The 222Rn and CO2 soil gas distribution at Lembang Fault Zone, West Java - Indonesia

Abstract

Series of 222 Rn measurements had been employed around the Lembang Fault Zone, West Java – Indonesia to investigate its relationship with the fault zone. Furthermore, the mechanism of 222Rn transport is also investigated by performing soil gas CO2 and its δ13C measurements. The evaluation of 222Rn concentration shows that 34% data are within threshold values with an average of 14,117 Bq/m3, with anomalous concentration greater than 20,000 Bq/m3. The concentration of CO2 in soil gas is varied from 72 ppm to 13,241 ppm and consisted of three populations, with 40% of the data above 655 ppm. The spatial distribution pattern shows that most of Lembang Fault Zone segment coincides with high 222Rn concentration indicating high permeability zone. Furthermore, the average 222Rn concentration in western part of the fault is higher than the eastern part and this may be correlated with higher seismic activities. In contrast to 222Rn, CO2 concentration shows less correlation to the fault structure. Based on δ13C values, the source of soil CO2 is dominated by atmospheric CO2, with minor mixing of biogenic origin. Although Lembang Fault Zone is located in the south of an active volcano, there is no indication of magmatic CO2 origin. The lack of correlation between 222Rn and CO2 suggests that there is no indication of 222Rn transport by CO2 as carrier gas.