N.Y. Comp. Codes R. & Regs. Tit. 6 §§ 363-6.3
In cases where the base of the constructed liner system is less than five feet above the seasonal high groundwater elevation, the department will require additional groundwater suppression systems to ensure that groundwater does not come in contact with the lowest portion of the landfill liner. At sites where perched water is encountered, the department will determine with respect to groundwater separation distances whether separation distances will be measured from the perched zone or the non-perched water table. The nature of the materials making up this separation, whether natural or backfilled, is subject to department approval. This minimum five feet separation requirement may be reduced or waived upon demonstration of selection of a suitable landfill site, as defined under section 363-5.1(a) of this Part and, that the proposed activity will have no significant adverse impact on the overall stability of the landfill, the environment, or natural resources and that the landfill's performance will be consistent with that which is expected from the application of this Part. In these cases, the department will require additional groundwater suppression systems to ensure that the seasonal high groundwater table does not come in contact with the lowermost portion of the landfill liner during construction, and until the hydrostatic pressures are equalized by weight of the liner system and/or waste.
N.Y. Comp. Codes R. & Regs. Tit. 6 §§ 363-6.3