Decision-makers should have a clear understanding of the initiation and development of a chemical contamination incident for effective remediation of hazardous chemical contamination. They will need to take into account the characteristics of the hazardous chemicals released, the nature of the prevailing environmental conditions under which the incident is occurring, and the characteristics of the materials impacted by the hazardous chemical release.
All hazardous chemical incidents begin with the release of a hazardous chemical. The release occurs from a source into an environment, system or building resulting in environmental contamination and the potential for human exposure to the hazardous chemical. In turn, exposed persons may experience one or more adverse (or undesirable) effects. In hazardous chemical incidents, primary concerns are: (1) significant injury or death, and (2) loss of the use of property or infrastructure. The objective of remediation is to: (1) mitigate casualties or severe injury, (2) address environmental concerns, (3) achieve decontamination of environmental materials and surfaces (e.g., soil, ground water, surface water, and drinking water) and infrastructure such as subways, buildings, stadiums, and offices, and, if possible, (4) return private and public property to their pre-incident uses. Remediation accomplishes this by reducing the amount of the hazardous chemical (and any toxic chemical degradation byproducts) in the environment to an acceptable level and, thereby, reducing contact between the hazardous chemical and the population.
In order to more accurately estimate the contamination spread from a nationally significant or large-scale release of hazardous chemicals, the physical and chemical properties of the chemicals released, the atmospheric and environmental conditions, the composition and nature of the impacted media, and the presence of other chemicals must be evaluated. This information can support emergency response activities; determine sampling locations; direct evacuation, shelter-in-place or restricted-use actions, and; provide data for air dispersion plume or water modeling efforts. Additionally, knowledge of the nature and composition of impacted indoor materials and surfaces can determine the most appropriate decontamination and/or waste management approaches to use. It is also important to consider that a covert release of a hazardous chemical may result in the spread of the chemical due to unintentional cross-contamination and/or fomite transport.
Physical and Chemical Properties. Key physical and chemical properties of hazardous chemicals will determine the dispersion and path(s) that the hazardous chemicals could take from the release source to the impacted site or a population. These paths will determine the potential exposure routes of concern for affected populations. Table 7 provides a partial list of key physical and chemical properties that are important to consider.
The persistence of a hazardous chemical should also be considered. Chemical persistence is determined by the rate at which a chemical volatilizes, breaks down, or dissipates. Persistent chemicals may continue to pose a hazard for days, weeks, or longer after a release by remaining as a contact hazard, or by slowly volatilizing and becoming an inhalation or ocular hazard. Persistent chemicals in solid, liquid, or droplet form may remain for even longer periods depending on environmental conditions such as temperature, moisture, and the types of materials contaminated. For example, HD, VX, and FGAs may persist for days, weeks, or longer.
Table 7: Key Physical and Chemical Properties
| Properties | Comments |
|---|---|
| Vapor Pressure | The pressure exerted by a vapor when it is in equilibrium with its liquid or solid form; chemicals with high vapor pressure tend to be non-persistent. |
| Boiling Point | The temperature at which a liquid boils and turns to vapor. |
| Freezing/Melting Point | The temperature at which a liquid turns into a solid when cooled, or a solid will melt when heated. Some solids can sublime directly from the solid to the vapor phase. |
| Flash Point | The lowest temperature at which a liquid gives off vapor within a test vessel in sufficient concentration to form an ignitable mixture with the air near the surface of the liquid. The lower the flash point, the easier it is to ignite a liquid solvent. |
| Vapor Density | Density of the vapor compared to air (air = 1) is the tendency for a vapor to rise or sink in the air column. The vapor density is affected by barometric pressure. |
| Specific Gravity | Density of liquid compared to water (water = 1) is the tendency for a liquid to rise or sink in the water column. |
| Log Kow | The octanol-water partition coefficient determines the extent to which a liquid will partition into either the aqueous (water) or organic phases. |
| Solubility in Water | A measure of the amount of chemical substance (liquid or solid) that can dissolve in water at a specific temperature. |
The environmental component of this parameter, however, considers the form and amount of the chemical along with the environmental conditions. That is, the actual amount of time that a chemical remains on surfaces or within environmental materials and surfaces after a single release will depend on the characteristics of the hazardous chemical as well as the amount released, environmental conditions, and the nature of the contaminated materials and surfaces. For example, soils from specific locations will have differing properties. These differences must be considered during cleanup and may significantly affect technical aspects related to cleanup (e.g., if analytical data consistently fails quality control, soil matrix may need to be considered as a cause for such failures) and sampling and analysis plans may need to be modified accordingly.
