DNAPL
Encyclopedia
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.
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:
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. Since that time, a considerable effort has been extended to improve our ability to locate and remediate 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. 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
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
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.
Liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...
that is both denser
Relative density
Relative density, or specific gravity, is the ratio of the density of a substance to the density of a given reference material. Specific gravity usually means relative density with respect to water...
than water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
and is immiscible in or does not dissolve in water.
The term DNAPL is used primarily by environmental engineers and hydrogeologists to describe contaminants in groundwater
Groundwater
Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock...
, surface water and sediments. DNAPLs tends to sink below the water table
Water table
The water table is the level at which the submarine pressure is far from atmospheric pressure. It may be conveniently visualized as the 'surface' of the subsurface materials that are saturated with groundwater in a given vicinity. However, saturated conditions may extend above the water table as...
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 trichloroethyleneTrichloroethyleneThe chemical compound trichloroethylene is a chlorinated hydrocarbon commonly used as an industrial solvent. It is a clear non-flammable liquid with a sweet smell. It should not be confused with the similar 1,1,1-trichloroethane, which is commonly known as chlorothene.The IUPAC name is...
, tetrachloroethene, 1,1,1-trichloroethane1,1,1-TrichloroethaneThe organic compound 1,1,1-trichloroethane, also known as methyl chloroform, is a chloroalkane. This colourless, sweet-smelling liquid was once produced industrially in large quantities for use as a solvent...
and carbon tetrachlorideCarbon tetrachlorideCarbon tetrachloride, also known by many other names is the organic compound with the formula CCl4. It was formerly widely used in fire extinguishers, as a precursor to refrigerants, and as a cleaning agent... - coal tarCoal tarCoal tar is a brown or black liquid of extremely high viscosity, which smells of naphthalene and aromatic hydrocarbons. Coal tar is among the by-products when coal iscarbonized to make coke or gasified to make coal gas...
- creosoteCreosoteCreosote is the portion of chemical products obtained by the distillation of a tar that remains heavier than water, notably useful for its anti-septic and preservative properties...
- polychlorinated biphenylPolychlorinated biphenylPolychlorinated biphenyls are a class of organic compounds with 2 to 10 chlorine atoms attached to biphenyl, which is a molecule composed of two benzene rings. The chemical formula for PCBs is C12H10-xClx...
(PCBs) - mercuryMercury (element)Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...
- extra heavy crude oil, with an API gravityAPI gravityThe American Petroleum Institute gravity, or API gravity, is a measure of how heavy or light a petroleum liquid is compared to water. If its API gravity is greater than 10, it is lighter and floats on water; if less than 10, it is heavier and sinks...
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
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
, 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. Since that time, a considerable effort has been extended to improve our ability to locate and remediate 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
United States Environmental Protection Agency
The U.S. Environmental Protection Agency is an agency of the federal government of the United States charged with protecting human health and the environment, by writing and enforcing regulations based on laws passed by Congress...
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. 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
Resource Conservation and Recovery Act
The Resource Conservation and Recovery Act , enacted in 1976, is the principal Federal law in the United States governing the disposal of solid waste and hazardous waste.-History and Goals:...
Groundwater remediation
Groundwater remediation
Groundwater remediation is the process that is used to remove pollution from groundwater. Groundwater is water present below the ground surface that saturates the pore space in the subsurface. At least one half of the population of the United States depends upon groundwater as a source of...
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)
- potassium permanganatePotassium permanganatePotassium permanganate is an inorganic chemical compound with the formula KMnO4. It is a salt consisting of K+ and MnO4− ions. Formerly known as permanganate of potash or Condy's crystals, it is a strong oxidizing agent. It dissolves in water to give intensely purple solutions, the...
W - hydrogen peroxideHydrogen peroxideHydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...
(with or without an iron catalyst) - ozoneOzoneOzone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...
sparging - persulfatePersulfateThe term persulfate refers to ions or compounds with more oxygen than normal sulfates.These do not have sulfur in a different oxidation state; rather, they contain peroxide units, where two oxygens take the place of one in a normal sulfate; the oxygen atoms are in oxidation state −1.The main forms...
- potassium permanganate
- in situ enhanced reductive dechlorination
- in situ surfactant flushing
- air sparging
- heating
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
Light non-aqueous phase liquid
A Light Non-Aqueous Phase Liquid is a groundwater contaminant that is not soluble and has a lower density than water, which is the opposite of DNAPL. Once LNAPL infiltrates through the soil, it will stop at the height of the water table since LNAPL is less dense than water...
(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
Coal gasification
Coal gasification is the process of producing coal gas, a type of syngas–a mixture of carbon monoxide , hydrogen , carbon dioxide and water vapour –from coal...
plants or manufactured gas plants where the tar mixtures can be neutrally buoyant and can change with time.
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 liquidLight non-aqueous phase liquidA Light Non-Aqueous Phase Liquid is a groundwater contaminant that is not soluble and has a lower density than water, which is the opposite of DNAPL. Once LNAPL infiltrates through the soil, it will stop at the height of the water table since LNAPL is less dense than water...
s) - water immiscible liquids that are lighter than water.