Need information on standards for ground water quality monitoring through borewells

We are from the petroleum refining industry and we have borewells to monitor ground water contamination by our petroleum products.
We have 4 bore wells inside the premises of our refinery installed to monitor ground water pollution and are used only when sample is taken for analysis. The depth of borewell currently followed is till we  get water,  that is up to top of water table. During a site visit it was suggested that the test sample should be taken from a level equivalent to the depth of sludge pit whereas sample is taken from very low level around 40 feet. This has prompted my query.

Is there any standard for bore well dug for monitoring ground water quality?