Monitoring and modelling of butyltin compounds in Finnish inland lake

In this study we measured the total concentration of BTCs using grab water sampling, dissolved concentration with passive samplers, and particle-bound fraction with sedimentation traps in a Finnish inland lake. The sampling was conducted from May to September over two study years. In grab water samples the average concentration of MBT at sampling sites varied between 4.8 and 13 ng L−1, DBT 0.9–2.4 ng L−1, and TBT 0.4–0.8 ng L−1 during the first study year and 0.6–1.1 ng L−1, DBT 0.5–2.2 ng L−1 and TBT < LOD-0.7 ng L−1 during the second year. The average BTC concentrations determined with passive samplers varied between 0.08 and 0.53 ng L−1 for MBT, 0.10–0.14 ng L−1 for DBT and 0.05–0.07 ng L−1 for TBT during the first study year and 0.03–0.05 ng L−1 for MBT, 0.02–0.05 ng L−1 for DBT and TBT 0.007–0.013 ng L−1 during the second year. The average BTC concentrations measured in sedimented particles collected with sedimentation traps were between 1.5 and 9.0 ng L−1 for MBT, 0.61–22 ng L−1 for DBT and 0.05–1.8 ng L−1 for TBT during the first study year and 3.0–12 ng L−1 for MBT, 1.7–9.8 ng L−1 for DBT and TBT 0.4–1.2 ng L−1 during the second year. The differences between sampling techniques and the detected BTCs were obvious, e.g., tributyltin (TBT) was detected only in 4%–24% of the grab samples, 50% of the sedimentation traps, and 93% of passive samplers. The BTC concentrations measured with grab and passive sampling suggested hydrological differences between the study years. This was confirmed with flow velocity measurements. However, the annual difference was not observed in BTC concentrations measured in settled particles which suggest that only the dissolved BTC fraction varied. The extreme value analysis suggested that grab sampling and sedimentation trap sampling results contain more extreme peak values than passive sampling. However, all high concentrations are not automatically extreme values but indicates that BTCs are present in surface water in trace concentrations despite not being detected with all sampling techniques.