2.5 In our 2014 report (Figure 1) we outlined an expectation that the Environment Agency would set evidence-based standards, and apply due diligence where it relied on others' work. We also expected the Department to understand how the European Commission interprets compliance with the Directive. We did not evaluate against these criteria in our 2014 report as competitions for the construction and financing of the Tunnel were still ongoing.
2.6 The Department met with the European Commission in 2007 and secured informal agreement that the four spills criterion would be compliant. The Commission nonetheless initiated court proceedings against the UK in 2010 as the Department's 2008 forecast was that the Tunnel would be complete by June 2020, but compliance had been required by the end of 2000.
2.7 Laboratory experiments measuring the impact on fish of reduced oxygen levels underpinned the dissolved oxygen standards. Research showed that dissolved oxygen levels below 4 milligrams per litre were associated with avoidant behaviour in some fish, and levels below 3 milligrams per litre were associated with increased mortality risk.7 The Environment Agency's judgement was used to set thresholds for the 'Health' and 'Aesthetic' criteria.
2.8 The dissolved oxygen standards underwent independent peer review in 2003, which found the methodology scientifically robust. A working group of Environment Agency staff and external experts reviewed and upheld the thresholds in 2012. The dissolved oxygen standards are comparable to standards contained in other UK guidance to manage dissolved oxygen levels in UK waterways. The thresholds for identifying unsatisfactory CSOs were primarily based on Environment Agency judgement and were only reviewed internally (and upheld) in 2011.
2.9 The Environment Agency will require Thames Water to monitor the duration of discharges at all CSOs connected to the Tunnel, allowing annual spill frequency to be counted. The Environment Agency can also monitor compliance against the dissolved oxygen standards through its 8 monitors stationed across the Tideway. It has no plans to monitor performance against the Health or Aesthetic criteria.
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7 Thames Water, Needs Report: Appendix F, 2010, p. 11.