Groundwater plays a critical role for rivers worldwide and many aquifers are in trouble

National Geographic, 2 October 2019:

There’s more fresh water hidden below Earth’s surface in underground aquifers than any other source besides the ice sheets. That groundwater plays a critical role for rivers worldwide, from the San Pedro to the Ganges, keeping them running even when droughts bring their waters low. 

But in recent decades humans have pumped trillions of gallons out of those underground reservoirs. The result, says research published Wednesday in Nature, is a “slow desiccation” of thousands of river ecosystems worldwide. Already, somewhere between 15 and 21 percent of watersheds that experience groundwater extraction have slipped past a critical ecological threshold, the authors say—and by 2050, that number could skyrocket to somewhere between 40 and 79 percent. 

That means hundreds of rivers and streams around the world would become so water-stressed that their flora and fauna would hit a danger point, says Inge de Graaf, the lead author of the study and a hydrologist at the University of Freiburg. 

“We can really consider this ecological effect like a ticking time bomb,” she says. “If we pump the groundwater now, we don’t see the impacts until like 10 years further or even longer. So what we do right now will impact our environment for many years to come.” 

Groundwater holds up modern life 

The last undammed river in the U.S. Southwest, the San Pedro of southwestern Arizona, used to gush and roil. Birds chirped and splashed on its banks when they stopped by on their migrations. Rare fish swam in its pools. 

But in the 1940s, wells started to pop up in the nearby area, sucking clean, cool water out of the region’s underground aquifers. 

It turned out that a good portion of the water that flowed through the river came not from rain and upstream snowmelt, but from those underground sources. The more water that got pumped out of the aquifers, the less flowed into the river—and the wetlands, cottonwood stands, fauna, and rushing waters of the San Pedro all suffered. 

Groundwater is the hidden scaffold propping up much of modern life. Globally, about 40 percent of the food we grow is watered with liquid extracted from below Earth’s surface. 

But many of the aquifers from which this water is extracted took hundreds, or even tens of thousands of years to fill: The water inside may have percolated through cracks in the earth when giant ice sheets last covered New York City 20 thousand years ago. 

Much of that water is being removed much faster than it can be replenished. That has enormous potential consequences for people who want to drink water grow and crops in areas that don’t get enough rain. But far before those impacts emerge, the effects will—and in fact already have—hit rivers, streams, and the habitats around them. 

“Think of an aquifer like a bathtub full of water and sand,” explains Eloise Kendy, a freshwater scientist at the Nature Conservancy. Then, imagine running your finger lightly through the top of the sand, creating a little trail. That little trail fills up with water that percolates through the sand into the “stream.” 

“If you pump out just a little bit of water out of the bathtub, that stream is going to dry out, even though there’s plenty of water still left in the bathtub,” she says. "But as far as healthy rivers go, you’ve destroyed it. But because rivers don’t scream and shout, we don’t necessarily know that they’re in trouble.” 

Read the full article here.