Certain agents such as VX and L break down in the environment to other substances that pose potentially significant environmental or human health concerns as well. Separate sampling and cleanup for these constituents may be necessary. Other agents such as FGAs are more soluble and stable in water, maintaining their toxicity when dissolved. In addition, other hazardous constituents (e.g., dusts containing lead, dioxins, polychlorinated biphenyls, asbestos), though typically not acutely toxic, may be produced as collateral hazards if explosions or fires are involved. Time and weathering may be less effective in degrading persistent hazards, especially if large quantities are deposited. In such cases, more thorough sampling and decontamination may be necessary in order to allow resumed use/re-occupancy. The potential for cross contamination and longer durations of exposure are important considerations in cleaning up persistent chemicals. Collateral hazards may also include chemical impurities present as a result of manufacturing, laboratory synthesis processes, byproducts of long-term chemical storage, chemicals added to enhance physical or chemical properties or by-products of the decontamination process itself.
Atmospheric and Environmental Parameters. These parameters help to determine the spread of the hazardous chemicals in the environment. Reactions or interactions with the hazardous chemicals and the environment can increase or decrease overall toxicity or fate in the environment. Decision-makers should have available to them appropriate modeling programs that can be used to predict the migration, extent, and fate of the hazardous chemical in the environment. These models incorporate the key physical and chemical properties of the hazardous chemicals with the site-specific environmental conditions and the nature of the impacted materials and surfaces itself to construct a representation to predict future spatial movement, direction, and concentrations of the hazardous chemicals. Atmospheric models incorporate environmental and topographic conditions and the prevailing meteorological data to construct a three- dimensional representation of the impacted outdoor areas. Many agencies have successfully used models to predict the movement of hazardous chemicals in environmental media. Modeling can be useful to direct cleanup activities and sampling efforts at all stages of an incident, as well as to advise decision- makers on possible evacuation, shelter-in-place, or restricted use actions. DHS has established the Interagency Modeling and Atmospheric Assessment Center (IMAAC) as the national resource for atmospheric modeling during national significant incidents. The National Oceanic and Atmospheric Administration and EPA have developed the Computer-Aided Management of Emergency Operations (CAMEO) modeling program—another atmospheric modeling program that is easy to use, incorporates simple assumptions and data inputs to calculate theoretical plume “footprints,” and is used by many fire departments and first responders across the U.S. Similarly, surface water, ground water, and vadose zone (soil above the permanent ground water level) modeling programs can be used to construct models of impacted water, aquifer, and soil column matrices. The EPA and the United States Geological Survey (USGS) have numerous modeling programs to predict the movement of hazardous chemicals through surface waters, ground water aquifers, and the vadose zone of the soil column.48 Consideration must also be given to environmental conditions that may have adverse effects on various sampling technologies and techniques. Tables 8 and 9 list some of the important atmospheric and environmental parameters to consider.
Characteristics of Impacted Materials and Surfaces. Decision-makers should be familiar with the key characteristics of the impacted materials and surfaces, both outdoors and indoors. A list of some examples of these characteristics can be found in Table 9. Decision-makers can use that information to determine the interaction of the hazardous chemicals with the impacted materials and surfaces, as well as predict potential sinks or reservoirs of materials that can influence the persistence of these chemicals in the environment or incident site. Indoor modeling programs can use floor diagrams; air exchange rates; HVAC system specifications; and building design criteria to construct a three-dimensional representation of the impacted infrastructure. The National Institute of Standards and Technology (NIST) has developed the CONTAM program, which is an indoor modeling program used to predict the migration of a vapor plume inside a building.49
Presence of Other Hazardous Chemicals. Other hazardous chemicals at the incident site, especially industrial sites, may react with and change the characteristics of the released hazardous chemical. These interactions can be either antagonistic or synergistic, reducing or increasing their impact at the site. The decontamination materials may cause degradation to other hazardous chemical species. Understanding of these processes and the resultant hazardous chemical byproducts will assist risk managers in the complete and appropriate evaluation and management of the hazardous chemicals that may be present at different phases of the response.
