# IDAPA 58.01.03.008: Section 008. Standard Subsurface Disposal Facility Design And Construction > Idaho · Regulations · In force URL: https://www.frixlaw.com/law-library/statutes/STATE_ID_IDAPA_58_01_03_008 ## Section - **Citation:** IDAPA 58.01.03.008 - **Heading:** Section 008. Standard Subsurface Disposal Facility Design And Construction - **Jurisdiction:** Idaho - **Kind:** Regulations - **Status:** In force - **Text as of:** August 14, 2026 - **Source:** Compiled text - **Location:** Idaho Administrative Code / IDAPA 58 (Environmental Quality, Department of) / Chapter 58.01.03 / Section 008 ## Text 01. Site Suitability. The area where a standard drainfield will be constructed must meet the followin g conditions: (7-1-25) a. The natural slope of the site will not exceed twenty percent (20%). (7-1-25) b. Suitable soil types will be present at depths corresponding with the sidewalls of the propose d drainfield and at depths between the bottom of the proposed drainfield and any limiting layer (effective soil depth). Soil Textural Design Soil Group USDA Field Test Textural Classification 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 Soil Textural Design Soil Group USDA Field Test Textural Classification Classification Silty Clay Clay Clay Clay Clay soils with high Clay shrink/swell potential Organic mucks Claypan, Duripan, Hardpan (7-1-25) c. Effective soil depths, in feet, below the bottom of the drainfield must be equal to or greater tha n those values listed in the following table. EFFECTIVE SOIL DEPTHS Design Soil Group Limiting Layer A B C Impermeable Layer 4 4 4 Fractured Bedrock, or Extremely Perme- 6 4 3 able Material Normal High Groundwater Level 6 4 3 Seasonal High Groundwater Level 1 1 1 (7-1-25) d ripan, Hardpan (7-1-25) c. Effective soil depths, in feet, below the bottom of the drainfield must be equal to or greater tha n those values listed in the following table. EFFECTIVE SOIL DEPTHS Design Soil Group Limiting Layer A B C Impermeable Layer 4 4 4 Fractured Bedrock, or Extremely Perme- 6 4 3 able Material Normal High Groundwater Level 6 4 3 Seasonal High Groundwater Level 1 1 1 (7-1-25) d. The drainfield must be located so the separation distances are maintained or exceeded according t o the following table: Soil Types Feature of Interest A B C All Public Water Supply 100 All wells and Other Domestic Water Supplies 100 Water Distribution Lines: (not double-encased) 25 (double-encased) 10 Permanent or Intermittent Surface Water other than 200 125 100 Irrigation Canals & Ditches Temporary Surface Water and Irrigation Canals and 50 Ditches Soil Types Feature of Interest A B C All Downslope Cut or Scarp: Impermeable Layer Above Base 75 50 50 Impermeable Layer Below Base 50 25 25 Building Foundations: Crawl Space or Slab 10 Basement 20 Property Line 5 (7-1-25) 02. Subsurface Disposal Facility Sizing. The size of a subsurface disposal system is determined by the following procedures: (7-1-25) a. Daily flow estimates are determined in the same manner as flow estimates for septic tank sizing i n Subsection 007.08. (7-1-25) b. Total required absorption area is obtained by dividing the estimated maximum daily flow by a value below or as specified in the TGM. Design Soil Group A B C Absorption Area - Gallons/Square Foot/Day 1.0 0.5 0.2 (7-1-25) c. An acceptable site must be large enough to construct two (2) complete drainfields in which each ar e sized to receive one hundred percent (100%) of the design wastewater flow. (7-1-25) 03. Standard Subsurface Disposal Facility Specifications. Additional design specifications for ne w subsurface sewage disposal facilities Group A B C Absorption Area - Gallons/Square Foot/Day 1.0 0.5 0.2 (7-1-25) c. An acceptable site must be large enough to construct two (2) complete drainfields in which each ar e sized to receive one hundred percent (100%) of the design wastewater flow. (7-1-25) 03. Standard Subsurface Disposal Facility Specifications. Additional design specifications for ne w subsurface sewage disposal facilities. STANDARD SUBSURFACE DISPOSAL FACILITY SPECIFICATIONS Item All Soil Groups Length of Individual Distribution Laterals 100 Feet Maximum Grade of Distribution Laterals and Trench Bottoms Level 1 Foot Minimum Width of Trenches 6 Feet Maximum 2 Feet Minimum Depth of Trenches 4 Feet Maximum Total Square Feet of Trench 1,500 Sq.ft. Max. Undisturbed Earth Between Trenches 6 Feet Minimum Undisturbed Earth Between Septic Tank and Trenches 6 Feet Minimum STANDARD SUBSURFACE DISPOSAL FACILITY