Mercury fate and transport in urban watersheds

Mercury (Hg) is a pollutant of global concern due to its high toxicity at low concentrations. Most emissions of Hg are to the air, however, the majority of human exposure to Hg is through the consumption of contaminated fish. Therefore, to link human exposure of Hg to emissions requires understanding its environmental cycle. The focus of this research is on fluxes of Hg in the urban environment with a particular focus on Hg air-to-surface deposition, surface-to-air emission, and surface-to-water washoff. The main study location was Toronto, Ontario, however samples were also collected from Austin, Texas. Mercury was found to deposit and accumulate on urban surfaces at a linear rate over time when not exposed to influence of washoff. The accumulation of Hg on urban surfaces was spatially variable throughout the urban environment (0.82 ng/m2/d to 2.8 ng/m2/d) and significantly higher compared a rural location (0.58 ng/m2/d). The surface-to-air emission of Hg from pavement was spatially variable throughout the city (median range from below the detection limit to 5.2 ng/m2/h), and was related to the concentration of Hg associated with street dust. The amount of solar radiation was also shown to have a significant influence on the emission of Hg from the surface. During rain events, high Hg concentrations were measured at the beginning of runoff (up to 76.0 ng/L), which quickly decreased during the remainder of the event. The flux of Hg was driving by the discharge and the largest Hg loads were always associated with the period of peak discharge. Depending on the rain event characteristics, the urban surface was a net sink or a net source of Hg. Simulated rain experiments on an urban roadway showed that the number of antecedent dry days was associated with the amount of Hg mobilized in runoff. The simulated rain events also determined that the Hg in rain became quickly bound to surface particles and that the majority of Hg in runoff was derived from the surface.