The Paper
You've got to check out this paper:
Tom Gleeson, Kevin M. Befus, Scott Jasechko, Elco Luijendijk & M. Bayan Cardenas, The Global Volume and Distribution of Modern Groundwater. Nature Geoscience (2015) doi:10.1038/ngeo2590
Click on the graphics to enlarge them, or see the paper.
Abstract
Groundwater is important for energy and food security, human health and ecosystems. The time since groundwater was recharged—or groundwater age—can be important for diverse geologic processes, such as chemical weathering, ocean eutrophication and climate change. However, measured groundwater ages range from months to millions of years. The global volume and distribution of groundwater less than 50 years old—modern groundwater that is the most recently recharged and also the most vulnerable to global change—are unknown. Here we combine geochemical, geologic, hydrologic and geospatial data sets with numerical simulations of groundwater and analyse tritium ages to show that less than 6% of the groundwater in the uppermost portion of Earth’s landmass is modern. We find that the total groundwater volume in the upper 2 km of continental crust is approximately 22.6 million km3 , of which 0.1–5.0 million km3 is less than 50 years old. Although modern groundwater represents a small percentage of the total groundwater on Earth, the volume of modern groundwater is equivalent to a body of water with a depth of about 3 m spread over the continents. This water resource dwarfs all other components of the active hydrologic cycle.
Here is a CBC article on the research.
'Environmental' Tritium and My Stone-Age Approach
Gleeson and his colleagues (I know only Soctt Jasechko well) have done a meticulous, remarkable job using the environmental tracer tritium (the radioactive form of hydrogen with a half-life of about 12.32 years) and numerical modeling on a global scale to estimate the volume of global groundwater.
Note that most of the atmospheric tritium that has entered subsurface water was produced by atmospheric thermonuclear bomb testing, much of which ceased in 1963, when the USA, the then-Soviet Union, and the UK agreed to cease such testing. The other major atmospheric testers, France and China, continued until 1974 and 1980, respectively. Others - presumably (?) Pakistan, India, and Israel - continued atmospheric testing.
You may question the application of the word 'environmental' applied to bomb tritium but that refers to the fact that the tritium is introduced into the groundwater by natural processes - rainout and subsequent infiltration and recharge.
I am partial to the groundwater tracer tritium since part of my PhD dissertation, defended 40 years ago on 19 November 1975, used a primitive mixing-cell model that traced tritium in the Edwards aquifer in Texas to characterize groundwater ages and recharge rates to the aquifer. Yeah, here it is: Finite-state models of transport phenomena in hydrologic systems. The title? Pretentious? Moi?
But I digress....
Back to Gleeson's Good Stuff
Yes, the Gleeson et al. approach provides estimates, but it's the best one I can think of. They deserve a lot of credit. As they point out, their data indicate nothing about the quality of the groundwater nor its recoverability. Like its fluid counterparts oil and gas, groundwater cannot be 100% extracted.
The authors state that based on the groundwater that is 'modern' - recharged during the past 50 years - only 6% of the groundwater in the upper 2 km of Earth's crust is renewable. What bothers me is that social media and the press like to pick up on this figure without qualifying that it is based upon groundwater that is no older than 50 years (based upon the global average life expectancy). Why? Simple - it makes things look 'bad', and that looks good for the 'water crisis'.
Still, that 6% amounts to a huge amount of water:
I should add that the CBC article corrected its omission of the 50-year qualification.
Had Gleeson et al. used their higher estimate of young groundwater - 5 million cubic kilometers (see abstract), then the renewability percentage would be 22%, not 6%. Had they used their lower estimate of 0.1 million cubic kilometers the percentage would have been just 0.4%. Go figure.
My Ten Cents and a Modest Proposal
Groundwater renewability, as defined by Gleeson et al., needs to be qualified according to the age of the groundwater that's renewable. So why not discuss groundwater renewability in terms of its time scale, i.e., the 50-year GWR (groundwater renewability), 100-year GWR, 200-year GWR, etc. It might not be as satisfying to some people as the worst case scenario, but it is more appropriate for a resource that is renewed on time scales often exceeding human lifespans.
End of story.
End of my day.
The following quote seems most appropriate, especially if you read my dissertation.
"...essentially, all models are wrong, but some are useful." - G. E. P. Box and Norman R. Draper
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