2000 Volkswagen Golf water leaking into footwell - rear passenger side

A considerable amount of water is gathering I. The footwell of the rear passenger side. A check of the undercarriage shows that the costing was worn away near a seal directly under where the water gathers. I presume the water is getting there but such in the amount of water I can't see how so much can make its way up from underneath especially when the car is parked up and ther has been only a small amount of rain.