For a tank you mainly want it out of a water table. They should be watertight settling chambers.

Now your SSAS, your drainfield, where effluent is being applied to soils you’re going to want sands and as little clay as possible. More importantly than soil type aside from excessive gravels is the intended application rate of effluent into the soil.

A larger drainfield applying effluent at a slower rate becomes more necessary as the soil type gets more smaller particles.