Houston, Texas, has flooded every year for the past five years. At the same time, Texas is also known for dire water shortages. What if people were to capture the floodwater and store it for later in aquifers — underground layers of permeable rock, gravel and sand that allow water to pass through?

A recent study by researchers from the University of Texas at Austin found that coastal aquifers from which water has been pumped for use in farms and cities, have enough space to store two-thirds of the water from high-flow events from 10 Texas rivers, reducing the impacts of both floods and droughts — if we figure out a way to get the excess water into them.

Actively moving water underground, a practice known as managed aquifer recharge (MAR), is increasingly popular today. There are about 1,200 managed aquifer recharge projects in 62 countries, according to the International Groundwater Resources Assessment Centre (IGRAC) based in Delft, the Netherlands. In addition to helping manage water over- and under-supplies, MAR can be used to restore depleted aquifers, rehabilitate ecosystems and cleanse polluted water. But there are challenges as well.

Water in the Bank

Storing water underground for future use is increasingly popular due to growing volatility in supply because of climate change as well as to the downsides of the alternative: damming rivers to create surface reservoirs.

For one thing, in developed countries, many rivers are already dammed. For another, dams cause myriad environmental problems, such as preventing sediment from replenishing coasts, blocking fish migrations, and destroying river habitat by slowing down water and allowing it to warm. Reservoirs lose up to a quarter of stored water to evaporation, and sometimes have to release water to make room for big storms. And surface storage can lead to overuse of water because the sight of it gives people a false sense of water security. It also costs around double the price of groundwater recharge.

Saving heavy flows underground for higher demand times has been the practice for the barrier island of Wildwood, New Jersey, since the 1960s, says Steven Phillips, a hydrologist and groundwater specialist with the U.S. Geological Survey in Sacramento. A popular resort area, Wildwood hosts numerous visitors in summer. Water managers store excess winter water underground and then pump it out for use during the high season.

California is looking to scale up this strategy. The snowpack that historically has supplied water into the dry spring and summer is predicted to largely disappear with the climate crisis. And its winter storms are predicted to grow more intense. Water managers and scientists, led by the California Department of Water Resources, are looking for the best places to move water from winter storms underground for use during the dry summers.

Other Benefits

Water’s natural tendency, in many places in many seasons, is to linger on the land. When surface waters slow down, some can seep underground and recharge aquifers.

But in many places we have blocked the opportunity for natural aquifer replenishment. We have cut off rivers from floodplains with concrete channels and levees; drained wetlands and paved over them with sprawling, impervious cities; and eliminated beavers. As a result, stormwater ends up running off into surface waters and away from the region instead of soaking into aquifers.

In fact, a 2015 study of NASA satellite data found that more than half of the world’s major aquifers were overdrawn or overstressed.Combined with the fact that in many places groundwater is being drawn up for surface uses, this situation has produced a net loss of underground water. In fact, a 2015 study of NASA satellite data found that more than half of the world’s major aquifers were overdrawn or overstressed.

This not only threatens water security, it can also cause the land above to sink, as it has in California’s San Joaquin Valley, Beijing and Mexico City. And pumping from an aquifer near the coast can decrease water pressure to the point where saltwater can push in underground, tainting the freshwater.

MAR can help reduce such problems. San Jose, California, began using MAR many decades ago after the downtown sank around 13 feet (4 meters). And Los Angeles and Orange counties were among the first places to use MAR to push back saltwater intrusion. In the Hampton Roads area of Virginia, officials are planning a “fairly large-scale groundwater recharge to push back saltwater coming into their groundwater,” says Bill Alley, director of science and technology for the National Groundwater Association. The saltwater intrusion is partly due to rapid sea-level rise.

Because surface water and groundwater are often linked by gravity and hydraulic pressure, recharge can also bring new life to wetlands, springs, creeks, and other surface-water ecosystems that dry up, get too warm or become choked with algae after people deplete the surface water or groundwater that feeds them. In the Sacramento Valley, The Nature Conservancy cut a gap in a levee so high river flows would once again flood Cosumnas River Preserve and seep down into the aquifer, where the water could provide ongoing sustenance for the ecosystem.

Moving water into the ground can often clean it, depending on the pollutants and the composition of the substrate.But sometimes it’s too late. In some parts of the San Joaquin Valley, water tables have fallen more than 500 feet (150 meters) below ground from overpumping, says Phillips. “Water levels are deep enough that we’re not likely to see rivers benefiting from groundwater input for possibly ever.” Such aquifers could still be used for water storage — although that’s not always possible because sometimes subsidence causes permanent loss of capacity in an aquifer.

Moving water into the ground can often clean it, depending on the pollutants and the composition of the substrate. Gilbert, Arizona, discharges wastewater into recharge ponds, including a riparian preserve that offers recreation space for people and habitat for wildlife, where it percolates down into the aquifer for future use. Pond water is also used directly for irrigation and other nonpotable uses, reducing demand on drinking water.

How It’s Done

A long-used approach to moving water underground, still practiced in rural places around the world, is to harvest or funnel rainfall into a shallow basin or trench and allow it to soak into the soil.

