For decades, intensive groundwater pumping has caused ground beneath California's San Joaquin Valley to sink, damaging infrastructure, particularly between 2007 and 2015, and this may have increased the concentration of arsenic in the water source, according to a modelling study published in Nature Communications on Tuesday. The California Central Valley accounts for around 20% of groundwater withdrawal in the United States, and groundwater is the main source of drinking water for approximately 1 million people in the San Joaquin Valley which is an area of the Central Valley. Arsenic is a natural contaminant in groundwater but is found in high concentrations in trapped pore waters in aquifer clay beds. Aquifer is a layer of water-bearing the bed rock. Under safe groundwater pumping levels, arsenic remains within the clay. The paper suggests that as pumping makes the ground sink, it also unleashes an invisible threat to human health and food production because it allows arsenic to move into groundwater aquifers that supply drinking water to 1 million people and irrigation for crops in some of the nation's richest farmlands. According to the paper, satellite-derived measurements of ground sinking could predict arsenic concentrations in groundwater. This technique could be an early warning system to prevent dangerous levels of arsenic contamination in aquifers with certain characteristics worldwide. The lead author of the study, Ryan Smith, a doctoral candidate in geophysics at the Stanford School of Earth, Energy, and Environmental Sciences, said that arsenic in groundwater has been a problem for a very long time. It is naturally present in the Earth's crust and a frequent concern in groundwater management because of its ubiquity and links to heart disease, diabetes, cancer, and other illnesses. But he clarified that the idea that overpumping for irrigation could increase arsenic concentrations is new. During their research, the team of researchers found signs that aquifers that were contaminated as a result of overpumping can recover if withdrawal stops. Areas that showed slower sinking compared to 15 years earlier also had lower arsenic levels. The team also analysed arsenic data for hundreds of wells in two different drought periods alongside centimetre-level estimates of land subsidence, or sinking, captured by satellites. They found that when land in the San Joaquin Valley's Tulare basin sinks faster than three inches per year, the risk of finding hazardous arsenic levels in groundwater as much as triples. Aquifers in the Tulare basin are made up of sand and gravel zones separated by thin layers of clay. The clay acts like a sponge, holding tight to water as well as arsenic soaked up from ancient river sediments. Unlike the sand and gravel layers, these clays contain relatively little oxygen, which creates conditions for arsenic to be in a form that dissolves easily in water. When pumping draws too much water from the sand and gravel areas, the aquifer compresses and land sinks. “Sands and gravels that were being propped apart by water pressure are now starting to squeeze down on that sponge,” Fendorf explained. Arsenic-rich water then starts to seep out and mix with water in the main aquifer. When water pumping slows enough to put the brakes on subsidence—and relieve the squeeze on trapped arsenic—clean water soaking in from streams, rain, and natural runoff from surfaces can gradually flush the system clean. Study co-author Rosemary Knight, a professor at the School of Earth Sciences and an affiliate at the Stanford Woods Institute for the Environment, warns against banking too much on a predictable recovery from overpumping. “How long it takes to recover is going to be highly variable and dependent upon so many factors,” she said. The researchers said overpumping in other aquifers could produce the same contamination issues seen in the San Joaquin Valley if they have three attributes: alternating layers of clay and sand, a source of arsenic, and relatively low oxygen content, which is common in aquifers located beneath thick clays.
