Monitored Natural Attenuation Research Articles

Monitored Natural Attenuation of Chlorinated Solvents: Assessment of Potential and Limitations

Chlorinated solvent contaminations in groundwater and soil are a widespread problem. Their remediation either by active and/or passive measures is complicated though, due to their characteristic features of long plumes and DNAPL. In this study Monitored Natural Attenuation (MNA) is evaluated regarding its applicability to remediate chlorinated solvent sites. At first a short overview of legal requirements is given, by whose means the efficiency of MNA is supposed to be demonstrated and assessed. Thereby the focus is set on the circumstances in the United States and Germany, as in the first MNA is applied as remediation option, whereas in the latter implementation and definition of MNA is under discussion. Based on criteria given by several German guidelines investigation methods and suitable parameters are discussed with the objective to evaluate their potential and limitations to demonstrate MNA at a chlorinated solvent site. Regarding the description of source location/inventory and source emission the legal requirements cannot be accomplished mainly because of missing methods. With regard to the characterization of plume migration, and evaluation of the efficiency of NA processes the combination of hydrogeochemical data and model approaches can accomplish most of the legal requirements. Applicability of microbiological and isotope methods is limited which is also reflected by evaluation of field studies, where these methods have been used at less than 10% of the sites. With regard to chlorinated solvents current German guidelines should be modified to enable the feasible application of MNA as an alternative or additional remediation option.

Application of the operating window concept to remediation‐option selection

AbstractAn 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.

Natural Attenuation Rate Clarifications: The True Picture is in the Details

The term “Natural Attenuation” (NA) has been defined as naturally occurring processes in soil and groundwater environments that act without human intervention to reduce the mass, toxicity, mobility, volume, or concentration of contaminants in those media. Monitored natural attenuation (MNA) protocols generally involve the collection of biogeochemical data from groundwater monitoring wells at sites. The data are correlated in time and space with the various chemicals of concern (COC's) to establish predominant biodegradation mechanisms. Modelers using the first-order decay expression typically use the rate coefficient as a calibration parameter and adjust it until the transport model results match field data. With this approach, uncertainties with a number of parameters (e.g., dispersion, sorption, biodegradation, etc.) are lumped together in a single calibration parameter. The problems associated with the lumped parameter approach are illustrated using two commonly used models, BIOSCREEN and Buscheck/Alcantar Analytical Solution, in a variety of practical examples. The natural attenuation decay rate estimated using the lumped parameter approach is distinguished from a biodegradation rate established by isolating processes and examining biodegradation lines of evidence. The half-life determined from empirical data using the lumped parameter approach is often mistakenly interchanged with a biodegradation half-life when it is an all-encompassing half-life based on the interaction of numerous processes. Isolation of the processes, as they are represented in the governing transport equation, and a rationale approach at parameter estimation to avoid the potential pitfalls of the all-inclusive “attenuation rate,” are provided.In closing, it is imperative to implement the following steps to dissern lumped process degradation rates from biodegradation half-lives: (a) be sure the rate/half-life processes are clarified as to what they encompass, (b) establish exactly how the rate/half-life was determined, (c) make certain other processes, such as dispersion, were estimated correctly, and (d) if the half-life is presented as a first-order biodegradation rate, examine the available lines of evidence to substantiate it.

A Site-Specific Approach for the Evaluation of Natural Attenuation at Metals-Impacted Sites

Consideration of monitored natural attenuation (MNA) as a remedy component for metals-contaminated sites can be achieved using a site-specific screening approach, followed by application of one or a series of sequential extraction measurements. Hazardous waste sites contaminated with metals can be screened for the implementation of monitored natural attenuation on the basis of contaminant-specific soil chemical characteristics (i.e., Kd's, solubilities, and nonexchangeable sorbed fraction). Field cases are used to demonstrate the screening approach and to outline the primary considerations involved in accurately applying sequential extraction procedures to support the of MNA for site remediation.The results of these case studies provide strong evidence that site-specific screening and the use of sequential extraction procedures are effective methods for evaluating natural attenuation for metals impacted sites.

NATURAL ATTENUATION OF VOLATILE ORGANIC COMPOUNDS IN GROUNDWATER REMEDIATION SITE

Recently, the assessing of the natural attenuation capacity in the aquifer, which is called the monitored natural attenuation (MNA), is attempted as a remediation technique for groundwater contamination by the volatile organic compounds (VOCs). The natural attenuation includes the processes of biodegradation, dispersion, dilution, sorption, and volatilization of contaminants. Especially the biodegradation process is often recognized as a dominant destructive attenuation mechanism. In this paper, the characteristics of biodegradation in the aquifer and the effect of natural attenuation capacity were evaluated by the field survey and the numerical simulation at the groundwater remediation site. As a result, the biodegradation rates of trichloroethylene (TCE) and dichloroethylenes (DCEs) were calculated and it was cleared that DCEs were degraded faster than TCE.

