Identifying nitrate transport mechanisms and export dynamics using water age modeling and concentration-discharge relationships

Tile-drained agricultural catchments in the Midwestern U.S. are a significant source of nitrate (NO3-) loss to surface waters. Water flow paths and residence times determine the contact length between NO3- and sub-surface material, controlling the rate of biogeochemical reactions and nitrogen removal via denitrification. A Nitratax N- NO3- sensor was installed at the outlet of the Upper Embarras River (UER) basin and collected 16 months of high-resolution NO3- and discharge data from August 2022 to November 2023. An ISCO Automatic Sampler located at the UER outlook collected daily river water samples from April 2022 to November 2023, which were measured for water isotopes alongside weekly precipitation samples. The overall goal of this study is to connect hydrology with NO3- concentration-discharge (C-Q) relationships using high-frequency NO3- measurements and water age modeling. The first part of this study identified NO3- export and hysteresis patterns of delineated storm events and analyzed correlations between C-Q metrics and event characteristics and antecedent conditions. I found that events predominantly show NO3- mobilization export and anti-clockwise hysteresis, indicating that NO3- is transport-limited in the UER catchment. Few environmental controls were correlated with C-Q metrics, though NO3- flushing response on the hydrograph rising limb increased with antecedent moisture. The a parameter, representing the intercept of the NO3- C-Q power law, was highly associated with multiple variables representing antecedent moisture, event magnitude, and seasonality. The second part of this study investigated catchment-scale water mixing and preferential removal during events. To investigate active flow paths, I used a StorAge Selection-based water age model to partition river discharge into three flow components (riparian flow, slow matrix flow, fast overland flow). Flow from riparian areas significantly contributed to low-flow events in the summer and fall, while overland and sub-surface fast-flow pathways were activated only during extreme-flow events in the spring and winter. Comparing C-Q metrics with water age and activated flow pathways showed that parameter a, representing NO3- concentrations, significantly correlated with the fractions of discharge from the riparian area and faster overland flow. Young water from riparian areas during dry conditions was associated with very low NO3- concentrations, but young water from overland or sub-surface flow during wet conditions was associated with very high NO3- concentrations. My study shows young water fraction and active flow paths correlate with stream NO3- across seasons and catchment wetness.

Name thesis:degree_name

M.S.

Level thesis:degree_level

Thesis

Discipline thesis:degree_discipline

Natural Res & Env Sciences

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2024

Author dc:creator

• Miller, Sabine

Contributors dc:contributor

• Yu, Zhongjie

• Nitrate
• Hysteresis
• Export

Statement dc:rights

• Copyright 2024 Sabine Miller

Language dc:language

eng, en

Handle dc:identifier

https://hdl.handle.net/2142/124451

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/124451