CARBON-14 AGE DATING OF GROUND WATERS IN MINNESOTA PALEOZOIC ROCKS

Over the past 30 years we have collected samples of DOC for analysis of ¹⁴C and δ¹³C from over 400 water wells and springs across Minnesota. Samples were collected with a novel dual bag system eliminating the traditional, and large, separatory funnel. The double bags are supported within a 110 or 200 liter plastic lined fiber barrel. Sample size is designed to collect 3 to 5 grams of dissolved carbon. The dual bag system allows the precipated barium or strontium carbonate to settle and the supernatant to be siphoned off. The resulting preciptate is then easily collected for isotopic analysis by scintallation counting after conversion of the carbonate to benzene.

The isotopic data, in combination with cation and anion analyses, has allowed the creation of dead carbon correction factors for common geochemical settings in the Paleozioc rocks of Minnesota. Specifically, models have been developed for simple carbonate dissolution, sulfide oxidation, dedolomitization, quartz sandstone systems, and methane production. Additional analysis of δ¹⁸O, δD, and δ³⁴S have been used to further refine the dead carbon correction factors. Ground water ages range from modern recharge to off-scale (>40,000 years). Long theorized waters of isotopically light hydrogen and oxygen resulting from glacial melt waters have not been found. The lack of isotopically light waters suggests that glacial recharge was not an important factor in Minnesota ground water. Instead, recharge during interglacial periods appears to dominate the ground water flow systems. 

The resulting ground water ages have found wide application in the Minnesota County Geologic Atlas program. The ages provide an estimate of ground water recharge rates and vulnerability for county and state planning agencies.