SPECIFICATIONS Item All Soil Groups 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 (7-1-25) 04. Wastewater Distribution. Systems must be installed to maintain equal or serial effluen t distribution. (7-1-25) 05. Excavation. Trenches must not be excavated during the period of high soil moisture content whe n that moisture promotes smearing and soil compaction. (7-1-25) 06. Soil Barrier. The aggregate must be covered throughout with a synthetic filter fabric (geotextile) o r other acceptable permeable material. (7-1-25) 07. Aggregate. The trench aggregate must be crushed rock, gravel, or other acceptable, durable an d inert material that is free of fines and has an effective diameter from one-half (1/2) to two and one-half (2 1/2) inches. (7-1-25) 08. Impermeable Surface Barrier. A treatment area trench or replacement area may not be: (7-1-25) a. Compacted. (7-1-25) b. Covered byan impermeable surface barrier; or (7-1-25) c. Used for parking or driving on, and must be adequately protected from such activities d inert material that is free of fines and has an effective diameter from one-half (1/2) to two and one-half (2 1/2) inches. (7-1-25) 08. Impermeable Surface Barrier. A treatment area trench or replacement area may not be: (7-1-25) a. Compacted. (7-1-25) b. Covered byan impermeable surface barrier; or (7-1-25) c. Used for parking or driving on, and must be adequately protected from such activities. (7-1-25) 09. 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. (7-1-25) a. Except as specified in this section, rules applicable to a standard subsurface disposal system appl y to an absorption bed facility. (7-1-25) b. Sites with slopes in excess of eight percent (8%) are not suitable for absorption bed facilities. (7-1-25) c. Distribution laterals within a bed may not be spaced on greater than six (6) feet centers, and an y sidewall may not be more than three (3) feet from a distribution lateral. (7-1-25) 10. Vehicle and Machinery Traffic. Rubber-tired vehicles and machinery may not be driven on th e bottom surface of any excavation or on the top of any drainfield. (7-1-25) 11. Failing Subsurface Sewage Disposal System. If the Director determines a system is failing an d replacement of the system cannot meet the current rules the replacement system must meet the intent of the rules b y using a system design specified by the Director. (7-1-25) ## Nearby sections - [IDAPA 58.01.03.000 Section 000. Legal Authority](https://www.frixlaw.com/law-library/statutes/STATE_ID_IDAPA_58_01_03_000.md) - [IDAPA 58.01.03.001 Section 001. Scope, Conflict, And Responsibilities](https://www.frixlaw.com/law-library/statutes/STATE_ID_IDAPA_58_01_03_001.md) - [IDAPA 58.01.03.002 Section 002. Referenced Material](https://www.frixlaw.com/law-library/statutes/STATE_ID_IDAPA_58_01_03_002.md) - [IDAPA 58.01.03.003 Section 003. Definitions](https://www.frixlaw.com/law-library/statutes/STATE_ID_IDAPA_58_01_03_003.md) - [IDAPA 58.01.03.004 Section 004. General Requirements](https://www.frixlaw.com/law-library/statutes/STATE_ID_IDAPA_58_01_03_004.md) - [IDAPA 58.01.03.005 Section 005. Permit And Permit Application](https://www.frixlaw.com/law-library/statutes/STATE_ID_IDAPA_58_01_03_005.md) - [IDAPA 58.01.03.006 Section 006. Registration Permits For Installers And Service Providers](https://www.frixlaw.com/law-library/statutes/STATE_ID_IDAPA_58_01_03_006.md) - [IDAPA 58.01.03.007 Section 007. Septic Tanks Design And Construction Standards](https://www.frixlaw.com/law-library/statutes/STATE_ID_IDAPA_58_01_03_007.md) - [IDAPA 58.01.03.008 Section 008. Standard Subsurface Disposal Facility Design And Construction](https://www.frixlaw.com/law-library/statutes/STATE_ID_IDAPA_58_01_03_008.md) - [IDAPA 58.01.03.009 Section 009. Other Components](https://www.frixlaw.com/law-library/statutes/STATE_ID_IDAPA_58_01_03_009.md) - [IDAPA 58.01.03.010 Section 010. Variances](https://www.frixlaw.com/law-library/statutes/STATE_ID_IDAPA_58_01_03_010.md) - [IDAPA 58.01.03.011 Section 011. Inspections](https://www.frixlaw.com/law-library/statutes/STATE_ID_IDAPA_58_01_03_011.md) - [IDAPA 58.01.03.012 Section 012. Violations And Penalties](https://www.frixlaw.com/law-library/statutes/STATE_ID_IDAPA_58_01_03_012.md) - [IDAPA 58.01.03.013 Section 013. Lsas](https://www.frixlaw.com/law-library/statutes/STATE_ID_IDAPA_58_01_03_013.md) --- Source: Frix Law Library, https://www.frixlaw.com/law-library/statutes/STATE_ID_IDAPA_58_01_03_008. Check the current official text before relying on it. Not legal advice.