Spatial Data for Septic Assessment

Spatial Data For Septic Assessment Project

In 2001, a U.S. Environmental Protection Agency 319 Grant through the Montana Department of Environmental Quality provided funding for the Gallatin Local Water Quality District (GLWQD) and other local agencies to address the need for readily accessible and comprehensive spatial data for the following purposes:

  • to aid in understanding water quality issues within the GLWQD
  • to help Gallatin County Department of Environmental Health (EHS) put septic system applications into perspective, and
  • to assist property owners and developers in understanding and planning for septic system limitations.

Today, much of this information is included in an updated format on the GLWQD and EHS websites, as well as the EHS and GLWQD online mappers.  Additional information on septic assessment topics from this project are available below.  Please note that the GLWQD boundary has expanded since the completion of this project.  For metadata and assistance interpreting these maps, please contact the District at (406) 582-3168.

These maps are designed to provide general information about soils in the area of interest.  Maps should not be a substitute for a site analysis, but rather to provide information about areas of concern to be explored further with the Gallatin Department of Health or with septic system installation professionals, such as whether a system is possible, and whether special design or siting are needed.

  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

Septic Limitation Index  (This index is a score built by assigning a numeric rank to all limitations for an area [3 = severe, 2 = moderate, 1 = slight, 0 = no data/other] and adding to produce the total score.  There are eight restrictive features so the maximum possible score is 3 X 8 = 24)

Number of Severe Limitations (This is a sum total of “severe” rankings for the eight restrictive features.)


  1. Hydrologic Unit Map
  2. Hydrostratigraphic Column
  3. Transmissivity
  4. General Hydrogeology
  5. Aquifer/Aquitard
  6. Surficial Geology