐step process to remove As from groundwater.  相似文献   

12.
This study investigated the development of pesticide pollution two, three, and 17 years after spills of the herbicides dichlorprop, mecoprop (MCPP), MCPA, 2,4‐D (phenoxy acids), simazine, and terbutylazine (triazines) in a former orchard machinery service yard. The spills had occurred over several decades on a 23‐m thick, mainly anaerobic fractured clayey till aquitard. Angled monitoring wells were installed in the aquitard 3 years after the spills ceased in 1989. In 1993, monitoring revealed that high groundwater concentrations of dichlorprop (677 µg/L) and MCPP (139 µg/L) were accumulated as a zone of maximum pollution in anaerobic and largely immobile pore water at 5 to 6 m depth in the aquitard profile. In contrast, 2,4‐D was determined in only one water sample, and MCPA and simazine and terbutylazine were determined only in low concentrations (below10 µg/L), although these pesticides had been handled at the site in greater amounts than dichlorprop and MCPP according to detailed historic information obtained for the site. Repeated monitoring in the same wells after a further 14 years in 2007 revealed that no identifiable degradation of MCPP had occurred, while dichlorprop had degraded by 75% to 80% (estimated half‐life of approximately 5 years). Furthermore, degradation products related to the phenoxy acids had accumulated, especially 4‐CPP with a maximum concentration of 218 µg/L. In the same zone, MCPA and simazine had almost disappeared. As the pollution was mainly accumulated in largely immobile pore water of the aquitard clayey matrix, and the groundwater recharge was low (30 to 60 mm/year), only minor vertical displacement of the zone with maximum pollution zone had occurred during the 15 years of monitoring. However, concentrations of dichlorprop (0.01 to 0.02 µg/L), MCPP (0.1 to 0.2 µg/L), and 4‐CPP (0.6 to 0.7 µg/L) had spread along textural heterogeneities in the aquitard into the underlying sandy aquifer at 23‐m depth.  相似文献   

13.
The maximum contaminant level for arsenic was reduced by the U.S. Environmental Protection Agency (U.S. E.P.A.) for Drinking Water Standards from 50 micrograms per liter (µg/L) to 10 µg/L, effective January 23, 2006. The subject site is a double-lined sanitary landfill facility located in the mid-Atlantic region of the United States. Arsenic was reported above the maximum contaminant level in a downgradient monitoring well (MW-18) in July 2005. Since July 2005, arsenic levels in MW-18 fluctuated above and below 10 µg/L. This research focuses on determining whether reducing conditions in groundwater, enhanced by subsurface landfill gas emissions, were causing naturally-occurring arsenic to mobilize from the native variably-saturated vadose zone soils into groundwater. The groundwater data collected from the impacted well (MW-18) were compared to an upgradient well (MW-8) to determine whether significant differences existed during the time period of April 2004 to April 2007. Linear regression analysis was also used to determine whether other parameters had a significant relationship with the arsenic concentrations detected in MW-18. The groundwater located in MW-18 was consistently more reduced than groundwater located in the upgradient/background well MW-8, and this was most likely attributed to the presence of subsurface landfill gas in the area. According to the U.S. E.P.A., oxidation-reduction potential (ORP) values in groundwater less than 50 millivolts (mv) suggest that a reducing environment may be present. The data presented in this study indicate that arsenic can mobilize into groundwater under moderately reducing conditions, with ORP measurements averaging 53 mv.  相似文献   

14.
The area surrounding the Colorado Department of Transportation Materials Testing Laboratory in Denver was the subject of intense investigation, involving the collection of thousands of ground water, soil-gas, and indoor air samples in order to investigate indoor air impacts associated with a subsurface release of chlorinated solvents. The preremediation portion of that data set is analyzed and reduced in this work to ground water–to-indoor air attenuation factors (αgw= the ratio of the measured indoor air concentration to the soil-gas concentration predicted to be in equilibrium with the local ground water concentration). The empirical αgw values for this site range from about 10−6 to 10−4 with an overall average of 3 × 10−5 (μg/L indoor air)/(μg/L soil gas). The analysis of this data set highlights the need for a thorough data review and data screening when using large data sets to derive empirical relationships between subsurface concentrations and indoor air. More specifically, it is necessary to identify those parts of the data that contain a strong vapor intrusion pathway signal, which generally will require concentrations well above reported detection levels combined with spatial or temporal correlation of subsurface and indoor concentrations.  相似文献   

