The potentiometric surface map of the Madison Group of Mississippian age in Montana indicates that water in this aquifer generally moves northward. The principal recharge areas of the Madison are the Little Belt and Big Snowy Mountains of central Montana, the Pryor and Bighorn Mountains of south-central Montana and northern Wyoming, and the Black Hills uplift of Wyoming and South Dakota. The dissolved-solids concentration of the water ranges from a few hundred milligrams per liter near mountainous recharge areas to about 300,000 mg/L in the

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northeastern part of the state (Williston basin). The ratio of sodium plus potassium plus chloride to dissolved-solids concentration is greatest in northeastern Montana where the Madison contains salt beds, and decreases toward recharge areas. The ratio of sulfate to total anions is greatest in north-central and southeastern Montana where anhydrite probably is the source of the sulfate. The ratio is smallest in northeastern and northwestern Montana; however, the concentration of sulfate, in milligrams per liter, can be greater in northeastern Montana than in other areas because of the greater concentration of all dissolved solids.

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