Natural and Enhanced Remediation Systems - Chapter 3 pot

Suthersan, Suthan S. “Monitored Natural Attenuation” Natural and Enhanced Remediation Systems Edited by Suthan S. Suthersan Boca Raton: CRC Press LLC, 2001 ©2001 CRC Press LLC CHAPTER 3 Monitored Natural Attenuation CONTENTS 3.1 Introduction 3.1.1 Definitions of Natural Attenuation 3.2 Approaches for Evaluating Natural Attenuation 3.3 Patterns vs. Protocols 3.3.1 Protocols for Natural Attenuation 3.3.2 Patterns of Natural Attenuation 3.3.2.1 Various Patterns of Natural Attenuation 3.4 Processes Affecting Natural Attenuation of Compounds 3.4.1 Movement of Contaminants in the Subsurface 3.4.1.1 Dilution (Recharge) 3.4.1.2 Advection 3.4.1.3 Dispersion 3.4.2 Phase Transfers 3.4.2.1 Sorption 3.4.2.2 Stabilization 3.4.2.3 Volatilization 3.4.3 Transformation Mechanisms 3.4.3.1 Biodegradation 3.5 Monitoring and Sampling of Natural Attenuation 3.5.1 Dissolved Oxygen (DO) 3.5.2 Oxidation–Reduction (REDOX) Potential (ORP) 3.5.3 pH 3.5.4 Filtered vs. Unfiltered Samples for Metals 3.5.4.1 Field Filtration and the Nature of Groundwater Particulates 3.5.4.2 Reasons for Field Filtration 3.5.5 Low-Flow Sampling as a Paradigm for Filtration 3.5.6 A Comparison Study References ©2001 CRC Press LLC …natural attenuation (NA) is not a “no action (NA)” alternative. Monitored natural Attenuation (MNA) defines the required monitoring parameters to dem- onstrate that the ongoing natural processes will continue to meet the remediation objectives… 3.1 INTRODUCTION The term monitored natural attenuation (MNA) refers to an approach to clean up subsurface contamination, specifically in groundwater, by relying on natural processes and monitoring. MNA is also referred to as natural degradation and intrinsic or passive remediation . Natural attenuation processes include a variety of physical, chemical, or biological processes that, under favorable conditions, act without human intervention to reduce the mass, toxicity, mobility, volume, and concentration of contaminants in groundwater. Depending on the geologic conditions, types of contaminants, and con- taminant mass and distribution at a given contaminated site, MNA could emerge as the preferred choice of remediation approach. Natural attenuation relies on the assim- ilative capacity of the ecosystem for the reduction of contaminant concentration and mass. This approach has been utilized by environmental engineers for a long time to control industrial and municipal wastewater discharges into surface waterbodies and maintain acceptable water quality standards. 3.1.1DeÞnitions of Natural Attenuation A variety of organizations have espoused the following definitions of natural attenuation due to the emerging popularity and preference of MNA as the remedi- ation method of choice at many contaminated sites across the country. 1 Environmental Protection Agency 2 : This policy directive defines monitored natural attenuation as the reliance on natural attenuation process (within the context of a carefully controlled and monitored site cleanup approach) to achieve site-specific remediation objectives within a time frame that is reasonable com- pared to that offered by other more active methods. The “natural attenuation processes” that are at work in such a remediation approach include a variety of physical, chemical, or biological processes that, under favorable conditions, act without human intervention to reduce the mass, toxicity, mobility, volume, or concentration of contaminants in soil or groundwater. These in situ processes include biodegradation; dispersion; dilution; sorption; volatilization; radioactive decay; and chemical or biological stabilization, transformation, or destruction of contaminants. American Society for Testing and Materials (ASTM) 3 : Its document titled Standard Guide for Remediation of Groundwater by Natural Attenuation at Petroleum Release Sites defines natural attenuation as the “reduction in mass or concentration of a compound in groundwater over time or distance from the source of constituents of concern due to naturally occurring physical, chemical, and biological processes, such as biodegradation, dispersion, dilution, adsorption, and volatilization.” ©2001 CRC Press LLC Air Force 4 : The first document, published in 1995, defines the process as resulting “from the integration of several subsurface attenuation mechanisms that are classified as either destructive or