Deep Hole Soil Test for Septic Design
Test pits dug, logged for mottling, groundwater and rock, and read by a New York licensed PE before the septic design is drawn. Onondaga, Oswego, Oneida, Madison, Cayuga and Cortland counties.
Soil Layers. Water Table. Rock.
What the test pit has to show before a septic plan is drawn.
Appendix 75-A requires at least one test hole at least 6 feet deep within or right next to the proposed leaching area. The NYS DOH design handbook says that for absorption fields the hole should preferably reach 4 feet below the bottom of the planned trenches. For seepage pits the hole goes at least 3 feet below the deepest proposed pit under Appendix 75-A.
At least one in or immediately next to the proposed absorption area. If the soil profile changes from hole to hole, more pits are dug and spaced to show whether there is enough useable soil. The handbook says one may also be required in the reserve area, and warns against digging so many pits that the natural soil in the treatment area is disturbed.
Mottling is a pattern of gray and rust colored spots in the soil. The NYS DOH handbook explains that gray colors form where soil stays saturated and yellowish brown colors where it stays aerated, so mottling marks the depth groundwater reaches during part of the year. That depth is treated as the seasonal high groundwater level for design.
Appendix 75-A requires at least 4 feet of useable soil above rock, unsuitable soil and seasonal high groundwater for a conventional absorption field. The highest groundwater must be at least 2 feet below the trench bottom.
Conventional trenches are out, but other designs may fit. The handbook's site table lists shallow trenches at 2 feet of useable soil, cut and fill at 3 feet, and raised systems and mounds at 1 foot of in place soil. For a mound the table also asks for 3 feet between the bottom of the absorption area and bedrock, with at least 2 feet of natural soil above the rock.
In some places, yes. In the Skaneateles Lake watershed a City of Syracuse watershed inspector must witness all site and soil appraisals. Madison County Public Health lists observing deep hole tests as one of its services. Onondaga County requires the soil profile to 6 feet to be shown on the plan.
Groundwater in New York is normally highest from March 15 to June 30, per the NYS DOH handbook. Holes dug in that window show free water at its worst. Outside it, the designer relies on mottling, which the handbook notes can be hard to see in some soils in summer.
Deep hole soil test for septic design
A deep hole test (also called a deep test pit or test hole) is an excavation at least 6 feet deep dug where the septic field will go. The designer climbs out with a log of every soil layer, the depth to mottling, any seeping water, and the depth to rock or dense clay. That log tells the Professional Engineer which systems the site can legally hold before a single perc number is looked at.
Every new septic design in New York needs at least one. Most Central New York lots need it before a mortgage, a building permit or a land closing goes forward.
What the deep hole has to find
The NYS DOH Residential Onsite Wastewater Treatment Systems Design Handbook says deep test holes or borings are used to find five things:
- Dense or restrictive layers. Clay, fragipan, shale, argillite and cemented limestone limit how far effluent can move down.
- Seasonal high groundwater. Appendix 75-A requires the highest groundwater level to be determined, including the type of water table: perched, apparent or artesian.
- Depth to bedrock. Fractured or channeled rock under thin soil can carry effluent to wells, streams and lakes too fast.
- The soil types penetrated. Texture (sand, loam, silt loam, clay loam, clay) and structure are logged layer by layer.
- Other features. Root systems, old farm drains and anything else that would affect the system.
How deep, and how many
Appendix 75-A, section 75-A.4(c), sets the rule: at least one test hole, at least six feet deep, within or immediately next to the proposed leaching area. If the holes show different soil profiles, more holes are dug and spaced to prove there is enough useable soil for the whole system. The design has to reflect the most severe conditions found.
- Absorption fields. The handbook says at least 6 feet, preferably 4 feet below the bottom of the proposed trenches or bed, so the sidewalls show mottling below the trench.
- Reserve area. At least one deep hole may also be required in the expansion area. Appendix 75-A asks for an extra 50 percent of useable area to be set aside whenever possible.
- Seepage pits. Appendix 75-A requires 3 feet of useable soil below the pit bottom, confirmed by a hole at least 3 feet below the deepest pit. The handbook says seepage pits shall not be used where trenches will work.
- Do not overdig. The handbook warns against digging too many pits inside the treatment area because it changes the natural soil.
Onondaga County's design guidelines add that the test pit results go on the plan as a soil profile description to 6 feet, with special attention to seepage, water tables, impermeable layers and bedrock.
Reading mottling and seasonal high groundwater
Most failed fields in wet Central New York ground were put too close to the seasonal water table. A septic field has to stay unsaturated so air can reach the soil. The deep hole is how the designer finds that line.
The NYS DOH handbook explains the color code. Gray soil colors form where iron and manganese sit in saturated, oxygen poor conditions. Yellowish brown colors form where the soil stays aerated. Soils with a high water table for part of the year show mottling at the high water mark and below. The handbook also warns that mottling can be hard to see in summer in some soils because of the soil's own color.
