Bacteria are not harmful in garden soil.
Actualy what is called "micro organism =microbes" are rsponsible for a lot of healthy things. They even live inside us.They make your bread rise, milk yogurt, grape juice wine, and alcohol to vinigar.
Sewage treatment is best done with the help of bacteria.
I have seen documentaries that the water coming out of sewage treated by bacteria and down the stream same water was drinkable.

But, in your case, you are not consuming anything from soil directly. The vegetables will not be affected by bacteria(living organisms), but they could be affected by chemicals (non-living things).
If I were you I would not worry about that sewage leakage into garden, way back when. That leaked material has been dealt with the benefitial bacteria and the balance has been restored. Even if it were not, your basils are smart enough not to take them in.

It depends...if you are consuming any roots or leaves or stalks grown in pathogen infested soil, chances are you are going to get sick. But I assumed it's been a couple of years already, so things have probably adjusted itself. If you want to be absolutely sure though, you should get your soil tested

I'd probably look into getting a soil test done, if you can find one that will test the right stuff.

I'd be more inclined to worry about the sewage shortly after the failure happened, and not at all after enough time had passed...but how much time is "enough"? I'm just not sure. Two years is an awfully good start.

To play it safe, why not just plant things like tomatoes and peppers, which are tall enough to avoid most "splash back" from the soil, in that spot?

If you still have any lingering doubts, cook the produce from that location.

While some harmful bacteria can actually colonize the insides of crops like lettuce, spinach and basil it has been a few years, so few of the really nasty ones can have survived that long. I think that cooking is the key, maybe even solarizing the soil this year to kill anything living in it that you might not want.

If it were me, I would never plant any edibles of any kind over a septic system. There is just too much risk. If the septic were very deep or a good distance away and the garden was above the water table. it might be fine though.

I have a concern that if tomatoes were planted in only 6 inches of soil that they would not be able to develop a good root system and fall over more.

Your body has probably adjusted to the types of bacteria in your soil. However I would plant flowers rather than food in that area.

I would plant flowers over a septic tank also. E-coli would be the booger to be afraid of over a septic tank in case of a runover mishap. I would'nt take a chance for myself.

Interestingly enough Dral The human body cannot survive with out E. coli. The big E. coli scares have been over E. coli O157:H7 primarily, it contains a gene from Shigela (Probably) that produces a protein not unlike Shiga toxin. A closely related toxin is responsible for the toxicity of Vibrio cholerae which causes Cholera. The Vibrio genus is to Marine animals what E. coli is to mammals, vital for proper digestion, and ubiquitous in the environment. Vibrio also have some cool bioluminescent qualities, but I digress.

Thanks for all of your input - We don't have a huge amount of herbs to plant and we have another sunny location, so I think I'm going to err on the side of caution and just let my daughter plant flowers over the septic area.