Isotopic variation in shallow groundwater across the United States – Insight into hydrologic and ecologic processes

Abstract:

Groundwater is critical to the maintenance of healthy ecosystems and environmental processes, thus characterizing aquifer hydrology is important to managing and preserving these resources. Groundwater isotopes provide insight into hydrologic and ecologic processes, however their application is limited to where measurements exist. To help overcome this limitation, we used the random forest algorithm to develop a predictive model for shallow groundwater isotopes in the conterminous United States. Our model uses environmental variables (e.g., temperature, elevation, precipitation isotopes) as predictors. We used our model to develop the first shallow groundwater isoscape of δ²H and δ¹⁸O for the conterminous United States. Using both our modeled isoscape and isotopic measurements compiled for this study, we describe patterns in shallow groundwater isotopes. We find that throughout much of the Eastern United States, groundwater isotopes are close to annual amount weighted precipitation, while across much of the High Plains and Western United States groundwater isotopes are significantly depleted relative precipitation. Furthermore, by comparing the measurements of groundwater isotopes compiled for this study to isotopes of precipitation, we determine the relative recharge efficiency (i.e., ratio of groundwater recharge to precipitation) between seasons and the proportion of annual recharge that occurs in a given season. Our findings suggest that winter recharge is generally more efficient than summer recharge; however, the dominant recharge season is more varied as it is the product of both seasonal recharge efficiency and the seasonal timing of precipitation. Parts of the central United States have summer dominant recharge, which is likely the result of heavy summer precipitation/nocturnal summer precipitation. Interestingly, parts of coastal California appear to have summer dominant recharge, which we suggest could be due to recharge from fog‐drip. Our results summarize spatial patterns in groundwater isotopes across the conterminous United States, provide insight into the hydrologic processes affecting shallow groundwater, and are valuable information for future ecologic and hydrologic studies.