Date of Award
Doctor of Philosophy (PhD)
Diane M. McKnight
Flooding is a critical driver of ecosystem productivity. By rapidly increasing stream stage and velocity, floods mix water and solutes from the stream, hyporheic zone, and floodplains/riparian areas. Such mixing may spur biogeochemical activity. In catchments underlain by permafrost, flooding is more common due to both the potential for rapid ice melting and minimal storage potential in frozen soils. High latitude environments are often underlain by permafrost and are also areas of biogeochemical interest, due to large stores of carbon (C) and nitrogen (N), and the potential for rapid cycling. The increased complexity in groundwater/surface water hydrologyduring floods requires rigorous hydrologic analysis before biogeochemical trends can be correctly interpreted. This research aims to accurately quantify the hydrology and biogeochemical cycling of C and N in two high-latitude catchments utilizing stream tracer additions, synoptic sampling, and surface water (sw), groundwater (gw), and coupled sw/gwflow models.
Two catchments, in Alaska and Antarctica represent very different ecosystems, both characterized by continuous permafrost and shallow aquifers. In Antarctica, coupled surfacewater / groundwater flow modeling and tracer additions identify sources of DOC (dissolved organic carbon) and locations of denitrification. Mass balance calculations identify heightened water / sediment interactions at high flows, and increased C and N uptake when solutes return to the stream during low flows. In Alaska, discharge correlates to DOC and nitrate concentrations, indicating leaching and flushing of organic material from the hill slope during high discharge, with a greater potential for microbial processing of this organic material during low flows. Multiple tracer additions demonstrate a seasonal trend, with the greatest C and N uptake early in the summer, potentially related to shallower flowpaths.
Differences between discharge, flooding, and C and N cycling in these two catchments indicate the importance of stream size and morphology. Using tracer dilution and major ion and uranium isotope chemistry, we identify preferential flow near and beneath the stream, indicating erosion of the stream bed via soil piping and thermokarsting. We propose that channel evolution will lead to decreased stream/catchment interactions and subsequently decreased C and N uptake potential in these high-latitude catchments.
Koch, Joshua Charles, "Hydrologic and Biogeochemical Implications of Flooding in Two Catchments Underlain by Continuous Permafrost" (2010). Civil Engineering Graduate Theses & Dissertations. 40.