by Julia Steenberg, Andrew Retzler, Jenifer McDonald, and Jacque Hamilton
Minnesota Geological Survey
Introduction
The Minnesota Geological Survey (MGS) recently competed year one of a two-year pilot project designed to support watershed planning efforts under the Groundwater Restoration and Protection Strategies (GRAPS) program. This project is funded through the Minnesota Department of Health. The goal of this project is to provide a compilation of geologic data within selected Board of Water and Soil Resources (BWSR) One Watershed One Plan (1W1P) boundaries in a format suitable for both modelers and the general public.
The GRAPS program helps local planning efforts prioritize groundwater quality and quantity concerns and provides strategies and actions for protection and restoration (https://www.health.state.mn.us/communities/environment/water/cwf/localimplem.html#HowDoesGRAPS). A GRAPS report is a collection of maps and data describing conditions in a watershed. Eighteen watersheds in Minnesota currently have a GRAPS report for local organizations to use for developing their watershed plans. Many state agencies (Minnesota Board of Water and Soil Resources (BWSR); Minnesota Department of Agriculture (MDA); Minnesota Department of Natural Resources (MNDNR); Minnesota Pollution Control Agency (MPCA)) work together to gather data and create these reports collaboratively. General geologic information exists in these reports, but they do not contain the most recent and detailed information on the distribution of sediments and rocks in the subsurface available from the MGS County Geologic Atlas (CGA) mapping program (https://cse.umn.edu/mgs/county-geologic-atlas). The County Geologic Atlas (CGA) program is run collaboratively by the MGS and MNDNR and provides information essential to sustainable management of groundwater resources through a comprehensive set of maps and information on the geologic and hydrogeologic conditions in a county. When planning at larger scales that involve several counties (i.e., a watershed) we recognize that it can be problematic for users to create seamless geologic and hydrogeologic datasets in a GIS environment. This pilot project was set up to address this need for the GRAPS program. The results may also guide us in producing seamless mapping datasets for other needs such as groundwater modeling.
Seamless geologic products were created for the Zumbro River Watershed for both the unconsolidated Quaternary sediments and the underlying bedrock layers based on a compilation of previously published MGS maps along with new mapping where necessary. A surficial geologic map was compiled and unit surfaces for the unconsolidated Quaternary and bedrock layers were produced. A unit surface is a GIS raster representative of the elevation of the tops and bottoms of the mapped units for use in GIS software. They are a powerful GIS tool used to accurately depict the subsurface geologic environment and provide a modeling framework to spatially analyze geology or groundwater flow. Quaternary units in Minnesota are generally thinner, more variable, and discontinuous than the bedrock units due to their depositional environment and repeated episodes of erosion. A texture-based point model was created to better visualize the unconsolidated Quaternary sediments at and below the ground surface, down to bedrock. These products were transferred into web-based 3D models so they could be readily visualized and used outside of a GIS environment by water planners, other state agencies involved in the GRAPS process, and the public. This point model can also be readily used for groundwater and surface water modeling. An MGS report describing the compilation methods and limitations associated with the subsurface modeling processes is available on our website (https://hdl.handle.net/11299/220567). All geologic datasets are provided in the supplementary files of this report. A separate report describing the results for the St. Louis Watershed in northern Minnesota will be available soon. Additional watersheds will be compiled in year two to complete this pilot project in 2022. CGA maps across the watershed are referenced and should be consulted for specific geologic information including the geologic setting, geologic data utilized, detailed map unit descriptions, and hydrogeologic properties.
Figure 1. Watershed map of Minnesota highlighting the location of the Zumbro River Watershed in southeast Minnesota. Gray lines show watersheds in Minnesota and red line depicts the Zumbro River Watershed.
Zumbro River Watershed
The regional geologic framework within the Zumbro River Watershed is referred to as a bedrock-dominated landscape. Unconsolidated sedimentary cover is thin, and the land surface largely reflects the topography of the bedrock. The bedrock formations are Paleozoic in age and made up of sandstone, shale and carbonate rock. The uppermost layers are dominated by carbonate bedrock and the deepest layers are dominated by sandstone. Paleozoic bedrock formations contain significant sources of groundwater that provide the water supply for the region. Properties within the bedrock formations control the direction and speed of groundwater flow. More permeable units such as sandstone or carbonate rock containing fractures and voids are aquifers and easily transmit water. Layers with more shale and fine-grained material are less permeable and are more difficult to transmit water through. These layers act as aquitards (confining units) and protect underlying aquifers. Groundwater is well-connected to surface water in parts of the watershed where unconsolidated sediments are thin to absent, have less clay content, and/or underlying bedrock aquifers are present at the land surface.
