STATE OF MINNESOTA COUNTY ATLAS SERIES DEPARTMENT OF NATURAL RESOURCES ATLAS C–11, PART B, PLATE 7 OF 10 DIVISION OF WATERS Bedrock Hydrogeology 92°45' R 18 W 93°00' R 17 W R 16 W BEDROCK HYDROGEOLOGY CROSS-SECTION EXPLANATION Dsom 56 6 6 1 6 1 1 Recent—Water entered the ground since 1953 (10 or more tritium units). Ogal 92°30' R 15 W R 14 W By Well screen color shows recent water. Odub er 6000 Mixed—Water is a mixture of recent and vintage waters Odub iv Dsom 16 R reek (0.8 to less than 10 tritium units). Well screen color shows mixed water. UDOLPHO 2500 C Moira Campion 7 ot Dsom ek o re 818,000 Ogal Dsom R C ’ Vintage—Water entered the ground before 1953 (less than 0.8 tritium units). 1 PLEASANT y A e 2002 Well screen color shows vintage water. A 1240 r Odub Waltham VALLEY 1260 a Dsom C Dcvl Very old—Water with carbon-14 age greater than 10,000 years before present. 16,000 63 Well screen color shows vintage water. 1 Sargeant T 104 N son 90 in Dsom b Potentiometric Contour Cross-Section Symbols ch o In feet above mean sea level. 1240 n R Waltham a If shown, ground-water age in years, Br Dsom I-90 Contour interval is 20 feet. Arrow 18,000 1220 1220 T 104 N Robinson indicates general direction of ground- estimated by carbon-14 SARGEANT Creek ( ) U.S. 63 ’ A Cedar River A water movement Dsom SV Vintage water shallower than mixed water C Dsom e 1300 1300 Upper Cedar Valley d WALTHAM RACINE Spring associated with stream a 20 Dcvl 218 r 1240 SV Lower Cedar Valley 1 6000 Ground-water flow into the cross section 1200 2500 1200 Spillville-Maquoketa Ground-water flow out of the cross section North 36 31 Racine 1100 SV 1100 R 36 31 56 36 31 36 31 31 iv e r ’ Geologic Units and Aquifers Elevation B (feet above sea level) 1000 1000 No r B t 6000 h Dsom Sand and gravel deposits Spillville-Maquoketa aquifer R o 1 900 b 6 1 6 1 6 6 1 6 erts ° Elevation LANSING 1 16,000 (feet above sea level) Undifferentiated till Dubuque Formation Lansing Creek Dcvl 7 Bear 43 45' 43°45' Fork 2 800 Creek 7500 Dsom 18,000 Upper Cedar Valley aquifer Galena Group aquifer Dsom 2 DEXTER Dsom 1220 Brownsdale 700 Chickasaw shale Decorah-Platteville-Glenwood map unit* 1260 Dcvl k Dcvl ee Cr Lower Cedar Valley aquifer St. Peter-Prairie du Chien-Jordan aquifer Dsom ear ek GRAND B Odub Wolf Cre Dclc MEADOW *The Decorah Shale and Glenwood Formation both act as confining units, but the intervening outh Fork S eek Platteville Formation is a thin aquifer. Combined, these units are treated as a confining unit. Ramsey 9000 er Cr T 103 N Mill Pond 1200 De T 103 N Dsom Grand Dexter 1280 Ogal Dcvu 16 Meadow 16 RED ROCK I-90 Racine Characterization of Bedrock Hydrogeology 35,000 Brownsdale k 16 e 20 Roberts Creek ( ) ’ e Dcvu Dclc ’ B Lansing B This plate displays the Mower County aquifer system in map and cross-section Mu r C r phy C Cedar River ( ) views. This presentation provides practical information about the hydrogeology in C 1200 C 1300 re 1300 e the county, which is crucial to understanding ground-water supply and management k 1240 FRANKFORD ins issues. The information also was essential to the development of the pollution b 1320 Dcvl b 1200 1200 o Dcvl Dsom sensitivity map on Plate 9. Concepts that require more detailed explanation are D 1300 7500 Dclp Maple View 1240 discussed on Plate 8 and in the Technical Appendix. 1100 6000 1220 8 1100 The interpretation of the bedrock hydrogeology was modified from a historical Dsom 36 31 14 one-aquifer system to a sequence of several aquifers and confining units. This 31 31 36 31 36 36 Elevation 31 (feet above sea level) 1000 1000 approach has been used by hydrogeologists in Iowa and Minnesota (see Plate 2, Part Dclc 1340 A). The presence of regional shale confining units provides protection from surface 1300 Elkton1280 900 contamination in areas with limited, low-permeability glacial cover. Mapping the 3000 Dclc 16 90 Dcvu 1260 1 shallow confining units at a countywide scale provides an opportunity to avoid 6 1 6 Elevation 1 6 1 6 1 (feet above sea level) 6 800 drilling unnecessarily deep wells for drinking water supply. Regulators who know 90 ek re 6000 the location of the regional shale units can target aquifers that may be hydrologically r C er ee iv protected from surface contamination. This can reduce drilling costs for municipalities Dclc Dcvl D R 700 3 Dcvu and residents who depend on ground