Environmental impact of CO2, Rn, Hg degassing from the rupture zones produced by Wenchuan M s 8.0 earthquake in western Sichuan, China.

Abstract

The concentrations and flux of CO2, (222)Radon (Rn), and gaseous elemental mercury (Hg) in soil gas were investigated based on the field measurements in June 2010 at ten sites along the seismic rupture zones produced by the May 12, 2008, Wenchuan M s 8.0 earthquake in order to assess the environmental impact of degassing of CO2, Rn and Hg. Soil gas concentrations of 344 sampling points were obtained. Seventy measurements of CO2, Rn and Hg flux by the static accumulation chamber method were performed. The results of risk assessment of CO2, Rn and Hg concentration in soil gas showed that (1) the concentration of CO2 in the epicenter of Wenchuan M s 8.0 earthquake and north end of seismic ruptures had low risk of asphyxia; (2) the concentrations of Rn in the north segment of seismic ruptures had high levels of radon, Maximum was up to level 4, according to Chinese code (GB 50325-2001); (3) the average geoaccumulation index I geo of soil Hg denoted the lack of soil contamination, and maximum values classified the soil gas as moderately to strongly polluted in the epicenter. The investigation of soil gas CO2, Rn and Hg degassing rate indicated that (1) the CO2 in soil gas was characterized by a mean [Formula: see text] of -20.4‰ and by a mean CO2 flux of 88.1gm(-2)day(-1), which were in the range of the typical values for biologic CO2 degassing. The maximum of soil CO2 flux reached values of 399gm(-2)day(-1) in the epicenter; (2) the soil Rn had higher exhalation in the north segment of seismic ruptures, the maximum reached value of 1976mBqm(-2)s(-1); (3) the soil Hg flux was lower, ranging from -2.5 to 18.7ngm(-2)h(-1) and increased from south to north. The mean flux over the all profiles was 4.2ngm(-2)h(-1). The total output of CO2 and Hg degassing estimated along seismic ruptures for a survey area of 18.17km(2) were approximately 0.57Mtyear(-1) and 688.19gyear(-1). It is recommended that land-use planners should incorporate soil gas and/or gas flux measurements in the environmental assessment of areas of possible risk. A survey of all houses along seismic ruptures is advised as structural measures to prevent the ingress of soil gases, including CO2 and Rn, were needed in some houses.