Causes of Elevated River Phosphorus during Low-Flow Conditions: A Synoptic Survey Approach.

The qualitative status of European waterbodies in relation to Water Framework Directive objectives is assessed relative to quantitative chemical thresholds. River concentrations of dissolved reactive phosphorus (DRP) exceeding 0.035 mg/l are considered to indicate ‘poor’ quality. Sub-hourly monitoring at the outlets of four contrasting Irish agricultural catchments over >7 years (2010-2017) revealed recurring exceedance of this threshold during low river flow. Hypotheses for the cause of these concentrations were identified as; a) lack of dilution, b) contributions from persistent point sources (e.g. dairy parlours, domestic waste water treatment) and c) biochemical mobilisation of phosphorus stored in banks or bed sediment. Lack of dilution did not appear to be the sole contributing factor, as P concentrations in some catchments increased during prolonged low-flow conditions, suggesting a delivery or mobilization component. The objective of this study was to identify areas of the river contributing to elevated P concentrations observed at the outlet and hence, to identify sources and pathways at those sites. This is a foundational study upon which determinative studies into the causes and mechanisms of the issue will be built.

Synoptic surveys were conducted in each of the four catchments during Spring, Summer and Autumn 2017, when mean daily discharge exceeded Q70 and no precipitation occurred in the preceding 24 hours. Streamwater samples (500 ml) were obtained from c. 60 locations per catchment, along with in-situ measurements of pH, temperature, redox potential, electrical conductivity, dissolved oxygen and turbidity. Samples were analysed for total phosphorus, DRP, total nitrogen, nitrate, total organic carbon and boron. A sub-set of samples were analysed for the presence of E. coli, Enterococci and Sulphite Reducing Clostridia. Microbial source tracking was employed using the BacHum and BacBov markers, to differentiate between potential domestic and agricultural sources. Results from this investigation will be forthcoming in early Autumn, 2017.