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Title: Assessment of the Impact of Nutrient Removal on Eutrophic Rivers
Author: B O L Demars
Author: D M Harper
Author: Environment Agency
Document Type: Monograph
Annotation: Environment Agency Project ID:EAPRJOUT_1061, Representation ID: 322, Object ID: 2334
1. The river Wensum, a calcareous lowland river in north Norfolk, is a site of Special Scientific Interest and a candidate Special Area of Conservation under the EU Habitats Directive as a representative of the calcareous Ranunculus lowland stream habitat. 2. The two main urban sewage works discharging into the Wensum, from Fakenham and East Dereham, had phosphorus stripped from their effluents by sedimentation with iron salts, from autumn 1999, funded under the AMP 3 investment programme for the water industry. 3. This project was initiated in order to understand the consequences it would have for the riveras physical and ecological condition. The specific objectives of the project are shown on pages 2-3 of this report. The project commenced one year before the phosphorus-stripping, to enable one yearas abeforea data to be collected. 4. The spatial pattern of total phosphorus in the Wensum showed that the effluents had clearly increased the water column total phosphorus. The mills weirs caused total phosphorus to accumulate in the sediment behind them. Bioavailable phosphorus (BAP) was related to sediment type. Sediment total phosphorus was probably bound to iron. 5. Background levels of soluble phosphorus from sub-catchments not known to be impacted by any effluents were 9-15 AMicro/L (of PO4-P) and total phosphorus 3977 AMicro/L. These represent the runoff from diffuse agricultural sources; no examples could be found of anaturala sites. 6. There were significant in-stream decreases of SRP and TP downstream from the two major STWs after phosphorus removal. The impact on sediment BAP was less convincing but this may be due to the high variability of BAP due to sediment heterogeneity. 7. The phosphorus control at Dereham and Fakenham led to a decrease of 64% and 76% respectively of the loads of total phosphorus (TP) in the River Wensum. Predicted TP loads were in accordance with observed loads and decreased relatively predictably after the phosphorus removal. The soluble reactive phosphorus (SRP) concentrations were relatively low over the post-stripping period, however this was also due to the high flow regime of the year 20002001. Concentrations of SRP at low flow (Q95) and mean flow regime (Q50) were predicted to be respectively extremely high (415-1039 AMicro/L of PO4-P) and very high (140-273 AMicro/L) below the STWs. 8. Aquatic plants were not found to be an indicator of the level of nutrients in the lowland rivers investigated, this over a wide gradient of soluble reactive phosphorus (11-3600 AMicro/L of PO4-P) and ammonium (16-434 AMicro/L of NH4-N) in a nitrate-rich environment (6.4-12.9 mg/L NO3-N). This is contrary to general opinion. The species composition of aquatic vascular plants was predicted by the R and D Technical report P2-127 ii niche theory and species demographic processes, which were also found to partially interact. 9. There were only marginal differences in species richness between rivers in the wider survey (Wensum, Nar, Bure) that could be attributed to phosphorus enrichment. The spatial species turnover along rivers was high, resulting from both natural and anthropogenic factors. 10. Fifteen recommendations for conservation, river management and future water management arising from this study are made. R and D Technical report P2-127 iii CONTENTS ACRONYMS v 1.
Publisher: Environment Agency
Subject Keywords: River; Sediment; Nutrient; Phosphorus; Aquatic plants; Ecohydrology
Extent: 119
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