New analysis highlights importance of groundwater discharge into oceans

An invisible flow of groundwater seeps into the ocean along coastlines all over the world. Scientists have tended to disregard its contributions to ocean chemistry, focusing on the far greater volumes of water and dissolved material entering the sea from rivers and streams, but a U.S. National Science Foundation-supported study finds that groundwater discharge plays a more significant role than had been thought.

The findings, published in Nature Communications, have implications for global models of biogeochemical cycles and for the interpretation of isotope records of Earth's climate history.

"It's really hard to characterize groundwater discharge, so it has been a source of uncertainty in the modeling of global cycles," said first author Kimberley Mayfield of the University of California, Santa Cruz. "It took a large effort by researchers around the world who came together to make this happen."

The scientists focused on five key elements -- lithium, magnesium, calcium, strontium and barium -- measuring concentrations and isotope ratios in coastal groundwater at 20 sites around the world and using previously published data from additional sites.

"Those elements are important because they come from the weathering of rocks, and weathering of silicate rocks accounts for a huge uptake of carbon dioxide from the atmosphere over long time scales," Mayfield said.

Co-author Adina Paytan of UC Santa Cruz said that groundwater is an important source of inputs to the oceans but has been easy to ignore because it is unseen and hard to measure.

"This is the first global assessment of groundwater discharge for most of these elements," Paytan said. "This information is important for our understanding of how weathering of rock is related to climate, not only in the present but also in the past."

The study estimated that the amount of these elements entering the sea from groundwater is at least 5% and up to 16% of the contributions from rivers, based on the latest global groundwater flux estimates.

"It's important to recognize that groundwater makes a difference globally, and now that we have this large data set, people can keep improving it with more sampling and develop better models of global groundwater discharge," Mayfield said.