Dense Nonaqueous Phase Liquids (DNAPLs)
Policy and Guidance
- Overview
- Policy and Guidance
- Chemistry and Behavior
- Environmental Occurrence
- Toxicology
- Detection and Site Characterization
- Treatment Technologies
- Conferences and Seminars
- Additional Resources
Specific DNAPL chemicals are regulated directly by several EPA statutes and regulations. Statutes that govern the cleanup of DNAPL releases or whose requirements are considered Applicable or Relevant and Appropriate Requirements under the National Oil and Hazardous Substances Pollution Contingency Plan (NCP) are discussed briefly below.
EPA has published issue papers and reports on technologies that can be used to characterize and remediate DNAPLs. These materials, which are informational rather than guidance, can be found in the appropriate sections (e.g., treatment technologies) of these pages.
Clean Air Act (CAA)
Several remediation technologies associated with the cleanup of DNAPL contamination strip the chemicals from the soil or water. The federal CAA, as well as state equivalents, may govern release of these chemicals to the atmosphere. A discussion of hazardous air pollutants can be found on the Hazardous Air Pollutants (HAPs) section of the EPA website. A list of DNAPL chemicals that are formally recognized by the EPA as hazardous air pollutants is also available.
Clean Water Act (CWA)
Among other things, the CWA and its state equivalents regulate the discharge of chemicals into surface waters. DNAPL releases, most frequently creosote, can move through the subsurface and discharge directly into a water body. A dissolved-phase plume of DNAPL chemicals can also discharge directly into a water body. Several DNAPL remediation technologies can produce a stream of contaminated water. The CWA and the state equivalents are the mechanisms that determine how much the water must be treated before it can be discharged to a receiving water. A list of DNAPL chemicals that are directly regulated by the CWA as priority pollutants is available. Discharge of DNAPL chemicals that are not on the priority pollutant list are regulated on a case-by-case basis.
Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA)
CERCLA is the basis for federal involvement in contaminated sites that are not covered by Resource Conservation and Recovery Act (RCRA) permits. The program has a removal component and a more long-term remedial component. Actions under both components are governed by the NCP. With the exception of polychlorinated biphenyls (PCBs), which are regulated specifically by the Toxic Substances Control Act (TSCA), and heavy-end oils, such as bunker C, which are regulated under RCRA, responses to releases of DNAPL chemicals to the non-surface water environment are governed by CERCLA. The emergency response component of CERCLA also requires that releases of listed chemicals above a chemical-specific quantity must be reported to the National Response Center. A list of DNAPL chemicals that have reportable quantities is available.
Resource Conservation and Recovery Act (RCRA)
RCRA and the state equivalents cover the treatment, storage, and disposal of listed hazardous wastes (F 
and K codes), toxicity characteristic wastes (D codes ), and the disposal of commercial chemicals (U and P codes). It also covers the disposal of cleanup debris and investigation-derived wastes (D codes) associated with CERCLA remedial or removal actions and RCRA corrective measures. The individual chemicals covered by RCRA are identified in a list of DNAPL chemicals and their associated RCRA codes.
Safe Drinking Water Act (SDWA)
Under the SDWA, EPA identifies chemicals that are hazardous and may find their way into the nation's drinking water. The Agency has set maximum contaminant levels (MCLs) for these identified chemicals. Aquifer cleanup efforts under CERCLA are required to meet these levels or to have the controlling regulatory agency make a determination that it is technically impracticable to do so. A list of DNAPL chemicals that have MCLs is available. Under the Underground Injection Control (UIC) Program, the SDWA and the state equivalents also control the injection of liquids into the ground that may affect water quality. Some DNAPL-related treatment and characterization technologies may fall under UIC jurisdiction, such as steam heating, hot water heating, in situ oxidation, biostimulation, surfactant/cosolvent flushing, tracer testing and re-injection of treated ground water.
Toxic Substances Control Act (TSCA)
TSCA governs, among other things, the cleanup and disposal of PCBs.
The following descriptions were taken from the EPA "Non-Aqueous Phase Liquids (NAPLS) and Ground Water" webpage. These older guidance and policy papers do not reflect advances in characterization and remediation technologies.
Superfund Ground Water RODs: Implementing Change This Fiscal Year, July 31, 1995.
OSWER Memorandum 9335.5-03P, EPA-540-F-99-005, NTIS Order Number PB99-963220, 2p.
