Declining numbers of contraventions of pesticide standards played a major role in the improvement. Only 0.2% of pesticide analyses failed to meet the standard in 1996 compared with 0.8% in 1995 and 1.2% in 1994. The DWI ascribes the improvement to the gradual completion of the water industry's £1 billion programme to remove pesticides at treatment works.
Despite generally improved compliance, an increasing number of supply zones failed to meet standards for coliforms, odour, PAHs and turbidity. 29 zones (1.2%) failed the coliform standard in 1996 compared with 16 in 1995. The DWI suggests that the increase may have been due to higher temperatures promoting the growth of biofilms in distribution mains, but also notes that most of the increase occurred in only two companies - South East Water and Welsh Water. Both reported failures in eight zones in 1996 compared with only one and two, respectively, in 1995.
Non-compliance on odour increased from 23 zones in 1995 to 71 in 1996 (2.9%). This "could be attributable to the growth of algae in impoundment reservoirs and the need to abstract from severely depleted reservoir stocks during the drought," the report says.
The number of zones failing PAH standards has increased by 45% since 1992. In 1996, 325 zones (13.4%) failed, largely due to fluoranthene - one of the less toxic of the six PAHs monitored. Levels were generally "very substantially lower than those known to be harmful," the DWI says.
One reason for the increasing number of PAH failures is the tendency of the sampling programme to focus on zones where failures occur, the report suggests. Another is flushing and cleaning of mains to solve other water quality problems which is likely to expose coal tar linings and increase contamination. Renovation of water mains to alleviate PAH failures remains a substantial part of companies' investment programmes. Since 1992, they have made undertakings to remedy PAH breaches in 194 zones - more than for any other parameter.
The DWI also released a special report on nitrate, pesticides and lead in July.2 The report covers monitoring in 1995 and 1996. Earlier reports in the series covered 1990-91 and 1992-94 (ENDS Reports 211 , pp 7-8 and 265, p 9 ).
Despite the improving trend in pesticide compliance, an increasing diversity of products is being detected above the 0.1µg/l standard. Ten new pesticides breached the limit in 1995-96, including the potato sprout suppressant chlorpropham, propachlor, heptachlor, and DDT and several of its metabolites.
The number of supply zones failing the nitrate standard rose from 135 in 1994 to 149 in 1995 and 147 in 1996. The report blames the increase on higher nitrate levels in surface waters during the drought. Little change in nitrate levels has occurred in groundwater sources.
The proportion of the population served by supplies failing the nitrate standard declined from 9.2% in 1991 to 6.7% in 1996 due to treatment programmes. However, most of these were completed by the end of 1995 and relatively few remain - suggesting that only minor improvements are likely in future.
Compliance with lead standards declined in 1995, with 3.85% of samples failing compared with 3.1% in 1994. But only 2.3% of samples failed last year. About half of the failures occurred in North West Water's area. The company is due to comply with undertakings to reduce plumbosolvency by the end of 1997 - an exercise completed by all other companies in 1995 (ENDS Report 263, pp 27-29 ).