The nature and extent of microbial reactions in formations targeted for geologic carbon sequestration (GCS), as well as in nontarget formations that may be impacted by potential CO2 migration, are key to understanding the fate of injected CO2. The dissolution of CO2 into formation waters drives predictable geochemical changes, including pH reduction, shifts in redox conditions, and increased mineral solubility. These changes can alter microbial community composition (e.g., favoring acid-tolerant taxa) and stimulate microbes capable of using CO2 as a carbon source. Resulting biotransformation processes can transfer CO2 into the mineral phase (e.g., microbially facilitated carbonate precipitation), gas phase (e.g., methanogenesis), or organic phase (e.g., biomass formation). However, the extent, rate, and significance of these processes in both target and nontarget environments are not well understood. This paper reviews current knowledge of CO2 biotransformation relevant to GCS, including reactions occurring in deep storage formations and those arising from potential CO2 migration into shallow groundwater aquifers, the vadose zone, and marine environments. Additionally, factors that influence these transformations are summarized, methods for monitoring microbial processes are discussed, and key research gaps that could facilitate improved prediction of the long-term fate of CO2 under varying environmental conditions are identified.

A landmark National Research Council report from 2013 emphasized the need to transition to passive or less resource-intensive management strategies like monitored natural attenuation (MNA) at sites where pump-and-treat and active remedies are providing diminishing returns. While this report is now more than a decade old, the process for implementing this type of transition assessment is still not well understood by practitioners. The objective of this study was to help address this gap by developing a software tool (TA2 Tool) that aids in gathering and analyzing data that are relevant for a site-specific transition assessment. The implications of site complexities on achieving remedial objectives are a key component of transition assessment. This free web-based tool has modules that perform quantitative assessment of concentration trends and project the remediation timeframe based on the current remedial approach. It has modules that predict how remediation timeframes are influenced by matrix diffusion to assess if additional remediation is warranted. Crucially, it includes modules that evaluate MNA as a transition technology, specifically by looking at plume stability, natural attenuation rates, and projections of plume concentrations at a downgradient point of compliance in the absence of further active treatment. This new tool complements existing resources on technology optimization and transitions, including ITRC guidance. The value of performing these types of assessments is highlighted through empirical data and case studies that show that it is not necessary to operate pump-and-treat systems in perpetuity and that many sites with these systems have either transitioned to other technologies or been closed.

While petroleum plumes and even many chlorinated solvent plumes stabilize in size on a time scale of a few to several years, perfluoroalkyl and polyfluoroalkyl substances, particularly perfluoroalkyl acids (PFAAs), in groundwater pose a specific challenge for site investigation and remediation due to their recalcitrance, which creates a potential for much longer-term plume expansion. Understanding the plume stability condition (i.e., is the plume expanding, stable, or shrinking) is essential for plume management and remedy selection, but resolving this trend using conventional monitoring methods can take 5 to 10years or longer at many sites. This paper explores the difficulties in determining plume stability condition and presents four tools for evaluating stability condition more quickly and more efficiently: (1) high-resolution spatial sampling, (2) high-volume sampling, (3) passive integrative samplers, and (4) statistically based high-frequency sampling. By employing these tools, site managers can optimize monitoring strategies to quickly discern if and how much PFAA plume expansion is occurring and make timely informed decisions regarding PFAA plume management.

In this study, the headspace vapor compositions of 37 unreleased petroleum products and field-collected non-aqueous phase liquid (NAPL) samples including crude oil, gasoline, diesel, kerosene, jet fuel, naphtha, tar, lubricating oil, and field NAPL were measured using an innovative high-resolution analytical method. Gasoline and some crude oils are highly volatile while diesel, kerosene, and jet fuel have much lower volatility. In unreleased gasoline and some light distillates, benzene, toluene, ethylbenzene, and xylenes (BTEX) constitute the majority of volatile aromatic hydrocarbons. As the BTEX compounds represent only a few of the known compounds in gasoline and light distillate products, monitoring only BTEX concentrations in vapor samples may not be sufficient to distinguish among lighter petroleum products nor to totally assess all toxicity associated risks in lighter fuels, both for unreleased and released weathered products. For crude oil vapor samples, however, we found a characteristic distribution of compound groups and carbon number range distribution despite varying widely in TVOC concentration, allowing clearer specific product identification. In addition, we found that total VOC composition in middle distillate fuel vapor samples was more variable indicating that vapor fingerprinting is less useful for identification of middle distillate fuel sources. Similarly, the ratios of individual VOCs in vapor samples could be used to identify specific gasoline and crude oil NAPL sources but was not applicable to many middle distillate fuel sources. This study provides valuable data on the maximum concentrations that could potentially partition from typical petroleum products into the vapor phase, as well as the compositional benchmarks of VOC vapors from petroleum products and NAPL samples.

