Thames Water Trials Aquifer Storage in Kent

Thames Water's £3.2M operational trial of aquifer storage and recovery (ASR) at Horton Kirby in Kent could help tackle future droughts by storing a bubble of clean water underground

Thames Water is progressing well with its operational trial of aquifer storage and recovery (ASR) in Kent, a project which could help ease future water shortages in South East England.

More commonly used in desert regions in the USA and Middle East, ASR involves drinking water being pumped into an aquifer where it can be stored and brought up again in times of need such as drought. Unlike other forms of aquifer recharge, the water is kept in a ‘bubble' underground, maintaining its integrity and ensuring it needs minimal treatment when it is brought up again for use in the water supply.

The £3.2M operational trial, the most advanced scheme of its type in the UK, is being carried out via a series of 250m deep boreholes into the Lower Greensand aquifer near Horton Kirby in north Kent. It builds on a pilot scheme carried out by the utility during AMP4 (2005-10) and is intended to create a substantial reserve of water which can be pumped out at a rate of 5 megalitres per day during drought conditions.

While Thames Water already recharges the Chalk Aquifer through its North London Artificial Recharge Scheme (NLARS), the Lower Greensand is more suitable than the Chalk for ASR, explains Thames Water's Groundwater Resources Manager Dr Mike Jones.

"The British Geological Survey did some work a few years ago where they ranked the Lower Greensand as the top target in the UK for ASR," Jones tells WWT. "That's simply because it's a much more homogenous aquifer, in the sense that it's layered, and not as fractured as the Chalk Aquifer is. If you try to store a bubble in a fractured chalk aquifer, it would shoot away from the borehole along the fractures and you would have no bubble integrity at all."

The Lower Greensand aquifer already contains groundwater - estimated by carbon dating techniques to be 6,000 years old - but this is too low quality to be put into use, containing quantities of iron, ammonia and arsenic. However, the pilot ASR programme has established that treated water can be pumped in (at a rate of 3 ML/day), allowed to rest and then pumped out (at 5 ML/day) without mixing with this groundwater. This process ‘conditions' or cleans the aquifer, so once it has been used for such a bubble it can then be used repeatedly in the same fashion.

Large-scale trial

But while the pilot programme only created a relatively small bubble, with around ten days worth of pumped water, the operational trial will be on a much larger scale.

"When we look at doing a trial on this operational scale then our recharge and recovery cycles are on the scale of months," says Jones. "The first time we do it we pump in for a month, and then we pump out for a month, then the second cycle will be 3 months pumping in and about 2 months pumping out. That's the sort of order of recharge and recovery cycle that's required to clean the aquifer."

The eventual aim is to store enough water through ASR to provide a 5 ML/day supply for 16 months - the length of time that it might be required during a severe and prolonged drought. However, the key task of the operational trial will be to establish how a bubble on such a huge scale will behave underground, and in particular the degree of movement that it will exhibit. The Lower Greensand at Horton Kirby has a natural flow from south to north, and Thames's team of hydrogeologists will use additional observation boreholes to monitor how the bubble of clean water is affected by this movement.

"The issue for us is that the Lower Greensand aquifer in this part of North Kent isn't that well developed - there's not many other abstractions - so when you try and model at a catchment scale to understand how your ASR operation would fit into that wider environment, you don't have enough data to develop a good quality groundwater model that you have confidence in," explains Jones. "With the operational scale trial, we will be stressing the aquifer over a larger volume than we were able to do in the pilot, with more observation points, so we'll be able to get more understanding of how our bubble behaves on a larger scale and over longer time periods."

The pilot scheme only had one observation borehole, situated about 50m from the production borehole where water was pumped in and out. The operational trial has expanded this with the addition of three more boreholes - one for production and two for monitoring - while it will also make use of another pre-existing borehole, giving a total of six points which the hydrogeologists can use to develop a picture of how the aquifer is performing.

Testing phase underway

Drilling started in September last year, and the production borehole was lined, cleaned and completed by January. Thames Water principal contractor MGJV, and specialist sub-contractor G Stow Plc, carried out this work.

Thames recently held a stakeholder engagement event in which it invited both the Environment Agency and local environmental group the Darent River Preservation Society (DRiPS) to look round the project; both have been supportive.

The testing phase will run throughout the rest of this year, and will be completed by spring or early summer 2016. If all then goes well, water stored in the aquifer could be used in supply by the end of the AMP6 in 2020.

Since Horton Kirby is also an existing extraction point for water from the Chalk aquifer, and Thames has recently reduced the amount it abstracts in the area, there is spare treatment capacity available there if needed. However, since the water stored using ASR will have already been treated before being stored, it is anticipated that when it is pumped out of the ground again it will only need disinfection.

There is 10 times more storage available underground in aquifers in the Thames Water region than there is above ground in reservoirs, and 70% of the Thames Valley's drinking water already comes from groundwater sources; so it is not hard to see why ASR has significant potential for Thames. Jones says that the utility has identified three possible similar projects in the Lower Greensand, a number which may increase as the team learns more about the aquifer. In the long term, there is also the prospect of investigating ASR in other aquifers such as the Cotswold Oolites.

Other water companies in the UK to have investigated ASR include Yorkshire Water, Wessex Water and Severn Trent, although none have yet progressed as far with it as Thames.

Source: WWT

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Taxonomy

• Aquifer
• Water Utility
• Water Management