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Slide 12

Next the driller lowers a large metal pipe called a “casing” down into the hole almost as far as he has drilled.

This casing alone will not protect the groundwater.  Pollution can still occur by mixing of good and bad ground water etc. between the outside of the metal casing and the inside of the hole. 

This space between the outside of the casing and the inside of the hole is called the “annular space”.

So another measure that is taken.