So, How Do You Actually Refill an Aquifer?

The world is slowly but surely running out of groundwater. A resource that for centuries has seemed unending is being lapped up faster than nature can replenish it.

“Globally speaking, there’s a groundwater crisis,” said Michael Kiparsky, director of the Wheeler Water Institute at UC Berkeley’s Center for Law, Energy, and the Environment. “We have treated groundwater as a free and limitless source of water in effect, even as we have learned that it’s not that.”

Aquifers are the porous, sponge-like bodies of rock underground that store groundwater; they can be tapped by wells and discharge naturally at springs or wetlands. Especially in places that have already been hard-hit by climate change, many aquifers have become so depleted that humans need to step in; the Arabian Aquifer in Saudi Arabia and the Murzuk-Djado Basin in North Africa, per a 2015 study, are particularly stressed and have little hope of recharging. In the U.S., aquifers are depleting fast from the Pacific Northwest to the Gulf, but drought-stricken California is the poster-child of both water stress and efforts to undo the damage.

In March, the state approved plans to actively replenish its groundwater after months of being inundated by unexpected levels of rainfall. While this move is not brand-new — the state’s Water Resources Control Board has been structuring water restrictions to encourage enhanced aquifer recharge since 2015 in the brief windows when California has water to spare — the scale of this year’s effort is unprecedented.

But just how will all that flood water get back underground? California’s approach, which promotes flooding certain fields and letting the water seep down slowly through soil and rocks to the aquifers below, represents just one potential technique. There are others, from injecting water straight into wells to developing pits and basins designed specifically for infiltration. It’s a plumbing challenge on an unprecedented scale.

Why recharge?

The act of putting water back into aquifers has a number of unglamorous names — enhanced aquifer recharge, water banking, artificial groundwater recharge, and aquifer storage and recovery, among others — with some nuanced differences between them. But they all mean roughly the same thing: increasing the amount of water that infiltrates into the ground and ultimately into aquifers.

This can have the overall effect of smoothing the high peaks and deep valleys of water supply in places dealing with extreme weather fluctuations. The idea is to capture the extra water that floods during periods of intense rainfall, and bank it for use during droughts. (While aquifers can also be recharged using any old freshwater, water rights are so complicated in the West that floodwater often represents “the only surface water that’s not spoken for,” Thomas Harter, a groundwater hydrology professor at U.C. Davis, told local television outlet KCRA.)

Recharge has the potential added benefit of protecting groundwater from saltwater intrusion. As water is pumped from a coastal aquifer, water from the ocean can seep in to fill the empty space, potentially poisoning the well for future use for agriculture or drinking water. It’s a risk that will only get bigger as the climate warms and sea levels rise.

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According to the Environmental Protection Agency, aquifer recharge is most often used in places where groundwater demand is high and increasing even as supply remains limited. These tend to be places with lots of people and lots of farms; the San Joaquin Valley, which is the focus of California’s current plan, checks all of those boxes. Aquifers are the source of nearly 40% of water used by farms and cities in California, per the Public Policy Institute of California, and more in dry years. And, until 2023, most recent years have been dry.

In response to this year’s sudden reversal of California’s water fortunes, the state’s Water Board — which regulates water rights — allowed local contractors of the U.S. Bureau of Reclamation to move up to 600,000 acre-feet of water, or well over a trillion gallons, to places that normally would be off-limits this time of year. Those contractors, who are largely farmers and other major landowners, have until July 30 to take advantage.

“California is essentially the pilot project for how we want to do this in the future,” said Erik Ekdahl, deputy director for the Water Board’s water rights division. It won’t be until the end of the year that the state will know exactly how much water was successfully banked, but Ekdahl said anecdotally that some contractors have already taken steps to put the spare water underground.

This comes as California’s enormous snowpack begins to melt: a potential boon for the aquifers that could also mean problematic and dangerous floods for the communities downstream of the runoff.

Options for aquifer recharge

How does enhanced aquifer recharge actually happen? It’s not as if the vast underground stretches of rock and sediment have faucets or even obvious holes leading to their watery depths. People aiming to reverse the centuries-long trend of drawing up water without actively replacing it have a range of artificial recharge options, which either speed along the natural seepage process or direct water straight to the aquifer below.

In the former cases, one option is to allow water to flood fields left fallow, a process known as “surface spreading,” as is beginning to happen in the San Joaquin Valley.

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Water can also be directed to dedicated recharge basins and canals. In both cases, excess water is absorbed by fast-draining soil, which encourages it to pass below ground. Aside from the technical challenge of redirecting water from typical flood patterns, these approaches tend to be low-tech.

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But in cases of aquifer depletion where those approaches are impractical — such as when the aquifer is under impermeable rock — injection wells represent a direct connection to the groundwater. These are either deep pits that drain into sedimentary layers above an underground drinking water source (like a traditional well functioning in reverse), or else webs of tubes and casing that blast water straight into the source.

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Cities are also experimenting with aquifer recharge on a smaller scale. For urban stormwater, the EPA promotes certain “green infrastructure” approaches that mold the built environment to mimic natural hydrology. For instance, shallow channels lined with vegetation, known as bioswales, redirect stormwater while encouraging it to seep through the ground. Permeable pavement — in use in several Northeastern states — works much the same way. Meanwhile, rain gardens designed to prevent flooding have the added benefit of replenishing groundwater.

Determining when and where to use different approaches to aquifer recharge, though, can be unclear. We are still a long way from widespread or coordinated adoption of these techniques, but researchers are working on weighing their costs and benefits.

Supported by a $2 million EPA grant, Kiparsky is part of a U.C. Berkeley team looking at how to make California-esque recharge work on a national scale. , including by developing a cost-benefit tool for water managers. Some of the geochemical and physical considerations are relatively simple to measure: Is the soil in question porous? Are there gravel-filled “paleo valleys” that could allow water to rapidly seep to the aquifers below, as one 2022 study found?

More complicated, potentially immeasurable, but no less important are the legal and regulatory considerations around water rights. It is, as Kiparsky put it, one of the quintessential modern examples of the tragedy of the commons. Whether the government will be able to entice individuals to use their own little corner of Earth to fill an aquifer for the benefit of the many is an open question.

But Kiparsky is fairly optimistic that recharge will take hold in years where there is water to spare, as the West recognizes that future drought must be prepared for, especially when it’s raining.

“Is recharge going to become a bigger part of water management? I would say absolutely,” he said. “I’m not usually in the game of making predictions, but I would predict the answer is yes. When we can figure out how to do it.”

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