STUDY ON GROUNDWATER BASELINE QUALITY, MONITORING INDICATORS AND CONTAMINATION TRACING METHODS RELATED TO SHALE GAS DEVELOPMENT: A CASE STUDY IN THE FULING GASFIELD IN SW CHINA

Environmental issues caused by hydraulic fracturing and wastewater reinjection during shale gas development have been paid more attention to. Because there are a wide variety of pollutants of the hydraulic fracturing, it is necessary to determine sensitive monitoring indicators for potential pollution tracing. This study takes the Jiaoshiba as an example, and has established the baseline values of groundwater environment(including water chemistry, isotopes and dissolved gas). Shallow groundwater is the typical karst water with TDS between 146 and 402 mg·L^-1, belonging to HCO₃-Ca·Mg water chemistry type. The groundwater samples are in the range of young groundwater(the contained ³H and ¹⁴C close to 100 pmC). The mass concentrations for CH₄ of all groundwater samples are less than 0.01 mg·L^-1 and the volumetric concentrations in dissolved gas of groundwater are less than 0.006 4%. The δ¹³C-CH₄ values for dissolved methane in groundwater are lower than-50‰, suggesting biogenic origin. The TDS of the shale water in the Jisoshiba shale gas field is about twice that of the seawater, but it shows the genesis of atmospheric origin, which is significantly distinguished from the shale water in North America that originates from the seawater(or evaporated seawater). According to the differences of hydrogeochemical characteristics between shale water and shallow groundwater, the framework for determining sensitive monitoring indicators has been established. In the Jiaoshiba gas field, the sensitive indicators have included 7 items. The potential groundwater contamination processes due to shale gas development are mainly from stray natural gas and dissolved solid constituents. For the contamination of stray gas, end-members have been established by the use of methane and noble gas(³He/⁴He) to identify gas contamination. The contamination from dissolved constituents can be identified by the determined sensitive indicators based on the baseline values of shallow groundwater. This study may hold great significance for the groundwater protection of shale gas development in China and contribute to the improvement of monitoring groundwater environment and tracing potential contamination during shale gas development.