SEASONAL ROLE OF SURFACE WATER-GROUNDWATER INTERACTION IN REGULATING STREAM WATER CHEMISTRY IN URBAN STREAMS

Urbanization is a major cause of stream degradation in the United States. Surface water –groundwater interaction may naturally mitigate impaired water quality associated with urbanization. Meadowbrook Creek, in Syracuse, New York, flows along a declining urbanization gradient, from headwaters that are highly channelized with armored banks to an outlet that is unconstrained and meanders through a broad, riparian floodplain. Biweekly, longitudinal stream samples were collected every 100 - 600 m during summer, fall and winter 2012-2013 and analyzed for conductivity and major ions. Seasonal sampling aimed to see the impact of leaf-off and salty road runoff on surface water. A five well transect in the downstream floodplain was also sampled. A constant rate tracer test was used to quantify groundwater inflow longitudinally during summer baseflow. Nitrate concentrations along the channelized reach were low to below detection (<0.09 – 1.48 mg/L NO₃^-) in summer; however, nitrate concentrations were consistently higher along the unconstrained reach, ranging from 2.96 - 4.03 mg/L NO₃^-. In the final 250 m of stream length, nitrate decreased by 0.04 – 0.18 mg/L. Chloride concentrations varied greatly between seasons, with stream concentrations similar to groundwater in the summer (97 - 304.71 mg/L Cl^- in surface water, 150.72 - 330.60 mg/L Cl^- in groundwater) and an order of magnitude higher in winter, during snowmelt events (805.12 - 4517.88 mg/L Cl^- in stream water). Low nitrate concentrations in the floodplain groundwater during the summer (0.02-0.29 mg/L), along with the drop in surface water concentrations, indicate surface water that has been denitrified as it travels through the subsurface is lowering surface water concentrations as it re-enters the stream. In winter, groundwater inflows dilute chloride and in summer they enrich chloride with distance. Overall, the geochemistry of the channelized portion of the stream is primarily controlled by surface runoff, while the unconstrained section of Meadowbrook is strongly influenced by discharging groundwater. Nitrate, however, increases with the presence of riparian vegetation, which has potential implications for stream restoration techniques that aim to increase organic carbon inputs to streams, as organic matter also adds nitrogen to the system.