Dense non-aqueous phase liquid

A dense non-aqueous phase liquid or DNAPL is a liquid that is both denser than water and is immiscible in or does not dissolve in water.^[1]

The term DNAPL is used primarily by environmental engineers and hydrogeologists to describe contaminants in groundwater, surface water and sediments. DNAPLs tends to sink below the water table when spilled in significant quantities and only stop when they reach impermeable bedrock. Their penetration into an aquifer makes them difficult to locate and remediate.

Examples of materials that are DNAPLs when spilled include:

• chlorinated solvents, such as trichloroethylene, tetrachloroethene, 1,1,1-trichloroethane and carbon tetrachloride
• coal tar
• creosote
• polychlorinated biphenyl (PCBs)
• mercury
• extra heavy crude oil, with an API gravity of less than 10

When spilled into the environment, chlorinated solvents are frequently present as DNAPL and the DNAPL can provide a long term secondary source of the chlorinated solvent to dissolved groundwater plumes. Chlorinated solvents are typically immiscible in water, having low solubility in water by definition, yet still have a solubility above the concentrations allowed by drinking water protections. Therefore, DNAPL which is a chlorinated solvent can act as an ongoing pathway for constituents to dissolve into groundwater. Common use of chlorinated solvents in manufacturing operations began during World War II, with the rate of usage for most solvents increasing into the 1970s. By the early 1980s, chemical analases becoming available that documented widespread contamination of groundwater with chlorinated solvents.^[2] Since that time, a considerable effort has been extended to improve our ability to locate ^[3][4] and remediate ^[5] DNAPL present as chlorinated solvents.

DNAPLs that are not viscous, such as chlorinated solvents, tend to sink into aquifer materials below the water table and become much more difficult to locate and remediate than non aqueous phase liquids that are lighter than water (LNAPLs) which tend to float at the water table when spilled into natural soils. The United States Environmental Protection Agency has focused considerable attention on the remediation of DNAPL which can be costly. Removal or in situ destruction of DNAPLs eliminates the potential exposure to the compounds in the environment and can be an effective method for remediation; however, at some DNAPL sites remediation of DNAPL may not be practicable, and containment may be the only viable remedial action.^[6][7] The United States Environmental Protection Agency has a program to address sites where DNAPL removal is not practicable for remediation projects under CERCLA under the Resource Conservation and Recovery Act^[8]

Groundwater remediation technologies have been developed that can address DNAPL in some settings. Excavation is not always practicable due to the depths of the DNAPL, the dispersed nature of the residual DNAPL, mobility caused during excavation, and complexities with near-by structures. Technologies that are emerging for treatment include the following

• in situ chemical oxidation (ISCO)^[9][10]
  • potassium permanganateW
  • hydrogen peroxide (with or without an iron catalyst)
  • ozone sparging
  • persulfate
• in situ enhanced reductive dechlorination ^[10][11]
• in situ surfactant flushing ^[9][10]
• air sparging^[9][10]
• heating^[9]

The density of complex mixtures can change over time as the mixture interacts with the natural environment. As an example, a heavy crude oil that is mixed with a lighter diluent may begin as an light non-aqueous phase liquid (LNAPL), but as the more soluble diluent is weathered from the mixture the density may increase and the oil may become a DNAPL. Conversely, some mixtures may start as an DNAPL and weather the lighter fractions to become a LNAPL. As an example, a mixture of cutting oils and chlorinated solvents may initially have a specific gravity greater than 1 and be a DNAPL, as the chlorinated solvents weather from the oils, the mixture may become less dense then water and become buoyant. Similarly changes can be seen at some coal gasification plants or manufactured gas plants where the tar mixtures can be neutrally buoyant and can change with time.^[7]

See also

• EPA's web page on DNAPL: http://cluin.org/contaminantfocus/default.focus/sec/Dense_Nonaqueous_Phase_Liquids_(DNAPLs)/cat/Overview/
• Interstate Technology & Regulatory Council (ITRC) page on DNAPL: http://www.itrcweb.org/guidancedocument.asp?TID=8
• LNAPLs (light non-aqueous phase liquids) - water immiscible liquids that are lighter than water.

References

1. ^ [1], USGS
2. ^ Pankow, James F., Stan Feenstra, John A. Cherry and M. Cathryn Ryan, "Dense Chlorianted Solvents in Groundwater: Background and History of the Problem" in Dense Chlorianted Solvents and Other DNAPLs in Groundwater ed. James Pankow & John Cherry, 1996.
3. ^ Dense Chlorinated Solvents and Other DNAPLs in Groundwater ed. James Pankow & John Cherry, 1996.
4. ^ Cohen R.M, and J.W. Mercer. 1993. DNAPL Site Evaluation. CRC Press, Boca Raton, FL. http://www.clu-in.org/download/contaminantfocus/dnapl/600r93022.pdf
5. ^ http://www.clu-in.org/contaminantfocus/default.focus/sec/Dense_Nonaqueous_Phase_Liquids_(DNAPLs)/cat/Overview
6. ^ U.S. EPA, 2003. "The DNAPL Remediation Challenge: Is There a Case for Source Depletion?" EPA/600/R-03/143. http://www.clu-in.org/download/remed/600R03143.pdf
7. ^ ^{a b} [ITRC, 2002. "DNAPL Source Reduction: Facing the Challenge" http://www.itrcweb.org/Documents/DNAPLs-2.pdf]
8. ^ U.S. EPA, 1993. "Guidance for Evaluating the Technical Impracticability of Groundwater Restoration" Directive 9234.2-25
9. ^ ^{a b c d} ITRC, 2000. "Dense Non-Aqueous Phase Liquids (DNAPLs): Review of Emerging Characterization and Remediation Technologies" http://www.itrcweb.org/Documents/DNAPLs-1.pdf
10. ^ ^{a b c d} Ruth M Davison, Gary P Weathhall and David N Lerner, 2002. Source Treatment for Dense Non-Aqueous Phase Liquids. Technical Report P5-051/TR/01. http://publications.environment-agency.gov.uk/pdf/SP5-051-TR-1-e-p.pdf
11. ^ ITRC, 2007. In Situ Bioremediation of Chlorinated Ethene DNAPL Source Zones: Case Studies. [2]