Groundwater Remediation
Groundwater remediation or clean-up is required when concentrations of contaminants exceed or are expected to exceed predetermined levels for the type of resource that is impacted (Grossman et.al., 2008). For example, lead levels in drinking water should not exceed the EPA action level of 0.015 mg/L. What caused the high levels of lead? How can the lead be removed from the aquifer? (Grossman et.al., 2008).
(Source: Wikipedia-Groundwater Remediation) - 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. The many and diverse activities of man produce innumerable waste materials and by-products; before the 1980s, the regulation of these wastes was less stringent and waste materials were often disposed of or stored on land surfaces where they percolated into the underlying soil and eventually were carried downward, contaminating the underlying groundwater and therefore jeopardizing the natural quality of it. As a result, contaminated groundwater became unsuitable for use. Current practices can still impact groundwater, such as the over application of fertilizer or pesticides, spills from industrial operations, infiltration from urban runoff, and leaking from landfills. Using contaminated ground water causes hazards to public health through poisoning or the spread of disease, and the practice of groundwater remediation has been developed to address these issues. Contaminants found in ground water cover a broad range of physical, inorganic chemical, organic chemical, bacteriological, and radioactive parameters. Pollutants and contaminants can be removed from ground water by applying various techniques thereby making it safe for use.
Content Table
Overview
Contaminated groundwater is a cleanup problem at most Superfund sites, virtually all sites with leaking underground storage tanks, and many sites designated for cleanup under the Resource Conservation and Recovery Act (RCRA) (see Figures 1 and 2) (USEPA, 2011a). It is now known that remediation efforts are taking much longer than originally anticipated (USEPA, 2011a). Once a source has been found, the most important first step toward remediation is to remove the source if feasible (Grossman et. al., 2008). Removal often involves excavation of leaky tanks and contaminated soil and employing soil remediation such as soil vapor extraction. Once the source is removed, the next step is to clean up contaminated water still in the ground (Grossmann et. al., 2008).
Dense Non-Aqueous Phase Liquid, DNAPL
DNAPLs have a relatively low solubility, a high specific gravity, a tendency to remain sorbed to organic materials in an aquifer and are not readily degraded (USEPA, 2011b). This makes DNAPLs difficult to locate and characterize in the subsurface. DNAPLs can migrate deep through the saturated zone in the subsurface and leave a trail of hydraulically trapped organic liquid (USEPA, 2011b).
Liquid Non-Aqueous Phase Liquid, LNAPL
An LNAPL is one of a group of organic substances that are relatively insoluble in water and are less dense than water (USEPA, 2010). LNAPLs, such as oil, tend to spread across the surface of the water table and form a layer on top of the water table (USEPA, 2010).
Figure 1. Example of Fate and Transport of LNAPL Contaminants through an Aquifer (Source: Grossman et.al., 2008)
Figure 2. DNAPL sinks in the bottom (Source: Grossman et.al., 2008)
Groundwater Remediation Techniques
Selection of the remediation method to be used in the event that human intervention is to be implemented starts with the consideration of applicable process options (Hyman et.al., 2001). Most Technologies used for groundwater remediation are as follows:
- Groundwater pump and treat
- Air sparging
- Permeable reactive barrier
- In-situ chemical oxidation
- Intrinsic and enhanced bioremediation
- Natural attenuation
- Phytotechnologies (or phytoremediation)
- Bioslurping
Groundwater Pump and Treat
Pump and treat involves pumping out contaminated groundwater with the use of a submersible or vacuum pump, and allowing the extracted groundwater to be purified by slowly proceeding through a series of vessels that contain materials designed to adsorb the contaminants from the groundwater (City Chlor, 2011). For petroleum-contaminated sites this material is usually activated carbon in granular form (City Chlor, 2011). Chemical reagents such as flocculants followed by sand filters may also be used to decrease the contamination of groundwater (City Chlor, 2011). Groundwater is the water that has collected underground in the spaces between dirt particles and crack within rocks (USEPA, 2001). Groundwater flows underground and may empty into rivers or lakes. Many people rely on groundwater as the source of their daily water needs (USEPA, 2001). According to United States Environmental Protection Agency (USEPA; 2001), pump and treat is quite safe when designed and operated properly. Since the polluted groundwater is pumped directly into holding tanks and from there into the treatment system, no one comes in contact with any harmful chemicals (USEPA, 2001). The harmful chemicals are destroyed or removed and disposed of properly (USEPA, 2001).
