Mapping the influence of groundwater on the world's oceans

Researchers at Ohio State University have created high-resolution maps of the world's water table meets the oceans – the first such analysis, providing important data points to communities and conservationists to protect drinking water and the seas.

In a study published June 3 in the journal Geophysical Research Letters, the team showed that nearly half of the fresh groundwater was flowing into the ocean near the tropics. They also found that areas close to the active fault lines – the area around the San Andreas Fault in California, for example – sent more water to the ocean than tectonically stable regions. . And, they found that dry, arid regions have very little groundwater discharge, opening limited groundwater reserves in these parts of the world to salt water intrusion.

The Ohio State team collaborated with researchers at NASA's Jet Propulsion Laboratory and the University of Saskatchewan to combine topographic data from satellites and climate models to show the flow of groundwater along the world's coasts.

The results could help coastal communities better protect and manage their drinking water.

"The discharge of freshwater and groundwater is a natural line of defense against saltwater intrusion," said Audrey Sawyer, assistant professor of earth sciences at Ohio State and co-author of l & # 39; study. "And the intrusion of salt water is a source of concern in areas like Miami, Georgia, Cape Cod, and this, up and down the coast. That's a problem because dry areas have as few groundwater discharges as they do because they search for groundwater to meet their freshwater needs. "

Their work, the first high-resolution, near-global, spatially distributed map of groundwater flow to the coast, could provide scientists with better clues about where to monitor groundwater discharge. Such monitoring is more difficult than that of the water quality of a river or creek, as groundwater, by its very nature, enters the oceans and lakes under Water – people can not see them from the ground.

When researchers look at the quality of coastal water and how it affects the biochemical composition of the world's lakes and oceans, they generally think of rivers and streams, for good reasons. Most of the water that goes to lakes and oceans comes from surface water sources. But groundwater also plays an important role in transporting minerals and, in some cases, pollutants to surface water bodies.

"If you've ever swam in a lake or in the ocean in the summer and you're going through an area of ​​cold, it's probably a place where groundwater escapes," Sawyer said. "And that's just one of the ways that groundwater affects surface water, in which case it affects the temperature, but it also affects the chemistry of the water. be difficult to measure on a large scale. "

That's why the Sawyer team started creating these images: the research group focused on groundwater and realized that there was little information showing – in detail – where groundwater was most likely to dump into the oceans.

The study found that in some parts of the world, groundwater could pollute oceans and lakes with nutrients and other chemicals.

Groundwater, for example, may contain higher concentrations of nitrates – a key factor in the types of harmful algal blooms that have caused problems for both fish and for drinking water in the region. Lake Erie, Chesapeake Bay and the Gulf of Mexico – as well as elevated levels of mercury. Understanding how and where groundwater reaches surface water could help decision makers develop better plans to improve these water bodies.

The study also revealed that climate has a strong influence on groundwater flow and that dryland cities are particularly vulnerable to contamination of aquifers by salt water.

"This study draws attention to the idea that surface water and groundwater are all related, and that if you start extracting groundwater, you are assigning that link to our masses of water. surface water, as well as the quality of surface water, "Sawyer said. . "There is a competition between the groundwater pushing towards the coast and the salt water that wants to enter, and if we do not have such a push to the outside because we remove that water Cool underground soil instead of letting that puts us in a more vulnerable position.It's not that we can not use groundwater, but we have to monitor our impact and remind ourselves that groundwater is not an inexhaustible resource. "