3 Conjunctive use
Conjunctive use is the combined use of surface water resources and groundwater, in a unified way, to optimise resource use and minimise the adverse effects of using a single source. It exploits the storage capacity of an aquifer and the ease of transport of water by a river. The aquifer is used to store surface water when there is an excess of it and it would otherwise be wasted, such as in winter. The river is used to transport water from the aquifer to where it is needed when the river discharge is too low on its own, as often happens in summer. Conjunctive use can also reduce abstraction from rivers when the discharge is low by using groundwater instead.
The storage of excess surface water underground in an aquifer is a type of conjunctive use called managed aquifer recharge. This makes the most of excess water by directing it into the ground where it can be stored for future use. Underground storage has many advantages over surface storage: no land is taken up by reservoirs, there is no evaporation loss, and capital costs are much lower. However, managed aquifer recharge is not a simple process, and it is difficult to do on a useful scale; it cannot absorb large volumes of flood water in a short time. It involves transferring water from the surface to underground, either by dispersing it over the surface to increase infiltration, or through aquifer injection wells.
Surface dispersal involves diverting the water into an unlined canal or shallow lagoon in permeable sediments or rock so that the water can percolate downwards into the aquifer (Figure 4). It works best in areas with highly permeable soils and unconfined aquifers, and where land is inexpensive.
Aquifer injection wells are used to recharge aquifers directly. Direct injection of water through wells is more expensive than basin recharge but is used when there is no suitable land for a recharge basin, or with confined aquifers. Aquifer storage and recovery (ASR) schemes (Box 3) 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.
Box 3 ASR in the Thames Valley
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 a day during drought conditions.
Storm runoff, which would otherwise be lost to the sea, can be used for managed aquifer recharge, especially in arid areas. In the Central Valley of California, storm runoff is trapped in alluvial sediments. On Long Island, New York, aquifers are recharged through sands and gravels, which also help to prevent flooding. In some areas it is possible to use sewage effluent for managed aquifer recharge, as the polluting substances in the water are removed by biological processes during infiltration. However, it is very easy to pollute an aquifer, especially if the effluent contains industrial waste. Another problem of artificial recharge is that fine sediment in the water can quickly clog the pores in an aquifer, reducing the natural rate of recharge from lakes, lagoons and wells.
Another type of conjunctive use is the use of groundwater to increase the flow of a river, called river augmentation (Box 4). 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.
Box 4 The Shropshire Groundwater Scheme
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 (Figure 5). 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. At the moment (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. For example, the abstraction of groundwater can reduce the baseflow contribution to rivers by lowering the water table. If carefully planned, however, the conjunctive use of rivers and groundwater can even out the seasonal variations in river flow. 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 (Figure 6, well A) for the drawdown 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.