Groundwater age dating key to sustaining aquifers

To help with long-term resource planning, governments and water utilities seek to understand surface infiltration or groundwater seepage. Natural aquifer recharge relies on precipitation to replenish water used by rural agricultural and urban communities.

The overwhelming majority of groundwater monitoring measures quantity and to a lesser extent, groundwater quality.

University of Utah geophysicist Douglas “Kip” Solomon believes that more critical data is missing: how much water is actually recharging the system. It is the linchpin to groundwater supply and demand, and in particular, approving or curtailing the amount of water extracted.

“A lot of (water) dating work is to help people understand sustainable yield,” said Solomon.

The university is one of handful in the world that can “determine how long the water has been in the ground.” Using a specialized mass spectrometer with parts custom made in the United Kingdom and Germany, Solomon analyzes water samples from the U.S. as well as from all over the world.

Water age is determined by measuring tritium and helium three. High concentrations of the radioactive isotope of hydrogen were once in the atmosphere, but have declined since a 1963 international ban on above-ground nuclear tests was enacted.

If five tritium atoms and five helium three atoms are measured from the water sample, Solomon knows the water is 12 years old. Most of the water he tests is less than 100 years old.

Solomon’s lab also uses the carbon-14 isotope to date “extremely old” water.

Working with the United States Geological Survey (USGS), Solomon and his lab have determined the groundwater age in the Utah-Nevada Great Basin to be roughly 40,000 to 50,000 years old.

In 2020, the Journal of Hydrology published findings by Solomon and fellow researchers who concluded that “the use of a combination of environmental tracers and groundwater geochemistry (can) significantly improve understanding of groundwater availability in a desert environment where water resources are scarce.” Their evidence suggested that previous research “overestimated [the] groundwater budget.”