Mobility and Bioavailability of Trace Metals in the Water-Sediment System of the Highly Urbanized Brunette Watershed

Selected trace metals were determined in stormwater runoff and sediments of the highly urbanized Brunette watershed in Metro-Vancouver. Surface sediment samples from three tributaries and a lake between 1974 and 1998 were analyzed for total and acid-extractable trace metals. Metal bioavailability was also investigated using Chelex-100 resin. Sediment geochemistry was determined by sequential extraction. Total trace metal concentrations decreased as stormwater moved through the hydrologic gradient of stormwater runoff, headwater stream to outflow river. The percentage of dissolved metals increased downstream largely due to disposition. Higher concentrations of particle-associated trace metals were flushed in stormwater runoff as the rainfall and total suspended solids transport increased. The highest trace metal levels were found in the lower reaches of a creek before entering the lake and in the lake where organic matter accumulated. Copper was associated with the organic/sulphur sediment components, whereas iron and manganese were mainly mineral-bound. Zinc concentrated in the easily acid reducible phase, augmented by increasing traffic and development. At least half of the sediment-bound lead was associated with the easily acid reducible and organic/sulphur-bound phases with an overall decrease as lead has been phased out as a gasoline additive.