J. Scott Harris

Exposure Pathway Assessment Using ArcView

Evaluating Possible Human Exposure Pathways to Populations Relative to Hazardous Materials Sites

ABSTRACT

Defining human exposure pathways is an important and often difficult task involved in the process of assessing risk to public health from hazardous material releases. This process is generally termed a public health assessment (PHA) and combines a number of steps to define the risk posed to susceptible populations. As an indispensable step in the PHA, the evaluation of the exposure pathways is the process of determining the relationship between hazardous material releases and adverse effects on public health. Human exposure pathways are complex systems which are constantly changing in terms of their spatial and temporal presence in the environment. When examining these human exposure pathways, the risk assessor must combine several sources of information to predict possible routes by which a contaminant could impact public health. Due to the extensive amount of geographic and demographic data which is required to effectively evaluate these pathways, geographic information systems (GIS) are often utilized in the effort. The extent of data available to the public health professional following a release of a contaminant to the environment is directly related to the accuracy with which he can predict exposure pathways. Ideally this process would be done quickly and in the field at the time of a reported release and the routes could be displayed in manner that the probable human exposure pathways are easily shown to pertinent individuals. This paper describes a relatively simple methodology utilizing GIS to predict human exposure pathways in a time-efficient and accurate manner. ArcView GIS 3.0, used in concert with required data sets and other software, provides the health professional with a powerful tool to accomplish this end.

INTRODUCTION

When determining risk to public health from releases at hazardous materials sites it is often necessary to conduct a public health assessment. A PHA is the evaluation of data and information on the release of hazardous substances into the environment in order to assess any past, current, or future impact on public health, to develop health advisories or other recommendations, and to identify studies or actions needed to evaluate and mitigate or prevent human health effects. In a nutshell, a PHA is a structured approach which pinpoints areas of risk to human health and suggests steps to reducing that risk. There are a number of steps involved in completing a PHA and the process can take a great deal of time and effort. One of the integral parts of the PHA is evaluating possible human exposure pathways. This is a difficult step due to the fact that, in general, exposure pathways are dynamic in nature and exist on several levels.

The requirement for a methodology to predict human exposure pathways came about when contamination at hazardous materials sites in Georgia was suspected to have been complicated by extensive flooding due to the Alberto Flood of 1994. These sites which were flooded needed to be identified and then analyzed for possible additional impact upon public health. Due to the fairly large number of sites which were believed to be affected and the extensive amount of data which was being gathered, GIS was selected to be used to assist in the analysis. Applying the use of GIS to this analysis allows the risk assessor to conduct a human exposure pathway evaluation in a quick and accurate manner. The ArcView project designed uses an extensive set of databases and coverages to assist the user in "visualizing" the possible exposure routes and applying that knowledge to a PHA. The application of this GIS-based methodology to the evaluation of possible human exposure pathways will provide assistance to Georgia health professionals who are faced with making on-site public health decisions in the field following a release to the environment.

FLOOD RECOVERY PROGRAM

The lower western and central part of the state of Georgia experienced extensive flooding in 1994 due to the tropical storm Alberto. This disaster affected fifty-five counties in the state (View 1). In response to possible health concerns caused by this flood the Flood Recovery Program was formed through a grant from the Centers for Disease Control and Prevention and organized under the Georgia Division of Public Health. The goals of Georgia's Flood Recovery Program are divided into four areas. The first is to fully assess the public health service needs in terms of environmental health, infectious disease, child health, and chronic conditions of residents in the flooded area. Secondly, the project aims to undertake environmental health activities to identify the remaining concerns due to flood damage to well and septic tank contamination, damage and displacement at known hazardous waste sites, and breeding of disease-causing insects throughout the fifty-five affected counties. The third goal of the program is to produce a comprehensive, state Division of Public Health-focused, disaster response field operations manual. Finally, the Flood Recovery Program will develop the skill capacity of local and state personnel to undertake disaster response activities and to provide them with the data and equipment necessary to do so.

This paper describes a task under the second goal of the program. Specifically, the ArcView project discussed in this paper was designed to identify the hazardous waste sites mentioned above and to assist in the analysis of further impact upon public health through identification of human exposure pathways. All hazardous materials sites in Georgia are required by the Georgia Environmental Protection Division (EPD) to be listed under the hazardous site index (HSI). Due to the extensive area of flooding throughout southeast and central Georgia, several of the HSI sites were believed to have been affected. Analysis of the sites using a USGS-interpreted flood extent identified twenty-five sites within this flood boundary (View 2). It was then necessary to research each of these sites extensively and evaluate possible human exposure pathways. The Georgia EPD maintains comprehensive files on all of the sites on the HSI which are available to the public for research. These files include information on site characteristics, limited geographic and demographic data, site assessments, sample data, contaminant profiles, legal documents, and remediation parameters (if applicable).

