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Biogeochemical characterization of the hyporheic zone below the Fraser River near Vancouver, British Columbia Zima, Michael K.


The aim of this thesis is to characterize the biogeochemical conditions in the hyporheic zone of the Fraser River in order to help assess the fate of creosote-derived contaminants in groundwater. By investigating hyporheic biogeochemistry, a framework is established for further studies which could quantitatively assess the potential for monitored natural attenuation as a remedial option for groundwater contamination. At our site on the North Arm of the Fraser, groundwater flows horizontally towards the river at a velocity of 0.1 to 0.2 m/day. Transverse mixing between fresh groundwater and underlying saline groundwater is believed to be responsible for cation exchange reactions. From intertidal monitoring wells out to the hyporheic zone (HZ), Fe and Mn concentrations are found to increase despite dilution, and this is believed to be a result of either cation exchange, or oxidation of organic matter (OM), H₂S, or CH₄. As groundwater reaches the river, it turns upward and discharges into the HZ. In the river channel, fresh groundwater discharges to the HZ at distances between 70 and 85 m from the shoreline’s high-water mark (HWM), while the deeper, saline groundwater discharges at distances greater than 100 m. A custom-made cryogenic probe was designed to collect representative, high-quality sediment samples from the HZ. A vertical characterization of biogeochemistry is presented. Chloride is used as a conservative tracer to determine a dilution factor of 170 in the top 85 cm of sediment, but dilution is believed to persist beyond this depth. DNA sequencing results infer a large variety of geochemical processes. Since multiple known aerobic microorganisms are found to have highest proportional abundances in the top 30 cm of the HZ, this depth range is considered to represent the portion of the HZ which is continually exposed to significant amounts of oxygen from the river. Porewater geochemistry also supports this delineation, as Fe(II) is non-detectable above the depth of 30 cm. The estimation of residence time in the HZ, the delineation of the aerobic portion of the HZ, and the characterization of dilution rates provide valuable information on the biogeochemical and physical components of natural attenuation in the HZ.

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