Travel time controls the magnitude of nitrate discharge in groundwater bypassing the riparian zone to a stream on Virginia's coastal plain

Abstract

Groundwater that bypasses the riparian zone by travelling along deep flow paths may deliver high concentrations of fertilizer-derived NO 3- to streams, or it may be impacted by the NO 3- removal process of denitrification in streambed sediments. In a study of a small agricultural catchment on the Atlantic coastal plain of Virginia's eastern shore, we used seepage meters deployed in the streambed to measure specific discharge of groundwater and its solute concentrations for various locations and dates. We used values of Cl - concentration to discriminate between bypass water recharged distal to the stream and that contained high NO 3- but low Cl - concentrations and riparian-influenced water recharged proximal to the stream that contained low NO 3- and high Cl - concentrations. The travel time required for bypass water to transit the 30-cm-thick, microbially active denitrifying zone in the streambed determined the extent of NO 3- removal, and hydraulic conductivity determined travel time through the streambed sediments. At all travel times greater than 2 days, NO 3- removal was virtually complete. Comparison of the timescales for reaction and transport through the streambed sediments in this system confirmed that the predominant control on nitrate flux was travel time rather than denitrification rate coefficients. We conclude that extensive denitrification can occur in groundwater that bypasses the riparian zone, but a residence time in biologically active streambed sediments sufficient to remove a large fraction of the NO 3- is only achieved in relatively low-conductivity porous media. Instead of viewing them as separate, the streambed and riparian zone should be considered an integrated NO 3- removal unit. © 2011 John Wiley & Sons, Ltd.