Determining contamination and sources of sediment in response to recent and historical landscape disturbances

Landscape level disturbances occur in nearly every watershed throughout the globe, and as the climate changes, these disturbances will continue to have a significant impact on terrestrial and aquatic ecosystems. Wildfire, timber harvesting, and agricultural expansion are only a few of these disturbances, but each uniquely impacts the sediment regime. While sediment is a necessary and beneficial input to streams and rivers, it also can have negative impacts on the aquatic ecosystem because it can carry contaminants and also be physically detrimental when it settles, clogging spawning habitat. All of these disturbances have occurred historically and in the present in the Nechako River Basin (NRB), a large, regulated watershed in north-central British Columbia, Canada. In the NRB, chinook and sockeye salmon and the Nechako White Sturgeon are species that have been declining in population, in part due to the clogging of their habitat by sand and fine sediment. One way to determine sources of sediment is by using the sediment fingerprinting technique, whereby sediment samples and samples from potential sources are collected and analyzed for a series of physical or biogeochemical properties, and the proportion of sediment coming from each potential source is identified using an unmixing model. After catastrophic wildfires in 2018, research was undertaken to determine the spatial and temporal contamination of soils and sediment by polycyclic aromatic hydrocarbons (PAHs), to determine if burned areas were contributing more sediment than unburned areas to tributaries and the Nechako River mainstem, and to determine the suitability of PAHs as a novel fingerprint. The results found that concentrations of PAHs in the burned soils were elevated immediately post-wildfire, but decreased significantly in subsequent years, and concentrations in sediments were very low. While PAHs were deemed to be non-conservative properties, unmixing modeling using colour showed that burned sources were an important contributor to the tributaries, but less so in the mainstem Nechako River. Agriculture is an important and growing industry in the NRB and is also an important source of sediment. Results from fingerprinting research undertaken in Murray Creek, an important watershed due to its proximity to spawning habitat, found that agriculture was the primary source of sediment in the basin, though channel banks were also important. While the intention was to use compound specific stable isotopes of long chain fatty acids to more specifically pinpoint agricultural fields that were contributing more sediment to the watershed, this semi-novel tracer was unable to discriminate between C3 plant types on a large scale. Taking the entire disturbance regime of the NRB into account, a broader scale fingerprinting study found that sources of sediment are tributary specific, though banks and agriculture were consistently most important. This study also identified that the predicted shift to a rain dominated watershed and earlier freshet will lead to increased potential for erosion from various sources, and that increased incidence of wildfire followed by heavy precipitation may increase sediment loads. Therefore, a number of management changes are suggested, including improving farming practices, post-wildfire landscape rehabilitation, and altering water release practices.