Geology and ground-water resources of the Two Medicine unit and adjacent areas, Blackfeet Indian Reservation, Montana, with a section on chemical quality of water

Abstract

The Two Medicine Irrigation Unit, on the Blackfeet Indian Reservation of northern Montana, is irrigated by water diverted from Two Medicine Creek. Waterlogging because of overapplication of water and locally inadequate subsurface drainage is a serious problem.This study was undertaken by the U.S, Geological Survey in cooperation with the U.S. Bureau of Indian Affairs to evaluate the problem and to suggest remedies.For this study, the geology was mapped, and data concerning 129 wells and test holes were gathered. The water level in 63 wells was measured periodically. Three test holes were drilled and 4 singlewell and 1 multiple-well pump tests were made. Nineteen samples of ground water were collected and analyzed chemically, and applied irrigation water was analyzed periodically.The project area is underlain by more than 300 feet of soft shale and sandstone comprising the Two Medicine Formation of Late Cretaceous age. This formation and the underlying Virgelle Sandstone of Late Cretaceous age are the only bedrock aquifers of economic importance in the project area, and they commonly yield enough water for domestic and stock use. The Two Medicine Formation is not permeable enough to provide adequate subdrainage of irrigated land; where the formation is exposed, or is under a thin mantle of slope wash, adequate drainage for successful irrigation farming cannot be developed. Land on the northern edge of the project fits into this category.Unconsolidated deposits of sand, gravel, clay, and glacial till mantle the bedrock throughout much of the area. Of these, only stream alluvium and terrace deposits composed of sand and gravel are aquifers, and they underlie most of the irrigated land. The Seville Bench is the largest surface underlain by terrace deposits and makes up most of the irrigated acreage. Waterlogging is less serious on the Seville Bench than on other parts of the unit but it is, nevertheless, a problem. Tests of several wells on Seville Bench indicate that the coefficient of transmissibility of the underlying gravel beds ranges from 2,800 to 3,200 gallons per day per foot. Only about 125 gallons per minute of water is now (1957) moving under the bench at a rate of about half a foot per day. Thus the amount of water removed would not have to be great to improve the drainage many fold.Canal leakage, downward percolation of applied irrigation water, and precipitation are the principal sources of water causing the waterlogging.The concentration of dissolved solids in ground water from the Virgelle Sandstone, Two Medicine Formation, terrace deposits, and alluvium ranges from about 200 to 1,500 parts per million. The water is very hard, is of the magnesium bicarbonate or sodium bicarbonate type, and is generally suitable for domestic use. Water from some wells had more than 0.5 part per million iron and more than 20 parts per million nitrate.Water supplied by the Two Medicine Canal is of the calcium bicarbonate type, commonly contains less than 100 parts per million dissolved solids, and is suitable for irrigat...