2.10 Our analysis of Thames Water's 2016 CSO discharge data shows that several CSOs continue to spill all year round, suggesting a risk that London remains non-compliant with the Directive (Figure 5). Comparison of this data with dissolved oxygen measurements, however, suggests that achieving the four spills criterion is not essential for maintaining satisfactory dissolved oxygen conditions in the river. Hammersmith was identified in 2005 as being a large-volume, frequently spilling CSO with significant adverse effect on dissolved oxygen levels.8 For the last April to September period in which the Tideway has historically been at its most vulnerable to dissolved oxygen deficits, we estimate that Hammersmith spilled eight times, discharging 1.8 million cubic metres of waste water. Environment Agency data on dissolved oxygen showed no breaches of any dissolved oxygen threshold in the area during this period. The Department considers that inferences should not be made from a single year of data.
2.11 Three other European Union member states operate thresholds as high as 10 spills per CSO per year (Figure 6 on page 22). This may provide some assurance that if the threshold is met the Tunnel will be deemed compliant with the Directive. The Department considers thresholds adopted by other member states to demonstrate compliance are not relevant comparators, because of differences in the nature of overflows and water areas affected. The Department also considers that the 'four spills' level of protection reflects the Government's aim to achieve its environmental and legal objectives well into the future, having regard for the likely impacts of population growth and climate change.
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8 Environment Agency, An Assessment of the Frequency of Operation and Environmental Impact of CSOs, 2005, p. 9.