Significance and Use
4.1 Hydrophobic organic liquids (for example, petroleum hydrocarbons, coal tars) may exist in the environment for long periods of time as NAPLs. Standardized guidance and test methods do not exist to assess NAPL movement (both pore-scale mobility and NAPL body-scale migration) in sediment. Literature searches have resulted in a limited body of available and applicable research. Current research has focused on site-specific sediment NAPL movement evaluation approaches.
4.2 Standardized guidance and test methods currently exist for assessing NAPL mobility and migration at upland sites, from organizations such as ASTM International (Guides E2531 and E2856), Interstate Technology and Regulatory Council (2), and the American Petroleum Institute (3, 4). Approaches commonly used in upland sites may or may not be applicable for any given sediment site. This guide provides perspectives on the applicability of various methodologies for specific sediment conditions.
4.3 This guide describes various methodologies that are useful in sediment NAPL movement evaluation, such as laboratory test methods, calculation approaches, and field observation interpretation. The guide then provides frameworks to evaluate the data generated from these methodologies to determine if the NAPL observed in the sediments under in situ conditions exhibits movement of any kind.
4.4 Important exposure pathways in upland sites are usually not applicable to sediment sites. The U.S. Environmental Protection Agency notes, “Contaminants in the biologically active layer of the surface sediment at a site often drive exposure” (5). In aquatic environments, benthic organisms live in the surface sediment to maintain access to oxygenated overlying water. These benthic organisms are at the base of the food chain. If NAPL in subsurface sediment is not migrating, the NAPL will not move into the surface sediment and result in exposure to benthic organisms. NAPL that is stable and only present in subsurface sediment likely does not pose a risk to human or ecological receptors, because there is no completed pathway to exposure if the overlying sediment remains in place (that is, it is not dredged or eroded). With no completed exposure pathway, removal of the NAPL in the subsurface sediment may not be needed during any remedy. Therefore, understanding the potential for movement of NAPL in sediments is a key factor in the management of contaminated sediment sites. Knowledge of NAPL movement is required for developing effective remedial options for NAPL impacted sediments and for long-term management of sediment sites.
4.5 The user of this guide should review the overall structure and components of this guide before proceeding with use, including:
| Section 1 | Scope |
| Section 2 | Referenced Documents |
| Section 3 | Terminology |
| Section 4 | Significance and Use |
| Section 5 | NAPL Mobility and Migration Evaluation Framework |
| Section 6 | Tiered and Weight of Evidence NAPL Movement Evaluation Approaches |
| Section 7 | Centrifuge Test Methods |
| Section 8 | Water Drive Test Methods |
| Section 9 | Calculation Methods for Potential Vertical Movement of NAPL |
| Section 10 | Field Observation Methodologies |
| Section 11 | Keywords |
| Appendix X1 | Laboratory Analysis Methods Commonly Used in NAPL Movement Evaluations (non-mandatory) |
| Appendix X2 | Illustrative Examples of Tiered and WOE Approaches to Evaluate NAPL Movement (non-mandatory) |
| Appendix X3 | Case Studies (non-mandatory) |
| Appendix X4 | Additional Information on Centrifuge Testing Technology in NAPL Mobility Testing (non-mandatory) |
| Appendix X5 | Laboratory Handling and Preparation of Sediment Cores (non-mandatory) |
| Appendix X6 | Additional Information on Water Drive Test Methods in NAPL Mobility Testing (non-mandatory) |
| Appendix X7 | NAPL Net Vertical Gradient Calculation Method (non-mandatory) |
| Appendix X8 | NAPL Effective Hydraulic Conductivity Estimation Methods (non-mandatory) |
| References |
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4.6 Activities described in this guide should be conducted by persons familiar with NAPL-impacted sediment site characterization techniques and sediment remediation science and technology, as well as sediment NAPL mobility and migration assessment protocols and methodologies.
