Chances are.. if you are mowing your front yard with it.. the tractor is small enough... I have had my NH 1920 over my tank, and it weighs 35xx or so...
Keep in mind that there will be 4 weight bearing points ( tires ) and the weight from each of those sections will be applied connically down and out from the tire. If your tank is about 8" deep, the area the weight is being applied to is generally going to be larger* than the ground contact area of the tire. ( * soil material throws extra factor is to the load bearing calculation. )
To get down to the math side, most septic systems I've seen are cast out of 4000 psi concrete. The load carried at failure is going to be difficult to determine, because it is going to be different depending on where the load is carried on the tank lid. If carried near an endwall, the endwall will support a good portion of the load, decreasing as you move farther away from the endwalls.
Shear strength of concrete is going to be between 1/6 and 1/4 of the ultimate compressive str. Tensile strength ( though rarely tested in non-reinforced concrete, should be between 7 and 10% of the ultimate compressive str of the concrete.
Probably didn't help you any did it?/w3tcompact/icons/smile.gif
Soundguy