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Geochemistry in the hyporheic zone of the lower Fraser River Roschinski, Tilman Gordon

Abstract

The geochemistry of the hyporheic zone was investigated to reduction-oxidation (redox) conditions across the hyporheic zone as groundwater and river water mix in the shallow sediments of the lower Fraser River, a large, tidally influenced 9th order river. The site selected for study is located at the trailing edge of a postglacial river delta deposit near Vancouver, British Columbia approximately 30 km upstream from the mouth of the river. Pore water samples were collected with a drive-point profiler while sediment cores were collected with a freeze-shoe corer. Difficulties encountered with the freeze-shoe corer led to the development of a novel tool: a liquid nitrogen-cooled freeze corer capable of sampling undisturbed sediments in a deep river environment. Selective single-step chemical extraction procedures were used to analyze sediment samples for secondary iron mineral precipitates that commonly form in the hyporheic zone where iron-rich anaerobic groundwater mixes with oxygen-rich surface water. Results of sediment analyses show a significant amount of iron in the sediments, but no distinct peak accumulations indicative of secondary iron mineral precipitates and no accumulation of iron oxyhydroxides on the hyporheic zone sediments, suggesting that oxygen does not enter the hyporheic zone in significant concentrations. It is hypothesized that oxidation of dissolved organic carbon and methane could remove oxygen and therefore maintain reduced ferrous iron in solution. X-ray diffraction detected the presence of iron-bearing chlorite and a magnetic separation found concentrations of magnetite. The presence of these minerals is thought to be the cause for the high concentrations of iron in the sediments by chemical extraction. Though no significant amounts of oxygen from river water is thought to enter the hyporheic zone sediments, chloride concentration profiles indicate that riverwater does mix with groundwater in the hyporheic zone. Where the river sediments are silt-dominated, river water appears to penetrate to less than one meter depth, whereas in sand-dominated sediments river water penetrates to at least 1 to 1.5 meters depth.

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