Phosphorus legacies and water quality in the Somerset Levels and Moors, England

Abstract Inflows of phosphorus (P) to freshwaters from the wastewater and agriculture sectors of our food systems continue to reduce aquatic biodiversity and threaten human health. A range of analytical tools were applied to investigate the contributions of these two sectors to riverine P pollution in the catchments of the Somerset Levels and Moors (SLM) region of England. A substance flow analysis of current sector P use in 2021 identified a low food system P use efficiency (48–75%) and variable P input pressures of unused P of 2.1–8.5 kg ha −1 year −1 driven by livestock feed imports and a high population density. Despite recent reductions in wastewater P discharges, concentration (C) and flow (Q) analysis found both point source and diffuse source signals in the river P record. River soluble reactive P (SRP) and total P (TP) flux in a range of SLM sub-catchments ranged from 0.5–1.5 and 0.75–2.4 kg ha −1 year −1 , respectively. The flux of SRP and TP associated with diffuse sources (calculated by CQ analysis) averaged 0.3–0.5 and 0.5–0.95 kg ha −1 year −1 , respectively, and was significantly positively correlated ( r 2 0.6, p < 0.01) to the agricultural P surplus in these sub-catchments. The large intercept (0.28 kg SRP ha −1 and 0.56 kg TP ha −1 ) of this relationship suggested that the historical legacy P store was the main pollution threat from agriculture. An inventory of agricultural P inputs and P outputs over the last 150 years together with a catchment soil analysis programme confirmed a legacy of soil P accumulation amounting to an average 2 t ha −1 . An analysis of the potential impact of reductions in the sector P input pressure on river P concentrations suggested that on-going and planned increases in wastewater P removal efficiency must be supplemented with a major system change towards drawdown of legacy soil P reserves to lower the P pollution threat in this nationally important region.