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GEOLOGIC ATLAS OF CHISAGO COUNTY, MINNESOTA...Mud Lake Sunrise River North Center Lake Pioneer Lake...
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1 0 1 2 3 4 SCALE 1:300 000 1012 45678 3 9 KILOMETERS 5 MILES 1 0 1 2 3 4 5 MILES 1 0 1 2 4 5 6 7 8 3 9 KILOMETERS SCALE 1:100 000 VERTICAL EXAGGERATION X 50 800 750 725 775 850 900 825 875 Comfort Lake Green Lake Spider Lake St. Croix River 5.45 2.72 6.2 1.78 2.3 39 5.1 6.3 3.3 <cv _m _tc _tc _tc _w _w <cf _w _e _m _w _e _m _tc <nb _e <nb _e _tc _tc _j _e qu _tc _sl nu nu nu nu xt xt lc xt xt ce cr cr ce ce cr cr ce ce cr cr ce cr ce cr ce ce cr Recharge zone Recharge zone Focused recharge zone n 4 Elevation (feet above sea level) E’ 1000 800 850 900 950 750 700 650 600 550 500 450 400 350 300 250 200 Elevation (feet above sea level) E 850 800 750 700 650 600 550 500 450 400 350 300 250 200 150 900 950 100 qu qu qu Elevation (feet above sea level) D’ 1050 1100 1000 800 850 900 950 750 700 650 600 550 500 450 400 350 300 250 200 Elevation (feet above sea level) D 850 800 750 700 650 600 550 500 450 400 350 300 250 900 950 1000 Mud Lake Sunrise River North Center Lake Pioneer Lake Taylors Falls St. Croix River 2000 yrs _tc _m qu _w _w _w _tc _m _m _tc _tc _tc _w _tc _w _w _w _e _w _e <su <cv _e _tc <cf _e _tc _tc _tc _e 900 nu nu nu xt lc xt rt ce cr ce rt rt cr ce cr ce cr cr cr cr rt cr cr cr ce 850 900 875 23 9 90 10.5 5.8 6.5 5.9 1.09 4.1 4.3 Recharge zone Recharge zone Focused recharge zone 4 Elevation (feet above sea level) C’ 1050 1000 800 850 900 950 750 700 650 600 550 500 450 400 350 Elevation (feet above sea level) C 850 800 750 700 650 600 550 500 450 400 350 900 950 Hay Creek North Branch Sunrise River North Branch Sunrise River Sunrise River Dry Creek St. Croix River Douglas Fault <hs Focused recharge zone Focused recharge zone Recharge zone Unconfined conditions Unconfined conditions 500 yrs 1000 yrs 3000 yrs _m _w _e _w _w _m _e _e <cf <cv <su <cv _m _e _m _w qu qu qu _m 875 850 800 825 nu nu nu xt lc xt lc lc cr cr cr cr ce ce ce ce rt cr rt rt ce 6.1 46 14.3 5.3 5.7 1.71 Elevation (feet above sea level) 800 850 900 950 B’ 750 700 650 600 550 500 450 400 350 Elevation (feet above sea level) B 850 800 750 700 650 600 550 500 450 400 900 950 1000 1050 St. Croix River Goose Creek Goose Creek Goose Creek Fish Lake Rum River watershed Lower St. Croix watershed Recharge zone Focused recharge zone 2000 yrs _e _tc _m _e qu qu _e _e <cv _m _e _w <su _w _tc _tc _w _e _m <cv <su 850 875 nu nu lc lc lc cr ce cr ce cr ce cr 800 825 925 950 900 37 5.6 3.6 n 12.5 1.89 4 Elevation (feet above sea level) A’ 800 850 900 950 750 700 650 600 550 Elevation (feet above sea level) A 850 800 750 700 650 600 550 500 450 900 950 1000 Rush Lake Rush Lake Rush Creek Rush Creek Douglas Fault Rush Creek St. Croix River Rum River water- shed Lower St. Croix watershed Recharge zone Recharge zone 1000 yrs 3000 yrs <su _m <su <su _e <cv _e _e _m _m qu qu ostp-opdc _m _e 950 nu nu nu lc lc lc lc lc cr cr cr ce ce cr ce ce cr ce ce cr 900 875 850 825 925 800 11.5 n n n 9.6 6.8 8.1 21.1 58 3.4 7 7.2 4 95 8 95 35 61 61 35 98 361 8 Stacy Harris Shafer Wyoming Rush City Lindstrom North Branch Taylors Falls T. 36 N. T. 37 N. R. 19 W. R. 20 W. R. 21 W. R. 22 W. T. 35 N. T. 34 N. T. 33 N. (A) (B) (C) (D) (E) (A’) (B’) (C’) (D’) (E’) A B C D E A’ B’ C’ D’ E’ Elevation (feet above sea level) E” 1000 850 800 750 700 650 600 550 500 450 400 350 300 250 200 900 950 Distance (feet) 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000 100 50 Elevation (feet above sea level) 1000 800 850 900 950 750 700 650 600 550 500 450 400 350 300 250 200 E” _tc _w _e _m <nb 825 Quaternary aquitards grouped by texture Percent sand content indicates relative hydraulic conductivity (Plate 4, Table 1). sl sc se sx sr Aquifers grouped by stratigraphy Sedimentary bedrock aquifers Jordan Upper Tunnel City Wonewoc Mt. Simon Undifferentiated Mesoproterozoic Surficial aquifer Buried sand and gravel aquifers Surficial sand Enhanced-permeability zone Eau Claire Formation _e Mesoproterozoic Volcanic Rock–Clam Falls Volcanics Mesoproterozoic Volcanic Rock–Chengwatana Group Mesoproterozoic Volcanic Rock–North Branch Volcanics St. Lawrence Formation _sl Bedrock aquitards Aquitards and nonaquifers Bedrock nonaquifer units sp Primarily intergranular flow Intergranular and fracture flow * * ** * * * ** St. Peter/Prairie du Chien ** _w _m _tc _j <su Symbols and labels 21.1 If shown, nitrate-nitrogen concentration equals or exceeds 1 part per million. 7.2 If shown, groundwater residence time in years, estimated by carbon-14 ( 14 C) isotope analysis 3000 yrs Lake Peat Land or bedrock surface Geologic contact Approximate equipotential contour 925 Direction of fault movement, arrows indicate relative movement Tritium age Darker color in small vertical rectangle (well screen symbol) indicates tritium age of water sampled in well. Lighter color indicates interpreted age of water in aquifer. Mixed—Water is a mixture of recent and vintage waters (greater than 1 TU to less than 8 TU). Well was not sampled for tritium. Vintage—Water entered the ground before 1953 (less than or equal to 1 TU). Recent—Water entered the ground since about 1953 (8–15 TU). Cold War era—Water entered the ground during the peak period of atmospheric tritium concentration during nuclear bomb testing, 1958–1959 and 1961–1972 (greater than 15 tritium units [TU]). Percent sand Greater than 60 cr, ce, rt Greater than 40 and less than or equal to 50 nu Greater than 30 and less than or equal to 40 lc, xt Undifferentiated Pleistocene sediment, texture and relative hydraulic conductivity unknown qu Aquitard Groundwater leakage from an overlying buried aquifer to an underlying buried aquifer Groundwater conditions Infiltration through a thin layer of overlying, fine-grained material to an underlying aquifer Groundwater recharge from an overlying surficial aquifer to a buried aquifer Groundwater discharge to surface-water body Lateral groundwater flow If shown, chloride concentration equals or exceeds 5 parts per million. Naturally occur- ring chloride concentration greater than 5 parts per million is shown with a superscript n. <cf <cv <nb General direction of groundwater flow Groundwater movement out of cross section CROSS SECTION EXPLANATION Interpreted tritium age is indicated by background color Interpreted tritium age is indicated by pattern color Groundwater leakage through multiple aquifers and fine-grained layers 4 GEOLOGIC ATLAS OF CHISAGO COUNTY, MINNESOTA INTRODUCTION The five cross sections on this plate illustrate the horizontal and vertical extent of aquifers and aquitards commonly encountered in Chisago County. They also show groundwater residence time and the general direction of groundwater flow. These five were selected from a series of 50 west-to-east geologic cross sections constructed for Part A (Meyer, 2010). Figure 1 shows the location of the five cross sections which are coincident with those selected to show the Quaternary stratigraphy on Plate 4 of Part A. Each was slightly modified and combined with underlying bedrock geology to create the geological framework used. Select wells along these cross sections were sampled and analyzed for most of the groundwater chemistry data including major cations, anions, trace elements, stable isotopes, tritium, and carbon-14. The hydrogeology of Chisago County is influenced by the extensive sandy surficial geologic deposits in the central part of the county and by the deeply incised St. Croix River valley that