15.
A value of 0.001 is recommended by the United States Environmental Protection Agency (USEPA) for its groundwater‐to‐indoor air Generic Attenuation Factor (GAFG), used in assessing potential vapor intrusion (VI) impacts to indoor air, given measured groundwater concentrations of volatile chemicals of concern (e.g., chlorinated solvents). The GAFG can, in turn, be used for developing groundwater screening levels for VI given target indoor air quality screening levels. In this study, we examine the validity and applicability of the GAFG both for predicting indoor air impacts and for determining groundwater screening levels. This is done using both analysis of published data and screening model calculations. Among the 774 total paired groundwater‐indoor air measurements in the USEPA's VI database (which were used by that agency to generate the GAFG) we found that there are 427 pairs for which a single groundwater measurement or interpolated value was applied to multiple buildings. In one case, up to 73 buildings were associated with a single interpolated groundwater value and in another case up to 15 buildings were associated with a single groundwater measurement (i.e., that the indoor air contaminant concentrations in all of the associated buildings were influenced by the concentration determined at a single point). In more than 70% of the cases (390 of 536 paired measurements in which horizontal building‐monitoring well distance was recorded) the monitoring wells were located more than 30 m (and one up to over 200 m) from the associated buildings. In a few cases, the measurements in the database even improbably implied that soil gas contaminant concentrations increased, rather than decreased, in an upward direction from a contaminant source to a foundation slab. Such observations indicate problematic source characterization within the data set used to generate the GAFG, and some indicate the possibility of a significant influence of a preferential contaminant pathway. While the inherent value of the USEPA database itself is not being questioned here, the above facts raise the very real possibility that the recommended groundwater attenuation factors are being influenced by variables or conditions that have not thus far been fully accounted for. In addition, the predicted groundwater attenuation factors often fall far beyond the upper limits of predictions from mathematical models of VI, ranging from screening models to detailed computational fluid dynamic models. All these models are based on the same fundamental conceptual site model, involving a vadose zone vapor transport pathway starting at an underlying uniform groundwater source and leading to the foundation of a building of concern. According to the analysis presented here, we believe that for scenarios for which such a “traditional” VI pathway is appropriate, 10?4 is a more appropriately conservative generic groundwater to indoor air attenuation factor than is the EPA‐recommended 10?3. This is based both on the statistical analysis of USEPA's VI database, as well as the traditional mathematical models of VI. This result has been validated by comparison with results from some well‐documented field studies.  相似文献   

16.
Several regulatory agencies recommend screening petroleum vapor intrusion (PVI) sites based on vertical screening distance between a petroleum hydrocarbon source in soil or groundwater and a building foundation. U.S. Environmental Protection Agency (U.S. EPA) indicate the risk of PVI is minimal at buildings that are separated by more than 6 feet (1.8 m) from a dissolved-phase source and 15 feet (4.6 m) from a light nonaqueous phase liquid (LNAPL) source. This vertical screening distance method is not, however, recommended at sites with leaded gasoline sources containing ethylene dibromide (EDB) because of a lack of field data to document EDB attenuation in the vadose zone. To help address this gap, depth-discrete soil-gas samples were collected at a leaded gasoline release site in Sobieski, Minnesota (USA). The maximum concentration of EDB in groundwater (175 μg/L) at the site was high relative to those observed at other leaded gasoline release sites. Soil gas was analyzed for EDB using a modification of U.S. EPA Method TO-14A that achieved analytical detection limits below the U.S. EPA Vapor Intrusion Screening Level (VISL) for EDB based on a 10−6 cancer risk (<0.16 μg/m3). Concentrations of EDB in soil gas above LNAPL reached as high as 960 μg/m3 and decreased below the VISL within a source-separation distance of 7 feet. This result coupled with BioVapor model predictions of EDB concentrations indicate that vertical screening distances recommended by regulatory agencies at PVI sites are generally applicable for EDB over the range of anticipated source concentrations and soil types at most sites.  相似文献   

