DOI: 10.1594/pangaea.970402
title: Major and trace elements in water from Deming Lake, MN, USA (2022-2023)

description: Sampling and in situ sensor analysis was conducted from an anchored boat at the deepest location of the lake, approximately 20 m water depth, in May 2022, August 2022, March 2023, and August 2023. All samples for water column dissolved and particulate major and minor elements were collected using a Proactive Mini Monsoon pump with a low-flow controller that was attached to vinyl tubing and a power cable marked with 0.5 m depth increments. Waters for trace metal analysis were filtered with removable 25 mm diameter 0.22 μm PES membrane filters placed in polypropylene Swinnex filter holders that were removed and immediately placed in cleaned Savillex Teflon reactors until leaching and analysis. For August 2023, samples were also collected with syringes fit with 0.45 μm syringe filters. The dissolved trace metal samples were collected in 50 mL centrifuge tubes, precleaned with 10 % trace metal-grade HCl. All lake water samples were acidified at the end of the sampling day using Optima grade HCl to a molarity of 0.4 M to ensure complete dissolution of Fe oxyhydroxides that form following exposure of Fe-rich lake waters to air. All sample preparation was performed in the NIRVANA class 100 clean laboratory at WHOI unless otherwise indicated. All reagents used through all preparation steps were double distilled or purchased at Optima grade, and sample preparation steps made use of Savillex Teflon reactors unless otherwise stated. Water elemental compositions were determined using a Thermo Fisher iCAP-Q inductively coupled plasma mass spectrometer (ICP-MS) at the WHOI Plasma Facility following 200 dilution of a 10 μL aliquot in 2 % HNO3. Indium (In) was added to samples at a concentration of 1 ng/g prior to analyses to monitor and correct for instrument drift by normalizing to In intensities. Concentrations were calculated using a five-point calibration curve obtained by fitting of ion beam intensities measured for serial dilutions of a gravimetrically prepared multi-element standard. The relative standard deviation (RSD) for five measurements of each sample was ~10% on the iCAP-Q. The accuracy and precision of similar concentration measurements on iCAP-Q at WHOI have previously been determined to be ± 5-10 % (1SD) based on comparison with USGS reference materials AGV-1, AGV-2, BHVO-1, BHVO-2, BIR-1, and BCR-2 prepared and analyzed as unknowns during earlier runs (Jochum et al., 2016; Shu et al., 2017). Particulate elemental compositions were determined following the same measurement scheme, after leaching the PES membrane filters originally mounted on the Swinnex holders overnight in 2 M HNO3 at 80 °C in a volume of acid calibrated to be sufficient to leach all trace metals from the filter.For May and August 2022, and March 2023 sampling events, particulate Mn oxide abundance data were generated using the same methodologies employed in Gadol et al. (2023). Roughly 150 mL of Deming Lake water was passed inline during collection through 0.22 m pore size Sterivex PES filter cartridges. Cartridges were thereafter purged via syringe, sealed with Parafilm, and frozen until quantification. Abundances were quantified using the leucoberbelin blue (LBB) assay with a permanganate standard curve (Altmann, 1972; Oldham et al., 2015). Sterivex cartridge filters containing lake particles were filled with 2 mL LBB at a concentration of 0.004% in 0.1% acetic acid and then resealed with Parafilm. After approximately two hours, the liquid extract was removed and absorbance was measured at 620 nm on a UV-Vis spectrophotometer.