Soil Flushing Figure scanned from USEPA 1998 document – 1998 revised guidance document for the remediation of contaminated soils; cites origin of figure as USEPA (1991). Engineering Bulletin. In Situ Soil Flushing. EPA/540/2-91/021.
Soil Flushing - Uses • Soil flushing is most often used for the extraction of inorganic pollutants, particularly relatively soluble metals (e.g., hexavalent Cr). • It can also be used for the extraction of pesticides, fuels, and volatile organic contaminants, although more cost effective methods are likely to be available for these substances. • The effectiveness of the method is closely related to the hydrologic conditions at the site. • It is most effective where the extracting fluids can be continually cycled through the subsurface with little, if any, loss to the surrounding environment. • Effective circulation generally requires sediment that exhibits a permeability in excess of 1 x 10-3 cm/sec.
Figure 4, page 17 in Iskandar, I.K. and Adriano, D.C. (1997) remediation of soils contaminated with metals – a review of current practices in the USA. In: Iskandar, I.K. and Adriano, D.C. (eds.), Remediation of Soils Contaminated with Metals. Science Reviews, Northwood, UK. Pp. 1-26.
Soil Flushing • The most significant advantage of soil flushing is that the contaminants can be permanently removed from the sediment without needing to remove the surficial materials, or other wise significantly disturb the site’s surface. • Most significant limitation is that if the hydrologic system is not adequately understood, or if the extraction methods fail to recover all of the circulating fluids, solubilized contaminants can be leached into the groundwater or river system, thereby increasing the area of contamination.
Electrokinetic Separation is a treatment process in which one or more electrode pairs are inserted into the ground and a low-intensity direct electrical current is applied between them to mobilize contaminants in the form of charged particles. • The method has also been referred to as electroreclamation, electromigration, and electrokinetic soil processing. • It can be used to separate and remove a wide range of contaminants from sediment and soils including metals, radionuclides, and some forms of polar organic pollutants (Evans 1997).
Electrokinetic Remediation In: Innovative In Situ Cleanup Processes (1992) The Hazardous Waste Consultant, Sept./Oct.;
Advantages/Limitations • Its primary limitations are that is expensive to install, requires constant maintenance, and tends not to remove metal precipitates from the sediments unless some form of extracting agents are used • It may represent an effective alternative for silt- and clay-rich deposits of low-permeability which cannot be effectively remediated using other in situ methods. • To date, its application has been limited, although the method has been successfully applied at number of demonstration sites
In Situ Soil Capping • in situ capping as “the controlled, accurate placement of a clean, isolating material cover, or cap, over contaminated sediments without relocating or causing a major disruption to the original [channel] bed” • In theory, the cap is intended to: • (1) physically isolate the contaminated materials from direct contact with biota, including burrowing organisms, • (2) stabilize the contaminants in place to inhibit their remobilization by processes of erosion, and • (3) limit the potential remobilization of the contaminants in the dissolved phase by geochemical processes. • As of 2004, in situ capping had been selected as part of the remedial strategy at approximately 15 marine and freshwater Superfund sites in the U.S. (USEPA 2005), and had been applied to about the same number of sites elsewhere.
In Situ Capping • Several perceived advantages over sediment removal. • (1) Unlike dredging or excavation activities, resuspension is limited, and there is no residual contamination left along the water-sediment interface. Bottom-dwelling organisms are therefore immediately supplied with a clean substrate to recolonize, provided that the cap will not be re-contaminated by the transport and deposition of sediment-borne contaminants from upstream areas. • (2) It limits the potential impacts on local communities as there is no need to develop sediment staging or treatment facilities, nor is there a need to transport contaminated materials through residential environments. • (3) It can be implemented quickly using conventional equipment and locally available materials. Thus, capping tends to be one of the least expensive remedial technologies for addressing contaminated channel bed sediment • Primary problem is finding a large source of suitable capping material. Can also be rather expensive.
