18.21.160 - Soil technology and disposal method selection.
A. Prior to project building construction, a site evaluation, soil characterization study, and project component positioning must be performed by a registered professional engineer, or registered professional sanitarian. Site evaluation is a combination of field inspection, laboratory testing, and desk top analysis. It includes a thorough understanding of the codes and regulations governing the use of the site. The following are the required elements of the evaluation.
B. Topography. A site's topography relates to the changes in surface height over the site's total area. Topography can influence the choice of system used. For instance, pressure dosing may be the only type of system possible where the only acceptable location for the disposal area is at a higher elevation than the treatment unit. Grading plans to alter the topography may be used in order to evaluate whether the site can be used. Care must be taken to protect or replace the site's topsoil.
C. Soil Characteristics. The most important characteristics of soil are the ability to absorb fluid, provide adequate treatment, and convey the treated water underground. Permeability is the characteristic relating to ease of water movement through soil. The main properties indicative of absorption capacity are soil texture, (see Table 18.21.160(A)) structure, color, thickness of permeable strata and swelling characteristics.
1. Soil Properties. Soil properties are discussed in detail by the United States Department of Agriculture, Soil Conservation Service (USDA-SCS), in the soil survey report for El Paso County. This survey includes general soil maps which outline the areas of different soils onto aerial photos. General information is given regarding the soil's suitability for sewage effluent
[image omitted; see the official code]
disposal on a broad scale and can be used for preliminary evaluation.
2. Field Investigation. A careful field investigation must be made by persons trained or having qualified experience in soils science or on-site sewage disposal to determine the capability of a site to absorb and treat wastewater. The investigation should be done during the wettest season of the site. The soil evaluation should address the eight site characteristics listed in Table 18.21.160(B). A site plan must be submitted to the local Licensing Authority showing the proposed location of the various components of the on-site sewage system and the existing or future improvements, lot lines and any other item that restricts the choices of component locations.
D. Groundwater. Seasonal high groundwater (perched water) tables can exist in any area of the state. Under such conditions, it is possible to locate perched water near the ground surface during wet periods each year. This situation is the result of seasonal rain storm runoff permeating into a shallow soil mantle that lies upon an impermeable material like a solid rock or very dense clay. The mechanism for the removal of this water is by very slow draining to open channels or areas not influenced by clay or rock. Some moisture is taken upon by evaporation and transpiration of plants that are rooted into the permeable top soil.
E. Flooding. Usual site development requires that the structure be built on the highest portion of the site. The sewage system is developed in the remaining area of the site that is lower than the structure. Unfortunately, the lower area of a site may be subject to flooding as it naturally receives storm runoff from all areas upstream of it. Careful evaluation of flooding potential is necessary to determine whether flood preventative measures must be incorporated into the on-site sewage system. All of a soil absorption system must be constructed out of the flood-prone area, and not within areas subject to inundation or erosion by flood waters or rainfall runoff. An applicant for a permit to install a sewage system shall consult with the local flood plain ordinance administrator, county engineer, State Highways and Public Transportation Department, nearest river authority, Farmers Home Administration, Federal Emergency Management Agency, and other officials who may have information regarding the potential for flooding at the disposal site.
F. Solid and Fractured Rock. Solid or fractured rock underlying a thin absorptive soil mantle which is less than four feet thick poses two different problems to the on-site sewage system user. Solid material will reduce the absorptive capacity of a site while fractured rock may act as the mechanism for direct pollution of an aquifer that lies under the site. Percolation tests in these materials are unreliable and must not be used to size the sewerage system.
G. Caliche. Deposits of a white-to-pale yellow mineral form of calcium carbonate and related compounds of variable thickness and hardness that should be carefully inspected by a soils scientist or qualified local authority to determine site suitability for treated sewage absorption. Caliche has several forms that may or may not allow the site to be developed as a sewage absorption area. However, if a soils scientist or qualified local authority is unavailable, an evaportranspiration system can be installed if climatic conditions are appropriate.
H. Offsets from Property Lines. Minimum spacings from adjacent property owners must be adhered to. A common property line may be built upon with fencing or masonry walls. The area may serve as natural or artificial drainage for storm runoff. For these reasons, private on-site sewerage systems must not be built on these spacings, unless there is a written agreement between the adjacent property owners involved.
I. Clearances from Structures and Surface Improvements. Table 18.21.160(C) indicates clearance requirements relating to structures and surface improvements. Structure foundations or surface improvements, such as swimming pools, concrete curbs, landscaping, lawn sprinklers, concrete, asphalt, wood decks, or other types of materials must not be placed or planned for installation in any manner that will jeopardize the suitability of subsurface sewage disposal sites, unless a study by a registered professional engineer or a registered professional sanitarian is approved by the licensing authority.
