Section 58.01.03.008 - STANDARD SUBSURFACE DISPOSAL FACILITY DESIGN AND CONSTRUCTION01.Standard Drainfield. A drainfield consisting of an effluent sewer, one (1) or more aggregate filled trenches and a gravity flow wastewater distribution system. These standards will be the basis of acceptable design and configuration. Overall dimensions of a specific facility will depend upon site characteristics and the volume of wastewater.02.Site Suitability. The area in which a standard drainfield is to be constructed must meet the conditions stated in this subsection: a. Slope. The natural slope of the site will not exceed twenty percent (20%).b. Soil types. Suitable soil types must be present at depths corresponding with the sidewalls of the proposed drainfield and at depths which will be between the bottom of the proposed drainfield and any limiting soil layer (effective soil depth). Design Soil Group | Soil Textural Classification | USDA Field Test | Textural Classification |
Unsuitable | Gravel | 10 Mesh | |
| Coarse Sand | 10-35 Mesh | Sand |
A | Medium Sand | 35-60 Mesh | Sand |
| Fine Sand | 65-140 Mesh | Sand |
| Loamy Sand | | Sand |
B | Very Fine Sand | 140-270 Mesh | Sand |
| Sandy Loam | | Sandy Loam |
| Very Fine Loamy Sand | | Sandy Loam |
| Loam | | |
| Silt Loam | | Silt Loam |
C | Silt | | Silt Loam |
| Clay Loam | | Clay Loam |
| Sandy Clay Loam | | Clay Loam |
| Silty Clay Loam | | Clay Loam |
Unsuitable | Sandy Clay | | Clay |
| Silty Clay | | Clay |
| Clay | | Clay |
| Clay soils with high shrink/swell potential | | Clay |
| Organic mucks | | |
| Claypan, Duripan, | | |
| Hardpan | | |
c. Effective Soil Depths. Effective soil depths, in feet, below the bottom of the drainfield must be equal to or greater than those values listed in the following table. EFFECTIVE SOIL DEPTHS TABLE |
Site Conditions | Design | Soil | Group |
Limiting Layer | A | B | C |
Impermeable Layer | 4 | 4 | 4 |
Fractured Bedrock, Fissured Bedrock or Extremely Permeable Material | 6 | 4 | 3 |
Normal High Groundwater Level | 6 | 4 | 3 |
Seasonal High Groundwater Level | 1 | 1 | 1 |
d. Separation Distances. The drainfield must be located so that the separation distances given be maintained or exceeded according to the following Table: Feature of Interest | Soil Types All | A | B | C |
Public Water Supply | 100 | | | |
All Other Domestic Water Supplies including Springs and Suction Lines | 100 | | | |
Water Distribution Lines: Pressure Suction | 25 100 | | | |
Permanent or Intermittent Surface Water other than Irrigation Canals & Ditches | | 300 | 200 | 100 |
Temporary Surface Water and Irrigation Canals and Ditches | 50 | | | |
Downslope Cut or Scarp: Impermeable Layer Above Base Impermeable Layer Below Base | | 75 50 | 50 25 | 50 25 |
Building Foundations: Crawl Space or Slab Basement | 10 20 | | | |
Property Line | 5 | | | |
03.Subsurface Disposal Facility Sizing. The size of a subsurface disposal system will be determined by the following procedures:a. Daily flow estimates should be determined in the same manner as are flow estimates for septic tank sizing in Subsection 007.08.b. The total required absorption area is obtained by dividing the estimated daily flow by a value below. Design Soil Group | A | B | C |
Absorption Area - Gallons/Square Foot/Day | 1.0 | 0.5 | 0.2 |
c. Required Area. The size of an acceptable site must be large enough to construct two (2) complete drainfields in which each are sized to receive one hundred percent (100%) of the design wastewater flow.04.Standard Subsurface Disposal Facility Specifications. The following table presents additional design specifications for new subsurface sewage disposal facilities. SUBSURFACE DISPOSAL FACILITY TABLE |
Item | All Soil Groups |
Length of Individual Distribution Laterals | 100 Feet Maximum |
Grade of Distribution Laterals and Trench Bottoms | Level |
Width of Trenches | 1 Foot Minimum 6 Feet Maximum |
Depth of Trenches | 2 Feet Minimum 4 Feet Maximum |
Total Square Feet of Trench | 1500 Sq.ft. Max. |
Undisturbed Earth Between Trenches | 6 Feet Minimum |
Undisturbed Earth Between Septic Tank and Trenches | 6 Feet Minimum |
Depth of Aggregate: | |
Total | 12 In. Minimum |
Over Distribution Laterals | 2 In. Minimum |
Under Distribution Laterals | 6 In. Minimum |
Depth of Soil Over Top of Aggregate | 12 In. Minimum |
05.Wastewater Distribution. Systems shall be installed to maintain equal or serial effluent distribution.06.Excavation. Trenches will not be excavated during the period of high soil moisture content when that moisture promotes smearing and compaction of the soil.07.Soil Barrier. The aggregate will be covered throughout with untreated building paper, a synthetic filter fabric (geotextile), a three (3) inch layer of straw or other acceptable permeable material.08.Aggregate. The trench aggregate shall be crushed rock, gravel, or other acceptable, durable and inert material which is, free of fines, and has an effective diameter from one-half (1/2) to two and one-half (2 1/2) inches.09.Impermeable Surface Barrier. No treatment area trench or replacement area shall be covered by an impermeable surface barrier, such as tar paper, asphalt or tarmac or be used for parking or driving on or in any way compacted and shall be adequately protected from such activities.10.Standard Absorption Bed. Absorption bed disposal facilities may be considered when a site is suitable for a standard subsurface disposal facility except that it is not large enough. a. General Requirements. Except as specified in this section, rules and regulations applicable to a standard subsurface disposal system are applicable to an absorption bed facility.b. Slope Limitation. Sites with slopes in excess of eight percent (8%) are not suitable for absorption bed facilities.c. Vehicular Traffic. Rubber tired vehicles must not be driven on the bottom surface of any bed excavation.d. Distribution Lateral Spacing. Distribution laterals within a bed must be spaced on not greater than six (6) feet centers nor may any sidewall be more than three (3) feet from a distribution lateral.11.Seepage Pit. Seepage pit disposal facilities may be used on a case by case basis within the boundaries of District Health Department Seven when an applicant can demonstrate to the district director's satisfaction that the soils and depth to ground water are sufficient to prevent ground water contamination. The district director shall document all such cases. a. General Requirements. Except as specified in Subsection 008.11.b., rules and regulations applicable to a standard subsurface disposal system are applicable to a seepage pit.b. Other conditions for approval, sizing and construction will be as provided for in the seepage pit section of the Technical Guidance Manual for Individual and Subsurface Sewage Disposal, except that the site size restriction in condition two (2) of the Conditions for Approval will not apply.12.Failing Subsurface Sewage Disposal System. If the Director determines that the public's health is at risk from a failed septic system and that the replacement of a failing subsurface sewage disposal system cannot meet the current rules and regulations, then the replacement system must meet the intent of the rules and regulations by utilizing a standard subsurface sewage disposal design or alternative system design as specified by the Director.Idaho Admin. Code r. 58.01.03.008