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The loth National Conference The Hazardous Materials Control Research Institute U.S. Army Corps of Engineers U.S.Agency for Toxic Substances & Disease Registry U.S. Department of Defense Association of Engineering Geologists REPOSITORYTALI American Society of Civil Engineers National Environmental Health Association National Solid Waste Management Association
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Framework for Assessing Baseline Human Health Risks at Superfund Sites Contaminated with Chemical and Radiological Wastes EXPOSURE ASSESSMENT TOXICITY ASSESSMENT Bechtel Environmental, Inc. Oak Ridge, Tennessee Historically, U.S.Depanment of Energy waste disposal practices have resulted in the contamination of numerous sites with chemical and radiological wastes. In many instances, a single site wiIl contain both typcs of waste, presenting unique risk assessment challenges. Currently, protocols for uniformly assessing risk from both types of contaminants at such sites do not exist. Consequently, the approach presented in this paper was developed to pmvide a uniform assessment of the risk to human health from sites contaminated with both hazardous chemical substances and radiological materials. This paper presents a suggested framework for assessing risk at U.S. Department of Energy (DOE) federal facility Superfund sites contaminatal with both chemical and radiological v,a.sm.Historically, the principal contaminants of concern have been the radioactive materials, with less or no emphasis placed on hazardous chemical contaminants. However, with the passage of the SARA in 1986, and with greater concern for the characterizationand cleanup of federal faciities. both chemiII IDENTIFY EXPOSURE OF POTENTIAL CONCERN II RISK CHARACTERIZATION cal and radiologicalcontamination are now being assessed at these DOE The baseline human health assessment is an analysis of site conditions in the absence of remedial action (ix., "no action" akrnative). This evaluation requires an understandingof the nature of contaminant releases fromthe site, the pathways of human exposure and a m w u m of the potential risk to human health as a result of the releases'. The prccu.5 includes identification of potential exposure scenarios for mcdiaspecific pathways, selection of indicator contaminants, exposure ~FYSS. ment, toxicity assessment and risk characterization. As part of the baseline human health assessment, several types of effects can be evaluated relative to radioactive and chemical conlaminanu. This paper will address radionuclide carcinogenic (c.g.. stochastic) effects and chemical carcinogenic and non-carcinogenic of radioactive contaminants will not be discussed since they a n the result of relatively higher doses, hence arc only of interest in special and limiled risk assessment scenarios. Further. effects from radionuclidr and chemical conlaminants may be combined to obtain m i d wtc carcinogenic and non-carcinogenic efftas. However, consideration of mixed waste efftcu arc bcyond the scope of this discussion lind will Each of the five major componcnfi of the baseline human heallh assessment presented in Figure 1 is discussed in the followingsections for chemical and radiological contaminants. IDENTIFICATIONOF EXPOSURE SCENARIOS The exposure scenarios describe the components for potential human exposure pathways. The pathways describe the mechanisms by which a receptor may be exposed to contaminants originating from a site. An exposure pathway is comprised of the following components: source; mechanism of contaminant release: an environmental transport medium: likely mute of human intake or exposure; and potential human recep tor or exposure point. Figure 1 presents an example of the components used in the developmenc of an exposure scenario. To be considered 3$ Under the authority of CERCl;A.-thcU:5.-.EPA.,recluitcSt~Jt'jS i l e * specific risk assessment be conductcd to charxtcrizc currcnt ;ithi potcnnicnt of a ctirrcnt c.rp,sure scenario lor mih site (C.R.. rcsidcnt wlrc" a potential cxposure sccnxrio. all of thew components of i h L. F. i)urN-
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111 llir II III Chr it igu hi cli ctiiilriil\ leg, ptij\ic.il, Jccd. wguldury r n - Conkrminants of potcntial concern arc those contaminants that arc sitc rclatcd and for which data are of wfficicnt qualify for uhe in o quantitative risk assessment. For the human hcalih assessment at siics contaminalcd with both chemical and radiological substances, chemical and radionuclidecontaminant daw must bc evaluated and validaled prior to sclcction of the contaminants of potential concern. Exposure assessment is the determination or estimation (qualitative or quantitative) of the magnitude. frequency, duration and route of exposure. Numerous variables are used to quantify exposure. These include estimation of exposure point concentrations,estimation of contaminant intakedexposurcs and quantification of pathway specific Exposure point concentration is the average concentration contacted over an exposure period. Methods for estimating exposure point concentrations include direct u x of environmental media monitoring data and use of environmental fate and lranspon models. Thcse models help 10 p n d i a conaminant rcleasc and mi@on when monitoringor characterization data are unavailable at specified points of exposure. They also help assess future risks to ncepton from hazardous substances pmcnt in the environmental media. Chemical intake is the amount of contaminant at the exchange