Initially, the objectives of P&T applications were to reduce contaminant concentrations at levels not exceeding those of drinking water (Voudrias, 2001). It was not known, however, whether P&T is capable of achieving such objectives. Real world experience shows that groundwater remediation by P&T to drinking water standards may be accomplished, perhaps only for areas with simple geology and relatively simple contamination scenarios (Voudrias, 2001). In general, the effectiveness of pump and treat systems can be compromised by a number of factors that are related to the contaminants of interest and the characteristics of the site (Voudrias, 2001). As a result, it is, usually, impossible to reduce dissolved contaminants to below drinking water standards in reasonable time frames, e.g., less than 10 years at many sites (Mackay and Cherry, 1989; EPA, 1994a).
Air Sparging
Air sparging (AS) is an in situ remedial technology that reduces concentrations of volatile constituents in petroleum products that are adsorbed to soils and dissolved in groundwater (see Figure 3) (USEPA, 1994b). Air sparging is the process of injecting air directly into groundwater (CPEO, 2011a). Air sparging remediates groundwater by volatilizing contaminants and enhancing biodegradation. It is akin to blowing bubbles from a straw into a bowl of water (CPEO, 2011a). As the bubbles rise, the contaminants are removed from the groundwater by physical contact with the air (i.e., stripping) and are carried up into the unsaturated zone (i.e., soil) (CPEO, 2011a). As the contaminants move into the soil, a soil vapor extraction system is usually used to remove vapors (CPEO, 2011a).
All in situ air sparging systems should be designed and operated to optimize volatilization and biodegradation processes and to minimize the probability of adverse consequences, such as off-site migration of vapor or contaminated ground water (Callaghan et. al., 1996. Since air sparging systems are extremely site-specific, there is a high variability of design and operation from site to site (Callaghan et.al., 1996). Currently, both flow rate and well placement design parameters are based highly on operating experience (Callaghan et.al., 1996). The advantages and disadvantages of air sparging are provided in Table 1 below.
Table 1. Advantages and disadvantages (Source: USEPA 1994b)
Figure 3. Air Sparging Process Schematic (Source: Suthersan, 1999)
Permeable Reactive Barrier
Permeable Reactive Barrier uses a trench back-filled with reactive material such as iron filings, activated carbon, or peat, which absorb and transform the contaminant as water from the aquifer passes through the barrier as shown on Figure 4 (Grossman et.al., 2008). According to Grossman et.al. (2008), this works only for relatively shallow aquifers.
The most common of the permeable barrier walls is the Iron Treatment Wall. It is made up of zero-valent iron or iron-bearing minerals that reduce chlorinated contaminants such as trichloroethylene (TCE) and perchloroethylene (PCE) (CPEO, 2011b). As the iron is oxidized, a chlorine atom is removed from the compound using electrons supplied by the oxidation of iron (CPEO, 2011b). The chlorinated compounds are reduced to nontoxic by-products. Reactive walls are also used to immobilize metals such as uranium, chromium, and arsenic. A variety of materials have been used in pilot tests, including iron, peat, and bone char (CPEO, 2011b). Essentially, these materials either absorb the metals or precipitate them, similar to soil stabilization and precipitation technologies (CPEO,2011).
