In-situ treatment destroys, neutralizes, or reduces the mass of contaminants while leaving soil in place. In-situ technologies result in limited site disturbance with limited need for excavation, treatment, and/or handling of contaminated media. This limits risks to remedial construction workers, on-site employees, and site abutters that can occur during more intrusive removal activities. Available in-situ treatment technologies are described below.
Soil Vapor Extraction
Soil vapor extraction (SVE) is a common technique for in-situ soil remediation. SVE involves the installation of a series of vertical extraction vents or “wells” in the contaminated soil above the water table and placing a negative pressure (i.e., vacuum) on wells in the vadose zone to pull contaminated soil vapors from the subsurface soils. The resulting pressure gradient in this zone induces the vapors to migrate through the soil pores toward the vapor extraction wells, volatilizing VOCs with low boiling points and low vapor pressures. The vapors are extracted from the soils and conveyed through a piping network to a treatment system, where they are commonly treated prior to discharge using either granular activated carbon (GAC) or a catalytic oxidizer. An additional benefit of SVE is the transport of oxygen to the vadose zone, augmenting natural biodegradation processes by the indigenous microbiological community.
Air sparging (AS) is the injection of air or other gases into the aquifer in an attempt to volatilize the contaminants, which then enter the unsaturated zone where they are captured via soil vapor extraction methods and treated ex-situ. Injected air traverses horizontally and vertically in channels through the soil column, creating an underground stripper that removes contaminants by volatilization. This injected air helps to flush (bubble) the contaminants up into the unsaturated zone where a vapor extraction system is usually implemented in conjunction with air sparging to remove the generated vapor phase contamination. This technology is designed to operate at high flow rates to maintain increased contact between groundwater and soil and strip more ground water by sparging. Air sparging has a medium to long duration which may last up to a few years.
Biosparging is a technique that uses the injection of air/oxygen below the water table to promote subsurface biodegradation. This alternative is similar in operation to air sparging, except that lower injection air-flow rates are used. While air sparging is intended to cause volatilization, the purpose of biosparging is solely to provide oxygen to microbes below the water table to promote aerobic biodegradation. Biosparging can be used to reduce concentrations of petroleum constituents that are dissolved in groundwater, adsorbed to saturated soils, and within the capillary fringe. Biosparging is often combined with SVE or bioventing and can also be used with other remedial technologies. As with bioventing, biosparging may be more difficult at sites with a tight formation.
Bioremediation is a managed or spontaneous process by which microorganisms, fungi, and plants metabolize pollutant chemicals to less toxic or non-toxic forms, thereby mitigating environmental contamination. An engineered bioremediation system stimulates the biodegradation of contaminants through the introduction of electron acceptors (typically oxygen), electron donors (substrates or food source), nutrients, or microbes that are acclimated to the contaminated soil or groundwater. These amendments are either introduced to the subsurface in situ or are added to extracted groundwater or excavated soil. Microorganisms may flourish with lower concentrations of VOCs and semi-volatile organic compounds (SVOCs) but tend to diminish with high VOC or SVOC concentrations due to the toxicity of these chemicals.
Anaerobic bioremediation (reductive dechlorination) is a commonly applied treatment method that has been demonstrated effective for chlorinated solvents in the industry. The biology/chemistry of reductive dechlorination reactions is well understood and can often result in a very rapid decrease in contaminant concentrations. This technology provides remediation of chlorinated solvents by amending the groundwater to create reducing groundwater conditions for reductive dechlorination by bacteria. Naturally occurring microorganisms create hydrogen, which replaces chlorine on chlorinated ethenes. Natural biodegradation of chlorinated ethenes can be accelerated through the addition of a carbon source (as a food source and electron donor) and/or nutrients. The effectiveness for the site depends on the ability to deliver carbon source to the location of the contaminant (as for other in situ technologies) and the ability to establish/maintain suitable reducing (“redox”) conditions. Groundwater geochemical conditions become more reducing as aerobic microbes consume available dissolved oxygen (DO) through respiration of a portion of the high concentrations of carbon added. Proprietary microorganisms could be introduced into the subsurface to encourage enhanced degradation.
Permeable Reactive Barrier
Permeable reactive barriers (PRB) involve the injection of zero-valent iron (ZVI) filings or other media placed in-situ to treat specific contaminants as a barrier technology and plume migration technology. As groundwater flows through the reactive wall, the contaminants react with the iron filings and are converted to salts and water.
Solidification/Stabilization is an in-situ technology which utilizes Portland cement, jet grouting, or another binding agent to encapsulate inorganic chemicals to prevent leaching of the inorganic chemical.
In Situ Thermal Remediation (ISTR) is suitable for treating sites with significant contaminant mass (i.e., a source zone, free product, NAPL or hot spots). The ISTR remedial approach has been proven to successfully treat a wide variety of contaminants, including chlorinated volatile organic compounds (CVOCs). ISTR removes CVOCs from subsurface soil and groundwater by raising the temperature in the subsurface to a desired temperature (typically ~100ºC or greater). ISTR can achieve up to 99% reduction in a relatively brief period of time (i.e., 6 to 12 months). As the soil is heated, CVOC are vaporized and/or destroyed by a number of mechanisms, including: (1) evaporation into the subsurface air stream; (2) steam distillation; and (3) hydrolysis. The volatilized CVOCs and evaporated water as steam are captured by a vapor recovery (VR) system or multi-phase extraction (MPE) system for above ground treatment. Extracted materials are separated in the treatment system and treated prior to discharge to the atmosphere for vapor and approved location for water (i.e., public sewer, private facility). The ISTR options include steam enhanced extraction (SEE), electrical resistivity heating (ERH) and thermal conductive heating (TCH), where SEE is generally more applicable to permeable soil types.
In Situ Soil Flushing (ISSF) involves flooding a zone of contamination with an appropriate solution to remove the contaminant from the soil. Water or a liquid solution is injected or infiltrated into the area of contamination. The contaminants are mobilized by solubilization, formation of emulsions, or a chemical reaction with the flushing solutions. After passing through the contamination zone, the contaminant-bearing fluid is usually collected and brought to the surface for disposal, recirculation, or on-site treatment and reinjection. The effectiveness of flushing with water can be limited by the solubility of the contaminant, rate-limited desorption (i.e., when desorption of the contaminant from the solid phase to the aqueous phase is slow), and the presence of low-permeability zones and other subsurface heterogeneities. Chemically enhanced flushing solutions often can be tailored to address recalcitrant contaminants, and treatability studies are conducted to determine the feasibility of the approach; however, subsurface heterogeneities not detected during characterization or considered in implementation can still limit flushing effectiveness. Flushing solutions can be water, acidic aqueous solutions, basic solutions, chelating or complexing agents, reducing agents, cosolvents, or surfactants.