Isotopic variation in groundwater across the conterminous United States – Insight into hydrologic processes

Abstract

Groundwater supplies a significant proportion of water use in the US and is critical to the maintenance of healthy ecosystems and environmental processes, thus characterizing aquifer hydrology is important to managing and preserving these resources. While groundwater isotopes provide insight into hydrologic and ecologic processes, 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 US. 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 δ 2 H and δ 18 O for the conterminous US. We describe the patterns in groundwater isotopes using both observations and our modeled isoscape. We find that throughout much of the Eastern US, groundwater isotopes are close to annual amount weighted precipitation, while groundwater isotopes are significantly depleted relative precipitation across much of the High Plains and Western US. Furthermore, we compare the observations compiled for this study to isotopes of precipitation, which allows us to 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 US 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 This article is protected by copyright. All rights reserved. This is the author manuscript accepted for publication and has undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as

Using Stable Isotopes to Determine the Water Balance of Utah Lake (Utah, USA)

“…Indeed, all the well water samples plot on or close to the LMWL ( Figure 6), indicating no significant evaporation in groundwater. Furthermore, our result confirms those reported in other studies performed in Utah and adjacent states [8,42,43]. Possible alternatives to recharge by winter precipitation are represented by: (i) recharge by meteoric precipitation that falls during the spring, or summer, or fall within or in areas adjacent to the Utah Lake watershed but at elevations higher than our UVU monitoring site, and/or (ii) recharge by meteoric precipitation that fell during the spring, summer, or fall in periods with a colder climate (e.g., the last glacial maximum).…”

A 3-D groundwater isoscape of the contiguous USA for forensic and water resource science

“…Cross-validation prediction errors are similarly distributed for all depth intervals and are somewhat heavy-tailed (S4 Fig) . Mean absolute errors and root mean squared errors range from 0.51 and 0.80‰ for the 200-500 m interval to 0.80 and 1.25‰ in the 500-2,000 m layer; for all depth intervals excluding this deepest one, the values are less than or equal to 0.71 and 1.07‰. These error statistics are very similar to those reported for geostatistically-based predictions of CONUS stream water [23] and global precipitation δ 18 O values [37] and for a CONUS shallow groundwater isoscape produced using a random forest algorithm [28].…”

“…We produced a set of 2-d summary layers from the 3-d groundwater isoscape (including the mean and standard deviation of values from the subsurface layers represented at each geographic grid cell). Model fit was assessed by regressing observed tap water δ 18 O values against associated depth-averaged means from the groundwater isoscape; an equivalent comparison was made with values from the precipitation and Stahl et al [28] CONUS groundwater isoscapes. For each TWI site, we also evaluated whether the measured tap water value was contained within the mean ± z standard deviation (σ) groundwater δ 18 O prediction interval for each subsurface layer (where z was 1 or 1.96 to approximate 68% and 95% prediction intervals, respectively).…”

“…Despite the widespread use of H and O isotopes in the study of groundwater systems and the importance of groundwater as a source to many hydrological and drinking water systems, groundwater isoscapes have received relatively little attention. Only two studies (to our knowledge) have attempted to develop synoptic-scale groundwater isoscapes: one using spatial interpolation of groundwaters across Mexico [27] and a second [28] using machine learning methods to model shallow groundwaters of the contiguous United States (CONUS).…”

Connecting the Age and Reactivity of Organic Carbon to Watershed Geology and Land Use in Tributaries of the Hudson River

“…Hydrograph separation (Text S1 and Figure S1), along with the comparison of stream water stable isotopes (δ 2 H and δ 18 O) to local groundwater isotopes (Stahl et al, 2020) and local precipitation isotopes (Figure S2), reveal that the streams were sampled under baseflow conditions and thus the dominant source of streamflow was groundwater (Table S1). Across sites DOC concentrations ranged from 1.48 to 4.23 mg/L C and are negatively correlated (correlation coefficient = −0.80, p-value = 0.05) with the percent of agricultural land cover within a watershed (Figure S3), a finding that is consist with observations in other watersheds worldwide (Drake et al, 2019;Spencer et al, 2019).…”