THE cleanup plan for Tolo Harbour could fail because of mariculture zones in the area, according to the Director of Audit's report. In assessing the progress of the Tolo Harbour Action Plan devised in 1987, the Director of Audit, Brian Jenney, was told by the Environmental Protection Department (EPD) that the target to reduce pollution would not be achieved unless the nitrogen contributed by the mariculture zones was significantly reduced. The department proposed last April phasing out mariculture in Tolo Harbour in the next three years by not renewing existing licences. It said that as some $1.7 billion was being spent to clear the harbour, it would be a waste of funds if parallel control measures on mariculture were not implemented. The proposal was turned down by the Lands, Works, Transport, Housing and Environmental Protection Policy Group of the Chief Secretary's Committee, with the EPD dissenting. The committee considered the phasing out programme unrealistic and decided the EPD should concentrate on developing other alternatives. According to an EPD study, it was concluded that the nitrogen load in Tolo Harbour could not be reduced to the target of 600 kilograms a day since mariculture zones were producing 670 kg of nitrogen a day. But the Agriculture and Fisheries Department told the Director of Audit the nitrogen load from the mariculture zones should not exceed 80 kgs a day. It estimated the target for nitrogen would be achieved in the year 2000. However, the EPD maintained its estimates were derived from a 1990 study report on the environmental impact of mariculture, which had been endorsed by the Government and was the most authoritative one. EPD director Stuart Reed said he was not convinced there was a case for revising the estimates although he was willing to consider any new evidence. Mr Jenney recommended that Mr Reed should, in consultation with Director of Agriculture and Fisheries Lawrence Lee Hay-yue, re-examine the pollution load caused by mariculture. Apart from the uncertainty over the achievement of a target pollution load for Tolo Harbour, Mr Jenney was concerned that the capital costs of the entire plan had risen dramatically - 178 per cent in real terms - to top $1.7 billion by this year from an original estimate of $376 million in 1987. He recommended that, for future major action plans, more accurate cost estimates should be made before submission to the approving authority. The Tolo Harbour cleanup plan began after the Governor-in-Council declared Tolo Harbour and Channel should be a water control zone under the Water Pollution Control Ordinance. It had been estimated that 600 kg a day of nitrogen would be achieved in 1991. But the programme had slipped due to a delay in two projects and the finding of two new additional sources of pollution including the mariculture zones after the cleanup plan had been devised.