Home > 14 > 12

Title: 14 - Buildings and Constructions

Chapter: 12 - On-Site Sewage Disposal Systems

Section: 70 - Site evaluations

A site evaluation is the first step in the process of obtaining a permit to install a new on-site sewage system. Site evaluations are required to allow proper system design and to demonstrate compliance with site suitability criteria prior to issuance of a permit for a new on-site waste treatment and disposal system.

A.    General. The project proponent for any land division or any other project that proposes to use on-site wastewater treatment and disposal systems shall submit an evaluation report providing sufficient explanatory information to demonstrate the suitability of the project site for on-site sewage treatment and disposal.  The evaluation report must identify any assumption and limitations considered by the qualified professional preparing the evaluation.  The evaluation report shall be prepared pursuant to subsection (B) of this section.

B.    Site Specific Proposals. For project design, divisions of land and other site specific proposals, site evaluation reports shall comply with the provisions listed below and be based on two or more exploratory holes per parcel or proposed parcels, unless otherwise authorized by the health officer in writing.

1.    The engineer preparing the site evaluation report shall evaluate the site of the proposed system, shall consider all system options and shall provide a report of such evaluation.

2.    The site evaluation report shall be on or in a form approved by the county.

3.    The report shall contain, at a minimum the following:

a.    Narrative description. A narrative description of the proposed development including:

i.    Basic property description,

ii.   Proposed development,

 iii.   Site investigation methods and results,

iv.   Assumptions used in evaluation of the site or design of the on-site sewage system,

v.    Any significant site constraints or other notable features of the property,

vi.   Proposed water supply,

vii.  An estimate of the wastewater quantity and quality

b.    Site Evaluation Summary Sheet. This sheet should summarize technical information regarding the proposed building site with the following information:

i.    Property owner,

 ii.    Date, including date of all testing performed,

  iii.    Assessor\'s parcel map,

   iv.    Location,

v.    Lot size,

                 vi.    Water supply, including existing and proposed wells,

  vii.    Ground slope in disposal field and replacement area, including percent and direction,

   viii.    Setbacks to significant site features such as wells, streams, drainage channels, ponds or lakes and steep slopes or cut banks for the septic tank and leach field,

     ix.    Depth to groundwater or soil mottling,

x.    Other factors noted (unstable landforms escarpements, etc.),

   xi.    Test holes, soils texture results, percolation test results,

    xii.    Depth of suitable soil beneath the proposed leach drain field

c.    Plot Plan. A plot plan shall be attached showing to scale the following:

i.    Property boundary,

ii.    Test hole locations with distances to property lines and any significant identifying features, sufficient to plot these locations in the field and to be plotted on maps,

   iii.   Boundary of the primary and reserve leach field area,

                  iv.   Significant site features such as streams, wells, steep slopes, wet area, ponds, cut  

                        banks, etc

d.    System Design Sheet. A sheet showing the design parameters of the proposed primary and reserve leach field shall be provided with the following information:

 i.   Septic tank sizing,

ii. Trench length calculations,

iii. Trench cross-section,

iv. Design flow

e.    Site Testing Information. Attach all soils profiles, percolation test results and textural analysis reports completed on the parcel

f.    Site evaluation reports for all divisions of land shall be based upon an evaluation of each lot and shall clearly show that each proposed lot is suitable for an on-site sewage disposal system and meets the requirements of the North Coast Regional Water Quality Control Board Basin Plan in this chapter.

g.    Specific conditions or limitations imposed on an approved site shall be listed on the evaluation report

h.    Each page of each report shall contain a block in the upper right-hand corner which contains the follow­ing information:

i.    Assessor\'s parcel number,

  1.  Date of report
  2. iii.   Each site evaluation report shall contain a statement by the preparer stating that such report and design (if included) complies or fails to comply with the provisions of this chapter and the regional water quality control board current basin plan and such statement shall contain a "wet signature" across the registration seal of the preparer issued by the state. If not in compliance with this chapter, the report shall note what exceptions or waivers are necessary and include an opinion of the preparer that with the granting of the exception or waiver, the site would be suitable for on-site disposal.

C.    Site Evaluation Criteria. Site evaluation criteria shall be in accordance with the current guidelines contained in the applicable water quality control plan for the north coast region approved by the north coast regional water quality control board, and as listed below:

1.    Minimum setback distances for features of individual water treatment and disposal systems shall be as listed in Table A in Section 14. 12. 130.

2.    When the ten-year flood level cannot be determined, setbacks from streams shall be measured from the top of bank/bank dropoff to stream, whichever provides the greatest separation.

3.    Natural ground slope in all areas to be used for effluent disposal shall not be greater than thirty percent. Where less than five feet of soil exists below the trench bottom, ground slope shall not exceed twenty percent. All soils to be utilized for effluent disposal shall be stable.

4.a.  Soil depth is measured vertically to the point where bedrock, hardpan, impermeable soils or saturated soils are encountered

b.  Where ground slope is twenty percent to thirty percent, minimum soil depth immediately below the bottom of the leaching trench shall be five feet

c.  Where ground slope is less than twenty percent, a minimum soil depth of three feet immediately below the leaching trench may be permitted

d.  Lesser soil depths may be granted only as a waiver or for alternative systems.

5.    Minimum depth to the anticipated highest level of groundwater below the bottom of the leaching trench shall be determined according to soil texture and percolation rate as follows:


          Soil Texture*                             Depth to Groundwater

     (percent silt and clay)                        Below Leaching Trench

                                                          (in feet)

5 or less                                       40

6 to 10                                         20

11 to 15                                        10

Greater than 15**                               5

Greater than 15                                 2***


*     Must exist for a minimum of three continuous feet between the bottom of the leaching trench and groundwater.

