University of minnesota Duluth

 

 Laboratory for Spatial Analysis

in

The GeoSciences

 

 

Cass county - Ten Mile Lake association Water resource management tools

 


September 30, 2003

 

 

 

CASS COUNTY - TEN MILE LAKE ASSOCIATION WATER RESOURCE MANAGEMENT TOOLS

 

1.       Groundwater Susceptibility Analysis (GWSA)

2.       Surface Water Susceptibility Analysis (SWSA)

3.       Rational Method Runoff Calculator (RMRC)

4.       Aquifer Probability Coverage (APC)

5.       Groundwater Flow Model (GWFM)

6.       Water Budget Analysis (WBA)

 

 

 

 

September 2003

 

 

 

Prepared for:

Cass County

And

Ten-Mile Lake Association

 

Prepared by:

University of Minnesota Duluth

Laboratory for Spatial Analysis in the Geosciences (LSAG)

 

 
 

Executive Summary

  This report summarized the results of an analysis of surface water and groundwater in the Ten Mile Lake watershed and surrounding area.  Included with this report are a group of tools to aid the Ten Mile Lake Association and their cooperators in managing water resources for land-use planning.  There are six major components: groundwater susceptibility analysis (GWSA), surface water susceptibility analysis (SWSA), rational method runoff calculator (RMRC), a 3D aquifer probability coverage (APC), Groundwater flow model (GWFM), and a water budget for Ten Mile and Birch Lakes (WBA).

The GWSA is based upon a model of aquifer sensitivity to ground water contamination from pollutants introduced at or near the surface.  The model was built using DRASTIC: A standardized system for evaluating ground water pollution potential using hydrogeologic settings.  Inputs to the DRASTIC model consist of seven parameters that define the intrinsic characteristics of the hydrogeologic system including depth to water, recharge, aquifer media, soils, topography, impact of vadose zone, and hydraulic conductivity of the aquifer.  The model output consists of a grid coverage containing relative aquifer sensitivity rankings for the Ten Mile Lake Watershed and surrounding area.  The rankings are classified into four categories of sensitivity: 1) low, 2) moderate, 3) high, and 4) very high based on the results of the analysis. 

The Surface Water Susceptibility Analysis (SWSA) determines relative runoff potential based on inputs of distance to water bodies (major rivers, lakes and streams), slope, land cover and soil parameters.

The Rational Method Runoff Calculator (RMRC) is an extension developed by UMD using Arcview Version 3.2.  The purpose of RMRC is to calculate the peak discharge (Q) utilizing the Rational Method from a user-defined watershed.  This is done in conjunction with another extension named “Basin1” which uses digital elevation data to derive watersheds based on a user-defined point.

The Aquifer Probability Coverage is derived from the water well drillers logs housed in the Minnesota County Well Index.  Stratigraphic information is extracted using a binary indicator.  If a stratigraphic unit is interpreted to be an aquifer it is given a value of 1, whereas units interpreted to be aquitards are given values of 0.  The water well data are then converted the locations of the wells, the material depths, and the binary indicator are converted to an x, y, z data file.  Geostatistical analysis is used to interpreted among wells to construct a 3D data set that reflects the probability that any particular location and elevation is an aquifer.  Numbers near 1 are likely aquifers and numbers near zero are likely aquitards.  The data used as input to groundwater flow models and are incorporated into an Arc/Info 3D ASCII grid file for import to 3D analyst in Arc/View or Arc/Info.

The Groundwater Flow Model is a numerical simulation of groundwater flow in the Ten Mile Lake Watershed and surrounding area.  The flow model was developed using MODFLOW with GMS (Groundwater Modeling System) as the data pre and post-processor.  The model is a steady-state representation of the groundwater system, which can be used to determine flow direction, flow rates, and to delineate wellhead protection areas. 

The Water Budget Analysis is an assessment of the hydrologic inputs and outputs to the Ten Mile and Birch Lake Watersheds.  The water budgets were compiled on an annual basis for the years 2000, 2001, and 2002.  The goal was to examine the magnitude of water exchange between Tenmile and Birch Lakes for use in managing lake levels and understanding surface and subsurface exchange of water in the complex topographic setting of the Ten Mile Lake area.

