Groundwater is an "invisible resource," writes environmental engineer Debra Perrone. It "flows slowly under our feet through cracks in rocks and spaces in sediments," she says, contrasting it with the more visible and obvious dams and rivers on the surface. This invisible resource is a quiet hero, supplying around a quarter of the US' daily freshwater needs.

Its distributed nature makes groundwater a challenging resource to manage. Unlike on the surface, where we can manage through public infrastructure like dams and reservoirs, groundwater is mostly tapped through millions of wells drilled by individuals, businesses, and farms. But current levels of groundwater use are not sustainable: resources are being steadily depleted as groundwater use outpaces natural replenishment.

This depletion means that shallower wells may run dry. Across the US, people are drilling deeper and deeper wells, report Perrone and her colleague Scott Jasechko in a paper in Nature Sustainability this week. That suggests that the easy-to-access water is already vanishing. But it's also not sustainable to keep going deeper.

Where the wells are

Perrone and Jasechko drew on 64 state-, regional- or county-level public databases recording the construction of wells, allowing them to pull together a database of nearly 12 million wells in the US. They looked across five large systems of water-carrying rock layers, called aquifers: the High Plains aquifer in the Central US and aquifer systems in California's Central Valley, Florida, the Mississippi embayment, and the Northern Atlantic Coastal Plain.

They found that wells across the US have generally been getting deeper since 1950. This trend toward deeper wells showed up across the majority of areas that were included in the database. Deeper wells and depleting groundwater may be related, but it's not necessarily that simple. There are other reasons why people might drill deeper wells—to avoid contamination in shallower wells, for example, or because improved technology or laxer legislation makes it possible where it wasn't before.

And even if people would like to drill deeper as shallower wells dry up, it's not always possible: as you drill deeper, groundwater may no longer be high quality or may not be as easy to extract in useful quantities.

To explore the relationship between groundwater depletion and deepening wells, the researchers used data on groundwater levels from the US Geological Survey. They found different patterns in different regions: for example, while depletion and well depth seemed to be moving in concert in California's Central Valley, the same was not true in the Floridan and North Atlantic Coastal Plain aquifers. The complex and varying features of different aquifers probably make deepening a viable response in some places but not others.

Deeper isn’t a long-term answer

Although shallower wells are at risk of drying out, drilling deeper comes with a host of concerns. For one thing, it's expensive: it may be an option available to some users, but it's not possible for everyone. It's also more energy-intensive to pump water from deeper wells, meaning that a trend toward deeper wells would likely increase the energy cost and carbon emissions of groundwater use.

Finally, there's just a limit to how far it's possible to go—because of poor-quality water at very deep levels, as well as the difficulty of extracting useful amounts of water in certain kinds of rock layers.

This study looks at the US, but this is really a global issue, the authors write. Not only is the US one of the world's largest food exporters (with a great deal of that food reliant on groundwater), but similar dynamics will be at play in other countries that rely on depleting groundwater, including China and India.

Ultimately, better management of this crucial resource requires data, which in turn can help to inform policy. That kind of policy is likely to become increasingly important as the climate crisis changes rainfall patterns: groundwater resources, write Perrone and Jasechko, "may become increasingly valuable because they are generally more resilient to short-term climate variations than surface waters."

Nature Sustainability, 2018. DOI: 10.1038/s41893-019-0325-z (About DOIs).