Surface water-groundwater interactions buffer seasonal changes in urban stream water quality
Surface water-groundwater interactions buffer seasonal changes in urban stream water quality
Previously Published Material: Some elements of this presentation have been published in a paper in Hydrological Processes (accepted and in press) and presented at prior Geological Society of America and AGU meetings. Other elements of the presentation are new findings.
Abstract ID#: 36630
English Abstract:
Urban streams commonly suffer from degraded stream water quality and limited surface water-groundwater (SW-GW) interaction due to channel incision and straightening, armoring of the stream bed and banks, and degradation of riparian floodplains and wetlands. Reestablishing hydrologic connection between urban streams and their hyporheic and riparian zones may be effective for naturally attenuating nutrients and salts. To investigate the impact of SW-GW interactions on urban stream water quality, we observed longitudinal and seasonal changes in stream water quality in an urban stream in Syracuse, New York. The stream is heavily degraded (channelized, armored, no riparian floodplain) for the upper 4 km and then transitions to being hydrologically connected to a broad, intact riparian floodplain, with a meandering channel, geomorphically complex bedforms, and large woody debris for the most downstream 1.5 km. We collected grab samples of stream water every 2 weeks from May 2012 to June 2013 at 24 sites positioned longitudinally along the stream and in a transect of riparian groundwater wells in the downstream floodplain. We also quantified the discharge profile along the 5 km stream in August 2012 using a constant rate injection of Rhodamine WT. Results show water quality in the urban stream is heavily impacted by road salt runoff and groundwater discharge along the lower, connected reach buffers seasonal changes in chloride. Groundwater discharge dilutes high in-stream chloride concentrations in winter; but, chloride storage in the riparian flood plain elevates in-stream chloride concentrations in non-winter months. Nitrate concentrations in the degraded reach are highly variable seasonally. In summer, direct exposure to sunlight, elevated stream temperatures, and rapid primary productivity promote nitrate uptake. In winter, stream nitrate concentrations increase as nitrate uptake by biota and denitrification are at a minimum. Groundwater modeling illustrates the mechanisms of salt storage in and discharge from the riparian aquifer. In-stream nitrate injection experiments quantify seasonal changes in nitrate uptake rates in the degraded and connected stream sections. Results indicate intact riparian floodplains can regulate urban stream water quality and should be preserved or restored.