Nene phosphate in sediment investigation - Environment Agency Project REF: 30258

This report details the results of research the British Geological Survey has undertaken for the UK Environment Agency on sediment and phosphorus dynamics in the main six Water Framework Directive (WFD) water bodies of the River Nene in eastern England. The sampled water bodies started in the head waters of the Nene near Daventry (water body 1) and continued to the Dog in Doublet lock to the east of Peterborough (water body 6). The project comprised of three parts. These were (i) sampling and laboratory analysis (ii) landscape evolution modelling and catchment erosion assessments to provide first order estimates of sediment inputs and transport in the River Nene and (iii) combining these results to determine sediment TP (TP) and sediment Olsen extractable phosphate (OEP) budgets for the river. Results showed that there appeared to be geological/soil parent material controls on the concentrations of TP in the sediments of the River Nene, with water bodies 1-3 containing less TP than water bodies 4-6. Analysis of OEP showed that sediments contained high concentrations (up to 100 mg kg-1 OEP) that could be utilised by macrophytes and also potentially desorb to the river water. Calculation of the Effective Phosphorus Concentrations (EPC0) in each of the water bodies suggested that sediments were currently most likely to act as a sink for soluble reactive phosphorus (SRP) in the river water, rather than as a source. However this is likely to remain dependent on how river water SRP concentrations vary in the long-term and how EPC0 concentrations vary with ongoing deposition and erosion of sediment. Calculations of sorption of SRP within the active zone of the river bed (the 10 cm of water above the sediment surface and the top 5 cm of sediment) suggests that up to 10 % of the SRP in this water layer could be sorbed by the sediment as the river travels over a distance of 1 km. Catchment erosion rates, river inputs and transport through the six water bodies were examined using the Caesar Desc Platform (CDP) landscape evolution model and compared to reported literature values. The CPD model gave first order estimations of natural baseline catchment erosion of ~0.5 t km2 yr-1. However, human impacts on erosion such as land drainage are not included within this estimate. Therefore, literature erosion rates were identified, with the most robust catchment erosion rate being ~6.6 t km2 yr-1. Using output variables calculated from the CPD model we applied these to this value to give a range of likely erosion and transport for each of the six water bodies based on typical annual precipitation rates. It was calculated that between 1000 and 10000 tonnes sediment would pass through the end of water body 6 (Dog in Doublet) each year. Water body 5 had the greatest quantity of sediment leaving it whilst greatest sediment deposition occurred in water body 6. Sediment associated TP and OEP transport and deposition corresponds to these sediment movements as well as their respective concentrations in the sediment. It was calculated that between 4 and 42 T of TP and 0.074 and 0.69 T of OEP attached to sediment passes through the exit of water body 6 each year, either as suspended sediment or bedload. Water samples were analysed and a strong correlation found between SRP and Boron, suggesting that SRP in the river waters at the time of sampling had a strong sewage treatment works (STW) signature. With EPC0 results suggesting that river sediments are currently active sorbents of SRP, the presence of sediment is likely acting to decrease the SRP in the river water. Thus, the greatest management task to improve water quality with respect to the concentration of SRP is preventing the sediment becoming a source of SRP if the river water concentration falls below the EPC0 concentration. This may represent a balance between de-silting (although this would involve removing a SRP sink), the harvesting of macrophytes to remove P in the biomass and a continued decrease in P inputs from Sewage Treatment Works in addition to Catchment Sensitive farming approaches to reduce diffuse P inputs.