LOCAL VARIATIONS IN SPRING GEOCHEMISTRY: EVIDENCE FOR POORLY INTER-CONNECTED FLOW PATHS IN FAULT ZONES

Thermal springs commonly occur along fault zones owing to enhanced vertical permeability afforded by fracture zones. Field and laboratory studies of fault zone materials document substantial heterogeneities in fracture permeabilities. Modeling and field studies of springs suggest that spatial variations in permeability strongly influence spring locations, discharge rate and temperatures. The impact of heterogeneous permeability on spring geochemistry is poorly constrained. We present stable isotope and ion data from a series of closely spaced thermal springs associated with the Hayward Fault, California. We suggest that substantial variations observed in δ¹⁸O and chloride values reflect subsurface fluid transport through a poorly connected fracture network in which mixing of subsurface waters remains limited. Our measurements highlight both the potential to document tectonically induced changes in fault zone plumbing with geochemical monitoring and the need to consider local variations when characterizing fracture zone fluid geochemistry from spring systems with multiple discharge sites.