This memo from Elliott Laws, EPA Assistant Administrator for Solid Waste and Emergency Response, to the EPA Regional Administrators discusses the importance of consistent national implementation of policies concerning Records of Decision (RODs) for sites with ground-water contamination. It emphasizes that OSWER expects that Technical Impracticability (TI) waivers will generally be appropriate for sites with contaminated ground water where restoration to drinking water standards is technically impracticable, for example, sites with DNAPLs. The memo states: "Beginning immediately, RODs addressing DNAPL contamination that do not follow the policy in favor of TI waivers at such sites must include a written justification for that departure from this policy."
Guidance for Evaluating Technical Impracticability of Ground-Water Restoration
September 1993.
OSWER Directive 9234.2-25, EPA/540-R-93-080, NTIS Order Number PB93-963507, 27p.
This interim final guidance clarifies how EPA will determine whether ground-water restoration at Superfund and RCRA sites is technically impracticable and if so, what alternative measures must be undertaken to ensure that a final remedy is protective. Topics include the types of technical data needed, the criteria for decisions, the types of documentation needed, and alternative remedial strategies for sites with DNAPLs.
Compilation of Information Relating to Early/Interim Actions at Superfund Sites and the TCE IRIS Assessment
U.S. EPA, Office of Superfund Remediation and Technology Innovation, 3 pp, 27 Aug 2014
In response to requests from U.S. EPA Regional offices, this memorandum compiles information regarding existing EPA guidance on early or interim actions at Superfund sites. The memo also provides current information about the toxicity of TCE. The information referenced in this document may be used to support Superfund decision-making at sites with actual or potential inhalation exposures to TCE.
Considerations in Ground-Water Remediation at Superfund Sites and RCRA Facilities — Update, May 27, 1992.
OSWER Directive 9283.1-06, NTIS Order Number PB92-963358, 13p.
This directive clarifies and expands OSWER's 1989 directive "Considerations in Ground-Water Remediation at Superfund Sites," especially with regard to NAPL contaminants. The directive recommends that the likelihood of NAPL contamination, especially dense NAPLs (DNAPLs), should be evaluated as early as possible; where NAPLs are likely, the potential nature and extent of contamination should be characterized to determine appropriate remedial actions; further contamination (dissolved or NAPL) migration should be minimized by using early response actions; early actions should be coordinated with later actions in a phased approach; and ground-water cleanup actions should be designed to include careful monitoring and provisions for modifying them over time to improve their effectiveness. For sites where it can be demonstrated to EPA that achieving cleanup standards is technically impracticable, EPA may issue a waiver for Superfund sites or modify the permit or enforcement order for RCRA facilities. For these cases, EPA will determine alternative remedial objectives that protect human health and the environment.
Estimating Potential for Occurrence of DNAPL at Superfund Sites, January 1992.
OSWER Publication 9355.4-07FS, NTIS Order Number PB92-963338CDH, 10p.
The presence of DNAPL in soils and aquifers can control the ultimate success or failure of remediation at a site, but because of the complex nature of DNAPL transport and fate, it may often be undetected by direct methods. This fact sheet provides a guide for estimating the potential for the presence of DNAPLs at a site based on historical site use information and site characterization data. It summarizes the definition of DNAPL and presents five conceptual models for its release and presence at sites. It includes a series of flow charts for deciding whether a site has a high, moderate, or low potential for containing DNAPL. It also includes step-by-step worksheets for calculating effective solubility of single-component DNAPL and for assessing the likelihood of DNAPL presence based on organic concentrations in soil samples, as well as a glossary of terms related to DNAPLs.
For the most part, states involved in sites listed on the National Priorities List will follow the guidance laid out in the NCP. For other types of sites that may be affected by DNAPLs (RCRA, UST, and state voluntary programs), many states have developed administrative codes or guidances using a three-tiered, risk-based approach for cleanup. These codes often have a DNAPL component. Listed below are examples of the codes from various states.
State Web Sites and Solid and Hazardous Waste Programs
Provides links to the US State and Territories and their hazardous and solid waste pages.
Connecticut Remediation Standard Regulations
These regulations (67 pp) address the steps the state wants a potentially responsible party to take to determine cleanup goals at a site. It discusses how the state's two general water quality designations (GA: drinking water source, and GB: historically degraded) are to be used in determining the cleanup goals. One section in these regulations is devoted specifically to NAPL issues.
