Site Evaluation: Large Septic Systems

Large On-Site Systems

Large on-site systems require a more-detailed site evaluation to more accurately determine the LT AR. A standard description of what a site and soil evaluation would be for a large system is not possible. Rather, this section describes a typical scenario that can aid in understanding the correct approach to large system site investigations.

The site and soil investigation must include more detailed descriptions and assessments of soil morphology and measurements of the saturated hydraulic conductivity (Ksat) of one or more soil horizons at that site. The location and number of Ksat measurements will vary with each site, depending upon the site conditions and the water flow regime at the site. See Section 4.5 under Additional evaluation factors - Large on-site systems for more details on using either the constant-head permeameter test or the auger-hole pump-out test to measure Ksat. Other tests may also be appropriate, such as aquifer pump tests, for large systems in coastal areas. The first part of Section 4.5, Guidelines for a site evaluation, provides guidance for performing a site evaluation for a large system.

Figure 4.6.3 illustrates a typical site in the Piedmont or Mountain region where a large on-site system is proposed. The soil here is deep and well drained. It has eight soil horizons down to a depth of 12 feet below the ground surface. The landscape position is an interfluve with a 4% slope of the ground surface.

  • A large, modified conventional system with trenches 2 to 3 feet deep is proposed for the site. A much deeper than usual assessment of soil properties is required- down to 12 feet - than is necessary for small, single family systems (usually only to 4 feet).

  • The horizons should be grouped on the basis of the similarity of their characteristics. For this example, there would be three major horizon groupings: horizon H1 (the Ap, BA, Btl, and Bt2 horizons), horizon H2 (includes the Bt3 and B/C horizons) and horizon H3 (includes the Cl and C2 horizons) .

    Three factors that could control the direction and quantity of flow of effluent from the proposed system were identified at this site.

    • The first factor is the rate of sewage effluent that can leave the trenches through the biomat at the infiltrative surface. This rate is determined by the LTAR of the biomat that forms in the BA and Btl horizons. The LT AR can be estimated from Tables 4.6.1, 4.6.2, or 4.6.3, or it can be estimated by measuring the Ksat of the horizons at the infiltrative surface (the BA and Btl horizons) and by assuming a percentage reduction in Ksat due to the biomat. Biomats typically reduce flow rates to 1-10% of the saturated hydraulic conductivity, although the percentage reduction can be even greater on some rapidly permeable sands. The amount of infiltrative surface (in this case, the amount of trench bottom area) needed to accommodate the maximum design flow from the proposed facility after a biomat forms should be determined. If pretreatment (such as with a pressure-dosed sand filter) is used to reduce BOD and suspended solids, the biomat will not form as extensively and the LT AR can be increased for the infiltrative surface. Likewise, if waste strength will exceed that of typical domestic wastewater, such as at food service facilities, the LTAR should be decreased.

    • Next, the most slowly permeable horizon in the soil should be identified because this horizon will potentially control the flow of wastewater through the site. In this example, the bottom of the Bt horizon and the transitional B/ C horizon were identified as the horizons most restrictive to vertical water movement. Therefore, Ksat measurements should be made in these horizons. Since the soil is well drained and these horizons are well above the water table, the constant-head permeameter test would be an appropriate method to measure Ksat. The number of Ksat measurements needed will depend upon the degree of variability of the soil horizons of interest (in this case, the Bt3 and B/C). If there is only a small amount of soil variability, then one to two Ksat measurements for each 1,000 ft2 of total drainfield area (not trench bottom area) may be sufficient. This is, however, just an estimate of the number of measurements needed for a site where a large system is proposed. Each site will be different.

    • The measured Ksat values in the most slowly permeable horizon (H2 in this example) should be compared to the estimated LT AR of the horizon where the infiltrative surface will be placed (H1 in this example). If the LTAR of H1 is more than two or three times greater than the Ksat of "2. then the ultimate rate of flow through the soil would be controlled by saturated flow through H2, not wastewater infiltration into H1• Therefore, the system size should be increased beyond the size indicated by the LT AR of H1. On the other hand, if the LTAR of H1 is nearly equivalent to or less than the Ksat of "2, then the ultimate rate offlow through the soil would be controlled by saturated flow through the biomat at the infiltrative surface, and all vertical flow through H2 and H3 beneath the infiltrative surface would occur as unsaturated down to the ground water table. In this case, the system size would be determined by the LTAR ofH1

    • The third factor that may be identified as potentially controlling flow at this site is the rate oflateral flow through H1 in the downslope direction. Perching of water on a slowly permeable horizon and flow over that horizon is more likely to occur as the Ksat of "2 becomes smaller in comparison to the Ksat of He Usually a 10-fold reduction in Ksat over a short vertical distance will cause some water to perch within and on the more slowly permeable layer. A 100-fold or greater reduction in Ksat will lead to more substantial perching of water. Both the slope of the slowly-permeable horizon and the amount of effluent proposed for disposal on a common slope must be considered, in addition to the horizon's LTAR rates, while evaluating the site's suitability for the system.

  • At this site the soil evaluation revealed that an approximate 10-fold reduction or less would be expected in the Ksat of "2 when compared to H1• However, close examination of the soil horizon boundaries in a backhoe pit indicated that no significant perching was currently occurring under natural rainfall conditions. While some perching on the H2 horizon could occur at low loading rates, it would be temporary and inconsequential since an adequate aerobic vertical separation would persist beneath the infiltrative surface. However, at high loading rates, one would expect more substantial perching to occur. In this case, the horizontal Ksat of H1 would need to be measured to allow calculation of the height of the perched water mound over "2 and the transmissivity of H1•

    In summary, measured Ksat values of the most slowly permeable horizon should be compared to estimated LT AR at the infiltrative surface. The lowest of the two values should then be used to determine the site loading rate. Examples are presented below.

From the North Carolina Onsite Guidance Manual