 

Table of Contents

1.0       Introduction. 8

1.1      Project Location. 8

1.2      Project Personnel 9

1.3      Water Resource Management Tools Definition and Purpose. 10

1.4      Software Utilized. 11

2.0       Primary Data Sources and Information. 12

2.1      Study Area Boundary. 12

2.2      Digital Elevation Model Data. 12

2.3      Land Cover Data. 12

2.4      Soils Data. 13

2.5      Geomorphology Data. 13

2.6      Well Data. 13

2.7      Lakes Data. 13

2.8      Streams Data. 13

2.9      Datum and Projection. 13

2.10    Model Resolution. 13

3.0       Water Management Tools Components. 14

3.1      Groundwater Susceptibility Analysis. 14

3.1.1        Overview of Drastic Model 14

3.1.2  Depth to Water 17

3.1.3        Recharge. 20

3.1.4        Aquifer Media. 23

3.1.5        Soil Media. 26

3.1.6        Topography (Slope) 31

3.1.7        Impact of Vadose Zone. 34

3.1.8        Conductivity (Hydraulic) 37

3.1.9        Results and Discussion. 40

3.2      Surface Water Susceptibility Analysis. 42

3.2.1        Overview of Surface Water Susceptibility Analysis. 42

3.2.2        Slope Factor Classification and Rating. 44

3.2.3        Distance to Water Factor Classification and Rating. 46

3.2.4        Land Cover Factor Classification and Rating. 48

3.2.5        Soil Factor Classification and Rating. 50

3.2.6        Results and Discussion. 52

3.3      Rational Method Runoff Calculator. 55

3.3.1        Overview of the Rational Method. 55

3.3.2        Overview AND PROCEDURES of the Basins Extension. 55

3.3.3        OVERVIEW AND Procedures for Rational Method Runoff CalculatOR EXTENSION.. 56

3.4      Overview of the Aquifer Probability Coverage. 58

3.4.1        Procedures For Producing Aquifer Probability Coverage. 58

3.4.2        Results and Discussion. 58

3.5      Groundwater Flow Model 59

3.5.1        Procedures for Building the Groundwater Flow Model 60

3.5.2        Results and Discussion. 62

3.6      Water Budget Analysis. 63

4.0       Conclusion. 64

5.0       Maintenance. 65

6.0       Recommendations. 66

7.0       References. 67

8.0       Appendix A – Water budget analysis. 69

Hydrologic Budget for Tenmile and Birch lakes. 70

9.0       Appendix B – BASIN1 EXTENSION DOCUMENTATION.. 80

 

List of Tables

Table 1, Primary Data Sources

Table 2, Weights assigned to DRASTIC parameters         

Table 3, DRASTIC Ranges and Ratings for Depth to Ground Water

Table 4, Ten-Mile Study Area Depth to Water Parameter

Table 5, DRASTIC Ranges and Ratings for Recharge (Net)

Table 6, Ten-Mile Study Area Recharge Parameter

Table 7, DRASTIC Ranges and Ratings for Aquifer Media

Table 8, Ten-Mile Study Area Aquifer Media Parameter

Table 9, DRASTIC Ranges and Ratings for Soil Media

Table 10, Ten-Mile Study Area Soil Media Parameter

Table 11, DRASTIC Ranges and Ratings for Topography

Table 12, Ten-Mile Study Area Topography Parameter

Table13, DRASTIC Ranges and Ratings for Impact of Vadose Zone Media

Table 14, Ten-Mile Study Area Impact of Vadose Zone Parameter

Table 15, DRASTIC Ranges and Ratings for Conductivity

Table 16, Ten-Mile Study Area Conductivity Parameter

Table 17, Results of Groundwater Sensitivity Assessment

Table18, Classification of Groundwater Sensitivity

Table 19, Slope Factor Classification and Rating                                                               

Table 20, Distance to Water Factor Classification and Rating                                 

Table 21, Land Cover Factor Classification and Rating                                         

Table 22, Hydrologic Soil Group Characteristics                                                  

Table 23, Soil Factor Classification and Rating                                                      

 

List of Figures

Figure 1, General Project Location Map                                                                          

Figure 2, Water Resource Management Tools Schematic   

Figure 3, Groundwater Susceptibility Analysis Flowchart

Figure 4, Depth to Water Map

Figure 5, Recharge Map

Figure 6, Aquifer Media Map

Figure 7, Soil Media Map

Figure 8, Topography (Slope) Map

Figure 9, Impact of Vadose Zone Map

Figure 10, Conductivity (Hydraulic) Map

Figure 11, Groundwater Sensitivity Map

Figure 12, Surface Water Susceptibility Analysis Flowchart 

Figure 13, Slope Map                                                                

Figure 14, Distance to Surface Water Map                                                           

Figure 15, Land Cover Map                                                                   

Figure 16, Soil Map                                                                               

Figure 17, Surface Water Susceptibility Map         

In support of the contract between the University of Minnesota, Duluth (UMD) Geological Sciences Department and Cass County, UMD is pleased to submit the Ten Mile Lake Association Water Resource Management Tools (WRMT).  This section of the report provides background information on the project location, personnel, definition and purpose of the tools and software utilized.

1.1          Project Location

The project location encompasses an area of approximately 92,466 acres and lies in portions of Cass and Hubbard counties, in north central Minnesota. Figure 1, General Project Location Map, depicts the location of the study area boundary and is presented on next page.  The study area is located in an area known as the Itasca/St. Croix moraine Interlobate area. 