These regulations (211 pp) address the steps the state wants a potentially responsible party to take to determine cleanup goals at a site. They provide for the use of three tiers (approaches) for determining cleanup objectives. Specifically:
A Tier 1 evaluation compares the concentration of contaminants detected at a site to the corresponding remediation objectives for residential and industrial/commercial properties contained in Appendix B, Tables A, B, C, D, and E. To complete a Tier 1 evaluation, the extent and concentrations of the contaminants of concern, the groundwater class, the land use classification, human exposure routes at the site, and if appropriate, soil pH, must be known. If remediation objectives are developed based on industrial/commercial property use, then institutional controls under Subpart J are required.
A Tier 2 evaluation uses the risk-based equations from the Soil Screening Level (SSL) and Risk Based Corrective Action (RBCA) documents listed in Appendix C, Tables A and C, respectively. In addition to the information that is required for a Tier 1 evaluation, site-specific information is used to calculate Tier 2 remediation objectives. As in Tier 1, Tier 2 evaluates residential and industrial/commercial properties only. If remediation objectives are developed based on industrial/commercial property use, then institutional controls under Subpart J are required.
A Tier 3 evaluation allows alternative parameters and factors not available under a Tier 1 or Tier 2 evaluation to be considered when developing remediation objectives. Remediation objectives developed for conservation and agricultural properties can only be developed under Tier 3. This approach requires the most information but allows for more flexibility in reaching cleanup goals.
The regulations address situations involving "free product."
Minnesota Risk-Based Site Evaluation Process Guidance Documents
The state of Minnesota has developed a series of draft guidance documents on how to proceed with evaluation and cleanup at voluntary cleanup program and state superfund sites. The approach is tiered.
Tier 1 Evaluation: Early site evaluation, sometimes considered a screening-level evaluation. Tier 1 includes the initial quantitative risk evaluation and will determine if site conditions warrant further investigation. This evaluation requires the least amount of site-specific information and consists of comparing site contaminant data to non-site-specific Tier 1 criteria to determine if individual contaminants or mixtures of contaminants pose a potential risk exceeding target risk levels. Sites presenting conditions that are not addressed by the generic Tier 1 assumptions about exposure pathways must undergo a more site-specific evaluation.
Tier 2 Evaluation: Site evaluation that typically relies on a limited amount of site-specific information including land or surface-water use determinations. The site contaminant concentrations are compared against Tier 2 criteria, which are generated using more site-specific information to determine if individual contaminants or mixtures of contaminants pose a potential risk exceeding the target risk levels.
Tier 3 Evaluation: Site-specific evaluation based on more complex modeling using site-specific data. Tier 3 evaluation is necessary in cases where remediation to Tier 2 target risk levels is not feasible or when site conditions require a unique approach to site investigation and setting remediation goals.
Several Minnesota manuals—e.g., the Risk-Based Guidance for the Soil/Human Health Pathway, Volume 2. Technical Support Document —have sections that address DNAPL issues.
TCEQ Regulatory Guidance: Risk-Based NAPL Guidance
Texas Commission on Environmental Quality, RG-366/TRRP-32, 68 pp, 2008
This document explains the risk-based management approach for non-aqueous phase liquid under the Texas Risk Reduction Program (TRRP) rule and presents a five-step process to address the rule requirements.
These regulations address the steps the state wants a potentially responsible party to take to determine cleanup goals at a site. They provide for the use of three tiers (approaches) for determining cleanup objectives. NAPL issues are addressed in the rule.
Tier 1 is a risk-based analysis to derive non-site-specific protective concentration levels (PCLs) for complete or reasonably anticipated to be completed exposure pathways. Tier 1 is based on default exposure factors and affected property parameters in the applicable PCL equations provided in the following figure and assumes exposure occurs at, above, or below the source area (i.e., no lateral transport).
Tier 2 is a risk-based analysis to derive site-specific PCLs for complete or reasonably anticipated to be completed exposure pathways utilizing site-specific exposure factors, as allowable, and/or affected property parameters and Tier 1 equations. Tier 2 PCLs may also include lateral transport considerations.
Tier 3 is a risk-based analysis to derive site-specific PCLs for complete or reasonably anticipated to be completed exposure pathways. Tier 3 PCLs are based on measured natural attenuation factors and/or natural attenuation factor models/equations other than those provided for Tier 1 or 2 and may also include site-specific exposure factors, as allowable, and/or affected property parameters.
