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Lumped parameter models of groundwater age, Spanish Valley Watershed, Grand and San Juan Counties, Utah

February 5, 2020

This data release documents two Microsoft Excel tables; one contains data for understanding groundwater ages in the Spanish Valley watershed, and one that describe the data fields. Mean ages and age distributions from 19 groundwater samples were estimated in support of an evaluation of the groundwater resources of the Spanish Valley watershed (Masbruch and others, 2019). Individual groundwater well and spring vulnerability to land-surface contamination and changes in hydraulic conditions (for example, water extraction or reduced recharge) can be assessed using environmental tracer-based groundwater age. The detailed interpretation of groundwater age reported here supplements the apparent tracer ages of Masbruch and others (2019). Multiple age tracers sampled in groundwater were fit using TracerLPM (Jurgens and others, 2012), with working knowledge of the well dimensions, hydrogeology, and geochemistry, to assign a unique age distribution. The age distributions describes the relative contributions of flow-paths of differing age and the extent of flow-path mixing in the sample. Concentrations of tritium (3H), tritiogenic-He (3Hetrit), chlorofluorocarbons (CFC-11, -12, -113), sulphur hexafloride (SF6), and geochemically corrected carbon-14 (14C) were fit by optimizing age distribution parameters. Details of 3Hetrit calculation, 14C geochemical correction, and noble gas based estimates of conditions which were used to correct CFCs and SF6 are described in Masbruch and others (2019). Lumped parameter models (LPM) are described for each sample by the mean age, LPM name, model parameters, and in the the case of binary mixing models (BMM) the mixing fraction and the mean age of the old component. Final mean age for BMMs is calculated as (Mean Age 1 * Fraction) + (Mean Age 2 * (1 - Fraction)). Jurgens, B.C., Bohlke, 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, 60p. Masbruch, M.D., Gardner, P.M., Nelson, N.C., Heilweil, V.M., Solder, J.E., Hess, M.D., McKinney, T.S., Briggs, M.A., and Solomon, D.K., 2019, Evaluation of groundwater resources in the Spanish Valley Watershed, Grand and San Juan Counties, Utah: U.S. Geological Survey Scientific Investigations Report 2019-5062, 86 p.,