Step 1: Identify areas where clay-rich glacial deposits may occur just beneath the land surface. The principle source for this information is the statewide coverage of landforms that was developed by the Minnesota Geological Survey and the Geology Department at the University of Minnesota at Duluth. Areas where till and superglacial deposits occur are the sediment assemblages selected to broadly define where clay-rich glacial sediments may occur within 30 feet of the land surface. These areas can be further defined by referencing the more detailed mapping of glacial sediments contained in the County Geologic Atlas Series or Regional Hydrologic Assessment Series that have been prepared by the Minnesota Geological Survey. Also, county soil surveys are useful for identifying whether clay-rich materials form the parent materials for the soil classifications used to prepare soil maps. In some areas, lacustrine deposits may be included following a determination of whether they are clay-rich or contain a significant component of sand and silt.
Step 2: Assemble data describing 1) where the depth to bedrock is less than 30 feet, 2) the locations of sinkholes, and 3) exposures of bedrock. These features are not present in all counties, but introduce a much more complicated set of geologic conditions where they occur. The data sources used to determine whether any of these features are present are1) the County Well Index database, 2) the statewide coverage of karst features, and 3) maps showing exposures of bedrock.
Using the data sources listed, a map is prepared to show the areas where the depth to bedrock is less than or greater than 30 feet. It is assumed that the capabilities for clay-rich glacial deposits to reduce contaminant risk from Class 5 automotive waste disposal wells are greatly diminished where bedrock occurs within 30 feet of the land surface. Therefore, these areas are considered to be sensitive.
Step 3: Identify the thickness of clay-rich glacial deposits that occur between 10 and 30 feet of the land surface. The methodology for determining where non-sensitive areas occur excludes any geologic materials that occur within 10 feet of the land surface because they are likely to be 1) fractured by frost and weathering, 2) disturbed by the construction of the Class 5 well, and 3) disturbed by plant roots and animal burrows. Therefore, this depth interval is not included because it is likely that the integrity of any clay-rich materials has been diminished.
Water well logs and test hole records are used to identify the geological materials that are present for the depth intervals of 10-20 and 20-30 feet below the land surface. Classifying the many terms used by drillers to reflect clay-rich materials is determined by a geologist and used in a statistical determination that produces map showing the probability that clay-rich conditions will occur.
Extrapolating between data points introduces uncertainty into the resulting probability map because of 1) the distribution of data points is often not uniform and 2) the detail describing geologic conditions in well logs varies between drilling contractors. For example, a drilling record may indicate that the thickness of glacial deposits is greater than 30 feet but the glacial deposits were lumped into a common term called “glacial drift”. This record was useful for determining the depth to bedrock for step 2 but is too general to be included in this step.
The final results of this step are 1) a coverage of points that indicate the cumulative thickness of clay-rich glacial deposits that occur at depths of 10 to 30 feet below the land surface and 2) a map showing the probability that clay-rich deposits occur at the same depth interval.
Step 4: Integrate the Results of the First Three Steps to Prepare the Initial Sensitivity Map. Areas where clay-rich glacial deposits occur (Step 1) are referenced to the depth to bedrock and local bedrock conditions (Step 2). The resulting map indicates the areas where clay-rich deposits should be at least 30 feet thick. Areas where these conditions are not present are considered to be sensitive to Class 5 automotive waste disposal wells.
Next, the remaining areas are referenced to the point coverage containing the cumulative thickness of clay-rich deposits that occur from 10 to 30 feet. Ideally, this thickness should be 20 feet, but small lenses of sand or gravel may occur within otherwise, clay-rich glacial deposits. These localized conditions should have little impact on the overall evaluation of the 10 to 30 foot interval where nearby data points indicate the full 20 feet consist of clay-rich materials. Therefore, the Minnesota Department of Health uses the probability evaluation to define areas where there is an 80% or greater probability that clay-rich deposits will occur within the 10 to 30 foot depth interval.
Step 5: Reference Site-Specific Data to Confirm Mapping Results. A geographic information system is used to reduce the time required to integrate the maps and data sets that are used to compile a sensitivity map. As a result, extrapolating between data points or overlaying and combining maps may introduce uncertainty into the final results, especially where subsurface geological conditions are variable. Therefore, a geologist must reference the final results to the initial point-source data and determine whether any changes are warranted to reflect any discrepancies that occur. Areas are removed from consideration as non-sensitive where point-source data conflicts with the interpreted mapping results.