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

Changes in the concentrations of dissolved ammonia (NH4'),+ nitrate(NO3-'), organic nitrogen (DON) were monitored along ground water flow pathsto determine the importance of the ground water system to the stream nitrogenbudget. The study site was located on a wide floodplain along a fourth-orderstream in the Oregon Cascades. A network of wells was installed during thesummers of 1989 and 1990. Water table elevations and nitrogen chemistry wassampled seasonally and within individual storm events.
Subsurface flows were dominated by the flow of advected channel waterthrough the gravel bar. Flow rates were correlated to stream discharge duringbase-flow periods, but did not increase during storms. In contrast, ground-waterflows through the aquifer beneath the floodplain were small during base flow, butnearly doubled during storm events. The mean residence time of water storedwithin the aquifer was long, exceeding 10 days for the gravel bar and 30 days forthe floodplain. Even though precipitation inputs to the aquifer during stormsequaled 12% of the water stored in the gravel bar and 23 % of the water stored inthe floodplain, the mean residence time of water remained long.
Subsurface flow through the aquifer adjacent to the stream was a netsource 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 thenitrogen per unit length of stream - accounting for approximately two-thirds ofthe 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 morenitrogen to the stream than did the floodplain. I estimate that 2 kg ha' yr' areleached from riparian forests into the aquifer, and transported to the stream. Thestudy site covered only one-half of the valley floor. Assuming that similar fluxesoccurred from the opposite side of the valley, I estimate that 17 g of nitrogen yr-1m' channel length are input to the stream. The stream is approximately 10 mwide, thus these inputs equal 1.7 g N M-2 streambed yr'.