Internal loading of phosphate in rivers reduces at higher flow velocity and is reduced by iron rich sand application: an experimental study in flumes.

Many lowland regions are afflicted with high phosphorus (P) peaks in rivers during the summer months. Static incubations of sediments have shown that reductive dissolution of ferric iron (Fe(III)) minerals in the sediment explain these P peaks. This study was set up to identify if that mechanism also dominates in a dynamic system, thereby testing the roles of water flow velocity and sediment Fe/P ratio. Decreasing flow velocity was suspected to lower the flux of dissolved oxygen (DO) towards the sediment. The role of the Fe(III)/P ratio was tested by amending iron-rich glauconite sand (GS) to the sediment, in this manner testing possible remediation techniques. Eight flumes (1.80 m long) were constructed with duplicates of four treatments of two laminar flow velocities over the sediment (0.05 m s-1 or 0.15 m s-1) that was either or not amended with GS (10% w/w). In all flumes a daily dose of sodium glutamate was added as a carbon source to mimic wastewater with high BOD, the flumes were operated for 28 days. A decreased velocity lowered the steady-state DO concentration and enhanced the sediment-water release of P by a factor 3. Sediment amendment with GS reduced solution P by factors 3 (low flow velocity) and 2 (high flow velocity). This effect is related to a combination of increasing binding sites for P and of lowering the DO consumption. These experimental data suggest that previously unexplained summer peaks of P in lowland rivers are related to low flow events that limit the DO flux. The internal loading of P requires management of DO in water and can be mitigated by enhancing sediment Fe.