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Groundwater data and age information from samples collected in Minnesota

May 4, 2022

Groundwater age distributions and susceptibility to natural and anthropogenic contaminants were assessed for selected wells, streambed piezometers, and springs in southeastern Minnesota. The data provide information to understand how long it will take to observe groundwater quality improvements from best management practices implemented at land surface to reduce losses of nitrate (and other chemicals) from agricultural practices. A total of eleven water samples were collected from four wells, three streambed piezometers, and four springs between October 2020 and May 2021. One field replicate sample was collected from one of the spring sites. Groundwater ages were estimated from dissolved gas (neon, argon, krypton, xenon, and nitrogen) and environmental tracer data (tritium, sulfur hexafluoride, chlorofluorocarbons, and tritiogenic helium-3) from field samples using the equations available in TracerLPM (an Excel workbook for interpreting groundwater age distriutions from environmental tracer data) and DGMETA (an Excel workbook for dissolved gas modeling and environmetnal tracer analysis); groundwater age estimates are reported in Table 1. DGMETA was used to compute the optimal water temperature, excess air, entrapped air, fractionation of gases, and excess nitrogen gas (mainly from denitrification) for the measured dissolved gases in a sample; condensed results are reported in Table 1 and these results are reported in detail in Table 2. These values were then used to convert the raw measured concentrations of environmental tracers into a form appropriate for age dating analysis; these results are reported in Table 3. Calculated concentrations of environmental tracers that were used in groundwater age calculations are the dry air mixing ratio of sulfur hexafluoride or chlorofluorocarbons, and tritiogenic helium-3, which is the concentration of helium-3 from the decay of tritium. Table 4 reports additional site information and field parameters. In addition to these four tables, two ancillary tables are included to provide more detailed information about the fields and the abbreviations used in tables 1-4. A readme file is provided that describes each table in more detail and processes to use the data in this data release to view age distributions in TracerLPM and to set up TracerLPM to run scenarios for other chemicals of interest.