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Site-average Natural Source Zone Depletion (NSZD) rates measured from 40 petroleum light non-aqueous phase liquid (LNAPL) source zone sites were compiled from researchers, project reports, and scientific papers. At each site, the following data were compiled: i) general site location; ii) LNAPL fuel type; iii) measurement method, number of locations, and number of measurements per location; and iv) calculated site-average NSZD rate in liters per hectare per year (L/ha/yr) per site and the associated measurement method (i.e., Gradient Method, Carbon Traps, Dynamic Closed Chamber (DCC), or Thermal Monitoring). The resulting dataset showed site-average NSZD rates that ranged from 650 to 152,000 L/ha/yr (70 to 16,250 gallons per acre per year (gal/acre/yr)), with a median value of 9,540 L/ha/yr (1,020 gal/acre/yr). The median site-average NSZD rate by type of fuel spill did not show a statistically significant difference between fuel types. When comparing the different NSZD measurement methods applied to the same sites, the site-average NSZD rates differed by up to 4.8 times (i.e., ratio of faster rate to slower rate), with a median difference of 2.1 times. No clear bias was observed between NSZD rate measurement methods. At four sites with calculations of NSZD rates by season, NSZD rates were typically higher during summer and fall compared to winter and spring. For these sites, Q

In recent years, there has been a growing recognition of the importance of sewers as preferential pathways for vapor intrusion. This pathway involves volatile organic compound (VOC)-impacted vapor migration through the interior of pipes or conduits towards the potential receptor. A number of research initiatives have been undertaken to improve the understanding of this pathway which, in turn, supports development of sampling methods to evaluate whether it is a concern at a given site. One aspect of the conceptual model that is important to understanding the potential risk of sewer vapor intrusion is the degree to which VOC concentrations vary over time. This paper presents results of field sampling conducted in Texas, Utah, and California to explore temporal variability of chlorinated VOC concentrations in sewer vapor over different timescales. These results, along with results from other studies, suggest that wide ranges of VOC concentrations can occur at given sampling locations. However, long-term temporal variability (months to years) is much higher than short-term variability (days). The timescale over which the variations occur has implications on sampling methods and frequency. The results suggest that multiple grab samples collected over a longer time period, such as on a quarterly basis, are more appropriate for characterizing the long-term average sewer vapor concentrations than single time-integrated samples collected over a period of hours to days. Because time-integrated samples can often be logistically more difficult to collect, an understanding of the timescale of variability may help practitioners balance field logistics considerations with data adequacy when characterizing VOC concentrations in sewers.

In recent years, a number of methods have been used to measure the biodegradation of petroleum light non-aqueous phase liquids (LNAPL) at petroleum release sites, a process known as natural source zone depletion (NSZD). Most commonly, NSZD rates have been measured at sites with unconsolidated geology and relatively shallow groundwater (50 ft. bgs,15 m bgs). For this study, we have used two methods (1. carbon dioxide flux measured using carbon traps and 2. heat flux based on subsurface temperature gradients) to measure NSZD rates at a petroleum release site in Hawaii with basalt geology and deep groundwater (300 ft. bgs,100 m bgs). Both methods documented the occurrence of NSZD at the facility and the two methods yield estimates of the NSZD rate that agreed within a factor of 2 (4600 to 7400 gal/yr; 17,000 to 28,000 L/yr for the flux method and 8600 to 13,000 gal/yr; 33,000 to 49,000 L/yr for the temperature method). Soil gas samples collected directly above the water table and at shallower depths within the vadose zone indicated aerobic conditions throughout the vadose zone (oxygen13%) and no detectable methane. These results indicate that NSZD occurs at this site through the direct aerobic biodegradation of LNAPL rather than the two-step process of anaerobic methanogenesis followed by methane oxidation at a shallow depth interval documented at other sites.

At sites impacted by volatile organic compounds (VOCs), vapor intrusion (VI) is the pathway with the greatest potential to result in actual human exposure. Since sites with VI were first widely publicized in late 1990s, the scientific understanding of VI has evolved considerably. The VI conceptual model has been extended beyond relatively simple scenarios to include nuances, such as biological and hydrogeological factors that may limit the potential for VI and alternative pathways, such as preferential pathways and direct building contact/infiltration that may enhance VI in some cases. Regulatory guidance documents typically recommend initial concentration- or distance-based screening to evaluate whether VI may be a concern, followed by a multiple-lines-of-evidence (MLE) investigation approach for sites that do not screen out. These recommendations for detailed evaluation of VI currently focus on monitoring of VOC concentrations in groundwater, soil gas, and indoor air and can be supplemented by other lines of evidence. In this Critical Review, we summarize key elements important to VI site characterization, provide the status and current understanding, and highlight data interpretation challenges, as well as innovative tools developed to help overcome the challenges. Although there have been significant advances in the understanding of VI in the past 20 years, limitations and knowledge gaps in screening, investigation methods, and modeling approaches still exist. Potential areas for further research include improved initial screening methods that account for the site-specific role of barriers, improved understanding of preferential pathways, and systematic study of buildings and infrastructure other than single-family residences.

We have utilized the California GeoTracker database to evaluate field duplicate variability and the significance of sample contamination for groundwater and vapor samples collected from contaminate...

