Geochemistry of water at Cajon Pass, California: Preliminary results

Samples of water and associated gases were collected from the Cajon Pass well using downhole samplers, and from the pipe stands at the completion of drill stem tests. The fluids were recovered from fracture systems in granitic rocks from two uncased test intervals located at 1,829 to 1,905 m and 1,829 to 2,115 m. Results of chemical analysis indicate major differences in the composition of water from different fracture systems. Water from one fracture system in the first test interval has a salinity of 2,150 mg/L dissolved solids and is relatively high in Cl, Ca and Fe, but low in HCO₃ and SO₄; water salinity from a second fracture system is only 950 mg/L and is dominated by Na, HCO₃ and SO₄. Most of the water from the second interval likely originated from one fracture system; it is alkaline, low in HCO₃, has a salinity of 1,150 mg/L, and is a NaSO₄ type water characteristic of pore water in the granitic rocks of the area. The differences in water composition indicate different evolutionary paths and isolation of water within relatively proximal fracture systems.