Institutional & Engineering Controls

Institutional actions include activity and use limitations (AULs) that restrict the use and/or activities at a Site and are recorded at the registry of deeds, which can be applied to protect public health and the environment.

Activity and Use Limitations (AUL)

Institutional controls are mechanisms to limit access to contaminated media and include alternatives such as site fencing and AULs in the form of deed restrictions. AULs are a legal tool to limit future site activities and uses in those areas that pose an unacceptable human health risk of exposure. While institutional controls do not eliminate contamination, they can provide an effective, low cost means for reducing human health exposure potential, and thus risk, in certain cases, if properly maintained and enforced.

Sub-slab Depressurization System (SSDS)

An SSDS is a system designed to eliminate or mitigate vapor intrusion into impacted structures. An SSDS creates a negative pressure field beneath the structure, inducing the flow of VOC vapors to one or more collection points, with subsequent discharge into the ambient air. The system diverts the subsurface VOC vapor migration pathway, reducing exposures to building occupants. An SSDS consists of a network of sub-slab piping that allows soil gas from the soil beneath a building to vent to the atmosphere. The building slab is typically installed with a vapor barrier which prevents contaminants from migrating into the building indoor air space. The SSDS typically is a passive system but can be supplemented with a blower to help create a negative vacuum beneath the building, specifically if a vapor barrier is not present.

Monitored Natural Attenuation (MNA)

MNA relies on naturally occurring processes such as volatilization, adsorption, dilution, oxidation, reduction, and biodegradation to reduce the mass, concentration, and/or toxicity of contaminants. Biodegradation is the transformation of organic compounds via metabolism by microorganisms. The rate of attenuation is determined by several factors related to the availability of required elements (i.e., carbon and oxygen), nutrients (i.e., nitrogen and phosphorus), and organic growth factors necessary for the growth of the microbial population. MNA is considered a passive remedial technology in that no active remediation is performed. Often, the rate and progress of natural attenuation is assessed via routine soil and/or groundwater monitoring (i.e., monitored natural attenuation [MNA]); such monitoring may include the assessment of surrogate indicators of attenuation processes. Under this approach, periodic monitoring would be conducted to assess the natural reduction in contaminant concentrations and to monitor potential migration.

Containment Technologies

The primary purpose of containment technologies is to isolate contaminated media and thus control potential exposure risks. Passive containment involves the placement of physical barriers to limit the potential for exposure to contaminated soil. Both engineered and non-engineered barriers are commonly used passive containment technologies.

Horizontal Engineered Barrier

Horizontal engineered barrier designs include impermeable cap barriers, which typically consist of a layer of asphalt pavement, concrete, or natural low permeability material such as clay. Impermeable cap barriers limit surface water infiltration into the contaminated media, thereby limiting the potential for contaminant migration. An engineered barrier, as defined in 310 CMR 40.0996, is a cap barrier that meets the following specifications: • Prevents direct contact with contaminated media; • Controls any vapors or dust emanating from contaminated media; • Prevents erosion and any infiltration of precipitation or run-off that could jeopardize the integrity of the barrier or result in the potential mobilization and migration of contaminants; • Is comprised of materials that are resistant to degradation; • Is consistent with the technical standards of RCRA Subpart N, 40 CFR 265.300, 310 CMR 30.600 or equivalent standards; and • Includes a defining layer that visually identifies the beginning of the barrier.

Physical barriers (or slurry walls)

Slurry walls or physical barriers are used to contain or isolate contaminated groundwater. These subsurface barriers consist of a vertically excavated trench that is filled with slurry. The slurry hydraulically shores the trench to prevent collapse and forms a filter medium which inhibits groundwater flow. Slurry walls often are used where the waste mass is too large for treatment and where soluble and mobile constituents pose an imminent threat to off-Site receptors. Slurry walls represent a full-scale technology that has been used for decades as a long-term solution for controlling seepage. They are often used in conjunction with horizontal barriers. Containment of groundwater requires a slurry wall or sheet piling encircling the source area and keyed into a confining layer so as to prevent groundwater above the confining layer from flowing into or out of the Site.

In-Situ Technologies

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

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

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

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.

Reductive Dechlorination

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

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.

Thermal Remediation

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.

Soil Flushing

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.

