Leach field is for all the liquds and junk... We don't garden over ours and keep it clear of trees because we don't want to damage the pipes. I would look into where your field is so that you know not to plant trees or to dig in those areas.
That said, most septic leach fields aren't going to be your entire back yard. Human waste in itself would be fine as a fertilizer as it breaks down, just like cow poop, chicken poop, and all poop really... but the drugs you take and are passed, chemicals of inorganic and organic backgrounds, surfactants like soaps and other junk you wash down the sink can accumulate in the soil around the field... and probably do run down into the pond/low lying areas. They use human waste to fertilize crops in other countries, just cook your veggies if you are that worried (or are traveling abroad). We have a huge black berry patch that grows over our field and we make juice and eat the berries every year for as long as I remember with no ill effects.
You might want to look into caring for your septic system so it can be "care free". We don't put toilet paper down the toilet if we can help it, and don't use a garbage disposal in the sink for anything other than undoing clogs. We also don't use any sort of drain clearer like drano in order to keep the septic tank bacteria happy and functioning. Our septic back home has been able to maintain 3.5 bath and a kitchen for almost 40 years without needing to be pumped, unlike some people who we know that have to have theirs pumped every 5 or so b/c of build up. Untreated city sewer ends up in rivers all the time when they over flow, so sometimes a well maintained septic is better as you know it is working.