DNAPLs

Nonaqueous phase Hquids (NAPLs) present special problems for soil and ground water cleanup. Contaminant transport through ground water depends in part on the water solubiHty of the compound. Because NAPLs cling to subsurface particles and are slow to dissolve in ground water, they hinder cleanups and prolong cleanup times. Dense nonaqueous phase Hquids (DNAPLs) migrate downward in the aquifer and can coUect in pools or pockets of the substmcture. Examples of DNAPLs are the common solvents tetrachloroethylene (PCE) and trichloroethylene (TCE) which were used extensively at many faciHties before the extent of subsurface contamination problems was realized. 

There are many cases of contamination by dense nonaqueous phase Hquids (DNAPLs) that have fmstrated pump and treat efforts. The general consensus is that pump and treat can reduce contamination or keep it from spreading, but it has failed ia many cases to remediate aquifers to stringent cleanup goals. 

Quinn J, C Geiger, C Clausen, K Brooks, C Coon, S O Hara, T Krug, D Major, W-S Yoon, A Gavskar, T Holdsworth (2005) Eield demonstration of DNAPL dehalogenation using emulsified zero-valent iron. Environ Sci Technol 39 1309-1318. 

Hot water injection via injection wells heats the soil and groundwater and enhances contaminant release. Hot water injection also displaces fluids (including LNAPL and DNAPL free product) and decreases contaminant viscosity in the subsurface to accelerate remediation through enhanced recovery. 

Phenomena Related to the Release of DNAPLs and Other Hazardous... 

PHENOMENA RELATED TO THE RELEASE OF DNAPLs AND OTHER HAZARDOUS SUBSTANCES... 

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... 

DNAPLs have higher densities than water, most between 1 and 2 g/mL, some are near 3 g/mL, for example, bromoform, which has a density of 2.89 g/mL. They have limited water solubilities, and are usually found as the free-phase immiscible with water or as residuals trapped by soil. Most DNAPLs are volatile or semivolatile Pankow82 has listed information on their physical and chemical properties, such as molecular weight, density, boiling points, solubility in water, vapor pressure, sediment/water partition coefficient, viscosity, Henry s law constant, and so on (see Tables 18.8 and 18.9). 

Similar to gasoline, the properties of DNAPLs such as immiscibility with water, volatility, and solubility of some of its components cause the presence of multiphase (pure product, solute, gas, and adsorbate) products and movement that is typical of the phenomena associated with DNAPL release. The theory associated with the interaction of gasoline with soil is applicable to DNAPLs. However,... 

Source U.S. EPA, Estimating Potential for Occurrence of DNAPL at Superfund Sites, EPA Publication 9355.4-07FS, U.S. EPA, Washington, January 1992. Many of these chemicals are found mixed with other chemicals or carrier oils. 

The remedial technologies83-85,90-93 described in previous sections for gasoline release are applicable, for the most part, for remediation of DNAPLs. For example, the pumping or trench method for free products, vacuum extraction, biodegradation, pumping and treatment, soil flushing, and soil excavation and treatment are suitable for cleanup of various phases of DNAPLs. Again, because of... 

Other enhanced DNAPL recovery techniques have been implemented utilizing both water flooding and well bore vacuum. Essentially, this minimizes drawdown, allowing a maximum pumping rate of the DNAPL/water mixture. 

DNAPL release has a higher tendency to sink deep down, entering cracks, biodegradation may be more significant for DNAPL release than for gasoline. 

FIGURE 18.21 A DNAPL recovery system where deliberate upwelling of the static coal-tar surface is used to increase the flow of product into the recovery wells. 

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)... 

DNAPLs), in soil and ground water treatment, 25 834 Dense symmetrical membranes,... 

Pankow, J. F., Feenstra, S., Cherry, J. A., and Ryan, M. C., 1996, Dense Chlorinated Solvents in Groundwater Background and History of the Problem In Dense Chlorinated Solvents and Other DNAPLs in Groundwater (edited by J. F. Pankow and J. A. Cherry), Waterloo Press, Portland, OR, pp. 1-52. 

Overall aquifer restoration and rehabilitation of dissolved hydrocarbons and associated contaminants (this would also include assessment of the potential of dissolved chlorinated hydrocarbons and DNAPLs) and... 

Slough, K. J., Sudicky, E. A., and Forsyth, P. A., 1999, Importance of Rock Matrix Entry Pressure on DNAPLs Migration in Fractured Geologic Material Ground Water, March-April, pp. 237-244. 

Organic compounds such as the chlorinated solvents also include a wide range of compounds and do not easily fit into the structural classification as described for petroleum hydrocarbons. Chlorinated hydrocarbons are commonly discussed in terms of their relative density (i.e., LNAPL or DNAPL) or degree of halogenation and degree of volatility (i.e., volatile, semi-volatile). 

DNAPLs are broadly classified on the basis of certain chemical properties such as density, viscosity, and solubility. Some of the more common DNAPLs are chlorinated solvents (i.e., trichloroethylene, TCE tetrachloroethylene, PCE and trichlo-roethane, TCA), creosote, and coal tar, that is, polycyclic aromatic hydrocarbons... 

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