Table 8: Key Atmospheric and Environmental Parameters
| Parameter | Comments |
|---|---|
| Temperature | Higher temperatures will increase the rate of evaporation of the hazardous chemical as well as increase any chemical and physical reactions or interactions with the environment or impacted materials and surfaces. |
| Wind speed/variability and direction | Higher wind speeds will increase the rates of evaporation of the hazardous chemicals. Speed, variability, and direction can be used to model/predict the movement of the vapor plume to assist in possible evacuation or shelter-in-place actions. Plume modeling can also direct ambient air monitoring and sampling efforts. |
| Relative humidity/precipitation | Presence of atmospheric moisture and/or precipitation will influence the characteristics of the vapor plume. Water can react with some hazardous chemicals (hydrolysis), either increasing or decreasing their toxicity. Products of hydrolysis themselves may be toxic. Precipitation may physically wash away from/or dilute hazardous chemical on the impacted materials and surfaces or drive soluble chemicals deeper into porous materials. |
| Barometric pressure | High- and low-barometric pressure conditions can influence the characteristics of the vapor plume. Higher barometric pressures can also decrease the rates of volatility of hazardous chemicals. |
| Solar radiation/cloud cover | Higher solar radiation (lower cloud cover) will increase the chemical reactions (ultraviolet photolysis) of hazardous chemicals, either increasing or decreasing their toxicity. Products of photolysis themselves may be toxic. Higher solar radiation will also increase the rates of volatilization of the hazardous chemicals. |
| Stability category (boundary layer) | Determines the atmospheric “mixing” of vapors within the air column. Unstable categories will result in more mixing of the hazardous chemical plume. This can act to disperse and therefore dilute the vapors. Stable categories will result in less mixing in the atmosphere and can predict a temperature inversion where the hazardous chemical vapor plume could be held stagnant over the incident site. |
Table 9: Key Characteristics of Impacted Outdoor/Indoor Materials and Surfaces
Outdoor/Indoor Materials and Surfaces
| Characteristic | Comments |
|---|---|
| Vegetation/soil cover/roughness | Can influence the hazardous chemical atmospheric plume by hindering movement, increasing mixing and reacting with vegetation surfaces. |
| Soil type/grain size/organic content | Physical and chemical characteristics of soils determine reactivity with hazardous chemicals, and control natural attenuation/biodegradation that may occur in soil media. |
| Topography | Features such as lakes, streams, hills, and valleys that can determine where the plume will move (i.e., a hazardous chemical with vapor density > 1 will sink and concentrate in valleys or low-lying areas in buildings). |
| Porous/nonporous materials | Porous surfaces and materials may adsorb more hazardous chemicals than nonporous surfaces and materials. This acts to retain the hazardous chemicals on porous surfaces and materials longer, adding to the persistence of the hazardous chemicals on the impacted media. |
| Organic/polymeric content | Materials with higher organic content, such as those with natural or synthetic polymers, may absorb more hazardous chemicals than those with lower organic/polymeric content. Absorption may be irreversible, complicating cleanup and decontamination efforts. |
| Anthropomorphic features | Man-made structures such as buildings, highways, bridges, and other infrastructure can obstruct movement of atmospheric plume and increase mixing. |
Indoor Materials and Surfaces
| Characteristic | Comments |
|---|---|
| Porous/nonporous materials | Porous surfaces and materials may adsorb more hazardous chemicals than nonporous surfaces and materials. This acts to retain the hazardous chemicals on porous surfaces and materials longer, adding to the persistence of the hazardous chemicals on the impacted media. |
| Organic/polymeric content | Materials with higher organic content, such as those with natural or synthetic polymers, may absorb more hazardous chemicals than those with lower organic/polymeric content. Absorption may be irreversible, complicating cleanup and decontamination efforts. |
| Status of HVAC system (on/off/materials) | Depending on whether the HVAC system was on or off during the incident and what materials ductwork is constructed from, HVAC can be a conduit for the spread of the hazardous chemical through vapor deposition. HVAC can also be a sink for chemicals. |
Outdoor/Indoor Materials and Surfaces
| Characteristic | Comments |
|---|---|
| Critical infrastructure | Structures or facilities that must be given priority during the cleanup and recovery phase due to their economic value, security, or other necessity. |
| Sensitive items/ electronics | Equipment or assets of a more sensitive nature, such as electronic, communication, or medical equipment, that may require less aggressive, less harsh decontamination methods and procedures. |
| Hot spots/surfaces | Areas or surfaces or high concentrations of contamination. May require more aggressive, harsher decontamination methods and procedures. |
| Large volumetric spaces | Large interior spaces such as auditoriums or transportation hubs that may require specific decontamination methods and procedures (e.g., fumigation, sprayers, or foggers). |