Natural gas boreholes and oil wells cross through many underground layers. which of the following is a threat or problem with these extraction processes?

a: boreholes and wells are very expensive because they follow strict safety standards

b: radioactive materials are brought to the surface

c: there are no threats or problems, fossil fuel extraction is completely safe

d: pipe casings can leak fossil fuels and fracking fluids into ground water aquifers

One of  the threat with the extraction process of Natural Boreholes & Oil Wells is :  D) Pipe casings can leak fossil fuels and fracking fluids into ground water aquifers.   Boreholes are drilled and then lined with steel tubes so that a mixture of water and sand with small quantities of chemicals, the fracking fluid, can be pumped into them at very high pressure.  Fracturing fluids contain chemicals which are known to be toxic, and widespread concern has been expressed over potential water contamination.  

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