This resource provides radon, uranium and strontium isotope data, along with select compositional data, for water samples collected in Hyalite Canyon, Montana. Computations in support of mixing models and monte carlo optimization are included, documenting groundwater contributions to streamflow.

Sampling sites were selected to represent potential contributions from rock units with distinct geochemical character. Sampling sites included surface waters in Hyalite Creek and five tributaries, a spring and associated spring channel in the bank of Hyalite Creek, a well and associated cistern in neighboring Hodgman Canyon, and a well in the uppermost alluvial fan formed by Hyalite Creek at the mountain front. Surface waters were sampled in February and August 2016-2018, when baseflow conditions were presumed to dominate stream flow generation based on hydrograph levels. Surface water samples were collected using a peristaltic pump (Geotech™ Denver, CO, United States) with platinum-cured Silicon tubing. Wells were sampled by purging three well volumes prior to water collection, employing the same filtration and field measures used at surface water sampling sites. Samples were filtered at the time of sampling using a 0.45 µm, mid-capacity capsule filter (Geotech™ Denver, CO, United States). In-situ temperature, pH, specific electrical conductivity (SC), and dissolved oxygen (DO) were measured at each sampling site using a handheld multimeter (YSI 556 Yellow Springs, OH, USA). Alkalinity was measured in the field using colorimetric titration (Hach™ kit; phenolphthalein/bromethymol blue and H2SO4).

Chemical and isotopic analyses were conducted at Montana State University (MSU) in Bozeman, MT, the Montana Bureau of Mines and Geology (MBMG) in Butte, MT, and the USGS Southwest Isotope Research Laboratory (SWIRL) in Denver, CO. Major cations and trace metal concentrations were analyzed by Inductively Coupled Plasma - Optical Emission Spectroscopy (ICP-OES; Perkin Elmer™ Waltham, MA, United States) at MBMG and the MSU Environmental Analytical Laboratory, and by inductively coupled plasma mass spectrometry (ICP-MS) at MBMG. U and Sr isotopic analysis followed procedures described in Ewing et. al. (2015) and Paces & Wurster (2014). Purified U aliquots were analyzed by TIMS using the USGS SWIRL ThermoFinnigan Triton™ equipped with a single secondary electron multiplier and a retarding potential quadrapole (RPQ) electrostatic filter. Purified Sr aliquots were analyzed at the USGS SWIRL by multicollector TIMS using either a ThermoFinnigan Triton™ or an Isotopx Phoenix™. Samples for radon isotope analysis were collected as described in Gardner et al. (2011) and analyzed at the University of Montana using scintillation counting.

We interpreted patterns in stream flow generation from groundwater aquifers along Hyalite Creek first by examining the longitudinal patterns in chemical and isotope characterizations with decreasing elevation and distance downstream. Longitudinal analysis allowed consideration of how geologic structures, geomorphology, and lithology influence the character of stream flow generation and surface-subsurface water interaction (Gardner et al., 2011). This sampling strategy allowed us to construct mixing models that quantify fractional inputs of groundwater to reaches of Hyalite Creek where geochemical data indicated notable influence of a given aquifer. These mixing models were tested and visualized using the analytical code provided with this resource.