Stone is not appreciably heavier than soil. Since the weight will be distributed, I see no problem. Tanks are often buried with 3 feet of covering soil.

The actual weight of the rocks is not a problem but you could run into a local code restriction.

The vegetation over a leachfield is an integral part of the septic tank treatment process. In some jurisdiction they will not allow you to even place lawn furniture over the septic tank or leachfield area because it would tend to increase foot traffic, which is detrimental to the continuing growth of the grass.

We can't put hardscape over a leach field, but we can over a septic tank.

I have a bluestone patio over most of my septic tank, it's dryset over compacted stone dust. I poured a concrete collar around the access holes to the tank.

Pull off a stone, dig out the stonedust packed within the collar, then pop off the cap to the septic tank to inspect or pump it out.

Ok So what can you put over a large leach field? No Hardscape?

No hardscape. You want shallow-rooted herbaceous plants, no bushes or trees.

"No Hardscape?"

If that is a question what that means, hardscape means solid surface finishings. http://en.wikipedia.org/wiki/Hardscape

You can grow small shrubs in your leach field and provide proper space for root growth so that it would not damage the drain lines.