Thomas Henderson 1985 <p><span>The Kootenai Formation in the Judith Basin, Montana, and the </span><span>Lance Formation and Fox Hills Sandstone in the Powder River Basin, </span><span>Wyoming, constitute two important sandstone aquifer systems in the </span><span>Northern Great Plains region. Ground waters in each of these </span><span>systems evolve from low dissolved-solids concentration, near-neutral </span><span>pH, predominantly calcium and magnesium bicarbonate types in </span><span>their recharge areas, to high dissolved-solids concentration, high pH, </span><span>predominantly sodium-bicarbonate types in the basins. Oxidation </span><span>potentials decrease as the waters flow downgradient under confined </span><span>conditions. Calculation of the saturation states of aquifer minerals </span><span>suggests several groups of mineral phases that could control ground-</span><span>water chemistry. Mass transfer modeling indicates, however, that the </span><span>observed behavior of major and minor dissolved species in both </span><span>systems can satisfactorily be explained only by equilibration with cal</span><span>cite, dolomite, or calcite and dolomite. The geochemistry of these </span><span>systems is probably controlled by the incongruent dissolution of dolo</span><span>mite to form calcite. This reaction appears to be driven by cation ex</span><span>change and the dissolution of carbon dioxide. Plausible carbon diox</span><span>ide sources include organic carbon oxidation and lignite coalification. </span><span>Aluminosilicates influence major element chemistry primarily as sub</span><span>trates for cation exchange, which, in combination with carbonate </span><span>equilibria, buffer ground water pH at values of 8.5 to 8.9. Dissolved-</span><span>iron concentrations are controlled by equilibration with amorphous </span><span>ferric oxyhydroxides in oxidizing waters, with amorphous ferric </span><span>oxyhydroxides and siderite in moderately reducing waters, and with </span><span>siderite and amorphous ferrous sulfide in strongly reducing waters. </span><span>Measured variations in dissolved carbonate isotopic composition </span><span>compare favorably with carbon isotopic evolution, calculated by </span><span>assuming dedolomitization. </span></p><p><span>Recharge areas of the two systems are characterized by ground </span><span>waters with high tritium and carbon-14 activities and relatively low </span><span>dissolved-solids concentrations, with calcium and magnesium as the </span><span>predominant cations. Recharge temperatures, calculated from </span><span>dissolved-argon concentrations and 5</span><span>18</span><span>0 and SD isotopic measure</span><span>ments, indicate that recharge is derived primarily from spring snow-</span><span>melt rather than late spring and summer storms. Ground-water flow </span><span>directions are generally parallel to trends of increasing dissolved </span><span>solids concentrations and decreasing divalent to monovalent cation </span><span>concentration ratios. However, these trends are sometimes obscured </span><span>in areas of leakage or mixing. Further indication of leakage between </span><span>aquifers is provided by abrupt changes in major element and isotopic </span><span>chemistry, which are not characteristic of normally observed geo</span><span>chemical evolution. Ground-water flow rates, calculated by adjusting </span><span>measured carbon-14 activities for carbonate mass transfer, are com</span><span>parable to values calculated from aquifer tests and potentiometric </span><span>data. These carbon-14 flow rates average 1.6 meters per year for the </span><span>Second Cat Creek sandstone of the Kootenai Formation, and 1.3 </span><span>meters per year for the Lance-Fox Hills aquifer. </span></p> application/pdf 10.3133/pp1402C en U.S. Geological Survey Geochemistry of ground-water in two sandstone aquifer systems in the Northern Great Plains in parts of Montana and Wyoming, North Dakota, and South Dakota reports