The use of measured boron concentration data from the GREAT-ER UK validation study 1996]1998/ to generate predicted regional boron concentrations

The concentrations of boron in the Aire and Calder which have been measured in order to validate the GREAT-ER model have been used to derive a predicted environmental concentration PEC. for boron in the Aire catchment in Yorkshire. The data have been obtained from 33 sites, with monthly sampling over a 2-year period. The 90th percentile of the means of measured concentrations 242 mg ly1. is higher than the 90th percentile predicted by the GREAT-ER model 186 mg ly1., if river stretches are normalised by their flow volumes, and unpolluted headwaters are included in the catchment averaging procedure. However, if the river stretches are normalised by the length of the respective stretch, and unpolluted headwaters are included, a lower regional PEC of 28 mg ly1 is obtained. If normalisation by stretch length is appropriate for regional risk assessment, then the 90th percentile of the means of the measured data will overpredict the regional boron concentration in a catchment such as the Aire, with a low natural level of boron in the headwaters. Monitoring data for boron in the effluents from sewage treatment works in the Aire catchment, and for boron in the Went and Rother catchments, with a higher background boron level, are also given.