The model should include a description of the source of contamination and what is known about the timing and quantity of the release. Most site characterizations begin by locating areas where chemical contaminants were originally released to the subsurface. In many cases, the distinct source of contamination is known to be a former underground storage tank, disposal pit, a leaking pipeline, a spill, etc; however, many industrial source areas are dispersed and sometimes difficult to delineate. For example, oil/water separators and sanitary and storm sewers have historically received chlorinated solvents from process operations or various plant maintenance shops. At such sites, it may be impossible to pinpoint the exact source of contamination.

The timing and amount of chemical contaminants released are equally difficult to estimate. Historical records on chemical uses are sometimes difficult to obtain, and if they exist are generally found in Phase I Installation Restoration Program documents developed in the United States in the early 1980s. Many sites as examples used chlorinated solvents like trichloroethylene (TCE), which was used for decades at some operations before it was phased out in the early 1980s. Such chemicals may not have been widely used at a facility for nearly 20 to 30 years. This fact is important when evaluating the fate and transport of chlorinated solvents or any chemical contaminant and is especially important when estimating degradation rates based on the breakdown products of certain chemicals like the chlorinated solvents.

The model must include a complete description of the site geology and hydrogeology. The descriptions should include the following at a minimum:

This may need to be updated to reflect current estimates of these properties based on site remediation experience. For example, the hydraulic properties of an aquifer can be more accurately estimated after a groundwater extraction system has operated several months. On sites where natural attenuation has been selected as the groundwater remedy, tracking the movement (or stability) of the contaminant plume provides essential information that can be introduced into an updated model.

The model should include a summary of the chemical, physical, and biodegradation properties of key contaminants of concern and describe their distribution, movement, and fate in the subsurface environment. Descriptions should include the following:

Under certain conditions, geochemical parameters may favor natural biodegradation of some chemicals. Examples of these can be found with chlorinated solvents. Geochemical indicators such as dissolved oxygen, nitrate, iron, manganese, sulfate, methane, and hydrogen ion concentrations should be reported in the conceptual site model. In the case of groundwater contamination caused by chlorinated solvents, the relative distribution of primary solvents such as tetrachloroethene (PCE) and TCE and daughter products such as 1,2-dichloroethane (DCE) and vinyl chloride should be documented in relation to the geochemical profile of the site.

Important considerations for defining risk-based goals are the following:

Once a conceptual site model has been devised, defining the source of chemical contamination, potential pathways, and potential receptors, the task of defining risk-based cleanup objectives may begin. This can be approached as a two-step process involving the following actions:

A two-step approach provides flexibility to replace potentially conservative, nonsite-specific exposure assumptions with site-specific information while still providing the same level of human health and environmental resource protection. The investigator is likely to encounter increasingly complex levels of data collection and risk along the process. The evaluation will need to be performed in order to establish the type and magnitude of remediation required to reduce or eliminate unacceptable risks at a particular site. This may be accomplished by replacing nonsite-specific (ie, default) assumptions about how chemicals behave in the environment and how receptors may be exposed, with site-specific data and assumptions that are more representative of actual site conditions and realistic exposure pathways for human and ecological receptors.

A screening level evaluation provides a means of identifying whether a particular chemical warrants additional risk evaluation. Screening levels are conservative (health protective), generic cleanup criteria that define the residual amount of a contaminant that can remain onsite and not present an unacceptable risk to potential receptors. For sites with the potential for discharge to surface waters, ecological screening levels are appropriate. Industrial screening levels for human receptors are generally based on reasonable maximum exposure assumptions and can be either health protective or designed to mitigate nuisances associated with chemical contamination (eg, taste and odor). In order to select (or develop) appropriate screening levels, information about the current and potential future land and groundwater uses at/or down-gradient from the affected site must be thoroughly documented in the conceptual site model. It is common practice to consider screening levels for industrial land use scenarios over prolonged periods of say 25 years and by taking into consideration exposure to all contaminated media. Many published industrial screening levels assume ingestion of onsite groundwater by a specific receptor group (eg, industrial onsite workers). Such assumptions may or may not be realistic; however, such conservative screening levels may be appropriate if groundwater use cannot be absolutely controlled through pumping restrictions or pump and treat strategies.

