Predicting accurate and ecologically relevant regional scale concentrations of triclosan in rivers for use in higher-tier aquatic risk assessments.

Triclosan (TCS) is used in a range of domestic products and may reach aquatic environments after disposal "down-the-drain". Reach-specific TCS concentrations were predicted using the stochastic water quality model LF2000-WQX for two heavily urbanised regions in England, encompassing some of the most densely populated areas (including London and Birmingham). The model was calibrated using effluent TCS concentration data derived from a national monitoring programme which included 21 sewage treatment plants (STPs). The assessment represents a significant improvement on previous higher-tier modelling studies of TCS because it is applied to a much wider area compared with that hitherto evaluated. Initial results suggest that total TCS concentrations exceed a conservative Predicted No Effect Concentration of 69 ng L(-1) in <15% of river reaches. However, TCS is an ionisable compound with a pKa of about 8. In many rivers, a significant fraction of TCS will exist as the phenolate anion, which has much lower toxicity than neutral TCS. Concentrations of the neutral form of TCS were, therefore, calculated using measured data on river water pH. Predicted neutral concentrations in most river reaches were significantly lower than those for total TCS, often by >50%, reducing the associated risks. Further work is needed to better characterise the aquatic toxicity of TCS to fresh water organisms under a range of environmentally relevant conditions.