MBMG 99 OCCURRENCE AND CHARACTERISTICS OF GROUND WATER IN MONTANA VOLUME 1 THE GREAT PLAINS REGION by Roger A. Noble, Robert N. Bergantino, Thomas W. Patton, Brenda Sholes, Faith Daniel and Judeykay Schofield 1982 ------- n«"* This report on characteristics of aquifers in Montana is a two-volume study; Volume I has been compiled for the Great Plains physiographic province and Volume II for the Rocky Mountains physiographic province of Montana. The division into two volumes was necessary in order to facilitate descriptions of the various aquifers that occur in these two distinct topographic and structural provinces. This report contains descriptions of thickness, potentiometric surface, structural configuration and water-quality data for the major aquifers within each province. These two volumes contain a compilation of existing hydrogeologic informa- tion for the State. Because statewide hydrogeologic investigations have only recently begun in Montana, there are many data gaps, especially for the deeper aquifers, and consequently some information is still conjectural. Demands on Montana's ground water are expanding because of increasing energy development and agricultural requirements (especially irrigation). For new developments, ground water is the only alternative left, as most of Montana's surface waters are already over-appropriated. Montana is currently quantifying its water use and consumption through a water-right adjudication program. This program is being implemented by the Department of Natural Resources and Conservation through Senate Bill No. 76. The completion date for the adjudication program is April 30, 1982; therefore, quantitative statistics for Montana's ground-water use will not be available until after this date. The ground-water use section is thus based on estimates of current trends. This study on aquifer characteristics in Montana was .funded by the U.S. Environmental Protection Agency through Contract No. GO-082-908-10, for the PREFACE "R 0 o 005 s" \M ii ------- Underground Injection Control Program. The U.S. Congress enacted the Safe Drinking Water Act (Public Law 93-523) for the purpose of protecting under- ground sources of water from contamination caused by well injection. This act mandated the U.S. Environmental Protection Agency to establish the Under- ground Injection Control Program for the purpose of preventing underground injections that endanger ground-water resources. The Montana Bureau of Mines and Geology's role in the Underground Injection Control Program is to identify and characterize the aquifers in the State of Montana. iii ------- TABLE OF CONTENTS CHAPTER Page PREFACE ii GENERAL STATEMENT 1 Purpose and Scope 1 Description of Montana 1 Previous Investigations and Sources of Information 5 I. INTRODUCTION TO THE GREAT PLAINS REGION 6 Physiography 6 Topography 6 Surface Drainage 7 Climate 8 Cultural Geography 11 Population 11 Land Use and Ownership 17 Geology 18 Stratigraphy 18 Structure 19 II. HYDROGEOLOGY BY AQUIFERS 23 Quaternary Unconsolidated Deposits 23 Early Tertiary Fort Union Aquifer 26 Fox Hills-Hell Creek Aquifers 28 Judith River Aquifer 29 Eagle Aquifer 31 Kootenai Aquifer 32 Swift Aquifer 33 Madison Group 34 III. GROUND-WATER USE 36 Municipal and Domestic 38 Agriculture 40 Irrigation 40 Livestock 41 Industry 41 ------- CHAPTER Page IV. WATER QUALITY 43 Data Sources 43 General Water Quality 44 Quaternary and Late Tertiary Unconsolidated Deposits 45 Fort Union and Wasatch Aquifers 48 Fox Hills-Hell Creek Aquifers 50 Judith River Aquifer 51 Eagle (Virgelle) Aquifer 54 Kootenai Aquifer 55 Jurassic Aquifers 57 Mississippian Aquifers 60 V. SUMMARY 68 VI. REFERENCES CITED 75 APPENDIX A: Well-numbering system 76 APPENDIX B: Glossary of terms 78 APPENDIX C: Montana Water Law 81 APPENDIX D: Printout of injection wells (available upon request) APPENDIX E: Printout of water quality analyses (available upon request) ------- LIST OF TABLES TABLE PAGE 1-1 Drainage area in Montana 1 1-2 River basin inflow and outflow 8 1-3 Population of counties and county subdivisions 12-15 1-4 Well use by county 37 1-5 Comparison of selected elements and ions in waters .... 63-65 ------- LIST OF FIGURES FIGURE PAGE 1-1 Major drainage basins 4 1-2 Mean annual runoff of major streams 9 1-3 County census subdivisions 16 1-4 Stratigraphic column 20 1-5 Tectonic map 21 1-6 Frequency of occurrence compared to dissolved solids in water from Quaternary unconsolidated deposits ... 46 1-7 Frequency of occurrence compared to dissolved solids in water from early Tertiary Fort Union aquifers ... 49 1-8 Frequency of occurrence compared to dissolved solids in water from the Fox Hills-Hell Creek aquifers ... 52 1-9 Frequency of occurrence compared to dissolved solids in water from the Judith River aquifer 53 1-10 Frequency of occurrence compared to dissolved solids in water from the Eagle aquifer 56 1-11 Frequency of occurrence compared to dissolved solids in water from the Kootenai aquifer 58 1-12 Frequency of occurrence compared to dissolved solids in water from Jurassic aquifers 59 1-13 Frequency of occurrence compared to dissolved solids in water from Mississippian aquifers 62 1-14 Diagram showing well-numbering system 77 ------- LIST OF PLATES Plate TF 211.11 E Hell Creek-Fox Hills Aquifers TF 211.29 E-W Judith River Aquifer TF 211.41 E-W Eagle-Aquifer TF 217.70 E-W Kootenai Aquifer TF 221.47 E-W Swift Aquifer TF 331.60 E-W Madison Group AT 211.07 E Hell Creek Aquifer AT 211.13 E Fox Hills-Hell Creek Aquifers AT 211.21 E Bearpaw Shale AT 211.29 E-W Judith River Aquifer AT 211.39 E-W Eagle Aquifer AT 217.32 E-W Basal Colorado Sandstone AT 217.70 E-W Kootenai Aquifer AT 221.47 E-W Swift Aquifer AT 331.60 E-W Madison Group DS 100.10 E Quaternary Unconsolidated Deposits DS 125.50 E Fort Union Aquifer DS 211.11 E Fox Hills-Hell Creek Aquifers DS 211.29 E-W Judith River Aquifer DS 211.39 E-W Eagle Aquifer DS 217.70 E-W Kootenai Aquifer DS 220.50 E-W Jurassic Aquifer DS 331.60 E-W Madison Group PS 331.60 E Madison Group Legend TF - Thickness of formation AT - Altitude of formation DS - Dissolved solids PS - Potentiometric surface 211.11 - Formation code E - Eastern half of Montana's 1:500,000 scale map W - Western half of Montana's 1:500,000 scale map AVAILABLE AT CURRENT COPYING RATES ------- GENERAL STATEMENT PURPOSE AND SCOPE This report was prepared by the Montana Bureau of Mines and Geology in order for the State of Montana to comply with Federal requirements relating to the Underground Injection Control Program. Existing hydrogeologic data were used for the aquifer characterization maps and the descriptive narrative. The aquifer characterization maps depict: (1) the areal and subareal extent; (2) surface configuration; (3) thickness; (A) potentiometric surface; and (5) water chemistry expressed as dissolved solids for the major aquifers in Montana. The narrative describes the lithology, general hydrogeologic parameters and potential well yields for individual aquifers. The inventory of injection wells was compiled from information obtained from the Montana Oil and Gas Commission. The inventory provides a listing of injection wells with locations, owners, affected aquifers and injection rates. The report also contains a section delineating well use by county. While broad in scope, this report is designed to meet the needs of Federal regulatory agencies responsible for writing and implementing regulations for underground injection. DESCRIPTION OF MONTANA Montana, the third largest state of the 48 contiguous United States, is vast and diverse. It has an area of 147,138 square miles and a population of 786,690 (U.S. Department of Commerce, 1980); the average population density is 5.4 people per square mile. Most Montanans live in the major cities that are geographically dispersed throughout the state. These cities are supported by the surrounding rural communities. Although Montana is sparsely populated, it - 1 - ------- is rich in natural resources and is a prime producer of agricultural staples for the nation. Montana's abundant natural resources include fossil fuels, minerals, timber and water. These resources, however, are either fully appro- priated or are being exploited rapidly. In 1980, Montana's low-sulfur coal reserves were estimated to be in excess of 120 billion tons (U.S. Bureau of Mines, 1976). Many coal deposits of the Fort Union Formation are easily accessible through strip-mining procedures and supply a substantial part of needed energy for the nation. Total coal production for 1980 was 29,905,627 tons (Cole and others, 1981), of which 90 percent was exported to other states. Montana also has projected oil reserves of 248 trillion barrels, an undetermined reserve of natural gas and unknown potential for uranium resources (Montana Department of Natural Resources and Conservation, 1980). Montana's mineral resources are of great economic importance to the State. Montana ranks among the top five states in the production of antimony, silver, copper, talc, vermiculite and bentonite (U.S. Department of Interior, 1979). In addition to these commodities, Montana has significant deposits of lead, zinc, tungsten, chromium, manganese, nickel, titanium, vanadium, platinum-group metals, molybdenum, arsenic, iron, antimony, thorium and other rare earths. Metallic and nonmetallic exploration activity in the State is increasing every year. Most of western Montana is heavily forested, and most of these forests lie within designated state and national forests or parks. Timber harvesting occurs on selected tracts within these forests and on privately owned land. The volume of timber harvested in Montana from 1976 to present (1982) has decreased because high mortgage rates have substantially reduced the number of new , - 2 - ------- buildings being constructed. Montana's water, both from ground-water reserves and surface-water flow, is one of the State's most valuable resources, because it is vital to agricul- ture, mining and power production. More than 43 million acre-feet of water flow from the State each year; 65 percent of it originates in Montana (Montana Department of Natural Resources and Conservation, 1976). Three major river basins—the Columbia, Upper Missouri and Yellowstone—account for 97 percent of this flow. Statistics concerning the drainage areas of the major river basins are presented in Table 1-1, with the major drainage basins displayed in Figure 1-1. TABLE 1-1 DRAINAGE AREA IN MONTANA River basin Area (sq. mi. Percentage of Montana's area Percentage of Montana's water Columbia Upper Missouri Yellowstone Little Missouri St. Mary 25,152 82,352 35,890 3,428 648 147,470 17% 56% 24% 2% 1% 100% 59% 17% 21% 1% 2% 100% Of the 15 million acres of cropland in production in the State, 12.5 million acres are dryland and the remainder are irrigated. Montana's major water use is the irrigation of these 2.5 million acres of cropland from both surface-water and ground-water diversions. Agricultural demands, hydroelectric generating facilities and instream-flow reservations have already claimed most of the surface water. This surface-water demand has resulted in over-appropria- tion of these waters, placing additional demands on ground-water resources. - 3 - ------- MAJOR DRAINAGE BASINS FIGURE 1-1 ------- Sources of potable ground water in certain areas are now limited. For the purpose of this report, the State has been divided into the Rocky Mountains region and the Great Plains region. Because geology, climate and aquifer characteristics of the Great Plains region are significantly different from those of the Rocky Mountains region, this natural physiographic division was used to facilitate the aquifer descriptions in this report. The line sep- arating the two divisions is not precisely the same as that used by geographers, because it follows the eastern edge of rocks that were severely disturbed by the Laramide orogeny rather than the actual mountain front, except where the two coincide. The following is a compilation of data for each of the major aquifers of the Great Plains region. PREVIOUS INVESTIGATIONS AND SOURCES OF INFORMATION The collection of data for this report was made possible by the cooperation of the U.S. Geological Survey, especially Richard D. Feltis and William R. Hotchkiss, who furnished essential information on particular aquifer units. Other data were compiled from oil-well logs of the Montana Oil and Gas Commission, various Montana Bureau of Mines and Geology and U.S. Geological Survey publica- tions, numerous theses and dissertations and unpublished information generated from water-well logs and records. Water-quality data in this report were obtained from Montana Bureau of Mines and Geology files. Additional analyses were collected from the U.S. Geological Survey. - 5 - ------- I. INTRODUCTION TO THE GREAT PLAINS REGION The Great Plains region of Montana extends from the eastern base of the Rocky Mountains between 45° and 49° north latitude to Montana's eastern border. Nearly two-thirds of the State's 147,138 square miles lie within this region. Along the western edge of the Great Plains is a zone, as much as 25 miles wide, that was tectonically disturbed during the formation of the Rocky Mountains. Although mountains did not develop in this zone, the structure is generally so complex that meaningful structural contour, isopach or potentiometric surface maps could not be produced. For this reason the Disturbed Belt has been in- cluded with the Rocky Mountains region in Volume II of this report, and the western edge of the Great Plains region thus begins at the eastern edge of the Disturbed Belt for this discussion. Agricultural trade, based on livestock and grain production, is the main economy of the region. Industry and retail marketing, however, are expanding in importance. Oil-well drilling and coal-mining operations have grown rapidly since 1974 when the need to develop additional domestic energy resources was recognized. These operations are adding significant strength to the economic base for the region, but are placing additional demands on the ground-water resources of the region. PHYSIOGRAPHY Topography The Great Plains region comprises almost two-thirds of Montana (roughly 92,400 square miles) east of the Disturbed Belt. This region is underlain by flat to gently dipping sedimentary rocks. The rocks that form the surface are - 6 - ------- generally quite soft and have been eroded into open, rolling plains. Near some of the major rivers and in areas of recent drainage changes, however, the plains have been sharply dissected into badlands and isolated, flat-topped buttes. Near the western edge of the plains, especially in central Montana, igneous intrusions and extrusions have formed minor mountain ranges such as the Sweet- grass Hills, the Bearpaw Mountains, the Little Rocky Mountains, the Big Snowy Mountains, the Bighorn Mountains and the Pryor Mountains. Although many of these folded mountains are high and rugged, they generally are not severely disturbed by faulting as are rocks in the Disturbed Belt. Because useful structural contour, isopach and potentiometric surface maps can be made up to the bases of these mountains, they are included in the