l Protection Agency (EPA) Industrial Source Complex (ISC) air dispersionmodel . This model uses meteorological data from the Paducah-Barkley Airport (1989-91), andparticulate emission and source data from the PGDP air discharge permit application .
The smelter emission rate (nickel particulates) used in the air dispersion model is based on measuredemission rates described in the air discharge permit application . Three trial runs provide anaverage emission rate of 160 g/hr (0.044 g/sec) based on an average smelter charging load of 1520kg/hr (3350 lbs/hr). Modeled emission rates were rounded up to 0.05 g/sec. The 20 million poundsof nickel smelted from late 1983 to 1986 is equal to an average of ~6.7 million pounds/year. This 3-year time frame represents the period of highest annual nickel output and will be used to evaluatepotential air concentrations.
Although the specific operating schedule of the smelter is not known, an average annual productionof 6.7 million lbs/yr divided by the loading rate of 3350 lbs/hr equates to an operating schedule of 8hrs/day for ~250 days/yr. Variation in the operating schedule, such as 16 hrs/day for 125 days/yr,will not change the instantaneous emission rate nor significantly change the annual dispersedconcentrations. Some variation in short term concentrations may occur as a result of specific weatherconditions during smelter operation. However, multiple model runs were conducted usingmeteorological data from three years (1989, 1990, and 1991) to ensure that the range of weatherconditions were evaluated for both short term (one and eight hour maximum averages) and longterm (annual average) concentrations. Meteorological data from the 1984 to 1986 years are notavailable on the EPA website .
In addition to emissions from the PGDP smelters, nickel and other metal particulates were (and arestill) released from combustion of fossil fuels at the PGDP steam plant and the Tennessee ValleyAuthority (TVA) Shawnee facility. Instantaneous emission rates from the TVA facility are estimatedfrom annual releases submitted to the Environmental Protection Agency Toxic Release Inventory(TRI) . The 1999 TRI indicates that 159 pounds of nickel compounds were released to air, the1998 release amount was 137 pounds. In order to account for yearly variations, this evaluationestimates an annual release of 170 pounds which, distributed over the course of an entire year,equals an emission rate of 0.0025 g/sec (released from a 244 m stack).
Airborne emissions released from the PGDP steam plant occur from two 34 m stacks that are inclose proximity to the each other and are modeled as a single source. Nickel emissions from thePGDP steam plant are estimated from maximum monthly coal and fuel oil usage (3300 tons/monthcoal and 400,000 gallons/year for fuel oil)  and standard emission factors . The estimatedinstantaneous maximum nickel emission rate from the steam plant (0.00009 g/sec) is more than 500times lower than smelter emissions. However, the steam plant was assumed to operate 24 hours/day,7 days/week for an annual emission rate about 100 times lower than the smelter emission rate.
The nickel releases from all sources are predominantly in a nickel oxide form [3;10]. Particulatereleases from the smelter are predominately about 30 microns in diameter with small fractions in the10 and 20 micron diameter size ranges . Nickel particulates from the PGDP steam plant andTVA facilities are predominately less than 5 microns in diameter [3,10].
The distance of the C-746A smelter building to the closest residence is more than 1000 meters (0.6 miles). Estimated air concentrations 1000 m from the source are used to evaluate potential residential exposures. Results of the air dispersion modeling are compared with the ATSDR health-based, non-cancer, comparison value (0.2µg/m3 chronic inhalation EMEG/MRL) [3,11] to determine if estimated long term air concentrations are at levels of health concern for residents. Potential cancer effects are evaluated by multiplication of the EPA unit risk for nickel subsulfide(1) of 4.8E-4 per µg