nondestructive. Biodegradation is the most important destructive attenuation mechanism. Nondestructive attenuation mech- anisms include sorption, dispersion, dilution from recharge, and volatilization.” Army 5 : Its report defines natural attenuation as “the process by which contam- ination in groundwater, soils, and surface water is reduced over time…[via] natural processes such as advection, dispersion, diffusion, volatilization, abiotic and biotic transformation, sorption/desorption, ion exchange, complexation, and plant and animal uptake.” In the past, the first question to be asked in consideration of the potential for natural attenuation at a contaminated site was whether biodegradation of the chem- ical contaminant had been reported. Oftentimes the question was, “Does the bio- geochemistry exist for ongoing degradation?” due to the assumption that the respon- sible microorganisms are ubiquitous in the subsurface. However, in this chapter the term “natural attenuation” will include all the processes that contribute towards the decrease in contaminant concentrations. 3.2 APPROACHES FOR EVALUATING NATURAL ATTENUATION Documenting that contaminant concentration has become very low or detectable in groundwater samples is an important piece of evidence that natural attenuation is working. However, such documentation is not completely sufficient to show that natural attenuation is protecting human health and the environment, for three primary reasons: • Monitoring of contaminant concentration reductions is not always precise due to the complex nature of groundwater systems. In some cases the total contaminant mass may have decreased, but the contaminant may have transformed to another, more hazardous chemical form. • In a few instances reactions that initially cause contaminants to attenuate may not be sustainable until reasonable cleanup goals are achieved. • Another situation of concern occurs when natural biogeochemical parameters, such as electron acceptors and electron donors that support attenuation, are used up before the treatment of contamination is complete. For these reasons, environmental regulators and others should not rely on simple rules of thumb (such as maximum contaminant concentration data or trends in these data over a relatively short time) in evaluating the potential success of natural attenuation. The decision to rely on natural attenuation and the confirmation that it will continue to work depend on linking monitoring data to a site conceptual model and “footprints” of the underlying mechanisms. Footprints are mappings of concentration changes in reactants (contaminant(s), electron acceptors, and donors) or products of the biogeochemical processes (such as Cl – ion, dissolved Fe 2+ ) that degrade or ©2001 CRC Press LLC immobilize the contaminants (Figures 3.1a, b, and c). Footprints can be measured to document that these transformation or immobilization processes are active at the site. An observation of the loss of a contaminant, coupled to observation of a few footprints, helps to establish which processes are responsible for the decrease in contaminant mass and concentrations. The three basic steps to document natural attenuation are as follows: 1. Develop a conceptual model of the site : The model should show where and how fast the groundwater flows, where the contaminants are located and at what concentrations, and which types of natural processes could theoretically affect the contaminants (Figures 3.2a and b). 2. Analyze site measurements : Samples of groundwater should be analyzed chemi- cally to look for footprints of the natural attenuation processes and to determine whether these processes are sufficient to control the contamination. 3. Monitor the site : The site should be monitored until regulatory requirements are achieved to ensure that documented attenuation processes continue to occur. Although the basic steps are the same for all sites, the level of effort needed to carry out these steps varies substantially with the complexity of the site. When site characteristics or the controlling mechanisms are uncertain, it will be difficult to develop the site conceptual model; thus, a large amount of data will be required to document natural attenuation. In these complex situations, computer modeling may be necessary, and data on footprints and site characteristics will have to be more than adequate to develop the model. Figures 