Significance and Use
4.1 Guidance on management of NAPL sites and a large body of research effort contributing to their development (for example, ITRC 2018 (1); CRC CARE 2018 (2); CL:AIRE 2019 (3) and CRC CARE 2020 (4)) point to the significance of natural attenuation and NSZD in the evolution of NAPL source and the resulting distributions of COCs in soil, groundwater and vapor.
4.2 Examples of reported ranges in estimated natural attenuation rates are 300 – 7700 gallons of NAPL/acre/year (Garg et al. 2017 (5)); and 0.4 – 280 metric tons of NAPL/year (CRC CARE 2020 (4)).
4.3 The intent of this guide is to provide a standardized approach for the estimation of natural attenuation rates for NAPL in the subsurface. The rates can be used for establishing a baseline metric for those involved in the remedial decision-making process. There is a need for a systematic approach and refinement in data collection and interpretation for quantifying the spatially and temporally variable rates. Providing quality assurance in estimation of this metric will enable the assessment of relatively more engineered remedies as compared to natural remedies or MNA (Fig. 1), as well as estimation of the remediation timeframe. This comparison, when performed through a standardized approach, can lead to actionable metrics for transition to sustainable remedies through well-defined and transparent criteria. In the context of a spectrum of remediation options in terms of engineered and natural remedies (Fig. 1), the transition is from a relatively more engineered (or active remediation) to a relatively more nature-based remedy. When considered in the remedial decision-making process, estimates of natural attenuation rates can be used:
4.3.1 Before active remediation (as baseline to assess whether active remediation is needed);
4.3.2 During active remediation (as performance/optimization metric); and
4.3.3 At the end of active remediation (support transition to MNA or site closure).
4.4 Since natural attenuation results in changes to the NAPL composition over time, methods to estimate the natural attenuation rate also inform NAPL forensics, and the risks associated with the NAPL such as in vapor intrusion, NAPL migration, and groundwater plume extent and stability.
4.5 In addition, understanding of the magnitude of natural attenuation rates can contribute to addressing overarching questions in NAPL sites management, following initial characterization and risk assessment, such as:
4.5.1 What is the remediation timeframe under natural attenuation and how does it compare with the remedial timeframe of engineered remedies?
4.5.2 What are the current and future estimates of NAPL mass (or volume) remaining on site? The remaining mass can impact compositional concerns.
4.5.3 Under what scenarios (for example, size of release and/or presence of NAPL); and site conditions are the rates of NAPL natural attenuation significant in terms of reaching remedial objectives in accordance with regulatory criteria and remedial timeframe?
4.5.4 How do the rate estimates of natural attenuation change over time?
4.6 Common challenges encountered in the management of NAPL sites are:
4.6.1 Sites that remain under engineered (active) remediation over extended periods of time without reaching an acceptable endpoint.
4.6.2 Understanding what the long-term fate of NAPL bodies would be with and without engineered remedies.
4.6.3 Understanding the long-term fate of NAPL-related dissolved organic carbon (DOC) plumes.
4.6.4 Understanding NAPL movement and demonstrating stability.
4.7 A major obstacle in answering the questions in 4.5 and addressing the challenges in 4.6 is the availability of methods for estimation of reliable and quantifiable NAPL attenuation rates that can be implemented and reviewed by site managers, site owners and regulators. To address this challenge, the intent of this standard is to describe the available methods and their selection and application based on site conditions.
4.8 It is important to understand the applicability and use of the NAPL natural attenuation rates in decision making with regards to the requirement for an endpoint of an engineered remediation system. A merited transition from engineered to natural remedy, including MNA would result in a more sustainable approach to site management. MNA in the context of this standard includes the monitoring of natural attenuation rates in both the saturated zone and the vadose zone and complements previous standards (Guide E1943) focused on MNA in the saturated zone by inclusion of methods related to the vadose zone (Section 6).
4.9 The natural attenuation processes (Section 5) can impact remedial objectives in terms of addressing NAPL saturation (mobility or migration) or composition (COC concentrations in soil, groundwater or vapor), and therefore need to be included in the CSM. Natural attenuation, including NSZD, can reduce both NAPL saturation and constituent-specific mass.
4.10 Integration of natural attenuation rate estimate at the early stages of site management (that is, in the CSM) can result in its proper application to the remedial decision-making process, since natural attenuation can result in exposure risk reduction, as well as overall source mass reduction.
4.10.1 In most cases, identifying the occurrence of natural attenuation at a site or measuring the rate at a site is not sufficient in itself to accomplish remedial goals and regulatory requirements.
4.10.2 This guide provides methods for identifying the occurrence of natural attenuation, measuring the rate of natural attenuation and demonstrating how this data can be used for achieving remedial goals and regulatory requirements.
4.11 The advantages of estimating natural attenuation rates at sites impacted by hydrocarbon-based NAPL including petroleum, coal tars, or creosote is evidenced by examples where one or multiple methods for the rate estimates have been applied.
4.12 US EPA and State regulations or guidance that highlight the significance of natural attenuation at NAPL sites include:
4.12.1 Role of natural attenuation and specifically biodegradation in the vadose zone is demonstrated through analysis of data sets to substantiate the applicability of screening distances for petroleum vapor intrusion (US EPA, 2015, ITRC, 2014 (6)).
