Estimating the Isotopic Altitude Gradient for Hydrogeological Studies in Mountainous Areas: Are the Low-Yield Springs Suitable? Insights from the Northern Apennines of Italy

Tazioli Tazioli,Cervi Cervi,Doveri Doveri,Deiana Deiana,Ronchetti Ronchetti,Mussi Mussi

doi:10.3390/w11091764

Abstract

Several prior studies investigated the use of stable isotopes of water in hydrogeological applications, most on a local scale and often involving the isotopic gradient (evaluated by exploiting the so-called altitude effect), calculated on the basis of rainwater isotopes. A few times, this gradient has been obtained using the stable isotopic contents of low-yield springs in a limited time series. Despite the fact that this method has been recognized by the hydrogeological community, marked differences have been observed with respect to the mean stable isotopes content of groundwater and rainwater. The present investigation compares the stable isotopic signatures of 23 low-yield springs discharging along two transects from the Tyrrhenian sea to the Po Plain of Italy, evaluates the different isotopic gradients and assesses their distribution in relation to some climatic and topographic conditions. Stable isotopes of water show that groundwater in the study area is recharged by precipitation and that the precipitation regime in the eastern portion of the study area is strongly controlled by a shadow effect caused by the Alps chain on the air masses from central Europe. Stable isotopes (in particular the δ18O and deuterium excess (d-excess) contents together with the obtained isotopic gradients) allow us to identify in the study area an opposite oriented orographic effect and a different provenance of the air masses. When the windward slope is located on the Tyrrhenian side, the precipitation shows a predominant oceanic origin; when the windward slope moves to the Adriatic side, the precipitation is characterized by a continental origin. The main results of this study confirm the usefulness of low-yield springs and the need for a highly detailed survey-scale hydrological investigation in the mountainous context.

Highlights

Over the last few decades, several studies involving stable isotopes of water have been performed worldwide, many with the principal aim of investigating the factors involved in the precipitation formation process [1,2,3,4,5]
The springs belonging to the northern transect showed δ18 O contents ranging between −9.4% and −5.7% and δ2 H contents ranging between −65% and −36% (Figure 4a, δ2 H values are not shown as being characterized by the same pattern as the δ 18 O content)
The low isotopic values found on the eastern side were the result of the shadow effect caused by the Alps chain on the clouds originating from the air masses of central Europe [38], which strongly controlled the precipitation regime of this part of the northern Apennines

Summary

Introduction

Over the last few decades, several studies involving stable isotopes of water have been performed worldwide, many with the principal aim of investigating the factors involved in the precipitation formation process [1,2,3,4,5]. Water 2019, 11, 1764 into hydrological processes occurring at the soil surface (i.e., evapotranspiration, runoff and infiltration). In this context, isotopic fractionation seems to be a key factor in understanding of hydrological processes connected to the entire hydrological cycle with a particular focus on precipitations. Fractionation is one of the most important properties of isotopes, and the isotopic composition of water vapor is modified during evaporation, freezing, condensation, and melting due to isotopic fractionation [6,7,8]. The Global Meteoric Water Line (GMWL) is used to depict the relationship between the δ2 H and δ18 O content in precipitations on a global scale [9,10]

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Conclusion