Figure 4 Permeable Reactive Barriers (Source: Grossman et.al., 2008)
In-Situ Chemical Oxidation (ISCO)
Remediation of groundwater contamination using ISCO involves injecting oxidants and other amendments as required directly into the source zone and down gradient plume (ITRC, 2000). The oxidant chemicals react with the contaminant producing innocuous substances such as carbon dioxide (C02), water (H20), and permanganate. Because this is an emerging technology, the number of laboratory and pilot scale tests exceeds the number of full-scale deployments (ITRC, 2000). This ratio is improving as the techniques are applied and gain acceptance (ITRC, 2000).
The following factors may limit the applicability and effectiveness of chemcial oxidation include (FRTR, 2011a) :
- Requirement for handling large quantities of hazardous oxidizing chemicals due to the oxidant demand of the target organic chemicals and the unproductive oxidant consumption of the formation.
- Some COCs are resistant to oxidation.
- There is a potential for process-induced detrimental effects. Further research and development is ongoing to advance the science and engineering of in situ chemical oxidation and to increase its overall cost effectiveness.
Figure 4. In Situ Chemical Oxidation (Source: Grossman et.al., 2008)
Intrinsic and Enhanced Bioremediation and Natual Attenuation
“Breakdown of a substance catalyzed by enzymes in vitro or in vivo. This may be characterized for purpose of hazard assessment as:
1. Primary. Alteration of the chemical structure of a substance resulting in loss of a specific property of that substance.
2. Environmentally acceptable. Biodegradation to such an extent as to remove undesirable properties of the compound. This often corresponds to primary biodegradation but it depends on the circumstances under which the products are discharged into the environment.
3. Ultimate. Complete breakdown of a compound to either fully oxidized or reduced simple molecules (such as carbon dioxide/methane, nitrate/ammonium, and water). It should be noted that the products of biodegradation can be more harmful than the substance degraded." (International Union of Pure and Applied Chemistry, 1993)
Bacteria and archaea can metabolize hydrocarbons and other contaminants, converting them to less toxic products (Grossman et.al., 2008). Specific organisms are injected into the groundwater, and in some cases, special nutrient are injected with the microbes. The method is especially useful for remediation of hydrocarbons in groundwater (Grossman et.al., 2008).
Natural bioremediation occurs when naturally occurring bacteria living in the aquifer degrade toxic contaminants into less toxic compounds (see Figure 6)(Grossman et.al., 2008).
Natural attenuation is not a "technology" per se, and there is significant debate among technical experts about its use at hazardous waste sites (FRTR, 2011b). Consideration of this option usually requires modeling and evaluation of contaminant degradation rates and pathways and predicting contaminant concentration at down gradient receptor points, especially when plume is still expanding/migrating (FRTR, 2011b). The primary objective of site modeling is to demonstrate that natural processes of contaminant degradation will reduce contaminant concentrations below regulatory standards or risk-based levels before potential exposure pathways are completed (FRTR, 2011b).
Figure 6. Bioremediation (Source: Grossman et.al., 2008)
Phytotechnologies
Phytotechnology is broadly defined as the use of vegetation to address contaminants in soil, sediment, surface water, and groundwater (see Figures 7 and 8) (USEPA, 2011c). Cleanup objectives for phytotechnologies can be contaminant removal and destruction, control and containment, or both (USEPA, 2011c). Phytoremediation (i.e., contaminant removal and destruction) is a phytotechnology subset (ITRC, 2009).
Six major plant mechanisms enable phytotechnologies to remove, destroy, transfer, stabilize, or contain contaminants (USEPA, 2011c):
- Phytoextraction
- Phytodegradation
- Phytovolatilization
- Rhizodegradation
- Phytosequestration
- Phytohydraulics
Figure 7 Phytoremediation (Source: USDA, 2011)
Limitations in using phytoremediation include the length of time required for remediation, pollutants at a level tolerable for the plants used, bio-availability of pollutants, and the level of cleanup required (USDA, 2011). Consult with appropriate environmental professionals to design an effective system (USDA, 2011).