4.12.2 Adoption of MNA as a means to ensure long-term containment and reduction of dissolved phase plumes (Guide E1943, WI-DNR 2014 (7), ITRC 2018 (1)).
4.12.3 Additional technical aspects of NSZD pertaining to forensic evidence and weathering patterns have previously been employed by environmental professionals, regulatory agencies and legal courts on site specific projects.
4.13 Comparison of the natural attenuation rates to the removal rates achieved through engineered remedies over time, if applicable, and defining a threshold for transition from more engineered to more natural remedies has the potential to improve remedial decisions as demonstrated through case studies presented in this standard guide. This includes termination of a relatively engineered remedy and reliance on MNA.
Scope
1.1 This is a guide for determining the appropriate method or combination of methods for the estimation of natural attenuation or depletion rates at sites with non-aqueous phase liquid (NAPL) contamination in the subsurface. This guide builds on a number of existing guidance documents worldwide and incorporates the advances in methods for estimating the natural attenuation rates.
1.2 The guide is focused on hydrocarbon chemicals of concern (COCs) that include petroleum hydrocarbons derived from crude oil (for example, motor fuels, jet oils, lubricants, petroleum solvents, and used oils) and other hydrocarbon NAPLs (for example, creosote and coal tars). While much of what is discussed may be relevant to other organic chemicals, the applicability of the standard to other NAPLs, like chlorinated solvents or polychlorinated biphenyls (PCBs), is not included in this guide.
1.3 This guide is intended to evaluate the role of NAPL natural attenuation towards reaching the remedial objectives and/or performance goals at a specific site; and the selection of an appropriate remedy, including remediation through monitoring of natural or enhanced attenuation, or the remedy transition to natural mechanisms. While the evaluation can support some aspects of site characterization, the development of the conceptual site model and risk assessment, it is not intended to replace risk assessment and mitigation, such as addressing potential impact to human health or environment, or need for source control.
1.4 Estimation of NAPL natural attenuation rates in the subsurface relies on indirect measurements of environmental indicators and their variation in time and space. Available methods described in this standard are based on evaluation of biogeochemical reactions and physical transport processes combined with data analysis to infer and quantify the natural attenuation rates for NAPL present in the vadose and/or saturated zones.
1.5 The rate estimates can be used for developing metrics in the corrective action decision framework, complementing the LNAPL Conceptual Site Model (LCSM) (Guide E2531).
1.6 The emphasis in this guide is on the selection and application of methods for quantifying rates of NAPL depletion or attenuation. It is assumed that the remediation endpoint has been defined for the site based on the remedial objectives to address composition or saturation concerns as defined in ITRC (2018) (1).2 While the rates can be used to estimate the timeframe to reach the remediation endpoint under natural conditions, methods for estimating the total NAPL mass and timeframe are beyond the scope of this standard.
1.7 The users of this guide should be aware of the appropriate regulatory requirements that apply to sites where NAPL is present or suspected to occur. The user should consult applicable regulatory agency requirements to identify appropriate technical decision criteria and seek regulatory approvals, as necessary.
1.8 ASTM standard guides are not regulations; they are consensus standard guides that may be followed voluntarily to support applicable regulatory requirements. This guide may be used in conjunction with other ASTM guides developed for sites with NAPL in the subsurface. The guide supplements characterization and remedial efforts performed under international, federal, state, and local environmental programs, but it does not replace regulatory agency requirements.
1.9 SI units are primarily used in the standard, however, units more commonly used in the industry are also represented.
1.10 The guide is organized as follows:
1.10.1 Section 2 lists referenced documents.
1.10.2 Section 3 defines terminology used in this guide.
1.10.3 Section 4 describes the significance and use of this guide.
1.10.4 Section 5 provides the conceptual model of natural attenuation processes and pathways.
1.10.5 Section 6 provides an overview and description of methods for the estimation of natural attenuation rates, including:
1.10.5.1 Description of methods and available technologies:
(1) CO2 efflux method
(2) Temperature gradient method
(3) Soil gas gradient method
(4) Groundwater monitoring method
(5) NAPL composition method
1.10.5.2 Screening or feasibility assessment of the method for the site conditions;
1.10.5.3 Background sources and correction methods;
1.10.5.4 Data interpretation, key considerations and challenges (for example, measurement frequency and locations and spatial/temporal averaging);
1.10.5.5 Applicability of methods for evaluating the performance of enhanced natural attenuation (bioremediation) systems;
1.10.5.6 Other method applications (for example, source delineation or estimating mass discharge rates).
1.10.6 Section 7 provides guidance on selection of a method or combination of methods applicable to site-specific conditions.
1.10.7 Section 8 provides example applications through case studies.
1.10.8 Section 9 lists keywords relevant to this guide.
1.10.9 Appendix X1 describes details of the CO2 Efflux Method.
1.10.10 Appendix X2 describes details of the Temperature Gradient Method.
1.10.11 Appendix X3 describes details of the Soil Gas Gradient Method.
1.10.12 Appendix X4 describes details of the Groundwater Monitoring Method.
1.10.13 Appendix X5 describes details of the NAPL Composition Method.
1.10.14 Appendix X6 provides details of case studies discussed in Section 8.
1.10.15 Appendix X7 provides example estimates of NAPL quantity.
1.11 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.12 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
