DNAPL liquid

Besides petroleum products, other hazardous substances (see Tables 18.7-18.9) are also stored in USTs. Among them, a common and important group is the dense nonaqueous phase liquids (DNAPLs). This group has some different physical properties from petroleum (especially gasoline) that make them behave differently in the way they move underground. This section presents the important factors associated with the cleanup of DNAPLs. 

DNAPLs are mainly liquid hydrocarbons such as chlorinated solvents, wood preservatives, coal tar wastes, and pesticides. Table 18.7 lists some common such chemicals.81... 

Villaume, J.F., Investigations at sites contaminated with dense non-aqueous phase liquids (DNAPLs),... 

An insoluble liquid or gas will separate from water, resulting in immiscible-phase separation. The behavior of nonaqueous-phase liquids (NAPLs) that may be lighter (LNAPLs) or denser (DNAPLs)... 

The problems associated with LNAPLs are well documented in the literature, ranging from small releases where just enough LNAPL is present to be a nuisance, to pools ranging up to millions of barrels of LNAPL and encompassing hundreds of acres in lateral extent. Subsurface migration of LNAPL (and DNAPL) are affected by several mechanisms depending upon the vapor pressure of the liquid, the density of the liquid, the solubility of the liquid (how much dissolves in water at equilibrium), and the polar nature of the NAPL. 

Adams, M. L., Sinclair, N., and Fox, T., 1998, 3-D Seismic Reflection Surveys for Direct Detection of DNAPL In Nonaqueous-Phase Liquids, Remediation of Chlorinated and Recalcitrant Compounds (edited by G. B. Wickramanayake and E. Hinchee), Battelle Press, Columbus, OH, pp. 155-160. 

Conner, J. A., Newell, C. J., and Wilson, D. K., 1989, Assessment, Field Testing and Conceptual Design for Managing Dense Non-Aqueous Phase Liquids (DNAPL) at a Superfund Site In Proceedings of the National Water Well Association and American Petroleum Institute Conference on Petroleum Hydrocarbons and Organic Chemicals in Groundwater Prevention, Detection, and Restoration, November, pp. 519-533. 

Sites suitable for conventional SVE have certain typical characteristics. The contaminating chemicals are volatile or semivolatile (vapor pressure of 0.5 mm Hg or greater). Removal of metals, most pesticides, and PCBs by vacuum is not possible because their vapor pressures are too low. The chemicals must be slightly soluble in water, or the soil moisture content must be relatively low. Soluble chemicals such as acetone or alcohols are not readily strippable because their vapor pressure in moist soils is too low. Chemicals to be removed must be sorbed on the soils above the water table or floating on it (LNAPL). Volatile dense nonaqueous liquids (DNAPLs) trapped between the soil grains can also be readily removed. The soil must also have sufficiendy high effective porosity (permeability) to allow free flow of air through the impacted zone. 

The Ferox process offers several potential advantages over conventional permeable barrier walls. For example, Ferox injection parameters may be modified to reflect the contaminant concentration heterogeneities present at most dense non-aqueous-phase liquid (DNAPL) sites. Unlike permeable walls, Ferox is not limited to the treatment of dissolved-phase contaminants and may be applied under structures. In addition, Ferox is not limited by depth and does not require the use of excessive quantities of iron powder. 

In situ chemical oxidation using potassium permanganate was also demonstrated to treat dense, non-aqueous-phase liquid (DNAPL) at the Canadian Forces Base Borden in Ontario, Canada, between 1996 and 1997. This application used a series of six injection and five oxidant recovery weUs. The total cost of the project was approximately 45,000 (D18766A, p.l3). 

Waterflood oil recovery is a commercially available, in situ techuology for the treatment of groundwater contaminated with dense non-aqueous-phase liquids (DNAPLs) such as oil. Water-flood oil recovery is tailored to specific site conditions and is generally used in conjunction with barrier technologies. All information is from the vendor and has not been independently verified. 

