Ecosystem Restoration and Denitrification in a Coastal Stream

Substantial removal of nitrate can occur in the riparian and hyporheic zones of small streams. Urbanization may lead to impairment of processes leading to this nitrate removal, however, by altering aboveground sources of carbon, and routing water to deeper flow paths away from denitrification "hot spots" in surface soil horizons. We have been quantifying denitrification in riparian and hyporheic zones of restored and unrestored reaches of an urban stream within the Chesapeake Bay watershed using an in situ 15N tracer technique. Recovery of conservative SF6 tracer showed that this "push-pull" technique could be applied to a wide range of locations using an incubation time of 4 hours. Rates of denitrification in hyporheic wells were higher in the restored reach of the stream as compared to the unrestored reach, which had less hydrologic connection to the stream channel. In the restored reach, sandy riparian areas, which were frequently flooded, showed significantly higher rates of denitrification as compared to less flooded areas. Maximum rates of denitrification in the restored reach ranged from 100-112 micrograms N/kg soil/day. These high rates of denitrification were consistent with significantly lower concentrations of nitrate in hyporheic water in the restored reach relative to the unrestored reach throughout different seasons. Mean concentrations of dissolved organic carbon also differed between the two reaches and were higher in hyporheic and stream water from the restored reach. Maintaining hydrologic connectivity between streams and adjacent ground water may be important when decreasing the amount of N transported to Chesapeake Bay. Considerable denitrification may occur in urban riparian and hyporheic zones following stream restoration.