ABSTRACT:

Soil samples were collected from 75 cultivated soils excavated in August-September 2012 in fallow fields, and two uncultivated soils excavated in August 2013, to 145 cm using a mini excavator, at three fields on key landforms in the watershed that were under similar management for non-irrigated wheat production. Samples were collected volumetrically in 15 cm depth increments (1500 cm3) and weighed. Field moist samples were dried at ~50°C for 24-48 hours, sieved to obtain the fraction <2mm, and subsamples of the fine fraction milled overnight in duplicate prior to weighing for total nitrogen and d15N analysis by combustion-IRMS (Delta V, Thermo) at the USGS Southwest Regional Isotope lab in Denver. Duplicates were included every six samples, and where concentration or isotopic differences were >10%, proximal samples were re-analyzed. Results are omitted for samples that contained insufficient nitrogen or where analysis was otherwise unsuccessful. Samples from 21 locations on two landforms, including two native range sites, were analyzed. A total of 62 analyses from these sites were successful, and results are presented as averages for each depth increment across all sites.

ABSTRACT:

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.

ABSTRACT:

Supplemental text and figures, analytical code, and full dataset documenting compositional differences and results of incubations for effects on dissolved organic carbon concentration and character.

Data and analysis in this resource describe stream samples collected in late summer of 2016 and 2017 from seven study regions selected to include Arctic, Boreal, and alpine ecosystem types and to represent a range of current and future climatic conditions in the permafrost zone (continuous, discontinuous, and non-permafrost). Water samples were collected from three or more locations within each study region. Selected sites were nested in river networks, except in interior Alaska, where the three sites came from independent streams. The seven sampled regions include broad variation in climate, geology, topography, and vegetation. In all permafrost-affected regions, various types of permafrost degradation have been observed, and other forms of less visible permafrost warming and degradation are also occurring. Though permafrost degradation is present in all the studied permafrost catchments, three of the seven regions were specifically chosen for their proximity to abrupt thaw features. Please see the primary manuscript for site details, complete methods description, statistical analyses, and citations to relevant literature.

Incubations were performed locally by each regional team, and samples were shipped to centralized locations for analysis. Stream water was filtered on site (0.7 m, Whatman GF/F) and refrigerated until laboratory incubations were initiated. We divided the filtered bulk stream samples into 200-mL aliquots and treated each aliquot with one of eight acetate (CH3COO-) and nutrient treatments (Table S1). We used acetate as the priming substrate in these experiments We used ammonium (NH4+), nitrate (NO3-), and phosphate (PO43-) as the inorganic nutrient substrates. Treatments were added only at the start of the incubations to simulate mixing of permafrost thaw products with modern DOM in stream networks.

Inorganic nutrients (NH4+, NO3-, NO2-, and PO43-) in unamended (background) stream waters were determined at µg L-1 levels on a QuAAtro39 continuous segmented flow analyzer (Seal Analytical, Inc.). We calculated dissolved inorganic nitrogen (DIN) as the sum of NH4+, NO3-, and NO2-. Acetate and other dissolved solutes in the treated incubation samples (NO3-, NO2-, Cl-) were measured at mg L-1 levels on an ICS 2100 Ion Chromatograph (Dionex, Thermo Scientific) equipped with an anion column (ASX-18 column). DOC and total nitrogen (TN) in all samples were determined using a V-TOC CSH Total Carbon Auto-Analyzer with a TNM-1 Total Nitrogen Module (Shimadzu Corporation).

We collected additional subsamples at t0 and t28 from a subset of the treatments (CT, A3, and AN) for optical analysis via fluorescence spectroscopy to evaluate indices of DOM composition. These subsamples were filter sterilized (0.22 µm, PES) into 40 mL amber glass vials and stored in the dark at 4˚C during shipment and until analysis. We measured the absorbance and fluorescence of these subsamples with a spectrofluorometer (Aqualog, Horiba Scientific, Edison, New Jersey). Detailed analysis of DOM chemical composition was performed for only the CT and A3 treatments at the t0 and t28 timesteps for a subset of sites (a total of 33 samples) via ultrahigh resolution mass spectrometry with a 21 T FT-ICR MS. These subsamples were filtered to 0.7 m (GF/F pre-combusted at 450oC for 5 hours) and stored frozen until analysis.

To calculate rates of acetate and background DOC consumption, we poured off and froze 15-mL subsamples immediately following the addition of treatments (t0), after 7 days (t7), and after 28 days (t28). We calculated change in background DOC and acetate for each replicate individually and then calculated the mean and standard deviation of ΔDOC and ΔAcetate across the three replicates for each site and timestep. We calculated change in optical properties (ΔOptical) and relative abundance (ΔRA) of molecular composition in the same way as ΔDOC and ΔAcetate. We calculated priming and nutrient effects for each site as the ΔDOC in each treatment minus the ΔDOC in the unamended (control) treatment. This yielded positive values for the nutrient and priming effects when the treatment resulted in greater background DOC consumption (i.e. positive priming) and negative values when the treatment DOC consumption was less than the control (i.e. negative priming).