Water companies are now required by the Drinking Water Inspectorate to install continuous monitoring for Cryptosporidium at all sources (surface water and groundwater) where there is deemed to be a significant risk of Cryptosporidium oocysts being present in the final water. This has substantial cost implications, not only for the one-off installation of monitoring equipment, but also for the daily collection and analysis of samples.

This paper describes a methodology that has been used by Southern Water to screen all their groundwater sources, in order to identify those which are at significant risk of Cryptosporidium contamination, and which therefore need continuous monitoring. The methodology is a semi-quantitative scoring technique, developed by the authors, which produces a ranked list of sources and enables action to be prioritized. The methodology is designed to operate with a level of information and data easily available to a water company, overcoming the problems of significant data requirements associated with probabilistic methodologies.

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