water is the import in the live
Effective case study.
Aquifer storage and recovery in the Thames Valley, UK.
Aquifer storage and recovery (ASR) schemes use the same borehole to inject and recover water. While most ASR systems are designed to store water during the wet season and recover it during the following dry season, some are established for water banking, where recovery may not take place for many years.
ASR is used in the Thames Valley area, north of London. Here water is often in short supply in summer, and it would be useful to be able to use more groundwater from the underlying Chalk and Basal Sands aquifer.
In the 1970s an artificial recharge scheme using injection wells was started in the Lee Valley area to the north of London, where the aquifer is intensively exploited. The water used for recharge is from the Rivers Thames and Lee at times of excess flow in winter.
It is treated to drinking water standards before recharge (when spare treatment capacity is available) so that there is no danger of polluting good-quality groundwater. The Scheme is designed to recharge the aquifer artificially over an area of 50 km2 and provide an extra resource of 105 m3 of water a day during drought conditions.
The Shropshire Groundwater Scheme, UK.
Another type of conjunctive use is the use of groundwater to increase the flow of a river, called river augmentation. Its advantage is that a river can be used to convey groundwater to its destination without the need to build a pipeline. The effect is similar to river regulation, except that the water is stored underground instead of in surface reservoirs.
A disadvantage is that the high-quality groundwater is mixed with poorer-quality river water and will require more extensive treatment before it can be used than would have been required had it travelled through a pipeline.
Triassic sandstones are the major aquifer in the English Midlands, and are exploited intensively for water supply. The only area with substantial unused reserves in the aquifer is in north Shropshire, in the Severn Basin.
The River Severn is the main component of the water resources strategy in the West Midlands. The river is regulated by water from the Clywedog and Vyrnwy reservoirs in Wales in the summer months, but the regulated flow is insufficient both in dry summers and to supply future demands.
A phased scheme of river augmentation to the River Severn from groundwater in the Triassic sandstones in north Shropshire was started in the 1970s. Since 2004 the scheme can supply up to 105 m3 a day during the summer months when necessary. The net gain is at least 65%.
Further phases of the scheme are due to be developed at a pace consistent with demand and if all phases are implemented it could supply 3.3 × 105 m3 a day. Not only is this scheme cheaper than building a new reservoir, but it is cheap to operate, is less environmentally destructive, and can be implemented in stages, depending on demand.
Groundwater and surface water are closely linked: groundwater maintains the baseflow of rivers, and water in rivers can infiltrate into the ground. The abstraction of surface water and groundwater cannot be planned in isolation - one will affect the other.
In the summer when the river flow is low, water is pumped from the aquifer into the river, so that more water can be drawn from the river downstream. The wells must be far enough from the river (Diagram, well A) for the draw-down around them not to make the water table slope away from the river, or water will flow back towards the well from the river (well B).
Pumping from wells also intercepts some of the natural baseflow to the river. The amount by which natural river flow is augmented by pumping is referred to as the net gain, usually expressed as a percentage of the pumped quantity.
The net gain is never 100% as some of the additional water in the river always infiltrates back into the aquifer. River augmentation schemes normally show a net gain to the river of between 40% and 70% of the water put in from the aquifer.
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