Nitrate concentration exceeding the European limit of 50 mg/L in groundwater requires detailed investigation in order to delimitate Nitrate Vulnerable Zones (NVZ). 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. The methodological approach consists in defining a hydrogeological model based on geostructural mapping and modelling, supported by hydrogeochemical and isotopic tools. The Logudoro basin, within a Burdigalian half-graben in northern Sardinia, was considered as a test site. The area 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 water-volume potential, recharge areas and aquifers interconnection. Water samples were collected from 13 springs, 28 wells, 3 surface waters and untreated and clean sewages. Geochemical features and isotopic analysis of δ2H - δ18O in water, δ11B, δ15N - δ18O in dissolved nitrate, and δ34S - δ18O in dissolved sulphate were determined to investigate groundwater recharge, processes occurring during flow path and identify nitrate origin. Groundwater show a wide range of geochemical compositions. A weak variation of isotopic composition with depth was observed. Piezometric contour lines combined with hydrogeochemical analysis allow to classify water groups and identify interaction processes within aquifers. Five hydrogeological units bounded by faults and aquitards, discontinuous in width and thickness, were outlined by coupling geochemical and geological modelling. The applied approach improves the groundwater flowpath comprehension leading to the delimitation of potential NVZ.

Designation of Nitrate Vulnerable Zones in northern Sardinia – Coupling geochemical data and geostructrural mapping and modelling to enhance the understanding of groundwater flow

Cyrill Labry;Luca Marcia;Riccardo Biddau;Rosa Cidu;Stefania Da Pelo;Antonio Funedda
2021

Abstract

Nitrate concentration exceeding the European limit of 50 mg/L in groundwater requires detailed investigation in order to delimitate Nitrate Vulnerable Zones (NVZ). 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. The methodological approach consists in defining a hydrogeological model based on geostructural mapping and modelling, supported by hydrogeochemical and isotopic tools. The Logudoro basin, within a Burdigalian half-graben in northern Sardinia, was considered as a test site. The area 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 water-volume potential, recharge areas and aquifers interconnection. Water samples were collected from 13 springs, 28 wells, 3 surface waters and untreated and clean sewages. Geochemical features and isotopic analysis of δ2H - δ18O in water, δ11B, δ15N - δ18O in dissolved nitrate, and δ34S - δ18O in dissolved sulphate were determined to investigate groundwater recharge, processes occurring during flow path and identify nitrate origin. Groundwater show a wide range of geochemical compositions. A weak variation of isotopic composition with depth was observed. Piezometric contour lines combined with hydrogeochemical analysis allow to classify water groups and identify interaction processes within aquifers. Five hydrogeological units bounded by faults and aquitards, discontinuous in width and thickness, were outlined by coupling geochemical and geological modelling. The applied approach improves the groundwater flowpath comprehension leading to the delimitation of potential NVZ.
NVZ, Sardinia, Isotopes, 3D-model
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11584/329673
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