Great Plains region. Altitudes within the area designated as the Great Plains region range from 1,864 feet above sea level at the Montana-North Dakota border where the Missouri River flows out of the State, to 11,214 feet above sea level at the summit of Crazy Peak in the Crazy Mountains. Surface drainage The upper Missouri River, the Yellowstone River and the Little Missouri River comprise the major drainage systems of Montana's Great Plains region. The upper Missouri River basin is the largest river basin in the State. It contains approximately 56 percent of the land area, yet it discharges only 17 percent of the water that annually leaves the State. Within the upper Missouri basin, there are 38 reservoirs that have storage capacities of 5,000 acre-feet or more. Fort Peck reservoir on the main stem of the Missouri River is the largest of these, having a storage capacity of 19,410,000 acre-feet. The net reservoir storage for the basin is greater than 25,000,000 acre-feet. - 7 - ------- The Yellowstone River basin in Montana includes 24 percent of the State's area and annually discharges 21 percent of Montana's surface water. Yellowtail reservoir (1,375,000 acre-feet) is the largest of seven reservoirs that have a storage capacity of 5,000 acre-feet or more. River-basin inflow and outflow figures for drainages in the Great Plains region of Montana are presented in Table 1-2. The mean annual runoff of the major streams for this region is represented schematically in Figure 1-2. TABLE 1-2 RIVER BASIN INFLOW AND OUTFLOW (IN ACRE-FEET/YEAR) Drainage Inflow Originating Leaving the Percentage origin- in the region region ating in the region Upper Missouri 4,513,000 3,420,000 7,933,000 43 Yellowstone 2,734,000 6,786,000 9,520,000 71 Little Missouri 55,930 132,500 101,430 70 Climate Warm-to-hot summers, cold winters and scant precipitation characterize the Great Plains region of Montana. In the Koppen system, the climate of the area is classed as "steppe" (BSk). In the Thorntwaite system, the plains are classed as "semiarid, microthermal, precipitation deficiency in all seasons" (DC'd). Because Thorntwaite's map is more detailed than most regional climate maps, additional climate zones are shown within the region. The mountains (based on the data available at the time of compilation) are shown as "subhumid, micro- thermal, precipitation deficiency in all seasons" (CC'd). Had the data been available, an additional class would have been added for the eastern outliers of the Rocky Mountains and classified as "subhumid, microthermal, adequate - 8 - ------- Scale 30 40 6Q so m.I*. I VO I SOURCE" U.3 0 S Wot«r R»tourc«> Data For Montana Port I, 1969 Ecflh Stitnc** D«portcB«nt, Montono 8 to fa U«l»#r«itj figure 1-2 MEAN ANNUAL RUNOFF OF MAJOR STREAMS Width of stream line corresponds to top width of channel. Mean annual discharge, in thousands of cubic feet per second, is represented by channel cross section. ~5\ ------- precipitation in all seasons" (CC'r). Also, on the1summit of the Crazy Mountains, additional classes of "humid, microthermal, precipitation adequate in all seasons" (BC'r) and "taiga" (D') would be shown. In the Bighorn basin, Thorntwaite shows a small area where the climate is "arid, microthermal, precip- itation deficiency in all seasons" (EC'd). The warmest average July temperatures occur along the Yellowstone and Powder rivers where July maximum temperatures exceed 90°F. The coldest average January temperatures occur in extreme northeastern Montana where average January minimums are approximately 0°F. The difference in temperature between the average monthly maximum and the average monthly minimum is about 80°F Average temperatures moderate toward the western edge of the Great Plains. The summers are cooler because of the altitude, and the winters are warmer because of the proximity of Pacific air masses and because of the occurrence of chinook (foehn) winds. Although precipitation averages about 14 inches per year throughout the plains region, amounts as low as 8 inches per year occur in some of the low- lying valleys and in the rainshadows of mountains, and amounts as much as 18 inches per year (enough to support the growth of coniferous trees) occur on some of the higher hills and plateaus. Most of the isolated mountain ranges receive more than 20 inches of precipitation per year, thereby supporting abundant conifers. Precipitation amounts of as much as 40 inches per year occur on the summits of the Big Snowy, Little Belt and Bighorn mountains, and 60 inches per year fall on the summit of the Crazy Mountains. Spring is the main season of ground-water recharge, because this is usually the only time when there is a surplus of precipitation, cool weather and little evapotranspiration. Snowfall is scant over most of the Montana plains; the average is about - 10 - ------- 40 inches per year. The average annual snowfall increases from east to west. At the eastern border of the State, the average is about 30 inches per year, but at the western edge of the plains, the average annual snowfall is about 50 inches per year. The summits of the isolated mountain ranges of the plains receive much more snowfall than the lower-lying plains : 100-200 inches per year on the Little Rocky, Judith and Moccasin mountains; 200-300 inches per year on the Bearpaw, Highwood and Bighorn mountains; and 300-500 inches per year on the Big Snowy and Little Belt mountains and Crazy mountains. The moisture content of a deep snowpack on these mountains helps assure abundant surface water during warm, dry summers. CULTURAL GEOGRAPHY Population Most of the Great Plains region is sparsely populated. The 1980 census showed 393,063 persons living in this region. The average population density of the region is 4.25 persons per square mile. Approximately 40 percent of these people reside in five cities of over 7,000 population: Billings, Great Falls, Miles City, Havre and Lewistown. The population distribution is sum- marized in Table 1-3, with the county census subdivisions represented in Figure 1-3. The largest city of the Montana plains region is Billings, with a population of 66,798 in 1980. Nearby subdivisions and towns raise the population of the Billings marketing area to nearly double that figure. Billings is a home base for most of the coal- and oil-exploration activity in the Powder River and Williston basins, and is also a hub for livestock processing for south-central and southeastern Montana. - 11 - ------- TABLE 1-3 POPULATION OF COUNTIES AND COUNTY SUBDIVISIONS OF THE GREAT PLAINS REGION, MONTANA County/County Subdivision 1980 1970 % Change Big Horn County Crow Reservation Division Hardin Division Northern Cheyenne Division Tongue River Division 11,096 5,645 4,249 1,013 189 10,057 10.3 Blaine County Chinook Division Fort Belknap Division Harlem Division 6,999 3,172 1,854 1.973 6,727 3,263 1,312 4.0 - 2.8 41.3 Carbon County Carbon East Division Fromberg-Bridger Division Joliet Division Red Lodge Division Roberts Division 8,099 658 1,753 1,782 3,082 824 7,080 1,613 1,384 753 14.4 8.7 28.8 9.4 Carter County Ekalaka Division Little Missouri Division 1,799 1,100 699 1,956 1,135 821 - 8.0 - 3.1 -14.9 Cascade County Belt Division Cascade Division Eden-Stockett Division Great Falls Division Great Falls North Division Monarch-Neihart Division 80,696 1,626 1,559 862 70,600 2,514 277 81,804 1,406 1,354 866 260 - 1.4 15.6 15.1 - 0.5 6.5 Choteau County Big Sandy Division Fort Benton Division Geraldine Division 6,092 9.998 2,866 1,228 6,473 2,127 3,066 5.9 6.1 6.5 Custer County Miles City Division Mizpah-Pumpkin Division North Custer Division Shirley-Ismay Division 13,109 11,846 511 383 369 12,174 7.7 - 12 - ------- TABLE 1-3 (CONTINUED) County/County Subdivision 1980 1970 % Change Daniels County 2,835 3,083 - 8.0 Daniels North Division 2,709 Fort Peck Division 126 Dawson County 11,805 11,269 4.8 Dawson North Division 1,552 Glendive Division 10,253 Fallon County 3,763 4,050 - 7.1 Baker Division 3,235 3,471 - 6.8 Plevna Division 528 579 - 8.8 Fergus County 13,076 12,611 3.7 Denton Division 820 977 -16.1 Grass Range Division 617 721 -14.4 Hanover Division 765 899 -14.9 Lewistown Division 10,046 Roy Division 405 437 - 7.3 Winifred Division 423 492 -14.0 Garfield County 1,656 1,796 - 7.8 North Garfield Division 1,204 1,309 - 8.0 South Garfield Division 452 487 - 7.2 Glacier County 10,628 10,783 - 1.4 Cut Bank Division 4,540 Golden Valley County 1,026 931 10.2 Lavina Division 438 Ryegate Division 588 —— Hill County 17,985 17,358 3.6 Gildford Division 910 Havre Division 13,738 Rocky Boy Division 1,778 Rudyard Division 998 Wild Horse Lake Division 561 Judith Basin County 2,646 2,667 - 0.8 Geyser Division 542 644 -15.8 Hobson Division 920 960 - 4.2 Stanford Division 1,184 1,063 11.4 Liberty County 2,329 2,359 - 1.3 Chester Division 1,839 1,851 - 0.6 Joplin Division 490 508 - 3.5 - 13 - ------- TABLE 1-3 (CONTINUED) County/County Subdivision 1980 1970 % Change McCone County 2,702 2,875 - 6.0 Circle Division 1,766 North McCone Division 936 Musselshell County 4,428 3,734 18.6 Klein Division 988 411 140.4 Melstone Division 656 623 5.3 Roundup Division 2,784 Petroleum County 655 675 - 3.0 Winnett North Division 189 457 -58.6 Winnett South Division 466 218 113.8 Phillips County 5,367 5,386 - 0.4 Belknap Division 206 Malta Division 4,242 Phillips South Division 390 Whitewater Division 529 —;- Pondera County 6,731 6,611 1.8 Blackfeet East Division 148 Conrad Division 4,522 Valier-Dupuyer Division 1,588 2,080 -23.7 Powder River County 2,520 2,862 -11.9 Broadus Division 1,321 1,442 - 8.4 East Powder River Division 725 928 -21.9 Otter Division 474 Prairie County 1,836 1,752 4.8 Terry North Division 270 259 4.2 Terry South Division 1,566 1,493 4.9 Richland County 12,243 9,837 24.5 Fairview Division 2,267 Lambert Division 753 Savage-Crane Division 1,341 Sidney Division 7,882 Roosevelt County 10,467 10,365 1.0 East Roosevelt Division 2,134 Fort Peck Division 8,333 Rosebud County 9,899 6,032 64.1 Ashland Division 564 Forsyth Division 3,516 Northern Cheyenne Division 2,651 Rosebud Division 3,168 - 14 - ------- TABLE 1-3 (CONTINUED) County/County Subdivision 1980 1970 % Change Sheridan County 5,414 5,779 - 6.3 Fort Peck Division 179 Medicine Lake Division 1,040 Plentywood Division 3,562 Westby Division 633 721 -12.2 Stillwater County 5,598 4,632 20.9 Columbus Division 2,387 Park City Division 1,223 822 48.8 Stillwater North Division 581 Sweet Grass County 3,216 2,980 7.9 North of Yellowstone Division 675 678 - 0.4 South of Yellowstone Division 2,541 2,302 10.4 Teton County 6,491 6,116 6.1 Choteau Division 3,481 Dutton-Power Division 1,198 1,298 - 7.7 Fairfield Division 1,812 1,719 5.4 Treasure County 981 1,049 - 8.2 North Treasure Division 288 427 -32.6 South Treasure Division 693 642 7.9 Valley County 10,250 11,471 -10.6 Fort Peck Reservation Division 1,283 Glasgow Division 6,636 Wheatland County 2,359 2,529 - 6.7 Harlowton Division 1,821 Judith Gap-Shawmut Division 538 Wibaux County 1,476 1,465 0.8 Pine Hills-St. Phillips Division 347 459 -24.4 Wibaux Division 1,129 1,006 12.2 Yellowstone County 108,035 87,367 23.7 Billings Division 86,493 Buffalo Creek Division 191 156 22.4 Huntley Project Division 2,905 2,179 33.3 Laurel Division 10,086 Northwest Yellowstone Division 1,669 Shepherd Division 2,550 1,226 108.0 South Yellowstone Division 4,141 1,320 213.7 Yellowstone National Park Division 275 64 329.7 - 15 - ------- COUNTY CENSUS SUBDIVISIONS ------- Great Falls, in north-central Montana, closely follows Billings, with a population of 56,725, and is supported by surrounding subdivisions that double its population. Great Falls is primarily a marketing outlet for grain and livestock production. The city also is the site for operations of Malmstrom Air Force Base, which contributes to the economy of the area. Although the population of the Great Plains region of Montana has increased by only 3.7 percent from 1970 to 1980, the population of Richland and Rosebud counties has increased 24.5 and 64.1 percent, respectively. The population growth in these counties can be attributed dominantly to oil exploration in Richland County and strip mining of coal in Rosebud County. This rapid growth usually produces a "boom" for the area and increases construction activity. Land use and ownership Roughly 60 percent of Montana's Great Plains region is used as pasture and range for livestock grazing (this includes the open woodland areas used as summer pasture as well as for timber operation). Twenty-five percent of the pasture and less than 1 percent of the range are irrigated. The Great Plains region has 13,138,066 acres that are classified as cropland, accounting for 22 percent of the land use in the region. According to the 1974 Census of Agriculture, 54 percent of those 13,138,066 acres is classified as harvested, and 5 percent is used as pasture (not included in the above class). Other activities such as mining and petroleum operations, human habitations and recreational areas account for the small remainder of land use. About 60 percent of the Great Plains region of Montana is privately owned, but State and Federal agencies administer large portions of certain counties. The federally administered lands include game ranges, national forests, Indian - 17 - ------- reservations and lands that were either never homesteaded or were withdrawn from homesteading at a later time. The State-owned lands include two sections (school sections) out of nearly every township and school sections that were traded out of Indian reservations, national forests, etc. The State land is administered by the Department of State Lands, whereas Federal land dominantly falls under the jurisdiction of either the Bureau of Land Management or the U.S. Forest Service. GEOLOGY Stratigraphy Sedimentary rocks of all geologic ages, from Precambrian to Quaternary, underlie parts of the Great Plains region of Montana. The seas that repeatedly covered Montana in the geologic past were comparatively shallow, but gradual subsidence of the region allowed a great thickness of sediments to accumulate. The thickness of sedimentary rock over Precambrian crystalline basement ranges from 4,000 feet along the Sweetgrass Arch in west-central Montana to 15,000 feet in the Montana portion of the Williston basin. The Precambrian sedimentary rocks are predominantly quartzite and argillite, belonging to the Belt Group. The Paleozoic sedimentary rocks are mainly lime- stone and dolomite, but shale is abundant also. Many of the Paleozoic units, especially the Madison Group, are targets for oil exploration in the Big Horn and Powder River basins and along the Sweetgrass Arch of the Great Plains region. The Madison Group is also one of the most productive deep aquifers in eastern Montana. Mesozoic sedimentary rocks are dominantly shale, but there are also several sandstone units of great areal extent that are generally good aquifers. The Fort Union Formation and Wasatch Formation are Cenozoic sedimentary - 18 - ------- units that contain abundant coal. Other important Cenozoic sedimentary forma- tions are the Flaxville gravel, glacial deposits and alluvium. Almost all of the Cenozoic formations are used for water supplies because they are at or near the surface. Glacial deposits sometimes yield as little as 2 gallons per minute (gpm), whereas alluvium may yield more than 1,000 gpm. The various stratigraphic units and their time relationship are shown in Figure 1-4. Structure Most of the Great Plains region of Montana is underlain by sedimentary rocks that have eastward dips, usually less than 5°. Reversals of this dip occur only in open anticlines, synclines and domes. Near the mountains and adjacent to the Porcupine Dome and the western flank of the Cedar Creek anti- cline J dips exceed 30° for a few miles. The major structural features of the Montana plains are shown in Figure 1-5. The plains region has not undergone appreciable deformation since the end of the Laramide in Eocene times. Small-scale, open folds occur in Oligocene and Miocene formations of southeastern Montana. This deformation may be evidence of small-scale compression or tectonism since these sediments were deposited. Epeirogenic uplift of 1,500 feet has occurred in extreme eastern Montana, and 4,000 feet of uplift has occurred near the Rocky Mountains. This uplift is thought to have occurred since mid- or late-Pliocene, and may have been accompanied by regional tectonic deformation. There have been few recorded seismic events in eastern Montana since the days of organized records in 1805. The absence of seismic events, however, does not necessarily imply tectonic quiescence. A fault zone extending from Froid, in northeastern Montana, through Brockton, on the Missouri River, to south of - 19 - ------- 22x 5 22.5 37 55 635 141 195 230 280 310 345 395 435 500 570 900 1400 Pliocene QUATERNARY Colorado Group Quadrant, Tensleep Amsden Big Snowy Group Wasatch Fort Union Formation Hell Croek Fox Hills Bearpaw Judith River Claggett Eagle Telegraph Creek • Basin-filt Deposits Montana Group Ellis Group (Swift, Rierdon, Sawtooth! Madison Group (Charles, Mission Canyon, Lodgepote) Red River, Winnipeg Bighorn Pilgnm Meagher Wolsey Flathead [\\NNN] Periods of non-deposition or erosion. Belt Group Figure 1—4 - 20 - ------- I N> LEGEND MAJOR FOLD TRENDS NORMAL FAULT HIGH-ANGLE REVERSE FAULT SHOWING DIP STRUCTURE CONTOURS ON BASE OF COLORADO SHALE 25 0 25 50 75 IOO MILES I .... 