Subsurface physical barriers : what if we stored water underground?

What is a subsurface physical barrier?

A subsurface physical barrier is a civil engineering structure built underground to retain water in aquifers. It allows water to be stored in its natural environment, protected from evaporation and without the risk of micro-organism proliferation, while at the same time allowing it to be replenished so that it can be made available at a later date, particularly during periods of drought. The water can then be collected by drilling wells and distributed to users. An SPD can also be used to protect a coastal aquifer from seawater intrusion or to keep a wetland covered in water. The effectiveness of subsurface physical barriers can be enhanced by combining them with controlled aquifer recharge techniques, which encourage the infiltration of good quality surface water.

Where and since when have SPBs been built? 

SPBs have been used for decades in regions where there are strong rainfall contrasts (Maghreb, Turkey, Middle East, Brazil, Africa, Japan, China, etc.). In France, there is only one such structure, built in Massillargues-Attuech in the Gard region in 1993, to raise the water level in the aquifer.

Construction methods depend on the length and depth of the SPB, the nature of the geological formations, the techniques available and the regulations in force. Most SPBs stretch over a few hundred metres and penetrate to less than 30 metres, although some are much longer, up to 14 kilometres in China, and deeper, up to 70 metres in Japan!

Why this interest in SPBs today? 

Climate change will reduce available resources and increase our need for water. To meet this challenge, we need to develop more ways of reducing consumption, but also of deploying new solutions, including SPBs. BRGM studies these techniques with the aim of developing and perfecting them and defining the conditions for their use from hydrogeological, social, economic and environmental perspectives.

Can an SPB have a negative impact? 

Yes, like all hydraulic infrastructures. However, it is possible to limit the potential impacts – which vary according to the type of SPB, its purpose and the context in which it is installed – through appropriate design. For example: prevent the water table from rising too close to the surface upstream of the SPB; renew the retained water sufficiently to prevent the accumulation of pollutants upstream; take account of the needs of the environment and those of downstream users, etc.

What do the BRGM studies on SPBs contribute? 

The inventory of SPBs around the world provides valuable references in terms of objectives, construction techniques, effectiveness and environmental impact. BRGM also carries out numerous virtual experiments, using digital modelling, to test different SPB configurations in various hydrogeological contexts, while also exploring the possibilities of reversing and draining the structures. These studies not only raise awareness of the potential of this technique for storing water in aquifers and protecting them from saline intrusion, but also help to develop public expertise on SPBs, which can be used to assess future projects.