"If his system is 6' down, you could grow anything on top with out ever being concerned the the roots will ever get near the contaminated water. In fact, if you dug down 4' I would bet that the ground would be quite dry."
Agreed, if you really have 6' to the top of rock then you could grow most anything with relatively shallow roots. I still have a hard time believing the 6' thing but it is entirely possible. Really, you could treat the ground like any other except I still think I would avoid deep rooting plants, those roots may seek the nutrient rich effluent and plug things up. Cottonwood trees are especially bad about this.
Heck, at 6' of true depth you could drive on top of it.