This paper explores the application of natural resource assessment and valuation procedures as a tool for developing groundwater remediation strategies that achieve the objectives for health and environmental protection, in balance with considerations of economic viability and conservation of natural resources. The natural resource assessment process, as applied under U.S. and international guidelines, entails characterization of groundwater contamination in terms of the pre-existing beneficial services of the impacted resource, the loss of these services caused by the contamination, and the measures and associated costs necessary to restore or replace the lost services. Under many regulatory programs, groundwater remediation objectives assume that the impacted groundwater may be used as a primary source of drinking water in the future, even if not presently in use. In combination with a regulatory preference for removal or treatment technologies, this assumed exposure, while protective of human health, can drive the remedy selection process toward remedies that may not be protective of the groundwater resource itself or of the other natural resources (energy, materials, chemicals, etc.) that may be consumed in the remediation effort. To achieve the same health and environmental protection goals under a sustainable remediation framework, natural resource assessment methods can be applied to restore the lost services and preserve the intact services of the groundwater so as to protect both current and future users of that resource. In this paper, we provide practical guidelines for use of natural resource assessment procedures in the remedy selection process and present a case study demonstrating the use of these protocols for development of sustainable remediation strategies.

The role of sewer lines as preferential pathways for vapor intrusion is poorly understood. Although the importance of sewer lines for volatile organic compound (VOC) transport has been documented at a small number of sites with vapor intrusion, sewer lines are not routinely sampled during most vapor intrusion investigations. We have used a tracer study and VOC concentration measurements to evaluate the role of the combined sanitary/storm sewer line in VOC transport at the USEPA vapor intrusion research duplex in Indianapolis, Indiana. The results from the tracer study demonstrated gas migration from the sewer main line into the duplex. The migration pathway appears to be complex and may include leakage from the sewer lateral at a location below the building foundation. Vapor samples collected from the sewer line demonstrated the presence of tetrachloroethene (PCE) and chloroform in the sewer main in front of the duplex and at multiple sample locations within the sewer line upstream of the duplex. These test results combined with results from the prior multi-year study of the duplex indicate that the sewer line plays an important role in transport of VOCs from the subsurface source to the immediate vicinity of the duplex building envelope.

The temperature sensitivity of microbial populations is reflected in measured source attenuation rates at hydrocarbon-impacted sites. The objective of this study was to evaluate the correlation between temperature and source attenuation rates (concentration vs. time attenuation rate over many years) of benzene and toluene by analyzing groundwater monitoring data from >2000 hydrocarbon sites. Historical monitoring records were obtained from three databases, processed to yield long-term multiyear source attenuation rates, and then compared with representative temperatures at each site. Statistically significant and positive relationships between temperature and source attenuation rates were established for benzene and toluene, indicating that temperature does impact hydrocarbon degradation, but is one of many factors that contribute to source attenuation. There was an observed 1.1 to 1.6 times increase in attenuation rates per 10 °C increase in temperature, which is less than the rate increases predicted by the Arrhenius equation. The temperature dependence on attenuation rate is consistent with several lines of evidence that methanogenesis plays a key role in the rate of hydrocarbon source zone attenuation rather than being controlled strictly by the availability of electron acceptors. First, methanogenesis is known to be strongly influenced by temperature, with significantly higher rates up to about 35 °C. Second, the temperature-degradation rate relationship was stronger at sites with deeper water tables (>30 ft) that are less susceptible to oxygen influx than sites with shallow water tables (

Different types of data can be collected to evaluate whether or not vapor intrusion is a concern at sites impacted with volatile organic compound (VOC) contamination in the subsurface. Typically, groundwater, soil gas, or indoor air samples are collected to determine VOC concentrations in the different media. Sample results are evaluated using a “multiple lines of evidence” approach to interpret whether vapor intrusion is occurring. Data interpretation is often not straightforward because of many complicating factors, particularly in the evaluation of indoor air. More often than not, indoor air sample results are affected by indoor or other background sources making interpretation of concentration-based data difficult using conventional sampling approaches. In this study, we explored the practicality of compound-specific isotope analysis (CSIA) as an additional type of evidence to distinguish between indoor sources and subsurface sources (i.e., vapor intrusion). We developed a guide for decision-making to facilitate data interpretation and applied the guidelines at four different test buildings. To evaluate the effectiveness of the CSIA method for vapor intrusion applications, we compared the interpretation from CSIA to interpretations based on data from two different investigation approaches: conventional sampling and on-site GC/MS analysis. Interpretations using CSIA were found to be generally consistent with the other approaches. In one case, CSIA provided the strongest line of evidence that vapor intrusion was not occurring and that a VOC source located inside the building was the source of VOCs in indoor air.

