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

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 10^4.4 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.