USGS: Merced County land sinking due to overpumped ground water

The Modesto BeeNovember 21, 2013 

— So much ground water is being pumped from the San Joaquin Valley that it's causing a massive swath of Merced County's surface to sink at an alarming rate, U.S. Geological Survey researchers revealed Thursday.

Parts of Merced near El Nido have dropped more than 21 inches in just two years. And researchers warn the deepening sinkhole now is spreading across 1,200 square miles -- from the cities of Merced on the north, to Los Banos on the west, Madera on the east and Mendota on the south.

That's a much larger region than previous studies had ever documented.

USGS officials said they fear sinking ground levels will wreak havoc on economically vital man-made structures like the Delta-Mendota Canal, the California Aqueduct and irrigation canals that serve Merced and Madera counties.

The sinking area includes part of the San Joaquin River and most of the Eastside Bypass, which is the primary flood control channel east of the river.

The sinking landscape has been accelerated by farmers who are pumping ground water to support crop expansions, area water leaders said earlier this year.

The farmers, who are east of the San Joaquin River in Madera County, have no other source of water. They did not realize their deep-water pumping was creating a problem.

Area water districts noticed the changes when they couldn't capture as much water as usual at nearby Sack Dam on the river.

Area farmer Cannon Michael said he questions the wisdom of growers who plant permanent crops, such as nut trees, on land that has no access to river or canal water. With no surface water, the grower must pump ground water to keep trees alive.

"Once you've made the investment, you have a hard demand for water," he said. "It's not sustainable."

When aquifers are being depleted, layers of clay collapse beneath the surface. That causes compression of the land above. Once that happens, the aquifers can never be refilled.

That's bad news for future ground water supplies. It's also bad for surface water supplies.

Researchers said the sinking ground is reducing the capacity of canals that transport floodwater and deliver water to agriculture, cities, industry and wildlife refuges. They warn that falling surface levels could cause infrastructure damage in local communities.

Over pumping of San Joaquin Valley aquifers has caused subsidence for decades. But USGS researchers report the current rate of decline is among the highest ever measured, and they discovered the problem's epicenter has shifted north to Merced County.

Comparing images and data from 2008 with 2010, they measured the subsidence and discovered the bowl of depression is much larger than originally believed. The worst area near El Nido is falling at a rate of nearly one foot per year.

A copy of the study is posted at www.modbee.com/groundwater.

Fresno Bee staff writer Mark Grossi contributed to this report.

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