HUMAN EXPOSURE PATHWAYS

There are countless scenarios where routes exist through which the public can be exposed to substances released to the environment. These human exposure pathways are generally considered to be composed of the following five elements (with examples):
  1. Source (landfill, spill)
  2. Transport media (groundwater, air)
  3. Exposure point (water well, food source, shower)
  4. Route of exposure (ingestion, inhalation)
  5. Receptor population (families, schoolchildren)
Exposure pathways are very complex and are constantly changing. In most cases, there are multiple exposure pathways present which may affect a number of different populations. Generally, the source of contamination is singular but may effect a variety of media. The transport media are determined through sampling efforts at the site where directional effects in terms of contaminant movement can complicate identification of exposure points. The exposure point is possibly the most difficult element of the exposure pathway to identify due to the great number of these points which can exist. The route of exposure is easily defined once the exposure points have been determined. Profiles of the contaminants which are present at these exposure points can help to more accurately determine the route of exposure. The receptor population is the last element of the exposure pathway and is determined only after all other elements are fully understood. Once exposure pathways are defined, steps can be taken to analyze the exposure to the public in terms of each exposure route.

The accuracy of the exposure pathway prediction is highly dependant upon the amount of data which is available to the assessor. Investigation of the site and the surrounding area is highly beneficial to the exposure pathway evaluation. If the assessor has an opportunity to examine the site or at least has access to detailed site information then the determination of each element of the pathway is strengthened, especially the exposure point. It is important to mention that human exposure pathways are only predicted and evaluated through the methodology described in this paper. The actual confirmation of a human exposure pathway is accomplished through a careful exposure assessment of populations. Exposure of contaminants to the public can only be confirmed only through extensive testing of off-site media, biota studies, and other analyses throughout the remaining steps of the PHA.

DATA REQUIREMENTS FOR GIS

An effective evaluation of human exposure pathways requires the initial gathering of extensive information. This information includes individual site research, maps, a number of databases, and GIS coverages. The following data is the information used by this method to support the GIS analysis of sites and to predict human exposure pathways: The HSI database includes locations, contamination data, and other information on each of the hazardous materials sites in the flood area. The census database has important information, arranged by block group, including population density, private/public well water data, and other relevant demographics. County highway maps and topographic quad sheets are helpful in assisting the user in locating a site with which he is unfamiliar. DOQQ's are greyscale aerial photographs and have a number of obvious applications to evaluating human exposure routes, especially in terms of the receptor populations. DEM's are models which provide information on flow direction and general topology. The landfill database provides locations for possible contributing sources of contamination or future additions to the HSI. The USGS flood extent was used to define the HSI sites which fall within the flooded area. The hydrologic and reach file coverages provide river, lake, and stream locations. Finally, the individual site research details specifics about each HSI site and the contaminant profiles are detailed descriptions of each site contaminant relevant to public health. Using the ArcView project described in this paper, this text information is made available to the user through hotlinks to a FrameReader application. FrameReader allows the user to move through documents by hyperlinks to get the necessary data needed for the analysis.

EXPOSURE PATHWAY EVALUATION USING GIS

Applying the use of GIS to the identification of human exposure pathways is done through a step-by-step analysis following the five elements mentioned earlier. The first three elements of the exposure pathway are the areas where GIS is used most effectively. The final two steps are determined through toxicological and epidemiological evaluation, respectively. To illustrate the method of GIS analysis on the first three elements, a groundwater contamination example will be used. Contamination of groundwater systems is a common threat to public health, especially in areas where public water is not readily available. Where private wells may be used for drinking water, the identification of possible human exposure pathways at the time of a reported release is especially vital. Making use of the data mentioned previously a scenario has been developed to illustrate exposure pathway evaluation using GIS. In this scenario there has been a reported release of a contaminant to the groundwater due to a leak in an underground storage tank (UST) in area where private wells exist will be examined.

The first element of the human exposure pathway, the source, is usually known at the time of release. The details of the release, contained within the site summary or from on-site investigation, suggest to the public health professional what event has occurred at the site. Also, when there is minimal information available about the site or the release, media sampling data is needed to identify not only the source but also the contaminant(s) which are involved. In the example scenario, the source is a leaking underground storage tank which was reported by the land owner to the health professional. The source of contamination, once defined, can be identified geographically and located on a view of the area.(View 3).

