A couple of questions here.

1) How long does a leach field last in an average septic system? Does it
have to be replaced every so often even if it was used properly, or is
improper use the only reason the leach field goes bad?

2) In an older septic system, how much impact force can the leach field
take before damaging it? I have to fell a totally rotten tree and it will
unfortunately land right on the field. I can of course minimize the blow by
chunking the tree up rather than just chainsawing it down in one big cut.
Even still do I have to treat the leach field delicately or can it tolerate
moderate blows?