The National Research Council has estimated that over 126,000 contaminated groundwater sites are unlikely to achieve low ug/L clean-up goals in the foreseeable future. At these sites, cost-effective, long-term monitoring schemes are needed in order to understand the long-term changes in contaminant concentrations. Current monitoring optimization schemes rely on site-specific evaluations to optimize groundwater monitoring frequency. However, when using linear regression to estimate the long-term zero-order or first-order contaminant attenuation rate, the effect of monitoring frequency and monitoring duration on the accuracy and confidence for the estimated attenuation rate is not site-specific. For a fixed number of monitoring events, doubling the time between monitoring events (e.g., changing from quarterly monitoring to semi-annual monitoring) will double the accuracy of estimated attenuation rate. For a fixed monitoring frequency (e.g., semi-annual monitoring), increasing the number of monitoring events by 60% will double the accuracy of the estimated attenuation rate. Combining these two factors, doubling the time between monitoring events (e.g., quarterly monitoring to semi-annual monitoring) while decreasing the total number of monitoring events by 38% will result in no change in the accuracy of the estimated attenuation rate. However, the time required to collect this dataset will increase by 25%. Understanding that the trade-off between monitoring frequency and monitoring duration is not site-specific should simplify the process of optimizing groundwater monitoring frequency at contaminated groundwater sites.

The recent boom in shale gas development in the Marcellus Shale has increased interest in the methods to distinguish between naturally occurring methane in groundwater and stray methane associated with drilling and production operations. This study evaluates the relationship between natural methane occurrence and three principal environmental factors (groundwater redox state, water type, and topography) using two pre-drill datasets of 132 samples from western Pennsylvania, Ohio, and West Virginia and 1417 samples from northeastern Pennsylvania. Higher natural methane concentrations in residential wells are strongly associated with reducing conditions characterized by low nitrate and low sulfate ([NO3- ] < 0.5 mg/L; [SO42- ] < 2.5 mg/L). However, no significant relationship exists between methane and iron [Fe(II)], which is traditionally considered an indicator of conditions that have progressed through iron reduction. As shown in previous studies, water type is significantly correlated with natural methane concentrations, where sodium (Na) -rich waters exhibit significantly higher (p

The HydraSleeve is a sampling device for collecting groundwater from the screened interval of a monitoring well without purging that uses a check valve to take in water over the first 3 to 5 feet of an upward pulling motion. If the check valve does not perform as expected, then the HydraSleeve has the potential to collect water from an incorrect depth interval, possibly above the screened interval of the well. We have evaluated volatile organic chemical (VOC) results from groundwater samples collected with the HydraSleeve sampler compared to other methods for sampling monitoring wells at three sites. At all three sites, lower VOC concentration results were observed for samples collected using the HydraSleeve. At two of these three sites, the low concentration sample results were most strongly associated with monitoring wells with more than 10 feet of water above the monitoring well-screened interval. At the site with the largest dataset, the median bias for samples collected with HydraSleeve was −20% (p < 0.001). At this site, a bias of −26% (p < 0.001) was observed for the subset of monitoring wells with greater than 10 feet of water above the screened interval compared to a bias of −7% (p = 0.21) for wells screened across the top of the water table. In addition to lower VOC concentrations, the monitoring records obtained using the HydraSleeve were more variable compared to monitoring records obtained using purge sampling methods, a characteristic that would make it more difficult to determine the long-term concentration trend in the well.

The objective of this study is to better understand the effect of mobile LNAPL on source-zone attenuation at sites using a statistical evaluation of 3,523 leaking underground fuel tank (LUFT) sites from GeoTracker, an extensive database of chemical release sites in California. Our analysis indicates that sites with mobile LNAPL (i.e., sites with measurable LNAPL thicknesses in one or more groundwater monitoring wells (LNAPL sites)) have higher maximum dissolved groundwater constituent concentrations and significantly slower source-zone attenuation rates (i.e., changes in maximum concentrations over time) compared to sites with a history of no measurable LNAPL thickness (non-LNAPL sites). However, the evaluation indicates that, for mobile LNAPL sites, physical recovery (skimming and bailing) does not increase source attenuation rates. The results suggest a need for more careful evaluation of the potential benefits of physical LNAPL technologies.

Determining whether changes in groundwater methane concentration are naturally occurring or related to oil and gas operations can be complicated by numerous sources of variability. This study of 10 residential water supply wells in Northeastern Pennsylvania evaluates how i) sampling from different points within the water well system, ii) purging different water volumes prior to sampling, and ii) natural variation over time, affects concentrations of naturally occurring dissolved methane and other water quality parameters. Among the population of wells, all had dissolved methane concentrations >1mg/L. Regardless of the volume of water purged or the timing between events, the maximum change in methane concentration (ratio of maximum to minimum concentration) among samples from a single well was 3.2, with eight out of ten wells exhibiting a maximum change less than a factor of two (i.e.

The role of sewer lines as preferential pathways for vapor intrusion is poorly understood. As a result, these pathways are often not considered when developing vapor intrusion investigation or mitigation plans. Neglecting this pathway can complicate data interpretation, which can result in repeated, and potentially unnecessary, rounds of sampling. Although a number of recent studies have highlighted the importance of sewers as preferential pathways at individual buildings, there is currently little specific technical or regulatory guidance on how to address it. The purpose of our study, therefore, was to conduct systematic testing to better understand the sewer vapor intrusion conceptual model. Through sampling at >30 different sites, the degree of interaction between impacted groundwater and the sewer lines were identified as the main factor when determining the degree of risk for sewer vapor intrusion at a given site. Higher risk sites are those with direct interaction between the subsurface volatile organic compound (VOC) source, such as groundwater, and the sewer line itself. This information can be used to prioritize sites and buildings to test for this particular exposure pathway.

