Casing material is chosen by the well driller based on local geology, well use, local and state laws concerning casing, and cost. Plastic casing, typically polyvinyl chloride (PVC), is a relatively new casing material that has gained acceptance in well installation over the past few decades. PVC is resistant to corrosion and is non-conductive, this gives PVC a distinct advantage over steel casing in terms of durability. However, they are often susceptible to heat related deformations, which can occur during well installation, and in the past have been thought to introduce contaminates to groundwater through leaching. Issues associated with plastic cased wells are typically related to improper construction, such as cracks in the casing, improper jointing, and incorrect seating of the plastic casing in bedrock.
In the above video you can see at the bottom right, where the plastic casing has not been properly seated into the bedrock. A large piece of casing has been broken off and part of it can be seen in the mirrored section of the video. In this scenario, if the casing is not properly sealed surface water can travel unimpeded, down along the outside of the casing and into the well.


In this video, you can see black staining below a joint in the plastic casing. If the joint isn't sealed properly or cracked during installation water can seep into the well.  The black staining indicates that water may be seeping into this well.


This video shows a screened section of a well.  The screen is a slotted section of casing located at the bottom of the well.  The slots allow groundwater to enter the well.  If a well is constructed properly, only groundwater from screened interval will enter the well.


In this video the plastic casing does not seem to be well seated into the bedrock.  Glistening bedrock below the casing may indicate that water is seeping between the bedrock and casing.