Ex-Situ Treatment Technologies

Ex-situ technologies that treat contaminated soil onsite, including biological, physical, and chemical technologies, would involve the excavation and on-site management and treatment of contaminated soil. Ex-situ treatment of groundwater requires groundwater to be pumped to the surface and subsequently treated using biological, physical, or chemical technologies. Following treatment, the water may be discharged back into the aquifer using injection wells or leach fields, discharged to surface water, or discharged to a storm or sanitary sewer system. Ex-situ removal of groundwater may be accomplished with groundwater depression pumps or dual phase extraction and may be used for groundwater source remediation or to contain a plume and prevent off-site migration.

Groundwater Pumping and Treatment

Pumping and treatment is a technique that withdraws groundwater (and also LNAPL, if present) from the subsurface with the intent of establishing hydraulic control, recovering LNAPL that may be present, or the capturing groundwater impacted by dissolved petroleum constituents. In the case of groundwater pumping, dissolved constituents are typically removed from the extracted groundwater using activated carbon or an air stripper. Where present, LNAPL is removed and pumped to a holding tank for subsequent off-site disposal. Treated groundwater may be discharged to a local sanitary and/or storm water sewer system, surface water, or back to the aquifer through a recharge or infiltration gallery. Groundwater/LNAPL Recovery involves placement of one or more recovery wells to facilitate extraction of groundwater directly from the subsurface and into a holding (frac) tank via a submersible pump. The frac tank acts as a settling tank to allow the sediments and particulate matter to settle to the bottom of the tank while product will be allowed to float to the surface where it can be recovered via skimmer. The water is then pumped from the frac tank via a submersible pump through bag filters (in series) to remove additional particulate matter and then through two (2) liquid phase granular activated carbon units (LGAC) piped in series, prior to being discharged to stormwater.

High Vacuum Extraction

High Vacuum Extraction (HVE) is a technology that is effective at remediating both dissolved and separate phase constituents in both saturated and unsaturated zone soils and groundwater. HVE is performed by applying a vacuum to selected wells for the purpose of simultaneously extracting groundwater, NAPL, and vapor. Contaminants are removed through a combination of mechanisms: (1) groundwater and NAPL are removed directly under vacuum; and (2) once the water table has been lowered, contaminants in the unsaturated zone soils and dewatered saturated zone soils are removed via volatilization. HVE is more effective in soils that have a lower permeability.

Excavation and Offsite Disposal/Recycling

Excavation and off-site disposal involve the physical removal and treatment, or off-site disposal, of impacted soils. This remedial technology is often used at sites where significant volumes of contaminants are present in soil located near the surface, and which are likely to be a continuing source of contaminant migration. Commonly, excavation is performed prior to or while implementing other remedies at a site. Excavation is used to remove/control the source of contamination, so that VOC constituents will not continue to migrate to the vadose zone or underlying groundwater. Site-specific characteristics, such as the presence of above ground or below ground obstructions, dictate the implementation of excavation. Locations where underground utilities or storage facilities exist may require extensive and time-consuming exploratory excavation or hand-digging. Excavation around or near buildings may require the use of underpinning or sheet piling to stabilize the structure and re-routing utility lines. Shoring and sloping may be required in sandy soil to maintain trench wall stability. Monitoring for air quality may also be required during excavation. Excavation equipment ranges from hand tools, such as pickaxes and shovels, to backhoes, front-end loaders, clamshells, draglines, depending on the amount of soil to be excavated, the total depth of the excavation, moisture content of the soil, and the space allowed for staging of excavation material. Backhoes and front-end loaders are the most commonly used equipment for excavation of shallow soils (less than 15 feet below grade).

Excavation and Ex-Situ Bio-Cell

Excavation and Ex-Situ Bio-cell involves excavation of significant quantities of OHM and LNAPL impacted soils followed by placement of the soils within an enclosure which allows for the introduction of ambient air or oxygen into the enclosure to promote biodegradation and volatilization of contaminants from the soils.

Excavation and Ex-Situ Surface Bioremediation

Excavation and Ex-Situ Surface Bioremediation involves the excavation and subsequent spreading of OHM and LNAPL-impacted soils onto polyethylene sheeting followed by the periodic mixing of the soils to promote volatilization with the option of adding bacterial enhancements.