42-17 Trends in Precipitation and Groundwater d-excess Across the Colorado Front Range: Variability and Comparisons to δ18O

Abstract:

Stable isotope measurements such as δ2H and δ18O have a long history of use as tracers for modeling the flow of water through hydrological systems and investigating the functioning of the water cycle from global to local scales. D-excess is a second-order stable isotope parameter that can be used as a tracer for water movement, as well as an indicator for the influence of various controls on the water cycle, including point-source relative humidity, regional moisture recycling, and evaporative influence. To examine the sources of precipitation and groundwater along the Front Range, we conduct an analysis of d-excess values in precipitation, groundwater, and stream waters across the Front Range. We utilize a water isotope dataset spanning from late 2021 to present, generated by a combination of samplers maintained by us as well as collaboration with citizen scientists from up and down the front range. Using this dataset, we analyze data across 14 discrete collection sites that include both precipitation and groundwater samples from the Colorado Front Range as well as one collection site from northern Wyoming. We find that d-excess in groundwater exhibits a greater degree of variability than δ18O, and that there is a tight correlation between the d-excess value of groundwater and elevation of the well head. A similar, but less constrained correlation is also observable in groundwater δ18O values. We also find that precipitation exhibits a unique signature in d-excess on the Front Range in which seasonal peaks take place in early Summer and early Fall, with a sharp drop to an annual minimum in the middle of Summer and another minimum occurring in the winter. These findings indicate that d-excess likely has a greater sensitivity to certain environmental/climatological influences than δ18O, indicating a need for further research into the function of d-excess in this region.

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