A shallow gravelless drainfield trench is typically 12" (wide) x 10" (deep) or smaller. So the laterals are right in the midst of the top 16" of soil stratum, where roots and 99% of soil biota are concentrated. Shallow gravelless drainfields can usually handle flows of high-quality effluent in excess of 5 gpd/ft2. Nitrates and other contaminants are removed by microbial activity and plant uptake. Soil structure improves and permeability actually increases over time.
Raised beds can be useful to enhance separation to groundwater. Drainfields can be landscaped, and water reuse is an option, via subsurface drip and spot irrigation.
