Internal loading of phosphorus in streams described by a Sediment-Water Exchange Model for Phosphorus (SWEMP): From lab to field scale.

The reaction of phosphorus (P) between sediments and water in streams strongly affects the surface water P concentrations. A new reactive transport model (SWEMP: Sediment-Water Exchange Model for Phosphorus) was developed to describe redox dependent P sorption in the sediment and vertical diffusive transport of solutes to the overlying stream. The model parameters were independently obtained to first predict P release in ten different sediment-water batch systems and in two flumes. Input parameters are the degree of P saturation of the sediment, its organic matter content, dissolved oxygen (DO) concentration and temperature. The dissolved P concentrations in the overlying waters ranged from 0.02 to 1.2 mg P L-1 in these systems and were correctly predicted by the model within, on average, a factor 1.3 (batch) or 1.1 (flume). The P flux from the sediment towards the overlying water increased with increasing sediment P:Fe ratio and respiration rates, and with decreasing DO and water pH. After validation of the model with experimental data, it was used to predict monthly P concentrations in Flemish rivers using the total P emission data, total discharge, average sediment properties and the monthly averaged water temperatures, DO concentrations and electric conductivity. The monthly average P concentrations oscillate annually between 0.24 and 0.73 mg P L-1 and predictions matched the long-term monitoring data within 10 % using only one adjustable parameter for the entire water system (N > 250,000). The model predicts that summer peaks in P are related to internal loading from the sediment under anoxic conditions rather than to emission-dilution effects, i.e. external input of P and/or its concentration at lower flow rates. This suggests that, surface water P concentrations can be lowered by enhanced DO in the water, the addition of Fe and Al rich binding agents to the sediments and by reducing P emissions.

Phosphorus immobilisation in sediment by using iron rich by-product as affected by water pH and sulphate concentrations.

Iron (Fe) rich by-product from drinking water treatment plants can be added to rivers and lakes to immobilise phosphorus (P) in sediment and lower eutrophication risks. This study was set up to investigate the P immobilisation efficiency of an Fe rich by-product as affected by the pH and sulphate (SO4) concentration in the overlying water. Both factors are known to inhibit long-term P immobilisation under anoxic conditions. A static sediment-water incubation was conducted at varying buffered water pH values (6, 7 and 8) and different initial SO4 concentrations (0-170 mg SO4 L-1) with or without Fe rich by-product amendment to the sediment. In the unamended sediment, the P release to the overlying water was highest, and up to 6 mg P L-1, at lowest water pH due to higher reductive dissolution of Fe(III) oxyhydroxides. The Fe rich by-product amendment to the sediment largely reduced P release from sediment by factors 50-160 depending on pH, with slightly lowest immobilisation at highest pH 8, likely because of pH dependent P sorption. The total sulphur (S) concentrations in the overlying water reduced during incubation. The P release in unamended sediments increased from 2.7 mg L-1 to 4.2 mg L-1 with higher initial SO4 concentrations, suggesting sulphide formation during incubation and FeS precipitation that facilitates release of P. However, no such SO4 effects were found where Fe rich by-product was applied that lowered P release to <0.1 mg L-1 illustrating high stability of immobilised P in amended sediments. This study suggests that Fe rich by-product is efficient for P immobilisation but that loss of Fe in low pH water may lower its long-term effect.

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.

Sediment respiration contributes to phosphate release in lowland surface waters.

High phosphate (PO4) concentration peaks in lowland rivers occur due to internal loading at low flow rates and low dissolved oxygen (DO) concentrations. However the mechanisms controlling this PO4 are not fully understood yet. This study was set up to identify additional factors affecting internal P loading, the hypothesis being that sediment respiration varies among sediments and might explain spatial variability in reducing conditions. The sediment of ten rivers was collected for a static sediment-water incubation experiment without aeration, to induce oxygen depletion by sediment respiration. In addition, four out of the ten sediments were selected and amended with mineral-N and OM in a full factorial design, to evaluate the impact of increased respiration rates and subsequent P release. The P release to the overlying water sharply increased if the average CO2 release rate exceeded 12 mmol CO2 m-2 day-1 over the first 15 days. However, the P concentration remained below environmental limits as long as the molar P/Fe ratio in the oxalate extract of the sediment was lower than 0.12. The P release increased with increasing sediment cation exchange capacity (CEC), which lowers solution Fe(II) and avoids trapping of PO4 in Fe-minerals. The PO4 release could be explained by a multiple regression model including CO2 release, oxalate extractable sediment P, Fe and Al and the CEC (R2 = 0.78), the R2 was only 0.41 for the molar P/Fe ratio in the sediment. This study shows that internal loading of P is enhanced under eutrophic conditions which boost sediment respiration and which may be attenuated when the CEC and Fe + Al oxide concentrations in the sediments are large.