Dang, Huy

Mineral feedstocks, including alkaline mine wastes, can sequester CO2 as a dissolved phase (e.g., HCO3-) or a solid carbonate via enhanced rock weathering (ERW). For this thesis, the release of easily accessible Ca and Mg from non-carbonate sources was determined for kimberlite residues from several diamond mines and commonly proposed ERW rock types, including wollastonite and olivine. Batch leaches determined the CO2 sequestration potentials of kimberlites to be in the range of 3–12 kg CO2/t, which was exceeded by most ERW feedstocks. Leaches also assessed the release of Ni and Cr, elements of concern in ERW settings, and P and K, which benefit agricultural soils. Year-long leaching columns were deployed using kimberlite from the Gahcho Kué and Venetia diamond mines, wollastonite skarn, and olivine sand from the initial assessment. The kimberlite residues sequestered 0.03 kg CO2/t as dissolved inorganic carbon and 0.6 kg CO2/t as solid total inorganic carbon. Weathering of wollastonite skarn resulted in CO2 removal rates via mineral trapping of CO2 of 6.31 kg CO2/t, while the olivine sand yielded rates of 0.5 kg CO2/t via solubility trapping. Both methodologies used in this study demonstrated value in the prediction and monitoring of drainage chemistry as it relates to ERW and CO2 mineralization. Implementation of these strategies can progress ERW efforts by providing confidence in feedstock selection and the verification of carbon offsets.

Author Keywords: CO2 mineralization, Drainage chemistry, Enhanced weathering, Mine wastes, Mineral trapping, Solubility trapping

In Ontario, farmers commonly use a MZ (Maize (Zea mays L.))-SB (Soybean (Glycine max))/WW (Winter wheat (Triticum aestivum)) – CC (mixed cover crop) rotation to maximize economic benefits. This study aimed to investigate the short-term impacts of the crop rotation phases and their associated management practices in this diversified cash crop rotation on soil health and the abundance of nitrogen (N)-cycling soil microbial communities (SMCs). Additionally, the abundance of N-cycling SMCs and plant-available N in both surface (0-5 cm) and rooting zone (5-15 cm) depths were characterized in tile-drained (TD) and non-TD fields. In the present study, soils collected under the CC phase had the highest labile carbon levels (10-17% higher) and water-stable aggregates (35-50% higher) compared to the other two crop phases. Lower nitrifying (amoA) gene abundances and soil NO3--N levels were observed in the CC phase compared to the MZ and SB-WW phases, suggesting a potential for decreased nitrification in the CC phase. The presence of SB potentially influenced the soil N concentration in the subsequent WW phase likely due to the release of symbiotically fixed N in the SB-WW phase. Further, higher amoA abundances and NO3--N in the SB-WW phase imply a potential for increased nitrification in the SB-WW phase. Additionally, higher amoA/nosZI and nirS+nirK/nosZI ratios were observed in the MZ phase than in SB-WW and CC phases, suggesting a potential capacity for increased N2O emissions from the reactions mediated by N-cycling SMCs in soils planted to MZ during fall sampling days. In the TD and NTD field study, higher NO3--N levels were observed in TD-SB-WW fields at 5-15 cm vs. 0-5 cm depths, which was possibly facilitated by tile drainage. The TD-CC fields displayed higher nosZI gene abundances and lower nirS+nirK/nosZI abundance ratios, suggesting a greater potential capacity for decreased N2O emissions in soils planted to CCs during the spring sampling days. When examining changes in plant available N by soil depth, reduced downward movement of NO3- through shallow soil depths (0-15 cm depth) was observed in the CC phase compared to cash crops. This short-term study highlights the potential contribution of the CC phase, particularly within TD agricultural fields, for improving soil health and reducing potential N2O emissions. Together, these results suggest that management-associated differences in crop rotation phases have temporary effects on soil health and the abundance of SMCs. Future studies linking N-cycling SMC's potential activity and field-scale N2O fluxes will provide a better insight into the longer-term sustainability of Ontario's cash crop management systems.

Author Keywords: denitrification, maize-soybean-winter wheat- cover crop rotation, nitrification, soil depth, Sustainable agriculture, tile-drainage

Road runoff is a vector for the transport of potentially toxic chemicals into receiving waters. In this study, selected tire-derived chemicals were monitored in surface waters of rivers adjacent to two high traffic highways in the Greater Toronto Area in Ontario, Canada. Composite samples were collected from the Don River and Highland Creek in the GTA during 5 hydrological events that occurred in the period between early October 2019 and late March 2020, as well as an event in August 2020. Grab samples were collected from these rivers during a period of low flow in August 2020, as well as during a storm event in July of 2020. Analysis was performed using ultra-high pressure liquid chromatography with high resolution mass spectrometric detection (UHPLC-HRMS). Hexamethoxymethylmelamine (HMMM), a cross-linker of tire material, was detected at elevated concentrations (> 1 µg/L) during rain events in the fall and winter of 2019-20 and during a period of rapid snow melt in early March of 2020. These samples were also analyzed for the tire additive, 6PPD, and its oxidation by-product, 6PPD-quinone, as well as 1,3-diphenylguanidine (DPG). In many samples collected from the Don River and Highland Creek during storm events, the estimated concentrations of 6PPD-quinone exceeded the reported LC50 of 0.8 µg/L for Coho salmon exposed to this compound. Temporal samples collected at 3-hour intervals throughout rain event the October 2020 showed that there was a delay of several hours after the start of the event before these compounds reached their peak concentrations. In addition, 26 candidate transformation products and precursor compounds of HMMM were monitored; 15 of these compounds were detected in surface waters in the GTA. The maximum total concentration of this class of methoxymethylmelamine compounds in surface water samples was estimated to be 18 µg/L. There is limited knowledge about the properties of HMMM, its precursor contaminants, and its transformation compounds, as well as their fate in the environment. COSMO-RS solvation theory was used to estimate the physico-chemical properties of HMMM and its derivatives. Using the estimated values for these properties (e.g., solubility, vapour pressure, log Kow) as inputs to the Equilibrium Criterion (EQC) fugacity-based multimedia model, the compounds were predicted to readily partition into aqueous media, with mobility in water increasing with the extent of loss of methoxymethyl groups from HMMM. Overall, this study contributes to the growing literature indicating that potentially toxic tire-wear compounds are transported via road runoff into urban surface waters. In addition, this study provides insight into the environmental behaviour of HMMM and its transformation products.

Author Keywords: 6PPD-quinone, COSMOtherm, Fugacity, Hexamethoxymethylmelamine, Road runoff, Tire wear