| Although commercial exploitation of shale gas can alleviate the shortage of oil and gas resources and meet the needs of economic development,we cannot ignore the huge environmental problems that shale gas mining may bring.In this study,the soil environmental quality of a shale gas mining area in Sichuan province was investigated,and the source and spatial distribution of inorganic pollutants were analyzed,to study the toxicity of the fracturing fluid to soil.In order to provide scientific basis for the biological diagnosis and risk assessment of contaminated soil,the laboratory experiments have been carried out to explore the toxicity of the exposed fracturing fluid to plant growth,the acute toxicity of earthworms and the activity of soil enzyme.The main results are as follows:(1)There were slight heavy metals exceed standard value in the soil around the shale gas field,especially the Cd elements.The most values of the spatial distribution of Ni,Cu,As,Zn,Cd and Cr are in c and d,and the most Pb content was mainly distributed in a and e.It was found that shale gas mining has no obvious influence on the heavy metals in the surrounding soil by evaluating the pollution of heavy metals inside and outside the wellsite.The total amount of water-soluble salt in the soil around the each platform wellsite was less than 1000 mg/kg.The shale gas mining did not cause soil salinization,and had no obvious influence on the content of water-soluble salt in the soil around the well site.(2)Shale gas fracturing fluid had a certain effect on plant growth and physiological and biochemical characteristics,was toxic to plants directly and indirectly.The direct exposure of the fracturing fluid had a slightly different effect on the germination of different seeds,but all of them were inhibited.The influence on the activity of alpha amylase in the seeds was low promotion and high inhibition,and the proline content in the radicle increased with the increase of concentration.The exposure of fracturing effluent to the roots and stem length of plants showed low promotion and high inhibition,which caused the increase of proline content,and had different effects on the contents of malondialdehyde(MDA)and peroxidase(POD)in different parts of plants.The fracturing fluid also inhibited the growth of plants in the soil,the proline content in leaves increased,MDA content and POD activity were not significantly different from those in control group,and the polluted soil did not cause obvious oxidative damage to plants.(3)There is a significant concentration effect relationship between the mortality of earthworms and the concentration of the fracturing fluid,in the filter paper test,the 24 h half lethal concentration(LC50)was 670.522 mg/L,48 h was 422.339 mg/L;in the artificial soil test,the 3d-LC50 80.265 mg/kg,7d was 26.444 mg/kg,14 d was 15.779 mg/kg.The difference of the weight of earthworms in soil was not significant.The effect of the fracturing fluid on the activity of the enzyme in the earthworm is different.(4)Under the exposure of different concentrations of fracturing fluid,the activity of catalase in soil decreased with the increase of time,and showed a certain inhibitory effect.The activity of soil dehydrogenase decreased with the increase of exposure time,but the difference was not significant.The soil urease activity increased first and then decreased with the increase of exposure time.Totally,fracturing fluid activated the activity of dehydrogenase in soil,and inhibited the activity of catalase and urease. |
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