Timing matters too. The handbook puts the normal spring high groundwater period at March 15 to June 30 and says droughts and wet years move it. In some soils mottling does not show at all, and the only accurate read is to watch free water in an open hole or shallow monitoring well during that spring window. Plants help confirm a wet site: the handbook names some ferns, cattails and purple loosestrife.
In the Skaneateles Lake watershed, 10 NYCRR 131.1 says required soil analyses should be done between about March 15 and June 30. If they are done at another time, mottling readings set the high groundwater level, and observation during a wet period may be required if questions come up.
Depth to rock and the separation each system needs
Here is the soil each system type needs, from Table 4 of the design handbook (Appendix 75-A site requirements):
| System | In-place useable soil needed | Separation from trench bottom to groundwater, mottling, rock or impermeable layer |
|---|---|---|
| Conventional trenches | 4 ft | 2 ft |
| Deep absorption trenches | 4 ft | 2 ft |
| Shallow absorption trenches | 2 ft | 2 ft |
| Cut and fill | 3 ft | 2 ft |
| Absorption bed | 4 ft | 2 ft |
| Seepage pit | 3 ft below pit | 3 ft below pit |
| Raised system | 1 ft | 2 ft if a dosing device is used (3 ft of useable soil without dosing, with local inspection program) |
| Mound | 1 ft | 2 ft to groundwater, 3 ft to bedrock, with at least 2 ft of natural soil over bedrock |
Over drinking water aquifers the local health department may require more separation to limestone, karst or shale bedrock, and the handbook says fields should not be built over visible cracks, crevices or sinkholes. Onondaga County's guidelines say up to 4 feet of suitable soil between the system bottom and bedrock may be necessary where needed to protect underground aquifers.
Problem and solution: what the pit log usually leads to
Mottling at 20 inches in silt loam. There is not 4 feet of useable soil, so conventional trenches are out. If there is at least a foot of useable in-place soil and the slope is in range, a raised system or mound builds the separation up with fill. On a sloped site the handbook shows curtain drains upslope of the field to cut off groundwater. See high water table.
Shale at 30 inches. A shallow trench design needs 2 feet of useable soil, and a mound needs at least 2 feet of natural soil over bedrock. See shallow bedrock.
Dense clay or fragipan under good topsoil. The perc may look fine in the top 2 feet, but the pit shows the restrictive layer that controls the design. Appendix 75-A says when perc results and the field soil disagree, more tests are run and the more restrictive one controls. See clay soil.
Different soils across the lot. More pits, spaced to map where the useable soil actually is, then the field goes in the best spot that still meets the setbacks in Table 2 of Appendix 75-A.
Safety on test pits
The NYS DOH handbook opens its design guidance with a construction safety chapter. It warns that excavations as shallow as 5 feet have caused injury and death, and that safe working conditions on site are the contractor's responsibility. Backwell digs test pits with an excavator, keeps people out of an unprotected pit, and backfills the hole when the log is done.
Who witnesses the pit
- Skaneateles Lake watershed (parts of Skaneateles, Spafford and Marcellus in Onondaga County, plus parts of Cayuga and Cortland counties): a City of Syracuse watershed inspector must witness all site and soil appraisals under 10 NYCRR 131.1.
- Onondaga County outside the watershed: the design professional is responsible for the site evaluation and soil testing, and county technicians may run confirming tests in suspect areas.
- Madison County: Public Health lists observing percolation and deep hole tests among its services.
- Cayuga County: the design professional must perform the deep test hole evaluation before preparing plans, and a county inspector visits the site to confirm conditions.
- Cortland County: the engineer conducts the percolation and deep hole tests.
Timeline and cost
The pit itself is dug, logged and backfilled in a day on most lots. The wait is the calendar: frozen ground, the spring high water window, and any witness scheduling. National cost guides put a deep hole percolation test by an engineer at up to $2,000 (HomeGuide, 2026). County fees vary. Oneida County lists $150 for an individual sewage site survey, and Onondaga County lists a $93 site investigation fee. The full fee breakdown is on the septic engineer cost page.
The pit log goes straight to the PE with the perc results from the perc test, and the design follows the steps on the septic engineering process page.
Designs are prepared and stamped by a New York licensed Professional Engineer (PE). Backwell coordinates the design, the health department submittal, and the install.
Sources
- NYS DOH, 10 NYCRR Appendix 75-A, Wastewater Treatment Standards: Residential Onsite Systems
- NYS DOH, Residential Onsite Wastewater Treatment Systems Design Handbook (includes Appendix 75-A and Fact Sheet #1)
- Onondaga County Health Department, Design Guidelines for Individual Sewage Systems
- 10 NYCRR 131.1, Skaneateles Lake watershed rules and regulations
- Madison County Public Health, Water and Septic: Individual Systems
- Cayuga County Health Department, Instructions for New Construction
- Cortland County Health Department, Sewage Systems
- Oneida County Health Department, Plan Review Fee Schedule (rev. 3/2022)
- Onondaga County Health Department, Fee Schedule
- HomeGuide, Septic Tank System Cost (2026)