Key Design Considerations (USDA, 2011):
- Select vegetation that is fast growing, easy to maintain, and capable of transforming the pollutants to a non-toxic form.
- May need to conduct screening studies and field plot trials to determine suitable plants.
- Avoid monocultures to reduce risk to disease and pests.
- Pollutants need to be within the upper rooting zone. Plants with different rooting types and depths may be used together to treat a greater soil depth. A fibrous root system is usually the most efficient.
- Determine and mitigate potential exposure risks for wildlife.
- Harvesting vegetation and proper disposal may be necessary.
Figure 8. Application of Phytoremediation (Chevron Corporation, 2010).
Bioslurping
Bioslurping is the adaptation and application of vacuum-enhanced dewatering technologies to remediate hydrocarbon-contaminated sites (see Figure 9) (FRTR, 2011c). Much like a straw in a glass draws liquid, the pump draws liquid (including free-product) and soil gas up the tube in the same process stream (CPEO, 2011c). Pumping lifts light non-aqueous phase liquids (LNAPLs), such as oil, off the top of the water table and from the capillary fringe (i.e., an area just above the saturated zone, where water is held in place by capillary forces) (CPEO, 2011c). Bioslurping can improve free-product recovery efficiency without extracting large quantities of ground water(FRTR, 2011c). In bioslurping, vacuum-enhanced pumping allows LNAPL to be lifted off the water table and released from the capillary fringe (FRTR, 2011c).
Figure 9. Bioslurping (Source: Anderson Group, 2011)
Related Articles
Groundwater and groundwater chemistry
Radiological Clean-up
Cryogenic Process for Soil and Surface Water Remediation'
Further Reading
1. Groundwater Remediation Technology Overview
2. Guidance for Design, Installation and Operation of In Situ Air Sparging System
3. Permeable Reactive Barriers: Lessons Learned/New Directions
4. Permeable Reactive Barrier Technology for Contaminant Remediation
6. Monitored Natural Attenuation of Petroleum Hydrocarbons
8. A Citizen's Guide to Bioremediation
11. Phytoremediation and Hyperaccumulator Plants
12. Phytotechnologies for Site Cleanup
14. Bioslurping
15. Bioaugmentation for Remediation of Chlorinated Solvents
16.New Approaches for Bioaugmentation as a Remediation Technology
17. BioAugmentation: An effective method for Reducing Contaminant Concentration
18. DNAPL
19. LNAPLs
21. The Basic Understanding of LNAPL
22. DNAPL Remediation Selected Projects Approaching Regulatory Closure
23. DNAPL Source Reduction: Facing Challenge
24. Technology Profile: Vacuum Mediated LNAPL Free Product Recovery
References
Anderson Group. 2011. Bioslurping. http://www.threer.co.uk/services_remediation_bioslurp.php (Accessed on July 23, 2011)
Callaghan, D., Blanchard, E. and A. Reif. 1996. Air Sparging. http://www.cee.vt.edu/ewr/environmental/teach/gwprimer/airsparg/airsparg.html (Accessed on July 22, 2011)
Center for Public Environmental Oversight (CPEO). 2011a. Air Sparging. http://www.cpeo.org/techtree/ttdescript/airspa.htm (Accessed on July 23, 2011)
Center for Public Environmental Oversight (CPEO). 2011b. Permeable Reactive Barrier. http://www.cpeo.org/techtree/ttdescript/permbarr.htm (Accessed on July 23, 2011)
Center for Public Environmental Oversight (CPEO). 2011c). Bioslurping.http://www.cpeo.org/techtree/ttdescript/bislurp.htm (Accessed on July 23, 2011)
Chevron Corporation. 2010. Guadalupe Restoration Project.http://www.guaddunes.com/northsouth.html (Accessed on July 23, 2011).
