Browsing University of Alaska Fairbanks by Subject "Groundwater"
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Groundwater flow in a vertical plane at the interface of permafrostGroundwater dynamics in discontinuous permafrost aquifers are complex. The topography of permafrost redirects flow in difficult-to-predict directions that can be tens of degrees off from the regional flow direction. Large zones of permafrost vertically separate aquifers into supra and sub-permafrost portions. The flow dynamics in each portion of the aquifer may be dissimilar due to different controlling boundary conditions. In areas of discontinuities in permafrost, known as open taliks, groundwater in the two portions of the aquifer may mix. These areas of mixing are the focus of this study, in particular, the groundwater dynamics in taliks located in the floodplain of lower reaches of rivers. The study hypothesizes that groundwater flow in floodplain taliks of lower reaches of rivers will bifurcate between the supra and sub-permafrost portions of a discontinuous permafrost aquifer. To test this hypothesis gradient, magnitudes and flow directions were determined at several depths ranging from the water table to 150 ft. (45.7 m) below ground surface, using a linear interpolation scheme in various locations in a floodplain talik. Errors in water level measurements due to instrument errors as well as vertically moving wells were propagated into the gradient calculations by Monte Carlo analysis. Results from this research show that a vertical divide in groundwater flow forms a short distance below the top of permafrost. Groundwater flow above the divide routes into the unconfined supra-permafrost portion of the aquifer. Water below the divide flows into the confined portion of the aquifer below permafrost. The position of the vertical groundwater divide may adjust in relation to the water table position. Additionally, a methodology is presented for stochastically propagating measurement errors into gradient analyses by Monte Carlo analysis. Understanding the flow dynamics in discontinuous permafrost aquifers is key to the understanding of contaminant transport, aquifer recharge, and resource development in subarctic environments.
Transport of CH₄ through open-talik lakes in discontinuous permafrost aquifersAs northern regions of the world experience warming climate, scientists look to permafrost, a crucial component of Arctic and subarctic ecosystems, as a source and sink of atmospheric carbon. It is well-known that the thawing of permafrost from above as a result of warming climate is a considerable source of greenhouse gases. However, few studies have considered the production of methane, a potent greenhouse gas, beneath the permafrost. A rugged permafrost bottom is proposed to favor the storage of gas in "pockets" that have been formed through permafrost thaw and degradation from below. Sub (below)-permafrost methane can migrate to reach the atmosphere when connections between the sub-permafrost and supra- permafrost open pathways from the pocket to the bottom of an open talik lake. We hypothesized that the migration of methane occurs through advection and diffusion as a dissolved gas and by movement as an immiscible fluid. Through measurement of environmental tracers in two thermokarst lakes in Goldstream Creek Basin, Fairbanks, Alaska, we found that advection was variable and was seasonally and climatically dependent demonstrating both upward and downward groundwater flow within our study lakes. Measurements of dissolved methane concentrations in the lakes demonstrated that diffusion of methane was not a significant transport mechanism in the groundwater-to-lake pathway due to the extreme temporal and spatial variability of methane concentrations. Immiscible flow of free-phase methane is likely the dominant transport mechanism but is dependent on the lake sediment composition and the formation of secondary pathways within the talik. Data obtained from this study allowed for a better understanding of methane transport and thermokarst lake dynamics.