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Atmospheric deposition is a largely unmonitored, but potentially large source of pollutants to aquatic ecosystems and their surrounding watersheds. The problem is further exacerbated in urban environments where the sources of pollutants to the atmosphere are numerous and the washoff of these contaminants once they deposit are enhanced due to imperviousness. The goal of this study was to estimate the deposition of trace metals to Santa Monica Bay and its watershed on varying temporal and spatial scales. Trace metals are responsible for numerous impaired waterbodies throughout the southern California region. Since atmospheric deposition represented a potentially important load, this study provided the opportunity to account for this source in total maximum daily load (TMDL) development. Trace metal loads from atmospheric deposition were estimated using a combination of aerosol sampling, water sampling and atmospheric transport and fate modeling.

The annual atmospheric deposition of chromium, copper, lead, nickel and zinc exceeded the estimated annual loads from industrial and power generating station effluents to Santa Monica Bay. Atmospheric deposition rivaled and, at times, exceeded the trace metal loads from municipal wastewaters to the Bay. In the Santa Monica Bay region, the majority of the atmospheric deposition loading occurred during dry weather. Interestingly, estimates of annual stormwater emissions from the Santa Monica Bay watershed was similar to the estimated annual dry deposition loading to the watershed. Steep gradients in deposition were observed, peaking near the downtown industrial and commerce center, decreasing towards the coast and further declining as one moved offshore. These gradients are consistent with the atmospheric flow patterns in the airshed. Based on modeling scenarios, it appears that sources of trace metals responsible for most atmospheric deposition in Santa Monica Bay arise! from within the Santa Monica Bay watershed.