BUNKER HILL MINING AND METALLURGICAL
KELLOGG, SHOSHONE COUNTY, IDAHO
As part of its 5-year review process, the U.S. Environmental Protection Agency (EPA) asked the Agency for Toxic Substances and Disease Registry (ATSDR) to perform a public health consultation on sampling data acquired from hillsides on the Bunker Hill National Priorities List site. The purpose of the consultation is to determine if sources of contamination or recontamination are present at levels of public health concern.
The Bunker Hill National Priorities List site is located in Shoshone County, Idaho, approximately 40 miles east of Coeur d'Alene, Idaho. The site covers about 21 square miles and includes the cities of Kellogg, Page, Pinehurst, Smelterville, and Wardner (1-4). This health consultation covers the unpopulated areas outside these cities. Over 7,000 people reside in the area. Cleanup in the populated areas has been ongoing since the smelter closed in 1981. About 6,000 acres of hillside property are suitable for some type of future development. Discharges from mining and smelting activities have caused the property to become contaminated with varying levels of heavy metals, including lead, arsenic, cadmium, and zinc. This public health consultation will focus on lead, because data are readily available.
Past studies and ongoing sampling at the Bunker Hill Superfund site indicate that mine tailings and mine related wastes have contaminated the area's hillsides over the past 100 years. The Bunker Hill facility is currently closed. Exposure and adverse health effects (resulting primarily from lead exposure) have been reported by area residents. Removal or remedial activities have not occurred in the unpopulated areas, to date.
It is possible that undeveloped property will be developed in the future. Soil samples were collected from various undeveloped properties in the site area at depths of 0-1, 1-6, 6-12, and 12-18 inches. The samples were analyzed by use of inductively coupled plasma lead analysis, x-ray fluorescence (XRF), or flame atomic absorption (FAA) spectrophotometry (3,4).
ATSDR evaluates hazards associated with environmental contaminants in two phases (5). The first screening phase consists of several prescribed steps, and the second phase is based on professional judgment. In the first step of the screening phase, ATSDR compares the concentrations of contaminants to which the community could be exposed to health comparison values. Exposures at levels less than or equal to these comparison values are not expected to make people sick and are thus considered to be "safe" levels, even under the conditions of maximum exposure. For example, comparison values for chronic-duration (long-term) exposures reflect lifelong, continual exposure. While it is unlikely that anyone would be exposed continuously or for a lifelong duration, it represents a worse-case scenario. If a contaminant concentration does not exceed a comparison value, ATSDR generally does not need to evaluate the contaminant any further, because even under the maximum conditions of exposure, an individual would not be expected to get sick.
If a contaminant concentration exceeds a comparison value but is below levels at which adverse health effects have been reported, ATSDR performs a more in-depth evaluation using realistic exposure scenarios. This evaluation describes the exposure and its possible health effect. It does not predict whether an individual will get sick. For example, if an individual were exposed to a contaminant half as much as another individual, the risk of the shorter exposure would be half as great. However this risk does not mean that the individual will get sick. ATSDR does not know if either individual will get sick. Rather ATSDR uses the evaluation method to assess an individual's risk of getting sick from a specific exposure.
In determining the estimated exposure, ATSDR uses the maximum concentration of contaminants. Use of the maximum concentration will result in the most protective evaluation for human health. For soil, the ingestion rates used are 50 milligrams per day (mg/day) for adults, 200 mg/day for young children, and 500 mg/day for toddlers. A small child (aged 1 to 3 years) may on occasion ingest up to 5000 mg/day (one teaspoon per day of contaminated soil). Standard body weights for adults, young children, and toddlers are 70 kilograms (kg), 16 kg, and 10 kg, respectively. Some exposures may be intermittent or irregularly timed. For those exposures, an exposure factor (EF) that averages the dose over the exposed period is calculated. When unknown, the biological absorption from the environmental medium is assumed to be 100%. Lifetime cancer risk (70 years) is also evaluated.
ATSDR comparison values are media-specific concentrations that are considered to be safe under default conditions of exposure (5). These concentrations are used as screening values in the preliminary identification of site-specific "contaminants of concern". The term contaminants of concern should not be misinterpreted as an implication of "hazard". As ATSDR uses the phrase, a "contaminant of concern" is merely a chemical substance detected at the site in question and selected by the health assessor for further evaluation of potential health effects. Generally, a chemical is selected as a "contaminant of concern" because its maximum concentration in air, water, or soil at the site exceeds one of ATSDR's comparison values.
Comparison values are not, however, thresholds of toxicity. While concentrations at or below the relevant comparison value may reasonably be considered safe, it does not automatically follow that any contaminant concentration that exceeds a comparison value would be expected to produce adverse health effects. The whole purpose behind highly conservative, health-based standards and guidelines is to enable health professionals to recognize and resolve potential public health hazards before they can become actual public health consequences. Thus, comparison values are designed to be preventative, rather than predictive, of adverse health effects. The probability that such effects will actually occur depends, not on environmental concentrations alone, but on a unique combination of site-specific conditions and individual lifestyle and genetic factors that affect the route, magnitude, and duration of actual exposure.