Nitrate flux from aquifer storage in excess of baseflow contribution during a rain event.

Nitrate flux from a bedrock aquifer due to a storm was calculated by hydrograph separation. The hydrograph generated by a 20-mm rain in Cedar River of Iowa was separated into the following three components: the constant baseflow, the rainwater, and the water released from aquifer in excess of baseflow. The separation was conducted by using oxygen isotopes, dissolved nitrate, and stream discharge. The peak discharge was 33h long. During this period, the water released from the aquifer storage in excess of baseflow was 36% of the total discharge. The rainwater and the pre-storm baseflow equivalents were 17% and 47%, respectively. The nitrate concentrations in the instantaneous discharge ranged between 8.6 and 10.0 mg/L. The average concentrations in the rainwater and the baseflow were 3.7 and 9.8 mg/L, respectively. A total of 3.6 x 10(5) mol of nitrate was transported by the stream during the 33 h of peak discharge. Approximately 35% (1.3 x 10(5) mol) of this mass was derived from the aquifer storage in excess of baseflow contribution. The rainwater and the constant baseflow equivalentswere 7% (2.6 x 10(4) mol) and 58% (2.1 x 10(5) mol), respectively. The release of a significant amount of nitrate from the aquifer suggests that the local geology is favorable for vertical recharge of rainwater which causes an increased fluid pressure within the aquifer forcing ground water to discharge laterally into the stream. Such observation also implies that the aquifer is effectively flushed out during storms, thus restricting long term build-up of nitrate from agricultural sources.