Dense nonaqueous phase liquids (dnapls)

Overview

Dense nonaqueous phase liquids (DNAPLs) are chemicals or mixtures of chemicals that have two major characteristics in common: they are heavier than water, and they are only slightly soluble in water. These two physical characteristics mean that when released into the environment in sufficient quantity, they can move through soils and groundwater until they encounter a sufficiently resistant layer that will impede further mass vertical movement and allow the liquid to pool. Depending upon the nature of the release, the movement through the subsurface soils can be quite complex as the liquid follows the path of least resistance. For example, soils considered homogenous often have subtle differences in layering that can cause a DNAPL to run and drop many times, creating a complex of thin horizontal and vertical ganglia. Both DNAPL soil residuals, which are the most common form of contamination/spill encountered, and pools become slowly dissolving sources of groundwater and soil vapor contamination. In addition, low conductivity areas into which the DNAPL mass/or and the dissolved-phase plume have diffused or migrated can in turn become sources of low-level contamination after the DNAPL mass has disappeared. Wolfe et al. (USGS 1997) contains a diagram showing the distribution of potential DNAPL-accumulation sites in a hypothetical karst setting (Figure 12). While the solubilities of these chemicals are very low (often hundreds to low thousands of parts per million), the level at which they can present a human health or ecological risk is considerably lower (few to hundreds of parts per billion). A fuller discussion of the fate and transport of DNAPLs is found in the Chemistry and Behavior section.

Cohen and Mercer (EPA 1993) identify over 70 DNAPL chemicals or mixtures that are or were commonly used solvents, feedstocks, and end products. Table 1 provides a list of the chemicals and some of their physical and chemical properties. For purposes of discussion in the following sections, these chemicals plus several others have been divided into major classes based on their presence at Superfund sites. The list of major classes below and the contents of this web site will be updated as additional DNAPL chemicals are discovered at Superfund sites:

Ethers
Halogenated alkanes
Halogenated alkenes

1,1-Dichloroethene
cis 1,2-Dichloroethene
trans 1,2-Dichloroethene
cis 1,3-Dichloropropene
trans 1,3-Dichloropropene
Tetrachloroethene
Trichloroethene

Halogenated monoaromatics
Polychlorinated biphenyls (PCBs)
Multi-Component Waste

Coal Tars
Creosotes
Heavy Oils

Other (aniline, benzyl chloride, etc.)

The chemicals most commonly found as DNAPLs at contaminated sites are chlorobenzenes, chloromethanes, PCBs (electrical transformer oils), tetra and trichloroethane (solvents), tetra and trichloroethene (solvents, degreasers), creosote (wood treaters), and coal tar (manufactured gas plants [MGPs]).

Many of these chemicals are regulated under various federal and state laws that will be discussed in the Policy and Guidance section. The chemical and physical properties of these chemicals are widely different and determine how they will travel in sediments and soils. For example, trichloroethene (TCE) is about 1.5 times as heavy as water and less viscous, which means it can move relatively quickly through permeable soils and water, while creosote is slightly heavier than water and very viscous, so its rate of movement is much slower. There are creosote plumes that are still moving even though the release occurred many years ago, such as at the McCormick & Baxter Creosoting Company Superfund Site, Portland, Oregon. These properties are discussed for each chemical in the Chemistry and Behavior section. Like the physical and chemical properties, the human health and ecological effects of these chemicals vary widely and are discussed in the Toxicology section.

Because of the way DNAPLs move and distribute themselves in the subsurface they are particularly difficult to detect and characterize. A detailed picture of the contaminant distribution (architecture) and an accurate site conceptual model are essential for selecting the appropriate technologies for addressing DNAPL contamination. Techniques, some specifically for DNAPL source zones, are discussed in the Detection and Site Characterization section.

Depending upon the source zone architecture and its location (e.g., above or below the water table, in deep or shallow unconsolidated soils or fractured bedrock), different treatment approaches may be appropriate. In the past, unless the source zone was amenable to excavation and treatment or to removal by soil vapor extraction, it was either recovered to the extent practicable or given a technical impracticability waiver and left in place with some form of containment system to prevent offsite migration of contaminated ground water. With the advance of treatment technologies, it has become possible to consider treating some source zones. Conventional treatment approaches as well as innovative technologies with case histories are presented in the Treatment Technologies section.

The cost of operating a pump and treat containment system for a source zone that may have an indefinite life span along with the appearance of technologies that may be able to considerably reduce the life span of the source area have generated discussion on if and when it may be practicable and cost effective to treat a source zone. In the summer of 2001, EPA's Office of Research and Development organized a panel of experts to examine issues associated with remediation of DNAPL source zones. Their report was issued in December 2003 (U.S. EPA 2003). In 2004, The National Academies of Science (NAS) issued a report of their own on Source Zone Assessment and Remediation (NAS 2004). Both of these reports focused on the technologies available for remediation, the ability to effectively characterize where the source zones are, how to measure success, and data gaps. Also in 2004, the Navy produced a document on optimizing site remediation that suggested that a treatment train approach that uses several technologies starting with source reduction might be the most cost effective way to clean up a site. Since these studies were done considerable experience has been gained in DNAPL source remediation and much more is known about the effectiveness of the various technologies.

Individual documents that are specific to a section topic are highlighted in the section with appropriate links to the document. The Additional Resources section is devoted to webpages maintained by EPA and other federal agencies that contain a number of downloadable DNAPL references.

A complete Site Map for this section of Contaminant Focus is also available.

Battelle Memorial Institute. 2010. Guidance for Optimizing Remedy Evaluation, Selection, and Design, User's Guide. UG-2087-ENV. Naval Facilities Engineering Command, 84 pp.