Isotopic Techniques For Identifying Active Groundwater Flow Along A Complex Fault Zone Associated With The Hayward Fault System
Abstract
A study of fluid flow along fractures in the Wildcat Canyon fault zone is being conducted to identify parameters that can be used to identify fault zones that are active conduits for groundwater flow to aid choosing sites for nuclear waste disposal. The geology of the site consists of highly fractured and permeable andesitic volcanic rocks of the Moraga formation overlying low-permeability Miocene sediments of the Orinda formation that unconformably overlie sequences of shaly chert and sandstone of the Claremont formation and permeable Cretaceous sediments of the Great Valley sequence. Groundwater recharge is believed to be primarily through the Moraga formation that is then channeled through faults in the Orinda formation. The Wildcat Canyon fault zone is located in the hills of Berkeley, CA. It is a part of the Hayward fault system, one of the most active fault systems in California. To date 4 core holes have been drilled through the fault system. The mineralogic, chemical and isotopic compositions of rock and groundwater samples retrieved from the holes are being analyzed. In this presentation, we will present preliminary data on the isotopic compositions of the groundwater (δD, δ18O, δ13C of dissolved inorganic carbon and 87Sr/86Sr of dissolved strontium) and carbonate minerals (δ13C, δ18O, 87Sr/86Sr) in the rocks. The δ18O values of groundwater samples collected from the boreholes averaged -6.5% (VSMOW), which is in oxygen isotopic equilibrium at 15°C with calcite with a δ18O value of -7.1% (VPDB). The δ18O values of secondary carbonate vein material collected from fractures in the fault zone ranged from -7.6% to +1.3%. The lowest values are concentrated in zones of highly fractured rock. The apparent oxygen isotopic equilibrium between the present-day groundwater and the secondary carbonates in these fractures is evidence that these zones are active flow paths for groundwater. Analyses of 87Sr/86Sr of the groundwater samples and a limited number of carbonate samples from the cores have also been done. The groundwater 87Sr/86Sr averaged 0.7072. The 87Sr/86Sr of the carbonates ranged from 0.7067 for a sample with a δ18O value of -6.8% to 0.7086 for a sample with δ18O of +1.3%. The close correspondence between the δ18O and 87Sr/86Sr values of carbonate minerals in these fracture zones and the isotopic compositions of the groundwater provide further evidence that they are active groundwater flow pathways and suggest that the isotopic compositions of the carbonate minerals do provide a useful tool for identifying zones of active fluid flow.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.H12A..04C
- Keywords:
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- 1022 GEOCHEMISTRY / Composition of the hydrosphere;
- 1040 GEOCHEMISTRY / Radiogenic isotope geochemistry;
- 1041 GEOCHEMISTRY / Stable isotope geochemistry;
- 1832 HYDROLOGY / Groundwater transport