Abstract
A wide range of hydrological, ecological, environmental, and forensic science applications rely on predictive "isoscape" maps to provide estimates of the hydrogen or oxygen isotopic compositions of environmental water sources. Many water isoscapes have been developed, but few studies have produced isoscapes specifically representing groundwaters. None of these have represented distinct subsurface layers and isotopic variations across them. Here we compiled >6 million well completion records and >27,000 groundwater isotope datapoints to develop a space- and depth-explicit water isoscape for the contiguous United States. This 3-dimensional model shows that vertical isotopic heterogeneity in the subsurface is substantial in some parts of the country and that groundwater isotope delta values often differ from those of coincident precipitation or surface water resources; many of these patterns can be explained by established hydrological and hydrogeological mechanisms. We validate the groundwater isoscape against an independent data set of tap water values and show that the model accurately predicts tap water values in communities known to use groundwater resources. This new approach represents a foundation for further developments and the resulting isoscape should provide improved predictions of water isotope values in systems where groundwater is a known or potential water source.
Highlights
The physics of moisture evaporation, transport, and condensation in the atmosphere create coherent spatiotemporal variation in the hydrogen (H) and oxygen (O) isotope ratios of meteoric waters [1,2]
Groundwater well depth (GWD) data for the 48 contiguous United States were compiled from state-level geodatabases or, where state-agency data were unavailable, from the U.S Geological Survey (Fig 1A)
Our analysis suggests that similar-magnitude offsets persist within deep aquifers of this region, which could reflect a consistent set of recharge-based controls across a wide range of depths or, perhaps more parsimoniously, can be explained if the precipitation model against which all ST: USA 3-D groundwater isoscape groundwater layers were compared is biased toward low δ18O values
Summary
The physics of moisture evaporation, transport, and condensation in the atmosphere create coherent spatiotemporal variation in the hydrogen (H) and oxygen (O) isotope ratios of meteoric waters [1,2]. This variation is frequently preserved and passed on through the hydrological cycle and the transfer of H and O into Earth materials and ecosystems, where it is widely useful in identifying and partitioning the sources of water and geological or biological materials. ST: USA 3-D groundwater isoscape number of these data are not available for redistribution by the authors or through the wiDB, in these cases the database record provides information on how the data can be obtained from 3rd party sources. We provide a list of the data sources from which these data were obtained as a supplementary table
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