forms the eastern border of the county. Since the end of the last glaciation, the St. Croix River has downcut into the glacial sediments and bedrock units creating the St. Croix River valley. Regional groundwater flow of both the Quaternary and bedrock aquifers is primarily east towards the St. Croix River. RELATIVE HYDRAULIC CONDUCTIVITY This plate identifies 1 surficial sand aquifer, 6 Quaternary buried sand and gravel aquifers, and 5 bedrock aquifers in Chisago County. The Quaternary buried sand and gravel aquifers are adequate for domestic wells, but variability in their thickness and extent can make them inadequate for muni- cipal or industrial supply wells that require higher pumping capacities. Most high-capacity wells are constructed in the bedrock aquifers. The hydraulic properties of the Quaternary buried sand and gravel aquifers and bedrock aquifers are discussed in more detail on Plate 8. The correlation of Quaternary aquifers and aquitards is shown in Figure 2. The relative hydraulic conductivity of the laterally continuous aquitards across Chisago County is depicted by shades of gray in Figure 2 and on the cross sections. Lighter shades represent higher hydraulic conductivity and darker shades represent lower hydraulic conductivity. These hues were assigned using the percentage of sand relative to silt and clay found in the matrix texture of each till unit (Part A, Plate 4, Table 1). The percentage of fine-grained particles of unconsolidated glacial material is a key control on the ability of the material to transmit water (Stephenson and others, 1988). Generally, glacial materials with higher percentages of silt and clay have less pore space available to transmit water. The lightest shade depicts the glacial sediment of the St. Croix (cr), Emerald (ce), and pre-Emerald (rt) phases of the Superior lobe. The tills of these phases are roughly 65 percent sand and 35 percent clay and silt. A medium shade depicts the New Ulm Formation (nu) of the Des Moines lobe. The till of this formation has a roughly even distribution of fines (clay plus silt) and sand. Darker shades represent sediment of the Automba phase of the Superior lobe (lc) and much older Winnipeg provenance sediment (xt). The tills of these phases are roughly 65 percent fines and 35 percent sand. Each cross section shows deeply buried undifferentiated Pleistocene sediments (qu) that are poorly understood because no wells pene- trate the sediments. The matrix texture of these sediments has not been adequately characterized; therefore, no relative hydraulic conductivity for the qu unit is presented. The sequence of bedrock geologic units, hydrogeologic units, and hydrogeologic properties is shown in Figure 3. The bedrock aquifers generally have good horizontal and vertical permeabilities. The confining units generally have low vertical permeability but may have relatively high horizontal permeability along bedding fractures that provide sufficient yield for domestic wells. Groundwater movement through the Jordan, Mt. Simon, and older undifferentiated aquifers is primarily through intergranular flow. Groundwater in the Upper Tunnel City aquifer mainly moves through fractures. Groundwater movement in the Wonewoc aquifer is through a combination of intergranular and frac- ture flow. An enhanced-permeability zone is generally found in the uppermost 50 feet of all Paleozoic sedimentary bedrock units that are either exposed at the land surface or directly covered by glacial sediment (Runkel and others, 2006). This enhanced permeability is attributed to the presence of frac- tures developed or enlarged during weathering when the bedrock surface was the land surface. The fractures in this enhanced-permeability zone generally increase the yield of aquifers. In addition, they will usually increase the permeability of aquitards, compromising their confining character. These fractures can allow units such as Mesoproterozoic volcanic rock to be used as a water source. GROUNDWATER RESIDENCE TIME The pink, green, and blue areas on the hydrogeologic cross sections represent groundwater resi- dence time. Groundwater residence time is the approximate time elapsed since precipitation infil- trated the land surface to when the water was extracted, either from a well or by natural discharge at a surface-water resource such as a spring. Groundwater residence time can be estimated by the amount of tritium ( 3 H) in the sampled water, as described on Plate 7. Recent tritium-aged waters are commonly found nearest to the land surface and vintage triti- um-age waters are commonly found at greater depths. Mixed tritium-age waters typically occur between shallow recent tritium-age waters and deeper vintage tritium-age waters, and are a blend of both. Cross section line E–E′ shows two circumstances that differ from this generalization. Vintage tritium-age water was found at a relatively shallow depth in a water sample from a well constructed in the sl aquifer near Comfort Lake. It is possible that this older water is flowing from the sandy cr aquitard that that occurs at a similar elevation to this well’s screen. The occurrence of vintage triti- um-age water at shallow depths in glacial sediments is not common, but does occur. In the vicinity of Spider Lake recent tritium-age water has migrated to depths not observed elsewhere in the county. In this vicinity, it appears that a steep groundwater gradient coupled with leaky aquitards has permitted recent tritium-age water to move to depths exceeding 250 feet below the ground surface. The concentration of several ions in groundwater can also be used to help understand ground- water age and aquifer pollution sensitivity. Elevated levels of chloride and nitrate can be used as chemical indicators of recent human influences on groundwater, including the application of road salts or fertilizer and the use of water softeners. Nitrate concentrations greater than 1 part per million (ppm) are greater than background condi- tions and indicate that an aquifer has possibly been affected by activities on the land surface (MDH, 1998 and Wilson, 2012). Nitrate concentrations greater than 3 ppm indicate that an aquifer has been affected by activities on the land surface (MDH, 1998). Chloride concentrations greater than 5 ppm can also be used to indicate that an aquifer has been impacted by activities on the land surface. However, some deep waters have naturally elevated levels of chloride. To differentiate, the ratio of chloride concentration to bromide concentration (Cl/Br) coupled with tritium results can be used to distinguish the chloride source (Davis and others, 1998; Panno and others, 2006). For this investi- gation, samples with chloride to bromide ratios below 270 have naturally elevated chloride concen- trations and are denoted on map figures with a superscript n. Groundwater samples with chloride to bromide ratios greater than 270 are interpreted as having elevated chloride due to human activity. The groundwater residence time of vintage tritium-age groundwater samples was further refined using carbon-14 ( 14 C) dating. This method uses the naturally occurring carbon-14 isotope and its half-life of 5730 years to estimate groundwater residence times from 100 to 40,000 years (Alexander and Alexander, 1989). Carbon-14 analysis was performed on 8 samples from this investigation and 8 previously collected for a joint investigation by the Minnesota Geological Survey and University of Minnesota Department of Earth Sciences (Lively and others, 1992). Of the 16 wells with modeled carbon-14 ages, 5 wells