 

                                              

1.2          Project Personnel

The following persons were involved in the production of this report.

·         Howard Mooers 

·         Dave Stark

·         Stacey Stark

·         Sue Hattenberger

·         Brennan T Mears

1.3          Water Resource Management Tools Definition and Purpose     

The WRMT were designed as a series of calculations, GIS analyses, and finally a groundwater flow model to assist with water resource management in Cass County and specifically to address issues in the Ten Mile Lake and Birch watersheds.  Figure 2, Water Resource Management Tools Schematic, is presented below and identifies the major components of this report. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


Each of the six components are defined below and addressed in individual sections in this report.

 

Groundwater Susceptibility Analysis (GWSA) – GWSA utilized the DRASTIC method to determine the susceptibility of specific areas to groundwater pollution potential.  DRASTIC is an acronym describing seven parameters controlling ground water pollution potential.  The seven parameters include Depth to ground water, Recharge (Net), Aquifer Media, Soil Media, Topography (slope), Impact of the Vadose Zone Media, and Conductivity (Hydraulic) of the Aquifer.  Rating factors and weights are assigned to each variable to determine the overall score.

 

Surface Water Susceptibility Analysis (SWSA) - SWSA describes the intrinsic factors of slope, distance to water, soil type and land cover and rates them in regards to their ability to produce surface water runoff that may lead to contamination in surface water bodies.   As with the GWSA, different runoff potential ratings and weighting factors are assigned to the variables to distinguish higher or lower runoff potential.

 

Rational Method Runoff Calculator (RMRC) – RMRC is series of tools used to determine peak discharge from a watershed.  With the provided input grids, watersheds can be calculated utilizing an Arcview extension.  Following this, a peak discharge is calculated for a specific rainfall event. 

 

Aquifer Probability Coverage (APC) –  APC is a map that describes the probability that any point within the study area is an aquifer. To develop an APC, stratigraphic information from 489 wells in the study area was extracted from CWI (file: Cass_well_raw_data.txt).  Elevations of all wells were determined from the 30 meter DEM.  The well data were imported to ArcView and the data table sorted by lithologic type.  All types deemed to be aquifer were coded with an indicator of 1, whereas all lithologic types deemed to be aquitards were coded with an indicator of 0.  Stratigraphic data for each 5-foot interval was extracted.  A total of 1244 stratigraphic intervals were coded.  Elevations and thickness of  each unit was converted to meters for geostatistical analysis.   Geostatistical analyses were completed using GSLib (Geostatistical Software Library and User's Guide by Clayton Deutsch and André Journel, 1992, 340 pp).  Vertical and horizontal variogram analysis was performed and the final results Kriged to a resolution of 100 x 100 meters horizontally and to variable thicknesess vertically.  The results were output to a 3D ASCII grid file for import to GIS software.

 

Groundwater Flow Model (GWFM) -  The purpose of a numerical model of groundwater flow is to provide a quantitative tool for groundwater flow in the Ten Mile Lake watershed and surrounding area.  The model is steady state, meaning it does not take into account time-dependent flow. It is regional in scale, but can be modified in the future to analyze site-specific applications.  Modeling was done using Groundwater Modeling System (GMS) developed by the Department of Defense.  GMS is a pre- and post-processor for MODFLOW, the groundwater flow model developed by the US Geological Survey. 

                                                                                                               

Water Budget Analysis (WBA) –  A hydrologic budget specifies inputs, outputs and storage changes over a reference period for a specific area.  The goal of the WBA was to examine whether there is significant exchange of water between Tenmile and Birch Lakes.  The simplest form of a hydrologic budget specifies inputs, outputs and storage changes over a reference period for a specific area.  The reference area for this investigation is the lake surface, the reference period is one year, and the study was done for the water years (Oct. 1 – Sept. 30) 2000, 2001, and 2002.  To most effectively determine the components of inflow and outflow to the lakes three separate hydrologic budgets were calculated.  First the watersheds of Tenmile and Birch Lakes were combined and an overall hydrologic budget was determined.  The relatively large size of the watersheds and the good record of stream outflow of the Boy River at Hackensack make it easier to calibrate coefficients used for determining surface runoff and evapotranspiration.  Over a one-year period the hydrologic budget should be relatively balanced with little change in storage.

 

These tools collectively present a variety of means to evaluate water resources within the study area boundary.  The GWSA and SWSA identify the intrinsic characteristics of the landscape that could lead to increased likelihood of surface or groundwater contamination.  The WBA and APC provided a framework for evaluating the groundwater flow from the Ten Mile to the Birch Lake watersheds and provided inputs for the GWFM.  The GWFM and RMRC can be utilized to predict surface and groundwater direction of flow and output.   

