On 30 September 2002, an operational problem at the Osmotherley site forced the company to drain its tanks of sodium hypochlorite used for chlorine dosing of drinking water. Staff mistakenly thought there would be enough water in the work's lagoons to dilute the chemical to acceptable levels before it discharged into the nearby Cod beck.
However, staff realised their mistake and Yorkshire Water called the Environment Agency to report the incident. Agency officers investigating found a number of dead fish downstream of the site.
The Agency pressed charges against Yorkshire Water, which appeared before Northallerton magistrates on 27 February to plead guilty to causing polluting matter to enter controlled waters, contrary to section 85(1) of the Water Resources Act 1991.
In mitigation the company said it was now spending £135,000 on improvements to the plant to prevent a recurrence. Yorkshire Water was fined £4,000 and ordered to pay the Agency's prosecution costs of £857.
Meanwhile, the company also appeared before Beverley magistrates on 3 March to plead guilty to another pollution offence under section 85 of the 1991 Act.
The court heard that the incident happened in June last year when untreated sewage from a combined sewer overflow at West Green, Pocklington, poured into Pocklington beck, killing around 2,000 fish. The Agency classed the incident as "serious" category 1, because of the large fish kill.
The incident came to the Agency's attention when a member of the public saw dead fish in the beck on 2 June. Dead fish were also found in the Pocklington canal downstream, a well-stocked coarse fishery and site of special scientific interest.
The overflow was found to be discharging because of a partial blockage of the sewer. It was only designed to operate in wet weather when there would be a high dilution.
Yorkshire Water said in mitigation that it had installed a telemetry system on the overflow to warn of future discharges and paid £10,000 to restock the watercourse.
The company was fined £3,500 and ordered to pay costs of £615.
The two cases break a long period of compliance by Yorkshire Water. The company appears not to have been prosecuted for any pollution offences during 2003.