Quality and Age of Water in the Madison Aquifer, Cascade County, Montana

The Madison Limestone occurs under most of central and eastern Montana. Where it is close to, or exposed at the surface it is a productive karst aquifer and an important source of water; it also is the source of many large springs, including Giant Springs at Great Falls, the 5^th largest spring in the US. Samples for major ion analysis were obtained from 58 Madison wells and springs; 16 of the sites were also analyzed for tritium, and samples from Giant Springs and a nearby Madison well were analyzed for chlorofluorocarbons (CFC’s) and carbon-14 activity.

            The results show that the water quality and ages are variable reflecting the irregular karstic porosity and potential impacts from the overlying Morrison Formation coal. The water composition differed between a Ca-HCO₃ and Ca-SO₄ type water. Total dissolved solids concentrations ranged from 192 to 5,992 milligrams per liter (mg/L), with a median concentration of 509 mg/L.

            Water from Giant Springs and 13 of the wells had detectable tritium; concentrations ranged from 1.4 to 20.4 tritium units (TU). Most of the values were in the 6 to 20 TU range and most likely represent water recharged within the past 30 years. Two samples did not have detectable tritium indicating recharge more than 60 years ago.

CFC’s and tritium were detected in the samples from Giant Springs and a nearby Madison well. The results returned “CFC ages” of 23 and 26 years, respectively. The ages are consistent with the tritium values of 10.1 and 10.2 TU. Radiocarbon dating of samples from the well and Giant Springs returned values of close to 50 percent modern carbon (47.48 and 49.38 PMC, respectively), reflecting that half of the DIC is derived from atmospheric CO₂ with modern ¹⁴C activity, and half from the carbonate (CaCO₃) that has no ¹⁴C activity.