Septic tanks are fine to plant over, gardens, lawn, raised beds, etc. I wouldn't plant trees on top of them tho. lol
Drain Fields on the other hand are not to be tampered with. Despite popular belief that the water seeps down into the soil, Drain Fields rely on evaporation. Depending on your specific location, and what type of Drain Field you have, (Gravity, Pressure, Sand Filter, etc.) A certain amount of the liquid does "seep" back into the ground, but at least half of it evaporates, meaning, it evacuates up through the soil, and returns to the atmosphere as a gas. Nothing should be planted on top of the Drain Field. Watering your Drain Field is a no-no. It won't cause your system to fail, but it will slow down the elimination process. Also, you should not drive a car, walk horses or cows (especially elephants) over your Drain Field.