Relative Importance of Surface and Subsurface Nitrogen Inputs to the Water Quality of a River in Coastal Georgia

It is commonly assumed that nutrient-related problems in surface waters are primarily due to contaminated runoff from non-point sources. A 12-month investigation of a black water river and its tributaries in an agricultural region of coastal Georgia was undertaken to determine the relative importance and timing of surface and subsurface inputs of nitrogen. Ammonia and nitrate concentrations were monitored along an 11-mile segment of the stream and its tributaries. Surface inputs were detected by ammonia concentrations in both the river and its tributaries and groundwater inputs by changes in nitrate concentrations along the main stream between tributaries. Results show that nitrogen concentrations were highest during low-flow periods in the summer, with the most important nutrient flux being localized groundwater baseflow, not diffuse surface runoff. Significant surface inputs did take place in the summer, but only sporadically as they were associated with discrete precipitation events, which caused nitrogen concentrations to rapidly rise and fall along the entire river. However, oxygen depletion, typical of nutrient loading, was found at a single location along the stream, not its entire reach. Moreover, data in the summer show relatively long periods of elevated nitrate levels upstream of the site of poor water quality. Because the area of elevated nitrate occurred between tributaries, the source of nitrogen was inferred to be a discrete flux of contaminated groundwater. Finally, water quality problems downstream of the entry point of nitrate were persistent and occurred only in the summer. Therefore, it is concluded that contaminated groundwater baseflow in the study area was more significant than non-point source runoff, especially since water quality problems took place during periods when baseflow was the most dominant and persistent component of stream discharge