The potential for phosphorus release across the sediment-water interface in an eutrophic reservoir dosed with ferric sulphate

Alton Water, Suffolk, UK is a pumped storage reservoir that has a history o f cyanobacterial blooms. Dosing of the input water with ferric sulphate to control external phosphorus loading has occurred since 1983. A detailed stu dy of the sediment chemistry of the site was carried out between May 1995 a nd July 1997. Sequential phosphorus fraction analysis indicated a decrease along the length of the reservoir in sediment labile phosphorus content fro m 0.62 to 0.08 mg P g(-1) dw and iron-bound phosphorus content from 3.22 to 0.46 mg P g(-1) dw. These gradients positively correlated with water colum n chlorophyll rr concentrations reported in a parallel study. Labile and ir on-bound sediment phosphorus contents were in a dynamic equilibrium due to diffusional release, contributing to internal loading to the water column. Equilibrium phosphorus concentrations (EPC) determined from phosphorus adso rption capacity (PAC) experiments were lower inside the bunded region (0.01 -0.03 mg P-PO4 l(-1)) where iron content was greatest compared to outside t he bund (0.15-0.20 mg P-PO4 l(-1)) suggesting greater potential for diffusi onal release of phosphorus at the latter site. PAC experiments indicated th at anaerobic and pH-mediated loadings were of less importance than diffusio nal release, although the latter map have contributed to internal loading i n the main reservoir. Sulphate concentrations may act to increase the poten tial for anaerobic internal loading near to the pumped input in microstrati fied sediment. Sediment iron content decreased from 250 +/- 13.1 to 51 +/- 4.0 mg Fe g(-1) dw across the line of a constructed bund at the north-west end near to the pumped input, which indicated successful control of dispers al of the fine ferric floc. The management implications with regard to phos phorus loadings indicated by these results are discussed. (C) 2001 Elsevier Science Ltd. All rights reserved