Dustfall deposition samples were collected during the months of June, July, and August for 2 years prior (1973 and 1974) and 2 years following (1976 and 1977) the addition of an oil-fired, 660 MW generating unit (No. 3) to two 330 MW coal-fired units (No. 1 & 2) at Chalk Point located in Prince Georges County, Md. Triplicate collectors were placed at 12 research sites located at distances of 1.6, 4.8, and 9.6 km in the north, east, south, and west directions from the power plant and were analyzed for water-insoluble particulate matter and soluble and water-insoluble Zn, Cu, Mn, Pb, and Ni contents.
The average rates of deposition for water-insoluble particulates declined from 4.73 ± 3.67 g m−2 month−1 during 1973 and 1974 to 1.9 ± 1.79 g m−2 month−1 during 1976 and 1977. The reductions in particulate deposition rates coincided with a 37% reduction in power generation output from Units 1 and 2 and the onset of Unit 3 operations.
Prior to Unit 3 operations, soluble and total Ni deposition rates were found to be significantly (p < 0.05) related to direction; however, the rates of Cu, Mn, and Pb deposition were observed to be significantly (p < 0.05) higher at 1.6-km than at 4.8- or 9.6-km distances from the facility. After Unit 3 began operating, rates for Cu, Mn, Zn, and Pb deposition were significantly (p < 0.05) reduced and showed no relationships to distance or directions from the source. The shifts in soluble Ni and soluble Pb deposition rates from pre- to post-Unit 3 operations were particularly noteworthy. The concentrations of Cu, Mn, and Zn in water-insoluble particulates were higher with increased distance and showed significant (p < 0.05) increases after Unit 3 began operation.
The relative rankings for average rates of total deposition for the five metals during pre-Unit 3 operations were Pb (191 µg m−2 day−1) > Cu ≅ Zn > Mn > Ni (11 µg m−2 day−1), which changed to Cu (98 µg m−2 day−1) > Pb ≅ Zn > Mn > Ni (12 µg m−2 day−1) after Unit 3 began operations.