The thickness of unconsolidated Quaternary sediments generally increases to the west across the watershed, ranging from absent to 275 feet (167 meters). Areas with the thickest sediments overlie deeply incised bedrock valley systems. Clay-rich tills, which can be considered aquitards, are present mainly in the western part of the watershed. Till is sediment directly deposited by glacial ice and is composed of a mixture of clay to boulder-sized fragments. Sand is mostly restricted to bedrock valleys. The texture of the unconsolidated Quaternary sediments is reported based on the average percent of sand, silt and clay as one of the twelve recognized United States Department of Agriculture (USDA) soil texture classes (Soil Science Division Staff, 2017). These include sand, loamy sand, sandy loam, sandy clay loam, loam, silt loam, silt, silty clay loam, clay, clay loam, sandy clay, and silty clay. We have also included gravel, sandy gravel, and rock in our descriptions, as these are important properties for modeling groundwater flow in the subsurface, despite not being recognized as official USDA textures. For the 3D geologic model, USDA textures of the unconsolidated Quaternary units were generalized into three categories (clay, mixed clay and sand and sand).
Methods
The Zumbro River Watershed spans six southeast Minnesota counties (Wabasha, Rice, Goodhue, Olmsted, Dodge, and Steele) and covers an area over 1400 square miles. Each of these counties has been mapped as part of the CGA program; however, they have been individually published over several decades, ranging from 1995 to present day, and vary in GIS data availability. Data produced for Rice, Goodhue and Wabasha CGAs are older (1995-2001) while the Steele, Dodge and Olmsted CGAs are more recent (2019 to in press). Our old and new products differ in GIS content and the degree to which mapping datasets align across county boundaries. Because methods for mapping Quaternary sediment and bedrock layers differ, the compilation process for these datasets also differed. Below is a brief description of the compilation methods used for both bedrock and unconsolidated Quaternary sediments.
Bedrock
To create bedrock unit surfaces, existing GIS data from the bedrock geology and bedrock topography maps of the six counties in the watershed were compiled (Hobbs and others, 1995; Setterholm and others, 1998; Runkel and others, 2001; Steenberg and others, 2019; Steenberg and others, 2020; Retzler and others, in press). Line work was edited along county edges and where new geologic data warranted changes. Revised mapping focused on the faulted area within Wabasha County to depict the geologic structure more accurately in the watershed. Faults in this area are nearly vertical planar features that mark a boundary where each side has either moved up or down relative to the other. These may have displacements of several hundred feet, enough to juxtapose several different formations to be in contact with each other. Contouring was done at 25-foot (7.6-meter) intervals. The bedrock layers are depicted in this dataset as a series of rasters representing elevations of the top and bottom of each of the seventeen individual bedrock formations and their thickness, as well as the bedrock topography. All raster surfaces can be viewed 2-dimensionally or 3-dimensionally in a GIS environment or through our online 3D map.
Quaternary (Unconsolidated) Sediments Compilation A seamless surficial geology map was created across the watershed by compiling the 1:100,000 scale GIS files of the statewide digital database D-1 (https://arcg.is/0CbHKG) with the GIS files of more recent maps completed in Dodge and Olmsted Counties (Steenberg and others, 2019; Steenberg and others, 2020). The Quaternary map units are distinguished from one another by texture, lithology of the very coarse-grained sand fraction (1-2 millimeters), stratigraphic position and landscape position. The Quaternary deposits are assigned to lithostratigraphic units defined in Johnson and others (2016). A total of 18 surficial geologic units are established for this watershed. We simplified these units for visualization purposes into three categories that represent sand, mixed (meaning the material contains variable amounts of both sand and clay) and clay. Surficial clay-rich tills are present mainly in the western part of the watershed and surficial sand is mostly restricted to bedrock valleys.
Figure 2. Surficial geology map of the Zumbro River Watershed simplified into three texture classes of clay, mixed (variable amounts of clay and sand) and sand as shown in the online 3D model
Unconsolidated Quaternary units are thin and more complex than the bedrock units due to their depositional environment and repeated episodes of erosion. Units tend to be discontinuous and variable in both thickness and elevation over relatively short distances. Subsurface Quaternary mapping was accomplished by creating a set of east-west cross sections to depict the variable layers. Twelve cross sections were constructed at regular (5-kilometer (3.1-mile)) intervals across the watershed. From these cross sections, unit surfaces were constructed for 11 subsurface Quaternary units including tills, loess and colluvium. Coordinates and elevations from the quaternary unit contacts are extracted from the cross sections in GIS and interpolated into unit surfaces (tops, bottoms and thicknesses). Sand and gravel units were not mapped for this project, rather they were depicted through an interpolation model based on current lithology data listed in the County Well Index (CWI) (Tipping, 2019). A texture-based point model was created to visualize textures at and below the ground surface, down to bedrock. The model points are at a 250-meter (820-foot) regularly spaced intervals with 5-foot (1.5-meter) vertical spacing for the watershed. The texture-based point model can be viewed 2-dimensionally or 3-dimensionally in a GIS environment or through our online 3D map (https://arcg.is/fevGS). The texture-based point model for our online 3D map uses the generalized textures (sand and gravel, mixed clay and sand, and clay) to show the general texture distribution of the unconsolidated Quaternary sediments that overly the bedrock formations.
