Historical analysis of monitored natural attenuation: A survey of 191 chlorinated solvent sites and 45 solvent plumes

A survey of experts in the application of natural attenuation was conducted to better understand how monitored natural attenuation (MNA) is being applied at chlorinated solvent sites. Thirty‐four remediation professionals provided general information for 191 sites where MNA was evaluated, and site‐specific data for 45 chlorinated solvent plumes being remediated by MNA. Respondents indicated that MNA was precluded as a remedy at only 23 percent of all sites where evaluated as a remedial option. Leading factors excluding MNA as a remedial approach were the presence of an expanding plume and an unreasonably long estimated remediation time frame. MNA is being used as the sole remedy at about 30 percent of the sites, and 33 percent are implementing MNA in conjunction with source zone remediation. The remaining sites are implementing MNA with plume remediation (13 percent), source containment (9 percent), or some other strategy (16 percent). © 2004 Wiley Periodicals, Inc.     

Let biodegradation promote in-situ soil venting

Although vapor extraction systems (VES) certainly help remediate volatile hydrocarbons (e.g., gasoline in unsaturated soils), recent studies have found that much of the related hydrocarbon removal is due to aerobic biodegradation, not simple volatilization. In many cases, more than 50 percent of the hydrocarbon removal by these systems is due to biodegradation. By emphasizing biodegradation and minimizing volatilization, the costs of system operation can be reduced, especially for off-gas treatment. Maximizing biodegradation also supports more efficient site remediation because not only are the volatile hydrocarbons cleaned up, but the less volatile contaminants are also cleaned up—by biodegradation. More complete site cleanups are possible through bioventing, especially when cleanup criteria are related to total petroleum hydrocarbons. This article explores the major environmental conditions that influence biodegradation, analyzes several bioventing case histories, and calculates biodegradation's remedial costs.     

Use of mixed technologies to remediate chlorinated DNAPL at a Brownfields site

A former chlorofluorocarbon manufacturing facility in northern New Jersey was purchased for redevelopment as a warehousing/distribution center as part of the New Jersey Department of Environmental Protection's Brownfields redevelopment initiative. Soil and groundwater at the site were impacted with dense nonaqueous‐phase liquids (chlorinated organic compounds) and light nonaqueous‐phase liquids (petroleum hydrocarbons). The initial remedial strategy (excavation and offsite disposal) developed by prior site owners would have been cost‐prohibitive to the new site owners and made redevelopment infeasible. Mixed remedial technologies were employed to reduce the cost of remediation while meeting regulatory contaminant levels that are protective of human health and the environment. The most heavily impacted soils (containing greater than 95 percent of the contaminant mass) were excavated and treated onsite by the addition of calcium oxide and lime kiln dust coupled with physical mixing. Treated soils were reused onsite as part of the redevelopment. Residual soil and groundwater contamination was treated via in situ injections of emulsified oil to enhance anaerobic biodegradation, and emulsified oil/zero‐valent iron to chemically reduce residual contaminants. Engineering (cap) and administrative (deed restriction) controls were used as part of the final remedy. The remedial strategy presented in this article resulted in a cost reduction of 50 percent of the initial remedial cost estimate. © 2008 Wiley Periodicals, Inc.     

Preliminary comparison of alternative designs for soil remediation by bioventilation

Computer simulation provides a powerful tool for comparing alternative designs of remediation processes. The software package FLUENT copes successfully with subsurface three-dimensional flow fields, multiple and directional permeabilities, wide spatial variations in subsurface gas velocities, and the presence of surface structures. FLUENT can provide an invaluable method for comparing remediation efficiencies among competing design alternatives. This article features case studies to illustrate the benefits that simulation studies provide before and during the initiation of remedial actions using bioventing.     

