Inferring Hydrological Information at the Regional Scale by Means of δ18O–δ2H Relationships: Insights from the Northern Italian Apennines

Federico Cervi,Alberto Tazioli

doi:10.3390/hydrology9020041

Federico Cervi, Alberto Tazioli

Open Access

https://doi.org/10.3390/hydrology9020041

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Abstract

We compared five regression approaches, namely, ordinary least squares, major axis, reduced major axis, robust, and Prais–Winsten to estimate δ18O–δ2H relationships in four water types (precipitation, surface water, groundwater collected in wells from lowlands, and groundwater from low-yield springs) from the northern Italian Apennines. Differences in terms of slopes and intercepts of the different regressions were quantified and investigated by means of univariate, bivariate, and multivariate statistical analyses. We found that magnitudes of such differences were significant for water types surface water and groundwater (both in the case of wells and springs), and were related to robustness of regressions (i.e., standard deviations of the estimates and sensitiveness to outliers). With reference to surface water, we found the young water fraction was significant in inducing changes of slopes and intercepts, leading us to suppose a certain role of kinetic fractionation processes as well (i.e., modification of former water isotopes from both snow cover in the upper part of the catchments and precipitation linked to pre-infiltrative evaporation and evapotranspiration processes). As final remarks, due to the usefulness of δ18O–δ2H relationships in hydrological and hydrogeological studies, we provide some recommendations that should be followed when assessing the abovementioned water types from the northern Italian Apennines.

Highlights

Oxygen (18 O and 16 O) and hydrogen isotopes (2 H and 1 H) of water are commonly used in surface and subsurface hydrology [1–3].h They are considered i environmental tracers in the Rsample RstandardPublisher’s Note: MDPI stays neutral form of δ18 O and δ2 H, where δ(‰) =with regard to jurisdictional claims in isotopic ratio (18 O/16 O or 2 H/1 H) in the water sample or in the standard
This study aims to verify whether such discrepancies in slopes and intercepts from different regression methods are present or not in four water types from the northern Italian Apennines
We presented the comparison of five different regression approaches applied to δ18 O

Summary

Introduction

Oxygen (18 O and 16 O) and hydrogen isotopes (2 H and 1 H) of water are commonly used in surface and subsurface hydrology [1–3].h They are considered i environmental tracers in the Rsample RstandardPublisher’s Note: MDPI stays neutral form of δ18 O and δ2 H, where δ(‰) =with regard to jurisdictional claims in isotopic ratio (18 O/16 O or 2 H/1 H) in the water sample or in the standard (usually V-SMOW, published maps and institutional affili.e., the Vienna Standard Mean Oceanic Water). If a multitude of water samples is collected from the same source (a rain gauge, a river, a spring, or a well), the corresponding δ18 O–δ2 H pairs in a Cartesian graph will be aligned along a regression line in the form of y = mx + q, where y is δ2 H, x is δ18 O, m is the slope, and q the intercept. This fact was first noted by [4]. This is due to the specific isotopic fractionation processes (i.e., vapour pressure and temperature conditions) controlling iations

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