Average concentrations

The reason for calling equation 8.3-1 a "Gaussian diffusion model" is because it has the form of the normal/Gaussian distribution (equation 2.5-2). Concentration averages for long time intervals may be calculated by averaging the concentrations at grid elements over which the plume passes. 

Time-Weighted Average (TWA) An allowable e,xposure concentration averaged over a noriiial 8-h workday or a 40-h workweek. 

There is limited information available regarding the distribution of methyl parathion after dermal exposure in humans. Two subjects, dermally exposed to methyl parathion, had 2.74 and 1.23 mg on their hands. Twenty-four hours after exposure, the serum levels were 0.027 and 0.032 mg/L, respectively (Ware et al. 1973). Twelve hours after cotton fields were sprayed, five men entered the treated fields for 5 hours. An average of 1.7 mg methyl parathion was detected on their hands. Serum concentrations averaged 0.156 mg/L in these subjects after 3 hours of exposure. Levels decreased to 0.1 and 0.002 mg/L at 2 and 24 hours postexposure, respectively (Ware et al. 1975). Although 0.5 mg methyl parathion was detected on the hands of four subjects, none was found in the serum (Ware et al. 1974). No information on the tissue distribution of methyl parathion in humans was found. 

Time-Weighted Average (TWA)—An allowable exposure concentration averaged over a normal 8-hour... 

Concentration averaged over the cross section of a tube cf (co) Laplace transform of the effective concentration field... 

No differences were observed in the Au concentration found in roots of plants treated with lCU40mgAuU (317-350 mg kg dry mass). However, the roots of plants exposed to 80, 160, and 320mgAuU, concentrated (average + SE) 998 + 63, 1779 161, and 5704+109, respectively [75]. On the other hand, it was observed that the translocation of Au from roots to shoots was higher in plants exposed to lOmgAuU (ratio shoot/root of 0.2) and lower in plants exposed to 320mgAuU (ratio of 0.04). 

The GC/FID conditions were as follows column, 1.5% OV-17 (2 m x 3-mm i.d.) glass column N2 carrier gas flow rate, 45mLmin temperature of injection port, column and detector, 240,235 and 235 °C, respectively. The recoveries of these amino derivatives with fortification level ranging from 0.5 to lO.Omgkg" were 62-101% for chlornitrofen, 62-101% for nitrofen and 58-101% for chlomethoxyfen, and satisfactory recoveries from soil were obtained at high concentrations, but the recoveries at lower concentration averaged about 66% for the least recovered compound. Interference from other substances in the soil extracts derived from the acetylation reaction was negligible. 

Although there is a distribution of residence times, the complete mixing of fluid at the microscopic and macroscopic levels leads to an averaging of properties across all fluid elements. Thus, the exit stream has a concentration (average) equivalent to that obtained as if the fluid existed as a single, large fluid element with a residence time of t = V/q (equation 2.3-1). 

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