In 2011, on a project in Tigray, Ethiopia, a mountainous, rural area, IGRAC hydrogeologist Arnaud Sterckx saw people building check dams in gullies, reforesting steep valley flanks, building terraces for agriculture, and digging ponds to capture rainwater. Such projects typically don’t appear in the MAR global inventory, he says, because these traditional methods don’t require permits, feasibility studies or environmental impact assessments.

Sterckx says the corralled water doesn’t even have to filter down to an aquifer to make this a useful practice. By lingering in the soil, it can help crops grow with reduced need for irrigation.

An urban version of this approach is cities’ increasing use of green infrastructure. Additions such as green roofs, bioswales, permeable pavement and parks along river banks absorb stormwater, reducing flooding and to retain water locally for future supply.

Water managers are also building large infiltration basins, such as Arizona’s 38-acre (15-hectare) Hieroglyphic Mountains Recharge facility, which stores Colorado River water for later use. Such basins are built above suitable geology for infiltration, typically a mix of sand, gravel and clay. But recharge basins can clog if the water has a lot of sediment and therefore need to be cleaned out periodically, warns Sterckx.

A more natural way to store water underground is to help creeks and rivers that have been engineered to stay within their banks to spread back out into their historic floodplains.

In an early example, local leaders in Los Gatos, California, built partial dams across a creek in the 1920s using burlap sacks filled with dirt to cause the water to slow down and spread out across flat areas adjacent to the creek. Today, inflatable dams are deployed in several northern and southern California counties to slow down rivers and streams during high flows to allow more infiltration in the natural channel, says Phillips.

The most industrial approach is to inject surplus water into the ground via a well or borehole. It’s more costly than passive methods because of the energy required. Nevertheless, it can be useful for moving water through a nonabsorbent clay layer into an aquifer below, or in places where there isn’t room for a spreading basin. This approach has been used in Rio Rancho, a suburb of Albuquerque, New Mexico, says Alley.

Another MAR method is called induced bank filtration. People dig a well several tens of meters away from the river so the hydraulic pressure will attract water from the river, moving it through sand and silt, which have good filtration properties, says Sterckx. Germany and the Netherlands have a lot of these projects and typically use it as a pretreatment for drinking water. Hungary gets about half of its public supply this way, says Alley.

Challenges

Despite the many benefits of MAR, there can be downsides as well. The main concern is pollution. Although putting water underground can clean it, in some cases, MAR can taint groundwater due to pollutants in the water or soil. “That’s why you need knowledge about aquifers, hydrogeology, the direction of the groundwater flow, the quality of water you’re infiltrating,” says Sterckx. “Otherwise you may have serious issues.”

Contaminants can come from urban runoff or from agriculture’s use of fertilizers and pesticides. Near Wichita, Kansas, says Alley, people have been pulling high flows from the Little Arkansas River to replenish the Equus Beds aquifers from which they draw their water. “They’ve had to deal with removing atrazine [an herbicide] from the water before they inject it.”

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Other pollutants are naturally occurring, such as arsenic, which is found in Florida’s geology. When recharging water there, it’s possible to prevent groundwater contamination, but it’s complicated, says Alley. “In the Everglades they had this massive idea of managed aquifer recharge [to resupply water that had been overdrained], and they haven’t followed through on that largely because of the chemistry problems with arsenic.”

In some cases, polluted groundwater can be diluted with recharged freshwater, cleaning it to a level where it is suitable for certain applications. In San Joaquin Valley, where some groundwater is polluted with nitrates from fertilizer, farmers measure the levels in the groundwater they’re pumping to irrigate their crops and reduce their fertilizer applications accordingly, a practice that could improve groundwater quality over time. Water can also be cleaned for drinking after being pumped from a well, although “it tends to be expensive,” says Phillips.

Recharging water for later use can also cause consternation over ownership. Getting access to water supply for recharge can be another issue. Although much water used to refill aquifers comes from wet season excesses, in some jurisdictions, it can be complicated due to legal or political considerations to get rights to that water or to ensure it’s of suitable quality, says Jim LaMoreaux, president of the U.S. branch of the International Association of Hydrogeologists.

Recharging water for later use can also cause consternation over ownership. Other people may pump out water that you inject, such as in El Paso and San Antonio, Texas, where, says Alley, it’s the “law of the biggest pump” — which essentially means that anyone can pump underground water if it lies beneath their property. That was the case until recently in California as well. But the state’s 2014 Sustainable Groundwater Management Act now requires water users in a watershed to work together to use their groundwater sustainably, a challenge that is motivating some districts to use MAR. They share the resource and are therefore accountable to each other.

Also, depending on the geology, some of the water moved underground can be “lost” to the wider environment. Accepting some loss requires a similar shift in thinking from an ownership mentality to one focused sustainability, in which recharging water supports ecosystem health, possibly requiring fewer human interventions to stave off collapse. And it’s worth stating again that above-ground reservoirs lose up to a quarter of stored water to evaporation, and sometimes have to release water to make room for big storms — so “loss” is already part of the status quo.

The world’s booming human population and our built environments are increasingly altering the natural water cycle. The impacts of those changes are exacerbated as climate chaos brings bigger floods and longer droughts. As one method to repair the water cycle, MAR will likely spread, especially as people learn from experience how to mitigate potential problems with the processes.