Understanding the mechanisms of natural source zone depletion (NSZD) will support an improved understanding of the long-term sustainability of NSZD as a site remedy and how NSZD rates may change over time. This is the first study that has quantified and compared the rate of three NSZD mechanisms (methanogenesis, vaporization, and aqueous biodegradation) between two chemically distinct light non-aqueous phase liquid (LNAPL) source zones (aliphatic-rich naphtha for Zone #1 vs aromatic-rich pyrolysis gasoline for Zone #2) within the same geologic and climate conditions. The rates of NSZD attributable to vaporization (400 mg C/m2/d vs. 300 mg C/m2/d) and aqueous biodegradation (92 mg C/m2/d vs. 67 mg C/m2/d) were similar for Zone #1 and #2; however, the rate of methanogenesis NSZD was 6x higher in Zone #1 (1000 mg C/m2/d vs. 170 mg C/m2/d). These results suggest that the aliphatic hydrocarbons content in an LNAPL source may be a factor in the rate of methanogenesis NSZD. For both Zone #1 and #2, total NSZD rate determined using this “three mechanism” measurement method was in reasonable agreement with two other methods used to measure total NSZD rates (CO2 Gradient Method and Dynamic Closed Chamber Method), validating the “three mechanism” method as a tool to measure the total NSZD rate at a site and to provide an improved understanding of the predominant NSZD mechanism. Overall, this study highlights the importance of LNAPL type and chemical characteristics in determining source zone natural attenuation mechanism and its total rates.

No field study has provided a detailed characterization of the molecular composition and spatial distribution of a vadose zone plume of petroleum volatile organic compounds (VOCs), which is critical to improve the current understanding of petroleum VOC transport and fate. This is study reports a high-resolution analysis of two distinct vapor plumes emanating from two different light non-aqueous phase liquid (LNAPL) sources (an aliphatic-rich LNAPL for Zone #1vs an aromatic-rich LNAPL for Zone #2) at a large petrochemical site. Although deep soil vapor signatures were similar to the source zone LNAPL signatures, the composition of the shallow soil vapors reflected preferential attenuation of certain hydrocarbons over others during upward transport in the vadose zone. Between deeper and shallower soil gas samples, attenuation of aromatics was observed under all conditions, but important differences were observed in attenuation to aliphatic compound classes. Attenuation of all aliphatic compounds was observed under aerobic conditions but little attenuation of any aliphatics was observed under anoxic conditions without methane. In contrast, under methanogenic conditions, paraffins attenuated more than isoparaffins and naphthenes. These results suggest that isoparafins and naphthenes may present more of a vapor intrusion risk than benzene or other aromatic hydrocarbons commonly considered to be petroleum vapor intrusion risk drivers. While the overall vapor composition changed significantly within the vadose zone, diagnostic ratios of relatively recalcitrant alkylcyclopentanes were preserved in shallow soil vapor samples. These alkylcyclopentanes may be useful for distinguishing between petroleum vapor intrusion and other sources of petroleum VOCs detected in indoor air.

At contaminated sites, groundwater monitoring results are commonly used (quantitatively or qualitatively) to predict remediation timeframes. If results are predictive, then there should be a strong positive correlation between attenuation rates for the first half of a temporal monitoring record and attenuation rates for the second half of the same record. We utilized the GeoTracker database to evaluate the power of historical groundwater monitoring results to predict future attenuation rates. For two data sets (petroleum and chlorinated solvent), we found a small negative correlation between the first-order concentration vs. time attenuation rate observed during the earlier part of the monitoring record and the later part of the monitoring record: benzene-correlation coefficient (r) = -0.11, methyl tert-butyl ether (MTBE)-r = -0.12, trichloroethene (TCE) = -0.12. For each data set, a small negative correlation between the first-order attenuation rate observed during the earlier part of the monitoring record and the later part of the monitoring record was also observed for a subset of monitoring records exhibiting the best model fits (R-2 > 0.8), a subset with a statistically significant (p < 0.05) positive attenuation rate for the first half of the monitoring record. For the TCE data set, this negative correlation was also observed for a subset of monitoring records with no change in site remedy during the monitoring period (r = -0.22). Our analysis suggests that the historical concentration vs. time attenuation rate for a contaminant at an individual site or monitoring well is a poor predictor of the future rate.

Currently, there are over 62,000 petroleum-contaminated sites undergoing remediation in the United States. Understanding attenuation rates for dissolved contaminant concentrations and factors that influence these rates is important to effectively manage these sites and move them towards closure. The GeoTracker and California Department of Public Health databases provide more than 15 years of groundwater monitoring results for tens of thousands of monitoring wells and public water supply wells in California. First-order maximum concentration attenuation rate constants (k(c-max)) were determined for sites with 5 or more years of monitoring data over the time period of 2002 to 2017. k(c-max) values vary by constituent; across more than 5000 sites, the median k(c-max) value ranged from 0.30 year(-1) for methyl tert-butyl ether (MTBE) (half-life of 2.3 years) to 0.076 year(-1) for naphthalene (half-life of 9.1 years). A focused evaluation of remediation technologies at 50 petroleum release sites indicates that dissolved contaminant attenuation typically increased during the time period of remediation. Further, the analysis indicates that biodegradation is an important attenuation process even during periods without active remediation (i.e., periods of natural attenuation). There was little evidence of petroleum and fuel oxygenate impacts to public water supply wells. Overall, the analyses indicate that both remediation and natural biodegradation contribute to site remediation and that these attenuation processes have been sufficient to prevent widespread impacts to public water supply wells.

