Thanks for all the replies. I guess I started a pretty hot topic. Anyway, I went and dug around over my leach field and the lines are only buried 6" below the surface. I'm not sure what is typically used for septic drain fields but we have open bottom plastic "tubs" for lack of a better description. They (the tubs) were about 24" wide and 12" deep and run the length of the leach field which is about thirty feet. We have very fine soil when it is loose but when compacted it can turn into a clay-type soil. I think what the contractor told me about not using a riding mower on it, etc... may have more to do with local codes and county recommendations rather than his personal recommendations. He told me he mows on his but has had to extend/rejuvinate it a couple times. He lives only ten minutes away but in a different county so different codes or whatever. Also, where we live ground conditions can really change from house to house. Anyway, I guess the best thing will be to plant grass over it and mow with a push mower to start and then play it by ear after that. Thanks for all the input.
Matt