1 I I 1 1 TECTONIC MAP OF THE GREAT PLAINS REGION FIGURE 1-5 ------- Weldon in McCone County (a distance of more than 100 miles) seems to have had displacement of several tens of feet since the Wisconsin glaciation. The recency of this faulting is suggested by the glacial deposits preserved in the central graben of the fault but eroded from its flanks. - 22 - ------- II. HYDROGEOLOGY BY AQUIFERS QUATERNARY UNCONSOLIDATED DEPOSITS Above the Mesozoic and early Tertiary formations are several types of un- consolidated deposits from which ground water may be derived. These unconsoli- dated aquifers include: alluvium, colluvium, terrace deposits, eolian deposits, glacial deposits, high-level gravels and the deeply weathered surface of some sandstone formations. Alluvium and terrace deposits are sinuous, river-lain sands, silts, gravels and clays within or adjacent to present-day drainage systems. Alluvium and terrace deposits are generally less than 30 feet thick along most drainages, but may be as much as 200 feet thick along some of the major rivers. Colluvium exists nearly everywhere, but it is rarely thicker than 15 feet except near the base of slopes undergoing active erosion. Eolain deposits are quite thin also (usually less than 10 feet thick) and sometimes are difficult to distinguish from colluvium unless good exposures are available. Eolian deposits are commonly found on the lee side of sandy hills and on the top of high river terraces. Extensive areas of eolian deposits occur in glaciated northern Montana, but the material does not resemble typical loess. Glacial deposits are found primarily north of the Missouri River. The deposits left behind by the ice that advanced as much as 50 miles south of the Missouri River have been largely removed by post-glacial erosion. Glacial deposits are usually less than 50 feet thick, but thicknesses greater than 100 feet occur in the Havre-Great Falls-Shelby area; in the extreme northeastern corner of Montana; and where extensive terminal or recessional moraines developed. High-level gravels are unconsolidated to semi-consolidated, Miocene and younger, fluvial deposits. High-level gravels adjacent to the isolated mountains of central Montana and those extending from - 23 - ------- the Beartooth and Bighorn mountains may have developed either as a result of recent uplift of these mountains or from climatic change. The high-level gravels 700 to 1,400 feet above the Missouri and Yellowstone rivers were de- posited by river systems ancestral to these two rivers. Differential uplift, glacial diversion and stream piracy have left many of these deposits as much as 75 miles from the present-day rivers. High-level gravels are generally less than 50 feet thick but may be as much as 100 feet thick. Although well yields of 1,000 gallons per minute or more have been obtained from high-level gravels, terrace deposits, reworked glacial deposits and some bedrock aquifers, alluvium typically yields more water to a well than any other aquifer. The probability of obtaining water from alluvium and the shallowness of a well neceissary to reach this water make alluvium the most-used aquifer in the Great Plains region of Montana. This high usage of alluvium for wells occurs because alluvial valleys often contain the best farmland and generally have the greatest population density. Terrace deposits within major river valleys adjacent to alluvium generally yield more water than terrace deposits outside the main valley or where isolated from alluvium. This difference in yield is largely a result of less ground- water recharge to the higher or more isolated deposits. Colluvium and eolian deposits are generally thin and rarely yield more than 10 gpm to a well. These aquifers were used mainly during the "homestead days" when dug wells were common. Despite the low yield of water from these deposits, the water was often satisfactory in quantity and quality to provide domestic and stock water to the homesteader. Well yields from glacial deposits are highly variable. Where the glacial deposits are mainly heterogeneous silt, clay and sand (till), water is rarely - 24 - ------- obtained, but where glacial deposits have been reworked by running water (glaciofluvial deposits, ice-marginal stream deposits, etc.), yields of more than 1,000 gpm have been obtained. Generally, however, a yield of 5 to 10 gpm may be expected from glacial deposits. The quality of water from unconsolidated deposits is as variable as the nature of the deposits themselves. Water from alluvium normally ranges from 300 milligrams per liter (mg/L) to 2,500 mg/L. The dissolved-solids content increases where alluvial deposits are in contact with Cretaceous shale or where it is influenced by salt migration from irrigated areas. Water from colluvium or weathered bedrock contains few dissolved solids if the parent material is a sandstone, but contains a large amount of dissolved solids if the parent material is a siltstone or contains much shale. Water from terrace deposits generally contains more dissolved solids than does water from alluvium. These higher concentrations of dissolved solids probably occur because there is less flushing of the terrace deposits by ground- water movement. Terrace deposits that have been cultivated for many years or that overlie shale also seem to contain water with higher concentrations of dissolved solids. High-level gravels often yield water with a low dissolved-solids content. Because areas underlain by these gravels are often quite flat and cultivated, water from these gravels is in demand for irrigation. Fine-grained glacial deposits (till, drift and lacustrine deposits) usually contain water with higher than average concentration of dissolved solids. This condition probably results from tne admixture of salt-rich Cretaceous shale into much of the glacial till. Glaciofluvial deposits, on the other hand, often contain water with few dissolved solids. - 25 - ------- Unconsolidated deposits are widely used as aquifers throughout the Great Plains region of Montana. The first well in the State was probably an alluvium well, because the water table in alluvium is usually close to the ground surface and the probability of obtaining water is very good—a strong consideration when a well was made with the pickax and shovel. More water is probably with- drawn from unconsolidated deposits (especially alluvium) than any of the other aquifers. These shallow ground-water systems are highly susceptible to contamin- ation and overuse. EARLY TERTIARY FORT UNION AQUIFER Only a few small patches of the Eocene Wasatch Formation are present in Montana, and the Cretaceous-Paleocene Willow Creek Formation of the northwestern Great Plains region has been little studied. For these reasons, the main forma- tion addressed is the Fort Union Formation. Continuous outcropping and maximum areal extent of the Fort Union Forma- tion occur primarily in the eastern third of Montana. A large area of Fort Union Formation that is separated from the main body lies in the Bighorn basin- Reedpoint syncline area of south-central Montana. Small isolated patches, often less than one square mile in extent, occur adjacent to the Bearpaw Mountains in north-central Montana. Erosion has removed the lower Tertiary deposits from much of Montana east of the Disturbed Belt and has beveled much, if not all, of these formations even where they are preserved. The lower Tertiary formations were not deposited as a uniform, continuous blanket as were most of the previously deposited formations. Tectonic activity of the Laramide orogeny was already producing major folds in the nearly horizontal strata of the Great Plains region, and these lower Tertiary - 26 - ------- deposits are accordingly thick or thin depending upon the tectonic pattern that was then current in their area of deposition. Over much of the Great Plains of Montana, restored thickness of lower Tertiary deposits indicates an accumulation of less than 1,500 feet. In the Bighorn and Powder River basins, however, lower Tertiary formations are more than 8,000 feet thick. The lower Tertiary formations in Montana were deposited entirely by fluvial systems. Channel sandstones are common in the Tullock and Tongue River members of the Fort Union Formation, and overbank siltstones and shales occur. Back- water swamps in which lush, subtropical vegetation grew, were plentiful and of long duration in southern Montana—especially during Tongue River time. Thick, extensive coal beds attest to the presence of these swamps. Because lower Tertiary sediments were deposited by fluvial systems, lithologic changes often occur over short distances. Ground water from lower Tertiary formations is obtained mainly from the sandstone units and from the coal beds, but some water is obtained from clinker. The water from the lower Tertiary formations is usually a calcium or magnesium bicarbonate type and dissolved-solids concentrations range from about 500 mg/L to more than 5,000 mg/L. The water quality and chemistry often reflect the lithologic changes that are the result of fluvial deposition. Yields of water from wells completed in lower Tertiary formations are typically less than 15 gpm, but wells yielding as much as 50 gpm have been reported. The Fort Union Forma- tion is the most widely used aquifer in eastern Montana; this is because of the great areal extent of the formation and because water is often available from it within 250 feet of the ground surface. - 27 - ------- FOX HILLS-HELL CREEK AQUIFERS These uppermost Cretaceous formations once extended from western Montana into the eastern Dakotas. Although erosion has removed much of the formations, they still are present in eastern Montana. These formations also occur just east of the Disturbed Belt, where they are referred to as the Horsethief Sand- stone and St. Mary's River Formation. Throughout most of its extent, the Fox Hills Sandstone is usually about 300 feet thick. In parts of east-central Montana, however, it was eroded and often completely removed during Hell Creek time. The Hell Creek Formation ranges in thickness from 500 to 1,100 feet. The Fox Hills Sandstone was the last marine formation to have developed in Montana. It was deposited as forset beds in a widespread delta that formed as the Bearpaw Sea withdrew. Sandstone is its most abundant component, but silt- stone and shale units are also present. The Hell Creek Formation is the upper- most Cretaceous formation to be deposited in Montana. It is a fluvial deposit that contains lenticular sandstone bodies and overbank silt and clay. Carbon- aceous shale lenses provide evidence that swampy conditions existed during the deposition of the Hell Creek Formation. Although the structural configuration of the surface of these formations was formed by the end of the Laramide orogeny, their present altitude was attained only during early Pleistocene as the result of epeirogenic uplift. In eastern Montana, these uppermost Cretaceous formations are sought as a preferred source of water. Wells commonly penetrate several other water-bearing zones that are close to the surface, but that water is cased off in order to obtain the softer water contained in the Fox Hills and Hell Creek formations. Total dissolved solids in these aquifers typically range from 500 mg/L to 1,100 mg/L. Yields of water from wells completed in these aquifers are also somewhat - 28 - ------- higher than those from aquifers nearer the surface. In many places, especially in southeastern Montana, wells penetrating Fox Hills-Hell Creek formations will have static water levels above that of the ground surface. Because many people like to have flowing artesian wells as their water sources, these aquifers are often preferred over shallower aquifers. JUDITH RIVER AQUIFER The Judith River Formation is a wedge of sandstone, siltstone and silty shale overlying the Claggett Shale and underlying the Bearpaw Shale; all these units are Cretaceous in age. Near the western margin of the Great Plains, the Judith River Formation grades into the Two Medicine Formation and is more than 700 feet thick. Near the eastern border of Montana, the Judith River Formation has thinned generally to less than 50 feet and is dominantly a silty shale. In much of north-central Montana, the lower part of the Judith River Formation is a fluvial, continental deposit and includes coal seams as much as 5 feet thick. In south-central Montana, the lower part of the Judith River Formation is a marine sandstone and is often designated as the Parkman Sandstone or Parkman Member of the Judith River Formation. Although the Judith River Formation thins eastward, east-west zones occur where the formation is considerably thicker than it is either to the north or south. These east-west zones probably mark the location of fluvial distributary channels or major, near-shore, submarine channels that were later filled with sand transported by longshore currents. The present structural