4.7 This guide may be used by various parties involved in sediment programs, including regulatory agencies, project sponsors, environmental consultants, toxicologists, risk assessors, site remediation professionals, environmental contractors, analytical testing laboratories, data validators, data reviewers and users, and other stakeholders, which may include, but are not limited to, owners, buyers, developers, lenders, insurers, government agencies, and community members and groups.
4.8 This guide is not intended to replace or supersede federal, state, local, or international regulatory requirements. Instead, this guide may be used to complement and support such requirements. Any remedial actions taken should meet the regulatory standards for the regulatory entity under which the corrective action is being performed.
4.9 This guide provides a framework based on overarching features and elements that should be customized by the user, based on site-specific conditions, regulatory context, and program objectives for a particular sediment site. This guide should not be used alone as a prescriptive checklist.
4.10 Assessment of NAPL movement in sediments is an evolving science. This guide provides a systematic, yet flexible, framework to accommodate variations in approaches by regulatory agencies and users, based on project objectives, site complexity, unique site features, programmatic and regulatory requirements, newly developed guidance, newly published scientific research, use of alternative scientifically based methods and procedures, changes in regulatory criteria, advances in scientific knowledge and technical capability, multiple line of evidence (LOE) approaches, and unforeseen circumstances.
4.11 Use of this guide supports multiple LOE approaches, using tiered or WOE evaluation frameworks, for the evaluation of NAPL movement in sediments.
4.12 Use of this guide is consistent with the sediment risk-based corrective action (RBCA) process that guides the user to obtain the appropriate data; acquire and evaluate additional data; and refine goals, objectives, receptors, exposure pathways, and the CSM. As the sediment RBCA process proceeds, data and conclusions reached at each step of the process help focus subsequent evaluation. This integrative process results in efficient, cost-effective decision-making and timely, appropriate response actions for NAPL-impacted sediments.
Scope
1.1 This guide discusses methodologies that can be applied to evaluate the potential for the movement (that is, pore-scale mobility or NAPL body-scale migration) of non-aqueous phase liquid (NAPL) in sediments. NAPL movement assessment in sediments is significantly different than in upland soils. As such, the frameworks for evaluating NAPL movement in upland soils have limited applicability for sediments. In particular, because upland NAPL conceptual site models may not be applicable to many sediment sites, this guide provides a framework to evaluate whether NAPL is mobile (at the pore scale) or migrating (at the NAPL body scale) in sediments.
1.2 Assessment of the potential for NAPL to move in sediment is important for several reasons, including (but not limited to) evaluation of risk to potential receptors, the need for potential remedial action, and potential remedial strategies. For example, if the NAPL is migrating, sensitive receptors may be impacted and this will influence the choice and timing of any remedy selected for an area of the sediment site. If the NAPL is not mobile or migrating, then remedial actions may not be warranted.
1.3 This guide is applicable at sediment sites where NAPL has been identified in the sediment by various screening methods and the need for a NAPL movement evaluation is warranted (Guide E3248).
1.4 Petroleum hydrocarbon, coal tar, and other tar NAPLs (including fuels, oils, and creosote) are the primary focus of this guide. These forms of contamination are commonly related to historical operations at refineries, petroleum distribution terminals, manufactured gas plants (MGPs), and various large industrial sites.
1.5 Although certain technical aspects of this guide apply to other NAPLs (for example, dense NAPLs [DNAPLs] such as chlorinated hydrocarbon solvents), this guide does not completely address the additional complexities of those DNAPLs.
1.6 The goal of this guide is to provide a sound technical basis to determine if NAPL at the site is mobile or immobile at the pore scale, and if mobile, whether it is stable or migrating at the NAPL body scale. The potential for NAPL movement in the sediment is a key component in the development of the conceptual site model (CSM) and in deciding what remedial options should potentially be chosen for the site to reduce potential risks to human health and ecological receptors.
1.7 This guide can be used to help develop, or refine, a CSM for the sediment site. A robust CSM is typically needed to optimize potential future work efforts at the site, which may include various risk management and remedial strategies for the site, as well as subsequent monitoring after any remedy implementation.