We have utilized the California Department of Public Health Water Quality Analyses database, with approximately 250000 results for methyl tert-butyl ether (MTBE), to evaluate the extent of MTBE in public water supply wells in California and how these impacts have changed over time. These data show that MTBE has never been detected in >98% of 13183 public water supply wells that have been tested for MTBE. The number of wells with first-time detections of MTBE peaked in 2000 and has decreased by 80% since that time. For the 188 wells in which MTBE has been detected at least once, MTBE was not detected in the most recent analysis of 142 of these wells. Tetrachloroethene, another common groundwater contaminant, has been detected more commonly, and new detections are declining more slowly. These results indicate that the impact of MTBE on public water supply wells has peaked and is declining.

Distinguishing between vapor intrusion and indoor sources of volatile organic compounds (VOCs) is a significant challenge in conventional vapor intrusion assessments. For this research project, the authors developed a step-by-step protocol to streamline building-specific investigations by using on-site gas chromatography/mass spectrometry (GC/MS) analysis and building pressure manipulation to determine the source of VOCs in indoor air during a 1-day field investigation. Protocol validation included implementation in industrial buildings and testing alongside conventional methods. The new protocol compares favorably to conventional approaches, yielding more definitive results in less time. This article presents three case studies which illustrate application of the protocol.

The results of comprehensive field testing of on-site vapor-phase-based groundwater monitoring methods are presented to demonstrate their utility as a robust and cost-effective approach for rapidly obtaining volatile organic compounds (VOCs) concentration data from a monitoring well. These methods—which rely on sensitive, commercially available field equipment to analyze vapor in equilibrium with groundwater—proved easy to implement and can be tailored to site-specific needs, including multilevel sampling. During field testing, low-flow groundwater concentrations could be reasonably estimated using submerged passive vapor diffusion samplers or field equilibration of collected groundwater (R2 = 0.85 to 0.96). These two methods are not as reliant on in-well mixing to overcome vertical stratification within wells as simpler headspace methods. The importance of well and aquifer-specific factors on concentration data (and therefore method selection) is highlighted, including the effect of changing in-well patterns due to seasonal temperature gradients. Results indicated that vertical stratification was relatively limited within the set of wells included in these studies, resulting in similar performance for short depth-discrete passive vapor diffusion samplers (constructed from 40-mL vials) and longer samplers (2.5 to 5 feet in length) designed to cover a larger portion of the screened interval. A year-long, multi-event evaluation demonstrated that vapor-phase-based monitoring methods are no more variable than conventional groundwater monitoring methods, with both types subject to similar spatial and temporal variability that can be difficult to reduce. Vapor sampling methods represent a promising approach for estimation of groundwater concentrations by reducing the cost liabilities associated with monitoring while providing a more sustainable approach.

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.

Our understanding of vapor intrusion has evolved rapidly since the discovery of the first high profile vapor intrusion sites in the late 1990s and early 2000s. Research efforts and field investigations have improved our understanding of vapor intrusion processes including the role of preferential pathways and natural barriers to vapor intrusion. This review paper addresses recent developments in the regulatory framework and conceptual model for vapor intrusion. In addition, a number of innovative investigation methods are discussed.

Seasonal changes in ambient temperature create vertical temperature gradients in shallow groundwater (less than 15 m). These temperature gradients can affect in-well flow dynamics that impact samples collected using no-purge sampling methods. In late winter, the shallower water is colder, resulting in thermally mixed conditions and uniform contaminant concentrations. In late summer, the shallower water is warmer, resulting in thermally stratified conditions and contaminant distributions in the monitoring well more consistent with the distribution in the surrounding aquifer. The importance of seasonal temperature gradients on in-well mixing was evaluated in two shallow monitoring wells in Houston, Texas. In each of the two wells, four vertically spaced passive diffusion samples collected in late winter showed a less than 1.3x difference in trichloroethene (TCE) concentration between depths, while the same sampling conducted in late summer showed greater than a 100x difference in TCE concentration between depths. A simple analytical model originally developed to predict vertical soil temperature profiles can also be used to predict the occurrence of thermally stratified and thermally mixed conditions in monitoring wells as a function of time and well depth. The results of this analysis and modeling suggest that shallow monitoring wells in most of the United States and Canada can have significantly different vertical concentration profiles within the well over the course of a year due to seasonal vertical temperature gradients. This can induce additional intra-annual temporal variability on passive no-purge sampling results from these shallow wells, potentially making it more difficult to discern true trends in the data. © 2012 Wiley Periodicals, Inc.

Long-term groundwater monitoring data for 48 retail gasoline sites were analyzed to define the characteristics of affected groundwater plumes containing benzene, methyl tert-butyl ether (MTBE), and tert-butyl alcohol (TBA). Results of this analysis were used to determine the observed range and statistical distribution of current plume lengths, plume stability conditions, constituent concentration trends and attenuation rates, and the remediation timeframe for this population of sites. The goal of this evaluation was to characterize plume behavior as observed across a variety of hydrogeologic settings, on the basis of detailed groundwater monitoring records, rather than to define the site-specific factors controlling plume behavior. The results indicate that MTBE plumes in groundwater underlying a majority of these underground storage tank sites that were monitored for five years or longer (1) have significantly diminished in concentration over time, (2) are comparable in length to benzene plumes, ...

