For sampling below the water table, it requires specialised equipment to hold the fluid samples and to prevent the hole collapsing.
One-off samples can be achieved using one of the special samplers but for continuous sampling, the hole will need Casing (a liner to prevent the hole collapsing) and if the material is fluid, a Sludger/Baler is required.
BELOW THE WATER TABLE INFORMATION To continue drilling or sampling below the water table most often requires casing the hole( a rigid lining). Any type of tubing which can be joined and advanced down the hole with the drilling process can be used.
The problems with the improvised casing are: joining the casing so that it does not hinder the installation; protecting the leading edge, and removing (or sacrificing) the tubing. It can be done and has been done on many occasions.
However, for us to sell a professionally made product we need to address the issues.
Our Casing is specially made so that the threaded joints are flush fitting on the outside while not impending work inside, there are hardened tooth Casing Cutters threaded to the leading edge and through design features, it is easier to recover our reusable casing.
We have customers who re-use our casing job after the job for years. The biggest risk with the casing is getting it stuck and not being able to recover all or part of it.