/m3  and the maximum off-site nickel concentration (and an exposure factor based on duration of exposure of nine years).
WKWMA workers may have been present in buffer areas from 500 to 1000 m from building C-746A. Concentrations at points 500 m from C-746A will be used to evaluate potential worker exposures. WKWMA worker exposures are evaluated using the National Institute of Occupational Safety and Health (NIOSH) Recommended Exposure Limit of 15µg/m3 for soluble or insoluble nickel (8 hour time weighted average(2) ) . Nickel soil concentrations are compared with estimated air concentrations to determine if estimated nickel dispersion and deposition are reflected in nearby soils.
Activities of radiological contaminants from nickel samples (Th 230, Th 232, U 235 and Tc 99) and stack emission monitoring (U234, U235, and U238)  are used to determine the quantity ofradionuclides released during nickel smelting. Calculated quantities and rates of radionuclideemissions from nickel smelting are about 10,000 times lower than airborne radionuclide emissionsfrom other PGDP process operations used in Appendix E. Consequently, radiological emissionsfrom the smelting operations did not constitute a significant radionuclide source and will not changepreviously estimated releases or exposures.
The estimated average annual concentrations of nickel particulates (based on 1984-1986 emission data and hourly weather data for 1990) are shown in Figure I-1. The highest estimated annual average nickel concentration of 0.18µg/m3 occurred approximately 200 m north of the C-746A building which reflects prevailing southerly winds. The maximum one hour average concentration is estimated to be 15µg/m3 and the maximum eight hour average is estimated to be 3.0µg/m3 with both maxima about 200 m north of the stack. Distribution of short term nickel concentrations, although higher to the north, are more uniformly distributed around the C-746A building and reflect short term weather conditions which may have winds from any direction. Model runs using hourly weather data(3) for the years 1989, 1990 and 1991 produced essentially identical results for both short term (hourly and daily) and long term (annual) nickel dispersion. All of the above maximum air concentrations occurred at areas within the DOE property.
The estimated nickel air concentrations at areas of potential off-site exposure (Table I-1) includeseveral notable trends. First, short term concentrations are much higher than long termconcentrations. This trend reflects how short term meteorological conditions control transport andhow averaging time affects concentration. One hour maxima are significantly greater than eight hourmaxima and much greater than annual average concentrations. Second, overall concentrations dropvery rapidly with distance away from the stack. At 1000 m, which is the distance to the closestresidence, all concentrations are about an order of magnitude lower than concentrations at 500 m.Third, the similarity between concentrations and distributions for different weather years indicatesthese data adequately capture the range of potential meteorological conditions.
The dispersion modeling indicates that most nickel deposition will occur within 500 m of the smelterbuilding. Measured soil nickel concentrations support the model results. Average soil concentrationsat locations within the security fence average 25 ppm. Areas within the buffer zone and off-site areashave statistically similar averages of ~12 ppm. These results indicate that onsite soils have elevatednickel concentrations relative to off-site areas but that there is no significant difference betweenbuffer area and off-site concentrations. Local nickel air concentrations and deposition to soil fromthe TVA plant may have been higher prior to the increase in stack height in the early 1970's .
The highest average annual nickel concentration at 1000 m from the smelter building is 0.004 µg/m3 which is about 50 times lower than the ATSDR comparison value of 0.2 µg/m3. Estimated long term exposures at 1000 m also represent n