A study of the geology and the groundwater conditions of the Helena Valley, in the west-central part of Montana, was made during the summer of 19*^80 The purpose of the study was to gather data on the occurrence of ground water and its relation to present and proposed irrigation in the area. With such data one may better predict the effects of the application of irrigation water to additional lands in the area. 2 GEOLOGY AND GROUNDWATER RESOURCES OF THE HELENA VALLEY, MONTANA Ground vaters in the valley contain moderate amounts of dissolved solids, the predominant ions being calcium, bicarbonate, and sulfate, Little difference is discernible in the composition of waters in the Quaternary and Tertiary deposits; the dilute nature of these waters indicates that the valley fill is derived from formations containing only small amounts of soluble minerals. The quality of the ground water appears to be little affected by recharge of the groundwater reservoir from return irrigation flows. Waterlogged areas may in time show an increase in mineralization as a result of evaporation. Both ground and surface waters are classified as "excellent to good" on the basis of Wilcox's criteria for irrigation waters. Boron, percent sodium, and dissolved solids are relatively low. Waters used for drinking or domestic purposes are generally considered satisfactory except, in some instances, for hardness. INTRODUCTION Purpose and Scope of Investigation The purpose of this investigation was to gather data on the occurrence, conditions, and potentialities of the ground water in the Helena Valley. Special consideration was given to the source of the ground water, to its direction of movement, to the fluctuations of water levels in wells, to the depth to the water table below the land surface, to the available supply, and to the present and potential extent of waterlogging and means for alleviating it. This study was a part of the program of the Interior Department for development of the Missouri River basin, and it is directly related to the proposed Canyon Ferry project which calls for diversion of water from the Missouri River into the Helena Valley to irrigate about 10,000 acres of land not now irrigated and to furnish additional water to about 5*000 acres of land already irrigated. This report is based principally on field work done by Howard W. Lorenz between July and early November 19^8. However, water-level measurements made during April, May, and June, 19^8, by the Bureau of Reclamation are incorporated in this report. The geology of the area was mapped on aerial photographs, and the field data were later transferred to a base map by use of a sketchmaster. An inventory was made of all wells in the area; water levels in observation wells were measured periodically; and a watertable contour map was constructed from instrumentally determined altitudes of water levels in wells. The field investigation was under the general supervision of A. N. Sayre, chief of the Ground Water Branch of the Federal Geological Survey, and G. H. Taylor, re...