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.

“Random” variability in groundwater monitoring data sets reduces the ability to identify long-term concentration trends. This, in turn, increases the time and cost required to evaluate the effectiveness of natural attenuation and other groundwater remedies. To better understand the factors influencing variability in groundwater monitoring results, we have analyzed three large groundwater monitoring data sets. For the three data sets, the long-term trend in contaminant concentration in each well accounted for an average of 30% to 40% of the overall variation in contaminant concentration. Understanding the causes of the remaining variability would support the development of improved groundwater monitoring methods. All three data sets show large differences in the temporal monitoring records between individual wells (e.g., coefficient of variation for monitoring results from individual wells ranges from 0.08 to 4.6) indicating that well and aquifer factors are more important contributors to variability than sample collection and analysis factors. However, the depth to groundwater (R2 = 0.020) and distance between water level and screened interval (R2 = 0.049) accounted for only a portion of the differences in variability between wells and other aquifer characteristics evaluated and were not correlated with the observed variability in monitoring results. Unidentified factors were apparently much more important contributors to variability than these factors. The monitoring data sets exhibited two distinct timescales for variability: Time-independent variability that was apparent even when wells were re-sampled within a few days and a long-term variability likely associated with the long-term concentration trend. The observation of time-independent variability suggests that frequent monitoring of contaminated monitoring wells serves primarily to characterize sources of variability unrelated to the long-term trend of primary interest.

Vapor intrusion is associated with subsurface sources of both chlorinated VOCs and petroleum VOCs; however, petroleum vapor intrusion has been reported to occur under a narrower range of hydrogeologic settings. Research conducted over the last several years including field studies, evaluation of large datasets, and modeling studies, has provided an improved understanding of the differences in vapor intrusion associated with chlorinated VOCs and petroleum VOCs. When oxygen is present in the vadose zone, aerobic biodegradation typically results in rapid attenuation of petroleum VOCs diffusing upwards from deeper sources. At many building sites, wind-driven advection and/or building pressure fluctuations provide sufficient oxygen transport below the foundation to support this aerobic biodegradation. In such cases, there is limited potential for vapor intrusion from dissolved sources of petroleum VOCs unless preferential migration pathways are present. These findings support a framework for the evaluation of ...

Recent improvements in field-portable analytical equipment allow accurate on-site measurement of VOCs present in air at concentrations of less than 0.1 parts per million volume (ppmv). The objective of this project is to determine if the use of these instruments for vapor-phase measurements of headspace in a monitoring well can serve as a reliable and accurate method for monitoring volatile organic compound (VOC) concentrations in groundwater under equilibrium conditions. As part of a comprehensive research project investigating the utility of this proposed monitoring method, the authors have completed a laboratory validation study to identify instruments and sample-collection methods that will provide accurate measurement of VOC concentrations in groundwater. This laboratory validation study identified two field-portable instruments (a gas chromatograph and a photoionization detector) with sufficient sensitivity to measure VOCs in groundwater at concentrations below typical monitoring standards (i.e., 1 to 5 μg/L). The accuracy and precision of these field instruments was sufficient to satisfy typical data-quality objectives for laboratory-based analysis. In addition, two sample-collection methods were identified that yield vapor-phase samples in equilibrium with water: direct headspace sampling and passive diffusion samplers. These sample-collection methods allow the field instruments (which measure VOC concentrations in vapor-phase samples) to be used to measure VOC concentrations in water. After further validation of these sample-collection methods in the field, this monitoring method will provide a simple way to obtain accurate real-time measurements of VOC concentrations in groundwater using inexpensive field-portable analytical instruments. © 2009 Wiley Periodicals, Inc.

We have utilized a simple method for the collection of gas samples from vapor intrusion investigation sites and the analysis of radon concentrations at an offsite laboratory. The radon data can be used to evaluate the movement of soil gas through building foundations at sites contaminated with volatile organic compounds (VOCs) where vapor intrusion is a concern. The use of radon as a tracer for the movement of soil gas into buildings minimizes the problem of indoor sources associated with the direct measurement VOCs. We have used this method to measure indoor to sub-slab attenuation factors in six buildings with the resulting values ranging from 0.0002 to 0.006 (dim). These values are similar to radon attenuation factors reported by other researchers, but are much lower than default values for VOCs recommended in regulatory guidance (United States Environmental Protection Agency [USEPA], 2002). The vertical radon concentration profile in soil gas can be used to evaluate soil gas mixing with the atmosphere...

Analysis of dissolved light hydrocarbon gas concentrations (primarily methane and ethane) in water supply wells is commonly used to establish conditions before and after drilling in areas of shale gas and oil extraction. Several methods are currently used to collect samples for dissolved gas analysis from water supply wells; however, the reliability of results obtained from these methods has not been quantified. This study compares dissolved methane and ethane concentrations measured in groundwater samples collected using three sampling methods employed in pre- and post-drill sampling programs in the Appalachian Basin. These include an open-system collection method where 40 mL volatile organic analysis (VOA) vials are filled directly while in contact with the atmosphere (Direct-Fill VOA) and two alternative methods: (1) a semi-closed system method whereby 40 mL VOA vials are filled while inverted under a head of water (Inverted VOA) and (2) a relatively new (2013) closed system method in which the sample is collected without direct contact with purge water or the atmosphere (IsoFlask® ). This study reveals that, in the absence of effervescence, the difference in methane concentrations between the three sampling methods was relatively small. However, when methane concentrations equaled or exceeded 20 mg/L (the approximate concentration at which effervescence occurs in the study area), IsoFlask® (closed system) samples yielded significantly higher methane concentrations than Direct-Fill VOA (open system) samples, and Inverted VOA (semi-closed system) samples yielded lower concentrations. These results suggest that open and semi-closed system sample collection methods are adequate for non-effervescing samples. However, the use of a closed system collection method provides the most accurate means for the measurement of dissolved hydrocarbon gases under all conditions.