Once the appropriate land use category has been defined, the types of exposure pathways to be considered in the screening evaluation should be defined. An essential step in defining appropriate screening levels is determining the risk target level. Acceptable target risk ranges for carcinogens (eg, chlorinated solvent contamination and in particular vinyl chloride) fall between 10^-6 and 10^-4.¹ The risk ranges are equivalent to an added lifetime cancer risk of 1 in 1 000 000 to 1 in 10 000 for people exposed to site contamination. Screening levels for carcinogens typically are based on an extremely protective 10^-6 target risk level that is referred to as a de minimis risk level, meaning that a 1 in 1 000 000 risk level is so small as to be of negligible concern. A 10^-6 target risk level should be considered health protective, given that the “normal background level” of cancer in the general population is about 1 in 3 persons (30%-35%). The US Environmental Protection Agency (EPA) reports that for
carcinogens, a 10^-6 target risk level for individual chemicals and pathways generally will lead to cumulative risks within the 10^-4 to 10^-6 risk range for the combinations of chemicals typically found at contaminated sites.⁴ In addition to potential human receptors, a screening level evaluation should consider potential ecological receptors and other environmental resources that could be impacted by site contaminants.⁵

Once applicable screening levels are identified, the evaluation process consists of comparing representative exposure-point concentrations from recent site sampling events to applicable screening levels. It is important to use the most recent site contamination data. It is good practice to evaluate the two most recent sampling events and a comparison of maximum detected site concentrations to applicable screening levels. The use of statistically averaged site concentrations may be appropriate at some sites.

If a discrete source zone containing DNAPL is identified and isolated, then a possible remediation strategy is the excavation of contaminated soil. Soil excavation methods are well established; after soil containing DNAPL has been removed from a source zone, it may be properly disposed, or treated—strategies include disposal in hazardous waste landfills, biodegradation cells, thermal desorption, chemical oxidation. Soils contaminated with chlorinated solvents generally cannot be disposed of in landfills unless they can be shown to be nontoxic based on the Toxic Characteristic Leaching Procedure.

Successful contaminant source removal/excavation starts with the collection of a sufficient number of soil/sediment samples to adequately characterize the extent of the contaminant source area, including variations in lithologies. A variety of equipment has been used to excavate contaminated soil or remove contaminated media. Typical machinery includes backhoes and track hoes for soil excavations and storage tank removals. Small track hoes are used inside buildings to excavate contaminated soils inside a building. Vacuum trucks are used to excavate contaminated soil, remove contaminated sediments from soakage pits and cleanout contaminated sludge and wastewater from septic tanks. Vacuum trucks can be used for shallow excavations under building floor slabs where access is limited. At some sites where remediation is performed indoors, a portion of the floor slab is cut out at contaminant source areas and the vacuum truck is parked near the facility, where the vacuum hose can access the excavation via the service door. Contaminated soil is removed down to the water table and then the excavation is backfilled with grout or sand and the floor slab restored.

Soil vapor extraction (SVE) strategies were developed to remove volatile contaminants from unsaturated soils. The SVE is a frequently used remedial strategy in situations where excavation is not feasible due to the presence of physical obstructions (eg, buildings, utilities, trees, etc), or where the extent of soil contamination is extensive. The SVE systems can be very effective at minimizing indoor vapor intrusion by keeping the contaminated soil vapors closer to the source area. The technology involves the application of a vacuum to slotted pipes in the vadose zone to draw air through contaminated material. The air flow volatilizes contaminants from the DNAPL, soil, and aqueous phases. The goal of most SVE operations at DNAPL sites is to remove sufficient contaminant mass so that water percolating through the vadose zone will no longer dissolve contaminants and transport them to the water table at concentrations exceeding regulatory limits. An SVE is capable of achieving the goal in relatively homogeneous, coarse-grained soils where air can rapidly move through the contaminant zone. An SVE is not as effective in mobilizing contaminants from the capillary fringe or in fine-grained or very moist strata. Slow diffusion of contaminants from residual DNAPL entrapped in these zones limit restoration rates.

Multiphase extraction refers to remediation strategies that incorporate combinations of groundwater pumping, DNAPL removal, and SVE using a single well to extract both liquid and vapor. Depending on the site, the depth at which SVE can be applied is increased by lowering the water table with groundwater extraction. At sites with mobile DNAPL, a low rate of groundwater pumping can be applied to lower the water table while applying SVE to enhance liquid migration toward the well and to remove volatiles from the expanded unsaturated zone. The multiphase extraction strategy is applicable at sites with permeable soils, where the majority of the DNAPL is trapped within the upper 10 ft (3.05 m) of the aquifer. Multiphase
extraction is less successful at sites with low-permeability soils because dewatering at the well may not result in a significant reduction in the water saturation level of the soil. In this case, the air permeability of the soil will not be improved by application of simple dewatering.

