Fresh water is arguably the most world’s most valued natural resource. It supports all live on Earth, and without it life simply wouldn’t exist. Globally, groundwater is one of the most overexploited natural resources, and because it is so vital to our future survival there is growing pressure to understand just how much remains available, and how long it will last before these precious reserves are depleted.
Now a team of international hydrologists have produced a data-driven estimate of the global groundwater supply. The study, the first of its kind, was recently published in the scientific journal Nature Geoscience. The main thrust of the study focuses on ‘modern’ groundwater. According to the report, less than 6% of groundwater found in the upper 2 kilometers of the Earth’s crust will be renewed within our lifetime.
“This has never been known before,” says lead author, Dr Tom Gleeson, of the University of Victoria. “We already know that water levels in lots of aquifers are dropping. We’re using our groundwater resources too fast–faster than they’re being renewed.”
According to Gleeson, with the world’s water demands constantly increasing, exacerbated further by climate change — this report provides vital information to policymakers and water managers, as well as hydrologists and scientists from related fields of study, giving them tools to sustainably manage groundwater resources.
Combining data from various sources, including data from approximately 1 million watersheds together with over 40,000 groundwater models, the authors estimate the world’s total groundwater volume to be around 23 million km3, of which approximately 0.35 million km3 is ‘modern’ groundwater that is less than 50 years old.
According to Gleeson, it is necessary to differentiate between modern groundwater and old groundwater as they both act in fundamentally different ways in terms of their role in both the water cycle and climate cycle. Old groundwater is located deeper than young groundwater, and is typically used by the agricultural and industrial sectors. It may contain contaminants such as uranium or arsenic, and in many cases may even be more salty than sea water — in fact, according to Gleeson, if it is very old and stagnant, it may in effect be considered non-renewable.
Modern groundwater by comparison forms the largest component of the water cycle, and is considered more renewable. However, because it lies closer to the Earth’s surface and moves faster than old groundwater, it is more susceptible to climate change and to contamination as a result of human activities.
The report’s maps reveal that most modern groundwater is found in mountainous and tropical regions of the world, with arid regions having the least.
According to co-author Dr Kevin Befus, while we would intuitively expect drier regions to harbor less young groundwater compared to humid regions, before undertaking this study, this was merely intuition. What we have now is a quantitative estimate that has been compared to geochemical observations, providing a more accurate picture of groundwater distribution.
The next phase in this research is to assess how rapidly we’re depleting our groundwater sources (both young and old) by analyzing volumes of available groundwater in relation to the amount of water being tapped off and depleted.