1.4          Software Utilized

The software utilized for this project included the following:

 

  • Arcview Version 3.2
  • Groundwater Modeling System (GMS) Version 5.0
  • MODFLOW 2000
  • Microsoft Word, Excel, PowerPoint

                       

The WMRT utilized a variety of GIS and other data sources for producing the outputs of the individual analyses.  Table 1, Primary Data Sources lists the data that was acquired or provided by Cass County, the scale of the data and a description of how it was utilized in the analysis.  Following the table the sections describe the data in greater detail.

 

 

Table 1, Primary Data Sources

 
Type of Data
Scale of Data
Data Utilization

Study Area Boundary

N.A.

All data clipped to this boundary.

Digital Elevation Model Data

1:24,000

Used for slope analysis in the GWSA and SWSA.  Utilized for inputs for deriving flow direction, flow accumulation and watersheds in RMRC.

Land Cover Data

1:100,000

Used for SWSA for coding land use with runoff potential ratings and in RMRC for coding runoff coefficients.

Soils Data

1:24,000

Used for inputs to both the GWSA and SWSA.

Geomorphology Data

1:100,000

Used for multiple inputs for the GWSA and the GWFM.

Well Data

Site specific

Used to develop the depth to water map for GWSA and for heads for the GWFM.

Lakes Data

1:24,000

Used for distance analysis in the SWSA and for general mapping purposes.

Streams Data

1:24,000

Used for distance analysis in the SWSA and for general mapping purposes.

2.1          Study Area Boundary

The study area boundaries for the WRMT were based on a shapefile produced by UMD. The name of this shapefile is Study_area.shp.  This shapefile incorporates all of the contiguous land areas within the Ten Mile and Birch watersheds.  In addition, the additional distance outside of the formal area of the Ten Mile Lakes area was needed for better definition of the regional groundwater regime being modeled in the GWFM.

2.2         Digital Elevation Model Data       

A 30-meter digital elevation model (DEM) was acquired from the National Elevation Data Set, which was accessed at the following web page http://gisdata.usgs.gov/NED/default.asp.   This data set includes seamless elevation data for the United States and was acquired at a resolution of 30 meter grid spacing.   This data set was clipped to the study area boundary defined by the file entitled Study_area.shp.  The name of the resulting DEM is Dem_nad83. 

2.3         Land Cover Data     

Land cover data was provided by DNR from their Data Deli (http://deli.dnr.state.mn.us, LandSat-Based Land Use-Land Cover (Vector)) and was also clipped to the study area boundary and was used for coding runoff potential ratings in the SWSA and for coding runoff coefficients for the RMRC. The name of the resulting shapefile is landcov.shp.                       

2.4         Soils Data

Soils data was provided by Cass County Environmental Services  and was also clipped to the study area boundary and was used for coding runoff potential ratings in the SWSA.   The name of the resulting shapefile is soilstenmile.shp. 

2.5         Geomorphology Data

Geomorphology data was acquired from the Minnesota Data Deli, which is accessed at http://deli.dnr.state.mn.us/.  This data was originally compiled under the direction of Dr. Howard Mooers and was utilized for components of the GWSA, APC and GWFM.  This data was also clipped to the study area boundary and the name of the resulting shapefile is geomorph.shp.

2.6         Well Data

Well data was acquired from the County Well Index maintained by the Minnesota Department of Health.  The well data was used in combination with the elevation of lakes in the study area to produce a depth to water map.  This map was utilized in the GWSA and the GWFM. This data was also clipped to the study area boundary and the name of the resulting shapefile is Wells.shp.

2.7         Lakes Data

Lakes data was acquired from the Minnesota Data Deli, which is accessed at http://deli.dnr.state.mn.us/.  The data was utilized in the distance analysis in the SWSA and for general mapping purposes.  This data was also clipped to the study area boundary and the name of the resulting shapefile is lakes.shp.

2.8         Streams Data  

Streams data was acquired from the Minnesota Data Deli, which is accessed at http://deli.dnr.state.mn.us/.  The data was utilized in the distance analysis in the SWSA and for general mapping purposes.  This data was also clipped to the study area boundary and the name of the resulting shapefile is rivers.shp.

2.9         Datum and Projection

All data was either acquired or converted to Universal Transverse Mercator (UTM) North American Datum (NAD) 83.  The standard unit of measure for this datum is meters.  Distance analysis was performed using feet as the unit of measure.

2.10       Model Resolution

The model resolution for the individual analyses are listed below:

 

Groundwater Susceptibility Analysis (GWSA) – 30 meter

Surface Water Susceptibility Analysis (SWSA) - 30 meter

Rational Method Runoff Calculator (RMRC) - 30 meter

Aquifer Probability Coverage (APC) - 100 meter horizontal and 2-6 meters vertical

Groundwater Flow Model (GWFM) - 100 meter horizontal and 2-6 meters vertical

Water Budget Analysis (WBA) – N/A Water budget scale is described in the report in Apendix 1.

