Iron-rich water treatment residuals effectively reduce internal phosphorus loading in peaty freshwater systems: a field study

Internal phosphorus (P) loading is a main cause for persistent eutrophication of shallow freshwater systems and can delay restoration for decades. Iron (Fe) amendment is often used to enhance P binding in the sediment and reduce benthic P fluxes. However, sufficient dosing using Fe salts is challenging due to acidification. Fe-rich water treatment residuals (Fe-WTR) are an attractive alternative, but their behavior in aquatic sediments is poorly studied. In this field study, a ditch in a peat polder was treated with ~ 2.5 kg Fe/m2 using Fe-WTR. Sediment porewater and solid phase analyses, including sequential Fe extraction, showed that the added Fe-WTR significantly increased the reactive Fe reservoir of the surface sediment. Sediment incubation experiments and surface water monitoring for one year indicated an efficient reduction of internal P loading. Redox cycling was found to redistribute the added Fe both laterally across the ditch and vertically towards the sediment surface. Reactive Fe phases were thus continuously replenished in the surface sediment and available for P retention via co-precipitation and adsorption, potentially increasing the longevity of the treatment. Loss of the added Fe to sulfidation was limited due to the large excess of available Fe. However, the initial P-content of the Fe-WTR also increased the sediment P reservoir by ~ 10%, potentially enhancing future internal P loading. This study shows that Fe-WTR are viable for freshwater restoration, however, in spite of detailed knowledge of the system, to judge longevity of the treatment remains challenging and long-term monitoring after treatment remains necessary.