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pollutants are VOCs, NOx, and PM2.5 and PM10. Moreover, the presence of high level of VOCs
andPM 2.5 and PM 10 is known to harm the human respiratory system [14]. Also, many types of
equipment used for hydraulic fracturing such as compressors and pumps require fuel combustion.
These activities increase atmospheric emission of air pollutants, which affects the local air quality.
During shale gas extraction process, another potential air pollutant is the crystalline silica dust
originated from the silica proppant. It is generated during the transportation and mixing of sand into
the fracturing fluid. The acute toxicity of silica is low to moderate. However, crystalline silica has
chronic effects to cause silicosis, which is a progressive lung disease which may result in lung cancer
in humans. In the United State, the National Institute for Occupational Safety and Health (NIOSH)
conducted a field study of silica exposure in 11 hydraulic fracturing sites in five different states [15].
Among the 116 samples collected, 79% samples have levels of crystalline silica that exceeded
NIOSH recommended exposure limits (REL, 0.05 mg/m
3
). 31% samples showed silica exposures >
10 times higher than REL and one sample is more than 100 times higher. The shocking results
highlighted the high occupational health risk associated with handling silica proppant.
After the hydraulic fracturing process, the fluid that returns to the surface before the well is put in
production is referred to as flowback water. The produced water is the fluid that returns to the surface
after the well is put to production. The flowback water and produced water are usually stored in on-
site tanks or pits before being treated, or transported for treatment and disposal. The storage tanks
and pits need to be properly managed to avoid potential leaks and storm water overflow. In addition
to surface and groundwater contamination, fugitive VOC emission from the flowback and produced
water might also be a concern for air pollution.
2.6. Induced earthquakes
It is well known that earthquakes can be induced by surface and underground mining, impoundment
of reservoirs, and injection or withdrawal of fluids and gases into or from the subsurface formation.
Both hydraulic fracturing and deep well injection of wastewater could result in induced seismicity
[16, 17].
The hydraulic fracturing process may induce a large number of micro-earthquakes which usually
lower than 1.0 magnitude. There are a few incidences when earthquakes large enough to be felt were
attributed to hydraulic fracturing activities; the biggest one happened at the Horn River Basin in
Canada in 2013 with a magnitude of 3.8, which did not pose significant risk. The investigation
carried out by British Columbia Oil and Gas Commission concluded that the event was caused by
fluid injection during hydraulic fracturing in proximity of pre-existing faults [18]. In 2014, a
hydraulic fracturing job was suspended in north England due to a potential link between its activity
and two seismicity reports of 2.3 and 1.5 in magnitude near the well [19].
Deep well injection of shale gas produced water has also been related to earthquakes. In
Youngstown, Ohio, 10 small earthquakes (all < 3.9 in magnitude) were recorded from April to
November, 2016. These earthquakes were linked to the operation of a Class II deep injection well
used to dispose of produced water as the fluid increased pore pressure along a pre-existing subsurface
faults located close to the wellbore [20]. Disposal wells are also linked to the increase in the seismic
activities in Oklahoma, including a 4.0 magnitude earthquake that occurred on October 15, 2013 [21].
3. Discussion and conclusions
Overall, the environmental impact associated with shale gas extraction mainly include habitat
division, fracturing fluid spills and leaks, local air quality, water resource, ground water aquifer, and
seismicity. Among them, the pollution on ground water aquifer and local air quality are the biggest
environmental issue caused by shale gas extraction. These problems should attract the government's
attention, since these environmental damage could harm the health of local residents, and these
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