Water companies are obliged to inform the DWI of incidents likely to affect the quality or sufficiency of water supplies, or which give rise to a significant risk to health. The DWI recently conducted a review of 347 incidents reported between 1990 and 1996 and sent the results to companies in an information letter. Some incidents in 1995 and 1996 are still under investigation so statistics for these years are incomplete.
The commonest causes of incidents were "operational deficiencies", cited in 64 cases. Raw water deterioration, equipment failure and ingress at service reservoirs each caused more than 30 incidents, while cryptosporidium contamination was cited in 15 cases.
The most frequently breached regulatory requirements were standards for water quality in water supply zones - breached in 60% of incidents. In one-third of incidents, companies failed to report incidents adequately to the DWI.
The requirement to supply only disinfected water was breached in 18% of incidents between 1990 and 1994. "As disinfection is the most important treatment process, this is a disturbingly high level," the DWI says.
The DWI also gives illuminating, if sketchy, summaries of all incidents included in the review. Notable cases include contamination of a supply with the solvent carbon tetrachloride after a waste disposal site polluted boreholes; contamination with the petrol additive MTBE; and contamination with trichloroethene due to poor storage at an industrial site. Another incident involved the mothproofer sulcofuron due to pollution of a supply river with effluent.
In a particularly unsavoury incident, a condom mysteriously appeared at a customer's kitchen tap. In another case, contamination with uncured epoxy resin occurred after a mains relining operation. The report gives no details, but this is likely to have involved bisphenol A - a compound known to have oestrogenic properties (ENDS Report 268, pp 26-29 ).