Septic Systems: Site and Soil Evaluation Concepts

The key to installing a reliable on-site system that minimizes pollution and disease is to identify suitable locations with a thorough site and soil evaluation. The evaluation determines suitability or points out site limitations. Only after a site evaluation has been completed can the proper on-site system be designed.

Site and Soil Evaluation Principles

This section provides guidelines for a thorough site and soil evaluation.

On-site systems must: (1) protect public health, and (2) minimize environmental impacts. To accomplish these goals, the state of North Carolina uses a site and soil evaluation to determine the suitability of a location for an on-site system and the type of system that can be installed.

This section discusses the purpose and offers guidelines for making a proper site and soil evaluation for a proposed on-site system.

Purpose of a Site and Soil Evaluation

The purpose of the site assessment is to understand the soil system and the hydrology of the site, to predict wastewater flow through the soil and into subsurface materials, and to design an on-site system to match the soil system and the hydrology of the site. The site and soil evaluation helps to predict how an on­site system will function at a site. How well the system functions depends on the soil's ability to absorb the wastewater, the probable flow paths of water from the site, and the treatment received by the wastewater.

The comprehensive site and soil evaluation used in North Carolina requires considerable expertise by the site evaluator. The site evaluator must have substantial knowledge about soil science, geology, sanitary engineering, and environmental health. Guidelines for a site evaluation are discussed below.

Guidelines for a Site Evaluation

The ten guidelines for a site evaluation can be grouped into the three components of a site evaluation:

  1. collecting information before the site visit,

  2. assessing the site and soil at the location, and

  3. recording site evaluation data for system design and relaying the information to the designer of the system and the applicant.

*This section, Site and Soil Evaluation, was written from existing unpublished materials by Michael T. Hoover (Department of Soil Science, North Carolina State University).

Collecting information before the site visit.

This component of site evaluation consists of preparation. Preparation includes learning about the sites and soils in the region and knowing what types of on-site wastewater systems can best fit a situation along with gathering information about the site to be evaluated.

FIRST GUIDELINE. Know the rules and know how to collect the needed information. The Laws and Rules for Sewage Treatment and Disposal Systems are established by the Commission for Health Services to protect public health and minimize the environmental damage from on-site systems. These rules provide performance criteria for on-site systems and consider the allowable risks to the environment and public health from constituents of the wastewater, such as bacteria, viruses, nitrate, phosphorus, and other pollutants. The rules also provide the legal support for a site and soil evaluation and set the standards for site suitability.

The standards in the rules determine the amount and level of information ihat will be collected for each site. An initial site assessment will determine the level of detail for the site investigation and the type of data that should be collected. For example, a site with relatively flat slope and deep, well-drained loamy soils will require less investigation than a site with a more complex slope and several different soil profiles.

SECOND GUIDELINE. Determine the wastewater flow rate and characteris­tics. Information on wastewater quantity and quality is used to determine the initial size and type of on-site system to be installed at a particular site. The information for determining wastewater quantity and quality can be obtained from the application for an Improvement Permit.

The type of activity and size of facility that the on-site system will serve determine the daily flow and the peak flow of wastewater, or wastewater quantity. Likewise, the strength of the wastewater, or wastewater quality, is determined by the activities in the facility and, to some degree, by the size of the facility and how and when the wastewater is created.

Wastewater quantity and quality affect the level of detail required for a site evaluation. For instance, a site proposed for treatment and disposal of wastewater from a school designed for 400 students would require a more extensive site evaluation than a system for a two-bedroom home.

THIRD GUIDELINE. Review preliminary site information. Existing, pub­lished information will help the evaluator understand the types of soils and their properties and distribution on the landscape.

  • Published documents, such as soil survey reports; soil catena diagrams; and geologic, topographic and plat maps should be used for initial information about the site.

  • Warning: soil survey maps are good for planning, initial decision making, and helping you understand what to expect when you visit the site. However, they are not detailed enough to make siting recommendations. A field investigation is necessary for a proper site and soil evaluation. There is NO substitute for field investigations.

FOURTH GUIDELINE. Understand the septic system design options. Site evaluators must understand how on-site systems function in order to assess trade­offs in design options. Additionally, the type of site investigation will also be determined by the system design options appropriate to the particular location. For instance, a different type of site investigation would be required for a modified conventional system using ground water interceptor drains than for a conventional on-site system.

  • The on-site system must be designed to allow a sufficiently deep aerobic zone beneath the treatment and disposal field to properly treat the wastewater before it enters the ground water.

  • Major design options include: the depth of the trench bottom or infiltrative surface; the loading rate used for sizing the system at the site; and the type of distribution system, such as gravity or pressure distribution and parallel or serial distribution.

  • The use of pretreatment options may be needed at the site.

Assessing the site and soil at the location.

FIFTH GUIDELINE. View the on-site system as part of the soil system and the hydrologic cycle. Typically, on-site systems serving single-family homes do not add enough water to the site to substantially change the site's hydrology.

SIXTH GUIDELINE. Predict wastewater flow through the soil and the under­lying materials. The soil morphological evaluation and landscape evaluation are important in predicting flow paths and rate of wastewater movement through the soil and underlying materials. These two evaluations are used for on-site systems in North Carolina because landscape position and soil morphology greatly influence wastewater flow from the site.