Another strategy is applying thermal enhancements to SVE system. In situ heating methods that have received attention are aimed at reducing soil moisture and thereby enhancing SVE effectiveness by extending SVE influence into shallow aquifers. In situ heating strategies offer potential to increase the permeability of silt and clay soils and increase the removal efficiency of SVE. Steam injection combined with vacuum extraction may also be considered an approach to remove volatile DNAPL from the vadose and shallow groundwater zones. When high pressure “dry” steam is injected into contaminated soil, volatile chemicals with boiling points lower than that of water are vaporized. The SVE wells are then used to remove the resulting vapor. This process has been applied at smaller sites with relatively uniform permeable soils but may not be as effective in layered soils. Other methods use resistive or radio frequency heating for increasing soil temperatures and volatilizing DNAPL residuals. The most promising of the resistive heating technologies is a sixphase system, which uses metal and graphite electrodes to set up an alternating electrical current in the soil.⁶ This strategy is limited to sites with soil moisture levels above 10% because water is required to conduct the electricity and to create resistive heating. Resistive heating can reach a maximum temperature of 100°C, which boils away the groundwater and can volatilize residual DNAPLs. Once volatilized, volatile organic compounds are removed by a concurrently operating SVE system. Radio frequency heating is a process that uses electrodes placed in the subsurface to deliver radio frequency energy that excite molecular motion and induce heating (in the same way that microwave ovens heat food). Radio frequency heating has the potential to heat soil to temperatures in excess of 200°C and can be used to volatilize higher boiling point compounds such as mixtures of jet fuel and solvents. The SVE wells are then used to remove the resulting vapor.⁷

In situ air sparging is the process of injecting air into the saturated zone with the objective of stripping the contaminants from the dissolved phase and transferring these compounds to a vapor phase. The strategy competes with conventional baseline technologies of pump-and-treat and pump-and-treat combined with SVE. Vertical well air sparging and in well recirculation technologies have been implemented at a number of sites.

An air sparge network consists of sparge points designed to deliver air to a specific zone of contaminated groundwater. Air compressors deliver contaminant-free air under pressure to the target zones. The vapor migrates upward from the saturated zone to the unsaturated zone. The vapor phase is then vented through the unsaturated zone to the atmosphere and typically uses an SVE in the unsaturated zone to more effectively control, treat, and remove the vapor plume from the unsaturated zone.

Volatile, semivolatile, and nonvolatile organic contaminants in dissolved, free-phase, sorbed, and vapor phases can be treated using air sparging. Air sparging is applicable for the treatment of less volatile and/or tightly sorbed chemicals that could not be remediated using vapor extraction alone. Contaminants affected by the volatilization and biodegradation processes of air sparging include various fuels (eg, gasoline, diesel, jet fuel), oils and greases, and the chlorinated solvents (PCE, TCE, DCE, etc). Some commercially available systems (eg, BioSparge^®) employs an ozone generator with the air sparging technique to extend the capabilities of the technology to chlorinated phenols (pentachlorophenol [PCP]), alcohols, ketones, and other industrial solvents. The injected ozone breaks the chlorine bonds, facilitating biodegradation of the resulting compounds.

Combinations of anaerobic and aerobic biological degradation represent a strategy that is theoretically capable of completely degrading chlorinated solvents to harmless by-products.¹ The strategy has been primarily applied to the dissolved phase of chlorinated solvent plumes. A promising biotechnology for DNAPL removal is enhanced reductive dechlorination. Under highly anaerobic conditions, chlorinated solvents such as PCE, TCE, and DCE are used as electron acceptors by subsurface bacteria. In this process, chlorine atoms are sequentially removed from the chlorinated solvent molecule.

Natural attenuation processes refer to biodegradation, dispersion, sorption, and volatilization, all of which affect the fate and transport of chlorinated solvents in all hydrological systems. When these processes are shown to be capable of attaining site-specific remediation objectives in a time period that is reasonable compared to other strategies, they may be selected alone or in combination with other more active remedies as the preferred remedial strategy. The EPA defines monitored natural attenuation as the reliance on natural attenuation processes (within a controlled and monitored cleanup approach) to achieve site-specific remedial objectives within a time frame that is reasonable compared to other methods.⁸ Natural attenuation processes include biodegradation, dispersion, dilution, sorption, volatilization, and chemical or biological stabilization, transformation, or destruction of contaminants. Such processes are typically used in conjunction with other active remediation measures or as a follow-up these that have already been implemented.