Replacement Drainfield Siting on a Sloping Lot With Redox Features
A homeowner wants to replace a failed 1970s trench drainfield on a 0.6-acre lot. The proposed 60-foot by 40-foot replacement area lies on an 8% slope. A test pit dug to 48 inches shows 12 inches of dark brown sandy loam over 10 inches of yellowish-brown sandy clay loam. Common, distinct redoximorphic concentrations begin at 16 inches; a firm fragipan begins at 34 inches. The soil scientist estimates the seasonal high water table at 14 inches. A percolation test in the upper horizon gives 45 minutes per inch. The nearest seasonal stream is 70 feet downslope, and the private well is 85 feet upslope. The local code requires 18 inches of unsaturated soil between the trench bottom and any limiting condition and 50-foot setbacks to streams and wells. The homeowner asks whether the area can be approved as-is.
1. Is the proposed replacement area suitable for a standard trench system as described? Explain your reasoning.
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No. The critical constraint is separation to a limiting condition. With the seasonal high water table estimated at 14 inches and redoximorphic concentrations beginning at 16 inches, a standard trench bottom at 24 to 30 inches would leave little or no unsaturated soil below the infiltrative surface. The code cited requires 18 inches of unsaturated soil between trench bottom and any limiting condition. The fragipan at 34 inches is a second limiting layer and may perch water above it. The 8% slope can be managed with contour trenching, but it increases the risk of downslope seepage and requires careful design. The 45 min/in percolation rate is workable but does not overcome the separation problem. Setbacks of 70 feet to the stream and 85 feet to the well exceed the stated 50-foot minimum, but local and state rules must still be confirmed. The area should not be approved as-is.
2. What additional evaluation and information would you require before issuing an opinion?
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Require a level 2 soil evaluation by a licensed soil scientist, including at least one additional test pit in the proposed area and one in a reserve area. Verify the seasonal high water table with monitoring wells or repeated observations, not a single morphological estimate. Confirm depth to bedrock or any other restrictive layer across the whole area. Evaluate slope above and below the area and the potential for surface runoff to pond. Check setbacks to property lines, water lines, wells, streams, floodplain, and any nearby drainage ditches. Percolation testing should be performed at the proposed trench bottom, not only in the upper horizon. Confirm that the area has not been compacted by equipment or used for parking. Obtain the local health department's current siting rules. A single test pit is not enough to approve or reject the entire 60-by-40-foot area.
3. How should the inspection report phrase the conclusion without overstepping?
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Use precise, conditional language: 'Based on one test pit, redoximorphic features at 16 inches and an estimated seasonal high water table at 14 inches do not provide the required 18 inches of unsaturated soil below a standard trench bottom. The area is not approvable as a standard trench system as proposed.' Then add: 'Final siting and design require a licensed soil scientist and a designer, and may require an engineered mound or alternative system with imported fill and pressure distribution.' Avoid words such as 'failed,' 'certified,' or 'approved.' State the data source, the depth and location of the test pit, and the limitation that conditions may vary across the area. Recommend that the homeowner reserve a separate replacement area and verify all setbacks with the local authority.