Quantifying the Source and in-Stream Gain/Loss of Nitrate Using Continuously-Collected Water Quality Data

Abstract:

High frequency in situ nitrate data from three streams spanning gradients in hydrologic conditions, land use, and watershed size were used to quantify the mass loading of nitrate to streams from two sources – groundwater discharged and event flow nitrate – at a daily time step for one year. Temporal variability in net in-stream loss or production of nitrate and environmental controls on nitrate processing were also quantified. Nitrate in groundwater discharge was identified as contributing approximately 70% of the total nitrate load to a large river and small agricultural stream, compared with 45% to a small urban stream. The greatest in-stream losses of nitrate occurred during the summer and fall months, with net in-stream losses of up to 70% of total nitrate load in the large river, 60% in the agricultural stream, and 50% in the urban stream. Photosynthetically active radiation was identified as the most likely mechanism of in-stream nitrate loss in the large river, whereas baseflow discharge and temperature were the most likely mechanisms of nitrate loss in the small agricultural and urban streams, respectively. As more high frequency nitrate data become available, the approach described here can be applied to further understand nitrate sources to streams and controls on in-stream nitrate production and retention.