Groundwater Age and Residence Time

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    Carbon-14 Age Dating Calculations for Minnesota Groundwaters
    (2018) Alexander, Scott C; Alexander, E Calvin Jr.
    Groundwater dating techniques can be applied to flow systems with time scales from hours to tens of millennia. For the purposes of this report age and residence time are used interchangeably. For waters with ages ranging up to about 30,000 to 40,000 years carbon-14 (14C), or radiocarbon dating, can be a useful technique (Han et al., 2012). Han and Plummer (2013, 2016) reviewed 14C groundwater dating models. In particular, converting a measured 14C activity to an “age” is complicated by exchange of carbon in surficial, soil, and groundwater environments. Groundwater age is, however, not defined by simple piston flow past an arbitrary point like a well. Mixing occurs at several scales from advection and dispersion along a single flow path, to mixing of multiple flow paths, to mixing within a borehole intersecting multiple aquifers. In practice all groundwaters are a mixture of waters with varying subsurface residence times (Bethke and Johnson, 2008; Cartwright et al., 2017). Efforts to reconcile complex geochemistry and flow paths with geochemical models and calculations have been made by many; classic efforts include Deines et al. (1974), Wigley et al. (1978), Plummer et al. (1990), ranging to work by Coetsiers and Walraevens (2009). This document outlines field and analytical techniques we have used to acquire the carbon isotopic data from nearly 700 wells in Minnesota. Determinations of the ages or residence times of Minnesota groundwaters are widely used in scientific and management studies all around Minnesota (Alexander and Alexander, 1989). In a typical county atlas about 100 wells are measured for groundwater chemistry, stable isotopes of hydrogen (H) and oxygen (O), and tritium content. A selection of about ten water wells with no measurable tritium are then resampled for the radioactive isotope carbon-14 (14C) and the stable isotopes carbon-13 (13C) and carbon-12 (12C). Three major groups of studies have been conducted in Minnesota. Many original analyses were done as part of research on groundwater age in the Mt. Simon aquifer and were extended with a radium study (Lively et al., 1992) with funding in large part by Legislative Committee on Minnesota Resources (LCMR). At this same time a variety of small-scale studies were conducted in a variety of geologic settings across Minnesota. The United States Geological Survey (USGS) oversaw several projects to define “flow tubes” in selected aquifers across Minnesota (Delin, 1990; Smith and Nemetz, 1996) also with LCMR funding. The Minnesota Department of Natural Resources (DNR) as part of the County Atlas Program and Mt. Simon recharge studies (Berg and Pearson, 2012) has conducted 14C age dating with significant 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.