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Using Septic Mounds as Components of Alternative Septic Systems for Difficult Sites InspectAPedia®  -

• How to Design & Maintain Septic Mounds as Components of Alternative Septic Systems for Difficult Sites
• Septic mound systems for rocky or thin soil sites
• Special septic mound system designs needed for steep slopes
• Alternative septic system design guidelines

This document discusses the use of septic mound systems as an alternative septic system design. A septic mound is an engineered system involving fill to create a soil absorption system for the disposal of septic effluent.

Septic mound systems are used where local soils are not suitable for effluent disposal. In the sketch shown here, effluent is discharged to the septic mound by a pumping chamber. (Source: US EPA) Alternatively a septic mound, if suitably located, may be supplied effluent by gravity feed. Citation of this article by reference to this website and brief quotation for the sole purpose of review are permitted. Use of this information at other websites, in books or pamphlets for sale is reserved to the author. Technical review by industry experts has been performed and is ongoing - reviewers welcomed and are listed at "References."

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What is a Septic Mound System?

Mound Septic Systems are a wastewater absorption trench system which has been constructed using "suitable soil-fill material" which has been placed on top of the natural soil on a building lot. Mound systems are often confused with "raised septic bed systems" but have different design requirements, are generally smaller in total size, and depend on the fill material for successful wastewater treatment.

Raised bed septic systems are constructed in fill over soil which can accept septic effluent below the fill. Mound septic systems are constructed in fill over soil which does not acceptably treat septic effluent below the fill - all of treatment occurs in the mound. Other requirements differ as well, as described, for example, in the New York standard for mound systems Appendix 75-A.9 reproduced below.

When are Septic Mound Systems Needed?

Site conditions such as sandy (permeable) soil with high groundwater levels, soils with poor percolation rate, and very rocky sites preclude installation of a conventional septic leach field as they lack adequate depth of proper soil in which effluent treatment could occur. For these conditions, additional fill is brought to the site and installed in an area and depth sufficient that all of the effluent treatment occurs in the mound.

Effluent may enter the mound by gravity if site slopes permit, or effluent may have to be pumped up into the mound from a pumping station. In this latter case the pumping station would usually be in a second septic tank which receives clarified effluent from the primary tank. Mound systems were developed by the North Dakota Agricultural College in the 1940's [Kahn, Allen, Jones cited at "More Information" below] with further studies of effective mound designs in the 1970's.

Common Septic Mound System Problems

Septic Mound Design Errors

Inspecting mound systems in New York State I have often seen improperly constructed mounds including efforts by the builder to save on fill cost by using the "mound" as a place to first pile up all of the trees and tree stumps which needed to be cleared from the site, covering this mess with a too-thin layer of fill soil in which the septic absorption trenches are installed. The result is a mound system with a short life. If you see trees poking out from the perimeter of your mound further investigation may be in order. Also watch for effluent breakout around the bottom edges of the mound.

Improper mound siting is found at some properties where the builder fails to consider site runoff or natural groundwater paths. One mound system was constructed using too little fill and placed over what had been a stream bed. My septic dye test very quickly produced red-dyed effluent at the low end of the mound where a seasonal stream continued to run under the mound in wet weather.

Placing a mound septic system on a steep slope, over a stream bed, over a natural drainage swale (photo of this defect), specifying an under-sized mound for the anticipated usage level, and poorly installed piping which does not slope properly or which becomes disconnected, and finally, use of improper soil which lacks the proper percolation rate all result in a costly installation with a too-short life.

What Will Cause Septic Mound or other Drain Field Damage

Traffic: As with any septic absorption system, heavy traffic over the mound can compress soil or break pipes, rendering the system inoperative and requiring costly repairs. At a recent inspection I found that the septic mound had been placed over a roadway connecting two properties owned by the same family. Family members continued to drive back and forth between homes right over the septic mound. Its future life was rather doubtful. Driving vehicles over the septic mound or other drainfield, even for a single project such as construction of a nearby structure or performing other site work, is likely to damage the system and lead to need for costly repairs. Kahn et als. also advise keeping grazing animals, horses, cows, etc. off of mound systems.

Structures, Paving, Patios: Covering a septic mound, such as by installation of paving, a patio, or weed-blocking solid plastic, will damage the mound and prevent proper operation by blocking both oxygen intake into the soil and also evaporation of effluent from the soil top. Blocking or sealing the ground surface will be a problem over just about any septic effluent absorption field. Don't build anything over a septic mound, no building, no fish pond, no patio, no tennis court, no parking area, no playground (compacts the soil).

Quick Guide to Planting Trees, Shrubs, and Ground Cover over Septic Mound Systems

Grasses, weeds, flower are OK: Many people have written to ask me what can be planted over a septic field. The best answer is grass or native grasses and weeds. Flowers are ok so long as they are not varieties which send down deep roots. Basically, any shallow-rooting planting will be ok over the surface of a septic mound or over most other septic drainfields.

Stay away from trees or shrubs which are likely to put down deep roots. The roots will quickly invade and clog the buried effluent lines and may also cause them to move, break, or become disconnected. Some experts (Kahn, Allen, Jones) also point out that if you're planting on the edges and lower toe of a septic mound, those plants need to be able to tolerate the higher moisture levels found in those parts of the mound system.

Trees are not OK: Kahn, Allen, Jones (book cited below) that trees such as bamboo, willow, eucalyptus, cypress, Monterey pine, pepper, and walnut have particularly aggressive and deep roots. Keep such trees at least 100 feet from the septic field.

Ivy, Pachysandra, Similar Ground Covers are not OK: because these plants will reduce effluent evaporation from the mound soils and because their roots often invade and clog effluent distribution piping.

