Definition of Release Mechanisms
The manner by which hazardous substances and their constituents (chemicals or radionuclides) are released or made to leave their sources or matrices of sources. Under CERCLA and RCRA, a potential area of concern or unit would not meet the definition of a release site or an SWMU unless a release of hazardous waste or hazardous constituents has occurred.
The release mechanism can be physical, chemical, thermal, or radiological. When a release mechanism is presented to the sources, transfer of the hazardous substances and their constituents to another medium or media is likely to occur, given the time and intensity of the release mechanisms. The physical and chemical nature of the hazardous substances or radionuclides in the source and its immediate environment (e.g., acidity, alkalinity, and sorptive properties of the receiving media) may influence the effectiveness of the release mechanism.
Several release mechanisms can be working simultaneously at a source to facilitate the release of hazardous substances or radionuclides. Unless the source is continuously being recontaminated, release from a source will likely result in lowering the original contaminant level(s).
This release mechanism modifies the effectiveness of other release mechanisms. During chemical transformation, a hazardous substance (chemical or radionuclide) changes its physical and chemical characteristics, making the hazardous substance more or less amenable to other release mechanisms. An example of chemical transformation is the conversion of lead to lead chloride or lead sulfate in acidic soil. The resulting lead compounds are more soluble than metallic lead, which could be subject to leaching action of infiltration. Other examples include the hydrolysis of many chemicals with water, the methylation of mercury, radiological decay, and photolysis.
Similar to dredging and tracking, this release mechanism refers to human or animal activities that cause the hazardous substance and/or their constituents to be exposed and subject to other release mechanisms. Construction activities that entail soil or sediment excavation, and burrowing actions by small mammals are examples of this release mechanism.
This release mechanism is caused by the frictional force posed by the act of dredging or tracking (vehicular or foot traffic). Under this release mechanism, the hazardous substances and/or their constituents are released from the matrix surfaces (solid, semisolid, or liquid) into air in the form of particulates or fugitive air emissions, or into water in the form of solubilized chemicals or suspended solids. Dredging or tracking actions also serve as migration pathways that transport the released hazardous substances and their constituents to other media or locations.
This release mechanism is most applicable to sources that have inadequate containment systems or poor integrity. Hazardous substances and their constituents may leak from the unit (e.g., tank, container, surface impoundment, waste pile, highly contaminated soil, waste disposal area or cell) to the soils adjacent to the unit or source. Under this release mechanism, the mass of the hazardous substances is pulled by gravity toward the subsurface strata through the path of least resistance.
This release mechanism refers to the water pressure that is being applied to the source which causes the hazardous substances and their constituents to leak out or to be released from the source or structures/barriers. A typical example is the leaching of hazardous waste or hazardous constituents through the liner of a surface impoundment or landfill.
This release mechanism refers to the movement of soluble chemicals via infiltration into surface soils. As a result of precipitation, runoff from storm, and/or groundwater recharge, the infiltration leaching action removes the hazardous substances and their constituents from the source (soils) to another medium (e.g., perched groundwater and aquifer). The release mechanism could be viewed as the combined mechanisms of gravitational force, hydraulic pressure, and solubility. Leaching also serves as a migration pathway that transports the released hazardous substances and their constituents to other media or locations.
This release mechanism refers to the physical force, posed by surface water moving in a downstream direction, that removes the hazardous substances and their constituents from the source and transports them to another medium. Runoff also serves as a migration pathway that transports released hazardous substances and their constituents to other media or locations. The effectiveness of this release mechanism is typically related to other release mechanisms, such as solubility and chemical transformation, that may be occurring. Runoff action is applicable to inland areas with measurable topographic relief, and is a predominant cause of release of hazardous substances in contaminated soils along swales, channels, river banks, or groundwater recharge areas. Flooding or overland flow may be considered a runoff mechanism on a larger scale.
This release mechanism refers to the forming of chemical bonds within the solvent or water matrices. Polar compounds form ionic bonds with the water molecules, acids, or bases. Non-polar compounds form weak intermolecular bonds with van der Waals forces in common solvents like kerosene, jet fuels, hexane, or isopropyl alcohol. Release stops when the solubility or saturation limit is reached within the solubilizing agent.
Like runoff, this release mechanism refers to the physical force posed by the movement of surface water caused by gravitational force from the moon (diurnal) or storms (episodic). Tidal action can act as both a release and a transport mechanism, removing the hazardous substances and their constituents from one medium to another, as well as from one location to another. Tidal action is applicable to large inland lakes, and marine and coastal areas.
This release mechanism refers to physical and/or chemical means occurring at the barrier between an organism and the contaminated media. Under this release mechanism, the hazardous substances and/or their constituents are released from the matrix (food, soil, sediment, and surface water) by physical means (e.g., molecular diffusion), biochemical means (e.g., active transport), or chemical means (stomach acids), or a combination of the above. The biochemical processes following ingestion and plant translocation mechanism at the root zone are examples of uptake release mechanism.
This release mechanism is dependent on the chemical characteristics of the hazardous substance (i.e., molecular weight, vapor pressure, Henry's Law constant, boiling point, etc.), ambient temperature, and wind velocity or air movement, where appropriate. Under this release mechanism, the hazardous substances and/or their constituents are released from the matrix surfaces (solid or liquid) into air (pores in soil or the ambient air). This release mechanism is most applicable to volatile organic chemicals and,to a lesser extent, semivolatile organic chemicals.
This release mechanism refers to the frictional force, posed by air movement near the earth surface, that removes the hazardous substances and their constituents from the source to air. Under this release mechanism, the hazardous substances and/or their constituents are released from the matrix surface into air. This release mechanism is most applicable to metals and semivolatile organic chemicals.