Dense non-aqueous phase liquid

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

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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 analyses 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 (USEPA) 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 USEPA 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] Dense nonaqueous phase liquids (DNAPLs), have low solubility and are with viscosity markedly lower and density higher than water-asphalt, heavy oils, lubricants and also chlorinated solvents-penetrate the full depth of the aquifer and accumulate on its bottom. [9] "DNAPL movement follows the slope of the impermeable strata underlying the aquifer and can move in the opposite direction to the groundwater gradient." [10]

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

Most DNAPLs remain denser than water after they are released into the environment (e.g. spilled trichloroethene does not become lighter than water, it will remain denser than water). However, when the DNAPL is a more complex mixture, the density of the mixture can change over time as the mixture interacts with the natural environment. As an example, a mixture of trichloroethene and cutting oil may be released and originally be denser than water—a DNAPL. As the mixture of trichloroethene and oil is leached by groundwater, the trichloroethene may preferentially leach out of the oil and the mixture may become less dense than water and become buoyant (e.g. the liquid may become an LNAPL). Similarly changes can be seen at some coal gasification plants or manufactured gas plants where the tar mixtures can be denser than water, be neutrally buoyant or be less dense than water and the densities can change with time. [7]

See also

Related Research Articles

Trichloroethylene Chemical compound

The chemical compound trichloroethylene is a halocarbon commonly used as an industrial solvent. It is a clear, colourless non-flammable liquid with a chloroform-like sweet smell. It should not be confused with the similar 1,1,1-trichloroethane, which is commonly known as chlorothene.

Environmental remediation Removal of pollution from soil, groundwater etc.

Environmental remediation deals with the removal of pollution or contaminants from environmental media such as soil, groundwater, sediment, or surface water. Remedial action is generally subject to an array of regulatory requirements, and may also be based on assessments of human health and ecological risks where no legislative standards exist, or where standards are advisory.

Bioremediation Process used to treat contaminated media such as water and soil

Bioremediation is a process used to treat contaminated media, including water, soil and subsurface material, by altering environmental conditions to stimulate growth of microorganisms that degrade the target pollutants. Most bioremediation is inadvertent, involving native organisms. Research on bioremediation is heavily focused on stimulating the process by inoculation of a polluted site with organisms or supplying nutrients to promote the growth. In principle, bioremediation could be used to reduce the impact of byproducts created from anthropogenic activities, such as industrialization and agricultural processes. Bioremediation could prove less expensive and more sustainable than other remediation alternatives.

Aqueous biphasic systems (ABS) or aqueous two-phase systems (ATPS) are clean alternatives for traditional organic-water solvent extraction systems.

A multiphasic liquid is a mixture consisting of more than two immiscible liquid phases. Biphasic mixtures consisting of two immiscible phases are very common and usually consist of an organic solvent and an aqueous phase.

Soil vapor extraction (SVE) is a physical treatment process for in situ remediation of volatile contaminants in vadose zone (unsaturated) soils. SVE is based on mass transfer of contaminant from the solid (sorbed) and liquid phases into the gas phase, with subsequent collection of the gas phase contamination at extraction wells. Extracted contaminant mass in the gas phase is treated in aboveground systems. In essence, SVE is the vadose zone equivalent of the pump-and-treat technology for groundwater remediation. SVE is particularly amenable to contaminants with higher Henry’s Law constants, including various chlorinated solvents and hydrocarbons. SVE is a well-demonstrated, mature remediation technology and has been identified by the U.S. Environmental Protection Agency (EPA) as presumptive remedy.

Groundwater remediation is the process that is used to treat polluted groundwater by removing the pollutants or converting them into harmless products. Groundwater is water present below the ground surface that saturates the pore space in the subsurface. Globally, between 25 per cent and 40 per cent of the world's drinking water is drawn from boreholes and dug wells. Groundwater is also used by farmers to irrigate crops and by industries to produce everyday goods. Most groundwater is clean, but groundwater can become polluted, or contaminated as a result of human activities or as a result of natural conditions.