Sustainable wind‐driven bioventing at a petroleum hydrocarbon–impacted site

Bioventing—the injection of air into the vadose zone to increase microbial activity—is a commonly used, proven technology for remediating volatile organic compounds present in the vadose zone. Passive systems driven by wind or solar power are both more cost‐effective and sustainable than conventional systems. Such a passive system is being applied successfully to remediate a site impacted with total petroleum hydrocarbons (TPH) and benzene, toluene, ethylbenzene, and xylenes (BTEX) in soil. Bioventing technology was approved by the regulatory agency as an interim remedial action to remove chemicals of concern (COCs) in the vadose zone. A bioventing pilot study was conducted to evaluate the effectiveness of COC removal and collect parameters for full‐scale design and implementation. To evaluate the potential to use wind‐driven bioventing technology, two mobile weather stations were installed at the site and monitored for one month for a wind speed study. Based on the pilot‐test data and wind speed research, 12‐inch diameter funnel/vane 360‐degree wind collectors were designed as passive wind‐driven air‐injection devices and connected to existing monitoring wells. The measured air velocity ranged from 20 to 110 feet per minute during the start‐up and the first three months of operation and maintenance. Monitoring indicated a 20 percent oxygen delivery and greater than 90 percent reduction in COC concentrations, demonstrating a successful sustainable remediation with no power requirement and minimal operation and maintenance. © 2012 Wiley Periodicals, Inc.     

Interim pump-and-treat remediation of a hydrocarbon-contaminated aquifer

The continuous and discontinuous release of petroleum hydrocarbons from an oil refinery in Alaska resulted in the contamination of an unconfined glacial outwash aquifer. Geologic conditions at the site allowed for the vertical migration of hydrocarbon product to the water table and subsequent formation of an areally extensive floating product layer. Since the petroleum hydrocarbon phase would provide a major source of BTX (benzene, toluene, xylene) contamination to the groundwater, interim product and groundwater recovery measures were initiated to limit aquifer degradation. Phase I remedial activities involved the operation of nine well pairs, with one well used for groundwater extraction and the other for product recovery. Phases II and III involved expansion of the recovery well network and use of a two-pump system. Petroleum product recovered was reprocessed at the refinery. Contaminated groundwater was initially treated using the refinery's wastewater treatment system, but treatment inefficiencies and continued system expansion necessitated use of a separate treatment unit. Performance evaluations indicate that the remedial phases have been successful in halting further contaminant migration and in recovering a significant volume of the released petroleum hydrocarbons.     

Closing the mass balance at chlorinated solvent sites: Sources and attenuation processes

Using detailed mass balance and simple analytical models, a spreadsheet‐based application (BioBalance) was developed to equip decision makers with a predictive tool that can provide a semiquantitative projection of source‐zone concentrations and provide insight into the long‐term behavior of the associated chlorinated solvent plume. The various models were linked in a toolkit in order to predict the composite impacts of alternative source‐zone remediation technologies and downgradient attenuation processes. Key outputs of BioBalance include estimates of maximum plume size, the time frame for plume stabilization, and an assessment of the sustainability of anaerobic natural attenuation processes. The toolkit also provides spatial and temporal projections of integrated contaminant flux and plume centerline concentrations. Results from model runs of the toolkit indicate that, for sites trying to meet traditional, “final” remedial objectives (e.g., two to three orders of magnitude reduction in concentration with restoration to potable limits), “dispersive” mechanisms (e.g., heterogeneous flow and matrix diffusion) can extend remedial time frames and limit the benefits of source remediation in reducing plume sizes. In these cases, the removal of source mass does not result in a corresponding reduction in the time frame for source remediation or plume stabilization. However, this should not discourage practitioners from implementing source‐depletion technologies, since results from the toolkit demonstrate a variety of measurable benefits of source remediation. Model runs suggest that alternative, “intermediate” performance metrics can improve and clarify source remediation objectives and better monitor and evaluate effectiveness. Suggested intermediate performance metrics include reduction in overall concentrations or mass within the plume, reduction of flux moving within a plume, and reduction in the potential for risk to a receptor or migration of a target concentration of contaminant beyond a site boundary. This article describes the development of two key modules of the toolkit as well as illustrates the value of using intermediate performance metrics to evaluate the performance of a source‐remediation technology. © 2010 Wiley Periodicals, Inc.     

Transmissivity as a Primary Metric for LNAPL Recovery—Case Study Comparison of Short‐Term vs. Long‐Term Methods