1.8 This guide considers the mobility of NAPL in sediments that originated from three broad categories of potential NAPL emplacement mechanisms (Guide E3248).
1.8.1 Migration of NAPL by advection (flow through the soil pore network) from an upland site into the pore network of sediments beneath an adjacent water body is one category of NAPL emplacement mechanism. This most commonly occurs within coarse-grained strata in the sediment.
1.8.2 Direct discharge of light NAPL (LNAPL) into a waterway, where it is broken down by mechanical energy to form LNAPL beads, is another category of NAPL emplacement mechanism. Oil-particle aggregates (OPAs) are formed when suspended particulates in surface water adhere to LNAPL beads. Once enough particulates have adhered to an LNAPL bead and the OPA becomes dense enough, it settles through the water column onto a competent sediment surface, where it forms an in situ deposited NAPL (IDN) and may be buried by future sedimentation.
1.8.3 The third category of NAPL emplacement mechanism is DNAPL flow (that is, direct discharge of DNAPL into a waterway), followed by settling through the water column and deposition directly onto a competent sediment surface, where it may be buried by future sedimentation.
1.9 Ebullition-facilitated transport of NAPL from the sediment to the water column by gas bubbles is not within the scope of this guide. The evaluation of ebullition and associated NAPL/contaminant transport is covered in Guide E3300. Transport of NAPL due to erosional forces (for example, propeller wash) is not within the scope of this guide.
1.10 This guide (see Section 5) presents an overall framework to evaluate if NAPL at the site is mobile or immobile at the pore scale, and migrating or stable at the NAPL body scale. It provides guidance on approaches and methodologies that address questions regarding NAPL movement evaluation.
1.11 This guide (see Section 6) discusses the use of data from various laboratory tests (Appendix X1), calculation methodologies, and other methodologies to technically evaluate if NAPL in sediment at various locations in the site is mobile or immobile at the pore scale, and stable or migrating at the NAPL body scale. This evaluation can be performed using tiered and weight of evidence (WOE) frameworks. For example, it may be possible that NAPL is mobile or migrating in one part of the site, but is immobile in other parts of the site. There are currently no industry standard tiered and WOE frameworks to evaluate if NAPL in sediment is mobile or migrating, but illustrative examples of such frameworks are presented in Appendix X2. Case studies demonstrating the application of the example tiered and WOE frameworks exhibited in Appendix X2 are presented in Appendix X3.
1.12 This guide (see Section 7) discusses applicable laboratory centrifuge testing methodologies that are used to evaluate NAPL mobility or immobility at the pore scale under the applicable test conditions (also see Appendix X4). Appendix X5 discusses the laboratory preparation of sediment samples used in centrifuge testing.
1.13 This guide (see Section 8) discusses applicable laboratory water drive testing methodologies that are used to evaluate NAPL mobility or immobility at the pore scale under the applicable test conditions. This section discusses both rigid wall and flexible wall permeameter testing (also see Appendix X6). Appendix X5 discusses the laboratory preparation of sediment samples used in water drive testing.
1.14 This guide (see Section 9) discusses calculation methodologies that provide insight into pore-scale NAPL mobility and NAPL body-scale migration at the site. To perform some of these calculations, NAPL property data such as density, viscosity, and NAPL–water interfacial tension are needed (see Appendix X1). The calculation methodologies include NAPL density versus hydraulic gradient calculations; pore entry pressure calculations; critical NAPL layer thickness calculations; and NAPL pore velocity calculations (also see Appendix X7 and Appendix X8).
1.15 This guide (see Section 10) presents other field observation approaches that are useful in evaluating pore-scale NAPL mobility and NAPL body-scale migration. These methodologies include vertical profiles of NAPL saturation (including isopach mapping of the thickness of unimpacted sediment above the NAPL zone); and installation of monitoring wells in sediment.
1.16 Units—The values stated in SI or CGS units are to be regarded as the standard. No other units of measurement are included in this standard.
1.17 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.
1.18 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.