3.1a Initial vinyl chloride plume at a landÞll site in Maryland with radial groundwater ßow from the center of the landÞll. ©2001 CRC Press LLC Figures 3.1b Natural attenuation effects on the vinyl chloride plume. Note: The signiÞcant reduction in vinyl chloride concentration and mass due to natural attenuation. Figures 3.1c Effects of the primary electron acceptor dissolved oxygen on the attenuation of VC and Mn along a North-South transect through the middle of the landÞll. Three-dimensional perspective plot of observed vinyl choride concentrations in groundwater -1996 Landfill boundary 500 200 150 100 20 5 1 0 1200 1000 800 600 400 200 0 Dissolved Oxygen, Redox, and Vinyl Chloride Distribution Landfill Vinyl Chloride Manganese Fe Redox Dissolved Oxygen 2+ Saprolite Bedrock Sand/Gravel Figures 3.2a A general site conceptual exposure model (adapted from ASTM, 1997). Current Domestic Water Supply Well Future Domestic Water Supply Well Confining Layers? Confining Layers? Confining Layers? Dissolved Plume Utilities Utilities Residual NAPL Abandoned Well? Current Municipal Water Supply Well Shallow Water Table ©2001 CRC Press LLC Figures 3.2b Site conceptual exposure models. Primary Sources Secondary Sources Transport Mechanisms Exposure Routes Receptors Chemical Storage Piping / Distribution Operations Waste Management Unit Soil or Waste Piles Lagoons or Ponds Other Residential Commercial/Industrial Construction Worker Relevant Ecological Recept o Residential Commercial/Industrial Construction Worker Relevant Ecological Recept o Residential Commercial/Industrial Affected Subsurface Soils (>3 ft depth) Affected Surface Soils (<3 ft depth) Dissolved Groundwater Plume Non-Aqueous Volatilization and Atmospheric Dispersion AIR Inhalation of Vapor or Particulates GROUNDWATER Potable Water Use SOIL Dermal Contact or Ingestion Wind Erosion and Atmospheric Dispersion Dissolved Groundwater Plume Leaching and Groundwater Transport Mobile ©2001 CRC Press LLC ©2001 CRC Press LLC 3.3 PATTERNS VS. PROTOCOLS 3.3.1Protocols for Natural Attenuation Within the past few years, many organizations have issued documents providing guidance on evaluating natural attenuation. 1 Among the 14 documents developed by a range of organizations from federal and state agencies to private companies and industry associations, the available technical protocols address two classes of organic contaminants only: fuel hydrocarbons and chlorinated solvents (with the exception of the Department of Energy (DOE) document). A large body of empirical evidence and scientific and engineering studies in recent years has been developed to support understanding of natural attenuation of these contaminants — mostly fuel hydro- carbons under certain conditions. However, the natural attenuation of polycyclic aromatic hydrocarbons, polychlorinated biphenyls, explosives, and other classes of persistent organic contaminants is not addressed in any protocol. 1 Furthermore, although the DOE document proposes a method for assessing natural attenuation processes for inorganic contaminants such as metals, such processes are extremely complex, and this document does not adequately reflect this complexity. 6 A recent effort was made to compare the guidelines currently available on natural attenuation against a list of characteristics of a comprehensive protocol. 1 The con- sensus was that a comprehensive protocol should cover three broad areas: • Community concerns : The protocol should describe a plan for involving the affected community in decision making, maintaining institutional controls to restrict use of the site until cleanup goals are achieved, and implementing contin- gency measures if natural attenuation fails to continue as expected. • Scientific and technical issues : The protocol should describe how to document which natural attenuation processes are responsible for observed decreases in contaminant concentrations, how to assess the site for contaminant source and hydrogeologic characteristics that affect natural attenuation, and how to assess the sustainability of natural attenuation over the long term. • Implementation issues : The protocol should be easy to follow and should describe the monitoring frequency and various monitoring procedures, in addition to the training and expertise required for the personnel carrying out the field implemen- tation. None of the current documents fulfills all the criteria defined above. 