Electrical resistance heating Environmental cleanup method

Electrical resistance heating (ERH) is an intensive in situ environmental remediation method that uses the flow of alternating current electricity to heat soil and groundwater and evaporate contaminants. Electric current is passed through a targeted soil volume between subsurface electrode elements. The resistance to electrical flow that exists in the soil causes the formation of heat; resulting in an increase in temperature until the boiling point of water at depth is reached. After reaching this temperature, further energy input causes a phase change, forming steam and removing volatile contaminants. ERH is typically more cost effective when used for treating contaminant source areas.

A light non-aqueous phase liquid (LNAPL) is a groundwater contaminant that is not soluble in water and has lower density than water, in contrast to a DNAPL which has higher density than water. Once a LNAPL infiltrates the ground, it will stop at the height of the water table because the LNAPL is less dense than water. Efforts to locate and remove LNAPLs is relatively less expensive and easier than for DNAPLs because LNAPLs float on top of the water in the underground water table.

In situ chemical oxidation (ISCO), a form of advanced oxidation process, is an environmental remediation technique used for soil and/or groundwater remediation to lower the concentrations of targeted environmental contaminants to acceptable levels. ISCO is accomplished by introducing strong chemical oxidizers into the contaminated medium to destroy chemical contaminants in place. It can be used to remediate a variety of organic compounds, including some that are resistant to natural degradation. The in situ in ISCO is just Latin for "in place", signifying that ISCO is a chemical oxidation reaction that occurs at the site of the contamination.

A permeable reactive barrier (PRB), also referred to as a permeable reactive treatment zone (PRTZ), is a developing technology that has been recognized as being a cost-effective technology for in situ groundwater remediation. PRBs are barriers which allow some—but not all—materials to pass through. One definition for PRBs is an in situ treatment zone that passively captures a plume of contaminants and removes or breaks down the contaminants, releasing uncontaminated water. The primary removal methods include: (1) sorption and precipitation, (2) chemical reaction, and (3) reactions involving biological mechanisms.

In situ chemical reduction (ISCR) is a new type of environmental remediation technique used for soil and/or groundwater remediation to reduce the concentrations of targeted environmental contaminants to acceptable levels. It is the mirror process of In Situ Chemical Oxidation (ISCO). ISCR is usually applied in the environment by injecting chemically reductive additives in liquid form into the contaminated area or placing a solid medium of chemical reductants in the path of a contaminant plume. It can be used to remediate a variety of organic compounds, including some that are resistant to natural degradation.

1,2,3-Trichloropropane Chemical compound

1,2,3-Trichloropropane (TCP) is an organic compound with the formula CHCl(CH2Cl)2. It is a colorless liquid that is used as a solvent and in other specialty applications.

Groundwater pollution Pollution that occurs when pollutants are released to the ground and seep down into groundwater

Groundwater pollution occurs when pollutants are released to the ground and make their way into groundwater. This type of water pollution can also occur naturally due to the presence of a minor and unwanted constituent, contaminant, or impurity in the groundwater, in which case it is more likely referred to as contamination rather than pollution. Groundwater pollution can occur from on-site sanitation systems, landfill leachate, effluent from wastewater treatment plants, leaking sewers, petrol filling stations, hydraulic fracturing (fracking) or from over application of fertilizers in agriculture. Pollution can also occur from naturally occurring contaminants, such as arsenic or fluoride. Using polluted groundwater causes hazards to public health through poisoning or the spread of disease.

The water associated fraction (WAF), sometimes termed the water-soluble fraction (W.S.F.), is the solution of low molecular mass hydrocarbons naturally released from petroleum hydrocarbon mixtures in contact with water. Although generally regarded as hydrophobic, many petroleum hydrocarbons are soluble in water to a limited extent. This combination often also contains less soluble, higher molecular mass components, and more soluble products of chemical and biological degradation.

Non-aqueous phase liquids, or NAPLs, are organic liquid contaminants that do not dissolve in, or easily mix with, water (hydrophobic), like oil, gasoline and petroleum products.

