SEASONAL VARIABILITY OF SOIL-GAS RADON CONCENTRATION IN CENTRAL CALIFORNIA

Radon concentrations in soil gas were measured by the track-etch metho d in 60 shallow holes, each 70 cm deep and supported by a capped plast ic tube, along several major faults in central California during 1975- 1985. This set of data was analyzed to investigate the seasonal variab ility of soil-gas radon concentration in an area which has various geo logical conditions but similar climate. The results show several diffe rent patterns of seasonal variations, but all of which can be largely attributed to the water-saturation and moisture-retention characterist ics of the shallow part of the soil. During the rainy winter and sprin g seasons, radon tended to be confined underground by the water-satura ted surface soil which had much reduced gas permeability, while during the sunny summer and autumn seasons, it exhaled more readily as the s oil became drier and more permeable. At several sites located on creep ing faults, the radon-variation patterns changed with time, possibly b ecause of disturbance of site condition by fault movement.