Abstract
Studies have been carried out to investigate the baseline radioactivity level (gross alpha, gross beta and 226Ra) of soil, rocks and groundwater in the Fuling block, Chongqing, the largest shale gas exploitation area of China. The results show that there is a general activity concentration trend of gross alpha, gross beta and 226Ra: shale > soil > limestone due to the high content of uranium, thorium and potassium in shale and low content in limestone. The average activities of shallow groundwater from a limestone aquifer are 0.14, 0.13 and <0.008 Bq/L for gross alpha, gross beta and 226Ra, respectively. The radioactivity concentrations of gross alpha, gross beta and 226Ra (4.37, 1.40 and 0.395 Bq/L, respectively) of the formation water were far lower than those of formation water in the Marcellus shale in the USA (with ranges of 86–678, 23–77 and 16–500 Bq/L, respectively). One polluted shallow groundwater source and its associated stream sediments had been polluted due to leakage of drilling fluid with relatively high radioactivity levels and high concentration of main ions. Overall, this study provides an important baseline radioactivity level to assess the impact of shale gas exploitation on a shallow environment.
Highlights
With the rapid development of horizontal drilling and large-scale hydraulic fracturing, the production of shale gas has substantially expanded [1]
Warner et al [9] found that the radioactivity level of 226 Ra in stream sediments at the point of discharge at a waste treatment facility in central Pennsylvania was nearly 200 times greater than those of upstream and background sediments
The activity concentrations of 226 Ra are all less than 0.008 Bq/L. These results show that the activity concentrations of gross-α, gross-β and 226 Ra in the shallow groundwater are all below the drinking water standards of China (GB5749-2006) and/or the WHO guideline values
Summary
With the rapid development of horizontal drilling and large-scale hydraulic fracturing, the production of shale gas has substantially expanded [1]. Concerns regarding potential environmental pollution from hydraulic fracturing have arisen [2,3], especially the potential contamination of shallow aquifers by hydraulic fracturing fluids and/or formation water from deep formations through induced and natural fractures [4], leaking from casings and cement or wastewater discharge [1,5]. Of the complex contaminants in hydraulic fracturing fluids and/or formation water, naturally occurring radioactive materials (NORM) are notable concerns [6,7,8]. Some studies have indicated that the high radioactivity in the flowback and produced waters can reflect naturally occurring brines, which are associated with the targeted formations [4,11]. There is a lack of studies on the radioactivity levels of these different reservoirs as well as the relationship of radioactivity between waters and their reservoirs
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