Groundwater isotopes across scales: continent-wide modeling and local field characterization

Groundwater is one of the world's most important natural resources. The use of stable water isotopes (𝛿2H and 𝛿18O) as natural tracers through the water cycle has provided a unique observational technique for characterizing hydrological processes and establishing connections between water distribution systems and their respective environmental sources. A predictive model for groundwater isotopes was developed across the contiguous United States (to combat pre-existing limitations of the lack of such data) using a random forest model based on environmental parameters. We find evident spatial coherence in the model, generally mirroring the signal of isotopes of precipitation, and highlight the potential for its application across hydrology and ecology. In addition, to demonstrate the applicability and versatility of groundwater isotopes, we investigated the local municipal water supply in Schenectady to understand the sources and seasonal variability in these sources. The Schenectady municipal well-field is sited less than a kilometer from the Mohawk River, making the interaction between surface water and groundwater highly complex and seasonally dependent. Schenectady tap water, which is drawn from local groundwater, and Mohawk River water were collected at regular intervals and analyzed in the Union College Stable Isotope Laboratory for stable isotopes of hydrogen and oxygen. The seasonal signal of isotopes can be approximated by sine waves, and the phase and amplitude of these signals can be used to calculate the average linear velocity (3.5 m/day) of the water moving into the aquifer and fraction of young water (57% < 2.7 months) in the local groundwater. Our results highlight the connection between the Mohawk River and the aquifer in the vicinity of the Schenectady well-field, and motivates further research to characterize the potential for vulnerabilities. Thus, this study not only provides an isoscape to detail the spatial distribution of isotopes regionally, but also demonstrates how we can leverage our understanding of isotopes for insight into the chemical and physical hydrology in a local water system.