Evaluation of EPA's Addendum to the Baseline Human Health Risk
Risk-based Concentrations for Exposure of Recreational Visitors
at Arrowstone Park to Arsenic in Soil and Tailings
Clark Fork Operable Unit of Milltown Reservoir Sediments
National Priorities List Site
MILLTOWN RESERVOIR SEDIMENTS
MILLTOWN, MISSOULA COUNTY, MONTANA
EPA provided ATSDR with an addendum  to their Baseline Human Health Risk Assessment  for the Clark Fork River operable unit of the Milltown Reservoir Sediment National Priorities List site. The addendum calculates risk-based concentrations (RBCs) for chronic and acute exposures of child and adult park visitors to arsenic in soil from Arrow Stone Park south of Deer Lodge, Montana. EPA requested that ATSDR comment whether the child chronic RBC of 684 parts per million (ppm) arsenic would be protective of public health. This document is an update of a July 2001 draft document, which commented on a provisional version of the addendum . The calculated chronic RBC is the same for both documents. This document also reflects the recent sampling effort undertaken by ATSDR and EPA to characterize soil arsenic levels in Arrow Stone Park, which is reported elsewhere .
This document evaluates the RBC for children's chronic exposures to arsenic. The chronic RBC is lower than the subchronic RBC and will therefore be protective for subchronic exposures as well.
We have not evaluated exposures for children exhibiting soil pica (high soil consumption) behavior in this document. Further discussion between EPA and ATSDR is necessary prior to evaluating this type of exposure and related RBCs. In Arrow Stone Park, the use by small children is not routine and/or concentrated in a well-defined area, so it is reasonable to use chronic exposure scenarios for general planning decisions.
- ATSDR concludes that the EPA RBC of 684 ppm for arsenic in soil is protective of public health for the chronic and subchronic exposures considered in the EPA addendum.
- The recently collected soil arsenic data allow a definitive comparison of arsenic levels at Arrow Stone Park with the chronic RBC. Soil samples were taken in specific usage areas of the park (with emphasis on areas frequented by children). Specific exposure areas with average arsenic concentrations less than 684 ppm were deemed safe for occasional use by children as described in the addendum.
- ATSDR's concurrence with the 684 ppm RBC excludes a consideration of pica exposures to arsenic, which may lead to mild, transient health effects. As a preventive measure, it is prudent to inform parents of the potential danger of small children consuming arsenic-contaminated soil.
- ATSDR would like to work with EPA to develop sampling plans to address specific activity locations within the park and to evaluate acute exposures at Arrow Stone Park through biomonitoring and observation of children's activity patterns in high use areas.
ATSDR evaluated the 684 ppm arsenic RBC using ATSDR's standard exposure dose formula and exposure assumptions as described below. In most cases, these assumptions match the assumptions in the EPA addendum. Slight differences in some of the values used by the two Agencies were examined to ensure that the different values did not significantly alter the conclusions.
For the chronic arsenic RBC, EPA assumed that a child ingested 200 milligrams (mg) or 0.0002 kilograms (kg) of contaminated soil per day for 48 days a year for 10 years . The average weight for this child was assumed to be 20 kg. A relative bioavailability of 50% was used; this is the value identified for soils from the Clark Fork River area . Allowing an exposure dose equal to EPA's chronic reference dose of 0.0003 mg/kg/day, the RBC is calculated as 684 ppm.
For ATSDR's evaluation, the arsenic RBC was used as the average soil concentration, and an exposure dose for each age of child from 1 to 15 years old was calculated. These exposure doses were then compared to ATSDR's minimal risk level (MRL) for chronic exposure to arsenic of 0.0003 mg/kg/day . Assumptions regarding exposure frequency and relative bioavailability were similar to those in the EPA addendum, except that children over 5 years old were assumed to ingest only 0.0001kg of soil per day. Values for ingestion rate and body weight were obtained from the EPA Exposure Factors Handbook . The ATSDR exposure dose formula is:
ed = c * ir * ef * af / bw , where
ed = exposure dose; c = concentration in media of interest; ir = ingestion rate; ef = exposure factor; af = absorption factor; and bw = body weight.
The exposure dose formula used by EPA in the addendum is of a different form, but similar to the above equation. Values used to calculated ed (exposure dose) in our evaluation are discussed below:
|c =||set at the chronic RBC of 684 ppm arsenic.|
|ir =||0.0002 kg/day for children under 5; 0.0001 kg/day for children 5 years and older.|
|ef =||0.132; equal to (48/365), fraction of days per year exposure occurs.|
|af =||0.5, equal to 50% relative bioavailability.|
|bw =||average body weight for 1-, 3-, 5-, 7-, 11-, and 15-year old children, equal to 11, 15, 20, 25, 40, and 65 kg, respectively.|
All the exposure doses calculated for children 5 years and older were below the chronic health guideline, indicating that health effects are unlikely, especially for intermittent exposure scenarios. For children under 3, the calculated exposure doses were higher than the health guideline. However, it is unlikely that health effects would occur for these exposures. The calculated exposure doses are more than 18 times lower than the lowest observed adverse effect level (LOAEL) in toxicological studies on arsenic . Health effects are unlikely because of the conservative (i.e., health protective) assumptions made by both ATSDR and EPA in their calculations. We did not evaluate exposures resulting from pica behavior. If pica behavior occurred, it could potentially result in higher exposures than those evaluated in this document.
There are a number of uncertainties associated with this evaluation. They are similar to those detailed in the Baseline Risk Assessment  developed by EPA. The key uncertainties are intake rates, exposure routes not quantified, and bioavailability.
Jill J. Dyken, Ph.D., P.E.
Environmental Health Scientist
Superfund Site Assessment Branch
Division of Health Assessment and Consultation
John R. Crellin, Ph.D.
Senior Environmental Epidemiologist
Superfund Site Assessment Branch
Division of Health Assessment and Consultation
- Syracuse Research Corporation. Baseline human health risk assessment for the Clark Fork River operable unit of the Milltown Reservoir Sediments National Priority List site. Addendum 1. Risk-based concentrations for exposure of recreational visitors at Arrowstone Park to arsenic in soil and tailings. Prepared for the US Environmental Protection Agency, Region VIII, Denver, CO; 2001 October.
- Roy F. Weston, Inc. Baseline human health risk assessment for the Clark Fork River operable unit of the Milltown Reservoir Sediments National Priority List site. Prepared for the US Environmental Protection Agency, Region VIII (Montana Office), Helena, MT; 1998 January.
- Syracuse Research Corporation. (Provisional) Baseline human health risk assessment for the Clark Fork River operable unit of the Milltown Reservoir Sediments National Priority List site. Addendum 1. Risk-based concentrations for exposure of recreational visitors at Arrowstone Park to arsenic in soil and tailings. Prepared for the US Environmental Protection Agency, Region VIII, Denver, CO; 2001 July.
- Dyken, Jill J., Crellin, John R. Environmental Media Investigation Report for Arrow Stone Park in Deer Lodge, Montana. US Department of Health and Human Services, Agency for Toxic Substances and Disease Registry, Atlanta, GA; 2002 (in preparation).
- ATSDR. Toxicological profile for arsenic. Update. US Department of Health and Human Services, Agency for Toxic Substances and Disease Registry, Atlanta, GA; 2000.
- EPA. Exposure factors handbook. US Environmental Protection Agency, Office of Research and Development. EPA/600/C-99/001, Washington, DC; 1999 February.