This article presents a case study and comparative analysis of light nonaqueous phase liquid (LNAPL) transmissivity estimated using short‐ and long‐term test methods at an active petroleum refinery. LNAPL transmissivity (Tn) is a recognized direct indicator of LNAPL recoverability with increasing acceptance by regulatory agencies. Historical releases at a refinery resulted in widespread LNAPL accumulations across the site and, as such, a focused approach is being implemented to enhance recovery, shorten remedial timeframes, and prioritize areas for recovery. Groundwater pumping systems operate continuously to maintain hydraulic containment of impacts, along with 12 LNAPL recovery systems. Transmissivity has been established as a primary metric and management tool for LNAPL recovery at the refinery. In this case study, estimated transmissivity values from short‐term data (baildown testing) and long‐term data (LNAPL skimming operations) from the same locations are analyzed and compared. Overall results are presented with respect to variations in transmissivities between the short‐ and long‐term tests, significance of data collection and quality, and consideration factors affecting transmissivity including fluid properties, soil types, hydrogeology, saturation levels, tidal effects, migration rates, and receptor risks. Additionally, the application of transmissivity as a metric for monitoring progress toward LNAPL recovery endpoints as part of the LNAPL remediation program development is discussed. ©2015 Wiley Periodicals, Inc.     

A method to compare groundwater cleanup technologies

DuPont has developed a method to compare, on a consistent economic basis, in situ remediation technologies. The methodology employs a template site with a perchloroethylene plume 1000 ft long by 400 ft wide, and incorporates various aquifer thicknesses and depths. Variables considered in the methodology include duration of the remediation; estimated engineering and flow/transport modeling costs; equipment costs; and operation, maintenance, and monitoring costs. In this article, substrate-enhanced anaerobic bioremediation, intrinsic bioremediation, in situ permeable reactive barriers, and pump-and-treat systems are evalutated. Cost metrics include present cost, cost per pound of contaminant removed, and cost per 1000 gals treated, using a discounted cash-flow analysis. Costs of the remedial alternatives increase starting from intrinsic bioremediation, to substrate-enhanced anaerobic bioremediation, to a biological substrate-enhanced anaerobic barrier, to in situ permeable reactive barriers, to pump-and-treat systems with air stripping and carbon adsorption.     

A deterministic approach to evaluate and implement monitored natural attenuation for chlorinated solvents

A US EPA directive and related technical protocol outline the information needed to determine if monitored natural attenuation (MNA) for chlorinated solvents is a suitable remedy for a site. For some sites, conditions such as complex hydrology or perturbation of the contaminant plume caused by an existing remediation technology (e.g., pump‐and‐treat) make evaluation of MNA using only field data difficult. In these cases, a deterministic approach using reactive transport modeling can provide a technical basis to estimate how the plume will change and whether it can be expected to stabilize in the future and meet remediation goals. This type of approach was applied at the Petro‐Processors Inc. Brooklawn site near Baton Rouge, Louisiana, to evaluate and implement MNA. This site consists of a multicomponent nonaqueous‐phase source area creating a dissolved groundwater contamination plume in alluvial material near the Mississippi River. The hydraulic gradient of the groundwater varies seasonally with changes in the river stage. Due to the transient nature of the hydraulic gradient and the impact of a hydraulic containment system operated at the site for six years, direct field measurements could not be used to estimate natural attenuation processes. Reactive transport of contaminants were modeled using the RT3D code to estimate whether MNA has the potential to meet the site‐specific remediation goals and the requirements of the US EPA Office of Solid Waste and Emergency Response Directive 9200.4‐17P. Modeling results were incorporated into the long‐term monitoring plan as a basis for evaluating the effectiveness of the MNA remedy. As part of the long‐term monitoring plan, monitoring data will be compared to predictive simulation results to evaluate whether the plume is changing over time as predicted and can be expected to stabilize and meet remediation goals. This deterministic approach was used to support acceptance of MNA as a remedy. © 2007 Wiley Periodicals, Inc.     

Operating windows to assess whether monitored natural attenuation is a technically feasible remediation option for BTEX‐contaminated groundwater

When used in combination with source management strategies, monitored natural attenuation (MNA) is likely to be a technically feasible remediation option if the contaminant persistence time along the flow path is less than (a) the transport time to the compliance point and (b) the time available for groundwater remediation objectives to be achieved. Biodegradation is often the most significant natural attenuation process for benzene, toluene, ethylbenzene, and xylenes (BTEX) in groundwater. While BTEX transport rates increase with groundwater velocity, examination of data obtained from the published literature for seven sites undergoing MNA revealed significant positive correlations between groundwater velocity and first‐order biodegradation rates for toluene (r = 0.83, P < 0.05), ethylbenzene (r = 0.93, P < 0.01), m‐ and p‐xylene (r = 0.96, P < 0.01), and o‐xylene (r = 0.78, P < 0.05). This is attributed to increased dispersion at higher velocities leading to more mixing of electron acceptors with the contaminant plume. There was no positive correlation between groundwater velocity and first‐order biodegradation rates for benzene due to noise in the relationship caused by variations in (a) the concentrations of electron acceptors in the uncontaminated groundwater and (b) the proportions of benzene in the total BTEX concentration in the source area. A regression model of the relationship between groundwater velocity and the first‐order biodegradation rate can be used to delineate operating windows for groundwater velocity within which the contaminant persistence time is less than the transport and remediation times for a given source concentration, target concentration, distance to compliance point, retardation factor, and remediation time. The operating windows can provide decision makers with a rapid indication of whether MNA is likely to be a technically feasible remediation option at a given site. © 2005 Wiley Periodicals, Inc.     

