SANDSTONE AQUIFERS Aquifers in sandstone are more widespread than those in The Cambrian–Ordovician aquifer system in the north- Ironton–Galesville and St. Peter–Prairie du Chien–Jordan aqui- all other kinds of consolidated rocks (fig. 4). Although the central United States (fig. 20) is composed of large-scale, fers are similar to that of the Mount Simon aquifer. The deeply porosity of well-sorted, unconsolidated sand may be as high predominantly sandstone aquifers that extend over parts of buried parts of the aquifer system contain saline water. as 50 percent, the porosity of most sandstones is considerably seven States and three segments of the Atlas. The aquifer Regionally, water in the Cambrian–Ordovician aquifer less. During the process of conversion of sand into sandstone system consists of layered rocks that are deeply buried where system moves from topographically high recharge areas, (lithification), compaction by the weight of overlying material they dip into large structural basins. It is a classic confined, or where the aquifers crop out or are buried to shallow depths, reduces not only the volume of pore space as the sand grains artesian, system and contains three aquifers (fig. 21). In eastward and southeastward toward the Michigan and Illinois become rearranged and more tightly packed, but also the in- descending order, these are the St. Peter–Prairie du Chien– Basins. A map of the 1980 potentiometric surface of the St. terconnection between pores (permeability). The deposition of Jordan aquifer (sandstone with some dolomite), the Ironton– Peter–Prairie du Chien–Jordan aquifer (fig. 23) shows this cementing materials such as calcite or silica between the sand Galesville aquifer (sandstone), and the Mount Simon aquifer general direction of movement. The map also shows that water grains further decreases porosity and permeability. Sandstones (sandstone). The aquifers are named from the principal geo- moves subregionally toward major streams, such as the Mis- retain some primary porosity unless cementation has filled all logic formations that comprise them. Confining units of poorly sissippi and the Wisconsin Rivers, and toward major withdrawal the pores, but most of the porosity in these consolidated rocks permeable sandstone and dolomite separate the aquifers. Low- centers, such as those at Chicago, Illinois, and Green Bay and consists of secondary openings such as joints, fractures, and permeability shale and dolomite compose the Maquoketa Milwaukee, Wisconsin. In and near aquifer outcrop areas, water bedding planes. Ground-water movement in sandstone aqui- confining unit that overlies the uppermost aquifer and is moves along short flow paths toward small streams. Movement fers primarily is along bedding planes, but the joints and frac- considered to be part of the aquifer system. Wells that penetrate of water in the underlying Ironton–Galesville and Mount Simon tures cut across bedding and provide avenues for the vertical the Cambrian–Ordovician aquifer system commonly are open aquifers is similar to that in the St. Peter–Prairie du Chien– Sandstone movement of water between bedding planes. to all three aquifers, which are collectively called the Jordan aquifer. Before development, all the water moved ei- Sandstone aquifers commonly grade laterally into fine- sandstone aquifer in many reports. ther toward surface streams where it discharged as base flow, grained, low-permeability rocks such as shale or siltstone. The rocks of the aquifer system are exposed in large ar- or downgradient, toward the structural basins, into deeply aquifers Many sandstone aquifers are parts of complexly interbedded eas of northern Wisconsin and eastern Minnesota, adjacent to buried parts of the aquifer where it discharged by upward leak- sequences of various types of sedimentary rocks. Folding and the Wisconsin Dome, a topographic high on crystalline Pre- age into shallower aquifers. faulting of sandstones following lithification can greatly com- cambrian rocks. From this high area, the rocks slope south- One of the most dramatic effects of ground-water with- plicate the movement of water through these rocks. Despite ward into the Forest City Basin in southwestern Iowa and drawals known in the United States is shown in figure 24. all the above limitations, however, sandstone aquifers are northwestern Missouri, southeastward into the Illinois Basin in Withdrawals from the Cambrian–Ordovician aquifer system, highly productive in many places and provide large volumes southern Illinois, and eastward toward the Michigan Basin, a primarily for industrial use in Milwaukee, Wisconsin, and Chi- of water for all uses. circular low area centered on the Lower Peninsula of Michigan. cago, Illinois, caused declines in water levels of more than 375 95° The configuration of the top of the Mount Simon sandstone feet in Milwaukee and more than 800 feet in Chicago from (that forms the Mount Simon aquifer) is shown in figure 22. 