Understand the impacts

This page addresses the following potential impacts. To skip to an individual topic, click on the numbered item in this index:

  1. Impacts on the Havant Thicket Reservoir
  2. Impacts on Langstone Harbour
  3. Impact on drinking water
  4. Other environmental impacts
  5. Impacts on leisure benefits

Impacts on the Havant Thicket Reservoir

If a minimum of 7.5 million litres of recycled treated effluent is discharged into the Havant Thicket reservoir every day what impact will that have on;

  • Reservoir water quality? Will there be dead spots that cause problems? How will they achieve 100% mixing? With a daily input is there any potential for accumulation?
  • Water quality and habitats in the wetland?
  • Will it increase the risk of algal blooms? (if so could this impact biodiversity and cause smell problems when it rots down?)
  • Will compounds build up in the sediment and be mobilised when they are disturbed?

If the daily discharge into the reservoir is increased to 15 million litres a day as is already planned, or 60 million litres a day as suggested could happen, how does that impact on water quality, the risk of algal blooms and the biodiversity potential of the reservoir? What will the impact of more regular drawdown events be?

Note: 7.5Ml/d is equivalent to three Olympic sized swimming pools, 15Ml/d is six and 60 Ml/d is 24. An Olympic sized pool is 50m x25m x 2m, with a volume of 2500m3.(ML/d means million litres per day.)

Unfortunately, we don’t know what the impact will be because Southern Water has not yet modelled the effects for 5, 15 or 60Ml/day. Despite this Southern Water are pressing ahead with the scheme.

If they are discharging to the reservoir everyday and keeping the reservoir topped up there will no longer be seasonal fluctuations in water levels to expose mud in the wetland, this will have an adverse impact on the potential for nesting and feeding areas for birds. This would adversely affect the promised biodiversity benefit of the reservoir.

The reservoir presented a real opportunity to create an unique chalk spring-water fed reservoir, which had the potential to be very special for wildlife. The proposal to discharge treated recycled effluent changes that. Will the biodiversity benefit promised at the reservoir site still be achieved?

The reservoir is being used by Southern Water as an environmental buffer lake to protect drinking water supplies if things go wrong, which provides an increased environmental risk to the reservoir. In other countries additional catchment controls are put in place to stop hazardous chemicals being discharged into the sewers and to monitor the sewers. What additional controls on commercial discharges and monitoring will Southern Water put in place? Would you trust Southern Water to ensure that there are adequate controls in place?

The Southern Water proposal is in two parts, the effluent recycling plans and a proposal to build a new 40km pipeline to Otterbourne, near Winchester. This will enable Southern Water to abstract more water from the reservoir more often, giving rise to more regular drawdown events. How often will drawdown events take place and what impact will this have on wildlife at the site?

Note: Southern Water are likely to pursue the 40km pipeline to Otterbourne even if the effluent recycling proposal was to be stopped.

Impacts on Langstone Harbour

In addition to there being a small spillway there will be a compensation flow discharged from the reservoir to the Riders Lane Stream, to the south of the reservoir site. This will flow on into the Hermitage Stream and from there into Langstone, which is designated as a Special Area of Conservation and Special Protection Area for birds. The reservoir was to have provided a benefit to Langstone Harbour by reducing the amount of nitrate discharged into the harbour. This benefit was a key driver for receiving regulatory support for the reservoir, as nitrate concentrations in the harbour are of significant concern.

Will the benefit of nitrate reductions still be delivered if Southern Water discharge recycled effluent into the reservoir?

Unfortunately we don’t know because Southern Water has not yet modelled the impacts. Despite this Southern Water are pressing ahead with the scheme.

What other downstream impacts might there be? For example, changes in temperature and metal content of the discharge out from the reservoir.

Will flows associated with emergency drawdown events/testing have any additional impacts?

Will the development of the dilute and disperse mixed waste landfill at Broadmarsh for the Water Recycling Plant present any increased risks to Langstone Harbour?

Will the WRP or tunnel and pipeline routes be vulnerable to sea level rise?

Impact on drinking water

The recycled water will be supplied to Portsmouth Water and Southern Water customers. While the water coming out of your tap your tap will be treated to meet the UK Drinking Water Standards and by law must be made safe to drink, it will taste different.

If the water you receive out of your tap tastes different will you think about where it has come from? Will you be comfortable drinking the ‘recycled’ water? Might this cause you to reject the water and buy bottled water instead?

The new treatment plant will be very expensive to operate, requiring a lot of energy, chemicals and regular replacement of the membranes used to filter the water.  Do you trust Southern Water to run and maintain the water treatment plant?

Other environmental impacts

The water industry have committed to net zero operational carbon by 2030. Why are Southern Water promoting a Reverse Osmosis treatment process which will have an enormous energy and carbon footprint at a time when they should be looking for more environmentally friendly solutions that work with climate change, not against it?

Climate change is predicted to give us wetter winters with more extreme flood events, why are they not looking for solutions that capture and store more winter water which would provide a double benefit?

Why is water going to be transferred 40km daily from the reservoir to Otterbourne? Surely there are solutions that can be adopted closer to where the water is needed? (especially when the 15Ml/d is only needed during a 1 in 200 year drought event.)

Impacts on leisure benefits

The spring-fed reservoir was to provide an enhanced green space for local people to enjoy with their families and get the health benefits from walking and cycling around the reservoir.

If the changes to the water source increase the risk of algal blooms that could make the reservoir a less attractive green space. At other reservoirs when algal mats rot down they produce pungent odours which detract from the leisure experience.

The Southern Water proposal is in two parts, the effluent recycling plans and a proposal to build a new 40km pipeline to Otterbourne, near Winchester. This will enable Southern Water to abstract more water from the reservoir more often, giving rise to more regular drawdown events. During drawdown large areas of mud will be exposed for long periods making the reservoir a less attractive place to walk round and explore with your family.

Even if plans for effluent recycling are rejected it is likely that Southern Water will still press on with their plans for a 40km pipeline. Pumping water 40km will take a lot of energy and carbon, at a time when energy prices are rising rapidly. It flies in the face of the stated policy from the water industry to achieve net zero operational carbon by 2030.

More regular drawdown events will expose more mud, more often. Does this give rise to increased health and safety concerns on this publicly accessible site, or concerns of smell issues from the mud?

[Content shared to Havant Matters by Havant Green Party.
Please refer queries by email to HGP]