DNAPLs are specifically addressed by this code. The regulation (300 pp) provides three options for establishing cleanup levels. The options and their applicability are described below.
Method A: Applicable Laws and Tables
How does it work? Method A provides tables of cleanup levels that are protective of human health for 25 to 30 of the most common hazardous substances found in soil and ground water. These levels were developed using the procedures in Method B. The Method A cleanup level for a substance must be at least as stringent as the concentration in the Method A table and the concentrations established under applicable state or federal laws. For soil, the Method A cleanup level must also be at least as stringent as a concentration that will not result in significant adverse effects on the protection and propagation of terrestrial ecological receptors (plants and animals), unless it can be demonstrated that such impacts are not a concern at the site. If neither the Method A table nor the applicable state and federal laws provide a value,
then the natural background concentration or the practical quantitation limit (PQL)
may be used as the cleanup level.
When is it used? Method A is designed for cleanups that are relatively straightforward or involve only a few hazardous substances. This method is typically used at smaller sites that do not warrant the costs of conducting detailed site studies and site-specific risk assessments.
Method B: Universal Method
How does it work? Cleanup levels under Method B are established using applicable state and federal laws and the risk assessment equations and other requirements specified for each medium.
Method B is divided into two tiers: standard and modified. Standard Method B uses generic default assumptions to calculate cleanup levels. Modified Method B provides for the use of chemical-specific or site-specific information to change selected default assumptions. For both standard and modified Method B, the human health risk level for individual carcinogens may not exceed one in one million. If more than one type of hazardous substance is present, the total risk level at the site may not exceed 1 in 100,000. Levels for non-carcinogens cannot exceed the point at which a substance may cause illness in humans (that is, the hazard quotient cannot exceed 1). In addition to accounting for human health impacts, Method B cleanup levels must account for any potential terrestrial or aquatic ecological impacts. Unless it can be demonstrated that such impacts are not a concern at the site, the cleanup level for each substance must be below a concentration that could adversely impact ecological receptors (plants and animals). Specific procedures are provided in the rule for assessing the impact of hazardous substances on terrestrial ecological receptors. The natural background concentrations and practical quantitation limits for a substance must also be considered when setting cleanup levels under Method B.
When is it used? Method B may be used at any site and is the most common method for setting cleanup levels when sites are contaminated with substances not listed under Method A. Sites that are cleaned up to Method B cleanup levels generally do not need future restrictions on the use of the property due to the small amount of residual contamination typically left on the property.
Method C: Conditional Method
How does it work? Method C is similar to Method B. Like Method B, Method C is divided into two tiers: standard and modified. The main differences are (1) cleanup levels are based on less stringent exposure assumptions, and (2) the lifetime cancer risk is set at 1 in 100,000 for both individual substances and for the total cancer risk caused by all substances on a site. As under Method B, potential terrestrial and aquatic ecological impacts must be accounted for in addition to human health impacts when establishing Method C cleanup levels. Unlike Method B, however, only the impacts on wildlife must be considered when conducting a terrestrial ecological evaluation.
As under Method B, the natural background concentrations and the practical quantitation limits for a substance must also be considered when establishing Method C cleanup levels.
When is it used? Method C cleanup levels may be used to set soil and air cleanup levels at industrial sites and to set air cleanup levels in manholes and utility vaults. For ground water, surface water, and air cleanup levels, Method C may also be used when Method A or B cleanup levels are lower than technically possible or area background concentrations, or when attainment of those levels may result in a significantly greater overall threat to human health and the environment than attainment of Method C cleanup levels, provided all practicable methods of treatment have been used and institutional controls are in place.
Integrated DNAPL Site Strategy: Technical/Regulatory Guidance
Interstate Technology & Regulatory Council (ITRC) Integrated DNAPL Site Strategy Team.
IDSS-1, 209 pp, 2011
Complete restoration of many DNAPL sites will require prolonged treatment and involve several remediation technologies. Making efficient progress requires an adaptive management strategy and might also require transitioning from one remedy to another as the optimum range of a technique is passed. Targeted monitoring should be used, and reevaluation should be done periodically. This guide was developed to assist site managers in developing an integrated DNAPL site management strategy.
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