1 To some extent, this reflects the various, and sometimes limited, purposes for which these documents were prepared. Some are detailed technical guides; others are intended to help ensure consistency in site evaluation within a particular organization (such as a private corporation or a branch of the military), and others are intended to guide policy. Nonetheless, key gaps in the existing body of protocols have to be addressed. The existing protocols provide little or no discussion of when and how to involve the public in site decisions and when and how to implement institutional controls. In the few instances where these matters are mentioned, the discussion is typically brief, almost in passing. Although most environmental regulatory agencies have separate policies that specify procedures for community involvement and ©2001 CRC Press LLC institutional controls, these procedures may be inadequate in cases where natural attenuation is selected as the remedy. Discussion of when and how to implement contingency plans in case natural attenuation does not work is also inadequate in many of the protocols. Further, the protocols do not provide sufficient guidance on when and how engineered methods to remove or contain sources of contamination benefit natural attenuation. A major shortcoming of some of the protocols relates to scoring systems used for initial screening to determine whether a site has potential for treatment by natural attenuation. Such scoring systems yield a numeric value for the site in question. If this value is above a certain level, the site is judged an eligible candidate for natural attenuation. Frequently, such scores are used inappropriately as the key factor in deciding whether natural attenuation can be a successful remedy at the site. Moreover, these scores often lead to erroneous conclusions about whether natural attenuation will or will not succeed, due to the complexity of the processes involved and the tendency of scoring systems to oversimplify them. In addition, the scoring systems developed for evaluating natural attenuation at petroleum sites are erroneously used to evaluate sites with chlorinated solvents by many practitioners of remediation. In summary, the existing body of natural attenuation protocols is limited in several important areas. 1 Where and how existing protocols can be used to meet regulatory requirements for documenting site cleanup — and whether such protocols are required at all — is also unclear. Guidance on the use of natural attenuation for remediation has to be developed to cover topics not addressed in existing protocols and to provide for the use of protocols in regulatory programs. 3.3.2 Patterns of Natural Attenuation Instead of relying on protocols and scoring systems, an educated screening tool should be to observe the patterns in reduction of contaminant concentrations. Nat- urally attenuating contaminant plumes can take a variety of forms: they might be expanding, stable, or shrinking, depending on the trends in the spatial variations of contaminant concentrations with time (Figures 3.3a, b, and c). Common patterns in all attenuating plumes are a decline in the dissolved contaminant mass with time, and a decline in contaminant concentrations downgradient from the source. Once these patterns are observed initially, the following list of questions should be devel- oped to collect additional data to develop a platform demonstrating that MNA is an ongoing and continuing process to meet the site cleanup objectives: •What chemical, physical, and biological processes are in effect to support natural degradation of the site-specific contaminants? •What site biogeochemical conditions are needed for these chemical, physical, and biological processes to work? Which types of site conditions are optimal? Which conditions inhibit natural attenuation? •What level of information is needed to characterize the site fully? •What breakdown products that may be more toxic, persistent, or mobile are created when the contaminants degrade? How does one prove that contaminants are degrading into harmless substances? [...]... grade Argon, an inert gas, into the subsurface to replace CO2 and CH4, followed by monitoring the production of CO2 and CH4. 23 Figures 3. 12a and b show the formula of BTEX compounds H C HC CH HC CH OR OR C H Figures 3. 12a Benzene formula and simpliÞed representations CH 3 CH 3 CH 3 CH 3 Benzene Toluene m-Xylene Figures 3. 