The simple idea that could save our groundwater.

This is a bank. You can't deposit a check, it doesn't have cash or gold, but it is filled with something arguably even more valuable. Water. And banking water is more important than ever. Around the world we're facing crop-destroying droughts and two billion people lack safely managed drinking water. So how does it work? And can you get one where you live? The water bank we're visiting is a pilot project in Berlin. It takes nearby river water and filters it into the city's groundwater supply. Alex, who heads research and development for Berlin's water services, was kind enough to show us around. Yeah, I mean, it's pretty unassuming. You could have basically walked right past it and not noticed it. What exactly is this? This is an infiltration trench. If you help me lift it. Cracking open the top reveals a steady, hypnotic trickle of water into a bed of gravel. You can have a look here. You have this trench, which is filled with a very porous material, and the water can easily seep into the underground. Groundwater supplies around half of the world's drinking water and 40% of water used for irrigation. It's stored in underground aquifers like this, layers of permeable rock or sediment that hold groundwater in pores and cracks. The water can be pumped above ground via wells. Groundwater makes up 30% of all freshwater, around 30 times more than all freshwater above ground. In many regions, that groundwater is at risk of running out, because people drink it, use it for industry, and especially for agriculture, which accounts for 70% of groundwater usage. The effects of climate change make all this worse. Drought and lack of rainfall can make surface water like reservoirs, lakes, and rivers run dry, forcing us to dip into the groundwater piggy bank more frequently. And that's why, back in Berlin, they're trying to fill that piggy bank. Once the water enters the trench, it does the rest of the work on its own, trickling down 7 meters until it's reached the groundwater. Then it naturally flows underground towards these pumps. These small kind of pipes you see here are monitoring wells which go into the groundwater and we have sensors in them. And we have a small pump in there, so we can just go there, press a button and we can take a water sample and get that into our laboratory. Enough water to fill about 65 bathtubs go through the filter every hour. The water takes a day to reach the first pump and a week to get to the end. In theory, the longer the water flows, the more time it has to be filtered and the cleaner it should be. They can constantly monitor how clean the banked water is and how its properties shift as it flows underground. We as a water producer have to make sure that the people can drink this water every single day until the end of their lives without any problems. But it is completely safe to drink. Thank you. Okay. I'm not going to drink this for the rest of my life every single day. This is one glass. Should be no problem. It's no problem. Cheers. It tastes like water. Delicious. Berlin has already invested in a number of major water banking projects, but drier summers are putting stress on the city's water supply. It's looking for new solutions. If we have now climate change and we need more water, we probably need more infiltration systems, and we want to know are other infiltration systems working and need less space so we can go to newer places where we haven't been infiltrating in the past. And saving every drop only becomes more important as human-driven climate change intensifies, but more on that later. Most of Berlin's drinking water is taken and treated from the region's rivers, but a good 10% of it is from water banking, also known as managed aquifer recharge, or MAR. Recharge is when surface or rainwater seeps into aquifers and turns into groundwater. It's what nature has always done, but techniques to spur it artificially, like Berlin's man-made trench, are an increasingly important way of building a buffer in wet seasons to secure water during dry summer months. We see more extreme weather phenomena. So we get longer dry periods and more extreme rainfalls. We just put more stress into our groundwater resources and storage the water and put it in a controlled way into the ground can be one part of the solution, I think. Around 70% of the world's major aquifers show sign of losing water in the long term. Pollution can threaten the remaining groundwater's quality, while desertification can lead to lots of salt entering groundwater. Some regions, like South Asia, are harder hit than others. By 2050, more than 1.5 billion people there are projected to face water scarcity. What climate change is doing is intensifying it. We have more short monsoon season, high extreme rainfall events happening. Within the monsoon, we have dry periods happening also, isn't it? And then we have this long dry season, lasting eight to nine months, where to have water storage becomes very essential. Dr. Faiz Muhammad researches groundwater usage in South Asia. Then you superimpose El Nino, which is currently happening. And it's already you see the effect. So for example, in India right now, the monsoon is 10% deficit from the long term average. Drought, erratic weather, and 2026's Super El Nino all mean the water supply is likely to be hit particularly hard in the near future, and it's only going to get worse. Berlin's test trench is a new approach in the German capital, but the city also has a number of a lot more rustic-looking filtration basins like these. Anna Koenig heads up Berlin's water