Variations and geologic significance of water quality in the Judith River Formation, north-central Montana

Geologic, hydrologic, and water-quality data are analyzed to explain water-quality anomalies in a large artesian basin. Geologic formations include the Judith River Formation, a sandstone and shale aquifer, and thick marine shales which confine the aquifer above and below. The structural feature causing artesian conditions is the Blood Creek syncline; the smaller Veasth syncline is also of major hydrologic importance. Potentiometric data show that water flows upward and eastward through the aquifer. Water quality in the aquifer does not vary as expected by consideration of basic water-quality controls. Sodium is the predominant cation; bicarbonate is generally the predominant anion, but locally high concentrations of total dissolved solids and sulfate occur. Nitrate and fluoride concentrations are relatively high; chloride content is low, indicating the aquifer has been flushed since deposition. Hydrochemical processes of major importance include shale-membrane filtration and base exchange of sodium for calcium. Shale-membrane filtration is effective except at or near axes of folds, where calcium sulfate is selectively passed to the aquifer. Breakdown of the filter locally occurs because of minute fracturing or disruption of clay particles at zones of maximum warping. Carbonate cementation is occurring at most places in the aquifer.hydrology collectio