To investigate the source of volatiles and their relationship to the San Andreas Fault System (SAFS), 18 groundwater samples were collected from wells near the Big Bend section of the SAFS in southern California and analyzed for helium and carbon abundance and isotopes. Concentrations of ⁴He, corrected for air-bubble entrainment, vary from 4.15 to 62.7 (× 10^{− 8}) cm³ STP g^{− 1} H₂O. ³He/⁴He ratios vary from 0.09 to 3.52 R_A (where R_A = air ³He/⁴He), consistent with up to 44% mantle helium in samples. A subset of 10 samples was analyzed for the major volatile phase (CO₂) — the hypothesized carrier phase of the helium in the mantle–crust system: CO₂/³He ratios vary from 0.614 to 142 (× 10¹¹), and δ¹³C (CO₂) values vary from − 21.5 to − 11.9‰ (vs. PDB).

³He/⁴He ratios and CO₂ concentrations are highest in the wells located in the Mil Potrero and Cuddy valleys adjacent to the SAFS. The elevated ³He/⁴He ratios are interpreted to be a consequence of a mantle volatile flux though the SAFS diluted by radiogenic He produced in the crust. Samples with the highest ³He/⁴He ratios also had the lowest CO₂/³He ratios. The combined helium isotope, He–CO₂ elemental relationships, and δ¹³C (CO₂) values of the groundwater volatiles reveal a mixture of mantle and deep crustal (metamorphic) fluid origins. The flux of fluids into the seismogenic zone at high hydrostatic pressure may cause fault rupture, and transfer volatiles into the shallow crust.

We calculate an upward fluid flow rate of 147 mm a^{− 1} along the SAFS, up to 37 times higher than previous estimates (Kennedy et al., 1997). However, using newly identified characteristics of the SAFS, we calculate a total flux of ³He along the SAFS of 7.4 × 10³ cm³ STP a^{− 1} (0.33 mol ³He a^{− 1}), and a CO₂ flux of 1.5 × 10¹³ cm³STP a^{− 1} (6.6 × 10⁸ mol a^{− 1}), ~ 1% of previous estimates. Lower fluxes along the Big Bend section of the SAFS suggest that the flux of mantle volatiles alone is insufficient to cause the super hydrostatic pressure in the seismogenic zone; however, results identify crustal (metamorphic) fluids as a major component of the CO₂ volatile budget, which may represent the additional flux necessary for fault weakening pressure in the SAFS.