Data for Distribution of Groundwater Age in Aquifers Used for Public Supply in the Continental United States, 2004 - 2017 (Version 1.1: June 2022)

This data release documents seven tables that contain environmental tracer data and lumped parameter model (LPM) results that are used for assessing the distribution of groundwater age in 21 Principal Aquifers of the continental United States. Groundwater samples were collected from 1,279 sites and analyzed for environmental tracers: tritium, tritiogenic helium-3, sulfur hexafluoride, carbon-14, and radiogenic helium-4. For each sample, groundwater age distributions were estimated by fitting LPMs to the environmental tracers using the computer program TracerLPM (Jurgens and others, 2012). Calibrated lumped parameter models provide the optimal distribution of ages in a sample that explain the measured concentrations of tracers. Information about each site, including well construction and location information, is presented in table 1. The mean of each sample’s age distribution, or mean age, as well as the fractions of Anthropocene (since 1953), Holocene, and Pleistocene water, are presented in table 2. Concentrations of the environmental tracers for each sample is presented in table 3. Detailed information on calibrated age distributions is presented in table 4. Table 4 contains all the information necessary to reproduce the model fit of each sample using TracerLPM. Table 4 includes the lumped parameter model name, the fitted parameters, the measured and modeled tracer concentrations, and the regional tracer histories in recharge used in the calibration process.

Tracer concentrations, except for tritium, require corrections to their analytical measurement before they can be used to calibrate age distributions. For sulfur hexafluoride, chlorofluorocarbons, tritiogenic helium-3 and radiogenic helium-4, corrections for contributions from solubility equilibrium and excess air components were made using the computer program DGMETA (Jurgens and others, 2020). Groundwater samples were also analyzed for dissolved gases (helium, neon, argon, krypton, xenon, and nitrogen), which provide an independent set of data to compute solubility and excess air component concentrations. Concentrations of these dissolved gases in water were fit to air-water equilibrium models using DGMETA. Calibrated air-water equilibrium models provide the optimal solubility equilibrium and excess air component concentrations that explain the measured dissolved gases in a sample. Table 5 contains the details of the air-water equilibrium model results for each sample, including the concentrations of dissolved gases. Table 5 includes information necessary to reproduce the modeling results using DGMETA. Table 6 contains the computations of environmental tracer concentrations using the air-water equilibrium model results. Tracer concentrations of samples listed in Table 3 can correspond to the average computed concentration in Table 6.

Carbon-14 (14C) concentrations were corrected for 14C-free sources using three methods: analytical models (Tamers, 1975; Han and Plummer, 2013), inverse geochemical models (Plummer and others, 1994; Parkhurst and Charlton, 2008), and scaling of the carbon-14 record. The first two are common carbon-14 correction methods whereas the scaling method is new. Table 7 presents calculated 14C corrections as well as an adjusted measured 14C concentration for all three methods to provide comparison of the methods.

This dataset is composed of new and previously compiled data. In some cases, the previous data were revised for consistency among modeled age distributions. For example, in past compilations a dispersion parameter of 0.1 was used as a fixed value in models. In the revised models, the dispersion parameter was set to 0.01. This value of the dispersion parameter is more consistent with the range of dispersion parameters obtained from model fits to the tracer data. The dispersion parameter was a fitted parameter in models of Anthropocene water when multiple tracers were available. The change in dispersion parameter results in a younger mean age than originally reported but is usually within 10%. Previously calculated tracer data, from which the models rely, were not revised. As such, this dataset presents a complete and consistent set of results for evaluating age distributions in groundwater nationally. References to the original source of tracer data and dissolved gas modeling results are listed in tables 3. In addition to these seven tables, three ancillary tables that provide references to original datasets (table 8), descriptions of the fields in each table (table 9) and abbreviations used throughout the tables (table 10). Please see processing steps in the general metadata file for more detailed information about the methods used to create the tables.

References:

Jurgens, B.C., Böhlke, J.K., and Eberts, S.M., 2012, TracerLPM (Version 1): An Excel® workbook for interpreting groundwater age distributions from environmental tracer data: U.S. Geological Survey Techniques and Methods Report 4-F3, 60 p., https://pubs.usgs.gov/tm/4-f3

Jurgens, B.C., Böhlke, J., Haase, K., Busenberg, E., Hunt, A.G., and Hansen, J.A., 2020, DGMETA (version 1)—Dissolved gas modeling and environmental tracer analysis computer program: U.S. Geological Survey Techniques and Methods 4-F5, 50 p., https://doi.org/10.3133/tm4F5.