Planning for the development of new lignite mines in the Ptolemais basin in western Macedonia, Greece, required the investigation of the hydrogeological characteristics of the adjacent karstic aquifer, since open-cast exploitation involves extensive and sustained groundwater table lowering (up to 260 metres). Review and evaluation of existing hydrological- hydrogeological data, in conjunction with the results of recent measurements, shows that groundwater flow is to the southeast in a single highly transitive karstified marble aquifer. An analytical regional flow model based on the finite element method is developed and used to assess the feasibility of the required groundwater table lowering and its effects on the hydrological budget of lake Vegoritis, nine kilornetres from the mines and in hydraulic continuity with the karstic aquifer. Model predictions show that the required ground water table lowering is not economically feasible at present and that any sustained large-scale dewatering scheme will significantly increase lake losses within a period of one to two years. It is thus concluded that exploitation of the new lignite- bearing horizons should not reach the karstic marbles at the boundary of the basin, but a zone of the relatively impermeable alluvium should be left to act as a seal. This scheme should be combined with partial lowering of the groundwater table to relieve the hydrostatic pressures on the seal, thus increasing the margin of safety against piping and slope failure.