OBSERVED SHIFTS IN δD, δ18O, AND Δʹ17O IN PRECIPITATION AT ALLENDALE, MICHIGAN: 2021–2023

Oxygen and hydrogen stable isotopes in precipitation have long been utilized as a tool for tracking atmospheric processes, including shifts in vapor source, weather patterns, and evaporative effects. Here we present meteoric water sample data from a downwind site east of Lake Michigan (the GVSU Allendale campus) collected from 2021–2023 by several undergraduate students. We measured hydrogen isotopes (δD), oxygen isotopes (δ¹⁸O), and the derived d‑excess parameter (d‑excess = δD – 8 × δ¹⁸O). We also measured the triple oxygen isotope parameter [Δʹ¹⁷O = ln(δ¹⁷O+1) – 0.528×ln(δ¹⁸O+1)], which can now be measured at high precision (± 0.01 ‰). Δʹ¹⁷O can add information beyond what can be inferred from the traditional mass balance approach, in part because it is temperature invariant relative to d-excess. These data were collected as part of a larger study investigating the possibility of isotopically quantifying evaporation from Lake Michigan. In this presentation, we seek to fundamentally describe observed trends in each parameter over several seasons and compare our results with other isotopic observations of precipitation in West Michigan. The data show that δ¹⁸O and δD data generally follow expected trends with high variability, with increases during summer and decreases during winter. Notably, d-excess and Δʹ¹⁷O data reach high values (>25 ‰ and >50 per meg, respectively) in winter, commonly associated with snow. Δʹ¹⁷O and d-excess are correlated overall but not always strongly, especially during winter months. This is likely due to the effects of lake evaporation being strongest during the cold season. Together, these data describe meaningful hydrologic variability in the region and provide insights into regional moisture recycling processes.