MERCER COMPLETES MILL FINANCING

Decisions to employ monitored natural attenuation (MNA) as a remediation strategy at contaminated field sites require a comprehensive characterization of the site-specific biodegradation processes. In the present study, compound-specific carbon and hydrogen isotope analysis (CSIA) was used to investigate intrinsic biodegradation of benzene and ethylbenzene in an aquifer with high levels of aromatic and aliphatic hydrocarbon contamination. Hydrochemical data and isotope fractionation analysis of sulfate and methane was used complementarily to elucidate microbial degradation processes over the course of a three year period, consisting of six sampling campaigns, in the industrial area of Weißandt-Gölzau (Saxony-Anhalt, Germany).Enrichment of 13C and 2H isotopes in the residual benzene and ethylbenzene pool downgradient from the pollution sources provided evidence of biodegradation of BTEX compounds at this site, targeting both compounds as the key contaminants of concern. The enrichment of heavy sulfur isotopes accompanied by decreasing sulfate concentrations and the accumulation of isotopically light methane suggested that sulfate-reducing and methanogenic processes are the major contributors to overall biodegradation in this aquifer. Along the contaminant plume, the oxidation of methane with δ13CCH4 values of up to + 17.5‰ was detected. This demonstrates that methane formed in the contaminant source can be transported along groundwater flow paths and be oxidized in areas with higher redox potentials, thereby competing directly with the pollutants for electron acceptors. Hydrochemical and isotope data was summarized in a conceptual model to assess whether MNA can be used as viable remediation strategy in Weißandt-Gölzau. The presented results demonstrate the benefits of combining different isotopic methods and hydrochemical approaches to evaluate the fate of organic pollutants in contaminated aquifers.

A case study for demonstrating the application of U.S. EPA's monitored natural attenuation screening protocol at a hazardous waste site

Natural attenuation assessment data, collected at a Superfund site located in Louisiana, USA, are presented. The study site is contaminated with large quantities of DNAPL waste products. Source characterization data indicated that chlorinated ethene and ethane compounds are the major contaminants of concern. This case study illustrates the steps involved in implementing the U.S. EPA's [U.S. EPA, 1998. Technical protocol for evaluating natural attenuation of chlorinated solvents in ground water, by Wiedmeier, T.H., Swnason, M.A., Moutoux, D.E., Gordon, E.K., Wilson, J.T., Wilson, B.H., Kampbell, D.H., Hass, P.E., Miller, R.N., Hansen, J. E., Chapelle, F.H., Office of Research and Development, EPA/600/R-98/128] monitored natural attenuation (MNA) screening protocol at this chlorinated solvent site. In the first stage of the MNA assessment process, the field data collected from four monitoring wells located in different parts of the plume were used to complete a biodegradation scoring analysis recommended by the protocol. The analysis indicates that the site has the potential for natural attenuation. In the second stage, a detailed conceptual model was developed to identify various contaminant transport pathways and exposure points. The U.S. EPA model and BIOCHLOR was used to assess whether the contaminants are attenuating at a reasonable rate along these transport paths so that MNA can be considered as a feasible remedial option for the site. The site data along with the modeling results indicate that the chlorinated ethene and chlorinated ethane plumes are degrading and will attenuate within 1000 ft down gradient from the source, well before reaching the identified exposure point. Therefore, MNA can be considered as one of the feasible remediation options for the site.

Evaluation of a petroleum hydrocarbon contaminated site for natural attenuation using ‘RBMNA’ methodology

This paper discusses a recently proposed methodology for risk-based monitored natural attenuation (MNA). The methodology is comprised of three interconnected modules. Module 2, the heart of the procedure, focuses on the fate and transport of the contaminant and its natural attenuation (NA). Computer automated tools are recommended to carry out each step of the methodology. The methodology is used in evaluating a site contaminated by the leaking of an underground fuel storage tank. The study observes that NA is effective at the site; however, it poses onsite risk for the initial 15 years. In order to use MNA as a remediation option, appropriate measures must be taken to reduce this initial risk. This paper offers a comparative evaluation of the two most commonly used NA modeling tools—BIOPLUME-III and BIOSCREEN—in studying the present case.

