589:
86:
407:
117:
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strips those gaseous components from the subsurface, allowing them to be extracted and treated. Less volatile compounds can have their volatility increased using the application of heat, which is then followed with SVE. Multiphase extraction involves an 18–26 inch mercury vacuum that can simultaneously extract gaseous, aqueous, and immiscible phases of NAPL. Additionally, SVE is thought to enhance aerobic degradation of NAPLs, improving cost effectiveness by reducing the amount of required above-ground treatment.
403:, aquifer testing, and remediation of various contaminants. "Pump-and-treat" is particularly effective for removing LNAPLs floating above the water table. Efforts must be taken during well drilling to minimize disturbances that might cause further infiltration of DNAPLs into the subsurface. It is easy to unknowingly drill through a DNAPL pool, causing the pool to drain down further into the aquifer.
155:
499:, and sequential aerobic and/or anaerobic biological processes to minimize the presence of NAPLs in the subsurface environment. Most bioremediation strategies rely on the presence of specific populations of bacteria/microorganisms and the addition of organic carbon to stimulate biodegradation. This organic carbon can be supplied via injection of soluble organic carbon sources such as
326:
available remediation techniques. The most mobile phases of NAPL are the volatilized/gaseous phase and the solubilized/aqueous phase, while the least mobile phases of NAPL are the adsorbed/solid phase and the immiscible liquid phase. Because of these complexities, flow is more difficult to measure in the unsaturated zone than in the saturated zone.
151:. This increased attention to groundwater contamination expanded research funds, and the studies that followed revealed widespread groundwater contamination in the United States. Subsequently, the understanding of transport mechanisms and the development of remediation strategies for organic contaminants, including NAPLs, have been expanded.
447:
saturated component of NAPL, allowing it to be displaced by continuous flooding. While surfactants are highly effective, resulting in recovery of 94% of the original DNAPL in case studies, they are also expensive and cost-prohibitive, also potentially adversely affecting the pH of the subsurface environment.
338:
In the saturated zone, the spaces between particles are filled with water. As such, a three-phase model of NAPL phase distribution is used in this zone, which excludes the gaseous phase. Once NAPLs reach the water table in the saturated zone, LNAPLs will float while DNAPLs will sink. Both LNAPLs and
446:
The purpose of surfactants is to mobilize various components of NAPLs by lowering their viscosity and interfacial tension. Solubilizing agents increase the solubility of NAPLs and transfer it to the aqueous phase, allowing it to then be extracted and treated. Mobilizing agents target the residually
385:
of the subsurface, complicating both their movement and the technology required for their mitigation. In a best-case scenario, the DNAPL is continuous and has collected as a reservoir above the impermeable layer. In this scenario, a recovery well can be drilled and installed. When it comes to DNAPL
175:
of NAPLs, which is counter-productive. While some experts have proposed that the complete removal of NAPLs from the subsurface environment is impossible, others view the challenge as an opportunity to expand and innovate remediation technologies. As a result, a variety of innovations to both detect
170:
Early remediation strategies focused on the restoration of aquifer quality via the construction of wells to extract and treat groundwater (the pump-and-treat strategy), but it soon became clear that the volume of water to be extracted and treated was unreasonably large and unfeasible. Additionally,
112:
provided a new method to detect groundwater contaminants imperceptible to the human senses. This development lead to the discovery and subsequent analysis of chlorinated solvents, one of the most deleterious forms of NAPL. It became understood that NAPLs are challenging both to detect and to remove
81:
effluent, and percolation from spills or landfills. The movement of NAPLs within the subsurface environment is complex and difficult to characterize. Nonetheless, the various parameters that dictate their movement are important to understand in order to determine appropriate remediation strategies.
103:
Groundwater has been a historically important source of water for public water systems, privately owned wells, and agricultural systems for generations. It had been commonly believed that as water traveled through soil, it was stripped of impurities before it could enter groundwater storages; as a
418:
While it is possible to study the direction and movement of groundwater flow via well drilling, this method is not always effective for determining the movement of NAPLs because they can flow in different directions. Some related strategies to determine the horizontal and vertical extent of NAPL
368:
A relatively small volume of NAPL can create toxic groundwater conditions, and NAPLs can remain in the subsurface, continually polluting groundwater, for decades or even centuries. Moreover, NAPLs are difficult to detect, particularly because of their multi-phase behavior. As a result, detection
455:
This form of remediation is possibly the most widely accepted in-situ technology for the removal of NAPLs in the unsaturated zone. Soil vacuum extraction (SVE) increases the volatility of NAPLs by using a vacuum that induces air flow. This process transforms NAPL into the gaseous phase and then
329:
Contamination of the unsaturated zone is dangerous because of both the potential to seep into the saturated zone, where aquifers are contained, and the potential to harm ecological life. Whether or not the NAPL reaches the saturated zone is determined by a parameter called residual saturation.
