**4.3 Water pollution risks**

Most of the public attention surrounding hydraulic fracturing concerns the risks of surface and groundwater pollution, especially in the context of drinking water wells. There are several pathways for such pollution [4], including:


 A study by Gross for Colorado found that water pollution from surface spills is a relatively frequent occurrence: groundwater were impaired in 77 surface spills that were reported between July 2010–July 2011 and representing ~0.5% of active wells in densely drilled Wells County, Colorado [38]. Such impairment occurs primarily when spilled fluids percolate through the soil into shallow groundwater aquifers.

 Vengosh et al. undertook a detailed study to understand which pathways were most likely for surface and subsurface migration. They distinguished between (i) the contamination of shallow aquifers with fugitive hydrocarbon gases (stray gas contamination, (ii) contamination of surface water and shallow groundwater from spills, leaks, and/or the disposal of inadequately treated shale gas wastewater, and (iii) the accumulation of toxic and radioactive elements in soil or stream sediments near disposal spill sites [3]. Using published data and studies from across the U.S. the results indicate that there is evidence for stray gas contamination, surface water impacts, and accumulation of radium isotopes at some disposal and spill sites. A critical issue in conclusively attributing the pollution of drinking water wells or other water sources to UOG operations is the lack of baseline data, i.e., data on

 water quality before UOG commenced. In particular, methane, heavy metals, and radioactive compounds may have been in the water long before the arrival of unconventional oil and gas production as a result of the aquifers' geology or due to other man-made activities. In order to better understand and attribute the sources of water contamination, states such as California now require water quality monitoring before and after unconventional well drilling and stimulation activities. Overall, the evidence for methane contamination of groundwater and drinking water wells from hydraulic fracturing remains controversial in many cases [39–41].
