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https://doi.org/10.5194/egusphere-2024-964
https://doi.org/10.5194/egusphere-2024-964
06 May 2024
 | 06 May 2024

Nitrate-nitrogen dynamics in response to forestry harvesting and climate variability: Four years of UV nitrate sensor data in a shallow, gravel aquifer

Ben Wilkins, Tom Johns, and Sarah Mager

Abstract. The leaching of inorganic nitrogen can adversely affect groundwater and hydrologically connected streams and rivers. Traditionally, these effects have been assessed using discrete water quality measurements. However, it is difficult to characterise the complex biogeochemical processes that control nitrate-nitrogen dynamics in groundwater using temporally sparse data. In this study, we installed a continuous UV nitrate sensor, downgradient of forestry land use in a shallow, gravel aquifer to understand nitrate-nitrogen dynamics in groundwater. We found that there were two mechanisms of nitrate-nitrogen pulses in groundwater from the upgradient forestry land use. The most prevalent were nutrient losses during winter months when plant uptake is lower. Outside of winter months, we observed a higher nitrate-nitrogen concentration (12 mg L-1) as a result of changing biogeochemical conditions after trees were harvested, compared to 5.9 mg L-1 when there was no harvesting. We used a novel hysteresis approach, comparing nitrate-nitrogen concentrations and groundwater levels after rainfall recharge to understand event scale variability. First flush events in winter had a larger average area (more hysteresis) of 0.65 compared to 0.35 (less hysteresis) for subsequent events. Peak concentrations occurred earlier in events during 2021 (wetter) compared to 2020 (dryer), highlighting slower drainage pathways in years with less recharge. Through this analysis we also found evidence that the mobilisation of nitrate-nitrogen shifted from rainfall recharge to rising groundwater levels after the surface supply was depleted from successive recharge events. Finally, the nitrate-nitrogen load analysis indicates that the leaching and export occurs in pulses, that discrete sampling cannot accurately characterise. For example, in 2021, over 80 percent of the exported load occurred during a quarter of the year and discharged when there were base flow conditions in the nearby Hurunui River. These findings have implications for forestry land management, the understanding of inorganic nitrogen dynamics in groundwater in response to rainfall recharge and can be applied to future climate projections where periods of drought and storm events are more frequent.

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Journal article(s) based on this preprint

03 Jul 2025
Nitrate-nitrogen dynamics in response to forestry harvesting and climate variability: 4 years of UV nitrate sensor data in a shallow gravel aquifer
Ben Wilkins, Tom Johns, and Sarah Mager
Hydrol. Earth Syst. Sci., 29, 2765–2784, https://doi.org/10.5194/hess-29-2765-2025,https://doi.org/10.5194/hess-29-2765-2025, 2025
Short summary
Ben Wilkins, Tom Johns, and Sarah Mager

Interactive discussion

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2024-964', Anonymous Referee #1, 18 Jun 2024
    • AC1: 'Reply on RC1', Ben Wilkins, 04 Jul 2024
  • RC2: 'Comment on egusphere-2024-964', Anonymous Referee #2, 03 Sep 2024
    • AC2: 'Reply on RC2', Ben Wilkins, 16 Sep 2024

Interactive discussion

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2024-964', Anonymous Referee #1, 18 Jun 2024
    • AC1: 'Reply on RC1', Ben Wilkins, 04 Jul 2024
  • RC2: 'Comment on egusphere-2024-964', Anonymous Referee #2, 03 Sep 2024
    • AC2: 'Reply on RC2', Ben Wilkins, 16 Sep 2024

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload
ED: Publish subject to revisions (further review by editor and referees) (23 Sep 2024) by Bob Su
AR by Ben Wilkins on behalf of the Authors (28 Oct 2024)  Author's response   Author's tracked changes   Manuscript 
ED: Referee Nomination & Report Request started (17 Nov 2024) by Bob Su
RR by Suzanne Jacobs (03 Dec 2024)
RR by Anonymous Referee #1 (02 Jan 2025)
ED: Publish subject to minor revisions (review by editor) (04 Mar 2025) by Bob Su
AR by Ben Wilkins on behalf of the Authors (10 Mar 2025)  Author's response   Author's tracked changes   Manuscript 
ED: Publish as is (07 Apr 2025) by Bob Su
AR by Ben Wilkins on behalf of the Authors (09 Apr 2025)

Journal article(s) based on this preprint

03 Jul 2025
Nitrate-nitrogen dynamics in response to forestry harvesting and climate variability: 4 years of UV nitrate sensor data in a shallow gravel aquifer
Ben Wilkins, Tom Johns, and Sarah Mager
Hydrol. Earth Syst. Sci., 29, 2765–2784, https://doi.org/10.5194/hess-29-2765-2025,https://doi.org/10.5194/hess-29-2765-2025, 2025
Short summary
Ben Wilkins, Tom Johns, and Sarah Mager
Ben Wilkins, Tom Johns, and Sarah Mager

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Short summary
We measured nitrate-nitrogen concentrations using a continuous UV nitrate sensor over four years in a shallow, gravel aquifer. We observed pulses of nitrate-nitrogen in response to forestry harvesting during subsequent rainfall recharge. Our novel hysteresis analysis helped elucidate event scale nitrate-nitrogen dynamics, while integrating continuous data improved nitrate-nitrogen load estimates. This work provides new insights into the variability of nitrate-nitrogen in permeable aquifers.
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