That means that if you're going to put in a septic system-- and a septic system basically means that human sewage from a facility or a house goes down into a pipe, it goes into a holding tank where solids settle out, and then fluid drains off and it leaches out through pipes that have holes in the bottom of them and then down through a bed of sand-- but if there is a clay area that lies beneath the sand, that means that the sewage is going to stay close to the surface, it's not going to dissipate, it's not going to break down, and it means that bacterial contamination of surface water supplies is much more probable. 