Standard Subsurface Disposal Facility Design And Construction

01. Site Suitability. The area where a standard drainfield will be constructed must meet the following 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 proposed drainfield and at depths between the bottom of the proposed drainfield and any limiting 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 IDAHO ADMINISTRATIVE CODE IDAPA 58.01.03 – Individual/Subsurface Sewage Department of Environmental Quality Disposal & Cleaning of Septic Tanks Rules Section 008 Page 17 (7-1-25) c. 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. (7-1-25) d. The drainfield must be located so the separation distances are maintained or exceeded according to the following table: Silty Clay Clay Clay Clay Clay soils with high shrink/swell potential Clay Organic mucks Claypan, Duripan, Hardpan EFFECTIVE SOIL DEPTHS Design Soil Group Limiting Layer A B C Impermeable Layer 4 4 4 Fractured Bedrock, or Extremely Perme- able Material 6 4 3 Normal High Groundwater Level 6 4 3 Seasonal High Groundwater Level 1 1 1 Feature of Interest Soil Types All A B C Public Water Supply 100 All wells and Other Domestic Water Supplies 100 Water Distribution Lines:  (not double-encased) (double-encased) 25 10 Permanent or Intermittent Surface Water other than Irrigation Canals & Ditches 200 125 100 Temporary Surface Water and Irrigation Canals and Ditches 50 Design Soil Group Soil Textural Classification USDA Field Test Textural Classification IDAHO ADMINISTRATIVE CODE IDAPA 58.01.03 – Individual/Subsurface Sewage Department of Environmental Quality Disposal & Cleaning of Septic Tanks Rules Section 008 Page 18 (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 in 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. (7-1-25) c. 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. (7-1-25) 03. Standard Subsurface Disposal Facility Specifications. Additional design specifications for new subsurface sewage disposal facilities. 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 Design Soil Group A B C Absorption Area - Gallons/Square Foot/Day 1.0 0.5 0.2 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 Width of Trenches 1 Foot Minimum 6 Feet Maximum Depth of Trenches 2 Feet Minimum 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 Feature of Interest Soil Types All A B C IDAHO ADMINISTRATIVE CODE IDAPA 58.01.03 – Individual/Subsurface Sewage Department of Environmental Quality Disposal & Cleaning of Septic Tanks Rules Section 009 Page 19 (7-1-25) 04. Wastewater Distribution. Systems must be installed to maintain equal or serial effluent distribution. (7-1-25) 05. Excavation. Trenches must not be excavated during the period of high soil moisture content when 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) or other acceptable permeable material. (7-1-25) 07. Aggregate. The trench aggregate must be crushed rock, gravel, or other acceptable, durable and 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 apply 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 any 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 the 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 and replacement of the system cannot meet the current rules the replacement system must meet the intent of the rules by using a system design specified by the Director. (7-1-25)