The distribution of phosphorus in sediment and water downstream from a sewage treatment works

Abstract

In order to ensure the good ecological status of a range of waterbodies, as set out in the EU Water Framework Directive, a clearer understanding of nutrient sources and delivery mechanisms is required. Therefore, this study aimed to investigate the quantity and distribution of phosphates in Cam Brook, downstream from Paulton sewage treatment works (STW). Nine sites along a 2.6 km stretch of the river were surveyed; including one upstream of the STW, in October 2008 and the following February to determine whether there is seasonal variation in phosphorus concentrations. Water samples taken from each site were analysed for orthophosphate although sediment samples were analysed for orthophosphate and total phosphate. In February, water samples were further analysed for total phosphorus. Other variables such as dissolved oxygen (DO), water velocity, substrate, width, depth and land use were also surveyed to determine whether they influence the distribution of phosphorus. The water orthophosphate concentrations in Cam Brook significantly increased downstream from the STW in both October and February, rising from mean concentrations of 0.07-0.23 mg l −1 . This suggests that the STW was the major contributor to water orthophosphate concentrations. There was also strong negative correlation between water orthophosphate and DO concentrations in October (r-value −0.414) which could be due to the high biological oxygen demand of decomposers in the Autumn. There were large concentrations of total phosphate in the sediment which can be attributed to the small particle size of the substrate (clay) which increases its adsorption capacity. There were no seasonal variations in the water orthophosphate or sediment total phosphate concentrations although sediment orthophosphate concentrations were significantly higher in February (p-value < 0.001). However, orthophosphate still only accounted for a small proportion, at 3.47%, of the total phosphate concentrations in the sediment. The orthophosphate entering Cam Brook from the STW appears to be subject to a complex set of storage and transformation mechanisms which results in its storage as the non-bioavailable fraction of the total phosphate. Further research needs to be undertaken to determine the phosphorus cycling mechanisms that cause seasonal variation in sediment orthophosphate concentrations.