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What Is Managed Aquifer Recharge?
Water supply resilience in the face of drought and climate change is a key concern for the UK, as it is indeed for the rest of the world.
It can be easy to fall into the trap of thinking that the UK won’t face water stress and scarcity issues, thanks to its famously damp climate, but experts have predicted that changes in weather patterns, rising temperatures, greater variability in rainfall, water mismanagement, urbanisation and so on will reduce water availability around the country.
The situation is also being exacerbated by population growth, which will drive water demand up at a time when supplies are already dwindling… and, unless action is taken now, water shortages are an absolute certainty.
In fact, these are already being seen this week (June 12th) in places such as Kent and Sussex, with temperatures reaching 26 degrees C and above.
Furthermore, a new report from economics consultancy Cebr and Kingfisher (owner of B&Q and Screwfix) has just revealed that seven regions in England are now in line to face severe water stress issues by 2030, so finding ways to improve water resilience in the natural environment is becoming more and more urgent as time goes on.
Groundwater resources
When we think of water, surface water is usually what first comes to mind, any freshwater that we see in the likes of lakes, streams, rivers, wetlands and so on. But a very important source of freshwater is groundwater, that which exists in aquifers deep below ground and which we can tap into in order to supplement our drinking water supplies.
Different parts of the country depend on different water sources, which means that water management will vary from region to region. For example, in the north of England, water is typically taken from surface water sources and it can be easily stored in reservoirs so that water can be supplied throughout the year.
But in the east of England, water supplies are more heavily dependent on underground aquifers, specifically chalk aquifers in this case. As well as supplying a large proportion of public water supply in the region, groundwater also feeds rivers and wetlands, while contributing to river flow… so it’s an incredibly important water source that shouldn’t be overlooked.
However, increasing amounts of pressure is now being put on groundwater resources around the country, with demand outstripping the rate of replenishment. Sufficient rainfall throughout the year is required to replenish both groundwater and surface water sources so as to ensure that abstraction continues to be sustainable.
This is now being threatened by climate change, which is having an impact on the UK’s weather patterns and we can expect to see hotter, drier summers and warmer, wetter winters in the future, as well as more frequent and more extreme drought conditions and flooding events… all of which will put even more pressure on surface and groundwater resources alike.
How can managed aquifer recharge restore the balance?
When it comes to groundwater management, one of the key practices is managed aquifer recharge (MAR). This is where water is purposefully recharged in these underground reservoirs so as to help support the natural environment or to be abstracted at some point in the future.
This way of working helps to balance out any seasonal decreases that may be seen in water availability with rising demand. There are various ways in which MAR can be delivered, everything from infiltration pods to seasonal flooding of permanent crops, rain gardens, rainwater harvesting and recharge, different landscaping techniques, recovery wells and so on.
The 2022 UN World Water Development report emphasised just how important MAR actually is, describing it as an “integrated approach that allows replenishment of aquifers to complement storage dams [that] provides a cost-effective alternative that minimises evaporation and environmental impacts.
“MAR can also be used to retain unharvested urban stormwater and recycled water to be made available for productive use when needed”.
It’s expected that MAR will come increasingly to the fore as growing populations around the world increase demand for water, forming part of a successful strategy to adapt to growing water supply variability as a result of climate change.
Currently, MAR is typically used to help restore the balance between supply and demand, whether that’s by recharging aquifers during wet seasons and aiding recovery during drier times of the year, helping support emergency water use (such as for fighting fires) or as a drought mitigation strategy.
MAR can also be used to help prevent further deterioration of groundwater levels, preventing land subsidence or controlling saltwater intrusion, as well as maintaining minimum stream and river flow.
Other potential uses for MAR include water quality enhancement and protection, water reuse, flood mitigation and ecosystem restoration and protection.
The different types of MAR
There are lots of ways in which aquifers can be recharged, ranging from recharge wells and water spreading to streambed channel modifications and bank filtration. Small-scale recharge can also be used, such as rainwater harvesting, featuring different infiltration methods or wells.
Streambed channel modifications
These systems include the likes of recharge weirs, where dams are built in ephemeral streams to detain water, which can then filter down through the bed to enhance aquifer storage to be extracted downstream.
Other options include underground dams (where a trench is built across the streambed and backfilled with low permeability material to help store flood flows), sand dams (where sediment is trapped when flow occurs and an aquifer is created after successive floods to be used in dry seasons), and recharge releases (where dams are used on ephemeral streams to detail flood water and release it slowly downstream).
Water spreading
These systems include the likes of infiltration ponds (where surface water is diverted into basins and channels so it soaks through the underlying unconfined aquifer), soil aquifer treatment (where treated sewage effluent is infiltrated through infiltration ponds) and dune filtration (where water from ponds is infiltrated at lower elevation to improve water quality).
Bank filtration
This involves the extraction of groundwater from wells near or under rivers and lakes to induce infiltration to improve water quality and make it more consistently good.
Recharge wells
Wells can be recharged in different ways, such as aquifer storage and recovery, where water is injected for storage and recovery at a later date. This can be of benefit in brackish aquifers where storage is the main concern.
Another strategy is aquifer storage, transfer and recovery, where water is infected into a well for storage, with recovery from a different well. This can help extend aquifer residence time beyond that of a single well.
And dry wells can also be used, which are shallow wells located in areas with very deep water tables, which allows very high quality water to infiltrate the unconfined aquifer at depth.
What are the pros and cons of MAR?
There are various long-term positive impacts that MAR systems can have on groundwater resources, as well as delivering a range of other socioeconomic and environmental benefits.
For example, when compared to dams, MAR boasts lower costs, prevention of algae buildup and mosquitos, reduced evaporation loss and ease of location in proximity to places with high water demands. Furthermore, these systems are scalable, which means they can be implemented in stages.
However, recharge rate in MAR systems can be constrained by porous material permeability and other losses aside from evaporation may take place in the aquifer, whereas reservoirs are able to store large volumes of water until they fill up. MAR-sourced water also needs a level of treatment in line with its intended use.
The UN report made the observation that when planning or MAR systems, costs, environmental impacts and energy requirements should be taken into account and compared against alternative supply systems.
It’s likely that MAR will be found to be the most economic, while reducing energy demand in comparison to other options. And in the instance that these systems do use more energy than other water supplies, emissions targets may be achieved through the provision of renewable energy sources from other sources of economic savings.
Do you want to find out more about water efficiency? Get in touch with the H2o Building Services team today to see how we can help.