Understanding hysteresis in high-frequency water quality data in rivers: adding value to targeted research using routinely collected operational data

Patterns and variability in the concentration-discharge relationship may be used to describe the complex interactions and combined effects of catchment processes affecting sources, mobilisation and transport of contaminants. Many concentration-discharge relationships display temporal variability on diurnal, event, seasonal and annual scales. This has been widely demonstrated through both routine regular sampling and targeted storm sampling.

The set-up costs of high frequency in-situ river and reservoir sensors is high, and operation and maintenance of a wide network is both time and resource intensive and the conditions for operation (e.g. environmental conditions, signal, power) are rarely ideal. Yet with technological advances and the growth in availability of high-frequency is-situ water quality sensors, the complexity of the water quality response to changes in flow, across multiple timescales, has become increasingly evident. The observed dynamics during events, and range of hysteresis patterns displayed, shows that the sources of contaminates, mechanisms for mobilisation, and transport times are highly variable both spatially and temporally. Furthermore, seasonal and interannual controls on catchment functioning are seen to result in pronounced differences in the behaviour of parameters between sites, and between individual events at the same site.

This study shows how routine high-frequency data, collected with an operational focus for source protection and in raw water at drinking water treatment works, provide opportunities when trying to identify sources and pathways for contaminants. Despite challenges, these data support the development of a baseline understanding for water quality within a specific catchment or region, and provide insight into catchment specific event-driven dynamics. In catchments where routinely collected data is the only source of multiannual high-frequency water quality data, these data may be crucial in building understanding of long term (decadal) variability and trends; in particular, gaining understanding the changing interactions and effects due to extremes in seasonal patterns across different years. However, the key limitations in the use of these data include undefined uncertainties and missing data, monitoring design, and limited metadata. Therefore, building on initial analysis of routine data, efficient monitoring campaigns for targeted research can be designed to investigate any previously unexplored or unidentified processes and pathways.

This study is part of a wider programme of research on the identification of sources and pathways for contaminants of concern in catchments supplying drinking water in the south west of the UK, and how water quality dynamics are impacted by meteorological and catchment conditions, including atypical events. It supports work on increasing resilience in drinking water source areas and reducing treatment demands and costs, through improving understanding of how water quality in rivers and reservoirs is affected by landscape and farm-based catchment interventions.