12b Structures of single-ring aromatic hydrocarbons ©2001 CRC Press LLC Ethylbenzene... Sectional View MW-2 MW-1 MW -3 t0 t1 t2 Plan View "Contaminant plume is continuing to grow and move downgradient from the source area" Figures 3. 3a Expanding plume • What kinds of specific monitoring and testing are needed to determine that the site and the contaminants are suitable for natural attenuation? Is extensive monitoring necessary? • How long is it reasonable to monitor to ensure that natural attenuation... 3. 11) In these zones, BTEX degradation processes are slower and less reliable than when oxygen is present Source Area Methanogenic 5 SO4 24 Reduction Fe3+/Mn4+ NO3 Reduction Reduction 3 2 Aerobic Zone O2 1 Groundwater Flow Direction 1 - Aerobic Zone 2 and 3 - Transient Anaerobic Zones 4 and 5 Figure 3. 11 Encroachment of the Aerobic Fringe - Core Anaerobic Zones Conceptualization of the dominant terminal... completely shut off natural attenuation of the chlorinated solvent Natural Attenuation Capacity (NAC): The manner in which natural attenuation and active remediation measures (such as source removal, pump and treat, chemical oxidation, or enhanced bioremediation) are combined depends on the natural attenuation capacity (NAC) of the system If the NAC is small, for example, active remediation measures... Sectional View t2 MW -3 MW-2 t0 t1 MW-1 Plan View Contaminant plume is almost stationary over time and concentrations at points within the plume are relatively constant over time with a slight declining trend Figures 3. 3b Stable groundwater plume and complete or partial removal or containment may be possible However, other common types of sources often are extremely difficult to locate and remove or contain... are reductions and oxidations, together known as REDOX reactions The reactions involve transfer of electrons from one molecule to another, which allow the microorganisms to generate energy and grow (Figure 3. 8) More discussions on REDOX reactions and microbial electron transfers are provided in Chapters 2 and 4 trons Elec Organic Contaminant and New Cells C + Energy Elec trons Figure 3. 8 n arbo Electron... of 1/time) [T–1] ©2001 CRC Press LLC (3. 16) Table 3. 2 Half-Cell Reactions for Some of the Common Electron Acceptors and Donors (adapted from Wiedemeier et al., 1999) Half-Cell Reaction DGro (kcal/mol e–) 4e– + 4H+ O2 Þ 2H2O Aerobic respiration –18.5 – 5e– + 6H+ + NO3 Þ 0.5N2 + 3H2O DenitriÞcation –16.9 2e– + 4H+ + MnO2 Þ Mn2+ + 2H2O Manganese reduction –8.6 e– + Fe3+ Þ Fe2+ Fe(III) reduction –17.8 2–... 0.5HS– + 0.5H2S + 4H2O Sulfate reduction 5 .3 8e– + 8H+ + CO2 Þ CH4 + 2H2O Methanogenesis 5.9 C2Cl4 + H+ + 2e– Þ C2HCl3 + Cl– PCE reductive dechlorination –9.9 C2HCl3 + H+ + 2e– Þ C2H2Cl2 + Cl– TCE reductive dechlorination –9.6 C2H2Cl2 + H+ + 2e– Þ C2H3Cl + Cl– cis-DCE reductive dechlorination –7.2 C2H3Cl + H+ + 2e– Þ C2H4 + Cl– VC reductive dechlorination –8.8 C2H3Cl3 + H+ + 2e– Þ C2H4Cl2 + Cl– TCA reductive... Carbohydrate oxidation –10 .3 –9.9 –10.0 12H2O + C6H6 Þ 6CO2 + 3O H+ + 3Oe– Benzene oxidation –7.0 14 H2O + C6H5CH3 Þ 7CO2 + 36 H+ + 36 e– Toluene oxidation –6.9 20H2O + C10H8 Þ 10CO2 + 48H+ + 48e– Naphthalene oxidation –6.9 4H2O + C2H3Cl Þ 2CO2 + 11H+ + 10e– + Cl– Vinyl chloride oxidation –11.4 12H2O + C6H5Cl Þ 6CO2 + 29H+ + 28e– + Cl– Chlorobenzene oxidation –8.0 ©2001 CRC Press LLC First-order rate constants... properties than water As shown in Figure 3. 4a, b, and c, LNAPLs can accumulate near the water table, DNAPLs can penetrate the water table and form pools along geologic layers, and both types of NAPLs can become entrapped in soil pores These NAPL accumulations contaminate the ©2001 CRC Press LLC Contaminated Zone Monitoring Well Cross Sectional View MW-2 MW-1 MW -3 t2 t1 Plan View t0 Contaminant plume . (Recharge) 3. 4.1.2 Advection 3. 4.1 .3 Dispersion 3. 4.2 Phase Transfers 3. 4.2.1 Sorption 3. 4.2.2 Stabilization 3. 4.2 .3 Volatilization 3. 4 .3 Transformation Mechanisms 3. 4 .3. 1 Biodegradation 3. 5 Monitoring. CONTENTS 3. 1 Introduction 3. 1.1 Definitions of Natural Attenuation 3. 2 Approaches for Evaluating Natural Attenuation 3. 3 Patterns vs. Protocols 3. 3.1 Protocols for Natural Attenuation 3. 3.2 Patterns. “Monitored Natural Attenuation” Natural and Enhanced Remediation Systems Edited by Suthan S. Suthersan Boca Raton: CRC Press LLC, 2001 ©2001 CRC Press LLC CHAPTER 3 Monitored Natural Attenuation