constructed in buried sand and gravel aquifers had groundwater ages ranging from 500 to 3,000 years, 8 constructed in the Mt. Simon aquifer had groundwater ages ranging from 500 to 7,000 years, 2 constructed in wells intersecting multiple bedrock aquifers had ages ranging from 200 to 2,000 years, and 1 sample provided a calibration value for recent tritium-age water. The ages determined through the carbon-14 dating of groundwater in Chisago County are relatively young for deep groundwater in Minnesota, indicating groundwater-flow systems that are recharged rela- tively quickly. This is consistent with the work of Berg and Pearson (2012), who found groundwater ages of the Mt. Simon aquifer to be generally younger in the northern Twin Cities metropolitan area west of Chisago County. HYDROGEOLOGY ILLUSTRATED BY THE CROSS SECTIONS Groundwater-flow Direction Groundwater moves from areas with higher potential energy to areas with lower potential energy. Groundwater-flow direction is indicated by the gray arrows in the cross sections on this plate and is interpreted from the equipotential contours constructed from measured water levels in wells. In Chisago County, the primary control on groundwater-flow direction is the elevation of the St. Croix River. The river serves as a key discharge area for both the buried sand and gravel and bedrock aquifers, greatly influencing groundwater-flow direction and gradients. Groundwater flow is perpen- dicular to equipotential contours; flow in Chisago County is both east and downward. Equipotential contours depicted in cross section D–D′ are more horizontal than in the other cross sections indicating groundwater movement is mostly downward. The gradient through this area appears to be controlled by the influence of a thick succession of basalt in the vicinity of Taylors Falls. The basalt has very low hydraulic conductivity, causing it to act as an impoundment which limits lateral flow to the east. To flow around this natural obstacle, groundwater must flow to the northeast or southeast. This condition is shown in mapview on Plate 8, Figures 2 through 4 and Figures 6 through 8. Near Taylors Falls the potentiometric surfaces of the buried sand and gravel aquifers and the groundwater-eleva- tion contours of bedrock aquifers show groundwater-flow direction is to the northeast and southeast around the volcanic basalt. The scale selected to show the cross sections on this plate limits the ability to show groundwa- ter-flow direction in the water table and perched groundwater systems of the county. Within these systems, groundwater flow is primarily influenced by surface landform and subsurface sediment texture characteristics. Localized groundwater-flow direction in these systems is typically toward secondary groundwater discharge areas such as smaller order streams, lakes, and wetlands. Aquifer recharge Precipitation is the source of recharge to the glacial sediments covering the county, which ulti- mately provides recharge to the deeper aquifers of the county. Recharge estimates to the surficial aquifers range from 3.4 to 8.6 inches per year (Lorenz and Delin, 2007). Recharge estimates to the buried sand and gravel aquifers and bedrock aquifers are generally less than one percent of average precipitation, or 0.3 inches per year (Delin and Falteisek, 2007). This recharge estimate varies and is dependent upon the matrix texture and thickness of the glacial sediment. Recharge rates can also be influenced by high-volume groundwater appropriation centers, which have the potential to locally steepen groundwater gradients and increase recharge. Important recharge areas exist in the county where sandy surficial sediment allows higher infiltra- tion rates. These recharge areas are primarily located in the central part of the county and are identi- fied on the cross sections as focused recharge zones. Surficial geologic units with higher clay and silt content in other regions of the county limit recharge and are denoted on the cross sections as recharge zones. Recharge also occurs in the areas of the cross sections not delineated with recharge zone labels. Stable isotopic data was collected for this atlas to better understand the interaction of lakes and open- water wetlands with groundwater. Analysis of stable isotopic data has shown that the Chisago Lakes area serves as an important groundwater recharge area (Plate 7). Aquifer discharge The St. Croix River is the major groundwater discharge feature for the buried sand and gravel and bedrock aquifers of Chisago County. Groundwater is also discharged from aquifers to other surface-water bodies such as the Sunrise River, Rush Creek, Beaver Creek, Lawrence Creek, and some wetlands and lakes. Groundwater interaction with lakes and wetlands depends on the physical setting of these features. Rush Lake and Goose Lake, located in the northwestern region of the county, receive groundwater discharge that provides perennial surface water flow from these lakes. Large wetland complexes, primarily located along the St. Croix River, receive groundwater discharge that supports native plant communities. In each of these cases, groundwater is recharged at a higher eleva- tion on the land surface and flows down gradient to these features. Groundwater discharge areas are identified on the cross section figures by . Perched groundwater The primary data used to create the water table map (Plate 7, Figure 1) is the minimum annual water depth from the Natural Resources Conservation Service (NRCS) soil survey for Chisago County. These data suggest that nearly the entire county has groundwater within 5 feet of the land surface. In some of these areas where groundwater is close to the land surface, it is likely present under perched conditions and is separated by unsaturated sediment or rock from the underlying regional water table. Perched groundwater conditions are very likely in eastern regions of the county where the land surface is much higher than the St. Croix River. Groundwater investigations in nearby Polk County, Wisconsin found evidence of water table mounding under lakes and the presence of dry conditions below lakes (Muldoon, 2000). REFERENCES Alexander, S.C., and Alexander, E.C., Jr., 1989, Residence times of Minnesota groundwaters: Minnesota Academy of Sciences Journal, v. 55, no.1, p. 48–52. Berg, J.A., and Pearson, S.R., 2012, South-central Minnesota groundwater monitoring of the Mt. Simon aquifer–phase 2: Minnesota Department of Natural Resources, 83 p. Davis, S.N., Whittemore, D.O., Fabryka-Martin, J., 1998, Uses of chloride/bromide ratios in studies of potable water: Ground Water, v. 36, no. 2, p. 338–350. Delin, G.N., and Falteisek, J.D., 2007, Ground-water recharge in Minnesota: U.S. Geological Survey Fact Sheet 2007–3002, 6 p. Lively, R.S., Jameson, J., Alexander, E.C., Jr., and Morey, G.B., 1992, Radium in the Mt. Simon- Hinckley aquifer, east-central and southeastern Minnesota: Minnesota Geological Survey, Information Circular 36, 58 p. Lorenz, D.L., and Delin, G.N., 2007, A regression model to estimate regional ground water recharge: Ground Water, v. 45, no. 2, 13 p. Meyer, G.N., 2010, Quaternary stratigraphy: Minnesota Geological Survey, Geologic Atlas of Chisago County, Minnesota, County Atlas Series C-22, Part A, pl. 4. MDH (Minnesota Department of Health), 1998, Guidance for mapping nitrate in Minnesota ground- water: 43 p., accessed online April 9, 2013, <http://www.health.state.mn.us/divs/eh/water/swp/ nitrate/reports/nitrateguidance.pdf>. Muldoon, M.A., 2000, Generalized water-table elevation map of