configuration of the Judith River Formation was essentially attained at the close of the Laramide orogeny. At that time, however, the top surface of the formation was generally well below sea level except along the major uplifts. The Judith River Formation was raised to its present altitude during the early Pleistocene as a result of regional epeirogenic uplift. - 29 - ------- Well yields from the Judith River Formation range up to approximately 100 gpm. There is usually a good correlation between yield and total sand- stone thickness for wells that fully penetrate an aquifer. Unfortunately, most wells completed in the Judith River Formation do not fully penetrate it, but stop when enough water for household or stock use has been obtained. Thus, the yields from this aquifer usually reflect water needs rather than true capacity of the aquifer. High drilling costs and low well yields have combined to prevent development of this aquifer where it is substantially beyond 500 feet below ground surface. Consequently, little is known about its water-yielding capabilities or potentiometric surface with distance from the outcrop areas. Water in the Judith River Formation is under sufficient pressure to cause it to rise in a well considerably above the level at which it enters the well. Flowing wells occur along the Missouri River between Little Rocky Mountains and Larb Creek and along the Musselshell River (and its preglacial course) from about Mosby to Beaver Creek. The quality of water from the Judith River Formation can range up to 27,500 mg/L dissolved solids. The water with the fewest dissolved solids is found close to recharge areas, and the more saline water is found in the eastern part of the State where for formation contains more shale. Ground water that stays in contact with salt-containing formations leaches the salts by solution. In structurally low areas, this water rarely moves laterally, and the dissolved- solids concentration of the water increases greatly. Wide ranges in water quality occur even near the outcrops. These quality variations may reflect lithologic differences. - 30 - ------- EAGLE AQUIFER The Eagle Formation is one of the main aquifers in the northwestern part of the Great Plains region of Montana. It is also commonly used in southern Montana west of 108° west longitude. The areas in which this aquifer is highly used correspond to the area of its outcrop and where the formation is less than 300 feet below the ground surface. The Eagle Formation is rarely more than 400 feet thick; its thickest section is near the western limit of the Great Plains region where the formation is predominantly sandstone. Siltstone and shale become dominant in the Eagle Formation with distance eastward from the Disturbed Belt. East of 107° west longitude, the Eagle Formation contains so much shale that it is often called the Gammon Formation or Gammon Shale and includes the equally shaley Telegraph Creek Formation. The Gammon Shale is exposed in the Black Hills uplift of southeastern Montana but contains so much shale that only about five wells are known to obtain water from it. The yields from these wells average less than 3 gpm, and the water can be used only for stock watering. The Eagle Formation receives little use as an aquifer in northeastern Montana because of its great depth below surface and scant yield of water. In northwestern Montana and near Bowdoin Dome, water from the Eagle Formation may contain natural gas. Cattle will drink this water after some of the gas has escaped, but humans who try to use this water complain of a sulfur taste. In the areas where the Eagle Formation is highly used as a source of water, the quality of its water is generally good. Dissolved-solids content of the water is usually less than 1,500 mg/L and often less than 1,000 mg/L. Yields of 500 gpm have been reported from the Cut Bank area, but yields generally average less than 50 gpm. - 31 - ------- KOOTENAI AQUIFER In latest Jurassic time, mountain uplift began in extreme western Montana and eastern Idaho with the primary intrusion of the Idaho batholith. The intensive mountain building of the Laramide orogeny was millions of years in the future, but the steepened gradients formed by these newly emergent hills produced streams that flowed eastward across swampy deposits that were to become the Morrison Formation. At first the streams cut into the eroded Morrison deposits, but as uplift ceased or was reduced in the west, the streams began to deposit sand along their channels. In time, the channels coalesced and migrated laterally. Eventually, sand blanketed most of eastern Montana to a depth of as much as 100 feet. This sand now forms the basal unit of the Kootenai Formation and is referred to as the Sunburst Sandstone, Cutbank Sand- stone, the Third Cat Creek Sandstone, the Pryor Conglomerate or the Lakota Sandstone. Following the deposition of this basal unit, crustal subsidence of the continental interior allowed the Cretaceous sea to enter eastern Montana. In this sea, the upper part of the Kootenai Formation was deposited. It consists of maroon or red and green shale with local bodies of sandstone. Locally, fresh- water limestone was deposited. The basal sandstone unit of the Kootenai Formation is the main aquifer, but in many places the upper sandstone units and the limestone unit produce enough water for stock or domestic use. Throughout most of the central and western Montana plains, where the Kootenai Formation is within 500 feet of the surface, the Kootenai Formation is sought for its ground water. Although yields as low as 10 gpm are sometimes obtained from the upper part of this formation, yields of 300 gpm have been obtained from the basal sandstone. In many places a well tapping the basal sandstone will produce flowing water. - 32 - ------- Chemical analyses of water from the Kootenai 'Formation show dissolved solids ranging from less than 204 mg/L near outcrop areas in the Little Belt Mountains to more than 14,000 mg/L in the Sweetgrass Arch near Cutbank. SWIFT AQUIFER The only Jurassic formation known to have sufficient water-bearing potential to be considered an aquifer is the Swift Formation. This formation is present throughout the Great Plains region of Montana except in the central areas of the plains mountains from where it has been eroded. The Swift Forma- tion generally thickens from west to east. It is about 100 feet thick along the east edge of the Disturbed Belt and 400 to 600 feet thick along Montana's eastern border. This greater thickness is caused by shale units which are more abundant in the east than in the west. Throughout most of central and western Montana the Swift Formation contains a total sandstone thickness of 40 to 150 feet. In eastern Montana, however, the total sandstone thickness is commonly less than 50 feet and in many places is less than 25 feet. Because the Swift Formation is at a considerable depth below land surface, few wells have been drilled into it solely to obtain water; thus, most informa- tion on its water-bearing characteristics comes from oil wells. This information indicates that where total sandstone thickness is more than 100 feet (western and central Great Plains region), wells will generally yield 50 gallons of water per minute. In eastern Montana, where the Swift Formation is much deeper below land surface and where the total thickness of sandstone is less, data are in- sufficient to evaluate its aquifer characteristics. Almost all chemical analyses of water from the Swift Formation are from western and north-central Montana. These analyses show that water from the - 33 - ------- Swift Formation commonly contains less than 500 mg/L dissolved solids with 10 miles of the outcrop, but dissolved solids increase rapidly with increasing dis- tance from the outcrop. Water with a dissolved-solids content of more than 4,000 mg/L is found in many places along the axis of the Sweetgrass Arch. MADISON GROUP The Madison Group aquifer extends across the entire Great Plains region. It is absent only in a few small areas at the center of mountain uplifts where it has been removed by emplacement of igneous rocks or by erosion. The Madison Group is dominantly limestone, but its uppermost unit, the Charles Formation, becomes increasingly an anhydrite with proximity to the center of the Williston basin. In some places the limestone has been largely dolomitized. Throughout most of eastern Montana, the Madison Group is more than 600 feet thick. A maximum thickness of more than 1,000 feet occurs along a trough that extends from the Big Snowy Mountains to the center of the Williston basin. Because of its great potential as an aquifer, the Madison Group has been the object of an extensive drilling and investigation program throughout the northern Great Plains. Data are currently becoming available to make a reasonably detailed evaluation of this aquifer, however, deep drilling depths have deterred extensive development. The test wells drilled through the Madison Group showed a great degree of variability in yield of water from this aquifer. The yield was highly dependent on fracture porosity, initial porosity and degree of dolomitization. Well yields ranged from 20 gpm to more than 1,000 gpm. Data from oil-well drill- stem tests are currently being compiled to determine the spatial variation in water yield. Because the Madison Group is recharged with ground water in the central areas of mountain uplifts, water in this aquifer is commonly under - 34 - ------- enough pressure to flow at the surface from any well that penetrates the aquifer. In some places the pressure is great enough to produce a static water level several hundred feet above the land surface. The quality of water in the Madison Group is highly dependent on the com- position of the Madison Group and the distance the water has traveled from area of recharge or, more specifically, the amount of time it has spent within the aquifer. Adjacent to the mountains, where the aquifer is recharged and where the Madison Group is dominantly limestone, dissolved-solids content of the water within the Madison Group is commonly less than 500 mg/L. Dissolved-solids con- tent of the water increases within a few miles of the 500 mg/L isoline to more than 1,000 mg/L, and then increases more slowly. Near the periphery of the Williston basin, where the Charles Formation is largely an anhydrite, dissolved solids again rapidly increase from 4,000-5,000 mg/L to more than 15,000 mg/L within about 10 miles. The dissolved-solids content of water from the Madison Group near the center of the Williston basin is greater than 300,000 mg/L, or about 10 times that of seawater. Water-bearing formations occur below the base of the Madison Group. Some of these formations have great potential as aquifers. The well depth required to obtain water from these formations exceeds 6,000 feet throughout most of eastern Montana. Well depths of 16,000 feet are necessary to reach the base of the deeper formations in the Williston basin. Because of these extreme drilling depths and the sparse data on aquifer characteristics, these pre-Madison Group formations are not included as aquifers in this report. - 35 - ------- III. GROUND-WATER USE Information on water use in Montana prior to 1980 is extremely limited because of a lack of accurate withdrawal-rate data. While communities have the best opportunity to record water use, in most instances only new delivery systems are equipped to measure discharge. Similarly, rural, agricultural and industrial water users often have no means of measurement, and only estimates can be made for these values. However, Montana is presently quantifying its water use and consumption through a water-right adjudication program. This program is being implemented through the Department of Natural Resources and Conservation under Senate Bill No. 76. All water-use applications are to be filed by April 30, 1982, and then will be reviewed and summarized. Better estimates of precise ground-water and surface-water use will become available after that date. Major uses of ground water in the Great Plains region are for irrigation, municipalities, industry, rural domestic use and livestock. Table 1-4 is a summary, by county, from MBMG's well appropriation files of the various well uses in this region. Most of these wells are completed in the Quaternary alluvium or Tertiary and Cretaceous aquifers, although deeper aquifers are exploited locally. The cumulative total of ground water withdrawn from the Great Plains region is approximatley 114.41 million gallons per day (mgd), or 351.24 acre-feet per day. This value for ground water represents about 2 percent of the total amount of water diverted within the Great Plains region, a value much lower than for most other western states. Even though 2 percent is a small percentage, ground water is the only viable source of potable water that can and will be further developed now that surface-water supplies are over- appropriated in this region. - 36 - ------- TABLE I - 4 WELL USE BY COUNTY IN THE GREAT PLAINS REGION COUNTY COM DOM D+S IRR IND PUB STK MU OTH NOT TOTAL RPT Bighorn 3 199 180 7 18 10 377 39 41 13 887 Blaine 1 109 233 23 6 8 374 14 ¦10 8 786 Carbon 1 611 269 13 2 13 202 55 63 8 1237 Carter 0 65 120 0 0 2 648 13 6 0 854 Cascade 22 904 417 14 5 U 233 62 53 1 1722 Chotcau 0 96 395 6 0 28 338 82 34 5 984 Cus ter 12 225 218 19 3 9 685 92 22 8 1293 Daniels 1 135 126 2 2 6 191 74 11 6 554 Dawson 13 362 338 17 17 14 693 89 30 4 1577 FalIon 0 62 235 3 8 7 568 28 8 6 925 Fergus 1 301 424 10 13 7 449 110 52 3 1370 Garfield 0 65 153 4 2 2 685 25 15 0 951 Golden Valley 0 50 115 3 0 0 276 17 3 0 464 Hill 0 345 319 12 9 21 226 70 25 4 1031 Judith Basin 2 175 150 15 3 8 308 60 15 1 737 Liber ty 0 66 102 2 0 3 116 33 7 1 330 McCone 2 133 167 10 0 7 486 25 12 851 Musselshell 0 235 311 6 2 9 823 33 14 21 1454 Petroleum 1 15 56 4 47 1 189 22 7 1 343 Phillips 2 162 435 20 8 7 476 29 21 1164 Pondera 0 36 148 2 (1 13 83 21 6 1 310 Powder River 1 226 216 28 3 9 1577 11 15 1 2087 Prairie 4 109 102 10 9 5 580 14 10 1 844 Richland 1 354 281 1 7 2 1 20 817 48 17 8 1586 Roosevelt 1 193 212 41 1 ! 