How much pressure is the world’s groundwater under, and can it bounce back?

Groundwater is the source for around half the world’s drinking water. Irrigation and agriculture are even more reliant on it. Extraction has increased – that we know – but by how much, and are there still freshwater surprises waiting for us underground?

A study in the past year by a team from Nature, the world’s leading multi-disciplinary science journal, set about analysing groundwater level trends across the world. The team checked 170,000 monitoring wells and 1,693 aquifer systems in countries that make up roughly 75% of the planet’s groundwater usage.

Comparing trends with late 20th century figures (where they were available) the study found that groundwater is caught in a pincer movement of pressure. Extraction has undoubtedly accelerated in these early years of the 21st century, but at the same time precipitation has fallen. Over extraction can not just delete supplies but has risky side effects such as saltwater intrusion or land subsidence.

The results show that not only are rapid declines in groundwater levels (>0.5 m year−1) widespread in the 21st century (especially in dry regions with extensive croplands) but critically, such declines have accelerated over the past four decades in 30% of the world’s regional aquifers.

With some potential good news, Nature’s analysis suggests that long-term groundwater losses are neither universal nor inevitable.

Policies to reduce agricultural demands, for example, do appear to have an effect, as do groundwater fees and pumping licences. (The study points to successful examples in Saudi Arabia and the Bangkok basin among others.)

And it notes than in 16% of the observed aquifer systems, groundwater level declines have actually reversed. In a further 13%, levels had risen in both the late 20th and early 21st centuries, following initial heavy exploitation prior to the 1980s.

However, while this might be positive news, the team also concluded that recovery – while possible – happens at a much slower rate than decline.

Meanwhile in Sicily…

…. further details have emerged on the discovery of a vast reserve of freshwater beneath Sicily’s Iblei mountains that could alleviate the territory’s steadily worsening history of drought.

In the last half of 2023, only 150mm of rain fell on the Italian island, prompting the declaration of a state of emergency. (With the bitter irony of climate change, that’s the same amount of rain Sicily saw in just six hours
when Cyclone Apollo hit in 2021.)

Since 2003, Sicily has seen a 40% reduction in rainfall levels, while an ageing irrigation and storage network has only compounded the problem. The island is heading towards one of its worst droughts in history, and the potential desertification of 70% of its land.

At the end of 2023 a group of researchers from the University of Malta, the National Institute of Geophysics and Volcanology (INGV), and the Roma Tre University revealed the presence of an “unprecedented underground water resource” in the Gela Formation, a Triassic carbonate platform in the subsoil of southern Sicily. Estimated at 17 billion cubic metres, the cache is twice the size of Scotland’s Loch Ness, and trapped somewhere between 700 and 2500m beneath the mountains.

The six-million-year-old pocket of fresh water is believed to date back to a far earlier drought that dried out the Mediterranean Sea, exposing the seabed. This allowed rainwater to trickle down into the earth’s crust. It’s theorised that the so called ‘Messinian Salinity Crisis’, which endured for 700,000 years, then ended suddenly, causing an abrupt rise in sea levels which locked the water in place.

However, accessing the water is fraught with problems, not least of which might be the societal, political, and funding questions caught up around it. Drilling could be difficult and lengthy, and the risk of making the area geologically unstable would also need to be carefully assessed.