Scientists recently estimated the volume and ages of groundwater using a combination of chemical measurements and mathematical models. The authors of the study compiled measurements of levels of tritium, the radioactive form of the element hydrogen, to estimate groundwater age. High tritium levels correspond to water that was exposed to nuclear testing in the past 50 years, or “young” water. The team found that about 6% of groundwater is “young” and that the “young” water is located near the surface. In general, shallow water is young because it is recycled more quickly. Deep water is recycled slowly because it moves much more slowly and also further underground . For water to travel about a mile into the Earth’s crust could take more than twelve million years! In addition to studying the age of groundwater, researchers estimated that Earth’s upper crust stores an incredible total volume of water – enough that, if spread evenly over Earth’s continents, the water would reach near the top of a 50 story building .
Although large, the reservoir of groundwater is finite. In the U.S., an average person uses close to 1,321 gallons of water per day! More than 85% of that usage comes from our reliance on agriculture, electricity, and other industries . Even if the average person gets most water from sources other than groundwater, our consumption in some regions far outstrips the rate at which groundwater is replenished. Furthermore, the availability of water depends on how deep underground the water is and how quickly it moves – deeper water recharges more slowly than shallower water. Because water travels so slowly under the ground, we can draw down water levels by trying to extract it too quickly.
Many thanks to graduate students Tamara Pico and Jacqueline Austermann, in Harvard’s Earth and Planetary Sciences, and to Prof. Vecitis and graduate student Marielle Remillard, in Harvard’s School of Engineering and Applied Sciences, for informative conversations and perspectives on groundwater.
 Gleeson, Tom, et al. “The global volume and distribution of modern groundwater.” Nature Geoscience (2015). http://www.nature.com/ngeo/journal/v9/n2/full/ngeo2590.html
 “Height Calculator.” Council on Tall Buildings and Urban Habitat. http://www.ctbuh.org/HighRiseInfo/TallestDatabase/Criteria/HeightCalculator/tabid/1007/language/en-GB/Default.aspx
 T. Winter, Harvey, J., Franke, O., and Alley, W. “Ground Water and Surface Water: a single resource.” U.S. Geological Survey Circular 1139 (1998). http://pubs.usgs.gov/circ/circ1139/pdf/circ1139.pdf
 “Water Use Today.” WaterSense (an EPA Partnership Program). http://www3.epa.gov/watersense/our_water/water_use_today.html