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Doctoral thesis2024Open access

Floodplain remediation in agricultural streams : improved process understanding for reduced eutrophication

Hallberg, Lukas

Abstract

Anthropogenically modified headwater streams are key pathways for high nutrient and sediment loads from agriculture, sustaining eutrophication and poor water quality in downstream aquatic ecosystems. To address this challenge, stream mitigation strategies are required that complement in-field measures, to intercept internal pollutant loading in streams. In this thesis, the capacity for instream nutrient and sediment mitigation by floodplain remediation was evaluated. Ten catchments in Sweden with streams subjected to floodplain remediation were studied, covering differences in soil texture distributions, land uses and floodplain designs. Reductions in particulate and total phosphoruslosses demonstrated that floodplains on both sides of the channel reduce erosion and improve downstream water quality, compared to both streams with floodplains on one side and trapezoid-shaped reference streams. Denitrification in floodplain sediments accounted for 33 % of total denitrification in remediated streams, controlled by floodplain inundation and stream nitrate concentration. However, at higher inundation frequencies, there was a concurrent increase in denitrification and soluble reactive phosphorus release from floodplain sediments, revealing a trade-off in solute nutrient mitigation. The overall impact on water quality by remediated streams was variable, with greater potential for sediment and associated phosphorus reductions compared to nitrogen. This was explained by high pollutant pressures and limitations in floodplain designs. To realize water quality improvements through floodplain remediation in agricultural streams, sitespecific dominant pollutant forms (nitrogen, phosphorus and sediments) need to be accounted for by appropriate floodplain designs. Yet, as floodplains primarily target instream mobilization and transport, their efficacy can be obscured by excessive pollutant inputs from agricultural soils.

Keywords

Stream remediation; constructed floodplains; two-stage ditch; denitrification; nitrogen; phosphorus; stream metabolism; agricultural stream; eutrophication

Published in

Acta Universitatis Agriculturae Sueciae
2024, number: 2024:39ISBN: 978-91-8046-342-3, eISBN: 978-91-8046-343-0Publisher: Swedish University of Agricultural Sciences

    UKÄ Subject classification

    Oceanography, Hydrology, Water Resources
    Environmental Sciences related to Agriculture and Land-use

    Publication identifier

    DOI: https://doi.org/10.54612/a.45i7o5ssmv

    Permanent link to this page (URI)

    https://res.slu.se/id/publ/128758