boundaries of an organism that is available for absorption. These data arc normalized for time and body weight and expressed as mg chemicallkg body wightday, The generic equation for calculating chemical intake is presented in Table 1. (e.g.. INGESTION Of WATER FROM WELL) Exposure Scenario Components For example, exposure can, in some cases, be more sensitive to individual activities (e.g., ingestion rates) than to contaminant concentration'. In many exposure assessments, adjusting all parameters 10 their limiting values would certainly maximizt exposure but may not have a realistic chance of happening in the rral world. For this reason, the concept of "reasonable maximum" scenario is used. In such cases. the exposures may be high, but the Combination of exposure paramelen for an individual arc those that arc more likely to occur in the actual population I. The U.S. EPA also rquircs consideration of all potential pathways of radiation exposure from a disposal system to a receptor. This may include inadvertent and intermittent human intrusion into a radioactive waste disposal system by exploratory drilling for resources'. For this scenario, the US. EPA assumes that passive institutional controls (e+, markers, intrusion barriers and fences) or the intruders' own exploratory procedum would be zdquatc for the intruders to detect. or be wmed of, the incompatibility of the area with their conhued activities. Other agencies also have established regulatory guidance pertaining (0potential receptor scenarios at radioactive waste disposal sites. The sible agency no longer has control of the site), an intruder may unintentionally access a closed waste disposal site and subsequently modify it for a specific purpose3. Ozfunali and Roles have identified several intruder scenarios relative to the NRC regulations. including a housing construction scenario and an agriculturc scenario'. As a result of Intrusion into the wstc. short- and long-term ndiation exposures to the individual could occur. I n addition. DOE has identified h ~ apened human rrccpror scenarios for asscssment of dose resulting from DOE on-site openilonat rcleascs from nuclear hcilities: the population and mximall! e q x w d individual c - Chenical concentratron a t the exposure point BY - Body veiqht o f exposad individual (kq) I - Annual intake of contaminated environmental .*drum C - A vera qe concantrstlon of contaminant In 0 - Othar pathway specific factors (m.q.. occupancy 0 - Radiolrqical aaay conctant for contaninant
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Spccific chcinical and radionuclide intakw can he calculated L t r inaiiy different exposure scenarios. A partial compilation for porsihlc residential exposures is presented in Table 2 . RMFible Hcsidentinl Exposure Srcnarins Ingestion of contaminants in drinking water Ingestion of contaminants in surface water while Dermal contact with contaminants in water Ingestion of chemicals in soil Dermal contact with contaminants in soil Inhalation of airborne(vapor phase) contaminants Ingestion of contaminated fish and shellfish Ingestion of contaminated fruits and vegetables Ingestion of contaminated meat and dairy products Toxicity assessment is the determination of the potential for adverse effects resulting from human exposure to the contaminants. If possible, an estimate of the relationship between the extent of exposure to a contaminant and the incidence of diseast also is pmided. Components of the toxicity assessment include estimation of effects from exposure to both chemicals and radionuclides. Effects from Exposure to Chemicals A reference dose, RfD, is the toxicity value used most often to evaluate noncarcinogenic chemical effects resulting from exposures at Superfund sites. Various types of RfDs are available, depending on the exposure route (oral or inhalation), the critical effect and the length of exposure being evaluated (chronic, subchronic or single event). The length of exposure may include exposures lasting 7 yr to a lifetime (Le.. exposures lasting less than 2 wk (i.e.. 1- or IO-day health advisories) and exposures from a single event (generally 1 day) (Le., developmental A slope factor and the accompanying weight-of-evidencedetermination are the toxicity data most commonly used to evaluate potential human chemical carcinogenic risks. The weight-of-evidence determination is used to determine the likelihood that the agent is a human carcinogen. The slope factor represents a toxicity value that quantitatively defines the relationship between dose and response. The slope factor is used to estimate an upper bound probability of an individual developing cancer as a result of a lifetime of exposure to a particular level of a potential carcinogen. Effects from Exposure to Radionuclides There are two broad classcs of effects resulting from radiation are those that have observable thresholds and that increase in severity with increasing dose. Examples of non-stochastic effects include cell death, lens opacification. cosmetically-unacceptablechanges in the skin and arncnorrhtm. Thcse effects. obwvable o n l y at relatively high dases. w d d only be of concern in ccrtain inadvertent intruder scenarios. Stochastic effects ;ire tliosc effccth ihat arc randoni (q.. probabilistic) i n nature, for which lincarit) i n dow rcxpone ir ahsulncd. this discussion only. the risk from carcinogenic stochastic cffecth u bc considered. Teratological effects data are insufficient to predict a linear dose-response relationship at low doses'*. The International Commission on Kadiological Protcction (ICRP) plam the risk of gcnctli effects at approximately a factor of two lower than thc risk of carcino. 