In Situ Capping - Limitiation • Where capping has been applied, it is often used as a further risk reducing practice following the removal of highly contaminated sediment, or at sites where dredging activities had difficulties in meeting the cleanup standard. • The primary limitation of in situ capping is that the contaminants remain in the aquatic environment, and if the cap is breached, they could become exposed and dispersed. • It is possible that contaminants in the dissolved form will migrate through the cap in significant quantities. Thus, the method lends itself to questions about its ability to provide a permanent solution to the problem, particularly in highly dynamic rivers.
Cap Designs • In situ caps can consist of a single layer or of several layers in which each horizon performs a specific function. • Commonly used horizons include: • (1) a layer of rocks or gravel at or near the top of the cap to armor it against erosion, • (2) a sandy layer which serves as a relatively stable substrate that resists burrowing and bioturbation, • (3) a fine-grained lower layer which promotes chemical isolation through sortion processes, and • (4) geotextiles that limit burrowing and material mixing
From Miller JR and Orbock Miller, SM, 2007, Contaminated Rivers, Springer
Soil and Sediment Capping • Similar to subaqueous capping • A perfectly designed cap is intended to: • (1) physically isolate the contaminated materials from direct contact with biota, • (2) stabilize the contaminants in place to inhibit their remobilization by wind or water, and • (3) limit the potential for contaminant remobilization by geochemical processes by reducing the infiltration of water to the subsurface. • A variety of materials have been used for soil and sediment capping. • They include all of those cited for in situ capping, plus such materials as asphalt and concrete.
Effective Use • Economically, soil and sediment capping is most applicable over relatively small areas. Thus, it is often utilized where there is a high potential for humans or other biota to come into contact with the contaminated particles. • Its application becomes less attractive over large areas, such as contaminated floodplains and terraces, where it is likely to be hindered by a lack of available capping materials near the site.
In situ solidification and stabilization • In situ solidification and stabilization is intended to decrease contaminant mobility and solubility by mixing soils and sediment with a binding agent that • (1) encases the material in a stabilized mass (solidification), or • (2) reacts with the material and any remaining water to reduce the solubility and mobility of the trace metals through changes in material chemistry (stabilization) • In most cases, the process reduces permeability of the contaminated sediment and, therefore, their contact with groundwater. • The binding agents that are used in the process are typically applied either by means of an injector system, or through the use of an auger which mixes the reagents in place.
In Situ Vitrification Figure scanned from USEPA 1998 document – 1998 revised guidance document for the remediation of contaminated soils; cites origin of figure as USEPA (1992). EPA Handbook: Vitrification Technologies for Treatment of Hazardous and Radioactive Waste. EPA/625/R-92/002.
Phytoremediation • Phytoremediation, or the use of plants to remove or contain contaminants in soil, sediment, and water • phytoextraction, which is intended to remove contaminants from sediment or soil, • phytostabilization, a process that retards the mobility of the contaminants in sediment and soil • rhizofiltration, a technique aimed at removing contaminants from surface- and groundwaters. • The current interest in these methods is primarily related to their ability to be applied to large areas with low-levels of contamination at a relatively low-cost. • Phytoremediation is an in situ treatment that does not require extensive disruption of the site. • It can be used as a means of ecological restoration and to prevent erosion, depending on the method(s) that are applied, while transforming the area into an aesthetically pleasing site
Plant Characteristics for Successful Project • Huang (2000) suggests that to be effective, the plant must be able to accumulate more than 1 % of the metal of interest, while generating at least 20 metric tons of aboveground biomass per hectare per year. • Many of the currently investigated species represent high biomass agronomic crops (that can be planted, grown, and harvested using conventional agricultural practices. • Fast growth with large biomass; • Suitable plant phenotype for easy harvest, treatment and disposal • Tolerance to site conditions.
Limitations • Its applicability, then, depends heavily on the time required for contaminant extraction by the plants, which in turn depends on the difference between the metal concentration in the sediment and the targeted level of cleanup (i.e., the remedial standard). • phytoextraction is likely to be most applicable to sites where it is not necessary to remove large quantities of metals from the soil or sediment, including sites covering vast areas.
