We investigated seven glacial watersheds in the Southcentral Alaska region, including Little Susitna, Moose Creek, Matanuska, Knik watersheds in the Chugach/Talkeetna Mountains and the Gulkana, Canwell, and Castner watersheds in the Eastern Alaska Range. To identify spatial and temporal trends in water chemistry across the glacial watersheds, we collected 345 water samples during late spring through early fall of 2022 and 2023 (Fig. 1). The sample sites included Knik (K1 through K4), Matanuska (M1 through M5), Moose Creek (MC), Little Susitna (L1 through L4), Gulkana (G1 and G2), Canwell (CW1 and CW2), and Castner (CT1 and CT2). The upstream sample site in each river was located as close as possible below the main glacier with the other samples extending to the end of the watershed. Given difficult access in the Moose Creek watershed, samples were only collected at the catchment mouth. We collected monthly samples at most of the 21 sites from May through September 2022 and 2023. The sample collection also included weekly samples at four USGS streamflow gauge sites (K3, M4, MC, and LS2). To compare water chemistry in the streams with potential water sources, we collected samples from supraglacial, subglacial, and periglacial streams, a proglacial lake, springs, and tributaries during July/August 2023.

Each sample included multiple subsamples for different analyses. For trace element, major cation, and major anion analysis, water was filtered through a 0.45 µm PES syringe filter using an acid-washed syringe. The trace element/major cation samples were acidified to 2.4% v/v trace metal grade HNO3 and stored in acid-washed 30 mL LDPE bottles. The major anion samples were stored in 15 mL HDPE bottles. Samples for water stable isotopes were collected in amber glass vials with Polyseal caps, leaving no headspace to prevent evaporation. All samples were refrigerated until analysis. Alkalinity, assumed to be HCO3- in the circumneutral samples, was measured on unfiltered samples using a Hach 2444301 Alkalinity Test Kit. Field parameters (pH, dissolved oxygen, conductivity, and oxidation-reduction potential) were measured on-site using a YSI Quatro multiparameter probe, calibrated regularly. Field blanks (n=28) were collected weekly by filtering Milli-Q water into an acid-washed 30 mL LDPE bottle and a 15 mL HDPE bottle. These field blank samples were then analyzed in the same way as other samples.

Water samples were analyzed separately for trace elements, major cations, major anions, silicon, and water stable isotopes (δ18O and δ2H). Trace element/major cations were analyzed using an Agilent 7500ce quadrupole inductively coupled plasma mass spectrometer (ICP-MS) for concentrations of Al, As, B, Ba, Be, Ca, Cd, Ce, Co, Cr, Cs, Cu, Dy, Er, Eu, Fe, Gd, Ho, K, La, Li, Lu, Mg, Mn, Mo, Na, Nd, Ni, Pb, Pr, Rb, Sb, Sc, Se, Sm, Sr, Tb, Th, Tl, U, V, Y, Yb, and Zn. The detection limit was determined as the standard deviation of each element measured on all blanks for each run. A calibration curve was developed from Inorganic Ventures single element standards. A National Institute of Standards and Technology (NIST) standard reference material (SRM 1643f) was repeatedly analyzed alongside water samples to serve as a continuing calibration verification. Major anions concentrations (F -, Cl-, NO3-, SO42-) were measured using a Dionex ICS-90 ion chromatograph (IC) with an AS40 autosampler (for 2022 samples) and a Thermo Easion IC with an AS-DV autosampler (for 2023 samples). Water stable isotope measurements (δ18O and δ2H) were measured using a Los Gatos Research Liquid Water Isotope Analyzer (model LWIA-24d). Values were reported relative to the Vienna Standard Mean Ocean Water (VSMOW) standard, with a precision of 0.2‰ and 1‰ for δ18O and δ2H, respectively. Silicon was measured using a Thermo iCAP 7400 duo inductively coupled plasma—optical emissions spectrometer (ICP-OES).