WATER RE-BORN Artificial recharge: innovative technologies for the sustainable management of water resources

\ LIFE10 ENV/IT/000394 WARBO

Life WARBO: area test di Copparo (provincia di Ferrara, coordinate geografiche: 44.911980°N,11.836217°E

Action: 7 Geophysical surveys at Copparo test site: Geoelectrical Resistivity Tomography Surface and borehole

Dr. Nasser ABU ZEID

Dipartimento di Fisica e Scienze della Terra Università degli Studi di Ferrara

Via Saragat 1 Blocco B 44122 Ferrara (FE)

Italia

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1. Introduction The geoelectrical methods, one of the oldest active, since 1926, and most widely used geophysical techniques for

the exploration of the subsurface at different depths: from few cm’s to thousands of meters depending, mainly, on the survey target. The geoelectric methods depend on the determination of the electrical resistivity. The Electrical resistivity based methods can be conducted on the ground surface, beneath the ground, from air and on/in water (mainly along, rivers, lakes and shorelines). In this short summary, I shall focus on ground-based active electrical methods. Of these, the Electrical Resistivity Tomography (ERT) and the less commonly used Electrical Induced Polarization (EIP) techniques. These techniques are employed to explore the subsurface in a completely indirect and non-invasive way providing 2D and 3D images of the distribution of both resistivity and chargeability. Such images, as shall we see later, allow for better visualization of lateral as well as vertical variations that can be interpreted in terms of lithology, saturation, alteration, physio-chemical modifications (Archie 1942; Telford 1990; Mualem and Friedman, 1991; Abu Zeid and Santarato, 2004, Ewing and Hunt 2006). The authors of the latter work presented ample evidences about the possibility to discern grain-size of salinized permeable sediments from impermeable fine-grained sediments. These achievements allowed authors to employ the same techniques for the subsurface characterization of Copparo (Ferrara, North Italy) test site [EU life+WARBO project /2011-2014/ (http://www.warbo-life.eu/)] not only for the geometrical mapping of the subsurface aquifers but also for mapping spatial and temporal variation of undergoing salinization processes and in monitoring the efficiency of the measures addressed to counteract these processes through the activity of Artificial Recharge (A.R.).

2. Data acquisition (WARBO test site at Copapro, North Italy) At Copparo’s test site, the subsurface around the artificial lake was explored with the help of 10 2D Electrical

Resistivity Tomography (ERT) and EIP profiles (Fig. 1). Similar investigation was conducted in the borehole “P9” which was initially planned for indirect and non-invasive of A.R. activity. Three georesistivity meters were employed for data resistance and chargeability data acquisition; these are: MTP DAS-1(USA, http://www.mpt3d.com/das1.html), ABEM SAS4000 Inc (www.abem.se) and LGM “Lippmann Geophysical Equipment” http://www.geophysik-dr-rauen.de/E_Index/e_index.htm). Both Wenner and Wenner-Schluberger electrode arrays were used. Resistivity and chargeability 2D inversion models were accomplished using the Res2Dinv commercial code (Geotomo, 2012 - www.geotomosoft.com/) which implements a non-linear Gauss-Newton optimization method that is based on OCAM’s concept (Oldenburg, 1994), (i.e. derived models are kept smooth while minimizing the root-Mean-Squares error RMS between experimental and theoretically derived apparent resistivity and chargeability data based on the best estimate of model parameters in terms of real resistivity and chargeability.

Figure 1. location map of Copparo, NE Italy, Artificial Recharge test site. Coordinates are in meters with respect to the European Datum 1950 (ED50). Read line: ERT-IP profile, green line: ERT profile No. 5 used for temporal monitoring of

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the A.R. activities, brown rhomb: phyto-depuration pit. Google images: two satellite snapshots of the artificial lake (old clay query) taken before 2003 and in 2013, photograph: present day lake, P9: armed piezometer for borehole ERT measurements, dot and circle: borehole (lithology), blue close curve: artificial lake approximate limits.

3. Results In this section, I shall focus on ERT-5 profile as it was used for monitoring subsurface

resistivity variations due to A.R. activities. As shown in Figure (2), four 2D resistivity models are reported. The first model (Fig. 2/Aprile 2013) refers to the prevailing conditions before the start of the A.R. activities. During the A.R. phase, two resistivity images were obtained (Fig. 2/June and August 2014) where a 4 meters depth increase of the brackish water-salt water interface have been observed. After the temporal suspension of the A.R. activities, a further resistivity image (Fig. 1/September 2014) has evidenced a nearly 4 meters decrease of the interface depth. The observed resistivity variation was surely linked to the A.R. activity. The obtained results, although based on only 4 acquisitions, have confirmed the efficiency of the ERT technique as an indirect and non-invasive low cost monitoring tool. In addition, the achieved results, supported also by direct measurements of surface and groundwater physio-chemical parameters, provided an additional support for the excavation of a permanent canal that continuously convey fresh water to the artificial lake passing through the phyto-depuration pit.

Figura 2: modelli di resistività relativi al profilo n. 5 ottenuti dall’inversione dei dati di resistività apparente acquisiti in quattro periodi diversi: aprile 2013, giugno, agosto e settembre 2014. Cerchio verde: fase di abbassamento dell’interfaccia acqua salmastra-acqua salata (in presenza di attività di R.A.). Cerchio rosso: fase di risalita dell’interfaccia (assenza di attività di R.A.).

 

April 2013 

August’14

Sept. 2014