17.
Monitored natural attenuation is widely applied as a remediation strategy at hydrocarbon spill sites. Natural attenuation relies on biodegradation of hydrocarbons coupled with reduction of electron acceptors, including solid phase ferric iron (Fe(III)). Because arsenic (As) adsorbs to Fe‐hydroxides, a potential secondary effect of natural attenuation of hydrocarbons coupled with Fe(III) reduction is a release of naturally occurring As to groundwater. At a crude‐oil‐contaminated aquifer near Bemidji, Minnesota, anaerobic biodegradation of hydrocarbons coupled to Fe(III) reduction has been well documented. We collected groundwater samples at the site annually from 2009 to 2013 to examine if As is released to groundwater and, if so, to document relationships between As and Fe inside and outside of the dissolved hydrocarbon plume. Arsenic concentrations in groundwater in the plume reached 230 µg/L, whereas groundwater outside the plume contained less than 5 µg/L As. Combined with previous data from the Bemidji site, our results suggest that (1) naturally occurring As is associated with Fe‐hydroxides present in the glacially derived aquifer sediments; (2) introduction of hydrocarbons results in reduction of Fe‐hydroxides, releasing As and Fe to groundwater; (3) at the leading edge of the plume, As and Fe are removed from groundwater and retained on sediments; and (4) downgradient from the plume, patterns of As and Fe in groundwater are similar to background. We develop a conceptual model of secondary As release due to natural attenuation of hydrocarbons that can be applied to other sites where an influx of biodegradable organic carbon promotes Fe(III) reduction.  相似文献   

18.
Vapor intrusion pathway evaluations commonly begin with a comparison of volatile organic chemical (VOC) concentrations in groundwater to generic, or Tier 1, screening levels. These screening levels are typically quite low reflecting both a desired level of conservatism in a generic risk screening process as well as limitations in understanding of physical and chemical processes that impact vapor migration in the subsurface. To study the latter issue, we have collected detailed soil gas and groundwater vertical concentration profiles and evaluated soil characteristics at seven different sites overlying chlorinated solvent contaminant plumes. The goal of the study was to evaluate soil characteristics and their impacts on VOC attenuation from groundwater to deep soil gas (i.e., soil gas in the unsaturated zone within 2 feet of the water table). The study results suggest that generic screening levels can be adjusted by a factor of 100× at sites with fine‐grained soils above the water table, as identified by visual observations or soil air permeability measurements. For these fine‐grained soil sites, the upward‐adjusted screening levels maintain a level of conservatism while potentially eliminating the need for vapor intrusion investigations at sites that may not meet generic screening criteria.  相似文献   

19.
Continuous remediation monitoring using sensors is potentially a more effective and inexpensive alternative to current methods of sample collection and analysis. Gaseous components of a system are the most mobile and easiest to monitor. Continuous monitoring of soil gases such as oxygen, carbon dioxide, and contaminant vapors can provide important quantitative information regarding the progress of bioremediation efforts and the area of influence of air sparging or soil venting. Laboratory and field tests of a commercially available oxygen sensor show that the subsurface oxygen sensor provides rapid and accurate data on vapor phase oxygen concentrations. The sensor is well suited for monitoring gas flow and oxygen consumption in the vadose zone during air sparging and bioventing. The sensor performs well in permeable, unsaturated soil environments and recovers completely after being submerged during temporary saturated conditions. Calibrations of the in situ oxygen sensors were found to be stable after one year of continuous subsurface operation. However, application of the sensor in saturated soil conditions is limited. The three major advantages of this sensor for in situ monitoring arc as follows: (1) it allows data acquisition at any specified time interval; (2) it provides potentially more accurate data by minimizing disturbance of subsurface conditions; and (3) it minimizes the cost of field and laboratory procedures involved in sample retrieval and analysis.  相似文献   

20.
Arsenic concentrations can be managed with a relatively simple strategy of grouting instead of completely destroying a selected interval of well. The strategy of selective grouting was investigated in Antelope Valley, California, where groundwater supplies most of the water demand. Naturally occurring arsenic typically exceeds concentrations of 10 µg/L in the water produced from these long-screened wells. The vertical distributions of arsenic concentrations in intervals of the aquifer contributing water to selected supply wells were characterized with depth-dependent water-quality sampling and flow logs. Arsenic primarily entered the lower half of the wells where lacustrine clay deposits and a deeper aquifer occurred. Five wells were modified by grouting from below the top of the lacustrine clay deposits to the bottom of the well, which reduced produced arsenic concentrations to less than 2 µg/L in four of the five wells. Long-term viability of well modification and reduction of specific capacity was assessed for well 4-54 with AnalyzeHOLE, which creates and uses axisymmetric, radial MODFLOW models. Two radial models were calibrated to observed borehole flows, drawdowns, and transmissivity by estimating hydraulic-conductivity values in the aquifer system and gravel packs of the original and modified wells. Lithology also constrained hydraulic-conductivity estimates as regularization observations. Well encrustations caused as much as 2 µg/L increase in simulated arsenic concentration by reducing the contribution of flow from the aquifer system above the lacustrine clay deposits. Simulated arsenic concentrations in the modified well remained less than 3 µg/L over a 20-year period.  相似文献   

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