Isotopes of helium, hydrogen, and carbon as groundwater tracers in aquifers along the Colorado River

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 cm 3 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 3 x 10-8 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.