1. GENERAL - The function of the leachfield is to dispose of the clarified wastewater from the septic tank into the ground. The relevant factors in designing a leachfield are: The depth from the surface of the ground to the water table (especially during the rainy season), the ability of the soil to soak up the water from the leachfield (percolation rate) and, the amount of land available on the subject parcel to install the leachfield. The following subsections list the requirements and procedures to be followed in designing a leachfield.
2. AMOUNT OF LEACHFIELD REQUIRED - Residential
The following table presents the minimum amount of leachfield required for residences per dwelling unit.
LEACHING AREA REQUIREMENTS - SQ. FT.
PERCOLATION RATE - MPI* MAXIMUM WATER USE
| |
1-5 |
6-30 |
31-60 |
61-120 |
(APPROX. GAL/DAY) |
| |
| 1 Bedroom |
500 |
600 |
900 |
2150 |
215 |
| 2 Bedrooms |
625 |
750 |
1125 |
2700 |
270 |
| 3 Bedrooms |
750 |
900 |
1350 |
3250 |
325 |
| 4 Bedrooms |
875 |
1050 |
1575 |
3750 |
375 |
| Additional Bedrooms |
125 150 225 ea. ea. ea. |
550 |
55 |
| |
*To the nearest whole MPI (Minutes Per Inch)
a. Determination of Soil Percolation Rate - The percolation rate range of the soil shall be estimated (if not already established by percolation tests) by the applicant on the application form for the purposes of estimating the proposed leachfield area required for the number of bedrooms desired. After the application is made, the parcel's soil types taken from the Soil Survey of Santa Cruz County will be noted by EHS staff on the application check sheet. A field visit to the parcel will be made by the EHS Specialist. A test hole excavation to observe soil texture characteristics (as well as a check for water table level) may be required. A percolation rate range will be assigned to the soils by the EHS specialist based on the Soil Survey and soil textures observed at the site. If concurrence on the estimated percolation rate range of the soil cannot be achieved among the inspector and the owner/contractor/consultant, a percolation test shall be performed by a licensed consultant or contractor familiar with the Santa Cruz County percolation test requirements. The results of the percolation test shall then be the basis for determining the leachfield area required.
b. Effective Leaching Area - The size of the leachfield is determined by the total of the areas of the trench bottom and sidewalls beneath the leach pipe. Example: A trench that is four feet deep, 1 ½ feet wide and 50 feet long with the leach pipe on top of 2 ½ feet of drain rock, has an effective leaching area of 325 sq. ft. There are 6 ½ sq. ft. (2 ½ + 1 ½ + 2 ½) of leaching area per lineal feet of trench:
6.5 sq. ft. per lineal ft. X 50 ft. = 325 sq. ft.
c. Expansion Area - In order to meet the requirements for a Standard System, a reserve expansion area for future leachfield repairs on the parcel must be designated on the approved plot plan for the septic system design. This expansion area must be capable of accommodating a duplicate of the approved leachfield. No construction of buildings, permanent swimming pools or other permanent structures shall be permitted over the expansion area.
3. AMOUNT OF LEACHFIELD REQUIRED - COMMERCIAL/INSTITUTIONAL The requirements for existing commercial/institutional establishments shall be determined based on an analysis of peak daily loading rates, using an absorption rate of 0.43, 0.36, 0.24 and 0.10 gallons per square feet of leaching area per day for soils percolating in the ranges 1-5, 6-30, 31-60 and 61-120 MPI, respectively. The applicant shall present at least two years of past water use records and a plausible projection of future peak daily wastewater flows if a change in property use which could result in increased wastewater loading is proposed. Note: For all large systems serving more than 5 residential units or having peak daily flows greater than 2500 gallons per day that are located within the San Lorenzo Watershed or a water supply watershed, enhanced treatment systems will be required. These systems will be considered Alternative Systems and can utilize increased application rates in the leachfield design (see Sections II.D and II.E).
4. LEACHFIELD CONSTRUCTION REQUIREMENTS FOR STANDARD SYSTEMS
Following are the requirements for a Standard Septic System. Owners of parcels that cannot accommodate all of the requirements of this section should refer to the Nonstandard System sections below. Please see Appendix A for a diagram of a standard leachfield.
a. Allowable Soil Percolation Rates
Soils in which the leachfield is constructed must percolate in the range of 1-120 MPI. Please see section B.2.a above for a description of the process to determine percolation rate range.