10 331 48 10 9 868 Rosebud 0 164 152 9 1 7 21 663 21 32 9 1088 Sheridan 0 112 224 5 b 12 185 26 19 4 593 Stillwater 3 453 215 7 5 6 394 40 30 1 1154 Sweetgrass 0 185 156 5 2 2 175 22 11 1 559 Teton 5 368 390 43 2 10 257 79 14 4 1172 Toole 0 23 88 4 1 1 1 123 47 14 2 313 Treasure 0 27 44 2 0 2 221 9 0 1 306 Valley 0 273 435 21 6 24 553 99 29 21 1461 Wheatland 1 64 57 8 2 2 288 26 10 0 458 Wibaux 0 51 188 5 2 7 405 23 12 1 694 Yellowstone 4 1711 544 152 17 '8 737 219 54 8 3464 Total 81 8664 8215 549 253 34 5 15732 1725 732 175 36471 COM- Community; DOM-Domestic; D+S-Domcstic and Stockwntor; IRR-1rngation; IND-Industrial; PUB-Public; STK- Stockwater only; MU-Multiusc; OTH-Othcr; NOT RPT- Not Reported. - 37 - ------- MUNICIPAL AND DOMESTIC A computer listing produced by the Montana Department of Health and Environmental Sciences (1980) shows that there are 89 communities in the Great Plains region of Montana that have a municipal water-supply system. The total number of public supply systems exceeds 250, including trailer courts, nursing homes and other institutions. Of the 89 communities, 13 rely exclusively on surface water, another 8 use both surface and ground water and the remaining 68 communities depend solely on wells or springs for their water supply. Of the 393,063 people who reside within the Great Plains region of Montana, approximately 264,300 live in municipalities. Of these, 91,400 (35 percent) depend exclusively upon ground water for their drinking and household needs; they withdraw a total of about 19.06 million gallons of ground water per day. Only a small percentage of this total is used to satisfy industrial or commercial needs. Although several different aquifers are used throughout the Montana plains for municipal water, no study or examination has been made to determine which aquifers are used. A cursory examination of the relationship of towns in the plains to geology indicates that perhaps as many as 90 percent of these water supplies derive their water from alluvium or other unconsolidated deposits. At least two towns (Broadus and Ekalaka) obtain water from either the Fox Hills or Hell Creek formations; several may obtain water from the Eagle and Kootenai formations; and Lewistown obtains its water supply from a spring that emerges from a limestone formation of the Madison Group. The quality of water used by many of the communities in the Montana plains region often exceeds the EPA's maximum recommended limits for dissolved solids. Several systems distribute water that contains more than 1,500 mg/L, and the - 38 - ------- well supplying the town of Jordan has dissolved solids of about 1,800 mg/L. Although these values greatly exceed the 500 mg/L recommended maximum, most communities have no other source. Iron is a problem for many community water systems. Most systems dis- tribute water that contains less than the recommended maximum of 0.3 mg/L for iron, but some greatly exceed it. Four community systems that distribute water exceeding 4.0 mg/L in 1980 were North Harlem Colony, Wibaux, Wolf Point and Wyola. Nitrates in water are a problem in some farming communities, both from application of fertilizers and from degradation of water supplies caused by saline seeps. The following community systems in 1980 delivered water with exceptionally high nitrate values: North Turner Colony (35 mg/L), Flaxville (28 mg/L), Conrad (21 mg/L), Coffee Creek (14 mg/L), and Denton (13 mg/L). In addition, the water from several trailer courts was analyzed and found to contain more than 7 mg/L. These latter high nitrate values may result from septic-system effluent or related pollution. Fluoride values are generally well below the EPA recommended maximum. Community systems that distributed water high in fluorides in 1980 were: Sidney (5.1 mg/L), Circle (4.7 mg/L), Richey (4.2 mg/L), and Lambert (4.0 mg/L). Several trailer courts also distributed water in 1980 that exceeded 2.0 mg/L. Trace metals in water from public supply systems were all usually within the recommended maximum. One exception was Lodge Grass with 0.805 mg/L of lead. Domestic water is that which is used by all persons not served by a munici- pal or community water system. For the most part, domestic wells primarily belong to rural residents, although some subdivisions have individual wells. There are approximately 17,000 domestic and stockwater wells in the Great Plains region. Ground water from these wells provides 95 percent of the rural population - 39 - ------- with a potable supply. Rural inhabitants withdraw 25.85 million gallons per day, most of which is consumed. Domestic wells are commonly drilled until sufficient amounts of relatively good quality water are reached, and for this reason, well yields are generally small. For most rural inhabitants of the Great Plains region, ground water is essential for their well being and livelihood. AGRICULTURE Agriculture is the largest ground-water user in the Great Plains region. The principal use of ground water is for cropland irrigation with a secondary use for livestock consumption. IRRIGATION There are about 1,347,740 acres of irrigated land in the Great Plains region. However, the percentage of this acreage that is irrigated in any given year is uncertain. Roughly 5.62 billion gallons per day (bgd) are diverted to this acreage, of which 1 percent is withdrawn from ground-water sources. Almost all of these irrigation wells are completed in the unconsolidated alluvial aquifer. The Madison aquifer is also receiving renewed interest as a deeper source of good-quality water. Requirements for diversion are more than double consumptive use, resulting in a return flow of 53 percent of the total diversion (DNRC, 1975). Consumptive use varies with irrigation efficiency, rates of application and other factors such as the crop, soil, precipitation, growing season and temperature. Nearly all irrigation is used to raise crops and feed to support the livestock industry. - 40 - ------- LIVESTOCK Stock consumptive use of ground water in the Great Plains region is estimated to be 19.71 million gallons per day, of which 50 percent is withdrawn from ground-water sources. Cattle and sheep account for most of the water being consumed, with average daily consumption values of 15 and 3 gallons per head per day, respectively. Pigs, horses and other livestock make up the remainder of stock water consumed. Stock water wells comprise the largest single category of permitted wells in this region; roughly 43 percent of the wells in the Great Plains region are used solely for stock-watering purposes. These wells tap all the aquifers within the region and are usually completed when a sufficient yield is obtained. In many cases these wells are the only viable source of water in the area for livestock ranchers. INDUSTRY Ground-water withdrawals by industry are separated into two distinct con- stituents: (1) the petroleum industry, which uses the largest quantity; and (2) self-supplied industry, which withdraws only a minor amount. Ground-water withdrawals by the petroleum industry fall into two major categories—fresh water, which is developed solely for use in the secondary recovery of crude oil; and produced water, which is withdrawn as a byproduct of primary and secondary oil recovery. Estimates of the total petroleum ground water withdrawn are highly variable, and for this reason an accurate value could not be predicted. However, records show that there are 659 injection wells re- injecting water for secondary recovery in the Great Plains region. According to Montana's Oil and Gas Conservation Division (1980), these wells have an - 41 - ------- average daily injection rate of 15.06 million gallons per day (mgd). Major aquifers used to produce water for secondary oil recovery include the Fox Hills, Judith River, Eagle and Kootenai formations and the Madison Group. The Madison aquifer has been the principal source of secondary recovery water for most oil fields in this region. Future projections of oil production within the Big Horn basin cannot be made reliably. Much oil remains to be produced by secondary and tertiary recovery methods, but economics will play a major role in whether or not this oil is produced. It seems likely that with rising prices, oil production in the basin, especially the percentage produced by secondary recovery methods, will continue to grow, as will petroleum-industry water consumption. The Montana Department of Natural Resources and Conservation defines self-supplied industrial water as that which is obtained from a source of supply by industry, as opposed to that provided by a municipality. An industry is also considered to be self-supplied if any of the water it uses is obtained from its privately owned water-supply facilities. The primary industrial use of water is for condenser cooling, while smaller amounts are used for processing, washing, conveying, air conditioning, boiler feeding and sanitation. Ground water accounts for about 10.5 percent of the total water used by industry in the Great Plains region. Approximately 3.47 million gallons of ground water are used daily as self-supplied industrial water. A large percentage of this water is consumed by industry. Examples of consumptive use are water that is canned or bottled in foods or beverages, and water absorbed or chemically combined into a manufactured product. - 42 - ------- IV. WATER QUALITY DATA SOURCES More than 3,000 water-quality analyses contained in the computer files at the Montana Bureau of Mines and Geology (MBMG) were reviewed for the Underground Injection Control project, with approximately 2,700 of these analyses from the Great Plains region. Additional analyses were located and reviewed from MBMG and U.S. Geological Survey (USGS) publications, including bulletins, memoirs, open-file reports, professional papers, hydrologic atlas maps and unpublished reports. USGS Open-File Report 76-40, by William Hopkins, provided much data relating to deeper aquifers in eastern Montana. The MBMG water-quality file contains water-quality analyses generated by the MBMG Analytical Division. Primary customers of this Division are the MBMG Hydrology Division, the USGS Water Resources Division and the U.S. Forest Service (USFS). The USGS Water Resources Division and the MBMG Hydrology Division furnish water samples taken from ground-water sources within the State of Montana to the MBMG laboratory for analysis, and the results of these analyses become part of an integrated data bank. Approximatley 2,000 ground- water analyses were completed and entered into computer's storage during the USGS's Northern Great Plains project of recent years. Additionally, the computer files contain data extracted from selected USGS and MBMG publications that existed prior to the creation of the data bank. Documents such as USGS Water- Supply Papers 599 and 600, covering ground-water resources for Rosebud, Treasure and Yellowstone counties, have been coded and included in the file. Many similar publications are yet to be included. Appendix V contains a tabulation of those analyses in the ground-water- quality system selected for this project. These analyses have been sorted by - 43 - ------- aquifer, and also according to township, range and section within the region. Many of these have been plotted on the Dissolved Solids Map series included with this report. Occasionally, points will appear on the listings that have not been plotted on the maps, and conversely, points will appear on the maps which are not contained in the tabulation. This has occurred because much of the previously published data are not computerized and because the listings may include data created since the compilation of the maps. GENERAL WATER QUALITY Ground-water quality data for eight aquifers or aquifer groups in the Great Plains region were compiled. These aquifers included: 1. Quaternary and Late Tertiary unconsolidated deposits 2. Fort Union and Wasatch Aquifers 3. Fox Hills-Hell Creek Aquifers 4. Judith River (Parkman) Aquifer 5. Eagle (Virgelle) Aquifer 6. Kootenai Aquifer 7. Jurassic Formations 8. Madison Group and other formations of Mississippian age The next sections will discuss the water quality of these eight groups in terms of their dissolved solids concentrations expressed as milligrams per liter (mg/L) and dominant cations and anions prevalent. - 44 - ------- Quaternary and Late Tertiary Unconsolidated Deposits The MBMG water quality file contains 575 dissolved solids values reported from 587 water quality analyses for water from Quaternary unconsolidated deposits in the Great Plains region. The data base for this aquifer represents a general overview of water quality for alluvial, glacial outwash, and glacial drift deposits. However, because the MBMG has historically been involved in research on subjects such as coal field and saline-seep hydrology, the data set presents a bias towards waters characteristic of these hydrologic situations. To lessen any bias, water analyses for waters produced by mine spoils have not been included and multiple analyses for water from research wells have been deleted. Even so, the water related to research sites is able to influence the entire data set. For example, the average sulfate ion concentration of 3097 mg/L for the unconsolidated formations includes approximately 50 analyses for saline- seep waters with sulfate ion concentrations ranging from 10,000 to 50,000 mg/L. Analyses such as these effectively increase the average dissolved solids concentration. The lowest dissolved solids concentration for water from the Quaternary deposits is from a 56-foot deep well located in T. IS., R. 14 E., section 21 ACAD in Sweetgrass County. This well produces a calcium bicarbonate type water having a dissolved solids concentration of 137 mg/L from an alluvial aquifer closely related to the Boulder River. The highest reported dissolved solids concentration is for water from a saline-seep research well at a saline- seep research site in Fergus County. This well is located in T. 17 N., R. 15 E., section 7 AAAB and produced magnesium sulfate water with 68,095 mg/L of dissolved solids for a water quality sample collected in 1979. Figure 1-6 is a histogram showing the number of analyses in this data file versus different ranges of dissolved solids concentrations. The most common analyses are for water with less than 2,000 mg/L of dissolved solids. The - 45 - ------- 175- 150- 125- ino 1001 2001 3001 4001 5001 6001 7001 8001 9001 >10000 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 DISSOLVED SOLIDS IN MG/L - 46 - ------- histogram also indicates some trends in water quality in terms of dissolved constituents. Waters with bicarbonate as the predominate anion were found in 127 of the 161 analyses within the dissolved solids concentration range of 0 to 1,000 mg/L. Above 1,001 mg/L of dissolved solids only 30 of 414 analyses were for bicarbonate type water. The sulfate anion is present in all of the dissolved solids intervals, but becomes exclusive in the higher concentrations. Calcium, magnesium, and sodium cations are dominant in waters with dissolved solids concentrations of less than 3,000 mg/L, but above 3,000 mg/L sodium becomes predominate. The late Tertiary unconsolidated rocks of the Great Plains regions consist principally of the Flaxville Formation and its equivalents. Only four analyses for waters from these deposits are in the data file. Dissolved solids for these analyses range from 250 to 900 mg/L. Of these, three analyses reported magnesium as being the dominant cation, and all four reported bicarbonate as the dominant anion. The fourth analysis had calcium as the dominant cation. Zimmerman (1960) reported four additional analyses from northern Blaine County, Montana. These analyses ranged in dissolved solids from 300 to 1,430 mg/L. Two of these waters were sodium bicarbonate, while the others were sodium sulfate and magnesium bicarbonate waters. The Flaxville Formation gravels underlie approximately 37 townships in northeastern and eastern Montana. The formation generally occurs in isolated patches except on the Big Flat north of Harlem and in the Plentywood-Scobey area of northeastern Montana. In these latter areas, the Flaxville Formation