Modeling Rain Isotopic Composition under Orographic Control: A Landscape Approach for Hydrogeological Applications

Paolo Madonia,Rocco Favara,Marianna Cangemi

doi:10.3390/hydrology8010022

Paolo Madonia, Rocco Favara + Show 1 more

Open Access

https://doi.org/10.3390/hydrology8010022

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Abstract

Oxygen isotopic composition is useful for individuating recharge areas of groundwater bodies by the comparison with those of local rainfalls. While on a global scale general relationships, such as the isotopic vertical gradient or continentality effects, efficiently describe spatial variations of the isotopic signature, hydrogeological applications need spatial models that are more focused on the effects of local topographic structures and/or subsoil geology. This work presents a case study in northeastern Sicily (Italy) characterized by complex geological and orographic structures, in which isotopic composition of rainfalls is governed by orographic effects and the varying initial composition of humid air masses. We used a black box approach, comparing the average isotopic composition of rain collected from a network of eight samplers with their spatial descriptors (elevation, latitude and longitude). We obtained the best correlation with the simultaneous use of all these variables, applying their multiple linear correlation equation to transform the 1 × 1 km digital elevation model (DEM) of the study area into a digital isotopic model (DIM). The reliability of the DIM was confirmed by its good agreement with the oxygen isotopic composition contour map of the local groundwater.

Highlights

The isotopic signature of natural waters, expressed in terms of δ18 O and δ2 H, is an important tool for understanding the complex relationships between rainfall and groundwater [1], with reference to the identification of the recharge areas of aquifers.If contributions from external hydrogeological structures, and/or re-evaporation of infiltrated water, due to high temperature/low humidity climatic regimes are negligible, the average groundwater composition of an aquifer is equal to the long-term average of the rain isotopic composition of its recharge area
This study proposed a simplified method for describing the complex isotopic fractionation processes affecting rain under orographic control by using a “black box” model based on few territorial variables and able to match the precision level needed in the hydrogeological approach
The proposed method was based on a geostatistical approach (MLRA) through which a digital elevation model (DEM) can be transformed into an equivalent digital isotopic model (DIM) of natural precipitations

Summary

Introduction

If contributions from external hydrogeological structures, and/or re-evaporation of infiltrated water, due to high temperature/low humidity climatic regimes are negligible, the average groundwater composition of an aquifer is equal to the long-term average of the rain isotopic composition of its recharge area. In making this assumption, the main problem is the estimation of the error in extrapolating and interpolating data acquired in from limited number of sampling points (rain gauges) to entire catchment areas.

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