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

Abstract

Abstract:Groundwater that bypasses the riparian zone by travelling along deep flow paths may deliver high concentrations of fertilizerderived 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.

Detecting groundwater discharge dynamics from point-to-catchment scale in a lowland stream: combining hydraulic and tracer methods

“…These differences in catchment runoff sources during large rain events are important to take into account in water management practices, since a significant transport of phosphorus and nitrogen has been shown to be associated with storm events (Jordan et al, 2005;Kronvang and Bruhn, 1996;Stutter et al, 2008). Furthermore, the travel time (Flewelling et al, 2012) and origin (Clément et al, 2003) of discharging groundwater are decisive for the possibility of nitrate reduction.…”

Legacy Nutrient Dynamics at the Watershed Scale: Principles, Modeling, and Implications

“…For example, Galavotti (2004) and Gu et al (2007) found that most of the groundwater nitrate was denitrified during transport through a 30-cm thick organic-rich layer located directly beneath the sediment surface. For shallow groundwater, a number of studies have highlighted the denitrification potential and nitrate removal occurring during transport through the subsurface riparian zone (Fisher and Acreman, 2004;Flewelling et al, 2012;Hinshaw and Dahlgren, 2016). Stable isotopic ( 18 O), radioisotopic ( 3 H), and dissolved gas (e.g., dissolved chlorofluorocarbon gases) analyses to determine groundwater transit times to surface water indicate a wide range of transit times from several years to decades 55 Legacy Nutrient Dynamics at the Watershed Scale McGuire and McDonnell, 2006).…”

Performance of Denitrifying Bioreactors at Reducing Agricultural Nitrogen Pollution in a Humid Subtropical Coastal Plain Climate

“…For example, interception of shallow groundwater by subsurface tile drainage systems, or groundwater flow paths that are deeper than the root zone, often cause "riparian bypass" [10], in which groundwater bypasses riparian buffers. The most appropriate practice for the many drained areas on the Delmarva is wetland restoration, but this option is not attractive to many agricultural producers because of the amount of land needed to be taken out of production.…”