Pumping an aquifer to the last drop squeezes out more than water.
(Image credit: Shutterstock) The group found that satellite-derived measurements of ground sinking could predict arsenic concentrations in groundwater.
“Groundwater must have been largely turned over,” said study co-author Scott Fendorf, a professor of Earth system science and a senior fellow at the Stanford Woods Institute for the Environment.
They found that when land in the San Joaquin Valley’s Tulare basin sinks faster than 3 inches per year, the risk of finding hazardous arsenic levels in groundwater as much as triples.
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.
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.
While well data is important to validate and calibrate satellite data, she explained, on-the-ground monitoring can never match the breadth and speed of remote sensing.
“You’re never sampling a well frequently enough to catch that arsenic the moment it’s in the well,” said Knight.
As in the San Joaquin Valley, areas of the Mekong Delta where land was sinking more showed higher arsenic concentrations.