In September 2000, the North American Commission for Environmental Cooperation (NACEC) released a report entitled "Long-range Air Transport of Dioxin from North American Sources to Ecologically Vulnerable Receptors in Nunavut, Arctic Canada" prepared for the NACEC by Dr. Barry Commoner and his colleagues at the Center for the Biology of Natural Systems (CBNS). This report details a computer modeling study of the transport of airborne dioxin (that is, poly-chlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans, also referred to by the acronym PCDD/PCDF) from widely distributed sources on the North America continent to the Canadian polar territory of Nunavut. This computer study suggests that dioxin formed during burning of high chlorine-content fuels, such as municipal solid waste (MSW), can be transported thousands of miles through the atmosphere before being deposited in the Canadian arctic." The City of Ames has asked you as the City's consultant to determine what the ground level concentrations for dioxin would be assuming the estimate from Commoner's study. They also want you to calculate the cancer risk is for an individual living at the point of maximum downwind concentration. The Commoner study assumed an emission of 58 g dioxin per year. The City (in conjunction with engineers at ISU) has estimated that the Commoner study greatly overestimated the dioxin emissions and has come up with its own estimate of 0.24 g dioxin per year. From the Iowa DNR permit (iowadnr.gov), we have determined that the stack height is 200 feet from ground level, the stack opening is 96 inches in diameter, and the exhaust flow rate is 142,000 cubic feet per minute at 252F. Assume that the atmospheric conditions are stable (Class D) at 101 kPa and 22C, and the wind velocity is 4.5 m/s. 1. Plot the ground level concentration of dioxin as a function of distance from the stack and determine maximum ground level concentration and the distance from the stack where the maximum concentration occurs. Plot for both the Commoner estimate (58 g/y) and the City estimate (0.24 g/y). . . 1 TR = RBSL. IR EF. ED SF, BW. ATC.365 d. 103 HS y mg . 2. Using the Commercial/Industrial (outdoor) default parameters, what is the target risk for an individual at the point of maximum concentration directly downwind from the plant? Calculate the risk for both the Commoner estimate and City estimate. The general risk model is: Risk model parameters Parameter Residential Default TR = target risk for example, 104 to 106 RBSL = risk based screening level, ug/m' chemical/site specific IR = inhalation rate, m /d (indoor) 15 (outdoor) 20 EF = exposure frequency, dly 350 ED = exposure duration, y 30 BW = body weight, kg 70 AT = averaging time for carcinogen, y SF; = slope factor for inhalation (mg/kg d)' chemical specific Commercial/Industrial for example, 104 to 106 chemical/site specific 20 20 250 25 70 70 70 chemical specific Sample slope factors for risk model Slope Factors SF Trichloroethylene (TCE) Benzene Benzo(a)pyrene 2,3,7,8-TCDD (dioxin) 0.006 0.029 7.3 1.16x10
