Michael Petruccelli says;
Not all water that enters a house comes from the roof or ground
surface. Subsurface water, or groundwater, which is typically the result
of a high local water table, may try to up-well into a basement through
the slab from underground. In a basement
this water is most easily controlled by a sub slab sump and electric
pump system that collects the under ground water and pumps it to the
surface and away from the foundation area. Occasionally, depending upon
the size of the basement, a second sump system may be required.
