28-7-14 In California, Demand for Groundwater Causing Huge Swaths of Land to Sink
Extensive groundwater pumping is causing a huge swath of central California to sink, in some spots at an alarming rate, the U.S. Geological Survey reports.
With California in the throes of a major drought and demand for groundwater rising, officials and landowners are racing to respond to the process known as subsidence. Some areas of the San Joaquin Valley, the backbone of California’s vast agricultural industry, are subsiding at the fastest rates ever measured, said Michelle Sneed, a U.S. Geological Survey hydrologist and lead author of the recent report.
While the bulk of the sinking 1,200-square-mile (3,108-square-kilometer) area in central California is subsiding only about an inch (2.5 centimeters) a year, one 2-square-mile (5-square-kilometer) area Sneed studied is subsiding almost a foot (0.3 meters) annually. At that pace, “lots of infrastructure can’t handle such rapid subsidence,” Sneed said, including roads, water canals, and pipelines. The drought is likely to exacerbate the situation, as less rain drives more pumping.
Sinking Lands Raise Flood Risk
The worst subsidence has already increased the risk of flooding in the sparsely populated region, including to the low-lying town of Dos Palos, population 5,400, said Christopher White, manager of the Central California Irrigation District.
That’s because portions of the area’s flood control system have sunk, reducing their ability to contain floodwater. Local flood officials are crafting emergency plans for where to place sandbags when big rains return.
“We’ve got some serious issues,” said Reggie Hill, manager of the Lower San Joaquin Levee District, which maintains part of the flood canal.
Other canals and dams that deliver water to irrigate the fields of hundreds of growers are also losing capacity as parts of them sink.
White oversees the local effort to respond to the subsidence. His irrigation district, which serves 1,900 growers, spent $5 million in recent years to raise canals and dams.
The federal Delta-Mendota Canal, which delivers water from northern California to growers and cities in the Central Valley, runs near the edge of the subsidence bowl and was the focus of the USGS study.
In 1969 the canal’s banks were raised four feet (1.2 meters) along a 15-mile (24-kilometer) stretch in response to subsidence. More renovations—including the raising of several two-lane bridges over the canal—will be needed in 20 years if the sinking in the area doesn’t slow, said Bob Martin, an engineer with the agency that oversees the canal.
Sneed said more research is needed to assess the impact of subsidence on cities around the Delta-Mendota Canal.
One permanent impact to the region may be lost groundwater storage. As groundwater levels drop, clay deposits move closer together and space for groundwater is lost. “You can never get the deposits to go back,” Sneed said. Groundwater provides about one-third of the area’s total water supply, even more in drought years, officials said.