Is the water table just below the layer of impermeable rock?

No, the water table is not necessarily just below the layer of impermeable rock. The water table is the upper surface of the saturated zone where groundwater is found, and its position can vary depending on local geology and hydrology. An impermeable layer, such as clay or solid rock, can restrict the downward movement of water, but the water table can be situated above, below, or at the level of this layer depending on the surrounding conditions.