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TECHNICAL REPORTS

Wetlands and Aquatic Processes

The Release of Phosphorus to Porewater and Surface Water from River Riparian Sediments

^a Catchment Science Centre, Kroto Research Inst., The Univ. of Sheffield, Broad Lane, Sheffield, S3 7HQ, UK
^b Centre for Sustainable Water Management, The Lancaster Environment Centre, Lancaster University, LA1 4YQ
^c Dep. of Geography, Queen Mary, Univ. of London, Mile End Road, London, E1 4NS

^* Corresponding author (b.surridge{at}sheffield.ac.uk).

Received for publication November 9, 2006. Sediments can be both a source and a sink of dissolved phosphorus (P) in surface water and shallow groundwater. Using laboratory mesocosms, we studied the influence of flooding with deionized water and simulated river water on P release to solution using sediment columns taken from a riparian wetland. The mesocosm incubation results showed that rather than retaining nutrients, sediments in the riparian zone may be a significant source of P. Concentrations of dissolved P in porewater reached more than 3 mg L^–1 and in surface water over 0.8 mg L^–1 within a month of sediment inundation. The reductive dissolution of P-bearing iron (Fe) oxides was the likely mechanism responsible for P release. Dissolved P to Fe molar ratios in anaerobic samples were approximately 0.45 when columns were flooded with water that simulated the chemistry of the adjacent river. This suggests there was insufficient Fe in the anaerobic samples to precipitate all P if the solutions were oxygenated or transported to an aerobic environment. If the anaerobic wetland solutions were delivered to oxygenated rivers and streams adjacent to the riparian zone, the equilibrium concentration of P in these systems could rise. The timing of P release was inversely related to the nitrate (NO₃^–) concentration in floodwater. This indicates that in riparian zones receiving low nitrate loads, or where NO₃^– loads are being progressively reduced, the risk of dissolved P release may increase. These findings present particular challenges for restoration and management in riparian areas.

Abbreviations: P, phosphorus • P_i, inorganic phosphorus • P_o, organic phosphorus • MRP, molybdate reactive phosphorus • NO₃^–, nitrate • Fe²⁺, ferrous iron • DW, core incubations using deionized water as the flooding solution • SRW, core incubations using simulated river water as the flooding solution