Standard Guide for NAPL Mobility and Migration in Sediment— Evaluating Ebullition and Associated NAPL/Contaminant Transport

Importancia y uso:

4.1 Ebullition is ubiquitous in sediment and is primarily a significant concern when there is associated NAPL/contaminant transport, resulting in exposure risk to humans, ecological receptors, or both. Ebullition may also be a concern when capping has been chosen as part of a site remedy.

4.2 Understanding the potential for ebullition-facilitated NAPL/contaminant transport in sediment is an important element of an overall conceptual site model (CSM) that forms a basis for (1) evaluating if (and how) human and ecological receptors may be exposed to NAPL/contaminants, and (2) assessing remedial alternatives. In addition, demonstrating the potential for (and extent of) ebullition-facilitated transport of NAPL/contaminants in sediments to regulators and other stakeholders has been historically hampered by the lack of standardized terminology and characterization protocols. The complexity of ebullition-facilitated NAPL/contaminant transport 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 at sediment sites. This has sometimes resulted in misleading expectations about remedial outcomes. The ebullition-facilitated transport mechanisms for NAPL/contaminants in sediments are different from advective transport mechanisms in sediment or in upland environments, due to a variety of physical, geochemical, and biological differences, thus necessitating this guide.

4.3 This guide is intended to serve as a stand-alone document to consider conditions that are unique to ebullition and ebullition-facilitated NAPL/contaminant transport, as well as to complement other guides used for CSM development at contaminated sediment sites (Guides E1689, E1739, E2081, E2531, and E3248). This guide will aid users in understanding the unique and fundamental characteristics of sediment environments that influence the occurrence of ebullition-facilitated NAPL/contaminant transport. Understanding the site characteristics that influence ebullition-facilitated NAPL/contaminant transport within the sediment column will aid in identifying specific data requirements necessary to investigate these conditions, which will enable further refinement of the CSM and provide a sound basis for remedy decisions.

4.4 Ebullition-facilitated NAPL/contaminant transport is the primary transport mechanism that is addressed within this guide.

4.4.1 In addition to ebullition-facilitated NAPL/contaminant transport, porewater advection may also facilitate NAPL/contaminant transport; however, this process is beyond the scope of this guide. Advective transport of NAPL in sediments is addressed in Guide E3248.

4.4.2 Processes associated with NAPL/contaminant transport due to erosion (for example, propeller wash) are not within the scope of this guide.

4.5 This guide identifies the relevant information necessary for a technically reliable and comprehensive CSM in support of the investigation or remediation of ebullition-facilitated NAPL/contaminant transport in sediments. It describes the conditions that lead to (or influence) ebullition-facilitated NAPL/contaminant transport, methods for quantifying the ebullition-facilitated NAPL/contaminant flux rate, considerations for field measurements, and use of field results in extrapolating the NAPL/contaminant flux rate. A technically reliable and comprehensive CSM will result in a more efficient and consistent investigation of ebullition-facilitated NAPL/contaminant transport in sediments to support remedy decisions. This guide may also be beneficial for evaluating ebullition alone at sites (for example, as input into sediment cap design).

4.6 Many materials (for example, chlorinated solvents, petroleum products, and creosote) enter the subsurface as an immiscible liquid, known as NAPL, which may flow as a separate phase from water. NAPL can contain contaminants, such as polycyclic aromatic hydrocarbons (PAHs).

4.6.1 Sheens may be observed on the surface of the water body from sources other than ebullition, such as natural/biogenic sheens, advective NAPL/contaminant transport, outfalls (for example, municipal and industrial), or vessel leaks. Identifying sources of sheens other than ebullition is not within the scope of this guide.

