Total and size-fractionated mass of road-deposited sediment in the city of Prince George, British Columbia, Canada: implications for air and water quality in an urban environment

Purpose The urban sedimentary system is attracting increasing interest because of its role in influencing air and water quality. A large amount of road-deposited sediment (RDS) lies on the road network of Prince George, a city of about 80,000 people, in British Columbia, Canada. The objectives of this study were: (1) to determine the total mass of RDS within the city, and how this varied over time and space; and (2) to determine the temporal and spatial variations in the particle-size fractions of the RDS. Materials and methods Samples of RDS were collected using a grid network during two different time periods in the snow-free season in 2009. Composite samples for each grid (n = 46) were fractionated into five grain-size classes (> 500, 250-500, 125-250, 63-125, < 63 mu m) using stainless steel sieves. Fractionated sediment samples were weighed to obtain a mass for each size class for each grid cell. Results and discussion The total amount of RDS (all particle-size fractions) in the city of Prince George was estimated to be 746 x 10(3) and 204 x 10(3) kg for the summer and fall 2009 sampling periods, respectively. Based on a total road length of 1,030 km within the sampled area of the city, this equates to an average of 724 and 198 kg per km of road, for the summer and fall sampling periods, respectively. The RDS was dominated by the > 500-mu m fraction, and there was a trend of decreasing amounts (by mass) for the finer particle-size fractions. In terms of the most important particle-size fraction from an air and water quality perspective, the < 63-mu m particle-size fraction accounted for, on average, 6.5% and 4.8% of the total RDS mass for the summer and fall 2009 sampling periods, respectively; it is estimated that an additional approximately 2% was lost to the air during sample collection, and thus values may be closer to 9% and 7%, respectively. This equates to averages of between 47-61 and 10-13 kg per km of road for the summer and fall periods, respectively. Total amounts of RDS were greatest in the city centre, compared to the outlying areas, reflecting the greater density of roads in the former, although there were some hotspots which may reflect land use activities such as light-industry and pulp and paper mills. Conclusions These findings have implications for air and water quality in the city and surrounding area, including the role of RDS in contributing to airborne fine particulates (i.e. PM10 and PM2.5) and the fine-grained sediment (< 63 mu m) transported within storm sewers to receiving waterways, such as the Fraser and Nechako Rivers.