Note: For systems in sandy soils (those identified by the Soil Conservation Service as Zayante or Baywood Series, or any other soil with fast percolation rates between 1 and 5 MPI) enhanced treatment will be required within the San Lorenzo Watershed and designated Water Supply Watersheds for any new system or any system serving a bedroom addition, a remodel adding more than 500 square feet, or other expansion of use which will result in an increase in volume or strength of wastewater flow. Such systems will be Alternative Systems and can utilize increased application rates in the leachfield design (see Sections II.D and II.E)
b. Groundwater Separation Below Leachfield
The minimum separation between the bottom of any leaching device and seasonally high groundwater shall be:
- 5 feet where the leaching device is between 50 and 100 feet from a stream, spring, or other waterbody.
- 3 feet where the device is over 100 feet from a waterbody.
(At distances greater than 250 feet from a waterbody, a system with groundwater separation below the leachfield less than 3 ft. may be approved as a Limited Expansion System provided however that a separation of at least 1 ft. must be maintained for at least 90% of the year. This will be considered a Limited Expansion System for the purposes of building additions, but will not be subject to annual inspections and an annual fee.)
The definitive determination of depth to groundwater in the area where a leaching device is proposed shall be through observation of depth to groundwater by the EHS Specialist in a test hole excavation (or monitoring well) during the wet weather testing period as described in Section 7.38.120.B. During the rest of the year, the depth to groundwater will be estimated based on observation of test hole excavations and groundwater records maintained by the EHS.
c. Trench Depth
The standard trench depth shall be a maximum of 4 feet from the ground surface (2 ½ feet maximum effective depth). Parcels with soils that percolate in the range 6-60 MPI may use a deeper trench to a maximum of 6 ½ feet from the ground surface (5 feet maximum effective depth) if space on the parcel prevents the use of a standard trench depth. Parcels that have surface soils that percolate slower than 60 MPI and deeper soils that percolate faster than 60 MPI may use a deeper trench but shall only receive credit for the area of the trench in the acceptable percolation rate range. However, in all instances where a trench deeper than 4 feet is utilized, the trench shall be as shallow as possible using the maximum lineal feet that can fit on the parcel while still reserving the required expansion area.
d. Credit for Existing Trenches Deeper than Standard Trenches
When upgrading a septic system, credit shall be given for any functioning, existing trenches, as verified by a licensed septic pumper's report, that meet all of the requirements herein, with the possible exception of trench depth. Deeper trenches may be allowed where the soils percolate in the range 6-60 MPI. Where the soils percolate in the range 1-5 MPI, no exception to the 2 ½ feet effective trench depth for existing trenches shall be given unless treatment for nitrogen removal (such as a sand filter or other approved treatment) is provided. This must be done through the procedures for a Nonstandard System, as described in Section II.
e. Trench Width - Trenches shall be 18" to 36" in width. The trench bottom area plus the areas of the two sidewalls beneath the leach pipe is the effective leaching area (see section B.2.a above).
f. Maximum Slope - The maximum slope where leachfields may be approved is 30%, except that leachfields may be approved on slopes up to 50% under the following conditions: the leach pipe must be buried at least 2 feet, there must be at least 5 feet of soil percolating in the range of 1-120 MPI below the leachfield, and conditions must otherwise be suitable to prevent lateral surfacing of effluent.
g. Minimum Trench Spacing - The minimum spacing between trenches (edge to edge) shall be twice the effective depth to a maximum of 10 feet. h. Trench Setbacks - The minimum setback requirements between a leachfield and the following are:
| Septic tank |
3 feet |
| |
| Property line |
5 feet |
| |
| Foundations/decks |
5 feet |
| |
| Potable water pipe |
10 feet |
| |
| Well |
100 feet |
| Embankment greater than 67% |
2 times the height up to 25 feet; if an impermeable layer is encountered, setback shall be 4 times the height up to 50 ft. |
| |
| Stream, spring, waterbody |
100 feet if space permits but no less than 50 feet. |
| |
| Seasonal drainageway |
25 feet |
| (flows no more than one week
after significant rainfall)
|
| |
| Swimming pool |
10 feet |
| |
i. Trench Construction Details
General installation guidelines: The leach trench shall be excavated to the appropriate depth perpendicular to any slope, following the contour of the land so as to maintain the same trench depth for the length of the trench. Any smeared trench sidewalls shall be raked to roughen the surface to enhance percolation. The trench inspection riser pipe shall be placed vertically in the end of the trench and held in place vertically while the clean drain rock is placed in the trench. The top of the bed of rock and the leach pipe shall be leveled using a builders level or transit.