                                                         

                                                                                                                                                           

                                   

3.1          Groundwater Susceptibility Analysis

DRASTIC is a standardized methodology used to evaluate the potential for ground water pollution potential in hydrogeologic settings (Aller et al. 1987).  A panel of managers, scientists, and private consultants developed the method. The panel included individuals representing federal, state, and local agencies, the Canadian government, and private industry.  Through a series of discussions, technical applications, and scientific reviews the panel developed what has become one of the most commonly used methods to evaluate ground water pollution potential in the United States (USEPA 1995).

 

The DRASTIC method was developed within the framework of the existing classification system of ground water regions of the United States.  Using this classification system it is possible to subdivide each ground water region into hydrogeologic settings  d on locally specific ground water characteristics.  A hydrogeologic setting is defined as a composite description of the major geologic and hydrologic factors, which affect and control ground water movement into, through, and out of an area (Aller et al. 1987).  The DRASTIC method is  d on the concept of hydrogeologic settings and is the acronym describing seven parameters controlling the ground water pollution potential of a specific hydrogeologic setting.  The seven parameters include:

 

Depth to ground water,

Recharge (Net),

Aquifer Media,

Soil Media,

Topography (slope),

 Impact of the Vadose Zone Media, and

Conductivity (Hydraulic) of the Aquifer. 

 

While these parameters do not include the infinite number of variables that can be used to describe the physical characteristics of a hydrogeologic setting they are considered the most important parameters for which data are available, and for assessing the ground water pollution potential of an area.

 

DRASTIC uses a numerical ranking system to assign a relative index of aquifer sensitivity (IAS) based on the following equation (Aller et al. 1987):

 

IAS = Dw*Dr + Rw*Rr + Aw*Ar + Sw*Sr + Tw*Tr + Iw*Ir + Cw*Cr                  

 

where w and r are weights and ratings assigned to each parameter. 

 

The weights assigned to each parameter are constant, ranging from 1 to 5, and based on the relative importance in evaluating ground water pollution potential as determined by the panel through a consensus approach.  Table 2, Weights Assigned to DRASTIC Parameters is located below.  In essence, the more important a variable is considered to be in evaluating ground water pollution potential the higher its weight will be.  

 

 

 

 

Table 2, Weights Assigned to DRASTIC Parameters

DRASTIC Parameter

Weight (relative importance)

Depth to ground water

5

Recharge (net)

4

Aquifer Media

3

Soil Media

2

Topography

1

Impact of vadose zone media

5

Conductivity

3

 

Numerical rating values for each of the parameters vary from 1 to 10, and are assigned using a range of values obtained by defining the physical characteristics of each parameter within the hydrogeologic setting.  The range of values represents data derived through either consulting existing sources of hydrogeologic information, or through conducting field-sampling programs.  Rating values for D, R, S, T, and C are assigned one value per range, whereas rating values for A and I are assigned a typical rating selected from a set of variable ratings.  However, the ratings for each parameter can be adjusted  based on specific knowledge of the hydrogeologic setting in question tempered by sound professional judgment.

 

 

Methods used to derive each factor are described in the following section of the report.   Figure 3, Groundwater Susceptibility Flowchart graphically displays the inputs, data assigned and analysis steps and is presented on the next page.

 

Figure 3, Groundwater Susceptibility Analysis Flowchart

 

Assign Values

 

 

 
 
 


Rasterize Files

 

 
 

Input Themes

 

 
 

Results

 

 
 
           

 

 

 

 

 

 

 

 

 

 

 


Soil texture field of geomorphology shapefile used to create coverage.

 
 

Aquifer_

media

Values

(3-8)
 

 

Aquifer_media.shp
 

Aquifer Media

Geomorph.shp
 
                               


An aquifer is a geologic unit that can store and transmit water at rates fast enough to supply reasonable amounts to wells (Fetter 1994).  In simpler terms, an aquifer represents a geologic unit in which all the pore spaces are completed filled (saturated) with water.  Ground water within an aquifer occurs in confined, unconfined, or semi-confined conditions.  Therefore, one must take care when selecting a value for depth to water based on the characteristics of the aquifer. 

 

In a confined aquifer ground water is generally under pressure; therefore, the elevation of ground water observed in a well can be higher than the elevation of the water table beneath the confining layer.  In this case, depth to water should be measured at the top of the aquifer, which also corresponds to the base of the confining layer.  Depth to water in a confined aquifer can be obtained by consulting geologic reports containing maps, cross sections, or well logs.