City Chlor. 2011. Remediation Technique Pump and Treat : How does it work? http://www.citychlor.eu/faq/remediation-technique-pump-and-treat-how-does-work.htm (Accessed on July 22, 2011)
Federal Remediation Technologies Rountable (FRTR). 2011a. Chemical Oxidation. http://www.frtr.gov/matrix2/section4/4_4.html (Accessed on July 23, 2011)
Federal Remediation Technologies Roundtable (FRTR). 2011b. Natural Attenuation. http://www.frtr.gov/matrix2/section4/4-32.html (Accessed on July 23, 2011)
Federal Remediation Technologies Roundtable (FRTR). 2011c). Bioslurping. http://www.frtr.gov/matrix2/section4/4-35.html (Accessed on July 23, 2011).
Grossman, E. and J. McGuire. 2008. Groundwater Remediation. http://oceanworld.tamu.edu/resources/environment-book/groundwaterremediation.html (Accessed on July 22, 2011).
Hyman, M., and R. Dupont. 2001. Groundwater and Soil Remediation: Process Design and Cost Estimating of Proven Technologies. American Society of Civil Engineers Press.
International Union of Pure and Applied Chemistry, 1993, Glossary for chemists of terms used in toxicology: Pure and Applied Chemistry, v. 65, no. 9, p. 2003-2122.
Interstate Technology Regulatory Cooperation (ITRC). 2000. Technical & Regulatory Guidance: In Situ Chemical Oxidation. http://redox-tech.com/ITRC%20Chemox.pdf (Accessed on July 23, 2011)
Interstate Technology Regulatory Cooperation (ITRC). 2009. Phytotechnology Technical and Reglatory Guidance and Decision Tree, Revised. http://www.itrcweb.org/Documents/PHYTO-3.pdf (Accesed on July 23, 2011).
Mackay, D. and J.A. Cherry. 1989. Groundwater Contamination: Pump and Treat Remediation, Environmental Science and Technology, 23, 630-636.
S. Suthersan. 1999. Remediation Engineering: Design Concepts-"In Situ Air Sparging". CRC Press. http://www2.bren.ucsb.edu/~keller/courses/esm223/SuthersanCh04AirSparge.pdf (Accessed on July 23, 2011)
United Stated Department of Agriculture (USDA). 2011. http://www.unl.edu/nac/bufferguidelines/guidelines/3_productive_soils/5.html (Accessed on July 23, 2011)
United States Environmental Protection Agency. 1994a. Methods for Monitoring Pump-and-Treat Performance, EOA/600/R-94/123, Washington DC.
United States Environmental Protection Agency. 1994b. Air Sparging. http://www.epa.gov/oust/pubs/tum_ch7.pdf (Accessed on July 23, 2011).
United States Environmental Protection Agency. 2001. A Citizens Guide to Pump and Treat. http://www.clu-in.org/download/citizens/pump_and_treat.pdf (Accessed on July 22,2011).
United States Environmental Protection Agency. 2010. Waste and Cleanup Risk Assessment Glossary. http://www.epa.gov/oswer/riskassessment/glossary.htm (Accessed on July 23, 2011).
United States Environmental Protection Agency. 2011a. Groundwater Cleanup .http://www.epa.gov/ord/lrp/quickfinder/gw-cleanup.htm (Accessed on July 22, 2011).
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United States Environmental Protection Agency. 2011c. Phytotechnologies. http://www.clu-in.org/techfocus/default.focus/sec/Phytotechnologies/cat/Overview/ (Accessed on July 23, 2011).
E.A. Voudrias. 2001. Pump-and Treat Remediation of Groundwater Contaminated by Hazardous Waste: Can It Really be Achieved? http://www.gnest.org/journal/Vol3_No1/voudrias.pdf (Accessed on July 22, 2011).
Links
1. Groundwater Cleanup: Overview of Operating Experience at 28 Sites
