Graduate Thesis Or Dissertation

Flux of ground water and nitrogen through the floodplain of a fourth-order stream

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  • Changes in the concentrations of dissolved ammonia (NH4+1), nitrate (NO3-1), organic nitrogen (DON) were monitored along ground water flow paths to determine the importance of the ground water system to the stream nitrogen budget. The study site was located on a wide floodplain along a fourth-order stream in the Oregon Cascades. A network of wells was installed during the summers of 1989 and 1990. Water table elevations and nitrogen chemistry was sampled seasonally and within individual storm events. Subsurface flows were dominated by the flow of advected channel water through the gravel bar. Flow rates were correlated to stream discharge during base-flow periods, but did not increase during storms. In contrast, ground-water flows through the aquifer beneath the floodplain were small during base flow, but nearly doubled during storm events. The mean residence time of water stored within the aquifer was long, exceeding 10 days for the gravel bar and 30 days for the floodplain. Even though precipitation inputs to the aquifer during storms equaled 12% of the water stored in the gravel bar and 23% of the water stored in the floodplain, the mean residence time of water remained long. Subsurface flow through the aquifer adjacent to the stream was a net source of nitrogen to the stream in all seasons of the year and during storms. Flows of water through the conifer forested floodplain supplied most of the nitrogen per unit length of stream - accounting for approximately two-thirds of the estimated flux, most of which is DON. The gravel bar was colonized by red alder, a nitrogen fixing tree, and on a unit area basis, supplied 2.5 times more nitrogen to the stream than did the floodplain. I estimate that 2 kg ha-1 yr-1 are leached from riparian forests into the aquifer, and transported to the stream. The study site covered only one-half of the valley floor. Assuming that similar fluxes occurred from the opposite side of the valley, I estimate that 17 g of nitrogen yr-1 m-1 channel length are input to the stream. The stream is approximately 10 m wide, thus these inputs equal 1.7 g N m2 streambed yr-1.
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