301:
into water, meaning that two liquid phases of NAPL (immiscible hydrocarbon and aqueous solute) can exist simultaneously. The gaseous phase of NAPLs is also responsible for the contamination of groundwater and soil; therefore, the distribution of NAPLs between its various phases is important to
437:
Because the pump-and-treat strategy involves the uptake of an unrealistically high volume of groundwater, the overall philosophy has shifted from "total capture" to containment strategies, which involve the use of physical structures to control the movement of aqueous-phase plumes. The highly
325:
This four-phase model is highly variable and can even change within a particular site during different stages of site remediation. As such, it is important to continuously monitor the phase distribution on a case-by-case basis. Each of these phases differs in terms of their mobility and their
208:
NAPLs are point-source pollutants, and they can be released from a variety of sources, including, but not limited to, improper chemical disposal, leaking underground storage tanks, septic tank effluent, and percolation from spills or landfills. Under high precipitation conditions, liquid will
372:
In order to determine site-specific characteristics e.g. soil material and water table parameters, drill cuttings and cores can be used. Soil gas surveys can be used as a preliminary screening procedure to determine the extent of contamination due to volatile components. Some of the current
107:
In 1960, organic contaminants, including petroleum hydrocarbons, coal tar derivatives, synthetic detergents, and pesticides, had been noted in an extensive literature survey of groundwater contamination that provided the first indication of NAPLs in the subsurface. By the early 1970s, the
280:
LNAPLs and DNAPLs can exist in multiple different phases simultaneously upon entering the subsurface environment. The composition of NAPLs is typically described using a multi-phase model that depends on a variety of complex and interrelated parameters, including, but not limited to,
369:
strategies, in addition to remediation strategies, are important in the effort to remove NAPLs from the environment. In this sense, it is important to quantify the geographic and phase distributions of NAPLs in addition to where they have been and where they may be going.
147:. The discovery of such high volumes of these contaminants, their widespread geographical extent, and their dangerous health effects eventually led to the passage of the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) and
296:
The liquid phase of NAPLs is characterized by a physical dividing surface that separates it from the liquid phase of water, indicating immiscibility due to NAPLs' organic structure. That said, some chemical compounds within the NAPL are capable of
475:
One challenge when it comes to chemical strategies is the existence of competitive reactions that limit treatment effectiveness. Another challenge is the presence of byproducts that might lead to the spreading of the targeted contaminant.
185:
356:. Contrarily, both the residually saturated and adsorbed DNAPL phases are relatively immobile and more difficult to remove. DNAPL movement in the saturated zone can also be influenced by anthropogenic activity, including unsealed
438:
corrosive nature of NAPLs can increase maintenance problems associated with these physical structures. Some examples of these structures include slurry barriers, vibrating beam barriers, jet grout walls, and geomembrane liners.
472:. The appropriate treatment depends largely on the specific contaminant. Chemical strategies are the most direct and fast method to remediate chlorinated solvents, which are one of the most prevalent types of NAPL.
537:
Yet another challenge is maintaining a sufficient population of bacteria/microorganisms in the face of competition from native bacteria and other external pressures. There is also regulatory pushback to the use of
339:
DNAPLs can remain in the water table for long periods of time, slowly dissolving and forming harmful chemical plumes; for this reason, remediation in the saturated zone is of particular importance to scientists.
347:
The liquid phases of DNAPLs will continue to move vertically downward through the saturated zone until either their volume is exhausted by residual saturation or their path is intercepted by the layer of low
192:
The behavior of NAPLs in the subsurface is guided by both the composition of the subsurface material and the various properties of the NAPLs. The subsurface can be categorized into two primary zones: the
546:, limiting the effectiveness of biodegradation strategies. In this sense, biodegradation may not be appropriate as a single solution, but it can certainly be used in conjunction with other strategies.
