Standard Guide for Development of Conceptual Site Models and Remediation Strategies for Light Nonaqueous-Phase Liquids Released to the Subsurface

Importancia y uso:

5.1 This guide will help users answer simple and fundamental questions about the LNAPL occurrence and behavior in the subsurface. It will help users to identify specific risk-based drivers and non-risk factors for action at a site and prioritize resources consistent with these drivers and factors.

5.2 The site management decision process described in this guide includes several features that are only examples of standardized approaches to addressing the objectives of the particular activity. For example, Table 1 provides example indicators of the presence of LNAPL. Table 1 should be customized by the user with a modified list of LNAPL indicators as technically appropriate for the site or group of sites being addressed.

5.3 This guide advocates use of simple analyses and available data for the LCSM in Tier 1 to make use of existing data and to interpret existing data potentially in new ways. The Tier 1 LCSM is designed to identify where additional data may be needed and where decisions can be made using existing data and bounding estimates.

5.4 This guide expands the LCSM in Tier 2 and Tier 3 to a detailed, dynamic description that considers three-dimensional plume geometry, chemistry, and fluxes associated with the LNAPL that are both chemical- and location-specific.

5.5 This guide fosters effective use of existing site data, while recognizing that information may be only indirectly related to the LNAPL body conditions. This guide also provides a framework for collecting additional data and defining the value of improving the LCSM for remedial decisions.

5.6 By defining the key components of the LCSM, this guide helps identify the framework for understanding LNAPL occurrence and behavior at a site. This guide recommends that specific LNAPL site objectives be identified by the user and stakeholders and remediation metrics be based on the LNAPL site objectives. The LNAPL site objectives should be based on a variety of issues, including:

5.6.1 Potential human health risks and risks to relevant ecological receptors and habitats;

5.6.2 Specific regulatory requirements; and

5.6.3 Aesthetic or other management objectives.

5.7 This guide provides a framework by which users specify benefit remediation metrics that are consistent and achievable given the conditions of the LCSM.

5.8 Guidance is focused on the information needed to make sound decisions rather than specific methods or evaluations that might be used in deriving that information. This guide is weighted toward field data rather than modeling, though modeling is clearly recognized as a useful tool in generating scenarios and bracketing conditions of the LNAPL body conditions. Limited examples of site specific data used to develop the LCSM are provided in Appendix X6.

5.9 By defining specific, measurable attributes of remedial actions acting upon an LCSM, users can determine which actions may be feasible and which likely are not, using an evaluation of a consistent set of factors and expectations.

5.10 A sound LCSM will lead to better decisions about remedial actions. The site management decision process premised on the LCSM is intended to result in more efficient and consistent decision-making about LNAPL risk evaluations and remedial actions.

5.11 The complexity of multiphase LNAPL issues and the wide variety of analysis and interpretation methods that are available has lead to uncertainty in decision-making regarding sites with LNAPL and has sometimes resulted in misleading expectations about remedial outcomes.

5.12 Current risk assessment methods often assume the LNAPL is an infinite source of chemicals of concern. The remediation decision-making may be better defined by considering the LNAPL as the source material for chemicals of concern by explicitly characterizing the chemical composition and physical characteristics of the LNAPL body.

5.13 When LNAPL presents the main source of risk, the LNAPL should be the primary target of remedial actions and those remedial actions should be determined by following the decision evaluations described in this guide.

5.14 LNAPL regulatory policies that define remediation metrics by small LNAPL thicknesses in wells are, on a site-specific basis, often inconsistent with risk-based screening levels (RBSLs) and with current technical knowledge regarding LNAPL mobility and recoverability. LNAPL remediation metrics should be connected to the current or potential future exposures and risks, as well as to other non-risk drivers present for a particular site.

5.15 The user of this guide is encouraged to identify the appropriate process for public involvement and stakeholder participation in the development of the LCSM and the site management decision process.

5.16 By providing a flexible framework, this guidance will continue to be applicable in principle while the many unknowns and uncertainties in LNAPL movement and the associated risks in all plume phases (for example, sorbed, dissolved, vapor) are studied through future research efforts. Like the LCSM itself, this is a “living” document that must embrace advances in knowledge and in technology.

Subcomité:

E50.04

Referida por:

E3248-20, E3300-21, E2081-22, E2856-13R21, E2205_E2205M-22, E3281-21A, E3361-22, E2616-09R20, E3282-22, E2856-13R21, E1689-20

Volúmen:

11.05

Número ICS:

13.080.05 (Examination of soil in general)

Palabras clave:

conceptual site models; corrective action decision framework; light nonaqueous phase liquids; petroleum releases; remedial action decision-making; risk-based corrective action;