Characterization of phosphorus sources in rural watersheds.

Correct identification of P sources in rural watersheds is critical for the development of cost-effective measures to combat agriculturally-driven eutrophication. The chemical composition of various storm runoff types (field surface runoff, field drain outfalls, roads, farmyards, and septic tanks) and the receiving streams in three micro (<10 km(2)) watersheds of varying agricultural intensity were monitored over a 2-yr period. Mean weekly stream soluble reactive phosphorus (SRP) and total phosphorus (TP) concentrations increased from 29 and 69 microg L(-1), respectively in the watershed with the lowest intensity agriculture to 382 and 503 microg L(-1), respectively in the watershed with high intensity agriculture and a village sewage treatment works. Concentrations of TP in storm runoff varied by up to two orders of magnitude reflecting the complex origins, routing, and composition of contributing source areas. Application of the DESPRAL test suggested field runoff TP concentrations were influenced by both P and organic matter in soil. However, runoff from impervious surfaces (farmyard and roads), and/or influenced by septic tank discharges, was significantly more concentrated (0.08-16 mg TP L(-1), mean >>1 mg L(-1)) than surface and subsurface runoff from cultivated land and pasture (0.02-3.6 mg TP L(-1), mean <1 mg L(-1)), and/or contained a significantly greater proportion (>50% vs. <50%) of P in dissolved forms. It is concluded that P sources associated with the functioning of rural communities (impervious surfaces, detergents, and wastewater) may be more ecologically relevant than those associated with agriculture and should be better quantified and controlled to avoid localized eutrophication impacts.

Characterising phosphorus and nitrate inputs to a rural river using high-frequency concentration-flow relationships.

The total reactive phosphorus (TRP) and nitrate concentrations of the River Enborne, southern England, were monitored at hourly interval between January 2010 and December 2011. The relationships between these high-frequency nutrient concentration signals and flow were used to infer changes in nutrient source and dynamics through the annual cycle and each individual storm event, by studying hysteresis patterns. TRP concentrations exhibited strong dilution patterns with increasing flow, and predominantly clockwise hysteresis through storm events. Despite the Enborne catchment being relatively rural for southern England, TRP inputs were dominated by constant, non-rain-related inputs from sewage treatment works (STW) for the majority of the year, producing the highest phosphorus concentrations through the spring-summer growing season. At higher river flows, the majority of the TRP load was derived from within-channel remobilisation of phosphorus from the bed sediment, much of which was also derived from STW inputs. Therefore, future phosphorus mitigation measures should focus on STW improvements. Agricultural diffuse TRP inputs were only evident during storms in the May of each year, probably relating to manure application to land. The nitrate concentration-flow relationship produced a series of dilution curves, indicating major inputs from groundwater and to a lesser extent STW. Significant diffuse agricultural inputs with anticlockwise hysteresis trajectories were observed during the first major storms of the winter period. The simultaneous investigation of high-frequency time series data, concentration-flow relationships and hysteresis behaviour through multiple storms for both phosphorus and nitrate offers a simple and innovative approach for providing new insights into nutrient sources and dynamics.