373:
strategies to detect and analyze NAPL presence include gas chromatography, high pressure liquid chromatography, and time domain reflectometry. That said, additional research in this area is warranted.
518:
and SVE. That said, the ability to supply sufficient oxygen is a limiting factor affecting the success of this type of remediation strategy. Also, many cases require the presence of inducers such as
188:
An depiction of the subsurface environment. The vadose zone and the saturated zone can be distinguished by the relative abundance of liquid water, and are separated by the capillary fringe.
318:. The rest of the space between these particles consists of air. Thus, NAPLs can either remain as an immiscible hydrocarbon, dissolve into water, adsorb onto solid porous material, or
483:. Ultimately, the most important factor that determines the viability of a chemical treatment approach is whether the subsurface conditions will allow for effective application.
113:
from the subsurface. Because NAPLs participate in a biological chain of degradation, they produce intermediate chemicals that create particularly acute dangers for human health.
162:
sites in the United States. Red sites are on the
National Priority List, yellow sites are proposed, green is deleted (usually meaning having been cleaned up). Data from
163:
85:
249:, while DNAPLs tend to sink downward and, in some conditions, pool at the bottom. Compared to LNAPLs, DNAPLs are more toxic and less biodegradable.
352:, at which point the DNAPLs will begin to migrate horizontally. if the lower permeability boundary is bowl-shaped, the DNAPL can form a pond-like
406:
468:, the most common of which include direct chemical oxidation, direct chemical reduction, secondary oxidation of reduction, and metal-enhanced
861:
718:
646:
381:
Mitigation of LNAPLs tends to be less complex and require simpler engineering strategies. Conversely, DNAPLs can seep into cracks in the
330:
Residual saturation is caused by capillary action, which immobilizes NAPLs and restricts their infiltration into the saturated zone.
89:
Underground fuel storage tank above ground. Leakage of underground storage tanks (LUSTs) are a common point-source of NAPL pollution.
176:
and mitigate NAPLs have been developed from the 1980s to the mid-2000s providing alternatives to the pump-and-treat strategy.
242:
238:
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Sufficient dissolved oxygen must be present for aerobic biodegradation, which can be supplied through strategies including
349:
269:
257:
124:
apparatus used to find unknown compounds. This type of equipment revolutionized the detection of subsurface contaminants.
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There are a variety of parameters specific to the subsurface environment that are important to consider in quantitative
82:
These strategies use NAPLs' physical, chemical, and biological properties to minimize their presence in the subsurface.
116:
70:. Strategies employed for their removal from the subsurface environment have expanded since the late-20th century.
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The microscopic properties of NAPLs determine their behavior in the field. If they enter the saturated zone, their
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quantify in order to assess the extent of contamination and to determine appropriate remediation strategies.
705:
Hemond, Harold F.; Fechner, Elizabeth J. (1 January 2023), Hemond, Harold F.; Fechner, Elizabeth J. (eds.),
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233:
relative to that of water will determine how they behave. As a result, NAPLs are categorized based on their
290:
74:
706:
534:, which are contaminants in and of themselves that are inherently harmful to the subsurface environment.
899:
842:"Chlorinated Solvent and DNAPL Remediation: An Overview of Physical, Chemical, and Biological Processes"
507:, cheese whey, etc. and placement of slow-release electron donors such as vegetable oil and soybean oil
121:
627:"Groundwater Contamination by Chlorinated Solvents: History, Remediation Technologies and Strategies"
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of the subsurface environment will determine the quantity that manages to enter the saturated zone.
539:
504:
496:
140:
109:
857:
799:
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642:
420:
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the construction of wells can be invasive to the subsurface environment and can cause deeper
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result, there wasn't much general concern about contamination of the subsurface environment.
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779:
634:
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261:
234:
38:
768:"A review of new TDR applications for measuring non-aqueous phase liquids (NAPLs) in soils"
633:, SERDP/ESTCP Environmental Remediation Technology, New York, NY: Springer, pp. 1–28,
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which consists of small particles, around which exist a thin film of water which acts as a
293:; the possible phases of NAPL include gaseous, solid, aqueous, and immiscible hydrocarbon.
543:
382:
166:
CERCLIS database, retrieved 12 February 2015 with last update reported as 25 October 2013.