 

In an unconfined aquifer the water table represents the expression of the surface below ground level where the pores spaces are completed saturated.  In this case, the water table is able to rise and fall under atmospheric pressure.  An unconfined aquifer can be present in any type of geologic media and may be seasonal or permanent in nature.  However, for the purposes of DRASTIC an unconfined aquifer is chosen as the depth to water table in a geologic unit that yields significant enough quantities of water to be considered an aquifer.

 

A semi-confined aquifer refers to aquifers that are overlain by a less permeable unit that restricts or retards the flow into or out of the aquifer.  Semi-confined aquifers exhibit characteristics ranging from confined to unconfined; therefore, the choice of depth to water is determined by evaluating which characteristic of the aquifer is most dominant and then follow the procedures outlined above. 

 

DRASTIC was designed for the evaluation of unconfined aquifers.  The ranges and ratings for depth to water are based on what are considered to be depths where the potential for ground water contamination significantly changes.  Table 3, DRASTIC Ranges and Ratings for Depth to Groundwater is presented below.  In cases where the depth to ground water is shallow the travel time for a contaminant released at the surface is shorter than ground water occurring at deeper levels.  Moreover, the potential for attenuation of a contaminant increases as depth to water increases.  These criteria are reflected in the assignment of ratings for the depth to water parameter.

 

 

Table 3, DRASTIC Ranges and Ratings for Depth to Groundwater

Range – Depth to Water (Feet)

Rating

0-5

10

6-15

9

16-30

7

31-50

5

51-75

3

76-100

2

100+

1

 

 

Depth to water in the study area ranges from zero feet to approximately 100 feet.  The calculated values for depth to water using DRASTIC range from a low of 5 to a high of 50.   These values are reflective of the variability of the hydrogeologic setting and overall characteristics of ground water flow in the study area.   Table 4, Ten Mile Study Area Depth to Water Parameter is presented below.

 

 

 

Table 4, Ten Mile Study Area Depth to Water Parameter

Range (FT)

Weight

Rating

Calculated DRASTIC Value

0-5

5

10

50

6-15

5

9

45

16-30

5

7

35

31-50

5

5

25

51-75

5

3

15

76-100

5

2

10

100+

5

1

5

 

A depth-to-water coverage of the study area was developed by using the elevations of surface water bodies, contouring those values and then subtracting the contoured water table elevations from the land surface elevation.  The depth-to-water grid was produced at the same resolution as the DEM, 30 meters. The resulting grid was used for assigning DRASTIC weight and rating factors and calculation of the depth to water parameter.    Figure 4, Depth to Water Map and the associated DRASTIC values are presented on the next page.


 

 

The primary source of ground water recharge is precipitation that infiltrates through the land surface and percolates into the aquifer.   The amount of water that recharges an unconfined aquifer is dependent upon three major factors: 1) the amount of precipitation not lost to evapotranspiration, 2) the vertical hydraulic conductivity of surficial deposits and stratigraphy of the unsaturated zone, and 3) the transmissivity of the aquifer and potentiometric gradient of ground water flow (Fetter 1994:512).  In a confined aquifer recharge occurs in areas where the confining layer is absent or a leaky confining layer is present.  Recharge may occur through down-flow from a higher aquifer, or through up-flow from a lower aquifer.    

 

In the DRASTIC model, net recharge is defined as the average annual amount of water that penetrates the ground surface and infiltrates to reach the aquifer.  However, it is a difficult parameter to measure and any quantification of aquifer recharge must be considered an estimate and not an exact measured value (Korkmaz 1990).  As such, the ranges and ratings used in DRASTIC provide some leeway for choosing values that are representative of the recharge for a given study area.   The amount of recharge for a given area determines the amount of water available to transport a contaminant introduced at the surface vertically to the water table and horizontally within the aquifer.  Moreover, the dispersion and dilution of a contaminant in the unsaturated zone is largely controlled by this parameter.  Table 5, DRASTIC Ranges and Ratings for Recharge (Net) are listed below.

 

 

Table 5, DRASTIC Ranges and Ratings for Recharge (Net)

Range – Net Recharge (Inches/Yr)

Rating

0-2

1

3-4

3

4-7

6

8-10

8

10+

9

 

The best estimates of recharge in the study area were calculate by St. George (1994) and range up to 12 in/yr. The calculated values for recharge using DRASTIC range from a low of 4 to a high of 24.  These values are reflective of the overall variability of the geomorphology of the region and characteristics of the vadose zone in the study area.  Table 6, Ten-Mile Study Area Recharge Parameter is listed below.

