Growing veg above old septic tank

At the bottom of my garden there is a disused septic tank (actually two concrete tanks from what I can see) and a drain field. The soil above is about 1 foot deep, and each tank is about 6 foot wide, and square, with a now open whole in the middle where the access cover was, and filled with mainly flints, and some soil. Over one I intend to grow a quince tree, and I think it will be able to establish roots over and around the tank, and into it via the access hole. I have dug the area over the drain field (where the outflow discharges) and I would like to grow fruit veg, specifically rhubarb, blackcurrants, perhaps jerusalem artichokes, beetroot etc. Could there be issues with diseases from the soil? The tanks have been out of use for ~20 years. The soil is curious. There is a lot of heavy clay and chalk bits near the surface. Below this there is some really lovely looking dark brown, almost black, soil, with a lot of small flints. It is very crumbly, and with no smell. I suspect it is enriched from the outflow. Could this have been clay that has broken down? Or was it originally different from clay?

Anyway, as I've said, my main concern is safety, and the possibility of diseases. I suspect faecal bacteria would be long gone, but what about viruses? Presumably anything picked above ground such as rhubarb is fine, but what about root veg?

Is anyone else in a similar situation?

By the way, the reason for cultivating this area is that it is at the bottom of the garden, away from the main sewer and a drain pipe from the road, hence I can grow things like a quince safely without worrying that roots will damage pipes. It also means it does not eat into the main lawn area, and is somewhat sheltered as there is a 2m fence nearby.