"Disturbed" Soil, issues in septic design
An important issue for percolation system design is avoiding so-called “disturbed” soil. Ideally, all soil involved with your percolation features is in its natural, original configuration, which would allow wastewater to pass downward in the most uniform and vertical manner.
If percolation were done in an area of filled soil, for example, percolating fluid may find easier routes laterally and potentially find daylight or create a “wet-spot” as could allow a vector for pathogens commonly found in sewage, or otherwise constitute a nuisance.
Filling is not the only variety of disturbance that the DEH takes into account. A ten-foot setback from utility line trenches must be observed, and the county considers any historical building slab-on-grade to also comprise a disturbed area to be avoided. In purchasing a lot, the buyer should verify that any historical grading be identified, as it could make a big difference in how the lot may be developed.
Disturbed soil is often not obvious and sometimes revealed only by bits of scrap found in soil from a drilled hole. The owner/client must communicate all information of this sort to the design consultant. Even tree-removals are likely to disqualify as disturbed areas. For many San Diego lots, this means that determining areas allowed for percolation will take precedence in layout of other onsite features, even dictating locations for a house and its driveway and hardscape; in other lots it may mean a more compact (and more expensive) system such as a seepage pit or Supplementary Treatment System will be necessary.