by Scott C. Alexander and E. Calvin Alexander Jr.
A recently updated report outlines field and analytical techniques used to acquire carbon isotope samples and provides documentation of the assumptions, equations, and models used to convert the measured carbon-14 from dissolved inorganic carbon to a groundwater age. A parallel database of nearly 700 well samples from Minnesota allows users to access the data. The report and database are available by clicking on this button.
Groundwater age and residence time are critical components to understanding recharge to aquifers. Residence time is the approximate time that has elapsed from when water infiltrates the land surface to when it is pumped from an aquifer or discharged to a water body. In general, short residence time suggests high recharge rates or short travel paths and long residence time suggests low recharge rates or long travel paths. In Minnesota, commonly used tools to understand residence time include dye tracing and tritium for short residence times and carbon-14 for long residence times.
This dataset began in 1982 with a sample from a deep Mt. Simon aquifer well located at the Schmidt Brewery in St. Paul, Minnesota, where Heileman brewery staff wanted to know the age of their water so they could proudly proclaim that their beer was “brewed with water from when the earth was pure”. Over the years, a large number of collaborators with the USGS, MPCA, DNR, MGS and others have contributed copious amounts of time and resources into generating this dataset. The Groundwater Atlas Program of the Minnesota Department of Natural Resources has been instrumental in collecting a significant portion of the database with support from the Minnesota Environment and Natural Resources Trust Fund as recommended by the Legislative-Citizen Commission on Minnesota Resources (LCCMR) with additional funding from the Clean Water Fund
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