Figure 3. Screen capture of the entire 3D geologic model for the Zumbro River Watershed. This image shows the texture-based point model for the Quaternary sediments in three generalized texture categories (clay, mixed and sand) as well as the underlying bedrock geologic units.
3D Model
The web-based 3D model (https://arcg.is/fevGS) is meant to be a visualization tool for water planners, other state agencies involved in the GRAPS process, and the general public, and is made readily accessible in a browser, requiring no GIS software. The model is separated into three parts: surficial geology, subsurface geology, and bedrock geology. The surficial and subsurface geology has been simplified into three textural categories of clay, mixed, or sand. Each category is a separate layer in the model that can be turned on/off independently of the others. The surficial geology is shown as 2D polygons and represents the unconsolidated Quaternary sediments within a few meters of the land surface. The polygons are shown with slight transparency to allow users to peer through them at underlying data. Below these polygons are the regularly spaced, 3D point data representative of the subsurface Quaternary geology from the base of the surficial deposits down to the top of bedrock. Below the regularly spaced point data lie the 3D bedrock layers representing the mappable Paleozoic units in the Zumbro River Watershed. Each bedrock unit is a separate layer in the model that can be turned on/off independently of the others. To better visualize thinner geologic units at this scale, the 3D model is exaggerated 20 times in the vertical and the surficial geology, subsurface geology and bedrock datasets are vertically offset from one another and the ground surface to prevent data overlap. Furthermore, a Geographic References layer and three different Basemap layers are included for reference. The Geographic References layer is an overlay of geographic boundaries, roads, city names and various other geographic features, so the user can readily identify or search by surface areas of interest.
Discussion and Future Work
The modeling process we use to create unit surfaces of the Quaternary units from 5-kilometer (3.1-mile) spaced cross sections (as opposed to manually mapping these surfaces in plan-view) has some unintended model artifacts. These include linearity along cross section lines in both elevation and map unit extent, unintended gaps in map units between cross sections and oversimplification of unit distribution between cross sections. Tipping (2019) evaluated these methods and offered solutions for improvement. We incorporated some of these solutions by creating unit surfaces of the Quaternary unit tops and bottoms of only till and fine-grained materials and superimposing the interpolated model to capture the most recent sand deposits. Although this improved the model output, some unintended modeling artifacts remained. These included oversimplifications of map unit distribution between cross sections, and gaps in the model with no assignment of map unit or texture. These gaps occurred locally across some units in plan-view, and between the bedrock surface and base of the Quaternary. To correct for gaps in the model near the base of the Quaternary, unit bases directly above the bedrock were set equal to the bedrock topography locally between 5-kilometer (3.1-mile) cross section lines. These adjusted unit bases were used to populate the map units and textures in the subsurface texture-based point model and filled all gaps in the model with a map unit and texture.
Next year, we intend to develop improved methods to map the distribution of Quaternary sediments in plan-view by manually contouring till unit bases in a similar manner to the way in which flat-lying bedrock geologic units are mapped. This will be a challenge due to the complexity of these deposits but is needed to correct for the oversimplification of map unit distribution between 5-kilometer (3.1-mile) cross section lines. Working in watershed areas that already have CGA Quaternary cross sections completed at 1-kilometer (0.6-mile) spacing should aid in developing this process.
The Zumbro River Watershed model is based on the USGS HUC-8 boundaries that differs slightly from BSWR 1W1P boundary. This was done to limit quantification of water budget – both surface water and groundwater, to the watershed boundary itself. Future efforts with the GRAPS program will also include putting together more supporting text from a hydrogeologic perspective to be presented along with the model for the general user to make sense of the information and how to apply to their resources conservation work.
References
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of Rice County, Minnesota, Part A: Minnesota Geological Survey County Atlas C-09, scale 1:100,000, 6 pls. Retrieved from the University of Minnesota Digital Conservancy, https://hdl.handle.net/11299/58514.
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Steenberg, J. R., Bauer, E. J., Retzler, A. J., Marshall, K. J., McDonald, J. M., Meyer, G. N., Lively, R.
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