Potential Exposure Pathways

The rate at which a contaminant is released from a source is a critical parameter in quantifying risk at contaminated sites. Picture two different extremes of release from the source In the first case, 1,000 kg of benzene (a confirmed human carcinogen) exists in a fined surface impoundment and is volatilizing at a rate of 0.5 kg d and is potentially inhaled by a nearby population. In the second case, the same mass of benzene is strongly sorbed to a clay soil rich in organic matter 1 m below the soil surface, from which the volatilization rate is 10 kg d. Obviously, the risk of hazardous chemicals depends on the rate of contaminant release. 

Quantifying contaminant release rate is an integral part of source release assessment. Because 

The goal of exposure assessment is to determine the concentration of contaminant to which the receptor organism is exposed. This procedure involves two steps (i) determining the concentration of the contaminant to which the population is exposed, and (ii) quantifying contaminant intake at the point of exposure. 

In some cases, direct measurements of contaminant exposure may be made, such as in the assessment of the steady-state release of an established hazardous waste source. Groundwater monitoring wells or air-sampling devices can be used to determine current exposure concentrations for exposed populations. Commonly, however, air, surface water, and groundwater sampling is neither a logical nor a practical choice. Typical situations in which sampling is not feasible include (i) the evaluation of future exposure and risk to potentially exposed populations and (ii) the potential risk from an event that has yet to occur (e.g. a hazardous waste spiU). 

Contaminant transport models are most commonly used for determining the concentrations of contaminants that reach the exposed population. Such models for the atmosphere, surface waters, and the subsurface have developed substantially their use has been described by Anderson and Woessner (1992), Watts (1998), and Zheng and Bennett (2002). 

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Collecting and analyzing existing data. Existing data (Table 16.1) are collected and analyzed to develop a conceptual site model that can be used to assess both the nature and the extent of contamination and to identify potential exposure pathways and potential human health or environmental receptors. 

Finding 3. Closure of any chemical agent disposal facility necessitates the identification of potential exposure pathways and environmental receptors in an initial conceptual site model. Closure of JACADS has been complicated because this was not done early and the end use will not be determined until much of the closure planning has been completed. 

Exposure assessment is a key phase in the risk assessment process since without an exposure route even the most toxic chemical does not present a threat. All potential exposure pathways are carefully considered. Contaminant releases, their movement and fate in the environment, and the exposed populations are analyzed. 

An expert system planned for Risk Assistant addresses the probability that various exposure pathways will be of concern at a hazardous waste site. This module guides users in considering factors that may increase or decrease the likelihood that a chemical will be released from a unit such as a landfill or surface impoundment to any environmental media. If release cannot be ruled out, factors affecting transport to an area of potential exposure are considered. If the presence of contamination in an area of potential exposure cannot be discounted, potential exposure scenarios are reviewed. The end result is a listing of potential exposure pathways that the user may have to consider for the site. 

Identification of Pathways. The expert system will guide a user to consider the variety of exposure pathways that should be considered for a site, and thus to perform a qualitative risk assessment. It helps to ensure that significant current or potential exposure pathways are not ignored. 

Pathways Considered - Were any significant current or potential exposure pathways excluded from analysis What is the consequence of including these pathways on total exposures Were any inappropriate pathways considered How much do these pathways contribute to the total exposure ... 

Following the 1985 edition of the Regulations, the application of the Q system as described here treats the derivation of each Q value, and hence each potential exposure pathway, separately. In general this will result in compliance with the dosimetric criteria defined earlier, provided that the doses incurred by persons exposed near a damaged package are dominated by one pathway. However, if two or more Q values closely approach each other this will not necessarily be the case. For example, in the case of a radionuclide transported as a special form radioactive material for which = Qg, the effective dose and skin dose to an exposed person could approach 50 mSv and 0.5 Sv, respectively, on the basis of the Q system models. Examination of Table 1.2 shows that this consideration applies only to a relatively small number of radionuclides, and for this reason the independent treatment of exposure pathways is retained within the Q system. 

Work is continuing in the area of vapor intrusion, and hopefully the level of precision in modeling will increase as we learn more about how specific vapors move through the subsurface environment and foundation cracks and wall seams. Given that several volatile chemicals are more toxic when inhaled than ingested (e.g., chloroform, toluene, methylene chloride, hexane, hydrogen cyanide, chlorine, methyl isobutyl ketone, naphthalene), rigorous and accurate evaluation of this potential exposure pathway is critical to adequately protect human health. 

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