Nitrate removal in deep sediments of a nitrogen‐rich river network: A test of a conceptual model

Abstract

[1] Many estimates of nitrogen removal in streams and watersheds do not include or account for nitrate removal in deep sediments, particularly in gaining streams. We developed and tested a conceptual model for nitrate removal in deep sediments in a nitrogen-rich river network. The model predicts that oxic, nitrate-rich groundwater will become depleted in nitrate as groundwater upwelling through sediments encounters a zone that contains buried particulate organic carbon, which promotes redox conditions favorable for nitrate removal. We tested the model at eight sites in upwelling reaches of lotic ecosystems in the Waupaca River Watershed that varied by three orders of magnitude in groundwater nitrate concentration. We measured denitrification potential in sediment core sections to 30 cm and developed vertical nitrate profiles to a depth of about 1 m with peepers and piezometer nests. Denitrification potential was higher on average in shallower core sections. However, core sections deeper than 5 cm accounted for 70% on average of the depth-integrated denitrification potential. Denitrification potential increased linearly with groundwater nitrate concentration up to 2 mg NO 3 -N/L, but the relationship broke down at higher concentrations (>5 mg NO 3 -N/L), a pattern that suggests nitrate saturation. At most sites groundwater nitrate declined from high concentrations at depth to much lower concentrations prior to discharge into the surface water. The profiles suggested that nitrate removal occurred at sediment depths between 20 and 40 cm. Dissolved oxygen concentrations were much higher in deep sediments than in pore water at 5 cm sediment depth at most locations. The substantial denitrification potential in deep sediments coupled with the declines in nitrate and dissolved oxygen concentrations in upwelling groundwater suggest that our conceptual model for nitrate removal in deep sediments is applicable to this river network. Our results suggest that nitrate removal rates can be high in deep sediments of upwelling stream reaches, which may have implications for efforts to understand and quantify nitrogen transport and removal at larger scales.Citation: Stelzer, R. S., and L. A. Bartsch (2012), Nitrate removal in deep sediments of a nitrogen-rich river network: A test of a conceptual model,

Interpreting spatial patterns in redox and coupled water–nitrogen fluxes in the streambed of a gaining river reach

“…These observations point to the need 107 to know more about the interactions of groundwater and 108 surface water and the implications of these interactions 109 for water quality improvement under baseflow condi-110 tions and a changing climate (Prudhomme et al 2012 Tonina and Buffington 2009;Krause 117 et al 2011). Water flow pathways through riverbeds 118 are complex and multi-dimensional, including lateral 119 (horizontal) inputs from the riparian zone (Ranalli and 120 Macalady 2010) and vertical, upwelling groundwater 121 (Stelzer and Bartsch 2012). In a gaining setting, these 122 pathways have the potential to supply nitrate through 123 the riverbed to surface waters.…”

“…574 be a conservative solute, and patterns in chloride 575 concentration in a riverbed and/or riparian setting are 576 generally due to the mixing of water from various 577 sources; for example due to mixing of surface and 578 ground water (Stelzer and Bartsch 2012;Pinay et al 579 1998) Figure 4 illustrates the patterns in pore water 607 chemistry that could arise from two different exchange 608 mechanisms in the riverbed. A decrease in 'source 609 function' score with depth in the river bed at 610 piezometer clusters H and I may be indicative of 611 mixing between surface and pore waters of distinctly 612 different chloride and sulphate concentrations.…”

“…Quantifying reach-scale spatial patterns of 144 redox-sensitive species (electron donors and acceptors) 145 in pore waters is important owing to its influence on the 146 spatial distribution of biogeochemical processes within 147 the streambed (Dahm et al 1998;Marzadri et al 2011Marzadri et al , 148 2012. Additionally, concomitant observations of con-149 servative chemical species together with redox species 150 offers further insights into sources of water, for example, 151 by aiding identification of preferential discharge loca-152 tions in the river bed (Stelzer and Bartsch 2012). 153 A number of field studies in gaining settings have 154 focused on the importance of upwelling groundwater 155 for nitrate flux and transformations in a streambed 156 (e.g.…”

“…153 A number of field studies in gaining settings have 154 focused on the importance of upwelling groundwater 155 for nitrate flux and transformations in a streambed 156 (e.g. Kennedy et al 2009a;Stelzer and Bartsch 2012;157 Flewelling et al 2012). Other research has focused on 158 other groundwater-surface water exchange mecha-159 nisms such as the role of hyporheic exchange flows 160 (Kasahara and Hill 2006) or meander bends (Zarnetske 161 et al 2011) for nitrogen transformations.…”

Buried particulate organic carbon stimulates denitrification and nitrate retention in stream sediments at the groundwater–surface water interface

“…Based on these observations, we developed and tested a conceptual model for NO 3 − removal in deep stream sediments in gaining stream reaches that receive oxic, NO 3 − -laden ground water (Stelzer and Bartsch 2012). In the conceptual model and throughout this paper, we use deep sediments to refer to sediments below 5-cm depth, a depth at which we have found 0 to minimal surface-water exchange in upwelling locations (Stelzer et al 2011a).…”