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1 Caratterizzazione degli acquiferi: lintegrazione di metodologie geoelettriche ed idrogeologiche D. Nieto Yàbar, A. Affatato,A. Bratus, G. De Carlo, E. Marin, D. Rapti- Caputo, G. Santarato, C. Vaccaro Istituto Nazionale di Oceanografia e Geofisica Sperimentale, Trieste Acque del Basso Livenza S.p.A., Annone Veneto (VE) Dipartimento di Scienze della Terra, Università di Ferrara GNGTS - 27° Convegno Nazionale Trieste 6-8 ottobre 2008

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2 Contents: 1.Scope of the work 2.Hydrogeological outline 3.Geophysical methods 4.Results 5.Discussion. GNGTS - 27° Convegno Nazionale Trieste 6-8 ottobre 2008 www.cami-life.net

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3 1. Scope of the work To delineate the whole sequence of aquifers, which spans several hundreds of meters To estimate effective porosity of the aquifers, where data of water conductivity are available, To monitorize the evolution of the aquifers with time GNGTS - 27° Convegno Nazionale Trieste 6-8 ottobre 2008

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4 2. Hydrogeological outline: the study area GNGTS - 27° Convegno Nazionale Trieste 6-8 ottobre 2008 Foto di Arno Mohl - WWFAustria

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5 unconfined indifferentiated aquifer (recharge area: pollution possible!) Conceptual Hydrogeological Model coarse conglomerates A resurgence line test area Sketch model hydr. section A Sketch model hydr. section B 4 Km 0 -100 100

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6 multi-aquifer system A0 A1 A2 Confined aquifers 20 50 180 200m 0m Unconfined aquifer A2 A1 A0 resurgence line Conceptual Hydrogeological Model coarse sand clay test area B Sketch model hydr. section A Sketch model hydr. section B

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7 Aquifer systems (100-507 m b.g.l.) Torrate sand gravel sandy-gravel clay aquifer system

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8 The test area: the Torrate exploitation field of Acque del Basso Livenza S.p.A.

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9 3. The test area: geo-electrical methods 2D and 3D Electrical Resistivity Tomography (ERT: 0 to 100 m b.g.l.) Transient (Time Domain) Electromagnetism (TEM- TDEM, about 50-500 m b.g.l.) In-hole resistivity measurements (60 to 507 m b.g.l.) GNGTS - 27° Convegno Nazionale Trieste 6-8 ottobre 2008

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10 ERT GNGTS - 27° Convegno Nazionale Trieste 6-8 ottobre 2008 Equipment: Syscal R2 Array: Wenner- Schlumberger Electrodes: up to 128 Electrode spacing: 5 m Acquisition mode: resistivity

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11 The 2D profiles (resistivity) Inversion method: smooth (Lokes RES2DINV) First confined aquifer A1 GNGTS - 27° Convegno Nazionale Trieste 6-8 ottobre 2008 clay gravel

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12 La forma del primo acquifero Inversion method: smooth (Lokes RES3DINV) First confined aquifer A1 The 3D ERT resistivity model -->Sub-horizontally layered geometry GNGTS - 27° Convegno Nazionale Trieste 6-8 ottobre 2008

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13 TDEM : layout of the first survey (October 2005) pumps 200 m Equipment: Geonics TEM57/PROTEM Frequency bands: 25, 6, 2.5 Hz Transmitter loop: 200x200 m, current 10 to 12 A (M~4.10 5 to 5.10 5 A.m 2 ) Receiver loop: 100 coils, 1 m diameter Receiver layout: centre and out of loop on each side 2D ERT profiles TD1_t

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14 TDEM: layout of the repeated survey (April 2006) pumps GNGTS - 27° Convegno Nazionale Trieste 6-8 ottobre 2008 200 m

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15 TDEM data quality: Data (10 A) noise sounding TD06/05 (central loop, high frequencies) sounding TD08/05 (out of loop, high frequencies) Data (10 A) noise GNGTS - 27° Convegno Nazionale Trieste 6-8 ottobre 2008

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16 TDEM: results (view from SW) Aquifer A1 Potential aquifer (unknown before this survey) Inversion: 1D smooth (Occam, unconstrained) 3D imaging via kriging interpolation of 1D models

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17 In-hole resistivity measurements (lateral array) sand gravel sandy-gravel clay aquifer system from Rapti-Caputo et al., Hydrogeology Journal, 2008, in print

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18 aquiferbulk resistivity ( m) water conductivit y ( S/cm) thicknes s (m) formation factor Estimated porosity (%; m=1.3) transmissivity (m 2 /s) hydraulic conductivity (m/sec) A1122*530*30±36.5241.5x10 -2 5x10 -4 A2225380138.5195.2x10 -3 4x10 -4 A325033438.3208.1x10 -5 2.7x10 -5 A6330296>209.817-- GNGTS - 27° Convegno Nazionale Trieste 6-8 ottobre 2008 Geophysical and hydrogeological parameters of the aquifers from Rapti-Caputo et al., 2008, in print on Hydrogeology Journal *october 2005 data. average value estimated by 1D constrained inversion with fixed geometry, using bore-hole direct information and seismic data (Giustiniani et al., 2008, Geophysical Prospecting). Effective porosity was estimated using Archies law: a=1, m=1.3.

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19 Pompe Area dei pozzi di alimentazione dellacquedotto Estimated effective porosity of aquifer A1

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20 Resistivity variations vs. time Occams 1D inverted models of Soundings at the centre of loop_2 (left) and at the centre of loop_3 (right). Blue line: first survey; red line: repeated survey. First confined aquifer A1: the resistivity increases about 13% Second confined aquifer A2: too low thickness-to-depth ratio (lack of resolution) Sixth confined aquifer: large variations of depth of its base, due to electrical equivalence and lesser data quality at the latest TDEM decay times. Additional information is needed. GNGTS - 27° Convegno Nazionale Trieste 6-8 ottobre 2008

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21 Resistivity variations vs. time Electrical Conductivity (E.C) of water in the first confined aquifer measured in situ, october 2005: about 530 S/cm in the test area E.C. measured on may 2006 in the test area : 460 S/cm Observed decrease: 13%, in excellent agreement with TDEM data. from Rapti-Caputo et al., 2008

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22 Conclusions Combining surface and in-hole resistivity data allows a satisfactory characterisation of a multi-aquifer sequence and of its main hydrogeological properties Combined ERT-TDEM are an efficient tool to image aquifers in an alluvial mattress TDEM has shown a sufficiently high sensitivity to resistivity variations, to be considered as a reliable tool for monitoring purposes; Aknowledgements: Research carried out with the financial support of the EC, contract LIFE04 ENV/IT/00500. GNGTS - 27° Convegno Nazionale Trieste 6-8 ottobre 2008

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23 M. Giustiniani, F. Accaino, S. Picotti and U. Tinivella: Characterization of the shallow aquifers by high-resolution seismic data. Geophysical Prospecting, 2008, 56, 655–666

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24 Based on the well known analytical relationship given by Niwas and Singhal (1981): T = KσR = KS/σ(2) where T = transmissivity, K = hydraulic conductivity, R = tρ (t and ρ are the thickness and resistivity of the aquifer layer) is the transverse resistance and S = t/ρ is the longitudinal conductance of the aquifer layer under study, the transmissivity in the whole surveyed area can be evaluated for aquifers A1, A2 and A3, bearing in mind that in areas of similar geologic setting and water quality, the product Kσ remains fairly constant.

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