Casing material is chosen by the well driller based on local geology, well use, local and state laws concerning casing, and cost. Steel casing has been the industry standard for the last century. It is durable, strong, and is resistant against high temperatures. Steel casing does have drawbacks though. In conditions where the groundwater's pH is too high or low, it can be corroded, leading to possible infiltration of surface water into the well and groundwater. Also where the groundwater has a high electrical conductivity a process called electrolysis can cause corrosion of the well casing. In these conditions it may be advantageous to use plastic casing. Also, depending on the depth of the well, the cost of using steel casing may be prohibitive.
In the above video, you can see where water is leaking into the well from the casing/bedrock interface. This could be due to poor sealing of the annulus.
In this video you can see that the steel casing is becoming corroded and flaking off. This could, if allowed to progress, contaminated water to create holes in the casing and infiltrate the well.
In the above video, you can see where water is leaking into the well between the casing and bedrock. It appears that the steel casing was not seated properly into the bedrock.
In this video you can see that the steel casing is becoming corroded and flaking off. If allowed to progress the steel casing could become compromised and surface water or shallow groundwater could enter the well above the screen.