Polk County, Wisconsin: Wisconsin Geological and Natural History Survey, Miscellaneous Map 48. Panno, S.V., Hackley, K.C., Hwang, H.H., Greenberg, S.E., Krapac, I.G., Landsberger, S., and O’Kelly, D.J., 2006, Characterization and identification of Na-Cl sources in ground water: Ground Water, v. 44, no. 2, p. 176–187. Runkel, A.C., Tipping, R.G., Alexander, E.C., Jr., and Alexander, S.C., 2006, Hydrostratigraphic characterization of intergranular and secondary porosity in part of the Cambrian sandstone aquifer system of the cratonic interior of North America–improving predictability of hydrogeo- logic properties: Sedimentary Geology, v. 184, 281–304 p. Stephenson, D.A., Fleming, A.H., and Mickelson, D.M., 1988, Glacial deposits, in Back, W., Rosenshein, J.S., and Seaber, P.R., eds., Hydrogeology: Boulder, Colorado, Geological Society of America, The Geology of North America, v. O-2, p. 301–314. Wilson, J.T., 2012, Water-quality assessment of the Cambrian-Ordovician aquifer system in the northern Midwest, United States: U.S. Geological Survey, Scientific Investigations Report 2011- 5229, 154 p. This map was compiled and generated using Geographic Information Systems (GIS) technology. Digital data products, including chemistry and geophysical data, are available from the DNR Ecological and Water Resources Division at http://www.mndnr.gov/waters. This map was prepared from publicly available information only. Every reasonable effort has been made to ensure the accuracy of the factual data on which this map interpretation is based. However, the Department of Natural Resources does not warrant the accuracy, completeness, or any implied uses of these data. Users may wish to verify critical information; sources include both the references here and information on file in the offices of the Minnesota Geological Survey and the Minnesota Department of Natural Resources. Every effort has been made to ensure the interpretation shown conforms to sound geologic and cartographic principles. This map should not be used to establish legal title, boundaries, or locations of improvements. Base modified from Minnesota Geological Survey, Chisago County Geologic Atlas, Part A, 2010. Project data compiled from 2010 to 2012 at a scale of 1:100,000. Universal Transverse Mercator projection, grid zone 15, North American Datum of 1983. Vertical datum is mean sea level. GIS and cartography by John Barry, Shana Pascal, and Holly Johnson. Edited by Carrie Jennings, Ruth MacDonald, and Holly Johnson. The DNR Information Center Minnesota Department of Natural Resources Ecological and Water Resources Division 500 Lafayette Road St. Paul, MN 55155-4025 For more information call 651-296-6157 or 888-646-6367 http://www.mndnr.gov/waters This information is available in alternative format on request. Equal opportunity to participate in and benefit from programs of the Minnesota Department of Natural Resources is available to all individuals regardless of race, color, creed, religion, national origin, sex, marital status, status with regard to public assistance, age, sexual orientation, or disability. Discrimination inquiries should be sent to MN-DNR, 500 Lafayette Road, St. Paul, MN 55155-4031 or the Equal Opportunity Office, Department of the Interior, Washington, DC 20240. © 2014. State of Minnesota, Department of Natural Resources and the Regents of the University of Minnesota. COUNTY ATLAS SERIES CHISAGO COUNTY ATLAS C-22, PART B, PLATE 9 of 10 Hydrogeologic Cross Sections Prepared and Published with the Support of the MINNESOTA ENVIRONMENT AND NATURAL RESOURCES TRUST FUND and the CLEAN WATER FUND STATE OF MINNESOTA DEPARTMENT OF NATURAL RESOURCES DIVISION OF ECOLOGICAL AND WATER RESOURCES FIGURE 2. Stratigraphic relationship in Chisago County. Quaternary buried sand and gravel aqui- fers are shown as patterns, aquitard units are shown as shades of gray, and undifferentiated Pleistocene sediment is shown in brown. Aquitards are low-permeability till and other fine-grained units that restrict groundwater movement. NOTE ABOUT THE VERTICAL EXAGGERATION Vertical exaggeration is used on cross sections A–A′ through E–E′ to allow the viewer to easily discern between aquifer units. Vertical exaggera- tion is a commonly employed technique used in geologic visualization that emphasizes vertical features that may be too small to discern relative to the horizontal scale. Vertical exaggeration is typi- cally used when the horizontal extent of the depicted terrain is significantly greater than the vertical extent. The vertical exaggeration used on the depicted cross section figures is 50x, or 50 times greater than the horizontal scale. While this technique is useful in cross section visualization, it can erroneously lead some readers to believe that aquifer units are thicker than reality. The figure in this inset depicts a small subset of cross section E–E′ with no vertical exagger- ation, allowing the reader to visualize the true thick- nesses of the aquifers and confining units relative to the horizontal scale. The depicted subset section of the E–E′ line is identified by the thin red outlined rectangle identified as E″–E‴. In addition to assisting with the visualization of thin geologic units, the vertical exaggeration technique is practical, as the E–E′ cross section would be approximately 45 feet wide if visual- ized at the 1:1 scale. HYDROGEOLOGIC CROSS SECTIONS By John D. Barry 2014 FIGURE 3. Sequence of bedrock geologic units, hydrogeologic units, and hydrogeologic unit properties in Chisago County. FIGURE 1. Location of hydrogeologic cross sections shown on this plate. The orange lines show the locations of the cross sections on this plate, which are coincident with the geologic cross section lines (shown in black) from Part A, Plate 4. Circles indicate the loca- tions of wells used to generate the hydrogeologic cross sections. Blue circles indicate wells displayed in the cross sections. Gray circles indicate wells that are not shown in areas of high well density. These wells were removed to show underlying detail. bs sl sx qu sr sp nu lc cr xt ce rt Surficial sand aquifer (unconfined) Aquitards Buried sand and gravel aquifers (confined) Undifferentiated Pleistocene sediment se sc LOCATION DIAGRAM Tunnel City Group Hydrogeologic Unit Properties Hydrogeologic Unit Geologic Unit Relatively high intergranular permeability Jordan Sandstone Mazomanie Formation Mazomanie Formation Jordan aquifer St. Lawrence Formation Lone Rock Formation Lone Rock Formation Relatively low intergranular permeability with high permeability bedding fractures Relatively low intergranular permeability with high permeability bedding fractures Confining unit, not shown on cross sections; included with _tc aquifer unit Confining unit, not shown on cross sections; included with _tc aquifer unit Relatively low permeability Moderate intergranular permeability Wonewoc Sandstone Wonewoc aquifer Eau Claire Formation Relatively low permeability Mt. Simon Sandstone Mt. Simon aquifer Moderate intergranular permeability Mesoproterozoic Sedimentary Rock Undifferentiated aquifer Moderate intergranular permeability Mesoproterozoic Volcanic Rock Volcanic nonaquifer unit Upper Tunnel City aquifer Upper Tunnel City aquifer Relatively low permeability Very low to low fracture permeability, very low yield Aquifer, relatively high permeability Aquitard, relatively low permeability Volcanic nonaquifer, very low yield High permeability bedding fracture
Transcript of GEOLOGIC ATLAS OF CHISAGO COUNTY, MINNESOTA...Mud Lake Sunrise River North Center Lake Pioneer Lake...