High variability in groundwater monitoring results makes it difficult to accurately evaluate remedy effectiveness and to predict remediation timeframes. In order to determine the effect of sample collection method on monitoring variability, five methods for collecting groundwater samples from monitoring wells were compared at two sites in Texas and California with chlorinated VOC plumes. The five sample collection methods evaluated included three variations of low-flow purge sampling: low-flow with purge to parameter stability (Low-Flow Standard), low-flow with fixed small volume purge (3 Liters), and low-flow with fixed large volume purge (18 Liters), and two no purge methods using commercial devices: Active No Purge (HydraSleeve) and Passive No Purge (SNAP samplers). Eight monitoring wells were sampled at each site six times using each sampling method at approximately two month intervals, resulting in a total of 480 groundwater samples. For the dataset as a whole, the five sample methods yielded relatively small differences in VOC concentration ( /-20 ), and with the exception of the Active No Purge (HydraSleeve) method at the California site, only small differences in variability between sample events. For the purge methods tested, results indicate that low flow purge sampling using a fixed purge volume provides results of equal quality compared to standard low flow purge sampling based on field parameter stability. The overall results suggest that for most monitoring well sampling methods tested, data quality was independent of method. In that context, factors such as cost, ease of implementation, and sample volume requirements become prominent deciding factors in sample method selection.

US EPA and many state regulatory agency guidance documents recommend below-foundation vapor sampling as a key element of site investigations to determine if vapor migration from underlying soil into buildings is a completed exposure pathway (USEPA, 2002; WIDHFS, 2003; San Diego County, 2004; PADEP, 2004). If volatile organic compounds (VOCs) are detected below the building foundation, then VOC migration from the subsurface is assumed to be occurring, and further investigation is needed to determine the extent of the VOC impact. These guidance documents are predicated on the assumption that VOCs detected in below-foundation samples have originated from deeper within the subsurface. However, detection of VOCs in below-foundation vapor samples alone is not sufficient to conclude that the VOCs are migrating from the subsurface upward towards a building. VOCs detected in below-foundation vapor samples can originate from indoor sources, migrating down through the slab by diffusion or advection. Commonly referen...

To further our understanding of the groundwater-to-indoor-air exposure pathway, a database of 270 paired groundwater and indoor air measurements of volatile organic compounds (VOCs) from 31 sites has been compiled and analyzed. Using regression analyses, these data have been analyzed to (1) detect evidence of indoor air impacts from dissolved petroleum hydrocarbons or chlorinated solvents in underlying groundwater, (2) estimate the true attenuation factor for volatilization from groundwater to indoor air, and (3) assess the utility of popular groundwater-to-indoor-air transport models for evaluating this exposure pathway. Key findings include the following: (1) > 95% of the indoor air concentrations fall within or below the range of national background indoor air values; (2) there is no correlation between petroleum constituent concentrations measured in groundwater and the concentrations of these VOCs in indoor air of overlying structures; and (3) for chlorinated solvent cases, a correlation between grou...

Under sponsorship from the U.S. Environmental Protection Agency (USEPA) and the American Society of Testing & Materials (ASTM), a nationwide study evaluated the impact of risk-based corrective action (RBCA) on the performance of state environmental cleanup programs. The RBCA process for management of affected soil and groundwater sites was originally detailed in ASTM Standard E-1 739-95, Standard Guidelines for Risk-Based Corrective Action at Petroleum Release Sites, published in 1995 and reissued in expanded form in 1998. With strong support from the USEPA, the RBCA process has been adopted by many state agencies in the U.S in an effort to improve the management of environmental cleanup efforts, as needed to address increasing case backlogs and rising costs associated with remediation of leaking underground storage tank sites. To evaluate the success of these RBCA programs, the Risk-Based Decision-Making (RBDM) Performance Assessment Study, initiated in 1997, involved detailed evaluation of five state en...

The reduction of the tetrazolium salt 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) to a blue formazan product is widely used for assaying cell survival and proliferation. The reduction reaction is catalyzed by dehydrogenases localized in the mitochondria of viable cells. As part of an analysis of the ability of glutathione S-transferase (GST) enzymes to protect cells from electrophilic compounds, we found extremely high background levels of the formazan product produced by cells that overexpressed the mouse GST P1-1 enzyme. Further analysis with purified GST enzymes confirmed the ability of these enzymes to reduce MTT in vitro. These data suggest that cytotoxicity assays using MTT should be interpreted with caution, especially when studying the effects of compounds that can influence GST expression.

