Nitrogen‐Loads to Streams: Importance of Bypass Flow and Nitrate Removal Processes

Steiness, Mads; Jessen, Søren; Veen, Sofie Gyritia Madsen van't; Kofod, Tue; Højberg, Anker Lajer; Engesgaard, Peter

doi:10.1029/2020jg006111

Cited by 8 publications

(6 citation statements)

References 73 publications

(166 reference statements)

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“…Preferential pathways (tile drains and other macropores, sand layers) that deliver groundwater to the CC and NC streams may be characterized by higher redox conditions (in general) and less effective denitrification compared to the low redox/less permeable zones represented by some of the groundwater samples. This interpretation is consistent with the concept of "bypass" via preferential pathways, including tile drains, through riparian zones that are characterized by enhanced denitrification (Hester & Fox, 2020;Jaynes & Isenhart, 2014;Steiness et al, 2021).…”

supporting

confidence: 86%

Nutrient delivery by groundwater discharge to headwater streams in agricultural catchments

Stempvoort

MacKay

Collins

et al. 2022

Hydrological Processes

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Nutrient concentrations in riparian groundwater are variable from site to site, making it challenging to use these data to quantify nutrient fluxes contributed by groundwater discharge to headwater streams. Instead, we define event‐excluded baseflow (BFEE), and use stream discharge data and stream samples collected during BFEE to estimate groundwater discharge and loading of nutrients to the streams. Based on this method, in two study catchments (Thames River basin, Ontario, Canada) groundwater discharge during BFEE contributed about 30% of the total streamflow and 20%–30% of the total loading of soluble reactive phosphorus (SRP) and nitrate. Previous estimates of baseflow using hydrographic separation techniques indicate that groundwater discharge likely contributes even larger fractions, ~50%–60% of flow in these catchments. We infer that groundwater likely contributes ~40% of the annual SRP load and ~ 50%–60% of the nitrate‐N load. These results indicate that groundwater discharge plays an important role in the nutrient loading to headwater streams. This reinforces the findings of earlier studies that have inferred a dominant influence of legacy reservoirs of nutrients in the subsurface of similar agricultural catchments on the loads of these nutrients in headwater streams.

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supporting

confidence: 86%

Nutrient delivery by groundwater discharge to headwater streams in agricultural catchments

Stempvoort

MacKay

Collins

et al. 2022

Hydrological Processes

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“…The likelihood of oxic conditions was lowest closest to the stream, with oxic conditions increasing quickly to about a fifth of the way up the watershed (LP = 2000) and then increasing more slowly ( Figure 3 ). A lower likelihood of oxic conditions when LP8 values were low is consistent with previous work showing the increased likelihood of suboxic conditions as streams are approached 79 , 80 due to an increase in electron donors in riparian zones. 81 However, it should be noted that these studies often documented suboxic conditions near lower-order streams than eighth-order.…”

Section: Results and Discussionsupporting

confidence: 90%

Predicting Redox Conditions in Groundwater at a National Scale Using Random Forest Classification

Tesoriero,

Wherry,

Dupuy

et al. 2024

Environ. Sci. Technol.

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Redox conditions in groundwater may markedly affect the fate and transport of nutrients, volatile organic compounds, and trace metals, with significant implications for human health. While many local assessments of redox conditions have been made, the spatial variability of redox reaction rates makes the determination of redox conditions at regional or national scales problematic. In this study, redox conditions in groundwater were predicted for the contiguous United States using random forest classification by relating measured water quality data from over 30,000 wells to natural and anthropogenic factors. The model correctly predicted the oxic/suboxic classification for 78 and 79% of the samples in the out-of-bag and hold-out data sets, respectively. Variables describing geology, hydrology, soil properties, and hydrologic position were among the most important factors affecting the likelihood of oxic conditions in groundwater. Important model variables tended to relate to aquifer recharge, groundwater travel time, or prevalence of electron donors, which are key drivers of redox conditions in groundwater. Partial dependence plots suggested that the likelihood of oxic conditions in groundwater decreased sharply as streams were approached and gradually as the depth below the water table increased. The probability of oxic groundwater increased as base flow index values increased, likely due to the prevalence of welldrained soils and geologic materials in high base flow index areas. The likelihood of oxic conditions increased as topographic wetness index (TWI) values decreased. High topographic wetness index values occur in areas with a propensity for standing water and overland flow, conditions that limit the delivery of dissolved oxygen to groundwater by recharge; higher TWI values also tend to occur in discharge areas, which may contain groundwater with long travel times. A second model was developed to predict the probability of elevated manganese (Mn) concentrations in groundwater (i.e., ≥50 μg/L). The Mn model relied on many of the same variables as the oxic/suboxic model and may be used to identify areas where Mn-reducing conditions occur and where there is an increased risk to domestic water supplies due to high Mn concentrations. Model predictions of redox conditions in groundwater produced in this study may help identify regions of the country with elevated groundwater vulnerability and stream vulnerability to groundwater-derived contaminants.

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“…Various factors may influence the relationships between surficial geology, denitrification and nitrate concentrations in the headwater streams. For example, in our study, the till terrain (NC) catchments are more extensively tile drained than the mixed-outwash-till terrain (CC) catchments; as a result, transport of nitrate-enriched groundwater from upland areas of the catchment to the streams via tile drains and other macropore networks may result in relatively efficient bypass of the anoxic 'hotspots' in riparian zones where extensive denitrification is focused (Blann et al, 2009;Steiness et al, 2021).…”

Section: Relationships Between Stream Nitrate Concentrations Land Cov...mentioning

confidence: 90%

Influence of wetlands on nutrients in headwaters of agricultural catchments

Stempvoort

MacKay

Collins

et al. 2023

Hydrological Processes

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We investigated the influence of land cover on nutrient concentrations (295 samples) in headwater streams over a 2-year period in 10 agriculture-dominated subcatchments (163-8373 ha) in southern Ontario Canada. In this region, monitoring and research on nutrient dynamics in headwater wetlands is sparse. Our results indicated a significant positive correlation (Pearson coefficient ρ = 0.320) between soluble reactive phosphorus (SRP) in the headwater streams and the percentage of wetlands in these agriculture-dominated catchments. This result suggests that headwater wetlands, and other wet riparian zones, are key sources of SRP in the headwater streams.Nitrate concentrations were positively correlated with % agricultural land cover (ρ = 0.316), consistent with previous studies, while SRP concentrations were negatively correlated with % agricultural land cover (ρ = À0.325). There was a significant positive correlation between SRP concentrations and discharge in some of the streams. Seasonal SRP trends appear to be closely related to temperature-dependent seasonal changes in redox conditions, including levels of dissolved O 2 . Surficial geology had some influence on nitrate concentrations in the streams, which tended to be higher in catchments dominated by glacial till (till terrain), compared to catchments with extensive areas of outwash sand, in addition to till terrain.

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Nitrogen‐Loads to Streams: Importance of Bypass Flow and Nitrate Removal Processes

Abstract: Nutrients applied in connection with agriculture may enter the groundwater and ultimately end up in streams. Riparian lowlands have shown to hold the potential to significantly decrease nitrate (NO 3 ) concentrations in groundwater flowing from uplands to streams (

Cited by 8 publications

References 73 publications

Nutrient delivery by groundwater discharge to headwater streams in agricultural catchments

Nutrient delivery by groundwater discharge to headwater streams in agricultural catchments

Predicting Redox Conditions in Groundwater at a National Scale Using Random Forest Classification

Influence of wetlands on nutrients in headwaters of agricultural catchments

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