12-12-1987 Planning Packet - Laserfiche WebLink

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* Soils Information To provide for proper sewage treat ment, the soil treatment system must be located in suitable soil. Presently 31 counties have a detail ed soil survey map. An accelerated soil survey program is underway to com plete detailed soil surveys for the en tire state by 1990. These show the occurrence and distribution of each kind of soil and are a good starting point for the lot owner. The delineated areas on the soils map are called "map ping units" which consist primarily of the soil for which the unit is named and soils with similar characteristics. In ad dition there are areas within the unit consisting of soils that are different. Soil types occupying an acre or more within the mapping unit are indicated by a series of map symbols. The mini mum size of the mapping unit, how ever, depends on the scale of the prim ed map. The smallest mapping unit area designated is about 3 acres. This means that the soils map itself cannot be used to determine the suitability of a specific site. Yet, the soil survey in formation is still useful as background and an indication of potential problems which may be encountered on the lot. The local Soil and Water Conservation District can often provide unpublished soils information if a county lacks a published soil survey. The crests of knolls and hills as well as slightly sloping portions of hills are likely areas for placement of waste treatment systems. Depressions, drain age swales that collect runoff from the surrounding area, and excessively steep slopes (figure 1) should be avoided. Future landscaping plans must be considered to provide access to the site not only during the construction phase but afterward so the septic tank can be pumped periodically. Indicating two or three potential waste treatment sites on the lot pro vides additional flexibility if the pri mary site is unsuitable. Some sanitary ordinances require locating two areas suitable for a waste treatment system on a lot. After this preliminary information has been obtained and evaluated, the site evaluation in the field can begin. Site Evaluation To locate the sewage treatment sys tem properly, soil texture, the presence of soil mottling, direct water table measurement, and land slope should be thoroughly evaluated. The first step in conducting the site evaluation is to make a soil boring at least 3 feet deep er than the bottom of the proposed seepage trenches at the approximate center of the proposed system. The first boring determines the presence of soil mottling or a water table. Soil mottling indicates the pres ence and depth of seasonally occurring water tables during dry periods. Water tables can be measured directly during wet periods by leaving the boring hole open for at least 24 hours. Well-drajned soil is often brown or red, while poorly drained soil is gray. The color difference is due to the peri odic water saturation. Mottled soils consist of spots of gray and brown or gray and red. Where mottled soil oc curs, it is assumed the soil is saturated during wet periods (unless the area has been artificially drained). Depth to soil mottling determines the depth at which soil treatment trenches can be placed, since there needs to be a 3 foot vertical separation between the trench bottom and sea sonal water tables (figure 3). Areas of periodically saturated soils can disrupt operation of the sewage treatment system. When the trenches are installed in a saturated soil zone, effluent entering the system during wet periods cannot m.ove away and will either come to the surface or back up into the house. These saturated zones often are unrelated to the existing re gional water tables or surface waters because zones of unsaturated materials generally separate the two. Land slope should be estimated for the proposed system. Knowledge of the land slope assists in the system's design and in selecting the effluent dis tribution metfiod. Steeply sloping land areas should be avoided to reduce con struction problems. However, some contractors will install trenches along the contour on slopes as great as 30 percent. While such construction is dif ficult and costly, the sewage treatment system will function properly when correctly installed. Soil texture can be used to estimate the percolation rate which is then used to estimate the size of the treatment area which will need to be investigated. Figure 3. Interpretation of soil mottling. WELL DRAINED CAN MEASURE WATER table directly (Soil IS Never Soturoted) (In Boring Hole Left open for 24 Mrs .* ASSUME SATURATION AND PRESENCE OP v;atertaqle tOufinq W.i Peftodi) - Cro» Woter foDle m Soil Becomes Safuroted O Soil Ones os Water raOie IS Lowered Soil Docomes Re-soturoted es se Bnghi Colors Iron finely Dissolved Tnroughour Ihe Profilfe Dull Colors Fine Iron Porficles ore Oissoi**: .1 in the water Alternating ol Oey and Red Colors witri Iron Precipitated Neor Soil Pores .1 3