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, Q10 values (a measure of the increase in NSZD rate associated with a 10 C increase in temperature) ranged from 0.8 to 15.1, with a median of 2.2. The implications of this study suggest that increasing mean annual soil temperature at a site using engineered methods could potentially increase the biodegradation rate (e.g., an increase of 10 C could double the NSZD rate). Finally, for five sites with site-average NSZD rates for multiple years, average NSZD rates varied by 1.1 to 4.9 times across years. Overall, the evaluation of NSZD rates measured at 40 LNAPL sites suggests that measurable NSZD occurs across a broad range of LNAPL sites. Although NSZD rates vary across sites, fuel type is not the primary factor explaining observed differences in rates.

Data mining as a research tool requires access to high quality datasets. Investigation and cleanup of contaminated sites yield large amounts of monitoring data; however, historically, these data have not been available in large, consolidated datasets. The California GeoTracker web site and database is a public repository for a wide variety of information related to investigation and remediation of cleanup sites in California. Under California regulations, responsible parties must submit laboratory analytical results for environmental samples in electronic form along with reports and other information. The GeoTracker website also supports public access to the entire database of laboratory analytical results, which, for some sites, date back to 2001. This database includes approximately 285,000,000 analytical records for more than 50,000 contaminated and formerly contaminated sites in California. Because of the large volume of publicly-available data, GeoTracker has been used as the primary data source for a number of data mining studies in the last 10 years. This article describes the GeoTracker origin story and how it has evolved to account for changes in regulatory priorities such as understanding vapor intrusion mechanisms and distribution of per- and polyfluoroalkyl substances (PFAS) in the environment, while maintaining database continuity. Finally, we review data mining projects that have utilized GeoTracker to better understand various aspects of contaminated site management. This review illustrates how long-term commitment to collection and sharing of environmental data can support the general public and the regulatory and research communities.

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.

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.

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 (oxygen >13%) 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.

ADVERTISEMENT RETURN TO ISSUEPREVViewpointNEXTFlux Chamber Measurements Should Play a More Important Role in Contaminated Site ManagementJie MaJie MaState Key Laboratory of Heavy Oil Processing, China University of Petroleum-Beijing, Beijing 102249, ChinaMore by Jie Mahttp://orcid.org/0000-0003-3719-3814, Thomas McHugh*Thomas McHughGSI Environmental, Houston, Texas 77098, United States,*Email: [email protected]More by Thomas McHughhttp://orcid.org/0000-0003-3035-0274, and Bart EklundBart EklundAECOM, Austin, Texas 78729, United StatesMore by Bart EklundCite this: Environ. Sci. Technol. 2020, 54, 19, 11645–11647Publication Date (Web):September 16, 2020Publication History Received22 June 2020Published online16 September 2020Published inissue 6 October 2020https://pubs.acs.org/doi/10.1021/acs.est.0c04078https://doi.org/10.1021/acs.est.0c04078article-commentaryACS PublicationsCopyright © 2020 American Chemical Society. This publication is available under these Terms of Use. Request reuse permissions This publication is free to access through this site. Learn MoreArticle Views4043Altmetric-Citations7LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail PDF (1 MB) Get e-AlertscloseSUBJECTS:Fluxes,Physical and chemical processes,Soils,Vinyl,Volatile organic compounds Get e-Alerts

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 GeoTracker database to evaluate field duplicate variability and the significance of sample contamination for groundwater and vapor samples collected from contaminated sites in California. Vapor duplicates are more variable than water duplicates with median percent difference in concentration of 25% compared to 7% for water samples. In addition, large differences in concentration were more common in vapor duplicates. For vapor analyte pairs, 20% of pairs had a percent difference in concentration of >300% while, for groundwater analyte pairs, only 3% had a percent difference of >300%. Contamination of samples during collection or analysis is also more significant for vapor samples. For water samples, sample contamination appears unlikely to result in false positive exceedances of drinking water standards; however, for vapor samples, sample contamination may result in false positive exceedances of indoor air screening values. For vapor samples, the use of reusable canisters and flow controllers is likely an important source of sample contamination.

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.

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., <±100%). Among water wells where methane concentration changed by ±50% or more, there was a strong correlation with changes in the concentrations of sodium, chloride, and other salinity indicators such as specific conductivity and TDS. This suggests that significant variability in methane concentration is predominantly related to changes in the relative volumes of sodium-rich fluids feeding the wellbore at any given time. Among study well locations with bladder and diaphragm pressure tanks, there was no significant difference in dissolved methane concentrations between samples collected either upstream or downstream of a pressure tank. There appears to be little benefit to purging multiple casing volumes of water from a well prior to sampling because such volumes tend to be much larger than those representative of normal residential use. We recommend purging a volume sufficient to remove standing water in the pressure tank and lines above the pump intake. This article culminates with additional recommendations for improving sample collection methods and interpreting sampling data.