The leach pipe shall be placed on the level rock bed with the holes pointed down and a cap placed over the end of the pipe. When the pipe is level, drain rock shall be brought up to 2" in depth over the drain pipe. The perforated inspection riser pipe shall be cut to the rock level and a section of solid pipe attached to extend through the ground surface. Untreated building paper or straw shall be placed over all of the rock surface of the leachfield. Earth backfill shall be placed over the leachfield 8" - 12" deep, and shall be mounded to ensure drainage away from the trench. If settlement occurs after installation, additional backfill must be added. The inspection riser pipe shall be capped. Any disturbed soils shall be protected from erosion by mulching with straw and seeding with erosion control seed mix.
Additional Construction specifications are as follows:
| Maximum length of trench |
100 feet |
| |
| Slope of leach pipe |
should be level, but 3" in 100 ft maximum |
| |
| Rock over pipe |
2" thick |
| |
| Size of Rock |
½" - 2 ½" washed drain rock - dirty loads
will be rejected
|
| |
| Type of leach pipe |
3" or 4" coextruded styrene pipe |
| |
j. Trench Bottom Setback to Impermeable Rock or Soil Layer - There shall be at least 5 feet of soil that percolates in the range 1-120 MPI beneath the leachfield if a stream, spring, or cutbank is located within 50 feet, or 3 feet of permeable soil if the horizontal separation is more than 50 feet.
k. Trench Inspection Riser Pipes - Each distinct leach trench shall have a pipe placed vertically in the end of the trench to provide a means of monitoring the water level in the leachfield. The pipe shall be a perforated pipe that transitions to solid pipe at the top of the rock before exiting the ground. The pipe shall be extended to the bottom of the trench and held in place vertically while the rock is placed in the trench. A cap or female adapter with a threaded plug shall be provided at the end of the pipe. Concrete boxes with lids shall be provided around the pipe end where a flush to grade pipe end is desired.
l. Distribution of Effluent to Multiple Trenches - When there is more than one leach trench, an approved effluent distribution device shall be used to distribute effluent to each trench (see Appendix B for details). "Pop-over" distribution is not permitted.
m. Geological Hazards - Where there is local evidence of, or a history of, landslide activity, the EHS Specialist may require a geologic hazards assessment to ensure that any proposed leachfield will not contribute to a slope failure. The Planning Department technical review staff shall review and provide comment on all such required technical reports which address potential impacts on slope stability from proposed septic systems to serve new or existing development. The applicant shall pay a fee for such review as established by the Board of Supervisors.
n. Sewage Easements - Locations on nearby properties or right-of-ways may be used for sewage disposal for system repairs only, if a recorded easement is obtained according to specifications in Section 7.38.060. No increase in bedrooms shall be approved if an easement is needed for sewage disposal.
o. Leachfield Repairs and Upgrades in Floodplains or Floodways - Leachfield repairs may be permitted for parcels in floodways. Upgrades to support building additions are not permitted in the floodways.
p. Drainage Improvements - Roof drains and surface runoff shall be directed away from the disposal area so as to reduce soil saturation.
q. Site Restoration and Erosion Control - At the completion of the job, the site must be restored, with proper, stable disposition of excavated material and measures taken to prevent any significant erosion of surfaces disturbed during installation of the system.
5. EFFLUENT PUMPING
When effluent pumping is necessary to deliver the effluent to the leachfield, a 1000 gallon effluent pumping station tank shall be provided. The pump control switches shall be set so that 750 gallons of emergency storage is available during power outages. Pump controls shall also be installed to limit the amount of effluent that can be pumped to the leachfield in one day to 120% of the daily design flow so as to prevent overloading the leachfield after an extended power outage. Any pump up system which has less than 300 gallons storage capacity in the pump chamber shall be considered a Low-Flow System. A system with 300-750 gallons storage capacity shall be considered a Limited Expansion System. A handout is available from EHS that describes the detailed effluent pumping requirements.
6. SEEPAGE PITS
The use of seepage pits to repair or upgrade a septic system for the purposes of bedroom additions shall only be permitted where there is a previously installed seepage pit and all of the soil, groundwater and other setback requirements above are met. However, if there is room to accommodate a standard leachfield, that shall be the preferred leaching device. Seepage pits must meet Basin Plan requirements. On parcels where a seepage pit has not been installed in the past, and standard leachfields cannot be installed, seepage pits may be installed provided they meet the requirements contained in the Regional Board's Basin Plan. In this instance, no bedroom increases shall be approved.
7. GRAVELLESS LEACHFIELDS
The use of gravelless leachfields may be permitted as provided in the chamber leaching regulations promulgated by the Health Officer. However, all standards regarding the location and placement of leaching devices shall apply to the gravelless method of effluent disposal.