Application of a core sampling methodology and a mechanistic model to examine the spatial distribution of non-ionized organic compounds in sediment microcosms

Dorn, Alexander; Hollert, Henner (Thesis advisor); Schäffer, Andreas (Thesis advisor)

Aachen : RWTH Aachen University (2022, 2023)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2022


Sediment risk assessment of pesticides was controversially discussed within the scientific and regulatory community in the last years. There are different water-sediment studies with respective purposes. On the one hand, the ecotoxicological potential of pesticides on non-target organism such as sediment dwellers can be investigated. On the other hand, determination of pesticide transformation and degradation could be the purpose of water-sediment studies. The European Food Safety Authority published a scientific opinion on sediment risk assessment, which contained amongst other a lack of knowledge concerning the assessment of ecotoxicological studies. The determined toxicological endpoints are based on the assumption, that test organism and test compounds were evenly distributed within the sediment. However, test organism such as the sediment dwelling larvae of Chironomus riparius reside preferably in the upper sediment layer, due to favorable living conditions. Furthermore, it is well understood that test compounds with lipophile properties tend to adsorb on the upper layer of the sediment, after application into the overlying water. Correspondingly, the spatial concentration differences are also not considered in water-sediment studies investigating the transformation. The distribution of transformation products and their conditions of formation are not respected.Goal of the present Dissertation was to elucidate the spatial distribution of three model compounds in different water sediment test systems by employing a special developed core sampling methodology. Experimental results were compared with simulated data by a developed Toxic Substances in Surface Water (TOXSWA) model contributing to a mechanistic understanding of the model compounds transport process within the sediment. By this comparison, the influence of different adsorption properties of the model compounds and organic mass of the sediment on diffusion should be elaborated. The model compounds were selected according to their adsorption properties, namely bixafen (KfOM: 2244 L kg-1), fluopyram (KfOM: 161 L kg-1) and N,N-dimethylsulfamide (KfOM: 0 L kg-1). The core sampling methodology was used for water-sediment test systems according to OECD 218 (Sediment-Water Chironomid Toxicity Using Spiked Sediment), OECD 219 (Sediment-Water Chironomid Toxicity Using Spiked Water) and OECD 308 (Aerobic and Anaerobic Transformation in Aquatic Sediment Systems) for examining the spatiotemporal behaviors of the model compounds with respect to ‘spiked water’ and ‘spiked sediment’ studies and different water: sediment ratios. Each of the water-sediment studies was implemented with artificial sediment, but as well with two natural water-sediments, respectively. The artificial sediment consisted of ca. 3% organic mass and was classified as sandy loam. The natural sediment from the Wiehltalsperre provided 13% organic mass and was identified as silt-loam, while the other natural sediment from Anglersee had a poor organic mass content (0.3%) and was sandy. In the OECD 218 (‘spiked sediment’) studies, the results of N,N-dimethylsulfamide showed that the majority (50 - 80%) the of the polar model compound migrated from all employed sediments into the overlying water within few days. Due to the moderate adsorption properties of fluopyram, the mass transfer from sediment to overlying water was performed more slowly and differences between the employed sediment could be observed. In the studies using artificial and silt loamy sediment, the fluopyram concentration within the sediment revealed a step-like distribution pattern with increasing concentrations from top to the bottom layer (e.g.: total concentrations of 91.6, 96.4 and 100.2 µg L-1). As the sandy sediment provided a poor amount of organic mass, the major part of fluopyram (50 - 60%) was recovered in the overlying water resulting in an even distribution within the sediment. Consideration of the strong adsorptive bixafen revealed no pronounced mass transfer from the sediment into the overlying water in the studies with artificial and silt-loamy sediment, while it was more pronounced within the sandy sediment studies resulting in an uneven distribution (total concentrations from top to bottom layer: 134.9, 155.3 and 166.3 µg L-1). The impact of adsorption properties of the model compounds and organic mass of the sediment was also revealed in water-sediment studies with ‘spiked water’ (OECD 219&308). N,N-dimethylsulfamide migrated unhindered through all sediments, while fluopyram and bixafen tended to adsorb on the sediment surface, respectively the top layer of the sediment. Governed by the organic mass content, the resulting concentration gradient of fluopyram was maintained in the artificial and silt loamy sediment (e.g.: total concentrations from top to bottom layer 9.3, 6.1 and 3.1 µg L-1), while it did not persist within the sandy sediment over the whole test duration. The strong adsorption of bixafen caused that the model compound remained unevenly distributed within all employed sediments. Besides, the dissipation of the model compounds was also governed by their respective adsorption properties and the organic mass of the sediment. Correspondingly, the dissipation of bixafen into the silt-loamy sediment provided the shortest dissipation half-live. Because of the smaller overlying water volume, the dissipation half-lives provided a shorter period within the OECD 219 water-sediment systems as compared to the respective OECD 308 systems. Experimental and simulated results showed good agreements. The core sampling methodology as well as a TOXSWA model can be used in frame of sediment risk assessment to determine the exposure concentration of a test compound on sediment dwelling organism. It was possible to follow the spatial-temporal behavior of the test compounds by employment of these measures and it was shown that diffusion retarded by adsorption is the relevant transport process within the sediment. Further, the TOXSWA model could be used to determine the dissipation half-lives of the model compounds within the different water-sediment systems. Consequently, a mechanistic explanation for biphasic dissipation kinetics was also revealed.