is an important water source, often providing the best quality water available in the area, but elsewhere it finds relatively minor use as an aquifer. - 47 - ------- Fort Union and Wasatch Aquifers Over 1,700 water quality analyses representing 1,330 individual wells or springs are found for Early Tertiary aquifers of the Great Plains region in the MBMG data bank. The data set represents a reasonable geographic overview for these aquifers, but portions of it represent densely concentrated data points in the coal fields of southeast Montana. The MBMG and USGS have had a historic interest in the coal hydrology of these areas and research sites have generated many water quality analyses to obtain detailed water quality profiles. As in the Quaternary unconsolidated deposits data set, analyses representing numerous samples from the same site or research well have been deleted leaving one representative analysis. Detailed water quality descrip- tions of many coal hydrology research sites are presented in Van Voast and others, 1977, Van Voast, 1974 and Van Voast, 1975. The lowest dissolved solids concentration of 111 mg/L was found in a sample from a 71 foot deep well located in T. IN., R. 41 E., section 17 BBBB in Rosebud County. This well produced a sodium bicarbonate water which was unused at the time of sampling. The highest dissolved solids concentration of 9,578 mg/L was found in a sample from a 94 foot deep research well located in T. 9 S., R. 44 E., section 7 BACD in Bighorn County. The water is unused and is a sodium sulfate type. Figure 1-7 is a histogram of frequency of occurrence compared to dissolved solids in mg/L for water analyses from the early Tertiary Fort Union aquifer in the MBMG system. This chart shows that the bulk of the analyses were for water less than 3,000 mg/L of dissolved solids and only 180 (14%) were for waters with greater than 3,000 mg/L of dissolved solids. Virtually all of the higher dissolved solids samples were sodium or magnesium sulfate waters. In the 0 to 3,000 mg/L range, as the dissolved solids - 48 - ------- 700- FIGURE ~ 1-7 FREQUENCY OF OCCURRENCE COMPARED TO DISSOLVED SOLIDS IN WATER FROM EARLY TERTIARY FORT UNION AQUIFERS, GREAT PLAINS REGION, MONTANA -700 600" 500- 400 300- 200- 100 -500 NaHC03 MgS04 NaS04 OTHERS (MgHC03) (CaHC03) (CaS04) (NaC03) l-MgS04 4g 5-NaS04 1-NaHCQ-^ 13 X 7 l-NaS04 2-MgS04 1-MgSO^ 2. I 2 / I-N3SO4 =*» 1001 2001 3001 4001 5001 6001 7001 8001 9001 >10000 600 -400 -300 •200 •100 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 DISSOLVED SOLIDS IN MG/L - 49 - ------- concentration decreases the bicarbonate anion becomes more common with sodium, magnesium and calcium the predominate cations. However, in the 0 to 1,000 mg/L dissolved solids range the number of analyses reporting bicarbonate waters becomes greater than the number of analyses reporting sulfate water. Figure 1-7 does not indicate where in the Early Tertiary formations a particular water type of dissolved solids concentration is likely to be found. Lee (1981), in USGS Water Supply Paper 2076 discussing the geochemistry of water in the Fort Union Formation within the Northern Powder River Basin, pro- vided a good description of water qualities in these rocks. Lee correlated 665 samples from springs, wells less than 200 feet, and wells greater than 200 feet in depth with water chemistry and found distinct differences between these three sources. Water from springs ranged from a low of 160 mg/L to a high of 5,260 mg/L and averaged 1,630 mg/L of dissolved solids primarily being a calcium sodium sulfate type water. For wells less than 200 feet in depth, the dissolved solids concentrations ranged from 110 to 6,300 mg/L, averaged 2,100 mg/L and were primarily magnesium sodium sulfate type waters. The third category, wells greater than 200 feet in depth, ranged in dissolved solids concentrations from 390 mg/L to 5,720 mg/L, averaged 1,400 mg/L, and were predominantly magnesium sodium bicarbonate type waters. Lee suggested that water qualities in the Fort Union formation of the northern Powder River Basin were controlled by a static regional system overlain by smaller dynamic recharge-storage-discharge cells. Fox Hills-Hell Creek Aquifers The regional Fox Hills and Hell Creek aquifers in eastern Montana are represented by 278 water-quality analyses in the MBMG's water-quality files. - 50 - ------- Ground water from these formations is generally soft and is of good quality, ranging in dissolved solids from 107 to 4,400 mg/L. The lowest dissolved- solids value reported is from an 80-foot well in Sweet Grass County, which is completed in the Hell Creek Formation. The water is used for domestic purposes and is a sodium bicarbonate type. The highest dissolved-solids value of 4,421 mg/L was measured in water from a 350-foot stock well in Musselshell County, and the water is a sodium sulfate type. Figure 1-8 is a histogram of the number of analyses of waters versus dissolved solids for the Fox Hills and Hell Creek aquifers. This histogram shows that 223 of the 276 samples (81%) were for waters with less than 2,000 mg/L of dissolved solids. There are no reports of waters containing more than 10,000 mg/L of dissolved solids in these aquifers. The frequency of higher dissolved solids from the Fox Hills and Hell Creek formations is more often a sodium bicarbonate type, with sodium sulfate becoming predominant as values above 1,000 mg/L occur. Judith River Aquifer MBMG's data file contains 221 analyses of Judith River Formation waters. Measurements range from a low dissolved-solids value of 161 mg/L for water from a 203-foot well in Wheatland County, to a high value of 27,500 mg/L for ground water from a 200-foot well in Liberty County. The low value represents a calcium bicarbonate type that is used for stock water. The high dissolved- solids value is a sodium sulfate type that is unused. Figure 1-9 is a histogram showing the number of analyses and dominant water type plotted against values for dissolved solids. The three most common water types in descending order are: sodium sulfate, sodium bicarbonate and - 51 - ------- 140- FICURE ~ 1-8 FREQUENCY OF OCCURRENCE COMPARED TO DISSOLVED SOLIDS IN WATER FROM THE FOX HILLS-HELL CREEK AQUIFERS, GREAT PLAINS REGION, MONTANA. - HO 120- - 120 116 ioo-;; 96 c/} w o a w ps o& P u o o o Pi Cx] 80- 60- 40- 20- 0 1001 - 100 NaHCO- NaSO/ CaHCO-; OTHERS (CaS04) (MgHC03) (NaCl) 2001 I*'p . j 3001 '4001 '5001 '6001 '7001 '8001 '9001 MOOOCf -80 r 60 C/} W o a w pd pi P o u o o pi w 40 - 20 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 DISSOLVED SOLIDS IN MG/L - 52 - ------- C/3 w o z w pi pi !=> o a o pu o o4 w 1001 2001 3001 4001 5001 6001 7001 8001 9001 >10000 1000 2000 3000 4000 5000 6000 7.000 8000 9000 10000 DISSOLVED SOLIDS IN MG/L - 53 - ------- sodium chloride. Other water types reported include magnesium sulfate (8 occurrences), magnesium bicarbonate (2 occurrences), and calcium bicarbonate (5 occurrences). The histogram shows that analyses with dissolved solids of less than 2,000 mg/L are dominantly sodium bicarbonate waters, with sodium sulfate becoming more common as dissolved solids increase. Variability in water types appears to decrease as values of dissolved solids increase. Of the 220 analyses, 162 or 74% contained less than 3,000 mg/L of dissolved solids, and four analyses were for waters in excess of 10,000 mg/L. All waters having dissolved-solids values of more than 5,000 mg/L were sodium sulfate waters. Eagle (Virgelle) Aquifer The Eagle Formation of north-central Montana has 93 analyses on file in the water-quality data bank. Waters range from a low dissolved-solids value of 285 mg/L in a 91-foot well in southern Blaine County, on the northwest flanks of the Little Rocky Mountains, to a high value of approximately 16,000 mg/L of a sodium sulfate water from a 91-foot flowing well in Toole County. A dissolved-solids value of 13,000 mg/L of a sodium chloride water recovered during a drill-stem test of the Shannon Sandstone in Bighorn County was also reported. Both occurrences of dissolved solids in excess of 10,000 mg/L are apparently related to oil and gas exploration work. The Shannon test was from ground water between 2,300 and 2,350 feet below land surface. Copious data points appear on the Eagle Formation DS map from the Cut Bank area—an area of primary use for this aquifer. Data for this region are con- tained in MBMG Bulletin 60, by E.A. Zimmerman. Analyses in this report are from water samples taken between T. 32 N. to T. 37 N., and between R. 3 W. to - 54 - ------- R. 6 W., in Toole and Glacier counties. Values of dissolved solids in this area ranged from 384 mg/L in a 160-foot well in T. 34 N., R. 5 W., section 35DC, to 5,210 mg/L in a 575-foot well in T. 36 N., R. 6 W., section 21CB. According to Zimmerman (1960), the quality of the water in the Eagle Formation varies locally in this area, but sodium is normally the predominant cation and bicarbonate or sulfate the predominate anions. Figure 1-10 is a histogram showing the number of analyses versus dissolved- solids values for the Eagle aquifer. Of the samples in MBMG's data files, 45 (58%) had dissolved-solids values of less than 2,000 mg/L. Most of these were sodium bicarbonate waters, with the second most prevalent type being sodium sulfate waters. Other ground-water types represented were calcium sulfate (2 occurrences), calcium bicarbonate (5 occurrences), magnesium sulfate (2 occur- rences), calcium bicarbonate (5 occurrences), and magnesium sulfate (1 occurrence). Above the 2,000 mg/L level, sodium bicarbonate waters become less common and sodium sulfate waters more prevalent. Sodium chloride waters also become more common above this level. Kootenai Aquifer The water-quality data bank contains 130 analyses of Kootenai Formation and Kootenai-equivalent waters. Equivalent formations included are the Cloverly, the Fuson, the Lakota and the Second and Third Cat Creek sandstones. In addi- tion to those analyses in MBMG's files, 20 analyses presented by Hopkins in 1976 were reviewed. Of the 130 samples, 108 are from wells or springs in an area bounded by T. UN. on the south side and T. 18 N. on the north side, and R. 9 E. and R. 35 E. on the west and east sides, respectively, in Judith Basin and Fergus counties. Therefore, only a limited areal extent of these aquifers - 55 - ------- 35- FIGURE — I-10 FREQUENCY OF OCCURRENCE COMPARED TO DISSOLVED SOLIDS IN WATER FROM THE EAGLE AQUIFER, GREAT PLAINS REGION, MONTANA. 30" :\U _,N .J -30 NaHC03 NaS04 - 25 OTHERS (NaCl) (MgS04) (CaS04) (MgHC03) (CaHC03) ~i 1 1 ' "t 0 1001 2001 3001 4001 5001 6001 7001 8001 9001 >10000 ¦35 - 20 r 15 10 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 DISSOLVED SOLIDS IN MG/L - 56 - ------- is represented, although this is their primary-use area. A low dissolved- solids value of 204 mg/L was reported in water from a 690-foot well in Judith Basin County; this water is a calcium bicarbonate type. A high dissolved- solids value of 10,500 mg/L, representing a sodium chloride water, was reported in drill-stem test water from an oil well completed in the Third Cat Creek Formation in Stillwater County. The histogram of Figure 1-11 shows that 121 (81%) of the 150 analyses have dissolved-solids values of less than 1,000 mg/L. These waters are primarily sodium bicarbonate (32%), calcium bicarbonate (37%), and calcium sulfate (18%). So few samples exist in the higher values ranging between 1,000 and 5,000 mg/L, that water-type breakdowns are not valid; however, it does appear that sodium chloride-type waters become moTe common. In the Cut Bank area, the lower Kootenai-equivalent sands are oil and gas producers and contain water ranging from 4,000 to 14,000 mg/L of dissolved solids. Jurassic Aquifers Of the 35 analyses (34 MBMG and 1 Hopkins) reviewed for aquifers in Jurassic-age rocks, one contained 36,100 mg/L of dissolved solids. This water was a sodium chloride-type from the Piper Formation and was obtained during a drill-stem test at a well in eastern Valley County. In MBMG's data bank, the highest value noted for dissolved solids was 4,245 mg/L from a 4,702-foot live- stock well in northern Rosebud County; this is a sodium sulfate type water. The lowest dissolved-solids value of 204 mg/L was for a calcium bicarbonate type water from a 249-foot well finished in the Morrison Formation in Judith Basin County. Figure 1-12 is a histogram of water analyses plotted against values of - 57 - ------- 140- 120 100- 80' 60- 40- 20- FIGURE — I-11 FREQUENCY OF OCCURRENCE COMPARED TO DISSOLVED SOLIDS IN WATER FROM THE KOOTENAI AQUIFER, GREAT PLAINS REGION, MONTANA. -140 21 Ullllllllllllllllll ly.v.v.vlv^ NaHCO-: CaHCOc CaSO/ OTHERS (MgHCOo) (NaS04) (NaCl) (MgS04) (NaC03) (INCLUDES 20 ANALYSES FROM HOPKINS,1976) """1 1 I 1 1 fuamaaa 0 1001 2001 3001 4001 5001 6001 7001 8001 9001 >10000 120 -100 CO w o ¦80 w -60 -40 -20 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 DISSOLVED SOLIDS IN MG/L - 58 - ------- 35- FIGURE ~ 1-12 FREQUENCY OF OCCURRENCE COMPARED TO DISSOLVED SOLIDS IN WATER FROM JURASSIC AQUIFERS, GREAT PLAINS REGION, MONTANA -35 30- -30 25- 20- 23 15-.' 10" NaHC03 NaSO, -25 CaHCO-: OTHERS (MgHC03) (MgS04) (CaS04) (NaCl) 5- 20 15 - 10 - 5 I i ¦ ¦ i i 0 1001 2001 3001 4001 5001 6001 7001 8001 9001 >10000 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 DISSOLVED SOLIDS IN MG/L - 59 - ------- dissolved solids. There are too few analyses represented to distinguish definite trends in water type. It does appear, however, that calcium bicarbonate-group waters predominate when dissolved-solids values are less than 1,000 mg/L. Mississippian Aquifers MBMG's computer file contains 37 water-quality analyses for water from Mississippian-age rocks. These analyses represent water sources in the western and central portions of the Great Plains region. Many of the water samples were obtained during research on warm-water wells and springs. Figure 1-13 is a comparison of dissolved-solids values to the number of analyses from waters from Mississippian-age rocks. Hopkins (1976) evaluated and classified an additional 75 waters from Mississippian-age rocks. Feltis (1980) mapped dissolved-solids values; ratios of sulfate to total anions; and ratios of sodium plus potassium plus chloride to dissolved-solids values from Madison Group rocks in the Great Plains region. These references plus others represent a wealth of information and data that has been gathered for the Madison Group because of the high interest in the water-yielding and water-quality character- istics of these rocks relative to their potential for industrial water develop- ment . The highest dissolved-solids values reported occur iji the extreme north- eastern corner of Montana, underlying portions of Sheridan, Richland and eastern Roosevelt counties. In this area, dissolved-solids values of 100,000 mg/L are common, and values approaching 300,000 mg/L are reported. These waters are sodium chloride brines associated with evaporite deposits within the Charles and Mission Canyon formations of this portion of the Williston basin. The highest dissolved-solids value in MBMG's files is for water from the Angela - 60 - ------- Hot Springs well in northeastern Rosebud County which has a value of 5,919 mg/L. This well produces a sodium calcium chloride sulfate water from well perforations placed between 8,152 and 8,183 feet below land surface- Waters obtained from sources near the outcrops of Mississippian-age rocks represent the other extreme and are very much lower in dissolved solids. The lowest dissolved-solids value noted was from rocks of the Big Snowy Group in Fergus County. A 225-foot well completed in these rocks produced a magnesium bicarbonate water of approximately 256 mg/L. This water is used for domestic purposes. Figure 1-13 shows that 30 of the 37 analyses in this small group had dissolved-solids values of less than 2,000 mg/L with calcium sulfate being the predominant water type. Most of these analyses represent waters in or relatively near the outcrop areas for Mississippian-age rocks. According to Feltis (1980), anion trends closely follow increases in dissolved-solids values. Waters with low sulfate concentrations are generally found near outcrops, but sulfate concentrations increase rapidly even a short distance from the outcrop. Over most of the Great Plains region, sulfate con- centration is greater than 50% of the total anion content of the water. In both the Williston basin and Sweetgrass Arch areas, chloride becomes the dominant anion. In these areas, Mississippian-age rocks produce oil and gas and have sodium plus potassium plus chloride ratios to dissolved-solids values of greater than 50 percent. Table 1-5 compares selected elements and ions to drinking water quality standards published by the U.S. Environmental Protection Agency (EPA). However, since no standard has been established for sodium plus potassium, an arbitrary value of 250 mg/L has been selected as a reference point. Based on these data, ground water in the Great Plains region is often of poorer quality than that recommended by the EPA's standards. In the eight - 61 - ------- 33- 30- 2!r m w u z w PS PS !