222Rn concentrations in deep formation waters and the geohydrology of the Cajon Pass Borehole

Abstract

Measurements of 222Rn, 226Ra, and 238U in formation waters derived from granitic rocks surrounding the Cajon Pass borehole indicate concentrations of about 700, 1, and 0.01 dpm/1, respectively. The radon profile in the borehole indicates that inputs are localized and that several fracture zones must contribute significant amounts of inflow. Radon concentrations in fracture fluids indicate a minimum fracture width of 0.3 µm, and this width may be combined with the measured permeability to show that fracture porosity does not exceed 1.5 × 10−3. It is likely that the calculated fracture width is too small because incongruent weathering enriches fracture walls in 238U and 226Ra relative to the bulk rock. However, radon concentrations are lower than those in formation waters from other crystalline rocks, suggesting that the wall enrichments here are less pronounced and that fractures have experienced relatively limited fluid flow.