In this data release, hydrogen-2 and oxygen-18 isotope ratios are presented for 158 unique sites in central and eastern Washington. There are 16 unique surface water sites with a total of 140 samples, and 142 groundwater sites with a total of 167 samples. Sample dates range from February 1993 to February 2021. Stable isotope analysis of water utilizes slight differences in the mass of water molecules due to naturally occurring stable isotopes of oxygen and hydrogen replacing the lighter and more common oxygen-16 and hydrogen-1. These differences in water molecule mass influence behavior during physical processes within the hydrologic cycle, such as evaporation and condensation. These mass differences lead to isotopic fractionation, where the lighter isotopes (for example: oxygen-16 and hydrogen-1) are preferentially enriched in the vapor phase, while the heavier isotopes (for example: oxygen-18 and hydrogen-2) are enriched in the liquid phase. The extent of fractionation is controlled by local factors such as temperature, relative humidity, and distance from the ocean which, combined, create an isotopic signature. Measuring these variations in isotopic ratios caused by local processes can be valuable for identifying groundwater sources and understanding hydrologic connectivity and flow paths in regional systems (Clark and Fritz, 1997). Stable isotopes of water (hydrogen-2 and oxygen-18) are particularly useful as groundwater tracers because they are part of the water molecule and move unhindered within the aquifer matrix. In addition to conservative transport, hydrogen-2 and oxygen-18 mix conservatively and can be used to assess mixture ratios of water parcels from distinct sources. References cited: Clark, I.D., and Fritz, P., 1997, Environmental Isotopes in . CRC Press, 342p, https://doi.org/10.1201/9781482242911