Section 58.01.03.013 - LARGE SOIL ABSORPTION SYSTEM DESIGN AND CONSTRUCTION01.Site Investigation. A site investigation for a large soil absorption system by a soil scientist and/or hydrogeologist may be required by the Director for review and approval and shall be coordinated with the Director. Soil and site investigations shall conclude that the effluent will not adversely impact or harm the waters of the State.02.Installation Permit Plans. Installation permit application plans, as outlined in Subsection 005.04, for a large soil absorption system submitted for approval shall include provisions for inspections of the work during construction by the design engineer or his designee and/or by the Director.03.Module Size. The maximum size of any subsurface sewage disposal module shall be ten thousand (10,000) gallons per day. Developments with greater than ten thousand (10,000) gallons per day flow shall divide the system into absorption modules designed for ten thousand (10,000) gallons per day or less.04.Standard Large Soil Absorption System Design Specifications.a. All design elements and applications rates shall be arrived at by sound engineering practice and shall be provided by a professional engineer licensed by the state of Idaho and specializing in environmental or sanitary engineering.b. Within thirty (30) days of system installation completion the design engineer shall provide either as-built plans or a certificate that the system has been installed in substantial compliance with the installation permit application plans.c. Effective Soil Depths. Effective soil depths, in feet, below the bottom of the absorption module to the site conditions must be equal to or greater than the following table: TABLE -- EFFECTIVE SOIL DEPTHS |
Site Conditions | Design | Soil | Group |
Limiting Layer | A | B | C |
Impermeable Layer | 8 | 8 | 8 |
Fractured Bedrock, Fissured Bedrock or Extremely Permeable Material | 12 | 8 | 6 |
Normal High Groundwater Level | 12 | 8 | 6 |
Seasonal High Groundwater Level | 2 | 2 | 2 |
d. Separation Distances. The disposal area absorption module must be located so that the following separation distances given, in feet, are maintained or exceeded as outlined in the following table: TABLE -- SEPARATION DISTANCES |
Feature of Interest | Design | Soil | Group |
| A | B | C |
All Domestic Water Supplies |
Sewage Volume - 2,500-5,000 GPD | 250 | 200 | 150 |
Sewage Volume - 5,000-10,000 GPD | 300 | 250 | 200 |
Property Lines |
Sewage Volume - 2,500-5,000 GPD | 50 | 50 | 50 |
Sewage Volume - 5,000-10,000 GPD | 75 | 75 | 75 |
Building Foundations - Basements |
Sewage Volume - 2,500-5,000 GPD | 50 | 50 | 50 |
Sewage Volume - 5,000-10,000 GPD | 75 | 75 | 75 |
Downslope Cut or Scarp |
Impermeable Layer - Below Base | 100 | 50 | 50 |
Separation Distance - Between Modules | 12 | 12 | 12 |
e. No large soil absorption system shall be installed above a downslope scarp or cut unless it can be demonstrated that the installation will not result in effluent surfacing at the cut or scarp.f. A minimum of two (2) disposal systems will be installed, each sized to accept the daily design flow, and a replacement area equal to the size of one (1) disposal system will be reserved.g. The vertical and horizontal hydraulic limits of the receiving soils shall be established and flows shall not exceed such limits so as to avoid hydraulically overloading any absorption module and replacement area.h. The distribution system must be pressurized with a duplex dosing system.i. A geotextile filter fabric shall cover the aggregate.j. An in-line effluent filter between an extended treatment system or lagoon system and the large soil absorption area shall be installed.k. Observation pipes shall be installed to the bottom of the drainrock throughout the drainfield.l. Pneumatic tired machinery travel over the excavated infiltrative surface is prohibited.m. The drainfield disposal area shall be constructed to allow for surface drainage and to prevent ponding of surface water. Before the system is put into operation the absorption module disposal area shall be seeded with typical lawn grasses and/or other appropriate shallow rooted vegetation.05.Large Septic Tanks. Large Septic Tanks shall be constructed according to Section 007, except as outlined in this Subsection: a. Length to width ratios shall be maintained at least at a three to one (3:1) ratio.b. Tank inlet shall allow for even distribution of the influent across the width of the tank.c. The width to liquid depth ratio shall be between one to one (1:1) and two and one-quarter to one (2.25:1).06.Monitoring and Reporting. Before an installation permit is issued, a monitoring and reporting plan shall be approved by the Director and shall contain the following minimum criteria: a. Monthly recording and inspection for ponding in all observation pipes.b. Monthly recording of influent flows based on lapse time meter and/or event meter of the dosing system.c. Monthly recording of groundwater elevation measurements at all monitoring wells if high seasonal groundwater is within fifteen (15) feet of the ground surface.d. Semi-annual groundwater monitoring at all monitoring wells.e. Monitoring shall conform to the requirements of all federal, state, and local rules and regulations.f. An annual "Large Soil Absorption System Report" shall be filed with the Director no later than January 31 of each year for the last twelve (12) month period and shall include section on operation, maintenance and monthly and annual monitoring data.07.Operation and Maintenance. Before an installation permit is issued, an operation and maintenance plan shall be approved by the Director and shall contain the following minimum criteria: a. Annual or more frequent rotation of the disposal systems, and whenever ponding is noted.b. A detailed operation and maintenance manual, fully describing and locating all elements of the system and outlining maintenance procedures needed for operation of the system and who will be responsible for system maintenance, shall be submitted to the Director prior to system use.c. A maintenance entity shall be specified to provide continued operation and maintenance. Approval of the entity shall be made by the Director prior to issuance of an installation permit.Idaho Admin. Code r. 58.01.03.013