Coupling Hydrogeochemical Data And Structural Modelling To Enhance Understanding Of Groundwater Circulation Controlled By Faults In Northern Sardinia (Italy)

Cyrill Labry;Luca Marcia;Riccardo Biddau;Rosa Cidu;Stefania Da Pelo;Antonio Funedda

2021-01-01

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Abstract

The complex flow pattern in groundwater originating from the geological structure often leads to difficult predictions of the main flow paths and potential mixing of groundwater. We describe a methodological approach based on structural mapping and modelling, fault sealing behaviour and hydrogeochemical tools to enhance understanding of groundwater circulation and to overcome the scarcity of data. The Logudoro basin was considered as a test site. It is a Burdigalian half‐graben that includes mainly subhorizontal continental to marine deposits such as conglomerates, sandstones, limestones and mudstones. The infill unconformably lies over an Oligo‐Miocene volcanic succession. Pleistocene basaltic flows unconformably overlie both volcanic and sedimentary Miocene rocks. The geological model was improved through detailed mapping (1:10.000). Outcrop geology and boreholes were used to constrain cross‐sections. The subsequently created 3D‐model validated the geological map and allowed to analyse recharge areas, aquifers interconnection and sealing conditions of the faults. Two fault sealing conditions were studied: 1) juxtaposition of different lithotypes with varying permeability along the fault section, 2) shale smear into a fault‐core membrane using the Shale Smear Factor (SSF). Faults show a barrier‐conduit behaviour, changing horizontal hydrogeological transmittivity. Comprehensive field surveys, including hydrogeochemical sampling and hydrogeological measures were carried out. Water samples were collected from 13 springs, 28 wells and 3 surface waters. Piezometric data were measured from 101 monitoring point. The major ion composition of water samples indicates a wide range of geochemical compositions of groundwater depending on aquifers heterogeneity and their partial interconnection. Hydrogeochemical features linked to hydraulic heads and the stratigraphic model allowed to classify water groups. Hence aquifer vertical interconnection and specific lithotype signature were determined. Furthermore, the lateral extension of an aquiclude, separating two different aquifers was identified. Adding fault seal properties allowed to understand lateral aquifers interconnections and/or disconnections. The fault sealing model confirmed the groundwater flow trend determined by the piezometric contour lines.