2012 NGWA Ground Water Summit: Innovate and Integrate

4He Dating of Groundwater in a Carbonate-Alluvial Aquifer System

Monday, May 7, 2012: 3:30 p.m.
Royal Ballroom A (Hyatt Regency Orange County)
Victor Michael Heilweil, U.S.Geological Survey;
Philip M. Gardner, U.S. Geological Survey;

Dissolved 4He concentrations in groundwater of the eastern Great Basin carbonate and alluvial aquifer system of western Nevada and eastern Utah often exceed concentrations from atmospheric sources (solubility equilibrium plus excess air) by up to two orders of magnitude. This excess helium is attributed to terrigenic (crustal or mantle) U/Th decay.  3He/4He ratios of groundwater samples containing elevated helium are as little as 10 percent of ratios expected for dissolved atmospheric helium. Previous studies have shown that this accumulation of 4He is a useful tracer for determining groundwater residence time. In an attempt to quantify age using groundwater helium, laboratory 4He release rates were measured on 11 borehole core samples, including Silurian through Permian-age carbonates and Quaternary-age unconsolidated alluvium. Helium release rates were measured after four heating steps, ranging from 60 to 250 degrees C. Extrapolating to an ambient average groundwater temperature of 15 degrees C yielded 4He release rates between about 10-11.6 and 10-9.2 cc He/cm3 rock/year. The three alluvial samples produced a much tighter range of 10-9.8 to 10-9.5 cc He/cm3 rock/year. Groundwater residence times of about 101 to 104.4 years were calculated using a geometric mean rate of 10-10.3 cc He/cm3 rock/year and assuming that all terrigenic groundwater helium is from U/Th decay within the aquifer matrix. These ages are generally similar to the range of corrected 14C ages of the aquifer system, providing an independent estimate of groundwater age. This improved confidence in calculated travel times is critical for accurately assessing groundwater development impacts.