Williams et al. (2009) assessed the risk of endocrine disruption induced by these steroid estrogen for the UK under current flow conditions at a catchment level. The concentrations of E1, E2, and EE2 were estimated using a geographical information systemebased model. The estimated concentrations were combined with effect levels to estimate the risk of endocrine disruption across England and Wales. A river network spreading over 21,452 km (10,313 individual reaches) and including more than 2000 STWs serving more than 29 million people was modelled. The study concluded that a very small proportion of the modelled reaches (1e3%) were predicted to be at high risk, and more than a third (39%) were at risk.