Friday, October 26, 2012: 11:30 a.m.
A large body of research has shown that groundwater-surface water exchanges within streambed sediments tend to be “hotspots” of chemical cycling. This interface, known as the hyporheic zone, exhibits high rates of denitrification (the reduction of nitrate and nitrite to N2O or N2 gas) because it accumulates abundant organic carbon and there is a high degree of exchange between groundwater and surface water. Based on hydrological and chemical similarities with riverbeds, it is likely that denitrification also occurs in fluvial islands and that they serve as a nutrient sink for nitrogen-rich streams. This study intensively monitored water chemistry and hydrology within one vegetated and one unvegetated fluvial island in the lower Platte River, Nebraska, over a three-month period. Utilizing osmotic pumps, continuous water samples were collected from each island and analyzed on daily timescales for major ions and stable isotopes of water. These samples were compared to weekly spot samples of river and island water. Analyses of denitrification indicators including Eh, dissolved oxygen, and dissolved organic carbon indicated strong reducing environments in both islands, and were supported by nitrate concentrations typically less than 1 ppm. River samples demonstrated higher pH, temperature, dissolved oxygen, and nitrate concentrations compared to island samples. Seepage meter tests confirmed the study area was a gaining reach. Based on these results, we conclude the islands in the study area are primarily groundwater-fed. Though little denitrification occurs during low-flow conditions, the islands exhibit high denitrification potential compared to the river, and likely act as a sink for nitrate during high-flow periods.