Evaluation of the release leads the assessor to the question of what media was impacted at the site. The transport media is easily identified by sampling data taken after the release. In broad terms, the media affected can be determined by inspection of the site and review of the release details. The problem with identifying the transport media lies in the variability of the contaminant presence in terms of spatial and temporal parameters. The availability of sampling information is a crucial factor in accurately defining not only the type of media which are impacted but, also, the extent of contamination which exists in that medium. GIS becomes a very useful tool in locating sample points in the vicinity of the contaminated site and defining an area which is initially affected by the release. The use of highway maps and quad sheets along with aerial photos aid the health professional in determining the actual impacted area in relation to landmarks with which he is familiar. In the UST release example, the transport media of concern is groundwater due to the presence of private wells in the area. A view of the site, incorporating the DOQQ for the area with a DOT highway road map, helps the assessor to identify the area which may be affected and apply sampling data to his evaluation (View 4).

Determination of possible exposure points is a difficult step in the process of predicting human exposure pathways. Exposure points can exist in such great numbers that the exactness of the defined contamination extent becomes a very influential factor upon the degree of accuracy with which the points can be predicted. The use of well point data and, in some cases hydrologic data, is useful when groundwater or surface water has been impacted. Census demographics can provide the health professional with population density data and drinking water source information for the pertinent blockgroups. DOQQ's and on-site inspection will aid in the identification of residences or other buildings which may be affected by the release. Site accessibility and other data obtained from the site summary and site inspection are also helpful. The use of digital elevation models and hydrologic reach file coverages become powerful tools when used with ArcView Spatial Analyst 1.0, especially in the case of soil contamination. Once the area of impact has been determined, areas around the site which may be affected in the future by surface flow can be predicted. This use of a Spatial Analyst 1.0 can help the health professional better define the possible exposure points over time. In the example of groundwater contamination, private well point data is useful in predicting possible points of exposure in the vicinity of the release. Plotted on the previous view of the site vicinity, the private well points can be examined in terms their relation to the release point (View 5).

The final two elements of the human exposure pathway are determined by non-GIS methods. The route of exposure is the link between environmental contamination and adverse effects on public health. Taking into account the contaminant profile(s), a toxicological evaluation provides important information on the route of exposure by which susceptible populations could be affected. In the case of the UST release where groundwater is contaminated and private drinking water wells exist, the probable route of exposure would be through ingestion. Knowledge of the physical and chemical characteristics of the contaminant as well as compound-specific human exposure dynamics can confirm the possibility that the public could be exposed. Once the routes of exposure are predicted, the receptor populations are determined through epidemiological analysis. GIS can provide some assistance in this final step if extensive census demographic data is available. The receptor population in the example mentioned previously would be the residents who are using the identified wells in the impacted area if the wells are found to be functional and are, in fact, being used for drinking purposes.

SUMMARY

The role of the health professional in assessing risk to public health has become increasingly important as public awareness of environmental contamination and identification of hazardous materials sites increases. Toxic releases to the environment at these hazmat sites are common and, when reported, require assessment of risk to populations in the vicinity of the site in a short time frame. In any case of a reported release, there are a number of individuals who demand to be informed of the risk to human health. While a comprehensive assessment of exposure to the public takes a great deal of time and effort, many questions can be answered by the identification of possible human exposure routes. Once these human exposure routes are defined, the process of evaluating the risk can begin. With the aid of GIS and sufficient data to support analysis, the public health professional can accurately and time-efficiently predict human exposure pathways at hazardous materials sites.

ACKNOWLEDGEMENTS

This analysis described in this paper was developed and supported 100% by federal funds from Centers for Disease Control and Prevention grant U1Q/CCU411900-01. Its contents are solely the responsibility of the authors and do not represent the official views of CDC.

REFERENCES

Agency for Toxic Substances and Disease Registry. (1994) Environmental Data Needed for
     Public Health Assessments, A Guidance Manual.

Derosa, Christopher T. (1996) Conference on Advances in Toxicology and Applications to Risk
     Assessment.  The Role of Health Guidance Values in Public Health Service.

Environmental Protection Division, Georgia Department of Natural Resources. (1995)
     Hazardous Site Inventory.
 
U.S. Environmental Protection Agency. (1994) TCSA Section 8(e) Triage Information Package.
 
Williams, P.L. and Burson, J.L. (1985) Industrial Toxicology: Safety and Health Applications in
     the Workplace.  New York, NY.: Van Nostrand Reinhold Company.

Williams, Stephen E.  (1996) Conference on Advances in Toxicology and Applications to Risk
     Assessment.  Environmental Risk Communication.
J. Scott Harris
Flood Recovery Program
Division of Public Health
2 Peachtree Street Room 2.509
Atlanta, GA 30303
Telephone 404.657.1945