I might be able to
answer this.
Just to be clear, the septic tank is not so much of your concern as the leach field, since that is where human waste
should be coming out. However, as you point out, sometimes pipes get leaky, so I would watch out for both. I know here, a septic tank or leach field can be no less than 100 feet from a well. That is rather close, but drilled
wells are sealed from ground run-off too...gardens are not.
So...
As a larger farmer I must have what is called a Comprehensive Nutrient Management Plan (CNMP for short) and it is a fancy title for saying, just where I can spread my animal
poop. In that, it specifies that I cannot spread manure 300 feet uphill of a private well or
water body, or or 100 feet down hill of a private well or water body. While you could certainly go farther away to be extra safe, I ASSUME the powers that derived these rules did some testing and were qualified in making this the minimum standards.
Not sure if my answer helped or not, but with animal manure at least, that is the USDA requirements for set-back distances.