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cr crcr cr
rt cr
crcrce
850
900
875
23
990
10.55.8
6.5
5.9
1.09
4.14.3
Recharge zoneRecharge zoneFocused recharge zone
4
Ele
vatio
n(fe
et a
bove
sea
leve
l)C’
1050
1000
800
850
900
950
750
700
650
600
550
500
450
400
350
Ele
vatio
n(fe
et a
bove
sea
leve
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C
850
800
750
700
650
600
550
500
450
400
350
900
950
Hay
Cre
ek
Nor
th B
ranc
hS
unris
e R
iver
Nor
th B
ranc
hS
unris
e R
iver
Sun
rise
Riv
er Dry
Cre
ek
St.
Cro
ix R
iver
Dou
glas
Fau
lt
<hs
Focused recharge zoneFocused recharge zone
Recharge zoneUnconfined conditions
Unconfined conditions
500 yrs
1000 yrs
3000 yrs
_m
_w
_e_w
_w
_m
_e_e
<cf<cv<su<cv
_m
_e
_m
_w
qu
ququ _m
875
850
800825
nu
nu
nu
xt
lc
xt
lc
lc
crcr
crcr
ce
ce
ce
ce
rt
cr
rtrt
ce
6.146
14.35.3
5.7 1.71
Ele
vatio
n(fe
et a
bove
sea
leve
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800
850
900
950B’
750
700
650
600
550
500
450
400
350
Ele
vatio
n(fe
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bove
sea
leve
l)
B
850
800
750
700
650
600
550
500
450
400
900
950
1000
1050
St.
Cro
ix R
iver
Goo
se C
reek
Goo
se C
reek
Goo
se C
reek
Fish
Lak
e
Rum River
watershed
Lower St. Croix watershed
Recharge zone
Focused recharge zone
2000 yrs
_e
_tc
_m_e
qu
qu
_e
_e
<cv
_m
_e_w
<su
_w_tc_tc
_w
_e
_m
<cv
<su
850
875
nunu
lc
lclc
cr
ce cr
ce
cr
ce
cr
800
825
925
950
900
37
5.6
3.6
n
12.5
1.89
4
Ele
vatio
n(fe
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sea
leve
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A’
800
850
900
950
750
700
650
600
550
Ele
vatio
n(fe
et a
bove
sea
leve
l)
A
850
800
750
700
650
600
550
500
450
900
950
1000
Rus
h La
ke
Rus
h La
ke
Rus
h C
reek
Rus
h C
reek
Dou
glas
Fau
lt
Rus
h C
reek
St.
Cro
ix R
iver
Rum Riverwater-shed
Lower St. Croix watershed
Recharge zoneRecharge zone
1000 yrs3000 yrs
<su
_m
<su<su
_e
<cv
_e_e
_m
_m
qu qu
ostp-opdc
_m
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950nu
nunu
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crcr cece
cr
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cr
900
875
85082
5925
800
11.5n
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n
9.6
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8.1
21.1
58
3.4
7
7.24
95
8
95
35
61
6135
98
361
8
Stacy
Harris
Shafer
Wyoming
Rush City
Lindstrom
North Branch
Taylors Falls
T. 36 N.
T. 37 N.
R. 19 W.
R. 20 W.
R. 21 W.R. 22 W.
T. 35 N.
T. 34 N.
T. 33 N.
(A)
(B)
(C)
(D)
(E)
(A’)
(B’)
(C’)
(D’)
(E’)
A
B
C
D
E
A’
B’
C’
D’
E’
Ele
vatio
n(fe
et a
bove
sea
leve
l)
E”1000
850
800
750
700
650
600
550
500
450
400
350
300
250
200
900
950
Distance(feet)
150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 100010050
Ele
vatio
n(fe
et a
bove
sea
leve
l)
1000
800
850
900
950
750
700
650
600
550
500
450
400
350
300
250
200
E”
_tc
_w
_e
_m
<nb
825
Quaternary aquitards grouped by texture Percent sand content indicates relative hydraulic conductivity (Plate 4, Table 1).
slscsesxsr
Aquifers grouped by stratigraphy
Sedimentary bedrock aquifers
Jordan Upper Tunnel CityWonewoc Mt. Simon Undifferentiated Mesoproterozoic
Surficial aquifer
Buried sand and gravel aquifersSurficial sand
Enhanced-permeability zone
Eau Claire Formation_e
Mesoproterozoic Volcanic Rock–Clam Falls VolcanicsMesoproterozoic Volcanic Rock–Chengwatana GroupMesoproterozoic Volcanic Rock–North Branch Volcanics
St. Lawrence Formation_sl
Bedrock aquitards
Aquitards and nonaquifers
Bedrock nonaquifer units
sp Primarily intergranular flow
Intergranular and fracture flow*
******
**
St. Peter/Prairie du Chien**
_w
_m
_tc
_j
<su
Symbols and labels21.1
If shown, nitrate-nitrogen concentration equals or exceeds 1 part per million.
7.2
If shown, groundwater residence time in years, estimated by carbon-14 (14C) isotope analysis
3000 yrs
Lake
Peat
Land or bedrock surfaceGeologic contact
Approximate equipotential contour925
Direction of fault movement, arrows indicate relative movement
Tritium ageDarker color in small vertical rectangle (well screen symbol) indicates tritium age of water sampled in well. Lighter color indicates interpreted age of water in aquifer.
Mixed—Water is a mixture of recent and vintage waters (greater than 1 TU to less than 8 TU).
Well was not sampled for tritium.
Vintage—Water entered the ground before 1953 (less than or equal to 1 TU).
Recent—Water entered the ground since about 1953 (8–15 TU).
Cold War era—Water entered the ground during the peak period of atmospheric tritium concentration during nuclear bomb testing, 1958–1959 and 1961–1972 (greater than 15 tritium units [TU]).
Percent sand
Greater than 60cr, ce, rtGreater than 40 and less than or equal to 50nuGreater than 30 and less than or equal to 40lc, xtUndifferentiated Pleistocene sediment, texture and relative hydraulic conductivity unknown
qu
Aquitard Groundwater leakage from an overlying buried aquifer to an underlying buried aquifer
Groundwater conditions
Infiltration through a thin layer of overlying, fine-grained material to an underlying aquifer
Groundwater recharge from an overlying surficial aquifer to a buried aquifer
Groundwater discharge to surface-water body
Lateral groundwater flow
If shown, chloride concentration equals or exceeds 5 parts per million. Naturally occur-ring chloride concentration greater than 5 parts per million is shown with a superscript n.
