Nitrate removal and young stream water fractions at the catchment scale

Abstract

Despite extensive research on nitrate export and removal, nutrient contamination remains a major threat to water bodies worldwide. At the local scale, nitrate removal is governed by biogeochemical conditions that vary in space and time, making integration to entire landscapes critical. Water transit times have often been used to describe solute transport, but the relation between water age and nitrate removal at the catchment scale is still poorly understood. We test the hypothesis that nitrate removal peaks when the fraction of young water in discharge is at its minimum, because nitrate removal occurs mostly under dry conditions where deeper, older groundwater dominates streamflow. We tested this hypothesis by exploring a detailed water quality record from the Kervidy-Naizin catchment (FR) and comparing the dynamics of nitrate to those of a conservative solute (chloride). We find that estimates of nitrate removal are consistent with previous estimates at the site and they show a good (inverse) correlation with the fraction of streamflow that is younger than 2.5 months. However, this young water fraction cannot be used to predict nitrate removal in the winter-spring period, when no removal is observed regardless of streamflow age. While this leads us to reject our hypothesis during the winter period, it also suggests that water age distributions and their correlation with nitrate removal can possibly reveal distinct sources of stream water at different hydrologic regimes and relevant biogeochemical reactions. K E Y W O R D Sagricultural catchment, chloride transport, denitrification, Naizin, nitrate removal, SAS function, water age, young water fraction

Depth of Solute Generation Is a Dominant Control on Concentration‐Discharge Relations

“…This can be the case of NO 3 , which often exhibits a mixed behavior. Nitrates can accumulate in the shallow soil layers in agricultural catchments and export under wet conditions; they can also percolate into the groundwater, where they undergo denitrification before reaching the channel through slow groundwater release (Abbott et al, 2018; Benettin et al, 2020; Dupas et al, 2016). The results here remark and provide numerical support to the importance of vertical distribution of solutes (Bishop et al, 2004; Koven et al, 2013) and solute transport activation in specific areas or depths shown before (Basu et al, 2010; Musolff et al, 2015; Seibert et al, 2009; Wen et al, 2020; Zhi et al, 2019).…”

Toward catchment hydro‐biogeochemical theories

“…In a global water chemistry data analysis comparing the relative controls of C–Q patterns from above (climate) and below (depth of solute generation), the depth of solute generation has been found to be predominant (Botter et al, 2020). Recent literature has seen a surge in exploring quantitative, causal linkages between C–Q relationships, transit time, and reaction rates (Barnes et al, 2018; Basu et al, 2010; Benettin et al, 2015; Benettin, Fovet, & Li, 2020; Musolff et al, 2016; Musolff, Fleckenstein, Rao, & Jawitz, 2017; van der Velde, de Rooij, Rozemeijer, van Geer, & Broers, 2010), although more in‐depth studies will be needed to further develop a general, quantitative framework for these connections.…”

Optimal Transport for Assessing Nitrate Source‐Pathway Connectivity

“…This rising limb typically happens during the fall (between October and December) when the soil and epikarst zones are reconnected to the subsurface conveyance channel and drain material which has accumulated during the dry summer season. This “flushing” pattern is often observed in nonkarst systems as well where a rise in the water table reconnects stored material to transport pathways (Benettin et al, 2020; Botter et al, 2020). Lastly, quick flow contribution only dominates during dry, baseflow periods when the magnitude of nitrate loading is relatively low to the rest of the year.…”

“…This temporal pathway contribution is plausible as residence time analysis of our modeling framework shows short residence in quick pathways (3 ± 2 days), but longer residence times and, implicitly, transfer in intermediate (47 ± 10 days) and slow (122 ± 9 days) pathways (Husic, Fox, Adams, Ford, et al, 2019). The extent to which transport occurs along these hydrologic pathways is heavily influenced by mean transit times (Benettin et al, 2020; Lutz et al, 2018). For example, since sinkholes and swallets pirate surface water fairly quickly, the time duration of quickflow connection is similar to that of surface runoff, which has been approximated to be no more than 2 days in the region (Mahoney et al, 2018).…”

“…Pathways of nitrate broadly include quick, intermediate, and slow, and are analogous to runoff, interflow, and baseflow, respectively. Time scales of transport for quick pathways are on the order of hours to days while they range from weeks to months and months to years for intermediate and slow pathways, respectively (Benettin et al, 2020; Husic, Fox, Adams, Ford, et al, 2019; Mellander et al, 2012). Tools to quantify these pathways include hydrograph separation (Husic, Fox, Adams, Ford, et al, 2019; Lutz et al, 2018), numerical modeling (Husic, Fox, Adams, Ford, et al, 2019; Sullivan et al, 2019), and high‐frequency sensing (Fenton et al, 2017; Mellander et al, 2012; Miller et al, 2017).…”