Rivers and lakes are subject to various forms of pollution from anthropogenic activity and natural sources. Many pollutants, such as those from agricultural runoff, are transported in rivers by binding to sediment. Sediment itself is a pollutant, and is often an indicator of terrestrial erosive processes. Common methods of establishing sediment provenance on a broad scale include the use of fallout radio nuclides and geochemistry. In 2008, Dr. Max Gibbs published a sediment tracing article on the use of compound-specific stable isotopes (CSSIs) of plant origin which identified sediment sources based on land use in sub-tropical New Zealand. The objective of the research undertaken here was to apply the CSSI concept to agricultural watersheds in a northern, temperate climate. Two watersheds were selected: the Horsefly River Watershed (HRW) near Horsefly, BC, Canada and the South Tobacco Creek Watershed (STCW), near Miami, Manitoba, Canada. The HRW represented a mostly pristine watershed. The STCW represented a heavily cropped agricultural watershed. The HRW samples were used to develop laboratory methodology, while the STCW samples were used to evaluate the CSSI technique using carbon stable isotopes. The dissertation addresses the following: (i) literature review of plant biomarkers and spatial/temporal variability of CSSIs due to biological, environmental and analytical factors; (ii) methodology, analysis and variability associated with bulk soil and sediment isotope determination; (iii) methodologies for processing soil and sediment from sampling to isotope analysis; and (iv) spatial and temporal variability of CSSI tracers. The CSSI tracers were evaluated to reveal spatial and temporal variability of VLCFA concentrations and isotope signatures at the point, transect and field scales. Weighted t-tests were used to differentiate sediment sources spatially and temporally. The use of bulk carbon as a proxy for VLCFA concentrations in source apportioning was also explored. The work presented here demonstrates the ability of CSSIs to differentiate sediment sources based on land use. The development of analytical methods and the resulting analysis of soil and sediment extracts have indicated that VLCFAs may be isolated and quantified to generate reliable isotope data. The methods will hopefully lead to the standardization of CSSIs protocols.
Vegetated buffer strips are a management practice implemented in agricultural landscapes because of their effectiveness in reducing the transport of phosphorus (P) to surface water. However, in northern climates, buffers can become a source of P when soils are frozen and vegetation is dead or dormant during the most significant runoff period. This research investigated buffer vegetation as a potential source of P at the Morden Research Station, Manitoba. Vegetation sampling in two new buffers and an established buffer in fall 2015 and spring 2016 showed biomass P loss of 32-47% and an increase in soil Olsen P of 25-43% over winter. Thus, it is likely that much of the leached P was retained in the soil. Laboratory experiments subjected timothy grass to zero, three or six freeze-thaw cycles (FTCs), followed by extraction to leach P. Results showed an increased number of FTCs resulted in increased concentrations of leached P.
The sediment source fingerprinting approach is based on the assumption that the potential sources of sediment within a watershed can be linked to in-stream sediment by using the inherent physical or biogeochemical characteristics of the sediment (i.e., sediment properties) as fingerprints. At present, one of the main limitations of the sediment source fingerprinting approach is the ability to link sediment back to their sources due to the non-conservative nature of many sediment properties. Ideally, sediment properties do not change as the sediment (i.e., transported unconsolidated soil or rock particles) move through a watershed allowing for a direct comparison between sources and sediment. However, sediment collected downslope or downstream from its source is often found to have a finer grain-size distribution and a higher organic matter content as compared to the source material as the smaller and less dense particles are preferentially mobilized and transported. Accounting for changes in both particle size distribution and organic matter content are important as many fingerprint concentrations are correlated with both properties, but it is unclear as to what is the best approach to account for these changes. In an effort to provide a more reliable and robust link between sources and sediment, a series of experimental and observational studies were conducted to investigate the factors that control particle size and organic matter selectivity and their subsequent effect on a broad suite of geochemical fingerprints. These studies investigated particle selectivity at the landscape scale, represented by a sequence of hillslope, riparian and fluvial environments. Processes within each of these three environments were found to preferentially mobilize and transport fine-grained and organic-rich particles. Many commonly used particle size and organic matter correction factors assume that the relation between both particle size and fingerprint concentrations are similar for all fingerprints. However, this research has demonstrated that these relations are fingerprint specific in terms of the magnitude, direction and linearity which suggests that the use of correction factors needs to be given careful consideration. In addition, a watershed-scale application of the sediment fingerprinting approach found that the scale of observation and the geomorphic connectivity of the watershed influenced the apportionment results. Overall, this research can be used to guide sampling design and protocols, fingerprint selection, data correction factors and the interpretation of apportionment results.
