Atmospheric deposition of trace metals in the Los Angeles coastal air basin (California)

Emissions of trace metals to the atmosphere and subsequent deposition represents a potential source of contamination to surface waters throughout coastal Los Angeles. However, few studies have quantified atmospheric deposition in Southern California. The purpose of this research was to increase our understanding of the role of atmospheric deposition as a potentially important source of trace metals to aquatic environments in coastal Los Angeles. The focus was on trace metals with detrimental environmental impacts on waterbodies in this region, including chromium, copper, lead, nickel, and zinc. Seasonal measurements of dry deposition were made at six urban and one non-urban site for one year. Dry deposition was significantly higher at urban sites compared with the non-urban site, while seasonal and spatial variability between urban sites was low. The estimated annual trace metal loads from dry deposition to the land surface of watersheds in coastal Los Angeles were far larger than the estimated annual trace metal loads in stormwater emanating from these watersheds. In addition, wet and dry deposition measurements made over one year in a small urban catchment indicate dry deposition dominates because of limited rainfall in Southern California. Based on the ratio of total deposition to stormwater runoff measured within the catchment, atmospheric deposition potentially accounted for as much as 40 to 70% of the total trace metal loads in stormwater within the catchment. Finally, because road dust was identified as a major source of particle-bound trace metals, measurements were made of the dry deposition gradient of trace metals near the I-405 freeway. Trace metal deposition was increased in the immediate vicinity of the freeway, and reduced to urban background within 450 m, especially for copper, lead and zinc. These data provide a first approximation of the contribution of atmospheric deposition to trace metal loadings in urban waterbodies in Los Angeles, and information on an important source of the deposited metals. The findings demonstrate atmospheric deposition represents a potentially large source of trace metals to urban waterbodies, and should not be ignored when assessing sources of trace metal pollution to contaminated waterbodies near urban centers.