Water & Wastewater Treatment

Project focus: water resource management 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 • Perspectives Ed Uden, Groundwater Team Leader at the Environment Agency: "This scheme is good news for the River Darent, a chalk stream flowing through Horton Kirby and other villages in Kent. Instead of water being taken from the environment in the summer, which can cause the river flow to be reduced, the ASR scheme will allow up to 5 million litres per day to be taken from the water stored underground and put into drinking water supply, protecting the river." Alan Williams, chairman of the Darent River Preservation Society: "We have monitored this project since its inception at the Horton Kirby site. Our site visit in December 2014 to the ASR 3 site demonstrated to us the progress that has been made. It certainly shows how clean water can be stored in times of plenty. DRiPS will continue to support the use of ASR for it is good for the river, the valley and the consumers." The team of hydrogeologists inspect rock samples taken from the Lower Greensand aquifer www.wwtonline.co.uk | WWT | APRIL 2015 | 21

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