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Imperviousness and trace metals in stream sediments : urbanisation in the Lower Fraser Valley between 1973-2006 Iwata, Oh


Impervious surfaces have been used as a good indicator of urban impact on aquatic health because urban runoff impacts the biodiversity of aquatic organisms. The impact of urbanization on metal concentrations in sediments was examined in 28 sub-watersheds in the Lower Fraser Valley. GIS techniques were used to determine rate of urbanisation in 1973 and 2006 and the metal concentrations in sediments in 2006 were then compared with historic data collected at the same sites. In 2006 the highest imperviousness was observed in the Alouette and N /W Vancouver sub-watersheds (50-70% and 40-60%). All sub-watersheds experienced increases in imperviousness, (Δ 1-30%o), which led to increases in trace metals concentrations in sediments (e.g., Cu, Pb, Zn, Ni). Zn and Ni were the most sensitive metals related to forest loss, increases imperviousness, and traffic, whose impacts were most evident within a 100 m riparian buffer zone. The % imperviousness forms a curvilinear relationship with aquatic biodiversity indices, with a threshold of 10-15 % of imperviousness in the early stages of urbanisation. An opposite curvilinear relationship exists between % imperviousness and several trace metals. The rate of increase in metals tends to be significantly higher in the early stages of urbanisation but flattens out once high level of imperviousness was achieved, possibly due to sediment saturation. Several factors account for significantly higher metal levels in N /W Vancouver than in Alouette, two sub-watershed with similar imperviousness: 1) higher traffic density, 2) enhanced storm sewer system connections, and narrower vegetative stripes within a buffer zone, 3) abundant phosphorus, and 4) higher organic matter. Imperviousness is not the direct cause of stream degradation, but is considered the conveyance system that brings pollutants directly into the stream. Complex interactions between these and other factors all influenced the high metal levels, and traffic was likely the principal source. KEY WORDS: Imperviousness, trace metal, stream sediment, land use, automobile traffic, zinc, riparian buffer zone, Lower Fraser Valley, GIS

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