  • Using the soil morphological characteristics, it can be determined whether water flow through the soil will occur primarily as vertical movement or as lateral movement in the horizontal direction.

  • Also, the soil morphological characteristics are used to estimate the long-term acceptance rate, or LTAR. This estimate of LTAR will determine the size of the area you must investigate. For example, if the first estimate ofLTAR for a site is 0.1 gallons per day per square foot (gpd/ft2) then you will need to evaluate four times the amount of land area than if the estimated LTAR had been 0.4 gpd/ft2 - For LTAR calculations see section 4.6.

  • Site and soil evaluations result in a more reliable prediction of wastewatermovement than a percolation or "perc" test. The perc test estimates saturated hydraulic conductivity by filling a borehole with water and measuring how quickly the water level falls. North Carolina formerly used the perc test to evaluate sites for on-site systems, but it has been shown that the perc test technique is inaccurate and unreliable for determining wastewater flow. Therefore, North Carolina discontin­ued the use of the perc test for evaluating sites for on-site systems several years ago.

SEVENTH GUIDELINE. Determine if additional informati.on is needed from the site. Site and soil conditions and the type of on-site system being considered determine whether additional evaluation is required. Some additional evaluations that may be required are: ground water mounding analysis, drainage analysis, hydrogeologic testing, linear loading rate evaluation, and hydraulic conductivity measurements.

  • For instance, if a large system serving a school is proposed at a location with ground water within 7 to 10 feet of the soil surface, you would want to identify whether there are any horizons limiting flow. Saturated hydraulic conductivity measurements in the least permeable horizon followed by ground water mounding analyses would be beneficial. This analysis helps predict whether unsaturated, aerobic conditions will still be present beneath the treatment and disposal field after operation begins and if ground water mounding occurs beneath the system.

  • In another example, if the soil is poorly drained, but sandy and located on a flat site, it may be possible to modify the seasonal high water table by using drainage. However, since the site is flat, additional investigations must be used to determine whether there is adequate elevation drop from the site to the proposed drainage outlet. This soil and site evaluation is necessary since drainage would not work effectively if there was not an adequate outlet available.

EIGHTH GUIDELINE. Assess the treatment potential of the site. The treat­ment potential of the site depends on the degree of soil aeration and the rate of flow of the wastewater through the soil. Wastewater is treated more effectively in well­aerated soils where wastewater flow is slow, which allows adequate adsorption and degradation of undesirable chemical and biological constituents. Thus, soil depth is crucial in determining the treatment potential of the site because there is a longer flow path through deeper soils. The longer flow path means more contact with the soil and soil organisms, and more time for degradation of pollutants.

  • In North Carolina, there must be 12 inches or more of separation between the bottom of the trench and any limiting soil condition such as restrictive horizons, wetness conditions, or bedrock. This separation provides a reasonable flow path and contact time for the pollutants to be removed.

  • The one exception is for Class I soils, which need a distance of 18 inches or more between the trench bottom and restrictive horizons, soil wetness conditions, or bedrock. These soils have a greater separation distance because wastewater flows more rapidly through them. The 18-inch separation provides more contact time for pollutant removal in these highly permeable soils.

NINTH GUIDELINE. Evaluate the site's environmental and public health sensitivity. Installing on-site systems in close proximity to community wells, near shellfish waters, in sole-source aquifer areas, or other sensitive areas may raise concerns regarding environmental and public health issues. When there are special environmental or public health concerns about a site, it may be necessary to obtain additional site information or perform certain evaluations to determine the degree of impact of the on-site system. In such cases adequate documentation must be kept to show that the site evaluation included the area of concern.

  • For instance, concerns about public health may be raised when large on-site systems are located adjacent to community wells. Here a detailed assessment of the ground water flow system is warranted. It is essential to determine whether the plume of wastewater from the treatment and disposal field will be intercepted by the cone of depression of the community well. If the cone of depression of the well is affected, then additional pretreatment of the wastewater may be needed to minimize any chance of polluting the community well.

Recording site evalua.tion data/or system design and relaying the information to the designer.

This component requires the site evaluator to communicate information gathered from the site evaluation to the person designing the system so that a proper design can be made.

TENTH GUIDELINE. Provide the system designer with soil/site descriptions and your recommendations. Based on the information gathered about the facility, and the actual site and soil evaluation, the last step is to suggest loading rates, highlight site and design considerations, and to point out special concerns in designing the on-site system.

  • The site evaluator should rank each site for the type of system that can be installed and provide specific soil and site data that will enable selection of the most feasible design options for the site. It is not enough to just provide the recommended loading rate or design. You must provide the data upon which these decisions are based.

  • In many cases, a single site and soil evaluation will be all that is necessary to design an appropriate system. However, on some sites, after collecting informa­tion about the site prior to and during the site visit, the evaluator may need additional information to determine the suitability of the site and the type of on­site system to be installed.

  • The process of data collection, evaluation, and design is often an iterative process. This means that the whole process is repeated several times, where each time new information or a new design is tried until a design is found that will fit the site. Some sites may require many repetitions before the final selection of an appropriate on-site system is made.

From the North Carolina Onsite Guidance Manual