Mice constitutively express glutathione S -transferase mGSTA3-3 in liver. This isoform possesses uniquely high conjugating activity toward aflatoxin B 1 -8,9-epoxide (AFBO), thereby protecting mice from aflatoxin B 1 -induced hepatocarcinogenicity. In contrast, rats constitutively express a closely related GST isoenzyme, rGSTA3-3, with low AFBO activity and, therefore, are sensitive to aflatoxin B 1 exposure. Although the two GSTs share 86% sequence identity and have similar catalytic activities toward 1-chloro-2,4-dinitrobenzene (CDNB), they have an approximately 1000-fold difference in catalytic activity toward AFBO. To identify amino acids that confer high activity toward AFBO, non-conserved rGSTA3-3 residues were replaced with mGSTA3-3 residues in two regions believed to form the substrate binding site. Twenty-one mutant rGSTA3-3 enzymes were generated by site-directed mutagenesis using combinations of nine different residues. Except for the E208D mutant, single mutations of rGSTA3-3 produced enzymes with no detectable AFBO activity. Generally, AFBO conjugation activity increased in additive fashion as mGSTA3-3 residues were introduced into the rGSTA3-3 enzyme with the six site mutant E104I/H108Y/Y111H/L207F/E208D/V217K displaying the highest AFBO activity (40 nmol/mg/min) of all the mutant enzymes. When this mutant enzyme was further modified by three additional substitutions (D103E/I105M/V106I) AFBO conjugation activity decreased 14-fold to 2.8 nmol/mg/min. Although wild-type mGSTA3-3 AFBO conjugation activity (265 nmol/mg/min) could not be obtained by our rGSTA3-3 mutants, we were able to identify six mGSTA3-3 residues; Ile 104 , Tyr 108 , His 111 , Phe 207 , Asp 208 , and Lys 217 that, when collectively substituted into rGSTA3-3, substantially increased (>200-fold) glutathione conjugation activity toward AFBO.

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.

Distinguishing between vapor intrusion (VI) and indoor sources of volatile organic compounds (VOCs) is a significant challenge in site assessments, greatly increasing the cost and complexity of investigations. Rapid on-site analysis of indoor air samples using a portable GC/MS allows the users to understand the distribution of VOCs in real-time, supporting identification of the source while in still the field. The overall objective of the demonstration was to develop and validate a step-wise investigation procedure using commercially-available off the shelf (COTS) on-site GC/MS analysis with real-time decision making as a tool to distinguish between vapor intrusion and indoor sources of VOCs. Use of on-site GC/MS analysis to distinguish between vapor intrusion and indoor sources of VOCs requires a field-portable analytical instrument with sufficient sensitivity to measure VOC concentrations in indoor air within the concentration range of regulatory concern (i.e., as low as 1 g/m3). A high degree of precision is also required because the protocol relies on measuring concentration gradients inside a building to identify sources of VOCs. For the demonstration, we utilized a HAPSITE portable GC/MS instrument. Although specific procedures in the investigation protocol were developed using the HAPSITE, any on-site instrument with sufficient sensitivity and precision may be used in the protocol. The field investigation program included application of the on-site GC/MS analysis protocol at four Department of Defense (DoD) sites. To evaluate the validity of this investigation approach, we also conducted conventional vapor intrusion and compound-specific stable isotope analysis (CSIA; ESTCP Project ER-201025) investigations concurrently at the study sites. Results from the three investigation methods were compared to evaluate the relative effectiveness of the different investigation approaches. This report presents the results from a total of seven demonstration buildings.

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.

The binding of two different reaction products (p-nitrobenzyl glutathione and the aflatoxin-glutathione conjugate) to mouse glutathione S-transferase A3-3 (mGSTA3-3) has been measured using equilibrium dialysis and a direct fluorescence quenching technique. As expected, p-nitrobenzyl glutathione was found to bind with a stoichiometry of 2.24 +/- 0.17 mol/mol of dimeric enzyme. However, the much larger aflatoxin-glutathione conjugate, 8, 9-dihydro-8-(S-glutathionyl)-9-hydroxyl-aflatoxin B1 (AFB-GSH), was found to bind with a stoichiometry of 1.12 +/- 0.08 mol/mol of dimeric enzyme. p-Nitrobenzyl glutathione bound mGSTA3-3 with a dissociation constant (Kd) of 59 +/- 17 microM while the aflatoxin-glutathione conjugate bound the enzyme with a Kd of 0.86 +/- 0.19 microM. Glutathione competitively inhibited binding of AFB-GSH to mGSTA3-3 with a Ki of 1.5 mM, suggesting that AFB-GSH was binding to the enzyme active site. Although AFB-GSH bound to mGSTA3-3 with a stoichiometry of 1 mol/mol of dimeric enzyme, AFB-GSH completely inhibited activity toward 1-chloro-2, 4-dinitrobenzene, indicating that AFB-GSH binding to one active site alters affinity for 1-chloro-2,4-dinitrobenzene in the active site of the other subunit. To our knowledge, this is the first report of a glutathione S-transferase reaction product which binds to the enzyme with a stoichiometry of 1 mol/mol of dimer.