<cf
<cv
<nb
General direction of groundwater flow Groundwater movement out of cross section
CROSS SECTION EXPLANATION
Interpreted tritium age is indicated by background color
Interpreted tritium age is indicated by pattern color
Groundwater leakage through multiple aquifers and fine-grained layers4
GEOLOGIC ATLAS OF CHISAGO COUNTY, MINNESOTA
INTRODUCTION
The five cross sections on this plate illustrate the horizontal and vertical extent of aquifers and aquitards commonly encountered in Chisago County. They also show groundwater residence time and the general direction of groundwater flow. These five were selected from a series of 50 west-to-east geologic cross sections constructed for Part A (Meyer, 2010). Figure 1 shows the location of the five cross sections which are coincident with those selected to show the Quaternary stratigraphy on Plate 4 of Part A. Each was slightly modified and combined with underlying bedrock geology to create the geological framework used. Select wells along these cross sections were sampled and analyzed for most of the groundwater chemistry data including major cations, anions, trace elements, stable isotopes, tritium, and carbon-14.
The hydrogeology of Chisago County is influenced by the extensive sandy surficial geologic deposits in the central part of the county and by the deeply incised St. Croix River valley that forms the eastern border of the county. Since the end of the last glaciation, the St. Croix River has downcut into the glacial sediments and bedrock units creating the St. Croix River valley. Regional groundwater flow of both the Quaternary and bedrock aquifers is primarily east towards the St. Croix River.
RELATIVE HYDRAULIC CONDUCTIVITY
This plate identifies 1 surficial sand aquifer, 6 Quaternary buried sand and gravel aquifers, and 5 bedrock aquifers in Chisago County. The Quaternary buried sand and gravel aquifers are adequate for domestic wells, but variability in their thickness and extent can make them inadequate for muni-cipal or industrial supply wells that require higher pumping capacities. Most high-capacity wells are constructed in the bedrock aquifers. The hydraulic properties of the Quaternary buried sand and gravel aquifers and bedrock aquifers are discussed in more detail on Plate 8.
The correlation of Quaternary aquifers and aquitards is shown in Figure 2. The relative hydraulic conductivity of the laterally continuous aquitards across Chisago County is depicted by shades of gray in Figure 2 and on the cross sections. Lighter shades represent higher hydraulic conductivity and darker shades represent lower hydraulic conductivity. These hues were assigned using the percentage of sand relative to silt and clay found in the matrix texture of each till unit (Part A, Plate 4, Table 1). The percentage of fine-grained particles of unconsolidated glacial material is a key control on the ability of the material to transmit water (Stephenson and others, 1988). Generally, glacial materials with higher percentages of silt and clay have less pore space available to transmit water. The lightest shade depicts the glacial sediment of the St. Croix (cr), Emerald (ce), and pre-Emerald (rt) phases of the Superior lobe. The tills of these phases are roughly 65 percent sand and 35 percent clay and silt. A medium shade depicts the New Ulm Formation (nu) of the Des Moines lobe. The till of this formation has a roughly even distribution of fines (clay plus silt) and sand. Darker shades represent sediment of the Automba phase of the Superior lobe (lc) and much older Winnipeg provenance sediment (xt). The tills of these phases are roughly 65 percent fines and 35 percent sand. Each cross section shows deeply buried undifferentiated Pleistocene sediments (qu) that are poorly understood because no wells pene-trate the sediments. The matrix texture of these sediments has not been adequately characterized; therefore, no relative hydraulic conductivity for the qu unit is presented.
The sequence of bedrock geologic units, hydrogeologic units, and hydrogeologic properties is shown in Figure 3. The bedrock aquifers generally have good horizontal and vertical permeabilities. The confining units generally have low vertical permeability but may have relatively high horizontal permeability along bedding fractures that provide sufficient yield for domestic wells. Groundwater movement through the Jordan, Mt. Simon, and older undifferentiated aquifers is primarily through intergranular flow. Groundwater in the Upper Tunnel City aquifer mainly moves through fractures. Groundwater movement in the Wonewoc aquifer is through a combination of intergranular and frac-ture flow. An enhanced-permeability zone is generally found in the uppermost 50 feet of all Paleozoic sedimentary bedrock units that are either exposed at the land surface or directly covered by glacial sediment (Runkel and others, 2006). This enhanced permeability is attributed to the presence of frac-tures developed or enlarged during weathering when the bedrock surface was the land surface. The fractures in this enhanced-permeability zone generally increase the yield of aquifers. In addition, they will usually increase the permeability of aquitards, compromising their confining character. These fractures can allow units such as Mesoproterozoic volcanic rock to be used as a water source.
GROUNDWATER RESIDENCE TIME
The pink, green, and blue areas on the hydrogeologic cross sections represent groundwater resi-dence time. Groundwater residence time is the approximate time elapsed since precipitation infil-trated the land surface to when the water was extracted, either from a well or by natural discharge at a surface-water resource such as a spring. Groundwater residence time can be estimated by the amount of tritium (3H) in the sampled water, as described on Plate 7.
Recent tritium-aged waters are commonly found nearest to the land surface and vintage triti-um-age waters are commonly found at greater depths. Mixed tritium-age waters typically occur between shallow recent tritium-age waters and deeper vintage tritium-age waters, and are a blend of both. Cross section line E–E′ shows two circumstances that differ from this generalization. Vintage tritium-age water was found at a relatively shallow depth in a water sample from a well constructed in the sl aquifer near Comfort Lake. It is possible that this older water is flowing from the sandy cr aquitard that that occurs at a similar elevation to this well’s screen. The occurrence of vintage triti-um-age water at shallow depths in glacial sediments is not common, but does occur. In the vicinity of Spider Lake recent tritium-age water has migrated to depths not observed elsewhere in the county. In this vicinity, it appears that a steep groundwater gradient coupled with leaky aquitards has permitted recent tritium-age water to move to depths exceeding 250 feet below the ground surface.
The concentration of several ions in groundwater can also be used to help understand ground-water age and aquifer pollution sensitivity. Elevated levels of chloride and nitrate can be used as chemical indicators of recent human influences on groundwater, including the application of road salts or fertilizer and the use of water softeners.
Nitrate concentrations greater than 1 part per million (ppm) are greater than background condi-tions and indicate that an aquifer has possibly been affected by activities on the land surface (MDH, 1998 and Wilson, 2012). Nitrate concentrations greater than 3 ppm indicate that an aquifer has been affected by activities on the land surface (MDH, 1998). Chloride concentrations greater than 5 ppm can also be used to indicate that an aquifer has been impacted by activities on the land surface. However, some deep waters have naturally elevated levels of chloride. To differentiate, the ratio of chloride concentration to bromide concentration (Cl/Br) coupled with tritium results can be used to distinguish the chloride source (Davis and others, 1998; Panno and others, 2006). For this investi-gation, samples with chloride to bromide ratios below 270 have naturally elevated chloride concen-trations and are denoted on map figures with a superscript n. Groundwater samples with chloride to bromide ratios greater than 270 are interpreted as having elevated chloride due to human activity.
