Siting a Replacement Absorption Field on a Sloping Lot
A single-family dwelling on a 0.6-acre lot has a failing septic system discharging to the surface downslope of the house. The lot slopes from north to south at roughly 12 percent. The homeowner wants a replacement absorption field. You walk the site and observe: (1) a shallow seasonal high water table indicated by redoximorphic features at 14 inches in the northern third of the lot; (2) a compacted, smeared zone from past equipment traffic near the driveway; (3) a property line 18 feet south of the only open area large enough for a field; and (4) a private drinking-water well 60 feet northwest of the proposed area. The local code requires 100 percent replacement area and minimum separation distances that you must verify against the adopted edition.
1. Before any percolation testing, what site and soil factors must you resolve, and in what order, to determine whether the lot can legally and physically support a replacement field?
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Resolve disqualifiers first, because they can eliminate the lot regardless of permeability:
- Setbacks and lot geometry. Measure the available rectangle against the adopted code's separation distances (property line, well, dwelling, water supply lines). The 18-foot property-line distance and 60-foot well distance may or may not comply depending on the edition in force - verify, do not assume.
- Depth to limiting condition. The redoximorphic features at 14 inches signal a seasonal high water table. Compare 14 inches to the code's required vertical separation beneath the infiltrative surface plus the system's depth. If the required separation is not met, that portion is out.
- Slope. A 12 percent slope may exceed the maximum allowed for conventional trenches; it may require a stepped or pressure-dosed design, or it may disqualify the area.
- Disturbance. The compacted/smeared zone can give falsely low permeability readings and must be avoided or reconditioned before testing.
- 100 percent replacement area. Confirm a second conforming area exists before approving the primary.
Only after these pass do you profile and test the soil in the area that remains.
2. The only open area meets setbacks but sits on the 12 percent slope with the water table at 14 inches. How do you structure your field investigation and reporting so the finding is defensible?
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Structure the investigation to document both the limiting condition and the usable depth:
- Profile pits, not just auger borings. Excavate at least one pit to the required depth in the proposed area and describe horizons, texture, structure, consistence, and the depth and character of redoximorphic features.
- Record the shallowest evidence, not the average. The 14-inch figure is the controlling number; note whether the features are depletions or concentrations and their abundance, which supports the interpretation of saturation.
- Account for slope in the vertical separation. On a slope, separation is measured from the lowest point of the infiltrative surface, so the downslope end governs.
- Test permeability only in undisturbed, representative soil, avoiding the smeared zone, and note the test method and date.
- Report the conclusion conditionally: state that the area is limited by seasonal saturation at 14 inches and by slope, cite the code sections for required separation and maximum slope, and recommend either a raised/alternative system with engineered design or a denial of the area. Document that the 100 percent replacement area was evaluated on the same basis.