Up to one in five wells worldwide may be at risk of running dry if underground water levels decline by just a few meters, according to a new study that is the first to create a map of the wells used to access water around the globe. 

The study, published April 22 in Science, analyzed construction records for approximately 39 million globally distributed wells across 40 countries and territories that represent about 40% of Earth's global ice-free lands, examining well locations, depths, purposes and construction dates. 

The researchers found that between 6% and 20% of Earth's wells are no more than five meters deeper than the water table — the upper level of an underground surface where soil or rocks are permanently saturated with water — indicating that millions of wells globally are at risk of running dry if groundwater levels decline by just a few meters.  

Groundwater is a globally reliable source of drinking water for billions of people — millions living in rural areas rely on wells to provide water to their homes, and thousands of cities use wells to provide residents with tap water, according to Scott Jasechko and Debra Perrone, the co-authors of the paper and assistant professors at the University of California, Santa Barbara.

"Wells are a very common infrastructure, and they're in every country around the world," Jasechko said. "As many as half of the world's people rely on groundwater for their drinking water supply."

Groundwater also provides roughly 40% of the water supply used to irrigate crops, making it a quintessential component in food production around the globe.

Human activities, such as the "unnatural process" of groundwater pumping via wells, are a driver of groundwater decline since it is changing the natural flow systems in the Earth's subsurface, according to Jasechko. Droughts and long-term climate change further exacerbate the issue since they also cause groundwater levels to decline, resulting in the drying up of wells — a phenomenon that also occurred in ancient times.

"If groundwater levels decline in places where groundwater well levels are close to the bottom of wells, wells will go dry, leaving people without reliable access to water," Perrone said.

Although newer wells tend to be constructed deeper than older wells in most areas, this is not always the case. In some areas that are experiencing significant groundwater declines, newer wells aren't being built deeper than older wells, meaning the newer wells might be at least as likely to run dry as older wells if groundwater levels continue to decline, according to Jasechko and Perrone. These areas include portions of Australia, Canada, France, Mexico, New Zealand, Sweden and the United States. 

The researchers also point out that constructing new, deeper wells merely provides a short-term solution since it is only feasible in areas where fresh groundwater is stored in deep, permeable aquifers. It is not a sufficient solution in places where aquifers contain saline water or yield little groundwater when pumped. 

Construction of newer, deeper wells also comes at high capital and operational costs since more energy is required to lift deeper leveled water from the bottom of the well to the surface. These high costs are already limiting users from accessing this groundwater, since well owners with sufficient capital to construct deeper wells can out-compete others in the area, according to the researchers. 

For example, in Punjab, India, the depths of wells are correlated with property size and crop production, which shows that, "Owners of large and lucrative farms are more likely than owners of smaller farms to be able to afford deep wells," according to Jasechko and Perrone, raising equity concerns in these areas. 

Further, people located in areas that lose access to a reliable water supply because a well ran dry may be forced to turn to less desirable options, such as selling their property and moving away from their homes, which in itself is difficult because it's "really hard to sell property if that property does not have access to water," Jasechko said. 

Because of these concerns, the researchers suggest the need for more sustainable solutions over the long term, and since groundwater depletion is a complex problem, many solutions taken together will likely be necessary to get humans on the path to sustainable groundwater management.

"We can reduce demand through behavioral changes, or through the adoption of water-saving technologies," Perrone said. "We can design and implement regulatory systems that are informed by the best available science, we can create water markets that support equitable and efficient groundwater use, and we can take advantage of excess water when it's available and use it to recharge our aquifers."

Jasechko and Perrone said groundwater "is one of our most resilient water supplies in the face of an uncertain future under climate change."

"This research just drove home the importance of protecting that resource, and ensuring it's available to help meet some of the really critical challenges we have in the future, such as realizing equitable and universal access to drinking water for humanity," they wrote.

The study "Global groundwater wells at risk of running dry," published April 22 in Science, was co-authored by Scott Jasechko and Debra Perrone, University of California, Santa Barbara.