135:
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Application techniques include injection via wells or the placement of a solid treatment
400:
197:, which includes small grains or particles surrounded by a thin film of water; and the
746:. Department of Civil and Sanitary Engineering, Massachusetts Institute of Technology.
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Henry, Susan M.; Hardcastle, Calvin H.; Warner, Scott D. (10 November 2002),
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Some of the purposes of well drilling include: personal use, measurements of
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These health concerns became more prevalent in the public eye after the 1976
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Liquid solution contaminants that do not dissolve in or easily mix with water
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766:
Comegna, Alessandro; Severino, Gerardo; Coppola, Antonio (1 October 2022).
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the unsaturated zone; if there is enough volume of liquid, it will then
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such as improper chemical disposal, leaking underground storage tanks,
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414:, Western Australia, one of a number dotted around the lake shore.
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268:, and ground water flow velocity. The collection of this data is
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Chemical Fate and
Transport in the Environment (Fourth Edition)
73:
NAPLs can be released into the environment from a variety of
848:, Washington, DC: American Chemical Society, pp. 1–20,
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and improperly sealed sampling holes and monitoring wells.
625:
McCarty, Perry L. (2010), Stroo, H.F.; Ward, C.H. (eds.),
201:, which contains important storages of groundwater called
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Expansion of groundwater contamination research after 1978
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of NAPL behavior. Some of these parameters include soil
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Attitudes about groundwater contamination before 1978
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remediation, the earlier it is removed, the better.
464:Chemical remediation strategies typically involve
631:In Situ Remediation of Chlorinated Solvent Plumes
743:Status of Knowledge of Ground Water Contaminants
542:bacteria. Furthermore, NAPLs may not be readily
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740:Stanley, William E.; Eliassen, Rolf (1960).
713:, Boston: Academic Press, pp. 223–316,
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306:Movement of NAPLs in the unsaturated zone
51:with water. Common examples of NAPLs are
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334:Movement of NAPLs in the saturated zone
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343:DNAPL behavior in the saturated zone
588:Huling, Scott G.; Weaver, James W.
25:
590:"Dense Nonaqueous Phase Liquids"
310:The unsaturated zone involves a
47:characterized by their relative
245:. LNAPLs tend to float on the
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217:into the saturated zone. The
108:technological development of
785:10.1016/j.envadv.2022.100296
225:Physical properties of NAPLs
639:10.1007/978-1-4419-1401-9_1
427:which uses NAPLs' relative
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854:10.1021/bk-2002-0837.ch001
425:time domain reflectometry
199:saturated (phreatic) zone
195:unsaturated (vadose) zone
31:Non-aqueous phase liquids
237:into two primary types:
272:and complex in nature.
772:Environmental Advances
451:Soil vacuum extraction
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487:Biological strategies
410:A monitoring well at
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377:Remediation of DNAPLs
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136:Niagara Falls Gazette
122:liquid chromatography
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846:ACS Symposium Series
540:genetically modified
419:presence use NAPLs'
180:Transport mechanisms
460:Chemical strategies
421:chemical properties
390:Physical strategies
322:into gaseous form.
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141:soil contamination
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110:gas chromatography
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53:petroleum products
863:978-0-8412-3793-3
720:978-0-12-822252-2
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654:13 November
442:Surfactants
266:wettability
247:water table
79:septic tank
61:chlorinated
884:Categories
869:9 November
778:: 100296.
726:28 October
599:28 October
550:References
429:electrical
423:, such as
291:volatility
287:solubility
211:infiltrate
145:Love Canal
139:report of
68:pesticides
804:2666-7657
509:emulsions
497:anaerobic
358:boreholes
354:reservoir
283:viscosity
215:percolate
160:Superfund
149:Superfund
57:coal tars
505:alcohols
320:vaporize
316:membrane
219:porosity
203:aquifers
64:solvents
532:toluene
528:ammonia
524:propane
520:methane
501:lactate
493:aerobic
231:density
158:Map of
94:History
39:organic
860:
802:
717:
645:
481:matrix
289:, and
254:models
66:, and
42:liquid
37:, are
530:, or
143:near
35:NAPLs
33:, or
871:2023
858:ISBN
800:ISSN
728:2023
715:ISBN
656:2023
643:ISBN
601:2023
241:and
18:NAPL
850:doi
790:hdl
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Text is available under the Creative Commons Attribution-ShareAlike License. Additional terms may apply.