Application of the operating window concept to remediation‐option selection

An Erratum has been published for this article in Remediation 14(4) 2004, 141. The selection of remediation options for the management of unacceptable risks at contaminated sites is hindered by insufficient information on their performance under different site conditions. Therefore, there is a need to define “operating windows” for individual remediation options to summarize their performance under a variety of site conditions. The concept of the “operating window” has been applied as both a performance optimization tool and decision support tool in a number of different industries. Remediation‐option operating windows could be used as decision support tools during the “options appraisal” stage of the Model Procedures (CLR 11), proposed by the Environment Agency (EA) for England and Wales, to enhance the identification of “feasible remediation options” for “relevant pollutant linkages.” The development of remediation‐option operating windows involves: 1) the determination of relationships between site conditions (“critical variables”) and option performance parameters (e.g., contaminant degradation or removal rates) and 2) the identification of upper‐ and lower‐limit values (“operational limits”) for these variables that define the ranges of site conditions over which option performance is likely to be sufficient (the “operating window”) and insufficient (the “operating wall”) for managing risk. Some research has used case study data to determine relationships between critical variables and subsurface natural attenuation (NA) process rates. Despite the various challenges associated with the approach, these studies suggest that available case study data can be used to develop operating windows for monitored natural attenuation (MNA) and, indeed, other remediation options. It is envisaged that the development of remediation‐option operating windows will encourage the application of more innovative remediation options as opposed to excavation and disposal to landfill and/or on‐site containment, which remain the most commonly employed options in many countries. © 2004 Wiley Periodicals, Inc.     

Actuarial Risk Analysis to Promote National Contingency Plan (NCP)‐Consistent Remediation

This article provides a case study of how green and sustainable remediation (GSR) concepts (including, but not limited to, worker risk) can be incorporated into the existing National Contingency Plan (NCP)/Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) remedy selection framework. The occupational risks of worker fatalities and injuries associated with two site remediation alternatives were calculated and compared. The results demonstrated that the increased worker risks associated with one of the remedy alternatives rendered it inferior based on the NCP “Balancing Criteria” of short‐term effectiveness. This type of approach is implementable at many sites by leveraging readily available information at the remedy selection stage using published methods and data sources. © 2014 Wiley Periodicals, Inc.     

Creosote DNAPL Recovery‐Well Design for Mass Removal

Sites with dense nonaqueous‐phase liquid (DNAPL) contamination present significant remediation challenges in terms of technical practicability and cost. Remedial approaches to DNAPL sites often follow a management approach rather than removal or eradication approaches, particularly due to the uncertainties associated with the benefits of partial source mass removal, as complete source removal is unlikely. Mass‐removal technologies should be evaluated for all DNAPL sites, although implementation of recovery technologies will be limited to a few sites based upon site‐specific factors. Sitewide remedial strategies that employ source reduction, where applicable, and incorporate associated risk‐reduction technologies, including monitored natural attenuation, are advised. Creosote DNAPL sites are particularly challenging, as they are predominantly composed of low‐solubility polycyclic aromatic hydrocarbons that form long‐term continuing sources. Additionally, the physical properties of creosote DNAPL, including high viscosity and relatively low density, result in significant migration potential and considerable dissolved‐phase groundwater impacts. An innovative creosote DNAPL source recovery well design was developed to achieve separate‐phase removal of pooled creosote DNAPL. The design presented herein employs modified circulation‐well technology to mobilize DNAPL to the engineered recovery well, where it is gravity‐settled into a sump to permit separate‐phase mass removal of the emplaced DNAPL source without groundwater production or treatment. A discharge mass flux protocol was developed to verify dissolved‐phase plume stability and the benefit of the source mass removal. © 2013 Wiley Periodicals, Inc.     

