Courtesy of Calif. Dept. of Water Resources
SACRAMENTO — Water regulators are considering restricting groundwater pumping in the San Joaquin Valley near the California Aqueduct and other man-made canals because of subsidence.
State Department of Water Resources officials say they’re considering legal options for preventing facility damage in light of a new National Aeronautics and Space Administration report on subsidence — the sinking of land because of a drawing down of the water table.
The Feb. 8 report asserts that two main “subsidence bowls” covering hundreds of square miles grew wider and deeper between spring 2015 and fall 2016 and that a seven-mile area in Fresno County has settled up to 20 inches.
Scientists say subsidence threatens flows in the Aqueduct, which supplies nearly 25 million Californians and nearly 1 million acres of farmland. The canal has dropped more than 2 feet in one area of Kings County, losing 20 percent of its carrying capacity, the report said.
The DWR is considering measures to curtail groundwater pumping, create groundwater management zones near critical infrastructure, and developing county ordinances, officials said. Some of these measures may be enacted through legislation, said Jeanine Jones, the agency’s interstate resources manager.
Thousands of wells exist near state facilities that could be contributing to subsidence, state officials contend.
“The question for us is, what’s the status of those wells?” Jones said. “Are they all causing subsidence? ... That’s the due diligence we need to go through.
“Perhaps the remedy is some kind of infrastructure protection zone” in which drilling couldn’t occur within a certain distance of a canal or could only go a certain depth, Jones said.
The DWR is conducting its own study of the effects of subsidence along the 444-mile Aqueduct, the State Water Project’s main artery, and other facilities and will identify potential actions in the coming months.
Much of the effort will likely be done through local entities as part of the Sustainable Groundwater Management Act, a 2014 law that requires local governments to regulate pumping and recharge. Local leaders face a July 1 deadline for setting up new groundwater management agencies.
NASA scientists compiled their latest report using radar satellite maps and additional aircraft-based images focused on the California Aqueduct, according to a news release.
The report follows the agency’s August 2015 study finding that all the drought-related groundwater pumping in the valley was causing land to sink at historic rates. That study showed land in the valley was sinking by nearly 2 inches per month in some places.
Since then, the University of California and others have begun numerous groundwater-recharge projects in the valley, and storms this winter have provided as much as double the normal rainfall in some places.
But whether the wet winter helps groundwater recharge in an area depends largely on the soil, Jones said. With “inelastic” surfaces such as fine grain clay soils, the groundwater that’s removed can’t be replaced, she said.
Groundwater recharge projects wouldn’t necessarily help the Aqueduct or other at-risk facilities unless they’re close by, she said.
A comprehensive rebuild of the Aqueduct to its original capacity would likely cost hundreds of millions of dollars, and even a focused triage to fix the most affected parts of the canal could cost tens of millions of dollars per location, officials said. Already, subsidence-related repairs have cost the SWP and federal Central Valley Project an estimated $100 million since the 1960s.
The DWR set aside $10 million from the $7.5 billion water bond voters approved in 2014 to help communities with depleted aquifers enact pumping ordinances and conservation plans. But no Proposition 1 funds were set aside for repairs of existing facilities, Jones said.
“The bond was really about building new things, not” operations and maintenance, she said.