From the air, scientists map ‘fast paths’ for recharging California’s groundwater table
Climate change has transformed precipitation systems across the American West in recent decades, making some places less likely to find enough water to grow crops or water wildlife. Scientists are now mapping the potential for California to build on its water resources by tapping into subterranean aquifers that hold as much as 100 billion gallons of water, though it takes decades of pumping to recharge the aquifers and bring them above ground.
The scientists, led by Stephen Holbrook, a geologist at the University of California at Riverside, focused on the California Channel Islands, a cluster of seven islands and three smaller islets that lie about 700 nautical miles (1,260 miles) west of mainland California.
Using radar sounding from the University of California at Irvine, they identified groundwater flow pathways that could be used as a source of fresh water. To map the underground rivers, the researchers mapped the surface layers of soil in each location, so that they could figure out how much water would flow through specific underground pathways. They then drilled wells on each island to sample the water and to calculate when it could be produced aboveground.
“We went through a number of experiments to figure out how the water was coming down to the ground, and how long it was going to stay there,” Holbrook said. “How long water stays in the ground isn’t actually the important thing. What matters are the pathways through which the groundwater is flowing. We found, for instance, that the groundwater on Midway is likely to flow through a number of different pathways – maybe a few dozen pathways.”
The scientists also drilled wells near the channel islands, which have been dry for more than a century. On Midway, they found that a water supply that flowed for about 1.5 meters (5 feet) a year could recharge the aquifer from less than 1.5 meters (5 feet) of precipitation, Holbrook said. If precipitation fell at a rate