The occurrence of intersex fish is widespread in the rivers of England and Wales. The extent of intersex in fish populations is believed to be strongly linked to their exposure to steroid oestrogens. This paper presents the first national, catchment-based, risk assessment for steroid oestrogens in the world. A Graphical Information System-based model predicted the concentrations of estradiol (E2), estrone (E1) and ethinylestradiol (EE2) which were combined and compared with known biological effect levels to predict endocrine disruption risk for 10,313 individual river reaches (21,452 km) receiving effluent from more than 2,000 sewage treatment plants (STPs) serving over 29 million people. The large scale of this assessment underlines the usefulness of computer based risk assessment methods. Overall, 39% of the modelled reaches (all percentages are expressed as % of the total river length modelled) in England and Wales were predicted to be not at risk from endocrine disruption (mean concentrations <1 ng/L E2 equivalents). There was a large range in the percentage of river reaches at risk in the various regions from 5% in Wales to 67% in the Thames catchment. Important factors influencing this proportion are the population density, particularly their location and the available dilution. A very small proportion of reaches, approximately 1 to 3% were predicted to be at high risk (>10 ng/L E2 equivalents). However, many of these high risk reaches were ditches which were composed almost entirely of sewage effluent. The model could equally well be applied to any other chemical of concern emanating from the human population and which would be impracticable to assess by measurement.\ud \u
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