Low permeability soil

The stationary in situ steam extraction system uses injection wells to introduce the steam, and recovery wells for removing it. Soil permeability is a major factor. Low-permeability soils require a far greater number of wells compared to high-permeable soil, driving up costs. To be effective... 

The U.S. EPA guidance6 discusses three types of liners FMLs, compacted clay liners (CCLs), and composite liner systems (an FML overlying a compacted low-permeability soil layer). Material specifications in the guidance for FMLs and CCLs are briefly reviewed below, along with regulations regarding all three liner systems. 

For compacted, low-permeability soil liners, the U.S. EPA draft guidance recommends natural soil materials, such as clays and silts. However, soils amended or blended with different additives (e.g., lime, cement, bentonite clays, and borrow clays) may also meet the current selection criteria of low hydraulic conductivity, or permeability, and sufficient thickness to prevent hazardous constituent migration out of the landfill unit. Therefore, U.S. EPA does not exclude compacted soil liners that contain these amendments. Additional factors affecting the design and construction of CCLs include plasticity index (PI), Atterburg limits, grain sizes, clay mineralogy, and attenuation properties. 

The guidance16 recommends a three-layer cap design consisting of a vegetative top cover, a middle drainage layer, and a composite liner system composed of an FML over compacted low-permeability soil. The final cover is to be placed over each cell as it is completed. 

Apparent/actual LNAPL thickness ratios can be very high at the perimeter of LNAPL pools, notably, under low permeability conditions. Once a well is installed, it can take several months before the LNAPL migrates from the formation into the well reflecting the presence of low-permeability soils in the zone of LNAPL occurrence. As clearly shown in Figure 6.8, a well screened at the perimeter of a known LNAPL pool initially had no detectable LNAPL until 4 months after installation, whereas upon detection the apparent thickness slowly increased with time up to 15.71 ft. 

FIGURE 6.8 Graph showing gradual presence of LNAPL in well with time in low-permeability soil along perimeter of LNAPL pool. (After Testa, 1994.)... 

All passive systems rely on the natural hydraulic gradient to transport LNAPL to the recovery location. Under most circumstances, the flow of LNAPL into this type of system is very slow. At open surface recovery sites (trenches and ponds) constructed in low-permeability soils, the LNAPL migrates in so slowly that free volatile product often evaporates before it accumulates sufficiently to be collected. High-permeability soils typically are subject to a low hydraulic gradient, which limits the rate of flow into the system. Conditions that are more favorable to passive recovery, shown schematically in Figure 7.1, include ... 

S Moderate Permeability Soil (Silty Sand) t — ] Low Permeability Soil (Clay and Silt) Water Table... 

Confirmatory soil sampling was subsequently performed. Three soil borings were drilled and samples retrieved from the impacted area. Soil from a depth of 30 ft and below were reported as nondetectable however, samples from a depth of 21 ft still contained significant gasoline components with TPH-gasoline ranging up to 3600 ppm. This zone of elevated hydrocarbons was anticipated due to the presence of clays and silt at this depth. Ventilation of these low-permeability soils was not deemed cost-effective, and significant reduction of the residual hydrocarbon concentrations unlikely. 

Trial and error is often the typical procedure that is used to implement enhanced biorestoration. In simple cases of gasoline cleanups, these may be appropriate however, when the chemicals involved are recalcitrant (difficult to degrade), toxic, or present in a complex geologic environment (i.e., low-permeability soil, lateral or vertical heterogeneities, etc.), enhanced biorestoration can be difficult, and risk assessment-type analyses may be more suited for a particular site. 

Carus Chemical Company offers CAIROX potassium permanganate for the in situ remediation of volatile organic compounds (VOCs) in groundwater and soil. The method of oxidant delivery during treatment is tailored to site conditions. For unsaturated, low-permeability soils, CAIROX is introduced using deep soil mixing. In areas where the site has high permeability or the treatment media is saturated with water, well injection or recirculation can be used. 

Electroremediation is a method for the in situ removal of heavy-metal and organic compounds from low-permeability soils by the application of direct-current electric fields. These fields induce the transport of water and contaminants to wells where they are pumped to the surface. Massachusetts Institute of Technology (MIT) developed the Electroremediation technology, which is commercially available through Corrpro Companies Incorporated. 

