Using concentration-discharge relationships to identify influences on surface and subsurface water chemistry along a watershed urbanization gradient

Urban development within watersheds impacts the hydrology and water quality of streams, but changes to groundwater-surface water interactions in this "urban stream syndrome" are not yet well understood. This study focused on three stream systems in a northern Virginia (USA) protected area with 14.2, 31.7, and 66.1% developed land in their watersheds. Surface water was sampled weekly for nutrients, dissolved metals, sulfate, ancillary water quality parameters, and discharge over two non-consecutive years with the hyporheic zone sampled during the second year. Concentration-discharge relationships revealed largely chemostatic behavior in surface water pollutants in the least urbanized stream, while in the two more urbanized streams, these relationships tended to have significant positive and negative slopes, indicating diverse delivery pathways depending on the constituent. Stream discharge had different effects on many water pollutants in the hyporheic zone of the least urbanized stream; the slopes of concentration-discharge plots were exclusively negative, indicating greater infiltration and dilution, while the other two urbanized streams maintained largely chemostatic behavior. Average specific conductance and nitrate + nitrite concentrations in stream surface water reflected an urbanization gradient, while sulfate, Ca, K and Sr concentrations suggested a threshold effect: the stream with a mostly forested watershed had the lowest concentrations, while the other two were similar. Similar to other "urban stream syndrome" studies, specific conductance indicated salinization of both surface and groundwater at the two more urban streams, possibly threatening aquatic organisms. Metal concentrations in surface and subsurface water were often positively correlated with specific conductance and negatively correlated with pH, suggesting that they may originate from road salt and/or be mobilized by acid precipitation. These results indicate the importance of monitoring both baseflow and stormflow as pathways for pollution and have implications for protection and restoration of both the Potomac River in Washington D.C. and the Chesapeake Bay.