There are two main types of groundwater: old ‘fossil’ water, sometimes in excess of one million years old, and young ‘modern’ water that renews within months to decades. While old water is often deeper and more stagnant, modern groundwater is more important to humanity – it replenishes itself quicker and interacts with the hydrological cycle. A new study by an international team of hydrologists, calculates and maps the amount of usable, modern water beneath our feet.
‘We found an approximate total of 22.6 million cubic kilometres of groundwater within 2km of the Earth’s crust,’ says Dr Elco Luijendijk, co-author of the study and hydrologist at the University of Göttingen. ‘Of this total, less than six per cent is modern groundwater – reserves that have recharged within the last 50 years or roughly a human lifetime.’
Although modern groundwater represents such a small fraction of the total, it is enough to cover the land surface of the continents in a flood three metres deep. ‘While we were surprised at how small the percentage is, modern groundwater is by far the largest component in the active hydrological cycle,’ says Luijendijk. Nonetheless, modern groundwater is more vulnerable to contamination and over-exploitation and is being used far quicker than it is replenished, the authors say.
‘Groundwater is an immense resource, however the amount of time that matters for it is much longer than we usually consider,’ says Dr Kevin Befus, hydrologist at the University of Texas and co-author of the study. Luijendijk says ‘because policy makers can’t see it, protecting water sources tends to focus on smaller aspects of the hydrological cycle, for example rivers, rainwater and lakes.’
Geographically, modern groundwater tends to be found in humid and mountainous areas. The map shows bigger stores beneath the Amazon, the Congo, Indonesia and the mountains of North America along the Rockies and the western cordillera to the tip of South America. The map largely excludes high latitude, northern regions of Asia and North America as this is where permafrost – which blocks groundwater recharge – exists.
‘Intuitively, we expect drier areas to have less young groundwater and more humid areas to have more, but before this study all we had was intuition,’ says Befus. ‘Now we have a quantitative estimate.’