Paper alert "How reliable are estimates of trace contaminants in rivers based on monthly grab samples?"

Surface water quality compliance assessment and other river basin management tasks still rely heavily on monitoring programs based on monthly sampling. This is despite the increasing awareness of the variability of contaminant emissions and transport in rivers and the inherent limitations of such an approach.
To systematically assess the implications of this problem, we conducted a one-year survey in two rivers using three sampling techniques in parallel, and compared low-frequency grab samples against time-proportional composite and flow-proportional composite sampling. In this way, we could quantify potential deviations in the estimation of chronic and acute exposure as well as in annual river loads. We did so for 450 contaminants, covering a broad diversity of emission patterns and environmental fate: potentially toxic elements, pharmaceuticals, biocides, pesticides and PFAS.

Our main conclusions:

• monthly grab sampling is only sufficiently reliable for contaminants being emitted fairly constantly and transported primarily in the dissolved phase. Even then, its reliability is mostly limited to assessing average concentrations and thus chronic exposure, while integrated composite sampling can significantly improve the accuracy of load calculations
• some contaminants were only detected by composite samples and were completely missed by grab samples
• from a management and regulatory perspective, the importance of selecting the correct monitoring approach depends on the objective and on how far the river concentrations are from the regulatory thresholds

Check all results in the openaccess paper by Nikolaus Weber et al. - Link, opens an external URL in a new window