 

Table 6, Ten-Mile Study Area Recharge Parameter

Range (inches/yr)

Weight

Rating

Calculated DRASTIC Value

0-2

4

1

4

2-4

4

3

12

4-7

4

6

24

7-10

4

8

32

10+

4

9

36

 

A recharge coverage of the study area was developed using a landform- based approach to estimation of ground water recharge (St. George 1994).  As discussed earlier it is a difficult parameter to measure for a number of reasons and any quantification of recharge must be considered an estimate.  Regardless, recharge values for the study area are based on regional geomorphologic characteristics of central Minnesota.  A digital polygon geomorphology coverage of Cass and Hubbard counties compiled at a scale of 1:100,000 by UMD was used to estimate recharge.  Landforms present within the geomorphology coverage were compared to those mapped by St. George (1994), and assigned recharge values using the same procedures.  The polygon coverage was then converted to grid coverage containing estimated recharge values for Cass and Hubard Counties at a spacing of 30 meters.  The recharge grid was then clipped to the boundary of the study area, and cell values reclassified as integers.  The resulting grid was used for assigning DRASTIC weight and rating factors and calculation of the recharge parameter.  Figure 5, Recharge Map is presented on next page.

Aquifer media refers to the consolidated or unconsolidated geologic material that yields sufficient quantities of water for use.  Water is contained in aquifers within the pore spaces of clastic sediment and rock and in fractures or solution cavities within non-clastic rocks.  Aquifers that yield water from pores spaces have primary porosity, whereas aquifers that yield water from fractures or solution cavities have secondary porosity.

 

The characteristics of ground water flow in an aquifer are controlled to a great degree by the porosity of the aquifer media.  Porosity is defined as the ratio of the volume of void spaces in a geologic unit to the total volume of the geologic unit.  Clastic sedimentary geologic units generally have primary porosity that is influenced by grain size, shape, and sorting all of the clastic materials and this contributes to the arrangement or packing of grains within the unit.  Packing is important because it largely determines the amount of void spaces available for water storage.  In general, sedimentary units that are poorly sorted typically contain a wide range of grain sizes and have lower porosities compared to sedimentary units that are well sorted and contain a small range of grain sizes.  Non-clastic rocks generally have secondary porosity and water is stored in and transmitted through fractures and solution cavities within the aquifer.

 

In DRASTIC the ranges of aquifer media types are given as descriptive names with rating values listed in order of increasing pollution potential.  Table 7, DRASTIC Ranges and Ratings for Aquifer Media is presented below.  The relative pollution potential of each media type is based on information obtained from observations made from studies conducted in various hydrogeologic settings.  The method allows for flexibility in selected a rating value based on professional expertise or specialized knowledge of the aquifer media present within a given study area.   

 

Table 7, DRASTIC Ranges and Ratings for Aquifer Media

Range – Aquifer Media

Rating

Typical Rating

Massive Shale

1-3

2

Crystalline Rock

2-5

3

Weathered Crystalline Rock

3-5

4

Glacial Till

4-6

5

Bedded Sedimentary Rock Sequences

5-9

6

Massive Sandstone

4-9

6

Massive Limestone

4-9

6

Sand and Gravel

4-9

8

Basalt

2-10

9

Karst Limestone

9-10

9

 

As a whole the aquifer media is dominated by the mixed sediments of the Itasca and St. Croix moraines.  The calculated values of the aquifer media parameter using DRASTIC range from a low of 3 to a high of 8.  Table 8, Ten Mile Study Area Aquifer Media Parameter is listed below.  These values reflect the complexity of the subsurface geology of the study area. 

 

Table 8, Ten Mile Study Area Aquifer Media Parameter

Aquifer Media Range

Weight

Rating

Calculated DRASTIC Value

Superglacial

3

4-6

5

Outwash*

3

5-7

6

Ice Contact

3

7-9

8

Till

3

2-4

3

*Note: a few small wetlands were included within outwash polygons and were coded as outwash.

 

An aquifer media coverage of the study area was developed using data derived from the geomorphology coverage of Minnesota.  The aquifer media types of the study area was compiled into a point coverage with a spaceing of 30 meters.  The point coverage was then converted to a grid of aquifer media that was clipped to the boundary of the study area.  The coverage was reclassified  based on lithology.  The resulting grid was used for assigning DRASTIC weight and rating factors and calculation of the aquifer media parameter.  Figure 6, Aquifer Media Map is presented on the next page.

 

Soil media refers to the uppermost weathered zone of the earth, which typically extends from the land surface to an average depth of 60 inches.  Soil formation is a complex process where the interaction and influence of climate, organisms, and topographic factors acting on the soil parent materials over time result in the development of a soil profile.  The soil profile contains a number of diagnostic surface and subsurface horizons that are classified on the basis of quantifiable physical and chemical criteria.  The genetic horizons potentially developed within a soil profile are typically arranged in the following sequence the O, A, E, B, C and R horizons (Buol et al. 1997).  There are a number of other potential arrangements and combinations of soil horizons; however, for the purposes of this project only the aforementioned horizons will be discussed.    

