Quantifying Nitrate Flux during Storm Events

Excess nitrate, mainly due to agricultural fertilizers, is a common problem in the surface water and groundwater of the Coastal Plain of the eastern United States. Baseflow conditions have been studied in an effort to understand nutrient loading to coastal waters, but there are many unanswered questions pertaining to nitrate flux during storm events. An automated stream sampler was deployed to collect water from a stream on the Eastern Shore of Virginia, USA, whenever stream stage rose following a rainfall event. Baseflow nitrate concentrations were typically 2.0–2.2 mg NO₃^–-N L^–1, and concentrations decreased rapidly with the onset of precipitation. Slightly in advance of peak discharge, the typical streamwater concentration was reduced to approximately 0.2–0.4 mg NO₃^–-N L^–1. Streamwater nitrate concentrations slowly returned to pre-storm levels after the discharge level returned to baseflow. Even though the nitrate concentration of surface water decreased during storms, the average storm nitrate flux was approximately a factor of 3 times greater than the average base flow nitrate. Large quantities of nitrate were quickly transferred during storms due to the large volume of water with low nitrate concentrations flowing in the stream. Insights to nutrient management, particularly considering rates of biological uptake of nitrate, are supported by this more complete understanding of the dynamic flux of streamwater nitrate to coastal waters.