1CRP states that risk assessment of the detriment duc to heredlo-) damage should be made over the total population. Most risk asw , ,. ment scenarios. however. will not include populations sufficiently 1: : to represent a potential threat to the gene pool. The ICRP and the National Council on Radiation Protection and Measurements (NCRP) make the assumption that the frequency of Occurrence of health effects per unit dose at lowdoses is the same at high doses. This linear. non-threshold hypothesis assumes that the risk of radiation-induced effects (i.e., cancer) is linearly proportional to dose, no matter how small the dose might be. Since no threshold associated with exposure to ionizing radiation is assumed, any dose, no matter how low, might give rise to cancer. Ideally, human epidemiological data and animal data regarding radiation-induced cancer are used in the calculation of numerical risk estimates. Howcver, since the epidemiological data are incomplete in many respects, mathematical models are used to estimate the risk. The result from the model is an effective dose equivalent for external radiation pathways or a committed effective dose equivalent for internal Risk characterization uses information from the exposure assessment and toxicity assessment to assess risks to human health from contaminants at a site. Components of the risk characterization include reviews of toxicity and exposure assessments. quantification of risk, from individual contaminants,quantificationof risks from multiple contaminants, combining risks acmss exposure pathways and assessment and presentation of unceminties associated with the estimation of risks. Quantification of Chemical Risks Chemical risks from carcinogenic substances arc estimated as the incremental probability of an individual d.;cloping cancer over a lifetime as a result of exposure to the potcntiz acinogen. For low risk levels (Le.. for estimated risks below 0.01). me linear lowdose cancer risk equation presented in Table 3 can be used to estimate risk. When risk levels a n greater than 0.01, the one-hit equation for high carcinp genic risk levels can be used (Table 3). The total cancer risk from multiple carcinogenic substances can be obtained by summing the individual substance risk estimates. Estimation or Carcinogenic Risks where: Risk - unitless probability of fatal cancers CDI - Chronic daily intake averaged o v e r 7 0 Year where: Risk - unitless probability of fatal cancers
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E and RID must: (1) be expressed in the same units (2) represent tho same exposure period (i.o., 1: rpn < 1, it is unlikely that won sensitive populations .=uld experience adverso health effects f r o n the contaminant. Quantification of Dose from Radionuclides ne annual dose for each radionuclide can be estimated as illustrated 3 Table 5. By summing the external and internal radiation doses awnal and internal dose to individuals is obtained. Funher, the annual aarnal and internal doses arc summed nsulting in a total individual mnual effective dose equivalent at an urposurc point. The annual effective dose equivalent may be conventd to a health risk by using the risk coefficient of 2 x IW risk of fatal cancer per rcrson-rem of radiation dose as calculated using the linear non-hshold nodel". The lifetime risk is based upon the funher assumption that dr exposure level is the same for each year of a Wyr lifetime. Combining Risks Across wthways Whether risks or hazard indices for two or mort pathways should k combined for a single total exposun pint can be determined by ansidering: (1) the identification of reasonable exposure pathway comht ions and (2) the like lih d that the same individuals would conSIstently face the reasonable maximum exposurc by more than one *\ray. If it is reasonable to combine risks across pathwtys. Be cancer risks and the noncancer r i s k s must be combined separately. There a n many uncertainties that arc inherent in the baseline risk Ussment process. Each component of the process. identified in ilpre I, has uncertainties associated with the input parameters, [he saluarion methodology and the results. Specifically, some sources of Jwcrtainty in the baseline risk assessment include input mriable un- ;emintics modcling uncertainties. scenario uncertainties. and risk Thc process of analyzing the uncertainty can be either quantitative qc31itative dcpendlng on the timc. resources and panmctcrs or 2 r K ~ ~ c sbeing analyzed. Selecting the appropriate u y to chanctcrizc uhcre: D - knnnnl Dorc to an individual from erternnb or intarnal rrposurr from a rrdlonuclide E r r - Envlronmmntal transport ractor lnhnlcd o r inqcsted (for inttrnal cxposure) o r the effoctlve doc. equivalent rate per concentration ofa radionuclide in the air, vater. or ground (for external exposure). -: Doso aquivalcnt is the product of ab8orb.d dore in tissue. a quality factor, and other Dodlfyinq factor.. mc ti ve DQ-: Cffe;tlvc dose equivalent ir the mum o f the products of dose eWlValent and wclqhtrnq rector for each v:The veiqhtinq factor i a thm decimal fraction of tho rick arirlng from irrrdlatlon of a selected tissue to the total risk vhen tho whole body i r irradiated uniromly to tho eaDe cqulvalent is the sum ol t h e couitted dose equivalents t o VariOU8 ticrues in ths body each nultiplied by the appropriate vciqhting : The coraltted dose equlvalont Is tha mvkl.nttoa tissu. or orqan over a specified tho period after an intake or a …