Ground water sensitivity

Three levels of pollution sensitivity assessment

Assessments of geologic sensitivity are conducted to identify where ground water is more easily affected by contaminants originating at or near the land surface. Knowing where sensitive areas occur and the conditions associated with them may encourage careful public and private land use decisions that protect both ground water and surface water resources.

In 1991, DNR Waters prepared a report providing criteria and guidelines for preparing and applying geologic sensitivity maps. The criteria are five overlapping time of travel classes. The guidelines describes three levels of assessment. Levels 1 and 2 are for assessments of surficial ground-water systems, and Level 3 is for assessments of deeper ground-water resources.

Level 1 - preliminary assessment

Ground water pollution sensitivity assessment level 1 diagram.

A Level 1 assessment examines geologic conditions to a depth of about 5 feet below the land surface. A Level 1 assessment uses soil survey information prepared by the Natural Resources Conservation Service. The assessment is based on the assumption that the material to a depth of about 5 feet below the land surface is representative of the entire vadose zone, the unsaturated material above the water table. Many factors or conditions that affect geologic sensitivity and that lie at depths greater than 5 feet cannot be identified by this assessment level. A Level 1 assessment addresses potential ground-water contaminants that originate at the land surface. It does not assess the pollution potential of contaminants that originate below the surface, such as underground storage tanks or improperly constructed wells.

DNR Waters has completed a statewide Level 1 preliminary assessment of geologic sensitivity. A Level 1 rating has been assigned to each of the approximately 9,000 soil map units listed as of May 1995 in the soil data base of the Natural Resources Conservation Service. The ratings, listed by county, are available by downloading the following file:

A summary of all ratings, listed by soil name, is available in the following file:

The criteria and guidelines report includes a map example of a Level 1 assessment for a portion of northwest Hennepin County. As shown in that example, ratings can be applied manually to soil survey maps that are available for most Minnesota counties. The pilot study for Scott County completed a Level 1 assessment map for that county as part of the study. GIS technology can be used to prepare a map from the ratings if a digital soil survey is available. An example is the Level 1 map for Nicollet County (PDF, 1.9MB) prepared using Geographic Information System (GIS) technology.

The ratings of Level 1 assessments are not intended for site-specific use, and additional subsurface characterization is needed for detailed assessments at all rating levels. In addition, as discussed in the pilot study for Scott County, the ratings tend to overestimate sensitivity somewhat; therefore more detailed information on sensitivity, such as a county geologic atlas, should be used instead if it is available.

Level 2 - assessment of water-table aquifers

Ground water pollution sensitivity assessment level 2 diagram.

A Level 2 assessment evaluates the sensitivity of ground water at the water table. It is based on an estimate of the vertical travel time of contaminants from the land surface to the water table. In most of Minnesota, the depth to the water table is from 10 to 30 feet below the land surface. The travel time of a particle of water from the land surface to the water table depends on the rate of downward movement, which is affected by the thickness and composition of the rock and sediment in the vadose zone. The vadose zone may be composed of various geologic materials, such as clay, sand, gravel, sandstone, or limestone. Other factors that may affect water movement in the vadose zone are not considered.

The vertical permeability of the vadose zone material is important in controlling the rate of downward migration. Vadose zone permeability is influenced by several factors. One factor is the texture (grain size distribution) of vadose zone materials. For example, sands and gravels have coarse textures and are more permeable than fine-textured materials like clay. If layers of low-permeability geologic materials are present, the cumulative or total thickness of the layers limits downward migration. A factor that poorly limits downward migration is the presence of fractures and joints in underlying bedrock material (sandstone or limestone). Fractures and joints act as conduits through which contaminants can migrate very quickly to the water table.

A Level 2 assessment is a more complete evaluation of the pollution sensitivity of ground water than a Level 1 assessment. However, determining the depth to the water table is difficult in Minnesota because maps describing local ground-water conditions are not available for most areas. As a result, a Level 2 assessment requires much more data and interpretation than a Level 1 assessment. For best results, the person conducting the assessment must be familiar with the geology in the area of concern and possess knowledge of principles of geology and ground-water hydrology.

Level 3 - assessment of deeper aquifers

Ground water pollution sensitivity assessment level 3 diagram.

A Level 3 assessment evaluates the sensitivity of deeper aquifers below the water-table aquifer. In some areas of Minnesota, there is a deeper aquifer below the water-table aquifer and an underlying, confining (low-permeability) layer. Deeper aquifers are defined here by the existence of one or more 10-foot-thick intervals of confining material below the water-table aquifer. This definition is used as a guideline; site-specific studies may show that in some places a thickness of less than 10 feet may also function as an effective confining layer.

The methodology used for a Level 3 assessment is similar to Level 2 in two respects:

  • the degree of sensitivity is based on the presence of low-permeability or confining layers that limit vertical movement of water into the aquifer and
  • a Level 3 assessment evaluates the cumulative thickness of low-permeability layers.

A Level 3 assessment can help identify the degree of natural protection afforded to deeper aquifers. The low-permeability layers act as confining units above or between deeper aquifers. If more than one deeper aquifer occurs in an area, each deeper aquifer is rated separately. If two deeper aquifers occur in an area, the lower of the two deeper aquifers will be less sensitive because the cumulative thickness of overlying confining layers will be greater for the lower of the two deeper aquifers.

Human activities will likely have little or no immediate impact on deeper aquifers protected by thick confining layers because infiltrating surface water and contaminants probably require a relatively long time to reach the aquifers. However, the long-term effects of lateral flow from recharge areas affected by human activities may be significant. Experience indicates that the principal sources of contamination to deeper aquifers protected by thick confining layers are improperly constructed or maintained wells and abandoned wells or test holes that have not been properly sealed.

The validity of a Level 3 assessment depends of the quality of the subsurface data and the expertise used to define subsurface hydrogeologic conditions. Mapping the geologic sensitivity of deeper aquifers in many areas is not realistic because of the general lack of high-quality subsurface data. Water well records provide a general understanding of deeper aquifer conditions but cannot provide the detailed understanding of subsurface geologic conditions that geologic test drilling, geologic mapping, and aquifer testing provide.

References

Geologic Sensitivity Workgroup, 1991, Criteria and guidelines for assessing geologic sensitivity of ground water resources in Minnesota: Minnesota Department of Natural Resources, Division of Waters, St. Paul, MN, 122 p.

H. O. Pfannkuch, 1998, Geological Sensitivity Assessment of Groundwater Systems: A Pilot Study for Scott County, Minnesota - Summary and Evaluation, Department of Geology and Geophysics, University of Minnesota, 61 p.

For more information:

Contact DNR Waters at (651) 259-5700.