I have 1000 gallon septic tank, deep buriel, 7 feet to the top, but I don't think your question revolves around the tank, as a filed is apt to be subseptible. That said, my field is a gravelk and perf pipe mound system that is 25 years old. Before I replaced my tank, it had a reduced line in a burst line and fed the field witout much pressure. After I replaced my tank, the edges of the raised field suddenly greened up over summer. Now bare in mind that a length or two of my pipes have risen to the surface and I have had to add a few yards of soil and grass seed to keep it under ground until I do a new field. Last year, I drove my BX on it all the time (unless satrated by heavy rains and soft) but at the end of the year when it begain to cool, I drove over a section I had soiled and reseeded and with the new dosing pressure of the larger lines, a great big wave beneath a mat of grass scalped itself under my tractor. I drove real slow to get off it. The next week, I dared try it again except this time it split open. Temporary repairs. You may have a chamber system with all those inspection ports. If you are supposed to pump out your field(?) perhaps you have chambers in trenches in a thick clay soil that has no absorbant charachteristics at all. By design chambers can hold over 15000lbs with the proper amount of native fill on top. That being said, any system will not benefit from having its media compacted.
Addidtives? University studies strongly urge letting your system do its own stuff and not upset the balance. I put some in recently when I let some bleach sit in my water lines when I put in a new water plant, but normally I avoid them (and bleach). Pump out when its warmer to allow the bacteria to flourish like in August.
Find out what type of system it is. Better yet, ask the old owner if he drove his old tractor on it. /forums/images/graemlins/cool.gif