water for drinking water, irrigation, and industry. Root The bedrock aquifers, confining units, and major mapped faults in the county 28,000 Dclc Dcvu 3 D’ are shown on the map, and a pattern overlies the area where till thickness is greater D Dclc 56 Branch than 75 feet. Cretaceous deposits are thin and discontinuous; they are not shown on Austin Dcvu I-90 Grand the map or cross sections. The potentiometric surfaces for the three uppermost 7 Dexter Meadow O 1240 South aquifers are shown with 20-foot contours of different colors, as well as the locations r c 3 h 105 CLAYTON T 102 N of wells used to develop the potentiometric contours. The locations of wells sampled a r ’ T 102 N d C for geochemistry and for residence time information from tritium and carbon-14 age Dcvl C AUSTIN 1200 Dcvu C r Maple View e MARSHALL dating are also displayed. The cross sections show the arrangement of aquifers and e BENNINGTON 1300 1300 k Cedar River ( ) confining units at depth and colored residence time interpretations based on the age- Dcvu 3 dating results from the sampled wells. All sampled wells have been projected onto 1200 SV 1200 Dcvl SV the closest cross section. No well is more than 2 miles from a cross section. See Dclc 43,000 Dcvu Dcvu 9000 Plate 8 for a description of radiometric dating of ground water. 4 1100 SV 1100 The map shows potentiometric contours of the Upper Cedar Valley, Lower er v r Varco Rose i 1320 ve Cedar Valley, and Spillville-Maquoketa aquifers. The small data points showing the r 4 4 R 9000 i e WINDOM Creek a R 1000 iv 1180 1000 well locations used to develop these surfaces are displayed in colors corresponding R R w a o o w s Creek I o Dcvu e r I to the contours. There are few locations where wells from shallower and deeper e p 36 900 Elevation 900 36 31 Dcvu (feet above sea level) aquifers are close enough to one another to be used as nests for a point-by-point Dsom 218 36 36 31 p 31 ar 31 31 U d 36 Dclc comparison of water-level information from different aquifers. Enough data points e ’ Elevation C E (feet above sea level) 800 800 exist, however, to show that the potentiometric surface of each aquifer is different. E 1200 56 7 Dcvl The potentiometric differences between the Upper Cedar Valley and the Lower Cedar Dclc nch 6 1 1 6 1 a 1 e 700 Valley aquifers are subtle in the southeastern part of the county. Greater differences 6 1 6 6 r tl it B L appear in the center of cross-section E–E’. The difference between the potentiometric h Adams t r 1320 600 surfaces of the Lower Cedar Valley and the Spillville-Maquoketa is most obvious o 8000 N in the center of the map and in the center of cross-section C–C’. Generally, ground Dclc Tritium units: less than 0.8 Dclc 5 35,000 Casing depth: 930 feet water flows from east-central Mower County to the south and west in the Upper and 1160 Taopi Depth completed: 1035 feet LE ROY 11 Lower Cedar Valley aquifers. In the northeast, ground water in the Spillville- 5 Maquoketa aquifer flows from the center of the county to the north. In the northwest, 7 Dsom ground water in the Spillville-Maquoketa aquifer flows south through potentiometric U 1200 1260 1300 p Dcvu p Elkton troughs toward the Cedar River discharge area. Generally, potentiometric differences er 1300 Root Dcvu River are smallest at the Cedar and Little Iowa rivers, which are regional discharge areas. Little 1280 Austin 1180 5 1280 T 101 N ’ Recent ground water was found in samples from 12 wells based on tritium LYLE 5 NEVADA D T 101 N Cedar dating. Nine of the wells with samples of recent water have till cover less than 75 1180 Dcvu D 1200 Dcvu LODI River ( ) U.S. 218 Dcvl 1240 Dcvu Cedar 56 feet thick. For most of the central and northeastern parts of the county, ground water 1220 Dcvu Dcvu 1300 1300 Dcvu 105 Iowa 14 gets progressively older with depth. This is a typical way for water to infiltrate from Dcvu ’ 6000 the land surface into the bedrock aquifers. However, there are areas shown on cross- F 1200 1200 R iver sections A–A’ and C–C’ where samples of vintage waters were found shallower F W Dclc o 3000 o than the samples of mixed waters in deeper nearby wells. In southwest and southeast d 1100 1100 b R 1280 ur Dcvu 1260 y iv Mower County, the distribution of recent, mixed, and vintage samples on the cross C O r er eek t ter 6 e 6 sections is different from the rest of the county.