The Parliamentary Under-Secretary of State, Department for Environment, Food and Rural Affairs (Baroness Hayman of Ullock) replied for HMG:

Reservoir operations must comply with water framework directive objectives. Water companies are required to follow the drinking water safety planning approach and must assess risks to drinking water from the water source that it is taken from until it reaches consumers at the taps. The planned raw water sources to supply the Havant Thicket reservoir have been monitored and mitigations have been proposed where required, including for emerging substances.

My follow up question: I thank the Minister for her Answer, but I am deeply unsatisfied with it, because I assume that it is Southern Water itself that has monitored and checked the sewage effluent that is going to be put into Havant Thicket reservoir. We need a reservoir. We desperately need more water sources, but we cannot trust Southern Water to audit its own discharges. Can the Government do what is being done in countries such as Singapore and create an expert panel to audit exactly what is going into that water supply? The forever chemicals that I have referred to are highly poisonous and pernicious.

Hansard record here: Havant Thicket Reservoir: Pollutants – Hansard – UK Parliament