Contributions of radioecology to the development of some modern concepts of ecotoxicology : From ecological half-life to monitored natural attenuation

Many of the modern concepts used in ecotoxicology have historical roots in earlier thinking from the field of radioecology. An example is the concept of Monitored Natural Attenuation (MNA), which is an approach to the remediation of contaminated waste sites, based on the natural degradation, sequestration or aging of toxic substances ; contaminant inventory reduction proceeds in the absence of engineered treatment or removal activities. Because of potentially reduced costs compared to other alternatives for waste site remediation, MNA has rapidly become a popular topic for research and development. Many years ago however, the field of radioecology recognized that biota and other ecosystem components contaminated with radioactive materials naturally decreased their contaminant burdens over time as the result of physical isotope decay and/or as the contaminants were sequestered or otherwise became unavailable for biological uptake. In the field of radioecology, this process is quantified as the ecological half-life (EHL), which is the time required for a population or other ecosystem component to reduce its contaminant burden by 50 % after first coming into a steady-state with contaminant levels in other parts of its environment. Because of the ease with which they can be quantified in living organisms and environmental media, gamma-emitting radionuclides have proved particularly useful for describing and quantifying EHLs in a variety of habitats. We summarize EHL studies based on more than 30 years of data for declines in 137 Cs levels in habitats at a former nuclear weapons production facility in the southeastern United States. These studies of EHL are then related to the development of MNA programs for non-radioactive contaminants in these same or similar habitats. Of particular importance is the finding that EHL's can vary between habitat types for the same contaminant. Even within the same habitat, EHL's can vary spatially across microhabitats as well as between ecosystem components (e.g. in plants vs. animals) or even between species within a single trophic level/taxonomic group.

Natural attenuation of chlorinated solvents at Area 6, Dover Air Force Base: groundwater biogeochemistry

Monitored natural attenuation (MNA) has recently emerged as a viable groundwater remediation technology in the United States. Area 6 at Dover Air Force Base (Dover, DE) was chosen as a test site to examine the potential for MNA of tetrachloroethene (PCE) and trichloroethene (TCE) in groundwater and aquifer sediments. A “lines of evidence” approach was used to document the occurrence of natural attenuation. Chlorinated hydrocarbon and biogeochemical data were used to develop a site-specific conceptual model where both anaerobic and aerobic biological processes are responsible for the destruction of PCE, TCE, and daughter metabolites. An examination of groundwater biogeochemical data showed a region of depleted dissolved oxygen with elevated dissolved methane and hydrogen concentrations. Reductive dechlorination likely dominated in the anaerobic portion of the aquifer where PCE and TCE levels were observed to decrease with a simultaneous increase in cis-1,2-dichloroethene ( cis-DCE), vinyl chloride (VC), ethene, and dissolved chloride. Near the anaerobic/aerobic interface, concentrations of cis-DCE and VC decreased to below detection limits, presumably due to aerobic biotransformation processes. Therefore, the contaminant and daughter product plumes present at the site appear to have been naturally attenuated by a combination of active anaerobic and aerobic biotransformation processes.

A Field Application of Hydrogen-Releasing Compound (HRCTM) for the Enhanced Bioremediation of Methyl Tertiary Butyl Ether (MTBE)