Septic Mound System Installation Costs

In New York State a septic mound over rocky soil may cost double a conventional leach field even where a pumping station is not required. (Perhaps $15,000 to $20,000). Septic mounds require a larger area than leach fields in good soil. If a pump station is required, some additional monitoring and maintenance is required, and of course if there is no electric power to run the pump, the size of the effluent holding tank will determine how many days of system operation are available in event of a power failure.

The following detailed specification for installation of a septic mound system is from: New York Appendix 75-A.9 Alternative Septic System Designs section 9.c Mound Septic Systems

Section 75-A.9 Alternative Septic Systems - © Mound Septic Systems Design Criteria

Title: Appendix 75-A.9 - Alternative Septic Systems [Regulation and System Design Criteria for Raised Septic Systems, Septic Mound Systems, Intermittent Sand Filter Bed Systems, Evaporation-Transpiration Septic Systems, Evaporation-Transpiration Absorption Septic Systems, and Other Alternative Septic Systems]
Effective Date: 12/01/1990

(1) General - Mound Septic Systems

A mound septic system (or septic mound system) is a soil absorption system that is elevated above the natural soil surface in a suitable fill material. It is a variation of the raised bed utilizing sandy fill material but not requiring a stabilization period prior to the construction of the absorption area.

On sites with permeable soils of insufficient depth to groundwater or creviced or porous bedrock, the fill material in the mound provides the necessary treatment of wastewater.

The overall size of the mound system will normally be substantially smaller than a raised bed.

[DF: Note: while they are similar in design "mound septic systems" discussed here are not identical to "raised septic systems" discussed at Raised Septics which have different site requirements.]

(2) Site Requirements for Mound Septic Systems

A mound system may be used where all the following conditions are found:

(i) The maximum high groundwater level must be at least one foot below the original ground surface.

(ii) Bedrock shall be at least two feet below the natural ground surface.

(iii) The percolation rate of the naturally occurring soil shall be faster than 120 minutes/inch.

(iv) The natural ground slopes shall not exceed 12%.

(v) All minimum horizontal separation distances can be maintained as described in Table 2.

(3) Design Criteria for Mound Septic Systems

(i) The designer shall consult with the health unit having jurisdiction regarding the method for detailing the hydraulic design.

(ii) The basal area of a mound system is defined differently than a raised bed. The basal area for a system on level ground includes all the area beneath the absorption trenches or bed and the area under the tapers.

Guide to Septic mound design notes for steep sloped sites

On a sloping site, the basal area includes only the area under the absorption trenches/bed and the lower or downhill taper. The basal area is designed upon the percolation of the naturally occurring soil. [Also see (iv) below for more steep slope design notes.]

Guide to Septic mound design notes for soils with too-fast percolation rates

Where the percolation rate is 60 min/in or faster, refer to Table 4B. For soils of 61 to 120 min/in, a rate of 0.2 gpd/sq. ft. shall be used for determining the minimum basal area required.

How soil percolation rates are tested for septic mound systems

(iii) Percolation tests for the fill material shall be conducted at the borrow pit. Only soils with a percolation rate between five and 30 minutes per inch shall be used for the fill material. Sands with greater than 10% by weight finer than 0.05 mm material must be avoided. At least 25% of the material by weight shall be in the range of 0.50 mm to 2.0 mm. Less than 15% of the material by weight shall be larger than a half-inch sieve. A sieve analysis may be necessary to verify this requirement. The required absorption area is based upon the percolation rate of the fill material as determined from Table 4B.

(iv) The system shall be designed to run parallel with the contours of the site. The width of the system (up and down the slope) shall be kept to a minimum, but in no case shall the absorption area be wider than 20 feet. In a distribution network using a center pressure manifold, distribution lines shall have a maximum total length of 200 feet.

In a network septic system mound drainage bed using an end manifold, distribution lines shall have a maximum length of 100 feet.

(v) Mound dimensions shall meet or exceed those required by the health unit having jurisdiction.

(vi) A pressure distribution network shall be required.

(vii) A dual chamber septic tank or two tanks in series in addition to the dosing tank shall be provided.

A gas baffle or other outlet modification that enhances solids retention is recommended.

(4) Construction Procedures for Mound Septic Systems

(i) Heavy construction equipment shall not be allowed within the basal area and area downslope of the system which will act as the dispersal area for the mound.

(ii) The vegetation shall not be scraped away, roto-tilled, or compacted. Excess vegetation shall be removed with trees cut at the ground surface but stumps left in place.

The area shall be plowed to a depth of seven or eight inches with a double bottomed blade/furrow plow and the furrow turned upslope.

(iii) The fill material is placed from the upslope side of the system to the full depth required in the design and shall extend to the edge of the basal area at a slope not to exceed one vertical to three horizontal.

(iv) The absorption area is then formed within the mound. A minimum of six inches of aggregate shall be placed beneath the distribution lines.

(v) The pressure distribution lines are placed parallel to the contours of the slope and a minimum of two inches of aggregate is placed above the lines.

(vi) A permeable geotextile is placed over the entire absorption area to prevent the infiltration of fines into the aggregate.

(vii) On sloping sites a diversion ditch or curtain drain shall be installed uphill to prevent surface water runoff from reaching the absorption area.

Curtain drains or intercept drains can protect septic drainfields in areas of wet soils or surface and subsurface groundwater

(viii) A minimum of six inches of finer materials such as clayey loam is placed over the top of the absorption area, and the entire mound including the tapers is then covered with six inches of top soil and seeded to grass.

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