Remediation and Management of Contaminated Soil - Sustainable Practices in Geoenvironmental Engineering - page 408

bedrock (USEPA, 2012a). Most fracturing for remediation purposes does not exceed depths
of 30 m. Existing issures are enlarged or new fractures are introduced—particularly
horizontally After fracturing, vapor extraction, or forced air injection is performed.
Technologies commonly used in soil fracturing include pneumatic fracturing (PF), blast-
enhanced fracturing, and Lasagna™ process (USEPA, 1996). Blast-enhanced fracturing is
used at sites with fractured bedrock formations. In the Lasagna™ process, in situ electro-
osmosis is combined with hydraulic fracturing to enhance sorption/degradation zones
horizontally in the subsurface soil. For the PF process, fracture wells in the contaminated
vadose zone are drilled and short bursts (~20 seconds) of compressed air are injected to
form fractures and repeated at various intervals. Overall, the cost range for pneumatic
fracturing has been estimated at $9 to $13 per metric ton (FRTR, 2007).
11.3.4 Soil Flushing
Soil remediation can be performed with or without excavating the contaminated soil by
soil washing or in situ lushing (Mulligan et al., 2001). Solubilization of the contaminants
can be performed with water alone or with additives. The solubility of the contaminant
is thus a key factor. Contaminants such as trichloroethylene (TCE), polycyclic aromatic
hydrocarbons (PAHs), and polychlorinated biphenyls (PCBs) are of very low solubility.
To remove nonaqueous phase liquids (NAPLs) from the groundwater, extraction of the
groundwater can be performed by pumping to remove the contaminants in the dissolved
and/or free phase NAPL zone in a pump and treat system. However, substantial periods
can be required and effectiveness can be limited. Drinking water standards of the extracted
water can be achieved after treatment with water treatment systems such activated carbon,
ion exchange, membranes, and other methods. To treat the contaminated soil, extraction
solutions can be introduced into the soil using surface looding, sprinklers, leach ields,
and horizontal or vertical drains to enhance the removal rates of the contaminants. Water
alone can be utilized for water soluble salts, and anions such as arsenate, arsenite, cyanide,
nitrate, and selenate. Surfactant or solvent solutions are utilized to solubilize and extract
the less water soluble contaminants as shown in Figure 11.3 in soil lushing. Additives can
include organic or inorganic acids or bases, water soluble solvents, complexing or chelat-
ing agents such as ethylenenitrolotetraacetic acid (EDTA), or nitrilotriacetic acid (NTA)
and surfactants.
To reduce further environmental problems due to the sorption of residuals during lush-
ing, the additives must be of low toxicity and biodegradable. Various factors including soil
pH, type, porosity, and moisture content, cation exchange capacity, particle size distribu-
tion, organic matter content, permeability, and the type of contaminants can inluence the
effectiveness of the treatment.
Highly permeable soils ( k values greater than 1 × 10 −3 cm/s) are more amenable for treat-
ment as the washing solution must be pumped through the soil by injection wells or sur-
face sprinklers or other means of iniltration. Depth to groundwater can increase costs.
The washing solution should be treated to remove and/or recover the contaminants and
reuse the water through recovery wells or drains. However, resultant spreading of con-
taminants and the luids must be contained and recaptured. Control of these iniltrating
agents may be dificult, particularly if the site hydraulic characteristics are not well under-
stood. Emissions of volatile organic compounds (VOCs) should be monitored and treated
if required. Recycling of additives is desirable to improve process economics and reduce
material use. Metals, VOCs, polychlorinated biphenyls (PCBs), fuels, and pesticides can be
removed through soil lushing.