Monitoring and Modelling of Butyltin Compounds in Finnish Inland Lake

Butyltin compounds (BTCs) in surface waters is seldom studied due to their low concentrations and limitations of analytical techniques. In this study we measured total concentration of BTCs with grab water sampling, dissolved concentration with passive samplers and particle bound fraction with sedimentation traps in Finnish inland lake. The sampling was conducted from May to September during two study years. The differences between sampling techniques and the concentrations were obvious. E.g. tributyltin (TBT) was detected only in 4-24 % of the grab samples when the detection with passive samplers was 93% and with sedimentation traps 50%. The dissolved 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 of settled particle.The extreme value analysis suggested that grab sampling and sedimentation trap sampling results contain more extreme peak values than passive sampling. This indicates that BTCs are present in surface water in trace concentrations despite they are not detected with all the sampling techniques. The assumption that WWTP, located in the study area, was the source of BTCs was not valid as elevated BTC concentrations were detected also at the reference site, located upstream of WWTP. Computational modelling and back tracking simulations also supported the concept that WWTP cannot be the only source but BTCs can come even from upstream of the sampling area where there is e.g. wood processing industry.