UC San Diego
Isotopes of helium, hydrogen, and carbon as groundwater tracers in aquifers along the Colorado River
- Author(s): Haber, Samuel Ainsworth
- et al.
Isotopes of hydrogen, carbon, helium, and neon in groundwater from 20 wells in the Lower Colorado River Valley (LCRV) are presented, with the aim of determining the age and mixing characteristics of the groundwater. Groundwaters from thirteen wells have measurable tritium and young ¹⁴C ages, suggesting mixing with at least a portion of recently-recharged (post-1952) groundwater. Groundwaters from the other seven wells are characterized as many thousands of years old and lack discernible tritium, which indicates no mixing with younger water. Carbon-14 ages of the groundwater indicate flow is away from the river (i.e. a leaking river). Dissolved helium concentrations were resolved into components associated with solubility equilibration, air entrainment, mantle- derivation, in-situ production within the aquifer, and extraneous crustal fluxes. Helium concentrations from the 20 groundwaters range from 0.55 to 69.8 x 10⁻⁷ cm³ STP g⁻¹ H₂0, or from close to air-equilibration values to 100- times saturation, with corresponding ³He/⁴He ratios falling between 5.07 and 0.17 times the atmospheric value. This large range of values is indicative of the two extreme types of groundwater : (1) young, tritium-rich waters and (2) old, ⁴He-rich waters. A whole crustal He flux, averaging 3x10⁻⁸ cm³ STP cm⁻² year⁻¹, was determined by constraining the ⁴He ages to match the ¹⁴C ages. A presence of mantle gases in which at least 1.4% of the helium is derived from a mantle flux is significant because the aquifers are at least 75 km from the San Andreas Fault Complex.