Thomas Kwader presents a list of potential sources of contamination that may appear in water samples that are not representative of the aquifer.
First, the drill rig and equipment obviously can be sources of contamination if they are not properly cleaned from the last site. Also, the mechanical equipment itself may be a source of leaking petroleum products (fuels, lubricants, hydraulic oils, etc.) that may inadvertently come into contact with tools or pipe going into the hole.