Take Your Time During Site Evaluation Process

If you start with bad soils data, the risk is high for design flaws that will cripple your new system

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We have been involved in troubleshooting many onsite waste treatment systems. We’ve observed that most problems with systems start with bad decisions about the site and soils.

If adequate time and effort has not been put into site evaluation, inadequate or wrong data are collected, leading to bad design decisions which then translate directly to installation practices in the field. This is true whether the site evaluator and designer are the same person or two separate people. The design and installation will only be as good as the information collected ahead of time and then used in the design.

A complete site evaluation involves three distinct phases: gathering preliminary information, evaluating the site in the field and reporting the results.

Information gathered during the preliminary phase can inform and direct field activities. The kinds of information gathered include legal descriptions of the property, setbacks, easements, well locations, soil and landscape characteristics and legal design requirements. Field evaluations locate these features and conditions on the site, collect site specific soil information and locate the system relative to other property improvements. Finally, the data and information collected is recorded and reported to the designer. This information is stored as part of system design and installation for future reference or review.

Soil survey is key

The importance of gathering information on soils and landscapes was brought home again when we were contacted by a state regulator to review and comment on a guidance document being prepared to send out to county regulators, site evaluators, designers and installers. The document covered identification, location and design solutions for sodium-affected soils.

It was interesting to hear about conditions that led to development of the guidance. In a certain county, for years there were reports from homeowners and installers that onsite treatment systems were failing shortly after installation. When soil survey information was accessed and reviewed in the state office, the area where failures occurred was mapped as having sodium-affected soils. Sodium-affected soils are generally characterized by very slow permeability and poor drainage. Both conditions, of course, are detrimental to long-term operation of onsite waste treatment systems.

Field visits confirmed this was the problem. The question was why it took so long to identify the problem when the answer was easily determined by looking at the soil survey information. This is a very good question, with probably not a single answer. In our education programs and presentations, use of soil survey information as a part of the site evaluation process has always been important.

Better information access

Soil surveys include a wealth of information about soils in an area, helping the site evaluator and designer determine their field activities and inform their design decisions. Information is provided on soil properties and conditions such as depth to limiting soil layers (saturation, bedrock, dense soil), texture, structure, saturated conductivity and permeability. In addition, there is information on site and landscape features (landscape position, slope, landform) and potential for flooding or ponding.

Long ago, the way to access this information was to obtain a printed copy of the county survey, which included soil maps on a photographic background and soil data in tabular form with written soil series and map unit descriptions. This was done by visiting the local soil and water district office, public library or county land and zoning office.

Because of this, access could be somewhat limited. In addition, depending on the year the survey was conducted in that area, the tabular data on soil properties, characteristics and interpretations could be outdated. In our opinion, all of this led to less use of this information to help inform the site evaluation. We happened to reside in a state where this information was more widely available, but this was not the case everywhere.

Fortunately, access to the information has changed dramatically and it can be accessed from anywhere through the internet through the web soil survey. In addition to easier access, any changes or updates to the survey are readily incorporated, so the information is up to date.

Check the database

The NRCS Soil Web Survey can be found at the following address: websoilsurvey.sc.egov.usda.gov/App/HomePage.htm. Within the NRCS Soil Web Survey:

  • Click on the green “Start WSS” button.
  • Under the “Quick Navigation” heading on the left side of the page, click on “address” and insert the address for your site.
  • Just below the “Area of Interest Interactive Map” tab, click on “AOI” (area of interest). This will allow you to draw a box around the general area of your site.
  • Click on the “Soil Data Explorer” tab near the top of the page. Then click on the “Soil Reports” tab just below to the right.
  • Select properties you are interested in. In the case of sodic soils, it is chemical properties.
  • Click on “View Soil Report.” This will provide a detailed chemical report for each soil name within your AOI.

Additionally, once you have defined your AOI, you could also click on the “Suitabilities and Limitations For Use” tab. This will provide information on the particular use of interest, in our case Septic Tank Absorption Fields. Be aware if you look at this information, it may not include all the design options available for this soil. We will address design options in a future column. Bottom line is if you have questions about soil conditions in your area, consult the soil survey. You will be happy you did.


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