1864 to 1980. Many of the wells in the Chicago–Milwaukee Figure 20. The Cambrian– The map shows that this aquifer, which represents the lower area obtain water from all three aquifers of the aquifer system, Ordovician aquifer system, part of the Cambrian–Ordovician aquifer system, is buried to and the water-level decline map, accordingly, is a composite which consists of predom- depths of 2,000 to 3,500 feet below sea level in these struc- map that shows the effects of withdrawals on the entire sys- inantly sandstone aquifers 90° tural basins. The configuration of the tops of the overlying tem. The declines extended outward for more than 70 miles separated by poorly permeable confining units, extends over a from the pumping centers in 1980. Movement of water in the large part of the north-central aquifers was changed from the natural flow direction (eastward United States. toward the Michigan Basin) to radial flow toward the pump- ° ing centers. Beginning in the early 1980’s, withdrawals from Duluth 85 MINNESOTA ● the Cambrian–Ordovician aquifer system decreased as some users switched to Lake Michigan as a source of supply. Water levels in the aquifer system had begun to rise by 1985 as a result of the decreased withdrawals. 45° Minneapolis ● ★ Saint Paul Figure 21. The three Green Bay ● aquifers of the Cambrian– Geologic Principal Hydrogeologic Ordovician aquifer system are nomenclature lithology nomenclature WISCONSIN separated by confining units. The Maquoketa confining unit at the top of the system creates Shale and artesian conditions wherever it dolomite 9 Madison MICHIGAN is present. Maquoketa ★ ● Milwaukee ★ Dolomite confining unit Lansing ● Sioux City Detroit ● Shaly Rockford Ordovician dolomite IOWA ● ●Chicago Des Moines Sandstone ★ Davenport ● St. Peter– Prarie du Chien– Dolomite Jordan aquifer and sandstone INDIANA ILLINOIS Dolomite and St. Lawrence- 40° fine-grained Franconia confining unit sandstone Springfield★ Indianapolis★ Ironton–Galesville Sandstone 10 Cambrian aquifer ● Cambrian–Ordovician aquifer system Kansas City EXPLANATION Shaly Eau Claire Jefferson City Saint Louis sandstone confining unit Cambrian-Ordovician aquifer system ★ ● MISSOURI Mount Simon 9 Atlas segment boundary and number Sandstone aquifer Precambrian 3 SCALE 1:7,500,000 Modified from Olcott, 1992 0 2550 MILES Base modified from U.S. 0 2550 KILOMETERS Geological Survey digital data, 1:2,000,000, 1972 95° Modified from Young, 1992b 95° Figure 22. The top of the Figure 23. Water in the Mount Simon aquifer slopes St. Peter–Prairie du Chien– from high areas near the Jordan aquifer moves from Wisconsin Dome downward 90° aquifer outcrop areas regionally 90° into the Michigan, Illinois, and toward the Michigan and Forest City Basins. The tops of Illinois Basins, subregionally S u p e Missis e r i the overlying Ironton–Galesville toward large rivers and major sip k o pi a r and St. Peter–Prairie du Chien– pumping centers, and locally L Jordan aquifers have the same 85° Duluth toward smaller streams. ° ● Duluth 85 general shape. MINNESOTA ● MINNESOTA 1000 750 R i 1000 500 v e 0 r 750 -500 1000 Wisconsin -1000 900 45° Minneapolis Dome -1500 800 ● ° ★ -2000 45 Minneapolis ● ★ Saint Paul Saint9 00Paul 800 800 ● n Green Bay r 800 ● e a 700 iv Green Bay R 600 g WISCONSIN 1000 i 7 900 50 WISCONSIN h Michigan c 800 Basin 700 i 700 900 M 500 MICHIGAN 900 800 Madison consin MICHIGAN ★ ● Milwaukee s 250 Wi Madison ★ ● Milwaukee ★ 900 450 0 800 ★ -250 Lansing 1100 ● 1000 e -500 Detroit ● Lansing Sioux City ● -750 k Detroit ● Sioux City 800 -1000 Rockford 900 -1250 ● 800 Rockford a 700 ● IOWA L -1 600 700 5 Chicago IOWA 700 00 ● Chicago Des Moines ● ★ -1750 Davenport Des Moines 0 ● ★ Davenport ● 100 500 300 Forest City 500 Basin -2000 -2500 INDIANA -2250 -3000 INDIANA 40° -2500 -2000 ILLINOIS 40° M ILLINOIS i -4000 Indianapolis s Springfield ★ s -1750 ★ i s Springfield Indianapolis★ s ★ -1500 Illinois ip p Basin i 500 EXPLANATION -1250 EXPLANATION ● -1000 Kansas City ● Kansas City Mount Simon aquifer 800 Potentiometric contour—Shows altitude at 500 Jefferson City Saint Louis ★ ● which water-level would have stood in Jefferson City Saint Louis ★ ● -250 Top-of aquifer contour—Shows altitude tightly cased wells in 1980. Dashed where of top of Mount Simon aquifer. MISSOURI approximately located. Hachures indicate R MISSOURI iv Contour interval, in feet, is variable. depression. Contour interval, in feet, is er Datum is sea level variable. Datum is sea level SCALE 1:7,500,000 Limit of St. Peter-Prairie du Chien-Jordan SCALE 1:7,500,000 0 2550 MILES aquifer 0 2550 MILES 0 2550 KILOMETERS 0 2550 KILOMETERS Direction of ground-water movement Base modified from U.S. Base modified from U.S. Geological Survey digital Modified from Olcott, 1992; and Young, 1992b Geological Survey digital data, 1:2,000,000, 1972 data, 1:2,000,000, 1972 Modified from Olcott, 1992; and Young, 1992b A10.