Table 18.21.160(B)
Criteria for Soil Absorption of Sewage Effluent Developed for Site Specific Evaluations
Characteristic
Site Characteristic | Suitable | Provisionally Suitable* | Not Suitable
Topography | Slopes 0—15% | Slopes 15—30% | Slopes greater than 30%; Complex slopes
Subsoil Texture | Sandy soils; Loamy soils | Clayey soils with low shrink-swell potential | Clayey soils with high shrink-swell potential
Subsoil Structure | Angular or subangular; Blocky | Platy structure; Weathered rock; Massive clayey soils
Soil Depth | Weathered rock or consolidated bedrock greater than 48 inches below ground surface | Weathered bedrock or consolidated rock from 36 to 48 inches below ground surface | Weathered rock or consolidated bedrock less than 30 inches below ground surface
Restrictive Layer | None within 36 inches of the ground surface | Restrictive horizon within 36 inches of the ground surface or below the trench bottom
Soil Drainage | No drainage mottles within 36 inches of the ground surface | Drainage mottles (chroma 2 or less) within 36 inches of the ground surface
Flooding | Areas subject to a possible flood; depressional areas without adequate drainage
Soil Permeability | Greater than or 5 min/inch but less than or equal to 60 min/inch | Less than or equal to 5 min/inch or greater than 60 min/inch. Unselective fill materials groundwater
* Soils may be reclassified from unsuitable to provisionally suitable under certain conditions using acceptable site or system modifications.
Table 18.21.160(C)
Minimum Required Distances in Feet for Conventional Systems
From To: | Sewage Treatment Tanks | Lined Evaporation Beds | Soil Absorption Systems or Unlined Evaporation Beds | Sewer Pipe with Watertight Joints
Private water wells, underground cisterns and pump suction pipes | 50+ | 150*+ | 150*+ | 20+
Public water wells | 50 | 150* | 150 | 20
Water supply lines | 10 | 10 | 10 | 9
Streams, ponds and lakes | 60 | 75* | 75 | 20
Sharp slopes, breaks | 5 | var. | 50** | 5
Foundations, structures and surface improvements | 5 | 5 | 15 | —
Property lines | 10 | 10 | 10 | —
Easement lines | 1 | 1 | 5*** | —
Soil absorption systems | 5 | 5 | 20 | —
Swimming pools | 15 | 15 | 15 | —
* When a leak detection system as described in Sec. 301.13.(c)(3)(A)(ii)(IV)(-a-) is used, the minimum required distance is 50 feet to existing private water wells, cisterns and pump suction pipes.
Var. These minimum required distances may be varied if an innovative lined system is required by the permitting authority to be designed by a Registered Professional Engineer or Registered Professional Sanitarian.
** The absorption system's bottom must be a minimum of 50 feet from any break or outcropping ledges, unless it is designed by a Registered Professional Engineer or a Registered Professional Sanitarian having hydrogeological data of the strata below the system's site. Greywater disposal areas may be installed to within 25 ft. of the slope face.
*** A drainage easement having sloped sides greater than 30% or grade breaks will require adherence to the 50 foot criteria indicated by **.
+ This distance may be reduced to a minimum of 50 feet, for existing private water wells only, if the space between the existing private water well casing and the surrounding ground is filled with cement slurry that is pumped through a tube that extends to the required depth of sealing. This depth shall be at least two times the horizontal encroachment measurement but not more than the depth to the water producing strata. A three-foot-square by six-inch-thick concrete slab shall be poured around the casing.
J. Spacing with Other Utilities. Utility companies may have special restrictions that can be enforced onto on-site sewerage system installations. Safety of operations has been cited as a reason to maintain distance from buried electric and gas conduits. Safety to public health requires the separation of potable water piping from sanitary sewerage systems. Table 18.21.160(C) lists commonly used criteria, but each utility company serving the parcel should be consulted prior to installing the sewerage system even if no potential problems are evident.
K. Disposal System Selection. In designing a private sewage facility, several options concerning subsurface disposal are available. Table 18.21.160(D) has been prepared to aid in the selection of the proper system based on site evaluation, percolation rate, and lot size considerations. The table includes the systems generally recommended for subsurface disposal which are drainfields, absorption beds, and evapotranspiration beds. The purpose of Table 18.21.160(D) is to indicate the most feasible type of system to construct taking into consideration lot size, soil absorptive capacity, water conservation practices, and local climate.
1. System Selection. After determining the sizing of appropriate treatment tankage, the licensing authority shall confer with the applicant regarding the calculations to determine the bottom area required for trenches, an absorption bed system, and an evapotranspiration system. Generally, the system having the least number of square feet of bottom area will be the most economical. In most cases where adequate room is available, a trench system will be less costly.
2. Low Soil Permeability. In areas where soils have low permeability, it is possible to design a system which combines both soil absorption and evaportranspiration. Such systems are somewhat complicated and should be designed by a person trained in sewerage system design.
(Ord. 9448 (part), 1988)