=> o u o ptl o peS w 20- 15- lO FIGURE — 1-13 FREQUENCY OF OCCURRENCE COMPARED TO DISSOLVED SOLIDS IN WATER FROM MISSISSIPPIAN AQUIFERS, GREAT PLAINS REGION, MONTANA NaHCO-: 15 15 CaSO/ OTHERS (CaHC03) (MgS04) (NaCl) (MgHC03) (NaS04) 0 1 1 1 1 1001 2001 3001 4001 5001 6001 7001 8001 9001 >10000 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 DISSOLVED SOLIDS IN MG/L - 62 - ------- TABLE 1-5 COMPARISON OF SELECTED ELEMENTS AND IONS IN WATERS OF THE GREAT PLAINS REGION, MONTANA TO DRINKING WATER QUALITY STANDARDS4 AQUIFER CONSTITUENTS NUMBER OF % GREATER % LESS AVERAGE AND VALUES THAN THAN CONCENTRATION STANDARDS REPORTED STANDARD STANDARD IN MG/L Unconsolidated Na+K(250)1 586 55 45 734. Deposits Fe(.3)2 547 27 73 1. Mn(.05) 448 51 49 1. CI(250)2 587 6 94 61. S04(250)2 587 80 20 3097. N(10)3 551 14 86 13. Ds(500)2 576 89 11 4860. Early Tertiary Na+K(250)1 1333 56 44 394. Formations Fe(.3)2 1318 20 80 , Mn(.05)^ 1278 30 70 • Cl(250) 1330 < 1 > 99 17. S04(250)2 1331 70 30 809. N(10)3 1328 2 98 1. Ds(500)2 1333 92 8 1765. Fox Hills-Hell Creek Na+K(250) 1 278 77 23 447. Aquifer Fe(.3)2 254 17 83 . Mn(.05)2 206 17 83 • Cl(250)2 278 2 98 35. SO4(250)2 278 48 52 432. N(10)3 258 < 1 > 99 Ds(500)2 262 85 15 1284. No standard has been set. A concentration of 250 Mg/L has been selected as a point of reference. 2 Secondary drinking water standard in Mg/L Primary drinking water standard in Mg/L Source: U.S. Environmental Protection Agency, 1976 ------- TABLE 1-5 CONTINUED COMPARISON OF SELECTED ELEMENTS AND IONS IN WATERS OF THE GREAT PLAINS REGION, MONTANA TO DRINKING WATER QUALITY STANDARDS4 AQUIFER CONSTITUENTS NUMBER OF % GREATER % LESS AVERAGE AND VALUES THAN THAN CONCENTRATION STANDARDS REPORTED STANDARD STANDARD IN MG/L Judith River Na+K(250)1 221 85 15 869. Formation Fe(.3)2 221 26 74 Mn(.05) 189 22 78 CI(250)2 221 26 74 284. S04(250)2 221 65 35 1102. N (10) 218 8 92 7. Ds(500)2 221 96 4 2756. Eagle-Virgelle Na+K(250)1 93 73 27 729. Formation Fe(.3) 93 18 82 Mn(.05)2 89 15 85 Cl(250)2 93 19 81 357. SO4(250)2 93 49 51 696. N(10)3 93 3 97 1. Ds(500)2 93 87 13 2265. Kootenai Na+K(250)1 130 20 80 202. Formation Fe(.3)2 130 46 54 1. Mn(.05) 121 28 72 Cl(250)2 130 4 96 78. SO4(250)2 130 22 78 156. N(10)3 130 0 100 a Ds(500)2 130 48 52 769. ^ No standard has been set. A concentration if 250 Mg/L has been selected as a point of reference. ^ Secondary drinking water O standard in Mg/L O Primary drinking water standard in Mg/L ^ Source: U.S. Environmental Protection Agency, 1976 ------- TABLE 1-5 CONTINUED COMPARISON OF SELECTED ELEMENTS AND IONS IN WATERS OF THE GREAT PLAINS REGION, MONTANA TO DRINKING WATER QUALITY STANDARDS4 AQUIFER CONSTITUENTS AND STANDARDS NUMBER OF VALUES REPORTED % GREATER THAN STANDARD % LESS " THAN STANDARD AVERAGE CONCENTRATION IN MG/L Jurassic Age Formations Mississippian Age Formations Fe (. Mn( CI Na+K(250)^ "-'.3)2 ...05)2 v.. .1.(250) 2 SO^(250)2 N(10)3 Ds(500)2 Na+K(250)1 Fe(.3)2 Mn(.05)2 Cl(250)2 SO4(250)2 N(10)3 Ds(500)2 35 34 33 35 35 35 35 37 36 35 37 37 36 37 17 44 27 3 40 0 63 19 28 26 5 68 0 81 83 56 73 97 60 100 37 81 72 74 95 32 100 19 129. 1.7 .09 64. 386. .7 951. 171. .8 .04 115. 739. .2 1448. No standard has been set. A concentration of 250 Mg/L has been selected as a point of reference. O Secondary drinking water standard in Mg/L ^ Primary drinking water standard in Mg/L 4 Source: U.S. Environmental Protection Agency, 1976 ------- aquifer groups, 68 percent of the samples had dissolved solids concentrations of greater than 500 mg/L; four percent of the samples had nitrate (as N) con- centrations greater than 10 mg/L; 65 percent of the analyses reported sulfate concentrations of greater than 250 mg/L; and eight percent of the chloride concentrations were greater than 250 mg/L. Manganese and iron are often greater than the standards with 27 percent and 28 percent of the reported values, respectively, exceeding them. Water-quality analyses for water from the Judith River aquifer quite often report dissolved solids concentrations greater than 500 mg/L. Water analyses from this aquifer showed 96 percent of the samples had dissolved solids con- centrations above that recommended by the standards. Early Tertiary, Quaternary unconsolidated and the Eagle-Virgelle aquifers followed with 92, 89 and 87 percent of their water-quality analyses reporting dissolved-solids values above 500 mg/L. Eighty-five percent of the analyses from the Fox Hills-Hell Creek aquifer reported dissolved solids concentrations above the recommended standard. Kootenai, Jurassic age, and Mississippian-age aquifers produced the best waters in our data set with less than 50 percent of their analyses report- ing dissolved-solids values greater than 500 mg/L. Average concentrations of elements and ions shown on Table 1-5 substantially vary between aquifers. The highest average sodium concentration was found for water from the Judith River aquifer with a value of 869 mg/L. The lowest average sodium concentration of 129 mg/L is reported for water from Jurassic- age rocks. The lower average sodium concentration in water from these rocks could be misleading and may actually be greater because of the limited number of analyses. Iron concentrations appear greatest in water from Jurassic-age rocks averaging 1.7 mg/L. Lowest average iron concentrations of .04 mg/L were found in water from Mississippian-age rocks. Manganese values varied from - 66 - ------- 1.1 mg/L in water from unconsolidated materials to .03 mg/L in the Fox Hills-: Hell Creek aquifer. Chloride concentrations for the region varied from 357 mg/L for water from the Eagle-Virgelle aquifer to 17 mg/L for water from that of early Tertiary aquifers. Chloride concentrations are generally very high in Mississippian-age rocks in the Williston Basin portion of the region where sodium chloride brines exist. Average sulfate ion concentrations range from 3097 mg/L for waters from unconsolidated deposits to 156 mg/L for water from rocks of the Kootenai formation. The sulfate ion concentration for the uncon- solidated deposits is elevated in our data set as the average contains concen- trations from a number of analyses collected during research projects on saline-seep waters and sulfate ion concentrations in saline-seep waters often range between 10,000 and 50,000 mg/L. Nitrate averages range from 13 mg/L in waters from unconsolidated deposits to .2 mg/L in waters from Mississippian rocks. High nitrate values can be characteristic of fertilized acreages and this average probably reflects their influence. - 67 - ------- V. SUMMARY 1. Eight major aquifer systems have been identified within the Great Plains region of Montana. These are the Quaternary unconsolidated deposits; the early Tertiary Fort Union Formation; the Fox Hills-Hell Creek, Judith River, Eagle and the Kootenai Formations, all of Cretaceous age; the Jurassic Swift Formation; and the Madison Group of Mississippian age. Thickness maps (isopachs), altitude of formation (structure contours) and dissolved solids maps have been prepared for each of these aquifers. In addition, a potentio- metric surface map was prepared for the Madison Group. Although considerable research has been completed to compile these maps, other parameters such as recharge rates, ground-water flow paths, the extent of interformational mixing and values for transmissivity and storativity are poorly known. Other data pertaining to the hydrochemical aspects of the formations are sparse, but are continually being accumulated. 2. Quaternary unconsolidated aquifers include: alluvium, colluvium, terrace deposits, eolian deposits, glacial deposits, high level gravels and the deeply weathered surface of some sandstone formations. Unconsolidated deposits are composed of uncompacted gravels, sands, silts and clays which can be either sorted or unsorted. Well yields are highly variable, ranging from a few gpm to in excess of 1,000 gpm, depending upon location. Development of these aquifers for drinking water, irrigation and stock uses has been extensive because shallow drilling depths allow for an easily accessible water source. Recharge to Quaternary unconsolidated deposits takes place through direct in- filtration of precipitation streamflow loss, upward leakage from underlying bedrock units and irrigation return-flow. - 68 - ------- Dissolved-solids concentrations range between 140 mg/L for alluvial ground waters to a high of 68,000 mg/L for ground water from saline-seep areas. 3. Throughout the eastern half of the Great Plains region the early Fort Union Formation is the most important source of ground water. This formation was deposited under a variety of sedimentary processes which accounts for the numerous lithologies present. Most of Montana's large coal reserves occur as seams in the formation. The Fort Union is developed extensively by shallow domestic and stockwater wells. Ground water is obtained primarily from the sandstone units and from the coal beds, but some water is derived from clinker. Aquifer properties are locally unpredictable because of the widely varying lithologies. The water quality also often reflects the changing lithologies and is therefore dependent upon location. Well yields are typically less than 15 gpm, but a few wells have been reported to produce 50 gpm. The shallow drilling depths and wide geographic extent of this aquifer propogates its widespread use. 4. Underlying the Fort Union Formation are the uppermost Cretaceous sand- stones, siltstones and shales of the Fox Hills-Hell Creek Formations and their equivalents. These formations form an areally extensive aquifer across most of the Great Plains region. Hydrogeologic data for this aquifer is sparse and therefore aquifer parameters are not definitely known. Wells penetrating the aquifer, especially in southeastern Montana, will likely have artesian con- ditions. The water quality is generally very good with dissolved-solids concentrations ranging between 100 and 4400 mg/L. The water is dominantly characterized as a sodium bicarbonate type and for this reason it is relatively soft. These favorable conditions make the Fox Hills-Hell Creek aquifer a preferable ground-water source of municipal and domestic wells. - 69 - ------- 5. From the western margin through the central part of the Great Plains region the Judith River Formation is present as a clastic wedge (more than 700 feet thick) thinning to the east. In much of north-central Montana, the lower part of the Judith River Formation is a fluvial deposit and includes coal seams as much as five feet thick. In south-central Montana, the lower part of the formation is a marine sandstone and is often designated as the Parkman Member. Because the lithologies of the formation and their thicknesses constantly change, regional hydrogeologic parameters are poorly known. Well yields from the Judith River Formation range from a few gpm to approximately 100 gpm depending upon location. In some areas, the ground water in the aquifer is under sufficient pressure to produce flowing artesian wells. The water quality of the aquifer is highly variable and ranges from 161 to 27,500 mg/L of dissolved solids. Seventy-four percent of the water quality analyses have values of less than 3000 mg/L, making the water suitable for domestic and stockwater purposes. 6. The Eagle Formation is a primary aquifer in the northwestern part of the Great Plains region of Montana. In central Montana the Virgelle sandstone occurs as the basal member of the Eagle Formation. Siltstone and shale are dominant in the eastern portion of the region where the formation is often referred to as the Gammon Shale. Decreases in well yields can be directly correlated progressing to the east, as the formation becomes more shaley. Yields of 500 gpm have been reported in the Cut Bank area, but generally average less than 50 gpm. Around central Montana, well yields range between 5 and 20 gpm. Toward the east, the formation becomes so shaley and impermeable that it is no longer recognized as an aquifer. - 70 - ------- Dissolved-solids concentrations of this aquifer range from 285 to 16,000 rag/L. The higher values of dissolved solids in excess of 10,000 mg/L appear to be related to oil and gas wells. The water quality of the Eagle Formation is generally good; 58 percent of the analyses reviewed have values of less than 2000 mg/L. Additional hydrogeologic data is needed to evaluate the potential of this aquifer as future demands increase withdrawals. The aquifer is extensively used in the Great Falls area where local overdrafts already have been reported. 