Most Limiting Surficial Material

Author:  Steve Custer
Last Modified 1/8/200; 02 January 2010

Back to Septic Assessment Home...

How do I Interpret the Limitations?

There are eight themes:
  1. Flooding Limitation
  2. Depth To Bedrock Limitation
  3. Ponding Limitation
  4. Wetness (Depth To Shallow Water Table) Limitation
  5. Percolation Limitation
  6. Filtration Limitation
  7. Slope Limitation
  8. Large Stones Limitation

There are eight additional soil properties that may be of interest:

Property Desricptions

  1. Hydrologic Group
  2. Steel Corrosion
  3. Concrete Corrosion
  4. Frost Action
  5. Soil SAR
  6. Shrink-Swell Potential from 0-60 inches
  7. Hydric Soils
  8. Any Hydric Soil

 

Example Questions the Database Can Answer

  1. What are the septic limitations from the perspective of flooding?
  2. What are the septic limitations from the perspective of depth to bedrock?
  3. What are the septic limitations from the perspective of depth to high water table?
  4. What are the septic limitations from the perspective of percolation rate for drain fields?
  5. What are the septic limitations from the perspective of filtration of microbes?
  6. What are the septic limitations from the perspective of slope?
  7. What are the septic limitations from the perspective of large stones (coarse fragments)?
  8. Where are the areas with hydric soils?
  9. Where are the high and low runoff areas?
  10. Where are the areas with high potential for steel corrosion which might impact underground pipes?
  11. Where are the areas with high potential for concrete corrosion which might impact concrete septic tanks?
  12. Where are the areas with high potential for frost action?
  13. Where are the areas with high SAR where drainfield soils may be sensitive to sodium from water softeners?
  14. Where are the areas with shrink-swell clays which may be sensitive to sodium from water softeners?
  15. What are the soil units in the area I am interested in?
This coverage focuses on most limiting surficial material. Each soil unit may contain more than one surficial material.  The word surficial material rather than soil is appropriate because one of the "soil units" possible for a mapped area is "rock outcrop".  Rock outcrop is not technically a soil, and does not have soil-property data, but is a limiting feature in that depth to bedrock is zero.  Because shallow bedrock (less than 40 inches) is a severe limitation for septic system installation and operation rock outcrop was included in the limitation list at the request of the Gallatin Environmental Health Department Staff and the LWQD Staff.  This decision identifies a severe rating for surficial materials that might otherwise be classified as moderate or slight if soils exclusive of outcrop are used as a basis for limitation decisions.  The most limiting soil was selected by examining each of the soils in a map unit for severe ratings in the order listed in U.S.  Natural Resources Conservation Service, 1996, National Soil Survey Handbook,  Part 620 NRCS 620-46  Soil Interpretations Rating Guides, Table 620-17 Septic Tank Absorption Fields The first soil that produced a severe listing was used for all further limitations.  For example, if there were three soils and none had severe flooding limitations, but one had depth to bedrock less than 40 inches, that soil became the soil used to define the most limiting restrictions.  Likewise,  if none of the three soils had a severe flooding limitation or depth to bedrock limitation, but one had a ponding limitation, the soil with the ponding limitation was selected to represent the most limiting conditions regarding installation and operation of septic systems.  The choice of soil was independent of the area the soil component represents in the map unit.  As a result the most limiting soil may represent as little as ten percent of the mapped unit.    The examples above illustrate questions that can be explored with this database.  The limitations are sometimes surmountable with sound engineering practice, but such work can be expensive and can substantially influence the development potential of the site from a cost perspective.  In all cases, the user must be aware that site specific work may reveal other limitations or show that the map unit has areas without the indicated limitations.  These coverages simply alert the user to problems which may be present based on second and third order soil surveys.

Map 1 -- Soil limitations for the most limiting surficial material in the map unit.

These maps present information flood, depth to bedrock, ponding, depth to water table (wetness), percolation, filtration, slope, and large stones.  These concepts are described more fully in the glossary.  Severity index and number of severe limitations for the polygon were purposefully left out because rock outcrop if selected as most limiting has little other data in the category and could easily lead to the erroneous conclusion that there are few other limitations when there simply is no data to reach this conclusion.  This coverage is best used to find areas of flooding and shallow depth to bedrock limitations.  Other limitations may or may not be as severe as the most limiting soil.  Finally hydric soils are mapped.  This information is not intended to be substituted for site investigations.

Map 2 -- Soil properties for the most limiting surficial material in the map unit.

These maps present information on hydrologic group (runoff characteristics), steel corrosion limitation, concrete corrosion limitation,  frost action limitations, sodium absorption ratio, and shrink-swell potential.  These properties are described more fully in the glossary.
 

Glossary

There are a variety of sources for information regarding septic systems in general and suitability criteria in particular.  A list of descriptions of the attributes is linked here.

About the Data

Mstlmsc.shp Metadata
The limitations are based upon Natural Resources and Conservation Service (NRCS) data for the most abundant soil mapped at a scale of 1:24,000 during a level second order soil survey and Gallatin National Forest data for the most abundant soil mapped at a scale of 1:62,500 during a third order survey. The data are projected in UTM meters NAD83.   The limitations were developed from NRCS 620-46 Part 620 Soil Interpretations Rating Guides (Table 620-17).    The most abundant soil may represent from 33 to 100 percent of the land area in the mapping unit since many of the soils represent a complex. The limitations for the most abundant soil differ from that for the most limiting soil.  This data should not be used for site assessment but is useful for planning and  generalized assessment of  areas larger than 10-20 acres.

The coverage for lowland agricultural soils was created from  soil tables in Rolfes, T., J. Brooker, B. Duncan, T. Keck, and R. Simms, 1998,  SSURGO Soil Survey Geographic Database, Gallatin County Area, Montana:  USDA-NRCS, Bozeman, Montana.  The coverage for the upland forest soils was developed from Davis, C.E., and Shovic, H.F., 1984 Soils survey of Gallatin national Forest Area, Southwestern Montana:  Interim Draft Report: USDA-Forest Service, Gallatin National Forest; and from Davis, C.E., and Shovic, H.F., 1996, Soil Survey of Gallatin national Forest, Montana: USDA-Forest Service, Gallatin National Forest, Bozeman, Montana.  The soils in the Big Sky area have been extrapolated from US Forest Service soil data; more detailed surveys have been conducted in this area that are not yet available digitally, but are available through the Gallatin NRCS office. The Gallatin National Forest soils were not originally interpreted for septic limitations, but such interpretations can be made as shown by Christner, W.T., Jr., 1999, Septic Interpretations from a Third Order Soil Survey:  Master of Science in Land Resources and Environmental Science, Montana State University, Bozeman, Montana.  Each soil unit may contain as many as three soils.  The most abundant soil was selected,  septic system limitation criteria were applied to the soil properties,  and severe, moderate, slight, or no data were assigned to each property for each most abundant soil.  The most abundant soil in each unit covers from 33-100 percent of the soil unit's area.  There might be hydric soils present as inclusions in the description which did not get hydric designation because the inclusion did not necessarily lead to a hydric designation in the original NRCS attribute table.  All these soils were identified in the NRCS data tables for Gallatin Valley (not Gallatin Forest) under the direction of Tony Rolfes at the Gallatin NRCS office, and used to update an attribute column titled ANYHYDRIC by Steve Custer.  This attribute (ANYHYDRIC)  may help those who would like to be alerted to any presence of observed hydric soils in the valley (not the mountain forests.

A tabular list of the attributes for soil coverages and their meaning may help you understand the attribute table.