In a first of its kind analysis, scientists have created high-resolution maps of points around the globe where groundwater meets the oceans, to trace the former’s influence on the latter.
Ancient groundwater remains largely unknown and uncharacterised; hence, this study attempts to make up for limited information on this score.
The team of researchers at Ohio State University claims this will help coastal communities better protect drinking water as well as the seas and tell scientists where to monitor the discharge.
Here’s what it says
The study, culminating in the first near-global and spatially distributed high-resolution map of fresh groundwater flow to the coast, was published June 3 in the journal Geophysical Research Letters.
It claims that nearly half of fresh submarine groundwater discharge flows into the ocean near the tropics. The team also found that regions near active fault lines pour greater volumes of groundwater into the ocean than tectonically stable regions do.
“Freshwater-groundwater discharge is a natural line of defence against saltwater intrusion,” said Audrey Sawyer, assistant professor of earth sciences at Ohio State and a co-author of the study.
The study shows that climate heavily influences groundwater flow. Dry and arid regions, such as the Sahara desert, Gobi desert, and central US, which have very little groundwater discharge, are more prone to have their limited groundwater reserves eroded by saltwater intrusion.
These are also the areas likely to be dug up for extracting groundwater and satiate drinking water needs. But this study properly establishes that surface water and groundwater are inextricably connected. Once we begin to extract groundwater, we will affect surface water quality too.
“There is competition between the outward push of groundwater towards the coast and the saltwater that wants to come in, and if we don’t have as much of an outward push because we’re taking that fresh groundwater out of the ground instead of letting it flow to the coast, it puts us in a more vulnerable position,” Sawyer explains.
The role of acquifers in shaping oceans
The Ohio State team worked with researchers at NASA’s Jet Propulsion Laboratory and the University of Saskatchewan to combine topographical data from satellites and climate models to show the flow of groundwater around the world’s coasts.
The findings will make monitoring it easier because unlike surface runoff, groundwater is invisible from land. Yet, groundwater plays an important role in constituting the biochemical make-up of the world’s lakes and oceans, carrying nutrients, minerals and, in some cases, chemical pollutants.
“If you’ve ever been swimming in a lake or in the ocean in the summertime, and you go through a cold patch, that is probably a place where groundwater is coming out,” Sawyer said, according to Science Daily.
“And that’s just one way that groundwater affects surface water—in that case, it’s affecting temperature, but it also affects the chemistry of the water. These effects can be hard to measure over large scales,” she adds.
For example, groundwater can carry higher concentrations of nitrates that catalyse the growth of harmful algal blooms causing eutrophication of freshwater bodies, as well as high concentrations of mercury, which causes arsenic poisoning.
Why this matters
Groundwater is a vital component of practically every system on Earth and has been used extensively in modern times for irrigation, potable water, and industrial activity. The Amazon rainforest in central South America and the Congo region in Africa have some of the highest volumes of groundwater, accounting for much of the ancient subterranean aquifers.
A 2015 study on recharge rates (or the time that groundwater takes to be replenished) found that the ages of reserves ranged from months to millions of years. Of the total groundwater volume in the continental crust of nearly 22.6 million km3, only 0.1–5.0 million km3 is less than 50 years old. This underlines the pressing need for smart irrigation and conservation systems today.
“It’s not that we can’t use groundwater, but we need to monitor our impact and remember that groundwater is not an inexhaustible resource,” Sawyer says.
We must effectively manage ephemeral surface water and modern (less than 50 years old) groundwater in order to limit strain on ecosystems.
Modern groundwater is most vulnerable to human activity and climate, and must be understood to effectively manage freshwater supplies around the world. The latest study by Sawyer and her team seeks to achieve just that.
Understanding how and where groundwater gets to surface water could help policymakers create better plans to improve those bodies of water and monitor unscrupulous drilling of groundwater aquifers.
In India, open sanitation is not the only threat to soil and groundwater pollution. Rapid urbanisation and gentrification are equally responsible for contaminating our rivers and, ultimately, the drinking water table.
Informal recycling of e-waste, open burning of solid waste, combustion of coal, and ship-breaking activities cause massive soil contamination in most Indian metropolitan cities, allowing unmanaged waste to leach into groundwater aquifers.
Studies show that 70% of the country’s water is contaminated, painting a dire picture that is abetted by droughts and toxic rivers flowing in several states.
According to a NITI Aayog report published in June 2018, more than 600 million people in India face high to extreme water crisis, placing the country 120th out of 122 countries on the water quality index.
As per latest reports, wells and hand-pumps have dried up in Bundelkhand, Uttar Pradesh; ponds have disappeared, the water table has receded pushing inhabitants to war over water. Water scarcity in India will intensify further as its 1 billion-plus population grows, as evinced in the large-scale migration to cities, from dried up villages of Telangana, Andhra Pradesh, Karnataka, Maharashtra, Gujarat and Bihar.
Prarthana Mitra is a Staff Writer at Qrius.