7. The basal sandstone unit of the Kootenai Formation forms the main aquifer of this formation. The unit is referred to by numerous local names such as the Sunburnt Sandstone, Cutbank Sandstone, Third Cat Creek Sandstone, the Pryor Conglomerate and the Lakota Sandstone. This sandstone unit is approximately 100 feet thick and is tapped as a ground-water source where it occurs within 500 feet of the surface. Yields of 300 gpm have been obtained from this aquifer, but generally average between 10 and 25 gpm. Flowing artesian wells occur where the aquifer is under a confined condition. This aquifer serves as the main source of ground water for many residents in central Montana. Water derived from this aquifer generally is of very good quality with 81 percent of the 150 analyses having dissolved-solids values of less than 1000 mg/L. 8. The only Jurassic formation known to have sufficient water-bearing potential to be considered an aquifer is the Swift Formation, although a few wells are known to produce water from the Morrison and Piper Formations. The Swift Formation is present throughout the Great Plains region of Montana - 71 - ------- except in the central areas of the plains mountains where it has been eroded. Wells penetrating Jurassic-age formations usually yield small quantities of marginal to poor quality water. Dissolved-solids concentrations vary from 204 to greater than 4000 mg/L. Use of Jurassic-age aquifers is restricted generally to outcrop areas, where the water quality has not deteriorated sub- stantially and is not deeply buried. 9. The Madison Group aquifer extends across the entire Great Plains region. It is absent in only a few small areas at the center of mountain uplifts where it has been removed by erosion or emplacement of igneous rocks. The Madison Group is dominantly limestone, but its uppermost unit, the Charles Formation, becomes increasingly an anhydrite with proximity to the center of the Williston Basin. Throughout the eastern portions of the Great Plains region the Madison Group is more than 600 feet thick, attaining a maximum thickness in the Big Snowy Trough of central Montana. Although the Madison aquifer appears to have excellent potential for producing large quantities of water, costs associated with deep drilling depths have deterred extensive development. Wells penetrating the aquifer are reported to yield from 20 gpm to in excess of 1000 gpm. Some of the wells are under sufficient hydrologic pressures which cause flowing artesian conditions. In central and north-central Montana, numerous large volume springs dis- charge from the Madison aquifer. Giant Springs flowing 300 cubic feet per second (cfs) and Big Springs flowing 160 cfs supply the municipalities of Great Falls and Lewistown, respectively. The water quality of the Madison aquifer depends on the water's residence time in the formation and distance from where the group outcrops. The lowest - 72 - ------- dissolved-solids value was obtained from a well close to where the Madison Group outcrops along the Big Snowy Mountains. These waters have a dissolved- solids value of 256 mg/L and are a magnesium bicarbonate type. The concentra- tion of dissolved solids increases rapidly toward the Williston Basin and attains values of approximately 100,000 mg/L, representing sodium chloride brines. Because of the extreme variability of waters from the Madison Group, more hydrogeologic information is needed to assess the full potential of the aquifer. 10. A precise tabulation of ground-water use by economic sector and aquifer source is limited by the lack of accurate withdrawal-rate data, and for this reason figures presented in this report are estimates of water use. However, Montana is presently quantifying its water use and consumption through the water-right adjudication program. Both surface- and ground-water claims shall be filed prior to April 30, 1982, and will be adjudicated after that date. Upon completion of the adjudication filings, the State will possess a written record of all water rights quantified according to time and volume of use. Municipal ground-water use totals 19.06 million gallons of ground water per day. Slightly more than three-fourths (76 percent) of the communities of the Great Plains region rely solely upon ground water for their drinking and household needs. Estimates show that approximately 90 percent of these water supplies derive their water from alluvium or unconsolidated deposits. Current estimates indicate that ground-water supplies approximately two percent (56.68 acre-feet per day) of the water used in the Great Plains region. Agriculture is the largest ground-water user within the region, mainly for irrigation needs. Roughly 5.62 bgd are diverted for irrigation, of which one percent is withdrawn from ground-water sources. ------- The petroleum industry is the largest commercial user of ground water in this region. Both fresh and saline ground water are withdrawn and either used for secondary recovery purposes or as a by-product produced during primary and secondary oil recovery. The average daily injection rate is estimated to be approximately 15.06 mgd. Domestic water is that which is used by people not served by a community system, usually rural residents. There are approximately 17,000 domestic and stockwater wells in the Great Plains region providing 95 percent of the rural population with a potable supply. - 74 - ------- VI. REFERENCES CITED Cole, G. A., Daniel, J. A., Heald, D., Fuller, and Matson, R. E., 1981. Oil and Gas Drilling and Coal Production Summary for Montana 1981. MBMG Open-File Report 59. Feltis, R. D., 1980, Dissolved Solids and Ratio Maps of Water in the Madison Group, Montana. MBMG Hydrogeologic Map 3, Montana Bureau of Mines and Geology. Hopkins, W., 1976, Water Resources Data for Deep Aquifers of Eastern Montana. U. S. Geological Survey Open-File Report 76-40, 37 p. Montana Department of Health and Environmental Sciences (1980). Montana Department of Natural Resources and Conservation, Oil and Gas Division: Annual Review for the Year 1980, Oil and Gas Volume 24, 44 p. Montana Department of Natural Resources and Conservation, Water Resources Division, October 1976, The Framework Report, Vol. One, 101 p. United States Bureau of Mines: 1976-present, Keystone Coal Industry Manual. United States Department of Commerce, Bureau of Census, PH680-V-28, Montana, 1980. United States Department of the Interior, Minerals in the Economy of Montana, 1979. Zimmerman, E. A., 1967, WAter Resources of the Cut Bank Area, Glacier and Toole Counties, Montana, MBMG Bulletin 60, 37 p. Zimmerman, E. A., 1960, Preliminary Report on the Geology and Ground-Water Resources of Northern Blaine County, Montana, MBMG Bulletin 19, 19 p. - 75 - ------- APPENDIX A WELL-NUMBERING SYSTEM ------- WELL NUMBERING SYSTEM In this report, locations are numbered according to geographic position within the rectangular grid system used by the U.S. Bureau of Land Management. The location number consists of as many as 12 characters. The first three characters specify the township and its position south of the Montana base line. The next three characters specify the range and its position east of the Montana principal meridian. The next two characters are the section number. The next three characters designate the quarter section (160-acre tract), quarter-quarter section (40-acre tract), and quarter-quarter-quarter section (10-acre tract), respectively, in which the well is located. The subdivisions of the section are designated A, B, C and D in a counterclockwise direction, beginning in the northeast quadrant. When more than one well is described within a 10-acre tract, consecutive digits are added to the well number. For example, as shown on Figure 1-14, well 05S54E16ACC is the first well inventoried in the SW^ SW% NE^ sec. 16, T. 5 S., R. 54 E. - 76 - ------- Diagram showing wall-numbering system. Figure 1-14 - 77 - ------- APPENDIX B GLOSSARY OF TERMS ------- Acre-foot - A unit for measuring the volume of water. It is equal to the quantity of water required to cover 1 acre to a depth of 1 foot, and is equal to 43,560 cubic feet or 325,851 gallons. Alluvium - A general term for clay, silt, sand and gravel deposited by running water as sorted to semisorted sediment. Aquifer - A formation, group of formations or part of a formation that contains sufficient saturated permeable material to yield significant quantities of water to wells or springs. Arid - A climate characterized by dryness, variously defined as rainfall insufficient for plant life or for crops without irrigation; less than 10 inches of annual rainfall. Artesian - Artesian is synonymous with confined. Artesian water and artesian water body are equivalent, respectively, to confined ground water and confined water body. An artesian well is a well deriving its water from an artesian or confined water body. The water level in an artesian well stands above the top of the artesian water body it taps. If the water level in an artesian well stands above the land surface, the well is a flowing artesian well; however, an artesian well does not have to flow. If the water level in the well stands above the water table, it indicates that the artesian water can and probably does discharge to the unconfined water body. Colluvium - A general term applied to a loose heterogeneous mixture of gravels, sands, silts and clays deposited at the base of a slope. Dissolved-solids concentration - The total dissolved minerals in water, expressed as the weight of minerals per unit volume of water, without regard to the type of minerals. Disturbed Belt - A zone roughly 25 miles wide along the eastern mountain front which was tectonically disturbed during the formation of the Rocky Mountains, but mountains did not develop in this zone. Eolian deposits - Sediments whose constituents were transported by the wind (i.e. sand dunes). Epeirogenic uplift - Primarily vertical movements which have affected large sections of land. Evaporite - A sedimentary rock dominantly composed of saline minerals which become concentrated by evaporation of a solvent. Fluvial - Pertaining to a river. - 78 - ------- Histogram - A vertical bar-graph representation of a frequency distribution in which the height of bars is proportional to frequency of occurrence within each class interval. Igneous extrusions - Molten magma which has been extruded onto the surface of the earth and cooled to a rock. Igneous intrustions - Molten magma which has been emplaced into pre-existing rocks and cooled to a rock. Injection well - A well into which water or other fluids are pumped for varying purposes such as disposal, secondary oil recovery or increased yield. Isopoch - A line drawn on a map through points of equal thickness of a designated stratigraphic unit. Laramide Orogeny - A time of deformation during which the Rocky Mountains were developed, extending from late Cretaceous until the end of the Paleocene. Lithology - The description of rocks in hand specimen and in outcrop on the basis of such characteristics as color, structure and mineralogic composition. Loess - A widespread, homogeneous, commonly nonstratified deposit of windblown dust that is generally believed to be Pleistocene age. Mean annual runoff - The average yearly flow from rainfall or melted snow which ultimately reaches a surface stream. Microthermal - Pertaining to a climate characterized by low temperature. Moraine - A mound, ridge or other distinct accummulation of unsorted, unstratified glacial drift, predominantly till, deposited by the action of glacier ice in a variety of topographic land forms. Potentiometric surface - A surface which represents the static head. As related to an aquifer, it is defined by the levels to which water will rise in tightly cased wells. Where the head varies appreciably with depth in the aquifer, a potentiometric surface is meaningful only if it describes the static head along a particular specified surface or stratum in that aquifer. More than one potentiometric surface is then required to describe the distribution of head. The water table is a particular potentiometric surface. - 79 - ------- Saline - A general term for naturally occurring soluble salts, such as sodium chloride, sodium carbonate, sodium nitrate, potassium salts, etc. Sedimentary - Formed by deposition or accretion of grains or fragments of rock- making materials. Applied to all kinds of deposits from the waters of streams, lakes or seas and in a more general sense to deposits of wind and ice. Semiarid - A type of climate in which there is slightly more precipitation (10-20 inches or 12-16 inches) than in an arid climate, and in which grasses are the characteristic vegetation. Static water level - The water level of a well that is not being effected by withdrawal of ground water. Steppe - An extensive, treeless grassland area generally considered drier than the prairie. Subhumid - A climate type that is transitional between humid and subarid types according to quantity and distribution of precipitation. Taiga - A swampy area of coniferous forest sometimes found lying between tundra and steppe regions. Unconsolidated deposits - Primarily clays, silts, sands and gravels that are loosely arranged and not cemented together. - 80 - ------- APPENDIX C MONTANA WATER LAW ------- The appropriative doctrine of water rights, "first in time, first in right," applies exclusively in Montana. Prior to 1973, a water right was acquired by simply making a diversion and posting notice of such diversion. A filing was to be made in the county office, but the courts had ruled that even this was not required. The appropriator did have to put the water to beneficial use. The appropriation of ground water to beneficial use prior to January 1, 1962 was recognized as a water right for the first time by the 1962 Montana Groundwater Law, but surface rights with a priority preceding that date were given priority over all prior or subsequent ground-water rights. The 1973 Montana Water Use Act established a uniform central system for the acquisition, administration and determination of all water rights. It also mandated, the adjudication of all existing rights. The appropriator is required to file for a permit with the Department of Natural Resources and Conservation (DNRC), to obtain a new water right if it involved construction of a new surface water diversion or impoundment, or a water well with an anticipated beneficial use of more than 100 gpm. The DNRC was directed to issue permits if applicants complied with certain conditions including the requirements that: (1) Unappropriated waters exist which the applicant can put to beneficial use in the amount and at the time proposed in the application. (2) The rights of prior appropriators will not be adversely affected. (3) The proposed means of diversion or construction is adequate. Beneficial use is defined as "a use of water for the benefit of the appropriator, other persons, or the public, including but not limited to, agricultural (including stock water), domestic, fish and wildlife, industrial, irrigation, mining, municipal power, and recreational uses..." who will appoint Water Masters to review the permit applications and that each - 81 - ------- water judge shall study and adjudicate all water rights granted thereby. Such judges have now been appointed and they estimate that the water rights, both surface and ground, for which applications are filed prior to April 30, 1982, will be adjudicated within 10 years. When this occurs, the State and its water users will, for the first time, have a written record of all water rights, in Montana, quantified in time and amounts of water. For a compilation on the rules and regulations pertaining to Montana's ground water, reference should be made to the Montana Code Annotated, Volume 13, Sections 85-2-501 through 85-2-520. - 82 - ------- |