It would probably be best to contact your County Extension Office, every county in Ohio has one, and see what they say about growing on a former septic field. If this is going to be market garden produce, many markets are requiring sellers to attend GAP (Good Agricultural Practices), and PSA (Produce Safety Alliance) classes, and provide your certificate of attendance, to be able to sell there.
I attended a GAP class, 3-4 years ago, and will be going to a PSA class here next month. Most is common sense items, but such as adding manure, they pretty well tell you it need to be composted, or spread on the area to be planted 120 days before tilled in, to prevent Listeria, etc. from even rain water splashing up on produce. If manure is brought in to form a composting pile, it needs to be so many feet away from crops grown.
Anymore, it's not simply tilling, planting, cultivating, harvesting, then off to market. If someone happens to get sick from produce bought at market, they want a paper trail to trace back to the grower, to find the problem. And most items would be the ones that are eaten in raw form, where normally cooking said item will kill the bacteria that would make a person sick. But, they are saying a lot of the liability requires the end user to wash items properly, before consuming. This would include peppers, tomatoes, etc.
