Overpumping of groundwater in the San Joaquin Valley, California, between 2007 and 2015 may have increased the concentration of arsenic in this water source, according to a modelling study published in Nature Communications this week.

The Central Valley of California accounts for around 20% of groundwater withdrawal in the United States, and groundwater is the main source of drinking water for approximately one million people in the San Joaquin Valley (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 (a layer of water-bearing permeable rock) clay beds. Under safe groundwater pumping levels, arsenic remains within the clay.

Ryan Smith and colleagues develop a quantitative model to predict arsenic concentrations, which is correlated to land subsidence rates caused by overpumping of groundwater in the San Joaquin Valley. The authors show that land subsidence is a result of clay deformation as pore waters containing high arsenic levels are released from the clay, which subsequently contaminate the main aquifer waters.

The team suggest that avoiding overpumping of aquifers in the San Joaquin Valley would improve water quality.