Isotope ratios of volatile organic compounds (VOCs) in the environment are often of interest in contaminant fate studies. Adsorbent preconcentration-thermal desorption of VOCs can be used to collect environmental vapor samples for compound-specific isotope analysis (CSIA). While active adsorbent samplers offer logistic benefits in handling large volumes of air, their performance in preserving VOCs isotope ratios was not previously tested under sampling conditions corresponding to typical indoor air sampling conditions. In this study, the performance of selected adsorbents was tested for preconcentration of TCE (for determination of C and Cl isotope ratios), PCE (C and Cl) and benzene (C and H). The key objective of the study was to identify the adsorbent(s) permitting preconcentration of the target VOCs present in air at low μg/m3 concentrations, without significant alteration of their isotope ratios. Carboxen 1016 was found to perform well for the full range of tested parameters. Carboxen 1016 can be recommended for sampling of TCE, PCE and benzene, for CSIA, from air volumes up to 100 L. Variable extent of isotope ratio alteration was observed in the preconcentration of the target VOCs on Carbopack B and Carbopack X, resulting from partial analyte loss via adsorbent bed breakthrough and (possibly) via incomplete desorption. The results from testing the Carbopack B and Carbopack X highlight the need of adsorbent performance validation at conditions fully representative of actual sample collection conditions, and caution against extrapolation of performance data toward more challenging sampling conditions.

Project ER-0707 has validated two improved methods for evaluation of vapor intrusion at corrective action sites: i) a Tier 2 screening procedure for evaluation of vapor intrusion from VOCs in groundwater at sites with fine-grained soils at the top of the water table and ii) a streamlined Tier 3 investigation program using building pressure control for application at sites that require building-specific investigations.

The use of measured volatile organic chemical (VOC) concentrations in indoor air to evaluate vapor intrusion is complicated by (i) indoor sources of the same VOCs and (ii) temporal variability in vapor intrusion. This study evaluated the efficacy of utilizing induced negative and positive building pressure conditions during a vapor intrusion investigation program to provide an improved understanding of the potential for vapor intrusion. Pressure control was achieved in five of six buildings where the investigation program was tested. For these five buildings, the induced pressure differences were sufficient to control the flow of soil gas through the building foundation. A comparison of VOC concentrations in indoor air measured during the negative and positive pressure test conditions was sufficient to determine whether vapor intrusion was the primary source of VOCs in indoor air at these buildings. The study results indicate that sampling under controlled building pressure can help minimize ambiguity caused by both indoor sources of VOCs and temporal variability in vapor intrusion.

The objective of the present study is an empirical validation of selected adsorbents for preconcentration of TCE, PCE and benzene in air samples containing low concentrations of these VOCs. For validation of adsorbent tube performance, we sought to: 1) using reference data, select adsorbent-analyte pairings that would be likely to offer good quantitative recovery of the target VOCs; and 2) for those adsorbent-analyte pairings, evaluate sampling conditions that are representative of sampling indoor air and soil gas with respect to sample volume, humidity, mass of target VOC, mass of non-target VOCs, and holding time between sample collection and analysis; 3) investigate the recovery of target analytes mass and any associated isotope effects. The study results demonstrated fracrtionation-free performance for Carboxen 1016. These results allow precise isotope ratio analysis into vapor intrusion site assessment protocols and other applications where VOCs of interest are present at low ug/m3 concentrations.

At buildings with potential for vapor intrusion of volatile organic chemicals (VOCs) from the subsurface, the ability to accurately distinguish between vapor intrusion and indoor sources of VOCs is needed to support accurate and efficient vapor intrusion investigations. We have developed a method for application of compound-specific stable isotope analysis (CSIA) for this purpose that uses an adsorbent sampler to obtain sufficient sample mass from the air for analysis. Application of this method to five residences near Hill Air Force Base in Utah indicates that subsurface and indoor sources of tricholorethene and tetrachloroethene often exhibit distinct carbon and chlorine isotope ratios. The differences in isotope ratios between indoor and subsurface sources can be used to identify the source of these chemicals when they are present in indoor air.

The purpose of this demonstration was to validate improved vapor intrusion field investigation methods to support cost-effective evaluation of the vapor intrusion pathway. Intensively monitored sites, such as the Borden Landfill in Canada, have greatly contributed to our understanding of the physical and chemical processes that control the transport of chemicals in groundwater. For this project, we have used a similar approach (i.e., intensively monitored sites with specially designed monitoring networks) to address the critical groundwater-to-indoor-air vapor intrusion pathway. The primary goal of the project has been to support the development of refined vapor intrusion guidance, stepwise screening, and cost-effective field investigation approaches. This will benefit facility managers by providing investigation results that support a defensible evaluation of vapor intrusion. Determination of the presence or absence of vapor intrusion impacts is important to the site management process. Definitive determination of the absence of vapor intrusion allows resources to be directed to other site impacts while avoiding presumptive mitigation, which can be burdensome from both financial and public relations perspectives. The primary objective of this demonstration study was to identify a cost-effective and accurate protocol for investigation of vapor intrusion into buildings overlying contaminated groundwater. Three performance goals were established, and all objectives were met, namely, *Collection of data representative of site conditions *Determination of vapor intrusion impacts at demonstration sites (i.e., indoor air concentration of chemical above risk-based screening limit, not attributable to background indoor air sources) *Development of a reliable vapor intrusion investigation approach (i.e., identify a limited scope investigation approach with higher accuracy than current approaches).