4.1 Understanding the potential emplacement and transport mechanism for NAPL in sediment is an important element of an overall conceptual site model (CSM) that forms a basis for (1) investigating the nature and extent of NAPL, (2) evaluating if (and how) human and ecological receptors may be exposed to NAPL, and (3) assessing remedial alternatives. In addition, demonstrating the potential movement of NAPL in sediments to regulators and other stakeholders has been historically hampered by the lack of standardized terminology and characterization protocols. The complexity of NAPL movement in sediment, and the lack of agreed upon methods for analysis and interpretation of site data, has led to uncertainty in corrective action decision-making. This has sometimes resulted in misleading expectations about remedial outcomes. The emplacement and transport mechanisms for NAPL in sediments are different from those in upland environments, due to a variety of physical, geochemical, and biological differences between sediment and upland environments, thus necessitating this guide.

4.2 This guide is intended to supplement the CSM developed according to the principles outlined in the contaminated sites conceptual site model Guide E1689, the standard guide for developing a CSM for Light Non-Aqueous Phase Liquid (LNAPL) sites Guide E2531, and the Risk-Based Corrective Action (RBCA) Guides E1739 and E2081, by considering conditions for NAPL emplacement and movement (that is, advection) that are unique to a sediment environment. This guide will aid users in understanding the unique and fundamental characteristics of sediment environments that influence the occurrence and behavior of NAPL in sediments. Understanding the sources of NAPL encountered in sediment, the mechanisms for NAPL to become emplaced in sediments, and the site characteristics that influence the advective movement of NAPL within the sediment column will aid in identifying specific data requirements necessary to investigate these conditions and to provide a sound basis for remedy decisions.

4.2.1 Advective transport is the primary NAPL migration mechanism that is addressed within this guide.

4.2.2 In addition to advective transport, biogenic gas bubbles moving through sediments (that is, ebullition) may also facilitate NAPL migration; however, this process is beyond the scope of this guide.

4.2.3 Processes associated with NAPL movement due to erosion (for example, propwash) are not within the scope of this guide.

4.3 This guide describes the emplacement mechanisms and advective processes, and identifies the relevant information necessary for a technically reliable and comprehensive CSM in support of the investigation and/or remediation of NAPL in sediments. A technically reliable and comprehensive CSM will result in more efficient and consistent investigation of NAPL in sediments (for example, assessment of risks associated with NAPL in sediment, and/or remedy decisions). The key elements in assessing the presence and mobility of NAPL in sediment include (1) the hydrological setting, (2) the physical and chemical characteristics of the sediment, (3) the physical and chemical characteristics of the NAPL, and (4) the physical extent of the NAPL zone. The means and methods for collecting this information, including evaluating the mobility of NAPL in sediments, is not addressed in this guide.

4.4 Many contaminants (for example, chlorinated solvents, petroleum products and creosote) enter the subsurface as an immiscible liquid, known as NAPL. NAPLs may flow as a separate phase from water. If the NAPL is denser than water (known as dense non-aqueous phase liquid, or DNAPL), it will sink under the influence of gravity. If the liquid is less dense than water (known as a light nonaqueous phase liquid, or LNAPL), it will float on water.

4.5 This guide provides a logical framework for the initial assessment of NAPL movement in sediment environments. It will help users understand the physical conditions and emplacement mechanisms that influence NAPL movement and aid in prioritizing methods for gathering data to support development of a CSM.

4.5.1 The elements of a CSM for NAPL at sediment sites describe the physical and chemical properties of the environment, the hydraulic conditions, the source of the NAPL, the emplacement mechanisms, and the nature and extent of the NAPL zone. The CSM is a dynamic, evolving model that will change through time as new data are collected and evaluated and/or as physical conditions of the site change due to natural or engineered processes. The goal of the CSM is to describe the nature, distribution, and setting of the NAPL in sufficient detail, so that questions regarding current and potential future risks, longevity, and amenability to remedial action can be adequately addressed.

4.5.2 The unique elements for a CSM for a NAPL sediment site (compared to an upland NAPL site) include, but are not limited to:

(1) Characteristics of the sediment and water body.

(a) Physical characteristics: hydrology (for example, river currents, tidal conditions), sedimentology (for example, native water body bottom characteristics, deposited sediment characteristics, sedimentation rates, erosive forces), and hydrogeology (for example, groundwater-surface water interactions).

(b) Geochemical: for example, redox conditions

(c) Biological characteristics: for example, presence of benthic community

(2) Characteristics of the NAPL release(s) including sources, mechanisms, and timing unique to surface water and sediment that affect the conditions under which the NAPL was emplaced in the sediment.

(3) Mechanisms of NAPL emplacement in sediments, which include:

(a) Advective transport from upland sources,

(b) Deposition on a competent sediment surface from direct releases to surface water, with potential burial by sediment deposition (applies to DNAPL only), and

(c) Formation and deposition of OPAs, with potential burial by sediment deposition.

(4) Indicators for the potential presence and extent of NAPL, including observance of seeps, droplets and/or sheens within a water body.

(5) The potential for human and ecological exposures to NAPL in sediment or by means of NAPL release to overlying surface water.

4.6 The user of this guide should review the overall structure and components of this guide before proceeding with use, including:

4.6.1 Section 1 - Scope;

4.6.2 Section 2 - Referenced Documents;

4.6.3 Section 3 - Terminology;

4.6.4 Section 4 - Significance and Use;

4.6.5 Section 5 - Unique Aspects of Sediment Sites;

4.6.6 Section 6 - NAPL Emplacement Mechanisms;

4.6.7 Section 7 - NAPL Movement Decision Analysis Framework;

4.6.8 Section 8 - Keywords;

4.6.9 Appendix X1 - Emplacement Models: Potential NAPL Interactions at Surface Water Boundaries and Effects on NAPL Movement;

4.6.10 Appendix X2 - Sedimentary Processes and Groundwater - Surface Water Interactions;

4.6.11 Appendix X3 - NAPL Movement Terminology.

4.7 This guide provides an overview of the unique characteristics influencing the presence and potential movement of NAPL in aquatic sediment environments. This guide is not intended to provide specific guidance on sediment site investigation, risk assessment, monitoring or remedial action.

4.7.1 This guide may be used by various parties involved in a sediment site, including regulatory agencies, project sponsors, environmental consultants, site remediation professionals, environmental contractors, analytical testing laboratories, data reviewers and users, and other stakeholders.

4.7.2 This guide does not replace the need for engaging competent persons to evaluate NAPL emplacement and movement in sediments. Activities necessary to develop a CSM should be conducted by persons familiar with NAPL impacted sediment site characterization techniques, physical and chemical properties of NAPL in sediments, fate and transport processes, remediation technologies, and sediment evaluation protocols. The users of this guide should consider assembling a team of experienced project professionals with appropriate expertise to scope, plan, and execute sediment NAPL data acquisition activities.