My first thought is "not really." And I asked my botanist son and he said, "The old septic area may provide some fertilizer, but no pathogens for humans will also infect the orange fruit." In other words, the tree will not uptake the bacteria and therefore the bacteria will not be hiding in the fruit. The risk is if you get the soil at the septic site on your hands.

Dear Louisiana gal : Thank you and your son for info. You saved a nice orange tree! I will harvest the oranges without them or myself touching the soil. Best regards, Jonathan