What is the spatial-temporal behavior of a low, medium and high adsorptive compound in two contrasting natural sediments in OECD 218/219 test systems?

Artificial sediment used in studies according to OECD 218/219 (Sediment Water Chironomid Toxicity Test Using Spiked Sediment/Water) does not necessarily mirror the characteristics of natural sediments. To investigate the influence of sediment characteristics on the spatial-temporal behaviors of bixafen (KfOM = 2244 mL/g), fluopyram (KfOM = 162 mL/g) and N,N-dimethylsulfamide (KfOM ≈ 0 mL/g), experiments according to OECD 218/219 with two contrasting natural sediments were conducted. The silt loam sediment provided a high content of organic matter (OM) (13.1%), while the OM (0.45%) of the sandy sediment was low. Diffusion into (OECD 219) or out (OECD 218) of the sediment was dependent on the extent of adsorption, which is linked to the model compounds ́ adsorption affinities and the sediments ́ OM. Consequently, N,N-dimethylsulfamide showed unhindered mobility in each experimental set up, while the high adsorption affinities of fluopyram and bixafen limited the diffusion in the respective sediments. Therefore, in experiments with the silt loam sediment, both compounds revealed a limited mobility and either accumulated in the top 5 mm of the sediment (OECD 219) or remained homogenously distributed over the sediment depth (OECD 218). A greater mobility was observed within the sandy sediment.The influence of OM as found in a study using artificial sediment could be confirmed. Moreover, the applicability of a TOXSWA model was reassured to predict the measured concentrations at different sediment depths. TOXSWA is used in the regulatory exposure assessment to simulate the behavior of pesticides in surface waters. Calibration of three driving input parameters by inverse modelling (diffusion-, adsorption coefficient and OM) revealed no potential for improvement. The core sampling technique used and the model may contribute to a more realistic determination of concentration to which the Chironomid larvae are exposed to. This applies to water sediment test systems where the test organisms do not evenly inhabit the sediment.