Dissolved oxygen measurements and water samples for dissolved arsenic were taken from a boat at approximately the deepest point in each lake in June 2019. Dissolved oxygen was measured every half meter using an In-Situ smarTROLL MP equipped with an optical Rugged Dissolved Oxygen (RDO) sensor. Water samples were collected using a peristaltic pump, filtered (0.45 µm Geotech cartridge filter), and then acidified with 1% HNO3 (v/v) in the laboratory. Concentrations of total arsenic in water samples were determined by inductively-coupled plasma mass spectrometry (ICP-MS) on an Agilent 7900 at the University of Washington Tacoma.

0 = Below detection limit