To ask the Secretary of State for Environment, Food and Rural Affairs (1) what the running costs were of the two bubbler boats used to oxygenate the Thames in the last year for which figures are available; and if he will make a statement; (106717)

[holding answer 30 November 2006]: The Thames estuary oxygenation vessels are operated and paid for by Thames Water on instruction from the Environment Agency (EA) London environment management team. The running costs of these vessels are therefore a matter for Thames Water.

The vessels are typically called out in response to reduced estuary dissolved oxygen levels caused by the breakdown of storm sewage (foul sewage and rainwater) discharged from the combined sewer overflows. This action is necessary to prevent environmental damage, with the most visible evidence being in the form of fish kills. Fish kills start to occur at dissolved oxygen levels below 20 per cent. saturation. Normal dissolved oxygen levels are over 70 per cent. in winter and over 50 per cent. in summer. The Operating Agreement between the EA and Thames Water specifies that the vessels may be called out when estuary dissolved oxygen levels fall below 30 per cent. saturation.

Dissolved oxygen levels are continuously monitored using a network of Automatic Water Quality Monitoring Stations along the estuary taking readings at 15-minute intervals. The vessels are used when the responsible EA officer considers that the estuary is at risk. Several criteria influence this judgement including water temperature, rainfall over the London sewerage catchments, freshwater flows to the estuary, tidal state, sewage works performance, recent dissolved oxygen levels and trends in the estuary. The relationship between these factors is complex, and a dynamic assessment is made of each event using real time monitoring, rainfall and flow information, to determine when bubbler boats should be used to oxygenate the Thames.