In California’s agricultural heartland, the San Joaquin Valley, excessive pumping of groundwater has resulted in subsidence, damaging crucial infrastructure, including roads, bridges and water conveyance. A study last year from NASA’s Jet Propulsion Laboratory in Pasadena, California, found overpumping of groundwater since the 1920s had caused parts of the San Joaquin Valley to sink as much as 28ft.

But groundwater overpumping may have another serious side effect, according to a study published June 5 in the journal Nature Communications. Researchers found that recent groundwater pumping caused an increase in the concentrations of arsenic in the aquifer. Arsenic has been linked to many kinds of cancer and can also cause stomach pain, paralysis and blindness. The scientists focused on the Tulare Basin, which stretches across the San Joaquin Valley from Fresno to just south of Bakersfield and is home to about 4 million people and a large agricultural economy.

“There is a huge societal impact behind this study, because not only is overpumping depleting our groundwater, it’s also contaminating it,” said Ryan Smith, coauthor of the study and a doctoral student in geophysics at Stanford University. “A lot of the groundwater pumping is being done by the agricultural industry, but the people that are being most strongly affected by it are those who are living in this area and need groundwater for their drinking water.”

The link between overpumping and arsenic concentrations in California is tied to the region’s geology. The Tulare Basin already has naturally occurring arsenic that originated in the rocks of the Sierra Nevada and coastal ranges and were deposited over millions of years in the valley by rivers. Some of this arsenic has ended up in groundwater, contaminating drinking water, and some of it is trapped in clay layers underground.

Under regular groundwater pumping conditions, the arsenic in the clay remains suspended, but when too much water is pumped, it becomes mobile, increasing the arsenic in the aquifer.

“When people pump a lot of groundwater it drops the pressure in the aquifers a lot and when the pressure is really low that pulls the water out of the clays, when normally it wouldn’t come out of the clays,” explained Smith. “You’re overstressing the groundwater system and that causes water to be drawn out of the clays and the water in the clays tend to have higher arsenic concentrations so you’re essentially bringing arsenic out of the clays into the groundwater system in the aquifer.”

At the same time, when the water is pulled out, the clay compacts and causes subsidence of the land surface.

“So therefore when you see subsidence, you have arsenic in the water,” said Rosemary Knight, study coauthor and a professor of geophysics at Stanford. “The link to both being through the clays.”

The study focused only on the Tulare Basin and the potential for the same mechanism to be active in other areas would depend on both existing arsenic sources and clay layers, said Smith.

A similar correlation has only ever been reported once before. A 2013 study by Stanford researchers found that excessive pumping of groundwater had increased arsenic in aquifers in southern Vietnam.

Modeling in the Tulare Basin study was done with an algorithm that used public well data on arsenic levels, and to measure subsidence the researchers tapped Interferometric Synthetic Aperture Radar (InSAR) data, which has been deployed for years to understand changes in groundwater levels. InSAR uses information from satellites that measure tiny changes in the elevation of the Earth’s surface.

“I think what’s most exciting [about this study] is this new way of using the remote sensing InSAR data,” said Knight. “We’ve always thought of subsidence in terms of water quantity issues, and this is the first study that is clearly showing that we can be using this remote sensing data as what I call an early warning system for water quality issues.”

The study concluded that, “Land subsidence due to overpumping increases the probability that groundwater is contaminated beyond the [World Health Organization] drinking water standard by a factor of two to three for the San Joaquin Valley.” The World Health Organization standard is 10 micrograms/liter (or 10 parts per billion), which is the same as California’s health standard.

The good news is that an increase in arsenic concentrations in groundwater doesn’t have to be permanent. “We need to stop pumping as much groundwater and we need to avoid overstressing the aquifer system,” said Smith. “Our study also indicates that if we can reduce pumping then the arsenic levels can return to safer levels within about 10–20 years or so.”

The bad news, however, is that groundwater is the main source of drinking water for about 1 million people in the study area. Many of them reside in small, low-income communities with limited resources for treating contaminated water sources. And others may be even worse off, including thousands of people that rely on essentially unregulated private wells there.

“The areas that are most at risk here are people who are using private domestic wells because they are not checking them for groundwater quality regularly and there is not a lot they can do if the arsenic levels exceed the safe standards and there are a number of those communities in this study area in the Tulare Basin,” said Smith.

Also of concern may also be crops irrigated with the water, he added.

Implementation of California’s 2014 Sustainable Groundwater Management Act (SGMA) aims to curb overpumping, especially in the San Joaquin Valley, but new agencies governing groundwater have two decades before sustainability goals need to be achieved.

SGMA, however, provides an important framework for Californians to think about how to achieve the sustainable management of groundwater, said Knight. “I’m a real advocate of data-supported science and policy,” she said. “This could be used as a guide to be thinking about how much pumping is reasonable, how much pumping is sustainable. I’m hopeful that these kinds of data can promote more sustainable groundwater management.”