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The environmental risk assessment of substances is introduced and the various controls used to protect the environment are outlined. The European notification system and the risk assessments required as part of the system are detailed. Through an examination of the existing European Union System for the Evaluation of Substances tool and sensitivity analyses based on variability in the measurement of physico-chemical properties for a substance, a spreadsheet model was developed to allow multiple risk assessments for the same substance to be calculated simultaneously. The development and testing of the NECXES spreadsheet tool are documented in detail. Data for the capacity and dilution factor at Sewage Treatment Plants (STPs) in England and Wales were collated and statistically analysed and compared to European default values used for generic risk assessments. The default capacity value for STPs (10,000 population equivalents) was protective of 70% of the STPs sampled. The remaining 30% however, a small number of large works, contributed 94% of the total effluent discharged from STPs in England and Wales. The STP data were used with the NEXCES tool to perform and compare probabilistic risk assessments to those calculated using deterministic methods for a number of test substances. The probabilistic calculations produced a lower median exposure concentration for water than the generic assessment for all of the test substances. Regression analysis allowed the probability of adverse effects to be quantified for the various deterministic risk values. The NEXCES tool was also used to develop a rapid assessment tool for new substances in the form of contour plots, which can be used to assess the risk of substances using minimal data. The main conclusions and contributions to the academic and industrial fields, as well as the field of environmental technology are presented. Areas where there are opportunities for further research are also outlined.