Angle Lake and Lake Killarney (western Washington State, USA) were sampled monthly or bi-monthly on a total of 29 dates between July 2016 and December 2017. Water column temperature and dissolved oxygen profiles were measured using a multi-parameter water quality probe (In-Situ smarTROLL MP) at approximately the deepest point in each lake. A peristaltic pump with acid-washed tubing was used to collect filtered (0.45 µm Geotech cartridge filter) water samples into acid-washed polypropylene bottles that were acidified with trace metal grade nitric acid (1% v/v). Phytoplankton samples were collected by net tow (20-153 micron). Sediment trap samples were collected at mid-depth (~7 m and ~2 m from bottom in Angle Lake and Lake Killarney, respectively) and near bottom (~1 m above bottom) in both lakes. Phytoplankton tow samples and sediment trap samples were filtered onto 5 µm polycarbonate membrane filters then subjected to a microwave-assisted (CEM MARS 5) total digestion protocol (modified EPA method 3015). Aqueous arsenic concentrations in water samples, plankton digestion solutions, and sediment trap digestion solution were determined by ICP-MS on an Agilent 7900 at the University of Washington Tacoma.
In-situ sediment and porewater sampling was conducted in Lake Killarney in July 2016, Aug. 2017, and Dec. 2017 and in Angle Lake in Sept. 2016, Aug, 2017, and Dec. 2017. Porewaters were collected using passive diffusive samplers (peepers) using sample vials (LDPE, 5 mL) filled with a de-oxygenated reverse tracer solution (200 µM KBr). Sediment cores were collected by in situ freezing, then sectioned under a N2 atmosphere. The solid phase of arsenic in sediments was investigated using a 3-step sequential extraction protocol: a phosphate-exchangeable fraction (PE) targeting strong adsorbed arsenic, a citrate-bicarbonate-dithionite (CBD) treatment targeting arsenic associated with iron oxides, and a microwave-assisted total nitric digestion targeting residual arsenic (R). Aqueous arsenic concentrations in porewater samples and sediment extraction solutions were determined by ICP-MS; bromide concentrations in peeper samples were measured using an ion selective electrode.
A temperature manipulation experiment was performed on sediment cores were collected from Lake Killarney and Angle Lake in January 2018. Duplicate cores from each lake were incubated at 10°C and 20°C and the concentration of arsenic in the overlying water was monitored at 7 timepoints over 48 days; aqueous arsenic concentrations were determined by ICP-MS.
