Leaching of Nutrients from Sediments into the Water Following the Application of Organic Amendments: Laboratory Scale Experiment

Sediment internal nutrient loading is a major cause of surface water eutrophication. Adding organic amendments to dredged sediments may improve their potential reuse but also risk enhancing nutrient leaching. This column study investigated the effect of adding compost (25% v/v), freshwater algae suspension (2.5% v/v), or their combination (22.5% compost + 2.5% algae) to reservoir bottom sediments on the leaching of orthophosphate (PO₄³⁻) and nitrate‑nitrogen (NO₃⁻-N) under three simulated weekly rainfall events. The control was unamended sediment. Leachates were analyzed for PO₄³⁻ and NO₃⁻-N. Compost‑amended sediment showed the highest PO₄³⁻ concentrations in leachates, but levels stabilized over time, whereas nitrate‑nitrogen concentrations decreased rapidly. Algae alone reduced both orthophosphate and NO₃⁻-N leaching compared to the control. The combination of compost and algae further decreased leachate volume and enhanced the retention of several elements but did not fully mitigate phosphorus leaching. The progressive hydration of organic material increased water holding capacity, reducing percolation. Compost promoted nitrogen transformation processes (e.g., immobilization or denitrification), while algae likely acted through nutrient uptake. We conclude that organic amendments have a dual role: compost increases phosphorus availability but stabilizes its release, whereas algae reduce leaching of both nutrients. The choice of amendment should align with specific water quality targets. Longer‑term and field‑scale studies are needed to confirm these trends.