Monitoring of micropollutants in rivers: Are national sampling strategies applied in the EU fit for purpose?

Monitoring the quality of water resources is the basis for protecting the environmental and human health from the adverse effects of diffuse and point source pollution. Organic and inorganic trace substances, also known as micropollutants, usually occur in surface waters in very low concentrations (nanograms or micrograms per litre). These very low concentrations require considerable analytical and therefore financial effort for monitoring. This limits the number of samples that can be analysed in monitoring programmes. In addition, the micropollutants relevant to water bodies belong to different groups, which differ in terms of their sources, emission pathways, transport dynamics and environmental behaviour in the catchments and rivers. Such complexity and variability pose significant challenges to effective monitoring. With the proposed extension of the EU Priority Substances List in 2022, the monitoring of micropollutants will become a higher priority, and therefore an accurate reflection of the situation in the river is crucial for effective monitoring of the quality of the EU's water resources. To address these challenges and provide a solid basis for future monitoring programs, a study was conducted to investigate the mean and maximum concentrations and annual loads observed in water bodies during a one-year water body monitoring programme and how these differ if different strategies for sampling are used.

A one-year programme applying three sampling methods, was implemented in parallel at two sites in the catchment of the Wulka river in Austria. Both monitoring sites are characterised by an agricultural catchment area, but only one is heavily influenced by discharges from urban wastewater treatment plants. Grab samples were taken every 14 days, while integrated composite samples were collected continuously over a 14-day period using two parallel-operated automated cooled samplers. These integrated composite samples utilized both time-paced (CTCV) and flow-paced (VTCV) sampling techniques. Four groups of trace contaminants were selected to represent different emission and transport dynamics in river catchments: trace metals (total and dissolved), pharmaceuticals, per and polyfluoroalkyl substances (PFAS) and pesticides.

For substances that are predominantly discharged continuously, such as widely used pharmaceuticals or dissolved metals, an initial assessment of the annual average concentrations can be made using twelve grab samples without major systematic deviations. Composite sampling methods, such as CTCV and VTCV, are advantageous in the case of temporal variability of emissions and riverine concentrations. Seasonal substances such as pesticides require special monitoring through extended composite sampling. Substances emitted during specific events (e.g. some pesticides, some PFAS, total metals) are difficult to record using grab samples. Composite sampling methods offer significant advantages by integrating samples during discharge-driven pollution events. An alternative approach is to grab samples specifically during such events using stratified sampling. If substances are emitted via pulses for only a short period of time (in this case study, this is likely to be the case for the insecticide lindane), these substances may not be detected at all by grab sampling. In such cases, the use of composite samples is necessary to ensure detection.