This technology is most advantageous in low-permeability soils, such as clay, where only a limited amount of groundwater can be withdrawn by traditional pumping methods. 

This technology is not suitable for very dense, low-permeability soils and sediments. However, electrokinetic transport could be used to remediate contaminated clay formations within a more permeable aquifer. 

The BLK, in contrast to many traditional venting methods, is capable of generating a directed circulation through the source of the contamination, especially in low-permeability soils. The circulation direction is reversible and can be adjusted according to the position of the contaminant in the soil. 

The process may not be effective in low-permeability soils or in soils with high natural organic content. 

ICE technology is not designed to remove or treat chlorinated vapors that can produce an offgas stream containing hydrochloric acid. This technology does not treat nonvolatile compounds or heavy metals. Areas with low-permeability soils where minimal flow rates are expected may not be appropriate for this technology. 

Low-permeability soils are difficult to treat with soil flushing. Surfactants can adhere to soil and reduce effective soil porosity. Reactions of flushing fluids with soil can reduce contaminant mobility. 

The pneumatic soil fracturing (PSF) technology is a commercially available, in situ technology that increases the airflow in low-permeability soils, such as clay, thus increasing the amount of volatile organic compounds (VOCs) withdrawn by vacuum extraction. Additional flow paths are created by injecting compressed air into soil, creating fractures around the injection point. 

Contamination in low-permeability soil is a problem of major importance in environmental remediation. Traditional treatments of contaminated soils include bioremediation methods, vapor extraction, and what are known as pump-and-treat methods. However, poor accessibility to the contaminants and difficulties in delivering reagents used for treatment make these current in situ methods very ineffective. Electroosmosis (combined possibly with one or more of the traditional techniques) can potentially serve as an alternative in situ treatment process, as shown in Figure 12.14. 

The term aquifer is used to denote an extensive region of saturated material. There are many types of aquifers. The primary distinction between types involves the boundaries that define the aquifer. An unconfined aquifer, also known as a phraetic or water table aquifer, is assumed to have an upper boundary of saturated soil at a pressure of zero gauge, or atmospheric pressure. A confined aquifer has a low permeability upper boundary that maintains the interstitial water within the aquifer at pressures greater than atmospheric. For both types of aquifers, the lower boundary is frequently a low permeability soil or rock formation. Further distinctions exist. An artesian aquifer is a confined aquifer for which the interstitial water pressure is sufficient to allow the aquifer water entering the monitoring well to rise above the local ground surface. Figure 1 identifies the primary types of aquifers. 

Abstract Contamination in low permeability soils poses a significant technical... 

One method of transporting solutions and compounds in low permeability soils is the application of an electric current to the soil using a process called Electroosmosis (EO). EO fluid flow is a result of ions movement in the double layer of clay surfaces. For this reason, EO is ideally suited to fine-grained, clay-rich soils. The magnitude of electroosmotic... 

The extension of SVE techniques to low-permeability soils was based on monitoring the vapor recovery rate and using the results of mini-pilot tests to adjust SVE system operation. The periodic mini-pilot tests provided information from individual SVE wells, including air flow, well vacuum, and hydrocarbon concentration in the extracted vapors, that was used to balance the flows. Wells with low hydrocarbon concentrations were shut off to focus remedial efforts on the most contaminated locations, and during some periods, wells with high flows were shut off to allow a more balanced flow from low flow wells or to provide hydraulic control along the periphery of the perched groundwater contaminant plume. 

Cherepy, N.J. and Wildenschild, D. (2003) Electrolyte management for effective long-term electro-osmotic transport in low-permeability soils. Environ. Sci. Technol. 37,3024-3030. 

The model was used to study the advection and dispersion of a trace contaminant load into a low-permeability soil column. An anisotropic case was simulated. The model estimated and showed the spatial distribution of the contaminant plume and visually depicted the concentration values in grayscale. The three-dimensional visualization provided by the model was shown to be very useful for identifying the extent and severity of the soil contamination due to the trace compound load under three different types of input load distribution (point source, line source, and two-point... 

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