4.7 This guide assumes that a CSM has been developed that includes the nature and extent of NAPL/contaminants in sediment. This CSM would include an understanding of (1) the hydrological setting, (2) the physical and chemical characteristics of the sediment and water body, (3) the physical and chemical characteristics of the NAPL/contaminants, (4) mechanism(s) of NAPL/contaminant emplacement, (5) the physical extent of the NAPL/contaminant zone, and (6) the potential for human and ecological exposures to NAPL/contaminants in sediment, or via NAPL/contaminant release to overlying surface water. The means and methods for collecting this information are not addressed in this guide.

4.8 This guide assumes that the user has developed a CSM that provides a framework for developing a conceptual model (CM) that is a component of the overall CSM, which addresses ebullition-facilitated NAPL/contaminant transport. This guide will help users understand the physical and chemical conditions and emplacement mechanisms that lead to (or influence) ebullition-facilitated NAPL/contaminant transport, as well as aid in prioritizing and executing methods for gathering field data and interpreting results to support the development of a CSM for the site.

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

4.8.2 The elements for the ebullition-facilitated NAPL/contaminant transport CM may include, but are not limited to:

4.8.2.1 Factors affecting the rate of gas production:

(1) Presence of microbial consortia capable of OM mineralization

(2) Presence of labile OM

(3) Geochemical conditions conducive to methanogenesis

(4) Sediment temperature

4.8.2.2 Factors affecting the nucleation of gas bubbles, bubble growth and migration through the sediment column:

(1) Availability of nucleation sites

(2) Sediment properties (for example, tensile strength, grain size, porosity, bulk density, cohesion, and heterogeneity)

(3) Porewater properties (for example, gas concentrations, salinity, pH, and geochemistry)

(4) Environmental setting (for example, hydrostatic pressure, atmospheric pressure, and groundwater seepage)

4.8.2.3 Presence and extent of the NAPL/contaminant zone, including identification of where it is collocated with active ebullition zones.

4.8.2.4 Ebullition-facilitated NAPL/contaminant transport rates, including spatial and temporal variability:

(1) Screening-level evaluations

(2) Quantitative evaluations

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

4.9.1 Section 1: Scope;

4.9.2 Section 2: Referenced Documents;

4.9.3 Section 3: Terminology;

4.9.4 Section 4: Significance and Use;

4.9.5 Section 5: Fundamentals and Considerations During Development of a Conceptual Site Model

4.9.6 Section 6: Initial Screening for Gas Ebullition and Ebullition Flux Measurement;

4.9.7 Section 7: Gas Ebullition Measurement;

4.9.8 Section 8: Quantification of Ebullition-Facilitated Transport of NAPL/Contaminants;

4.9.9 Section 9: Field Considerations in the Measurement of NAPL/Contaminant Fluxes;

4.9.10 Section 10: Keywords;

4.9.11 Appendix X1: Organic Matter Degradation and Microbiology of Biogenic Gas Production in Sediments;

4.9.12 Appendix X2: Carbon Source Identification Using Radioisotope Analysis;

4.9.13 Appendix X3: Bench Scale Testing for Biogenic Gas; and

4.9.14 References.

4.10 This guide provides an overview of the unique characteristics influencing ebullition-facilitated NAPL/contaminant transport in aquatic sediment environments. This guide is not intended to provide specific guidance on sediment site investigation, risk assessment, monitoring, or remedial action.

4.10.1 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.

4.10.2 This guide does not replace the need for engaging competent persons to evaluate ebullition-facilitated NAPL/contaminant transport in sediments. Activities necessary to develop a CSM should be conducted by persons familiar with NAPL/contaminant-impacted sediment site characterization techniques, physical and chemical properties of NAPL/contaminants 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 appropriate data acquisition activities.

Subcomité:

E50.04

Referida por:

E3382-24, E3282-22, E3163-24

Volúmen:

11.05

Número ICS:

13.080.01 (Soil quality and pedology in general)

Palabras clave:

contaminant; contaminated sediment; ebullition; ebullition-facilitated NAPL/contaminant transport; NAPL; sediment; sediment corrective action;