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Over the past decades, thousands of different per- and polyfluoroalkyl substances (PFASs) have been produced and applied in various industrial processes and consumer products. Their structural diversity has reached a level that cannot be covered by classical target screening methods for individual compounds. Large-scale contaminations of soil, however, require the need to adapt new analytical methods that can describe a contamination more comprehensively. While sum parameters such as the total oxidisable precursor (TOP) assay have been developed in the past years, they are not yet applied in the regulatory context of PFASs.In this commentary, we provide an overview on different approaches of the TOP assay as well as its benefits and disadvantages to other sum parameters for PFASs in soil samples. Furthermore, we elaborate its opportunities and its challenges that need to be tackled to implement the TOP assay as a regulatory tool. With several different approaches of the TOP assay being available, a sound and standardised method needs to be agreed upon and more research is necessary to better describe the method. Although the complexity of PFAS contaminations in soil cannot be fully covered by any analytical method alone, the TOP assay can provide valuable data to detect and characterise soil contamination as an inventory for subsequent remediation measures. Therefore, the TOP assay should be implemented as a useful tool both in research and in the regulatory context of PFASs.

Abstract. Recent research indicates that greenhouse gas (GHG) emissions from dry aquatic sediments are a relevant process in the freshwater carbon cycle. However, fluxes are difficult to measure because of the often rocky substrate and the dynamic nature of the habitat. Here we tested the performance of different materials to seal a closed chamber to stony ground both in laboratory and field experiments. Using on-site material consistently resulted in elevated fluxes. The artefact was caused both by outgassing of the material and production of gas. The magnitude of the artefact was site dependent -the measured CO 2 flux increased between 10 and 208 %. Errors due to incomplete sealing proved to be more severe than errors due to non-inert sealing material.Pottery clay as sealing material provided a tight seal between the chamber and the ground and no production of gases was detected. With this approach it is possible to get reliable gas fluxes from hard-substrate sites without using a permanent collar. Our test experiments confirmed that CO 2 fluxes from dry aquatic sediments are similar to CO 2 fluxes from terrestrial soils.

Abstract. Greenhouse gas emissions from dry aquatic sediments are probably globally relevant. However, they are difficult to measure because of the often rocky substrate and the dynamic nature of the habitat. Here we tested the performance of different materials to seal a closed chamber to stony ground both in laboratory and field experiments. Using on-site material consistently resulted in elevated fluxes. The artefact was caused both by outgassing of the material and production of gas. The magnitude of the artefact was site dependent – the measured CO2 flux was increased between 10 and 208 %. Errors due to incomplete sealing proved to be more severe than errors due to non-inert sealing material. Pottery clay as sealing material provided a tight sealing of the chamber to the ground and no production of gases was detected. With this approach it is possible to get reliable gas fluxes from hard-substrate sites without using a permanent collar. Our test experiments confirmed that CO2 fluxes from dry aquatic sediments are similar to CO2 fluxes from normal soils.

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