Background
Petroleum fuels have been in production and use for over one hundred years; and chlorinated solvents such as trichloroethylene (TCE) have been widely used as cleaning solutions and for other purposes by the Department of Defense (DoD) and industrial entities for over 50 years. As a result of the widespread production, transportation, use, and disposal of petroleum fules and solvents, these contaminants are present at numerous sites throughout North America and Europe. For example, fuel constituents such as benzene, toluene, ethylbenzene, and xylene isomers (BTEX constituents) and chlorinated solvents account for most of the top twenty organic contaminants detected most frequently at hazardous waste sites. In addition to releases resulting from inadvertent spills of fuels and solvents, prior to the promulgation of environmental regulations, waste petroleum fuels and solvents frequently were disposed of in landfills, sanitary and storm sewers, disposal pits, and fire training areas at hundreds of Air Force facilities. The Department of Defense (DoD) has identified thousands of fuel-spill sites, and has identified chlorinated solvents at nearly 50 percent of its contaminated waste sites (USEPA, 1997).
After petroleum fuels or chlorinated solvents have been introduced to the subsurface, their characterization and removal are problematic. Where possible, cost-effective remediation strategies should focus on identifying and delineating those parts of the subsurface environment containing the greatest mass of introduced chemicals, or where chemicals are present at high concentrations. These areas represent potential chemical source zones, from which contaminants can leach into groundwater, migrate to surface-water bodies, or volatilize into soil vapor. These zones can function as long-term contaminant sources, contributing chemical mass to the environment for decades. Identification and reduction of chemical source areas is essential, particularly when natural attenuation processes are slow, and the costs of containing resultant groundwater plumes are high. This page provides an overview of the properties governing the movement and fate of petroleum fuel constituents and solvents in the environment, discusses recent technical advances and regulatory developments in addressing source zones, and discusses in general terms the cost and performance associated with several technologies that are being applied to reduce the impact of source zones.
The NAPL Challenge
Petroleum fuels are distillates of crude petroleum comprising a complex mixture predominantly composed of paraffins, cycloparaffins and aromatic groups, together with other minor constituents added as octane enhancers, or to improve evaporation and condensation characteristics of the fuel. The aromatic BTEX compounds are primary constituents of petroleum-distillate fuels, and are chemicals of the greatest potential environmental concern in fuels because they are relatively toxic, and can be mobile in the environment (Newell et al., 1995). Solvents generally consist of volatile organic compounds (VOCs), together with an inert base. These are mixed in varying proportions depending on specific applications (Pankow and Cherry, 1996).
In the pure chemical state, petroleum fuels and most chlorinated solvents are immiscible fluids that do not mix well with water. Petroleum fuels all are less dense than water, and are known as light non-aqueous phase liquids (LNAPLS). Liquid solvents, or solvent mixtures having densities greater than water, are known as dense non-aqueous phase liquids (DNAPLs). Other organic mixtures, such as creosote and coal-tar derivatives, also can form DNAPLs. Chlorinated solvents comprise the principal category of DNAPL chemicals of concern to DoD.
Under particular conditions, chemicals can exist in the environment in any of four different phases - as pure compound or in a chemical mixture; dissolved in water; sorbed to soil particles; or as a vapor. Several transport processes control the physical movement of chemicals through soils, as non-aqueous phase liquid (NAPL), dissolved (aqueous) phase, gas (vapor) phase, and sorbed (solid) phase. When initially released to the subsurface environment, petroleum hydrocarbons and organic solvents are usually in the NAPL (oil) phase. Once a chemical has been introduced into the environment, it interacts with the surrounding soils. The major processes affecting chemicals in the subsurface include sorption to soil, diffusion, dissolution, chemical and biological degradation, and volatilization.
Subsurface transport of chemicals as NAPL, dissolved-phase, or vapor-phase, like movement of any liquid in the subsurface, is driven by potential gradients - gravitational, hydraulic, or chemical. In the unsaturated zone, gravitational and hydraulic potential gradients are primarily vertical, so that the direction of movement generally is downward. When released to the subsurface, NAPL migrates downward through pore spaces in the soil that are not filled with water. Each soil type has a certain capacity to retain the LNAPL in its pore spaces; this capacity is known as residual saturation. If the volume of NAPL (fuel or solvents) released is small, the entire NAPL volume may be adsorbed or trapped in soil in the vadose zone (soil above the groundwater table) without exceeding the residual saturation. If the volume of NAPL released is large, the residual saturation may be exceeded, and the mobile NAPL will continue to migrate downward until it encounters a less permeable layer or until it reaches the capillary fringe (that point in the soil column at which the soil is fully saturated, but hydrostatic pressure is less than atmospheric pressure) above the groundwater table. At that point, an LNAPL generally will form a lens of "free product" on top of the groundwater and DNAPL will spread until sufficient pressure (NAPL head) develops to enable the liquid to penetrate the capillary fringe and migrate to the water table. The downward movement of mobile LNAPL or DNAPL also can be impeded by a soil layer of lower permeability than surrounding material.
As water percolates through the unsaturated zone, chemicals present as non-aqueous phase, a sorbed phase, or a vapor phase, can be dissolved and migrate with the infiltrating water to the water table. Dissolved constituents are carried downward in percolating water ("advective transport"). Volatilized constituents move in response to chemical concentration gradients between soil moisture and air-filled pore spaces ("diffusive transport"). If the relative vapor density of the volatile phase is greater than that of air, some chemical migration in the vapor phase may be downward. In general, however, vapor-phase migration is from the subsurface to the atmosphere.
At many sites, the potential for serious long-term contamination of groundwater by NAPL chemicals is high due to their toxicity, limited solubility, and significant potential for migration in soil vapor and groundwater, or for NAPL migration as a separate immiscible phase. DNAPL chemicals, especially chlorinated solvents, are among the most prevalent groundwater contaminants identified at disposal sites. DNAPL phase has been identified or is suspected at over 50 percent of the solvent-contaminated sites recently examined by the United States Environmental Protection Agency (USEPA, 1999). Release of a NAPL to the subsurface introduces the potential for several risk factors to affect nearby receptors:
- Vapor-phase migration of volatile constituents from NAPL in the vadose zone to land surface,
- Dissolution of constituents from NAPL in the vadose zone into percolating water, and subsequent downgradient movement of those dissolved constituents after recharging water arrives at the groundwater table, and
- Release of NAPL mass in quantities sufficient to exceed the capacity of the vadose zone to absorb it, resulting in the formation of a mobile NAPL phase above (LNAPL or DNAPL) and/or below (DNAPL) the original groundwater table.
Additional information regarding the nature of the LNAPL problem can be found in the USEPA's Ground Water Issue - Light Nonaqueous Phase Liquids. Additional information regarding the nature of the DNAPL problem can be found in the Air Force Center for Engineering and the Environment (AFCEE) document entitled Remediation of Chlorinated Solvent Contamination on Air Force and Industrial Properties and in Pankow and Cherry (1996).
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