CESAR was developed to address the problem of locating, characterizing, and removing dense non-aqueous-phase liquids (DNAPLs) from contaminated aquifer systems. The process is particularly suited to remediating groundwater contaminated with chlorinated solvents, such as trichloroethylene (TCE), tetrachloroethene (PCE), trichloroethane (TCE), and carbon tetrachloride (CCE). According to the vendor, CESAR can also be applied to sites contaminated with creosote, polychlorinated biphenyls (PCBs), Freon 113, volatile organic compounds (VOCs),... 

The process of anaerobic biotransformation with steam injection is a technology for the in situ remediation of soils and groundwater contaminated with dense non-aqueous-phase liquids (DNAPLs). Using this approach for remediation, steam is injected into the soil to volatilize and remove DNAPLs, with the simultaneous introduction of nutrients. The resulting subsurface conditions are suitable for biotransformation of the dissolved phase, into compounds that are more easily removed by vapor and groundwater extraction. 

Reclaim is commercially available and suitable for the recovery of VOCs, vinyl chloride, trichloroethylene, carbon tetrachloride, dense non-aqueous-phase liquid (DNAPL) compounds, and, in particular, petroleum hydrocarbons. The vendor states this technology has been used in a wide variety of industrial applications, such as the remediation of groundwater at service stations, dry cleaners, herbicide production facilities, and municipal and industrial landfills, among others. 

ISOTEC was chosen to treat soils contaminated with dense non-aqueous-phase liquids (DNAPLs) at a Superfund site in Florida. With a projected cost of 340,000, ISOTEC was cheaper than the alternative technologies considered. The estimated cost for implementing six-phase heating at the site was 535,000, and the estimated cost for excavation and ex situ treatment was 835,000 (D21478I, pp. 10, 11). 

Abbreviations TCE, trichloroethylene DCE, dichloroethylene PCE, perchloroethylene TCA, trichloroethane cDCE, cis-dichloroethylene DNAPL, dense, non-aqueous-phase liquid. 

The vendor states that tetrachloroethane (PCE), trichloroethene (TCE), and other volatile compounds are difficult to remove from saturated soils because they are relatively insoluble. The vendor states that the technology is especially applicable to sites contaminated with dense non-aqueous-phase liquids (DNAPLs). Using the ISSZT technology creates an unsaturated zone from which these contaminants can be readily air stripped. Other contaminants such as polychlorinated biphenyls (PCBs) or metals can be isolated from groundwater and contained within barriers preventing the spread of contamination. 

During the field demonstration in 1997 at the Department of Defense s (DOD s) Operable Unit 2, Hill Air Force Base in Utah, approximately 908 gal of dense, non-aqueous-phase liquids (DNAPLs) were removed from contaminated soils. The cost of the demonstration was 230/yd of soil treated and 165/gal of DNAPL removed. These costs included the purchase of all the equipment and an extensive monitoring network. It is estimated that future applications at the site would require roughly half as many wells and the boiler rental would be restricted to a much shorter period. These conditions would lower the direct treatment costs to 103/yd of treated soil and 74/gal DNAPL removed (D18518W, p. 216). 

Terra Vac s dense non-aqueous-phase liquid (DNAPL) vaporization involves heating the subsurface, including both groundwater and soil, to vaporize the DNAPL. According to the vendor, this technology is appropriate for medium and large sites with separate pools of dense chlorinated solvents, such as chloroform, dichloroethane, dichloroethene, Freons, methylene chloride, and vinyl chloride. This technology is commercially available. 

The technology has been applied primarily at sites contaminated with petroleum hydrocarbons. The Vacu-Polnt technology can remove hydrocarbons and chlorinated solvents from contaminated groundwater and soil. The developer asserts that the technology can treat VOCs, and all phases of non-aqueous-phase liquids (NAPLS) and dense non-aqueous-phase liquids (DNAPLS). 

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