The ground-water conditions of the area covered by the Paintrock irrigation project, in north-central Wyoming, were investigated during the summer of 1947. The purpose of the study was to obtain a general evaluation of ground-water recharge, discharge, and storage in the area now irrigated and in the adjacent areas where additional lands are to be irrigated.

Much of the area covered by this report consists of flat to gently sloping stream terraces and alluvial-bottoms along Nowood, Paintrock, and Medicine Lodge Creeks. The stream-terrace materials consist of fluviatile sand, clay, and gravel. The alluvium is very fine grained and in general has low permeability. The materials underlying the stream terraces and the bottomlands became progressively finer grained and less permeable downstream.

The bedrock formations underlying the area studied range from the Madison limestone of Mississippian age to the Fort Union formation of Paleocene age. Beds have been folded into several prominent structures which trend northwest-southeast across the area. Several of the formations exposed in the area serve as aquifers and yield water to domestic and stock wells. The most important bedrock aquifers are the Fort Union, Lance, Meeteetee, Mesaverde, Frontier, Cloverly and Morrison formations , the Tensleep sandstone, the Amsden formation, and the Madison limestone. More than 7,000 feet of strata are exposed in the area, the older beds being exposed on the western flank of the Big Horn Range near the eastern end of the area.

The quality of the water in the project ranges within wide limits. The concentration of dissolved solids in seven samples of ground water ranges from 279 parts per million for a water in the Tensleep sandstone to 4,590 parts per million for a water in the Morrison formation. The hardness as calcium carbonate (CaCO₃) ranges from 13 to 1,680 parts per million. Limited data on the quality of water in Nowood and Paintrock Creeks indicate that these waters are suitable for irrigation. The water in Paintrock Creek near Tensleep is higher in mineral content and hardness than the water upstream at Hyattville as a result of return flow of the irrigation water that is applied to farm lands above Tensleep.