THE GEOLOGY AND GEOCHEMISTRY OF ANCIENT GROUNDWATER IN THE CENTRAL PEAK AQUIFER OF THE MANSON IMPACT STRUCTURE, IOWA

The area around the town of Manson in north-central Iowa has been known as an area of anomalous geology since 1905 when town drilled its first well and encountered unusually soft groundwater beneath 1000 feet of unknown strata. It was subsequently discovered that Manson lies above one of the best preserved complex impact structures on earth, and its aquifer is the crater’s central peak. So, when the town announced that it would drill a new well in 2011, an investigation of the anomalous water was initiated. Manson wells, and several domestic wells that also utilize soft water from the central peak, were sampled as were several wells that produce water from the overlying glacial drift. Water samples were analyzed for major ion chemistry, trace elements, and various isotopes including ¹⁴C, δ¹³C, δ¹⁸O, ³H, δD, and ³⁶Cl . Interpretation of data from these analyses indicates that the Manson water in the central peak is naturally soft (alkalinity of 66 mg/l as CaCO₃) and contains elevated fluoride concentrations (up to10 mg/l). ³⁶Cl analyses suggest that the water is over 2 million years old, apparently isolated from groundwater recharge by low-permeability crater fill materials. The Manson water also contains trace element concentrations of Li, Rb and Cs significantly higher than other water sources in the area. Over the last century of use, water pumped by the town of Manson from deeper in the central peak aquifer has been replaced by recent groundwater at the top of the central peak, a region where the aquifer is in contact with glacial drift. The drift water is hard and contains elevated levels of nitrate. The mixing of ancient and recent waters in the central peak aquifer is very complex and has not yet been fully resolved. Preliminary information from the new Manson well has introduced additional complexities by demonstrating that the Manson aquifer consists, in part, of a crystalline rock debris flow extending into the crater fill material that is in hydrologic communication with the main body of the central peak.