 

The O horizon is a generally associated with organic soils and is characterized as a soil layer dominated by organic materials formed or deposited on either an organic or mineral surface.  The A, E, B, C, and R horizons are associated with mineral soils. 

 

The surface A horizon is a soil layer formed at the surface or below an O horizon.  It is characterized by the accumulation of organic matter derived from the decay of plant and animal tissue, and various humic compounds.  Surface A horizons vary in thickness depending on the factors involved is soil genesis, but are generally thicker where grasses dominate.

 

An E (elluvial) horizon is a subsurface soil layer formed below the A horizon that is characterized by the elluviation or loss of clay, iron, aluminum and other compounds resulting in a concentration of quartz or other weathering resistant minerals in silt or sand size particles. 

 

The B (illuvial) horizon is a subsurface layer formed below the A and E horizons in which the dominant features are characterized by one or more of the following: 1) illuvial concentration of silicate clay, iron, aluminum, and other compounds alone or in combination, 2) evidence of removal of carbonates, 3) coatings on the faces of peds, 4) alteration of material from its original condition that obliterates the original rock structure, or 5) any combination of these. 

 

The C horizon is a subsurface layer that shows little evidence of alteration by soil forming processes and lack the properties of the O, A, E, and B horizons.  The C horizon represents the parent material for soil formation that may or may not be similar to the material in which the other horizons are formed.  The R horizon is a layer consisting of consolidated or incompletely weathered bedrock material.

 

Soils, when present, offer the first line of defense in the protection of an aquifer from contamination.  The soil has a significant impact on the timing and amount of water that infiltrates into the ground surface and is available for percolation to recharge the aquifer.  Moreover, the amount of organic matter present in the soil has a profound influence on the adsorption and complexation of contaminants released at or near the surface.  In DRASTIC the ranges of soil media are based on the soil textural classification chart and given ratings based primarily on grain size.  Table 9, DRASTIC Ranges and Ratings for Soil Media is presented below. In general, finer grained soils (e.g. clays, silts) have a low rating due to their ability to attenuate or slow the migration of contaminates as compared to coarse-grained soils (e.g., sands, gravels).

 

 

Table 9, DRASTIC Ranges and Ratings for Soil Media

Range – Soil Media

Rating

Thin or Absent

10

Gravel

10

Sand

9

Peat

8

Shrinking/aggregated Clay

7

Sandy loam

6

Loam

5

Silt Loam

4

Clay loam

3

Muck

2

Non-shrinking/aggregated Clay

1

 

 

 

The soils present across the study area are dominated by  sandy loam, loamy sand and muck distributed throughout the area.  The calculated values for the soil media parameter using DRASTIC range from a low of 4 to a high of 20.  Table 10, Ten Mile Study Area Soil Media Parameter is listed below.  The lower values correspond to fine textured soils that have low infiltration rates, whereas the high values correspond to coarse textured soils with high infiltration rates.    Overall the soils within the bounds of the study area are representative of the region as a whole.

 

 

 

 

 

 

Table 10, Ten-Mile Study Area Soil Media Parameter

 
Soil Name
HYDGRP
Texture
Weight
Rating
DRASTIC

Akeley-Debs

A

Loamy sand

2

0

16

Alstad

C

Fine sandy loam

2

6

12

Aqualfs

C

Clay loam

2

2

4

Arenic Eutroboralfs

B

Silty clay loam

2

2

4

Baudette

B

Silt loam

2

0

12

Bergkeller

B

Sandy loam

2

6

12

Bootlake-Graycalm

A

Sandy loam

2

0

12

Bowstring-Seelyeville

A

Muck

2

2

4

Cathro

A

Muck

2

2

4

Cathro-Seelyeville

A

Muck

2

2

4

Cromwell

A

Sandy loam

2

6

12

Cushing

B

Loam

2

5

10

Cutaway

B

Sand

2

10

20

Debs-Akeley

B

Silt loam

2

0

12

Demontreville

B

Loamy sand

2

8

16

Demontreville-Mahtomedi-Cushing

B

Loamy sand

2

8

16

Egglake

B

Loam

2

0

10

Fluvaquents

D

Sandy loam

2

6

12

Friendship

A

Sand

2

10

20

Glossaqualfs

B

Sandy loam

2

6

12

Graycalm

A

Sand

2

10

20

Graycalm-Bootlake

A

Loamy sand

2

0

16

Graycalm-Mengha

A

Loamy sand

2

0

16

Graycalm-Sanburn

A

Loamy sand

2

0

16

Greenwood

A

Peat

2

2

4

Haslie-Nidaros

D

Muck

2

0

4

Haslie-Seelyeville-Cathro

D

Muck

2

0

4

Histosols

A

Muck

2

2

4

Mahtomedi

A

Loamy sand

2

8

16

Markey

A

Muck

2

2

4

Meehan

B

Sand

2