Specific conductance (SC), estimated chloride (Cl), and other major ion data sets used in an analysis of the extent to which deicer applications affect groundwater quality in the Siskiyou Pass area, southwestern Oregon, 2018 to 2021. The analysis is documented in the following publication: Gingerich, S.B., Wise, D.R., and Stonewall, A.J., 2023, Assessing the effects of chloride deicer applications on groundwater near the Siskiyou Pass, southwestern Oregon, July 2018–February 2021: U.S. Geological Survey Scientific Investigations Report 2023–5107, 39 p., https://doi.org/10.3133/sir20235107. This data release contains seven child items: 1) Specific conductance and major ions measured at selected sampling locations during July 30 to August 1, 2018, including springs, streams, and wells, in the Siskiyou Pass area, southwestern Oregon.2) Measured specific conductance, water temperature, pH, and major ions at selected sampling locations from September 2019 to November 2020, including springs, streams, and a well, in the Siskiyou Pass area, southwestern Oregon. 3) List of selected sampling locations from July 2018 to February 2021, including springs, streams, wells, and a pond, in the Siskiyou Pass area, southwestern Oregon. 4) List of selected sites that were part of the study in the Siskiyou Pass area, southwestern Oregon. 5) Measured specific conductance, water temperature, Quantab units and estimated chloride concentration from National Water Quality Lab and Quantab field chloride titrators at selected sampling locations from July 2018 to November 2020, including springs, streams, and a well, in the Siskiyou Pass area, southwestern Oregon. 6) Mean annual salt and deicer chemical applications from winter 2016-17 through winter 2019-20. 7) Quality assurance data at selected locations, Oregon.