Urban atmospheric mercury: The impact of local sources on deposition and ambient concentration in Detroit, Michigan

Elevated levels of mercury in effluent from the Detroit Waste Water Treatment Plant led to a study of the impact of atmospheric deposition on the overall loading of mercury at the head of the plant. The data generated in this study has allowed the most comprehensive evaluation of mercury cycling in an urban environment to date, and has provided new insight into the differences between urban and rural pollutant behavior. Measurements were made of ambient vapor-phase and particle-phase mercury, mercury dry and wet deposition flux, as well as runoff mercury flux at three sites in Detroit, from April through October of 1996. An intensive evaluation of the behavior of trace elements in runoff was performed to investigate the importance of traffic on mercury levels in runoff. Comparison of the behavior of mercury in the urban environment to that of a suite of other trace metals sampled concurrently elucidates the unique aspects of the urban mercury cycle and the relative impact of different source categories (e.g. coal-burning, steel production, and waste incineration). The importance of the dry deposition of particle phase mercury to the urban mercury cycle was clearly defined. Dry deposition fluxes of mercury displayed a strong spatial gradient, directly reflecting the proximity of the sampling site to industrial mercury sources. The dry deposition flux of mercury at the most industrial sampling site averaged 51.1 ng m−2 day−1, while the mean dry deposition flux at the residential sampling site averaged 9.9 ng m−2 day−1 . In contrast, the wet deposition of mercury displayed little spatial gradient across the city over the course of this study, with the volume-weighted mean mercury concentration in precipitation ranging from 19.3 ng/L at the industrial site to 17.2 ng/L at the residential site. Dry and wet deposition contributed approximately equal amounts to the total mercury deposition in the most industrial area, whereas wet deposition mercury dominated at the residential site. The type of impervious surface (e.g. highway or street) appears to be a dominant factor in determining the magnitude of the impact of particle bound mercury in runoff in contrast to other trace metals, which is critical to consider in future studies of mercury deposition in urban environments.