Remediation of the phosphate excess in Flemish surface waters

The lowland rivers of Flanders exceed phosphate (PO4) limits in over 80% of the locations, the average P concentration exceeds the limit by about factor 5. This excess is commonly ascribed to the large emissions from intensive agriculture and from household emissions. Recently, a detailed analysis of the monitoring data challenged that general view by stating that the river P concentrations are unrelated to P emissions. Instead, P concentration are found to increase with reducing dissolved oxygen and with decreasing iron (Fe) concentration in the sediment. Both trends were experimentally confirmed and are explained by release of PO4 from the sediment after reductive dissolution of Fe(III) minerals, which fix P in the river sediment. The consequence of these findings is that excess P should be rather managed by ensuring larger dissolved oxygen in the water than by reducing P emissions. The objectives of this project are to use this new information to remediate the phosphate excess in Flemish waters by on field interventions, i.e. demonstration that increasing dissolved oxygen by aerating and/or by treating sediments with doses of Fe(III) salts in small rivers decreases the P. A cost estimation will be made to compare reduction of P excess in Flemish water by direct interventions compared with decreasing the P or BOD emissions. This work has undoubtedly impact on the agricultural industry in Flanders which is challenged by increasingly stringent limits on P emissions.