Recovery of Hyporheic Denitrification Potential Following Maintenance Events in Agriculturally Modified Streams

We are comparing the denitrification potential of hyporheic sediments in three agriculturally modified streams that have last experienced channel maintenance at various times over the past 30 years to an unmodified stream segment. Our goal is to identify the time scale of recovery of denitrification potential after a modification/maintenance event. Hyporheic samplers were installed in the upwelling and downwelling zone of a selected riffle at each location and in the adjacent point bar. Sampling began in May and will continue through late summer/early fall to capture seasonal influences and varying flow conditions. Samples will be analyzed for nitrate, ammonium, sulfate, chloride, and DOC. Field parameters include dissolved oxygen and temperature. Bromide tracer tests are scheduled for each location to characterize travel times through the riffles. Finally, detailed cross sections and measurements of thickness of coarse-grained sediments overlying low permeability glacial till will provide detailed information on the hyporheic zone. Preliminary results indicate that under relatively high flow conditions little to no loss of nitrate is occurring across the riffle at each location including the unmodified section. This may be due to short residence times in the subsurface, high dissolved oxygen, and low DOC concentrations. During the tracer test completed at the most recently modified segment, a decrease in nitrate of approximately 10% was observed consistently between the stream and the upwelling zone of the riffle. However, it appears this change may be due to dilution, based on the significant decline in bromide concentration across the riffle. The source of the water causing dilution is unclear at this time; however, water chemistry does not suggest an alternate source of water (i.e. groundwater). The upwelling zone of this riffle may serve as a discharge point for a deeper hyporheic flow path that has a longer residence time than the tracer test.