Due to a greater understanding of the behavior of the fuel oxygenate Methyl Tertiary Butyl Ether (MTBE) in groundwater, the United States Environmental Protection Agency (EPA) and the American Petroleum Institute (API) recently have acknowledged the need for the development and application of additional remedial strategies to address the more extensive, longer lived, and faster moving dissolved MTBE plumes often associated with oxygenated fuel releases (API, 2000 and USEPA, 2000a).The need for alternative methods for managing dissolved MTBE plumes is particularly evident in the case of the Upper Glacial aquifer of Long Island, New York. Hydrogeologic conditions in the this water table aquifer (i. e., high hydraulic conductivity, high average pore velocities, low organic carbon, and high rates of recharge) have been found to contribute to the formation of extensive, long, narrow, and three-dimensional dissolved MTBE plumes that plunge into the aquifer in response to recharge (Weaver et. al. 1999). The characteristics of MTBE plumes in the Upper Glacial aquifer in combination with abundant sensitive receptors (mainly drinking water supply wells), often renders monitored natural attenuation (MNA) plume management strategies inappropriate, resulting in the need for plume control, frequently via pumping and treating (NYSDEC, 2000). In such cases, remedial costs can rise well beyond those associated with similar fuel releases that did not contain MTBE (USEPA, 1998a). Consequently, the application of remedial technologies for MTBE other than MNA, or pumping and treating, are of great interest to those responsible for the management of dissolved MTBE plumes on Long Island or in similar hydrogeologic settings.An alternative strategy for the remediation of dissolved MTBE plumes was recently field tested at an oxygenated fuel spill site on Long Island. The strategy was enhanced biodegradation via the application of Hydrogen Release Compound (HRCTM). HRCTM is a form of polylactate ester that slowly releases biodegradation stimulating constituents into the aquifer and has been shown in other studies to foster methanogenic conditions that advance the reductive dechlorina-tion of perchloroethene (PCE) and trichloroethene (TCE) (Koenigsberg, 1998). Numerous reports have been written that discuss the biodegradation of MTBE under aerobic conditions, as well as microcosm studies in which MTBE biodegradation was observed under anaerobic conditions. However, there are limited reports that document the natural anaerobic biodegradation of dissolved MTBE (McLoughlin, 2000). Despite the lack of documented natural anaerobic biodegradation of MTBE, it has been observed that MTBE transport often occurs under anoxic conditions at oxygenated fuel releases as the result of the biodegradation of other fuel constituents, such as benzene, toluene, ethylbenzene and xylene (BTEX), which deplete the available dissolved oxygen as well as other electron acceptors (nitrate, ferric iron, manganese, etc.) (USEPA, 2000c and API, 1996). Therefore, an anaerobic biodegradation strategy is attractive due to its synergy with the existing geochemical conditions. Consequently, the study was conceived and designed to test the ability of HRC(tm) to foster the anaerobic bio-degradation of MTBE under methano-genic conditions (McLoughlin, 2000).The application of HRC(tm) did result in the formation of a large area of enhanced reducing conditions in the vicinity and down gradient of the application zone. However, under these site conditions, the HRC(tm) application did not induce measurable methanogenic conditions with the associated elevated dissolved hydrogen concentrations required for significant MTBE anaerobic biodegradation. The high hydraulic conductivity and high average pore velocity at the site were likely responsible. Despite this, the study can be viewed as a success since much was learned that can be used in future studies of anaerobic biodegradation of MTBE and the application of HRC(tm).

Petroleum Hydrocarbon Monitored Natural Attenuation: Essential Framework for Remedial Managers

Abstract Monitored natural attenuation (MNA) is an attractive remedy for petroleum-contaminated sites. However, to use MNA as a partial or total site remedy, the remedial manager must first present unequivocal evidence that natural attenuation will occur at the site and will meet public and regulatory concerns. A considerable body of scientific evidence of natural attenuation exists, as does a small library of American Society for Testing and Materials and U.S. Environmental Protection Agency documentation on use of MNA. This article clearly and succinctly presents the background of MNA, guides the new remedial manager through the process, discusses the pros and cons of MNA alone and in combination with other low-cost remedies (primarily source removal), and outlines regulatory and public concerns and how to meet them.

An approach to estimating hidden cost contingencies associated with monitored natural attenuation

Estimating the costs of Monitored Natural Attenuation (MNA)1 can be difficult because of the long time periods that may be required for remediation and the possibility that drastic changes in remedial action may be required at unknown times far into the future if the MNA does not prove effective. In this paper, we describe MNA, its advantages, the status of MNA application at cleanup sites and reasons for its increased application, and the need for better MNA cost estimates. A cost‐estimating methodology based on a Monte‐Carlo‐type simulation using decision trees representing the possible future alternatives is also presented. This methodology yields more meaningful estimates than conventional methodologies, because it can account for large uncertainties anda wide range of potential outcomes that are often overlooked and inherent in MNA.

The bubble stripping method for determining dissolved hydrogen (H2) in well water

The bubble stripping method was developed for use at field sites to measure the concentration of dissolved hydrogen (H2) in ground water. This information is useful in assessing the viability of employing monitored natural attenuation (MNA) as a strategy to influence the restoration of sites contaminated with chlorinated solvents. In laboratory studies, a reservoir containing water was employed to simulate a well. The system was constructed so that the concentration of dissolved H2 could be maintained at a constant level. The method was applied by pumping water from the reservoir into a sample cell, and then injecting 20 ml of nitrogen into the cell to produce a headspace (the “bubble”). Stripping was accomplished by pumping water through the cell, which produced agitation between the aqueous phase and the headspace. Pumping was continued for a length of time sufficient for dissolved H2 to partition between the two phases. Analysis of H2 in the headspace by gas chromatography enabled the concentration of dissolved H2 in solution to be calculated with the use of Henry's law. Two sample cell designs were compared in this study, the Microseeps Cell and the Chapelle Cell. Kinetics of equilibration studies were conducted with each cell, employing solution flow rates of 200, 300, and 400 ml/min, at 4 and 21 °C. The Microseeps Cell compared favorably with the Chapelle Cell with regards to kinetics of equilibration, with the added benefit of costing significantly less. © 2000 John Wiley & Sons, Inc. Field Analyt Chem Technol 4: 283–296, 2000