What does this sentence mean ?

"The crushed granite stone, processed from native Rhine riverstone, filters the rain water before it recharges the water table of the Rhine River." Can someone please explain this for me? I am learning about aquifer's and I am just not understanding this sentence. Like i get that the crushed granite acts as a filtration system for storm water but "filters the rain water before it recharges the water table of the Rhine River."........ I just don't get this part!