The groundwater residence time of vintage tritium-age groundwater samples was further refined using carbon-14 (14C) dating. This method uses the naturally occurring carbon-14 isotope and its half-life of 5730 years to estimate groundwater residence times from 100 to 40,000 years (Alexander and Alexander, 1989). Carbon-14 analysis was performed on 8 samples from this investigation and 8 previously collected for a joint investigation by the Minnesota Geological Survey and University of Minnesota Department of Earth Sciences (Lively and others, 1992). Of the 16 wells with modeled carbon-14 ages, 5 wells constructed in buried sand and gravel aquifers had groundwater ages ranging from 500 to 3,000 years, 8 constructed in the Mt. Simon aquifer had groundwater ages ranging from 500 to 7,000 years, 2 constructed in wells intersecting multiple bedrock aquifers had ages ranging from 200 to 2,000 years, and 1 sample provided a calibration value for recent tritium-age water. The ages determined through the carbon-14 dating of groundwater in Chisago County are relatively young for deep groundwater in Minnesota, indicating groundwater-flow systems that are recharged rela-tively quickly. This is consistent with the work of Berg and Pearson (2012), who found groundwater ages of the Mt. Simon aquifer to be generally younger in the northern Twin Cities metropolitan area west of Chisago County.
HYDROGEOLOGY ILLUSTRATED BY THE CROSS SECTIONS
Groundwater-flow Direction Groundwater moves from areas with higher potential energy to areas with lower potential energy.
Groundwater-flow direction is indicated by the gray arrows in the cross sections on this plate and is interpreted from the equipotential contours constructed from measured water levels in wells. In Chisago County, the primary control on groundwater-flow direction is the elevation of the St. Croix River. The river serves as a key discharge area for both the buried sand and gravel and bedrock aquifers, greatly influencing groundwater-flow direction and gradients. Groundwater flow is perpen-dicular to equipotential contours; flow in Chisago County is both east and downward. Equipotential contours depicted in cross section D–D′ are more horizontal than in the other cross sections indicating groundwater movement is mostly downward. The gradient through this area appears to be controlled by the influence of a thick succession of basalt in the vicinity of Taylors Falls. The basalt has very low hydraulic conductivity, causing it to act as an impoundment which limits lateral flow to the east. To flow around this natural obstacle, groundwater must flow to the northeast or southeast. This condition is shown in mapview on Plate 8, Figures 2 through 4 and Figures 6 through 8. Near Taylors Falls the potentiometric surfaces of the buried sand and gravel aquifers and the groundwater-eleva-tion contours of bedrock aquifers show groundwater-flow direction is to the northeast and southeast around the volcanic basalt.
The scale selected to show the cross sections on this plate limits the ability to show groundwa-ter-flow direction in the water table and perched groundwater systems of the county. Within these systems, groundwater flow is primarily influenced by surface landform and subsurface sediment texture characteristics. Localized groundwater-flow direction in these systems is typically toward secondary groundwater discharge areas such as smaller order streams, lakes, and wetlands.
Aquifer recharge Precipitation is the source of recharge to the glacial sediments covering the county, which ulti-
mately provides recharge to the deeper aquifers of the county. Recharge estimates to the surficial aquifers range from 3.4 to 8.6 inches per year (Lorenz and Delin, 2007). Recharge estimates to the buried sand and gravel aquifers and bedrock aquifers are generally less than one percent of average precipitation, or 0.3 inches per year (Delin and Falteisek, 2007). This recharge estimate varies and is dependent upon the matrix texture and thickness of the glacial sediment. Recharge rates can also be influenced by high-volume groundwater appropriation centers, which have the potential to locally steepen groundwater gradients and increase recharge.
Important recharge areas exist in the county where sandy surficial sediment allows higher infiltra-tion rates. These recharge areas are primarily located in the central part of the county and are identi-fied on the cross sections as focused recharge zones. Surficial geologic units with higher clay and silt content in other regions of the county limit recharge and are denoted on the cross sections as recharge zones. Recharge also occurs in the areas of the cross sections not delineated with recharge zone labels. Stable isotopic data was collected for this atlas to better understand the interaction of lakes and open-water wetlands with groundwater. Analysis of stable isotopic data has shown that the Chisago Lakes area serves as an important groundwater recharge area (Plate 7).
Aquifer dischargeThe St. Croix River is the major groundwater discharge feature for the buried sand and gravel
and bedrock aquifers of Chisago County. Groundwater is also discharged from aquifers to other surface-water bodies such as the Sunrise River, Rush Creek, Beaver Creek, Lawrence Creek, and some wetlands and lakes. Groundwater interaction with lakes and wetlands depends on the physical setting of these features. Rush Lake and Goose Lake, located in the northwestern region of the county, receive groundwater discharge that provides perennial surface water flow from these lakes. Large wetland complexes, primarily located along the St. Croix River, receive groundwater discharge that supports native plant communities. In each of these cases, groundwater is recharged at a higher eleva-tion on the land surface and flows down gradient to these features. Groundwater discharge areas are identified on the cross section figures by
U
.Perched groundwater
The primary data used to create the water table map (Plate 7, Figure 1) is the minimum annual water depth from the Natural Resources Conservation Service (NRCS) soil survey for Chisago County. These data suggest that nearly the entire county has groundwater within 5 feet of the land surface. In some of these areas where groundwater is close to the land surface, it is likely present under perched conditions and is separated by unsaturated sediment or rock from the underlying regional water table. Perched groundwater conditions are very likely in eastern regions of the county where the land surface is much higher than the St. Croix River. Groundwater investigations in nearby Polk County, Wisconsin found evidence of water table mounding under lakes and the presence of dry conditions below lakes (Muldoon, 2000).
REFERENCES
Alexander, S.C., and Alexander, E.C., Jr., 1989, Residence times of Minnesota groundwaters: Minnesota Academy of Sciences Journal, v. 55, no.1, p. 48–52.
Berg, J.A., and Pearson, S.R., 2012, South-central Minnesota groundwater monitoring of the Mt. Simon aquifer–phase 2: Minnesota Department of Natural Resources, 83 p.
Davis, S.N., Whittemore, D.O., Fabryka-Martin, J., 1998, Uses of chloride/bromide ratios in studies of potable water: Ground Water, v. 36, no. 2, p. 338–350.
Delin, G.N., and Falteisek, J.D., 2007, Ground-water recharge in Minnesota: U.S. Geological Survey Fact Sheet 2007–3002, 6 p.
Lively, R.S., Jameson, J., Alexander, E.C., Jr., and Morey, G.B., 1992, Radium in the Mt. Simon-Hinckley aquifer, east-central and southeastern Minnesota: Minnesota Geological Survey, Information Circular 36, 58 p.
Lorenz, D.L., and Delin, G.N., 2007, A regression model to estimate regional ground water recharge: Ground Water, v. 45, no. 2, 13 p.
Meyer, G.N., 2010, Quaternary stratigraphy: Minnesota Geological Survey, Geologic Atlas of Chisago County, Minnesota, County Atlas Series C-22, Part A, pl. 4.
MDH (Minnesota Department of Health), 1998, Guidance for mapping nitrate in Minnesota ground-water: 43 p., accessed online April 9, 2013, <http://www.health.state.mn.us/divs/eh/water/swp/nitrate/reports/nitrateguidance.pdf>.
Muldoon, M.A., 2000, Generalized water-table elevation map of Polk County, Wisconsin: Wisconsin Geological and Natural History Survey, Miscellaneous Map 48.