Aquifer thermal energy storage (ATES) is the storage and recovery of thermal energy in the subsurface. ATES is applied to provide heating and cooling to buildings. Storage and recovery of thermal energy is achieved by extraction and injection of groundwater from aquifers using groundwater wells. Systems commonly operate in a seasonal mode. The groundwater that is extracted in summer, is used for cooling by transferring heat from the building to the groundwater by means of a heat exchanger. Subsequently, the heated groundwater is injected back into the aquifer, which creates a storage of heated groundwater. In wintertime, the flow direction is reversed such that the heated groundwater is extracted and can be used for heating. Therefore, operating an ATES system uses the subsurface as a temporal storage to buffer seasonal variations in heating and cooling demand. When replacing traditional fossil fuel dependent heating and cooling systems, ATES can serve as a cost-effective technology to reduce the primary energy consumption of a building and the associated CO2 emissions.

<i>In situ</i> bioremediation

Bioremediation is the process of decontaminating polluted sites through the usage of either endogenous or external microorganism. In situ is a term utilized within a variety of fields meaning "on site" and refers to the location of an event. Within the context of bioremediation, in situ indicates that the location of the bioremediation has occurred at the site of contamination without the translocation of the polluted materials. Bioremediation is used to neutralize pollutants including Hydrocarbons, chlorinated compounds, nitrates, toxic metals and other pollutants through a variety of chemical mechanisms. Microorganism used in the process of bioremediation can either be implanted or cultivated within the site through the application of fertilizers and other nutrients. Common polluted sites targeted by bioremediation are groundwater/aquifers and polluted soils. Aquatic ecosystems affected by oil spills have also shown improvement through the application of bioremediation. The most notable cases being the Deepwater Horizon oil spill in 2010 and the Exxon Valdez oil spill in 1989. Two variations of bioremediation exist defined by the location where the process occurs. Ex situ bioremediation occurs at a location separate from the contaminated site and involves the translocation of the contaminated material. In situ occurs within the site of contamination In situ bioremediation can further be categorized by the metabolism occurring, aerobic and anaerobic, and by the level of human involvement.

Cosolvent

In chemistry, cosolvents are substances added to a primary solvent in small amounts to increase the solubility of a poorly-soluble compound. Their use is most prevalent in chemical and biological research relating to pharmaceuticals and food science, where alcohols are frequently used as cosolvents in water to dissolve hydrophobic molecules during extraction, screening, and formulation. Cosolvents find applications also in environmental chemistry and are known as effective countermeasures against pollutant non-aqueous phase liquids, as well as in the production of functional energy materials and synthesis of biodiesel.

Beth L. Parker is a professor at the University of Guelph known for her research on groundwater contaminants and the remediation of groundwater systems.

References

  1. ↑ , USGS
  2. ↑ Pankow, James F., Stan Feenstra, John A. Cherry and M. Cathryn Ryan, "Dense Chlorinated Solvents in Groundwater: Background and History of the Problem" in Dense Chlorinated 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. ↑ "CLU-IN | Contaminants > Dense Nonaqueous Phase Liquids (DNAPLs) > Overview".
  6. ↑ USEPA, 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. 1 2 [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. ↑ Manuel Ramâon Llamas; Emilio Custodio, eds. (2003). Intensive Use of Groundwater: Challenges and Opportunities. CRC Press. p. 478.
  10. ↑ Vrba, Jaroslav; Adams, Brian, eds. (2008). Groundwater Early Warning Monitoring Strategy A Methodological Guide (PDF) (Report).
  11. 1 2 3 4 ITRC, 2000. "Dense Non-Aqueous Phase Liquids (DNAPLs): Review of Emerging Characterization and Remediation Technologies" http://www.itrcweb.org/Documents/DNAPLs-1.pdf
  12. 1 2 3 4 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 Archived 2006-02-18 at the Wayback Machine
  13. ↑ ITRC, 2007. In Situ Bioremediation of Chlorinated Ethene DNAPL Source Zones: Case Studies.
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