Superfund Research Program
Investigating the Mass Transfer and Persistence of Nonaqueous Phase Liquids in the Subsurface Environment
A type of contaminant that is especially troublesome on Superfund sites is the group of chemicals known as nonaqueous phase liquids, or NAPLs. These compounds are so-named because, when they are in contact with water they do not readily dissolve in it, but instead form a separate layer or "phase." Differences in density and other chemical and physical properties are responsible for the physical boundary that forms between NAPLs and water. As a general rule, NAPLs that are denser than water (DNAPLs) sink to the bottom of aqueous media, while those that are less dense than water (LNAPLs) float on top.
In addition to being pervasive in the environment, NAPLs are problematic as contaminants because they are difficult to detect and treat, particularly in the subsurface environment. These latter problems stem from the complex behavior of NAPLs in subsurface environments. NAPL masses tend to be highly mobile in permeable materials such as porous soils, advancing quickly under the influence of gravity. Some of the more volatile NAPLs enter the vapor phase of soil systems. Another complication is that a fraction of most advancing NAPL masses tends to remain behind as isolated residual globules of varying sizes. This entrapped liquid often serves as a continual source of soil and groundwater contamination, hampering clean-up efforts and raising concern about the potential for NAPL contamination of drinking water supplies.
Although substantial progress has been made in recent years in understanding the fate and transport of NAPLs in the subsurface, these processes are still poorly understood due to the complex structural and spatial heterogeneity of the subsurface environment, and the unique behavior of NAPL plumes. Because NAPLs present a potential long-term source of groundwater contamination and human exposures, it is important to accurately describe their behavior in the subsurface environment.
Researchers at the University of Michigan are developing a better understanding of the factors that influence the behavior of NAPLs in the subsurface environment. Laboratory soil column experiments have evaluated the evaporation and persistence of tetrachloroethylene (PCE), a single component DNAPL, in a variety of sandy soils representative of the soil zone above the groundwater table. Emphasis was placed on quantifying the mass transfer rates of PCE. This work showed that the volatilization rates of entrapped PCE can be predicted from easily determined flow and soil parameters. Following the recovery of approximately 99% of the contaminant, low levels of the contaminant were found to persist in the soil column for extended periods. This contaminant persistence is believed to result from slow release of the contaminant from the soil and/or water phases.
Volatilization removal experiments were also conducted for two-component mixtures, which are more representative of conditions on hazardous waste sites. These latter experiments confirmed that the predictive methods developed to describe single component volatilization provide good predictions of the volatilization of two-component mixtures.
Experiments were also conducted to evaluate the influence of soil properties, namely chemical heterogeneity and grain size distribution, on the removal and degradation of PCE. Results suggest that even low percentages of organic-wet solids (i.e., solids saturated with organic matter) greatly enhance the breakdown of NAPLs.
The results obtained in these studies will help in predicting the environmental impact of NAPL contamination and provide information that can be used to improve the effectiveness of cleaning up NAPLs in soil and aquifer systems.
NAPLs include a wide range of compounds that contribute to their widespread presence in the environment. Examples of DNAPLs found on hazardous waste sites include halogenated solvents, polycyclic aromatic hydrocarbons, polychlorinated biphenyls, and certain pesticides. Common LNAPL contaminants include gasoline and oil. Approximately 60% of the sites on the National Priorities List are contaminated with DNAPLs, while LNAPLs are known to affect groundwater quality at thousands of locations across the country, including many Superfund sites.
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To learn more about this research, please refer to the following sources:
- Abriola LM, Bradford SA. 1998. Experimental investigations of the entrapment and persistence of organic liquid contaminants in the subsurface environment. Environ Health Perspect 106(4):1083-1095. PMID:9703497
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