Geochemical Markers as a Tool for the Characterization of a Multi-Layer Urban Aquifer: The Case Study of Como (Northern Italy)

Gilberto Binda,Paula J Noble,Roberto Gambillara,Alessandro M Michetti,Sara Trotta,Silvia Terrana,Davide Spanu,Maria F Ferrario,Francesca Frascoli,Franz A Livio,Andrea Pozzi

doi:10.3390/w14010124

Abstract

The analysis of geochemical markers is a known valid tool to explore the water sources and understand the main factors affecting natural water quality, which are known issues of interest in environmental science. This study reports the application of geochemical markers to characterize and understand the recharge areas of the multi-layer urban aquifer of Como city (northern Italy). This area presents a perfect case study to test geochemical markers: The hydrogeological setting is affected by a layered karst and fractured aquifer in bedrock, a phreatic aquifer hosted in Holocene sediments and connected with a large freshwater body (Lake Como); the aquifers recharge areas and the water geochemistry are unknown; the possible effect of the tectonic setting on water flow was overlooked. In total, 37 water samples were collected including water from two stacked aquifers and surface water to characterize hydrochemical features. Moreover, six sediment samples in the recent palustrine deposits of the Como subsurface were collected from cores and analyzed to understand the main geochemistry and mineralogy of the hosting material. The chemical analyses of water allow to observe a remarkable difference between the shallow and deep aquifers of the study area, highlighting different recharge areas, as well as a different permanence time in the aquifers. The sediment geochemistry, moreover, confirms the differences in trace elements derived from sediment-water interaction in the aquifers. Finally, an anomalous concentration of As in the Como deep aquifer was observed, suggesting the need of more detailed analyses to understand the origin of this element in water. This study confirms the potentials of geochemical markers to characterize main factors affecting natural water quality, as well as a tool for the reconstruction of recharge areas.

Highlights

Monitoring of groundwater chemistry and water resource management are important tools in sustaining human and environmental health, as these practices increase understanding of the main processes and stressors affecting water quality [1,2,3].Groundwater exploitation for human and industrial use requires the knowledge of the main natural and anthropogenic factors that can affect water quality
The cation plot clearly indicates an enrichment in Na+, K+, and Mg2+ with a decreasing trend of Ca2+ moving from the surface water toward the shallow and the deep aquifers
The enrichment in Na+, K+, and Mg2+ could be due to either a longer residence time in the deep sediment, or a recharge area that is less dominated by carbonate bedrock, in contrast with the karstically-derived shallow aquifer

Summary

Introduction

Monitoring of groundwater chemistry and water resource management are important tools in sustaining human and environmental health, as these practices increase understanding of the main processes and stressors affecting water quality [1,2,3].Groundwater exploitation for human and industrial use requires the knowledge of the main natural and anthropogenic factors that can affect water quality. Host rock geochemistry and tectonic structural features are natural attributes known to play a major role in groundwater chemical composition and flow path, in addition to the hydrology and geomorphology of the region [4,5,6,7,8] These attributes, can greatly affect geochemical background values and can cause high concentrations of potentially toxic elements (PTEs, e.g., As, Ni, Cr, Pb, U, Li), which in turn can negatively affect water quality for human use and consumption [3,9,10,11]. Geochemical markers are valid tools used to reconstruct groundwater circulation in these contexts Different chemical variables, such as major ions [12], trace elements [4], and water isotopes [13], can provide helpful insights for the estimation of groundwater circulation and the determination of recharge areas. Their combined use can provide information to develop and test conceptual models of groundwater circulation in complex systems, identify source waters and evaluate potential subsurface interconnections and mixing between local and regional aquifers [5]

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