Advanced site diagnostic tool 3D‐CSIA for in situ remediation

In situ remediation is inherently considered “green remediation.” The mechanisms of destruction by in situ technologies, however, are often unseen and not well understood. Further, physical effects of amendment application affect concentration data in an identical manner as the desired reactive mechanism. These uncertainties have led to the weight‐of‐evidence approach when proving viability: multiple rounds of data collection, bench studies, pilot studies, and so on. Skipping these steps has resulted in many failed in situ applications. Traditional assessment data are often tangential to the desired information (e.g., “Is contaminant being destroyed or just being pushed around and diluted?” and “What is the mechanism of the destruction and can it be monitored directly?”). An advanced site diagnostic tool, “Three‐Dimensional Compound Specific Stable Isotope Analysis” (3D‐CSIA), can assess the viability of in situ technologies by providing definitive data on contaminant destruction that are not concentration‐related. The 3D‐CSIA tool can also locate source zones and apportion remediation cost by identifying plumes of different isotope signatures and fractionation trends. Further, use of the 3D‐CSIA tool allows remediation professionals to evaluate effectiveness of treatment and make better decisions to expedite site closure and minimize costs. This article outlines the fundamentals of advanced site diagnostic tool 3D‐CSIA in detail, and its benefit is highlighted through a series of case studies at chlorinated solvent–contaminated sites. © 2010 Wiley Periodicals, Inc.     

Nature,frequency, and cost of environmental remediation at onshore oil and gas exploration and production sites

This article quantifies the nature, frequency, and cost of environmental remediation activities for onshore oil and gas operations, as determined from over 4,100 environmental remediation cases in Texas, Kansas, New Mexico, and Colorado. For the purpose of this article, “remediation'' refers to cleanup efforts that entail longer‐term site characterization, monitoring, and remedial action beyond the initial spill cleanup or emergency response stage. In addition, data are also presented regarding short‐term spill cleanup activities in two of the four states. © 2011 Wiley Periodicals, Inc.     

Removal,Handling, and Thermal Desorption Treatment Techniques for Manufactured Gas Plant Wastes

Due to the nature of contamination typically found at former MGP (manufactured gas plant) sites, excavation and thermal desorption of MGP wastes has proven to be an effective method for the remediation of MGP‐contaminated soil. The use of on‐site thermal desorption enables MGP sites to be quickly remediated at a low cost. Tar pits, holders, and other underground storage structures typically contain coal tar residuals and waste from former operations, and the areas around these structures are often significantly contaminated. Thus, excavation techniques, odor and vapor management, and material preparation for the treatment method are important factors to consider when developing a site remediation strategy. This article reviews typical excavation and handling methods associated with the remediation of former MGP sites and discusses the treatment of MGP wastes using on‐site thermal desorption technology. © 2001 John Wiley & Sons, Inc.     

Sustainable Remediation and Decision Analysis Practices at an Onshore Gas Well Site

Strategies for remediation of drilling mud wastes at a typical deep sour gas well site in the foothills of Alberta were assessed in terms of financial and social costs and benefits, in alignment with established sustainable remediation and decision analysis principles. Managers of contaminated sites containing historical drilling wastes are challenged with managing liability through several regulatory changes over time. Excavation and disposal of the contaminated soil from the site was the only means of securing regulatory release, with the nearest landfill located 150 km away. A perception exists that in many cases excavation and disposal inflicts unnecessary levels of site intrusiveness and public disturbance when other options achieving a similar risk end point may do so for lower social cost. The study tested this hypothesis to ascertain whether the currently accepted solution is the best option when the wider costs and benefits to society and the environment are included. Eight remedial strategies were assessed using cost–benefit analysis, including using environmental economics techniques to quantify social and environmental impacts. The economic model showed that methods such as capping in‐place or engineered encapsulation were superior to full excavation and disposal from financial and sustainability perspectives. Quantified external costs and benefits such as road damage, greenhouse gas emissions, public nuisance and safety, and community amenity value were influential in identifying superior options. It was demonstrated that $0.2 million of societal costs could be avoided by choosing capping over landfill disposal. This represents substantial implications when viewed in the context of this and other operators’ portfolios of hundreds of abandoned wells in the area. ©2016 Wiley Periodicals, Inc.