“Age Dating” of Groundwater in the Salt Lake Valley

Chuck Call, P. E.

April 20, 2001

Emigration Tunnel is located in Emigration Canyon, a small canyon east of Salt Lake City, Utah.

(Salt Lake City, Utah) – Last summer Andy Manning, a Ph.D. student at the University of Utah, performed age dating on water samples collected from several groundwater sources in the Salt Lake Valley and the Wasatch Mountains, including the Emigration Tunnel.  The Emigration Tunnel is an infiltration tunnel located in Emigration Canyon. Water from the Emigration Tunnel is over 50 years old. Samples from the rest of the mountain springs and tunnels are younger, however, having ages of less than 40 years. Groundwater withdrawn from wells in the northern Salt Lake Valley is typically a mixture of water that is older than 50 years and water that is younger than 50 years old.

Several methods are used to make “age dating” measurements.  By determining the dissolved concentrations of several harmless gasses in the water samples, the age of the water can be determined. Two of the dissolved gas measurements are chlorofluorocarbons (CFC-12) and helium-4.

CFC-12 Dating Method

Atmospheric concentrations of chlorofluorocarbons (CFC-12) have been increasing steadily since about 1940 due to human activities. These are chlorinated and fluorinated hydrocarbons that have been used in refrigerants, aerosols, cleaning agents, fire extinguishers, and insulation foams.  All atmospheric gases, including CFC’s are dissolved in precipitation.  The greater the concentration of gas in the atmosphere, the greater its dissolved concentration in precipitation.

After recharging groundwater moves below the water table, dissolved atmospheric gasses are “locked in” and cannot diffuse back out of the groundwater. Because CFC’s do not react with anything in the ground, the concentration measured at the discharge point is the same as that acquired during recharge.  A CFC-12 age is thus obtained by measuring the dissolved concentration of CFC-12 at the discharge point, then comparing it to atmospheric concentrations in the past to determine at what time the water must have recharged. Its main limitation is that it only works back to 1941 – water older than this cannot be dated.

CFC’s measurements were taken from two water samples taken in the Emigration Tunnel.  These samples were collected from two adjacent points near the end of the right-hand (more southern) branch of the tunnel. They were located roughly 450 feet into the tunnel from the entrance shaft. At each point, the water sampled was discharging into the tunnel from a fault or major fracture.  From these samples, it is estimated that this water entered the groundwater system prior to 1941.

Helium-4 Dating Method

Concentrations of helium-4 gas can also be used as an approximate indicator of how long the water has been in the groundwater system. This noble gas builds steadily as water moves through the ground.  The measured helium-4 concentrations in samples collected from wells in the northern Salt Lake Valley suggest that a significant fraction of the groundwater in this part of the valley (likely the deeper groundwater) is over 50 years old.