Storing water for when it’s needed through Managed Aquifer Recharge

In many parts of the world, people are encountering “extremer weather.” This includes bigger and more frequent storms, rain at unexpected times of year, and longer droughts. Heavier and more intense precipitation can mean that excess water just flows away, and due to longer dry times, there may not be enough water stored for agriculture, industry and humans overall.  

Throughout history, people have built dams to hold reservoirs that balance supply and demand for water. But dams come at a cost – it can be a huge financial commitment, precious land is covered with water, there’s the risk of a dam bursting or being overtopped, and much of the stored water may be lost through evaporation. In addition, many reservoirs around the world have water quality issues, including sedimentation filling, reducing their storage capacity. 

A World Bank report says that over the last 50 years, the global population has doubled, generating a constantly increasing demand for water and commensurate water storage. Yet the supply of natural water storage has decreased, declining by around 27,000 billion m³ due to melting glaciers and snowpack, and the destruction of wetlands and floodplains. 

So how can we better store water to make our communities more climate resilient? 

Depositing water into Mother Nature’s bank 

Part of the solution lies in Managed Aquifer Recharge (MAR). An aquifer is an underground layer of water-bearing material, consisting of permeable fractured rock or unconsolidated materials. Some aquifers are huge, covering thousands of square kilometres, while others are just small local formations. Aquifers are commonly used throughout the world to abstract water for irrigation, drinking water supply and industrial use. Whilst aquifers will often naturally be recharged by infiltration from rainfall or surface water, over-abstraction can lead to declining groundwater levels and reduced aquifer yield. 

MAR involves artificially recharging the aquifers at times of sufficient surface water flows, and using this stored water during drier periods. As such, MAR entails using aquifers as Mother Nature’s bank, making deposits of water at times of surplus, so that withdrawals can be made in times of shortage.  

In MAR, surplus water can be taken from rivers in their high-flow season, and diverted to where it can be released into the aquifer. Shallow aquifer can often be recharged by infiltration basins, which are ponds from which the water flows down into the aquifer. For deeper and confined aquifers, the water is delivered into the target aquifer by injection wells. When the water is needed, it can be pumped back to the surface from abstraction wells. 

MAR is not a new idea – one of the early applications of MAR was in the Netherlands in the 1960s, mostly to control saltwater intrusion into aquifers in the dunes near Amsterdam which are used to abstract water for municipal water supply. Another longstanding application is in southern California, where the Orange County Water District uses MAR as a key part of supplying the heavily-populated area with water. The District says it has purchased nearly 1,600 acres of land to maximize the capture and recharge of available water. More than two dozen infiltration basins slowly put water back into the underlying aquifer while naturally filtering and purifying it. 

MAR shows great promise for supplying water to the world’s population, industry and agriculture. Some points to consider:  

It can’t create water: Just like money must be deposited before it can be withdrawn from a bank, MAR only works if there are times when surplus water is available to be added to the aquifer. And, responsible management must be practiced so as to not over-draw water from the aquifer. 
 
It’s part of the solution, not the whole solution: MAR is often best combined with other sources to provide enough water to meet the needs. For example, as well as its MAR sources, Orange County relies on the Santa Ana River, stormwater, natural recharge such as rainfall, and imported water to supply communities and irrigators. 

Good understanding of aquifer characteristics needed: Each aquifer is unique. Some are unsuitable for MAR for various reasons. They may not provide sufficient capacity to store the required volume of water, there could be unwanted effects, and some aquifer don’t retain water very well – it flows right through them. Fortunately, in many parts of the world there’s already good knowledge of the local hydrogeological conditions, that can be applied to finding aquifers that will work for MAR. Airborne electromagnetic surveys, can further improve knowledge of underground conditions in combination with ‘traditional’ investigation technologies such as drilling and aquifer testing to assess the productivity of the aquifers.  

Imaginative solutions may be needed: WSP recently worked with a New Zealand city where MAR was previously thought inapplicable, because an aquifer in the area was too “transmissive” – it dissipated water too readily. WSP demonstrated that water pumped into a lower aquifer of lesser permeability, would result in “mounding” – pushing water upwards so it would keep the too-transmissive aquifer full, making MAR practical in that application. 

Maintenance matters: One of the challenges of MAR schemes has to do with clogging of infiltration infrastructure such as basins, injection wells or the pipeline network, with fine sediments. Sometimes, it’s necessary to temporarily take off-line some infiltration ponds so that they can be cleaned of silt and organic matter. Investing in maintenance is important to MAR success. For example, several MAR schemes in Southern Australia failed because of inadequate consideration of technical challenges, insufficient technical and management skills, and lack of feasibility because of high operational costs.  

MAR solutions can grow: Some water-storage schemes, such as dams, require big investments all at once. MAR solutions can be scalable. It’s possible to start with one well or just a few infiltration basins, learn from those, and then expand when appropriate and needed. 

You need a good business case: As in the Australian example, MAR schemes can fail. It’s important to build a good business case before starting, or even detailed planning.  

Having the right expertise can help – starting with finding out if MAR would work for you, given your local climate and other factors. WSP offers the necessary expertise worldwide in fields such as meteorology, hydrology, geology and hydrogeology, so please reach out to us for support through the evaluation stage and then implementation or MAR as part of your water solution.