management. These basins have been in operation since the 70s. They take up a lot more space than the pilot, but they're a tried and true method and easy to maintain by swapping out the top layer of sand at the bottom every year. Water is pumped in from the nearby lake, seeps through the sand at the bottom of the basin, and after a couple of months of filtering through aquifers, it's pumped back up into the nearby waterworks and enters Berlin's drinking water supply. Eight million cubic meters are filtered into these three basins annually, enough to fill more than 3,000 Olympic-sized pools. We don't take as much in the winter, but the same amount of water is filtered. Of course, the groundwater ladder recovers over the winter months. And then you have a kind of buffer. All this keeps Berlin's water flowing, no matter how dry the summers get. There are MAR projects all over the world, and since groundwater is used for agriculture, industry, and drinking, there are plenty of opportunities for expansion. Investment in the field has brought about new technologies that allow for better monitoring and filtering. Underground scanning also now gives us a better idea of how groundwater behaves, like this map showing major aquifers. In California, with both a massive population and a huge agricultural industry, recharge projects like this one in Orange County are key. It uses sophisticated microfilters and UV lights to clean pre-treated wastewater before recharging 100 million gallons into Southern California aquifers every single day. Water-starved Namibia's capital, Windhoek, uses a large aquifer to store water underground for times of shortage. It gets one-third of its drinking water from carefully treated wastewater. And India has a long history of managing its groundwater, like these traditional step wells or johad ponds used for storing and re-infiltrating drinking water. But as drilling wells became cheaper, more people started to tap into the aquifers right under their feet. So it's an unregulated sector. Everyone who owns the land owns the water beneath it and has the right to basically tap it. And that's why we have these 30 million wells tapping these aquifer systems across South Asia and this widespread depletion happening. Unlike some of the major MAR projects in Europe or the U.S., South Asia is taking a similarly decentralized approach to water banking. With lots of smaller initiatives like these recharge ponds in Uttar Pradesh or check dams, small barriers that slow water flow and encourage recharge, like these in Kerala or this in Tamil Nadu. These are not large schemes of MAR happening, one here, one there, one utility. What the case of MAR is millions of small, small MARs that are just happening across the South Asian landscape. Meanwhile, the pattern of intense floods followed by drought means banking would potentially help to reduce flood risk while better storing water for lengthy dry periods. I wouldn't think of managed aquifer recharge as a replacement for other approaches. It's more like one tool within a large water management. Alberto Casillas is a hydrogeologist at UCLA. We feel these surface reservoirs, conservation practices, water reuse, improvements in irrigation efficiency. And to be most effective, groundwater recharge should go together with other approaches to water saving, like rainwater harvesting, which can provide more water for the bank. But that's, of course, still limited by the amount of rain available in the first place. Because there are so many potential uses for MAR, you have to be very careful that you find a project tailored to where your water source is, its quality, and how you'll recharge it. Not every place has, you know, a surplus of water. Then you need the infrastructure to move that water into the right place. You need soils and geology that allow water to infiltrate at adequate rates. You also have to understand water quality because recharge can mobilize naturally to occurring contaminants or transport contaminants that are already present in the subsurface. And because everything around water treatment, pumping, and filtration is often managed by different institutions, there are bureaucratic hurdles to effective water banking. Or in some cases, a laissez-faire approach can bring its own challenges, like assessing the countless mini-MAR projects throughout South Asia, or maintaining the projects as they're established. And though using MAR to fight flooding has a lot of potential, it's also particularly tricky. Floods don't always occur in predictable locations, and sometimes flooding includes landslides and sediment that can clog up filtration systems. And of course, there's a challenge to banking we're all probably familiar with. If you keep spending like crazy, your bank can't magically make you rich. Berlin is already planning on expanding its MAR capacities. We want to go further up. So the new waterworks we're going to build will have an intensive groundwater enrichment. Anna expects MAR to climb from 10% to about 14% of Berlin's drinking water once the latest waterworks is completed. And it's only set to grow in other regions, too. MAR wants to eliminate droughts, but it can help us better take advantage of the ones that we have in the wet periods. So it prepares better when the dry period arrives. It may not be perfect for everywhere, and it can only be part of a wider water scarcity plan, but water banking will help soften the blow of droughts and hold onto every dear drop we've got. What's your local town doing to maintain its water supply? Thanks a lot for watching, and don't forget to subscribe.