The aquifer system studied in the upper Popular River basin in Montana ranges in age from Late Cretaceous to Holocene. Most wells obtain water from the Paleocene Fort Union Formation or younger rocks. The potentiometric surface of the Fort Union and overlying rocks indicates movement of water from the high interstream areas toward principal stream valleys. Recharge is principally through sand and gravel deposits of the Flaxville Formation, which occur as large terrace remnants, and the Wiota Gravel. Water in these formations locally recharges the underlying Fort Union Formation. Water in the gravel also emerges around the edge of the terraces as seeps and springs. Water samples from wells tapping the aquifers are of the sodium bicarbonate, magnesium bicarbonate, or calcium bicarbonate type. The range in dissolved-solids concentration for the various aquifers is: Fox Hills-Hell Creek aquifer, 940 to 1,300 mg/L; Fort Union Formation, 520 to 1,470 mg/L; glacial outwash deposits, 570 to 770 mg/L, and alluvium, 880 to 1,490 mg/L. The dissolved-solids concentration of one water sample from the Flaxville Formation was 400 mg/L. Seepage measurements of the Poplar River and East Fork Poplar River from the international boundary to the south edge of the study area in October 1977 showed a net gain of 2.84 cubic feet per second from a total flow of 11.7 cubic feet per second. This gain is discharge of water from the alluvium to the rivers. (Woodard-USGS)
