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 μm) using stainless steel sieves. Fractionated sediment samples were weighed to obtain a mass for each size class for each grid cell. Results and discussionThe total amount of RDS (all particle-size fractions) in the city of Prince George was estimated to be 746×10 3 and 204×10 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-μm 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-μm 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. PM 10 and PM 2.5 ) and the fine-grained sediment (<63 μm) transported within storm sewers to receiving waterways, such as the Fraser and Nechako Rivers.
A growing literature is demonstrating that platinum (Pt) is transformed under surface conditions; yet (bio)geochemical processes at the nugget-soil-solution interface are incompletely understood. The reactivity of Pt exposed to Earth-surface weathering conditions, highlighted by this study, may improve our ability to track its movement in natural systems, e.g., focusing on nanoparticles as a strategy for searching for new, undiscovered sources of this precious metal. To study dissolution/re-precipitation processes of Pt and associated elements, grains of Pt-Fe alloy were collected from a soil placer deposit at the Fifield Pt-field, Australia. Optical-and electron-microscopy revealed morphologies indicative of physical transport as well as chemical weathering. Dissolution "pits," cavities, striations, colloidal nano-particles, and aggregates of secondary Pt platelets as well as acicular, iron (Fe) hydroxide coatings were observed. FIB-SEM-(EBSD) combined with S-m-XRF of a sectioned grain showed a fine layer of up to 5 mm thick composed of refined, aggregates of 0.2 to 2 mm sized crystalline secondary Pt overlying more coarsely crystalline Pt-Fe-alloy of primary magmatic origin. These results confirm that Pt is affected by geochemical transformations in supergene environments; structural and chemical signatures of grains surfaces, rims, and cores are linked to the grains' primary and secondary (trans)formational histories; and Pt mobility can occur under Earth surface conditions. Intuitively, this nanophase-Pt can disperse much further from primary sources of ore than previously thought. This considerable mineral reactivity demonstrates that the formation and/or release of Pt nanoparticles needs to be measured and incorporated into exploration geochemistry programs.
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