Arsenic (As) toxicity is a global environmental and health problem. There are both natural (eg volcanic activity) and anthropogenic sources of As (eg lead arsenate and copper arsenate were commonly used pesticides in the 1900’s). Aqueous levels of arsenic in the Klamath Basin (CA, OR), which has a volcanic origin, can exceed at some locations both the Oregon Department of Environmental Quality human health water quality criteria (2.1 ug/L) (Sturdevant, 2011) and the US EPA drinking water limit (10 ug/L) (US EPA., 2001). In this study, dissolved concentrations of As, copper (Cu) and lead (Pb) were measured in more than 30 sites within the Klamath Basin between May and October. Results from samples collected between 2018 and 2022 are reported in this data release. References: Sturdevant, Debra., 2011. Water Quality Standards Review and Recommendations: Arsenic (Draft Report). State of Oregon Department of Environmental Quality. https://www.oregon.gov/deq/FilterDocs/AppEArsenicIssuePaper.pdf US EPA., 2001. Drinking Water Standard for Arsenic (Report No. EPA 815-F-00-015). United States Environmental Protection Agency. https://www.epa.gov/dwreginfo/chemical-contaminant-rules