**    Or a percolation rate slower than 5 MPI.

***   Granted only as a waiver or for alternative systems.

6.    The preferred percolation test method is the Ryan Standardized Test. Percolation test results in the effluent disposal area shall not be less than one inch per sixty minutes (sixty MPI) for conventional leaching trenches and one inch per thirty minutes (thirty MPI) for seepage pits. Percolation rates of less than one inch per sixty minutes (sixty MPI) may be granted only as a waiver or for alternative systems. Zone 2 soils may not be subject to percolation test requirements for the purpose of site suitability (see Section 14.12.070(D3)(5)(b)(2)).

7.    An adequate replacement area equivalent to and separate from the initial effluent disposal area shall be reserved at the time of site approval.

D.    Method of Site Evaluation. Site evaluation methods shall be in accordance with the current guidelines contained in the applicable water quality control plan for the north coast regional water quality control board.

1.    Site features to be determined by inspection shall include:

a.    Land area available for primary disposal system and replacement area

b.    Ground slope in the effluent disposal and replace­ment area

c.    Location of cut banks, natural bluffs, sharp changes in slope and unstable land forms with fifty feet of the disposal and replacement area:

d.    Location of wells, intercept drains, streams, and other bodies of water on the property in question and within one hundred feet on     adjacent properties.

2.    Soil characteristics shall be evaluated by soil profile observations. One backhoe excavation in the primary disposal field and one in the replacement area shall be required for this purpose. A third profile shall be required if the initial two profiles show dissimilar conditions. Augered test holes shall be an acceptable alternative:

a.    Where use of a backhoe is impractical because of access

b.    When necessary only to verify conditions expected on the basis of prior soils investigations or

c.    When done in connection with geologic investigations. Where this method is employed three test holes in the primary disposal field and three in the replacement area shall be required.

3.    In the evaluation of new subdivisions, enough soil profile excavations shall be made to identify a suitable disposal and replacement area on each proposed parcel. The following factors shall be observed and reported from ground surface to a depth of at least five feet below the proposed leach field system:

a.    Thickness and coloring of soil layers and apparent United States Department of Agriculture (USDA) classification

b.    Depth to and type of bedrock, hardpan, or im­permeable solid layer

c.    Depth to observed groundwater

d.    Depth to soil mottling

  1. Other prominent soil features such as structure, stoniness, roots and pores, dampness, etc.

4.    The anticipated highest level of groundwater shall be estimated:

a.    As the highest extent of soil mottling observed in the examination of soil profiles or

b.    By direct observation of groundwater levels during wet weather conditions.

Where a conflict in the above methods of examination exists, the direct observation shall govern. In those areas which, because of parent materials, soils lack the necessary iron compounds to exhibit mottling, direct observation during wet weather conditions shall be required. A dry weather test may be rejected by the county where an inconsistency exists between the report submitted and other information on file with the county, or field observation does not support in the judgment of the county, the con­clusions in the report submitted.

5.    Determination of a site\'s suitability for percolation of effluent shall be with one of the following methods:

a.    Percolation testing of soils within Zone 3 and 4 shall be conducted during wet weather conditions. Per­colation testing of soils falling within Zone 1 and Zone 2 may be conducted in non-wet weather conditions provided presoaking of the , test hole is accomplished with (1) a continuous twelve-hour presoaking, or (2) a minimum of four complete refillings beginning during the day prior to that of the conduct of the test.

b.    Soils from the limiting soil layer observed within the excavated soil profile shall be obtained and analyzed for texture and bulk density according to methods prescribed by the regional board. The results shall be plotted on the soil textural triangle of Figure 2 of the water quality control plan for the north coast region as per the indicated in­structions.

i.      Soils within Zone 1 shall be considered to have minimal filtration capabilities, requiring in­creased depths to groundwater as per Table 7 of the water quality control plan for the north coast region.

ii.     Soils within Zone 2 shall be considered suitable for effluent disposal without further testing.

iii.    Soils within Zone 3 and 4 shall require percolation testing as per Section 14.12.070(D) (5)(a) to verify suitability for effluent disposal.

6.          Wet Weather Criteria. Wet weather testing periods shall be determined by incorporating the following criteria as a minimum:

a. Between January 1st and April 30th and

b. Following ten inches of rain in a thirty-day period or after one-half of the seasonal normal precipitation has fallen.

     Extension of wet-weather testing beyond the limits of the above criteria may be made in accordance with a program of groundwater level monitoring approved by the regional board.

    The county may use wet-weather periods established by Humboldt County until such time as the county establishes its own program.

E.    Qualifications Necessary of Person Preparing Site Evaluation Reports. One of the following registrations with the state is required of the person(s) preparing a site evaluation report, who may prepare such evaluation reports to the extent permitted state law:

1. California Registered Civil Engineer

2. California Registered Environmental Health Specialist

3. California Registered Geologist or Soil Scientist.

F.    Validity - Technical Rule Changes Shall Not Invalidate a Site Evaluation. Design criteria changes may require use of a different kind of system or changes in the submitted design. Physical changes, such as but not limited to grading of the site, may invalidate a favorable evaluation.



(Ord. 2005-025B § 7, 8, 9, 10 2008)(Ord 2005-25B §§7,8,9 and 10, 2005 Ord. 88-34 § 2 (part), 1988.)