Panno, S.V., Hackley, K.C., Hwang, H.H., Greenberg, S.E., Krapac, I.G., Landsberger, S., and O’Kelly, D.J., 2006, Characterization and identification of Na-Cl sources in ground water: Ground Water, v. 44, no. 2, p. 176–187.
Runkel, A.C., Tipping, R.G., Alexander, E.C., Jr., and Alexander, S.C., 2006, Hydrostratigraphic characterization of intergranular and secondary porosity in part of the Cambrian sandstone aquifer system of the cratonic interior of North America–improving predictability of hydrogeo-logic properties: Sedimentary Geology, v. 184, 281–304 p.
Stephenson, D.A., Fleming, A.H., and Mickelson, D.M., 1988, Glacial deposits, in Back, W., Rosenshein, J.S., and Seaber, P.R., eds., Hydrogeology: Boulder, Colorado, Geological Society of America, The Geology of North America, v. O-2, p. 301–314.
Wilson, J.T., 2012, Water-quality assessment of the Cambrian-Ordovician aquifer system in the northern Midwest, United States: U.S. Geological Survey, Scientific Investigations Report 2011-5229, 154 p.
This map was compiled and generated using Geographic Information Systems (GIS) technology. Digital data products, including chemistry and geophysical data, are available from the DNR Ecological and Water Resources Division at http://www.mndnr.gov/waters.
This map was prepared from publicly available information only. Every reasonable effort has been made to ensure the accuracy of the factual data on which this map interpretation is based. However, the Department of Natural Resources does not warrant the accuracy, completeness, or any implied uses of these data. Users may wish to verify critical information; sources include both the references here and information on file in the offices of the Minnesota Geological Survey and the Minnesota Department of Natural Resources. Every effort has been made to ensure the interpretation shown conforms to sound geologic and cartographic principles. This map should not be used to establish legal title, boundaries, or locations of improvements.
Base modified from Minnesota Geological Survey, Chisago County Geologic Atlas, Part A, 2010.
Project data compiled from 2010 to 2012 at a scale of 1:100,000. Universal Transverse Mercator projection, grid zone 15, North American Datum of 1983. Vertical datum is mean sea level.
GIS and cartography by John Barry, Shana Pascal, and Holly Johnson. Edited by Carrie Jennings, Ruth MacDonald, and Holly Johnson.
The DNR Information Center
Minnesota Department of Natural ResourcesEcological and Water Resources Division500 Lafayette RoadSt. Paul, MN 55155-4025For more information call 651-296-6157 or 888-646-6367http://www.mndnr.gov/waters
This information is available in alternative format on request.
Equal opportunity to participate in and benefit from programs of the Minnesota Department of Natural Resources is available to all individuals regardless of race, color, creed, religion, national origin, sex, marital status, status with regard to public assistance, age, sexual orientation, or disability. Discrimination inquiries should be sent to MN-DNR, 500 Lafayette Road, St. Paul, MN 55155-4031 or the Equal Opportunity Office, Department of the Interior, Washington, DC 20240.
© 2014. State of Minnesota, Department of Natural Resourcesand the Regents of the University of Minnesota.
COUNTY ATLAS SERIESCHISAGO COUNTY
ATLAS C-22, PART B, PLATE 9 of 10Hydrogeologic Cross Sections
Prepared and Published with the Support of the MINNESOTA ENVIRONMENT AND NATURAL RESOURCES TRUST FUND and the CLEAN WATER FUND
STATE OF MINNESOTADEPARTMENT OF NATURAL RESOURCESDIVISION OF ECOLOGICAL AND WATER RESOURCES
FIGURE 2. Stratigraphic relationship in Chisago County. Quaternary buried sand and gravel aqui-fers are shown as patterns, aquitard units are shown as shades of gray, and undifferentiated Pleistocene sediment is shown in brown. Aquitards are low-permeability till and other fine-grained units that restrict groundwater movement.
NOTE ABOUT THE VERTICAL EXAGGERATION
Vertical exaggeration is used on cross sections A–A′ through E–E′ to allow the viewer to easily discern between aquifer units. Vertical exaggera-tion is a commonly employed technique used in geologic visualization that emphasizes vertical features that may be too small to discern relative to the horizontal scale. Vertical exaggeration is typi-cally used when the horizontal extent of the depicted terrain is significantly greater than the vertical extent. The vertical exaggeration used on the depicted cross section figures is 50x, or 50 times greater than the horizontal scale. While this technique is useful in cross section visualization, it can erroneously lead some readers to believe that aquifer units are thicker than reality. The figure in this inset depicts a small subset of cross section E–E′ with no vertical exagger-ation, allowing the reader to visualize the true thick-nesses of the aquifers and confining units relative to the horizontal scale. The depicted subset section of the E–E′ line is identified by the thin red outlined rectangle identified as E″–E‴. In addition to assisting with the visualization of thin geologic units, the vertical exaggeration technique is practical, as the E–E′ cross section would be approximately 45 feet wide if visual-ized at the 1:1 scale.
HYDROGEOLOGIC CROSS SECTIONS
By John D. Barry
2014
FIGURE 3. Sequence of bedrock geologic units, hydrogeologic units, and hydrogeologic unit properties in Chisago County.
FIGURE 1. Location of hydrogeologic cross sections shown on this plate. The orange lines show the locations of the cross sections on this plate, which are coincident with the geologic cross section lines (shown in black) from Part A, Plate 4. Circles indicate the loca-tions of wells used to generate the hydrogeologic cross sections. Blue circles indicate wells displayed in the cross sections. Gray circles indicate wells that are not shown in areas of high well density. These wells were removed to show underlying detail.
bs
sl
sx
qu
sr
sp
nu
lc
cr
xt
ce
rt
Surficial sand aquifer(unconfined)
AquitardsBuried sand and gravel aquifers
(confined)
Undifferentiated Pleistocene sediment
se
sc
LOCATION DIAGRAM
Tunn
el C
ity G
roup
HydrogeologicUnit Properties
Hydrogeologic UnitGeologic Unit
Relatively high intergranular permeabilityJordan Sandstone
Mazomanie Formation
Mazomanie Formation
Jordan aquifer
St. Lawrence Formation
Lone RockFormation
Lone RockFormation
Relatively low intergranular permeability with high permeability bedding fractures
Relatively low intergranular permeability with high permeability bedding fractures
Confining unit, not shown on cross sections; included with _tc aquifer unit
Confining unit, not shown on cross sections; included with _tc aquifer unit
Relatively low permeability
Moderate intergranular permeabilityWonewoc Sandstone Wonewoc aquifer
Eau Claire Formation
Relatively low permeability
Mt. Simon Sandstone
Mt. Simon aquifer Moderate intergranular permeability
MesoproterozoicSedimentary Rock
Undifferentiatedaquifer
Moderate intergranular permeability
MesoproterozoicVolcanic Rock
Volcanic nonaquifer unit
Upper Tunnel City aquifer
Upper Tunnel City aquifer
Relatively low permeability
Very low to low fracture permeability, very low yield
Aquifer, relatively high permeabilityAquitard, relatively low permeability
Volcanic nonaquifer, very low yield High permeability bedding fracture
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