Other Considerations • Commonly apply chemicals to increase the accumulation of the metals. Referred to as induced hyperaccumulation. Chemicals include EDTA, DTPA, and HEDTA. • Problem is that these chemicals may allow the metals to be leached from the soils and enter the groundwater system or migrate offsite. • Produces Ash/sludge that is highly concentrated in metals that must be disposed of in hazardous waste landfill; • The primary problem with this method is that it may require 100s to 1000s of years to decontaminated a heavy contaminated site using this method.
Phytostabilization • In situ metal inactivation through the use of revegetation practices. • This may or may not involve the use of soil/sediment additives. • Commonly, phytostabilization utilizes basic farming practices, using similar equipment, planting and cropping techniques Vangronsveld and Cunningham, 1998). • Primarily used for the reclamation of remediation of mining and milling wastes, or around areas devegetated by the effects of smelting (around smelters e.g., Copper Basin).
The Plants Perform Four Principal Functions • Protect the soil from wind and water erosion; • Reduce water percolation (leaching) through the soil (because more evapotranspiration occurs) to prevent leaching. • They may also stabilize the contaminants by accumulating and precipitating heavy metals in the roots, or by adsorption on root surfaces. • It is also possible that they will change soil characteristics by adding organic matter to the soil, or by changing pH.
Advantages and Disadvantages of Phytostabilization • Relatively easy to perform, with low cost ($0.20 to $1 per m3). • Can be applied to clay rich soils which are often difficult to remediate. • The method produces an aesthetically pleasing site during and following the remediation program and helps to restore a healthy ecosystem. • Negative is that it is not readily accepted as a remediation alternative.
Soil Additives • Assumption is that it is possible to reduce the mobility of various metals by changing the environmental conditions with which they are associated, or by change the sorption potential of the sediments • Both of the above can be done by adding various substance to the sediments/soils
Soil Additives • The practicality of using these methods depends on the chemical form (species) in which the metal occurs.
Types of Soil Additives • Those that increase pH include lime, gypsum, cement, wood ash, and coal fly ash. The latter two also have some sorption potential. • Those that increase sorption potential include Fe or Mn oxides (commonly as steel shot), or aluminum compounds including concentrated Al3+.
Types of Soil Additives • The CEC can be increased by adding mixed layer clays to the sediment such as montmorillonite or illite, or by adding zeolites. • The organic matter content of the soil or sediment can be increased by adding various materials rich in humic or fluvic acid or humus including manure, sludges, composts, or mulch.
Monitored Natural Recovery • Monitored natural recovery (MNR) relies on naturally occurring processes that contain, destroy, or reduce the bioavailability or toxicity of contaminated sediment • Similar to the no-action alternative as contaminated sediment is neither removed nor treated in any way. • However, it is not equivalent to the no-action approach • MNR, assumes on the basis of historical data and a variety of predictive methods that • (1) site recovery is currently underway as a result of natural processes, and • (2) the rate of recovery will result in the desired outcomes in an appropriate period of time. • MNR may be accompanied by an adaptive management approach where if continued monitoring of key indicators determines that natural recovery is not occurring, or not occurring at an acceptable rate, then other remedial technologies can be applied.
Means of Initiating Natural Attenuation • The natural processes which promote recovery of the aquatic environment can broadly be placed into three categories: • Physical • chemical, and biological. • All three of these process types are interconnected. • The outcome of these interactions may actually increase the environmental risks, rather than reduce it (as would be the case if the process were operating in isolation)
Physical Process Important to MNR • Physical processes which are commonly cited as being of interest to MNR include sedimentation, erosion, dispersion, bioturbation, advection, and volatilization. • Sedimentation, in this case, is envisioned to bury contaminated materials with clean or cleaner materials, thereby containing the contaminants in place, and reducing the potential for biota to come into contact with the more heavily contaminated sediment. • Dilution also important