Correlated radon and CO2 variations near the San Andreas Fault

Abstract

Correlations have been observed between groundwater radon and thoron concentrations and carbon dioxide discharges at the Lake Hughes station of the Caltech radon monitoring network. The Lake Hughes site is one of three radon monitoring stations located near the "big bend" segment of the San Andreas fault which began to record anomalous radon levels in August 1981. Two stations, Lake Hughes and Lytle Creek, recorded anomalous increases in radon while the third, Sky Forest, recorded an anomalous decrease. Several weeks after the onset of the anomaly, strongly correlated radon fluctuations began at Lake Hughes and Lytle Creek. These radon spikes also were found to be phase anti‐correlated with barometric pressure fluctuations. Analyses of gas grab samples showed relatively high levels of CO2 and ethylene in borehole air at Lake Hughes and Lytle Creek, while analyses of water samples showed relatively large increases in HCO3− at both sites. Isotopic analysis of one gas sample from Lake Hughes yielded a 13C δ value of −22 ‰, which suggests that the CO2 originates from the oxidation of organic material. The correlation in radon fluctuations at Lake Hughes and Lytle Creek and their common dependence on barometric pressure changes began shortly after the onset of the radon anomaly in August, and probably resulted from the simultaneous saturation of the water in these boreholes with carbon dioxide.