Many coastal areas are affected by aquifer salinization due to an unsustainable use of groundwater resources. Currently, few aquifer maps are available for areas subjected to salinization, since a quantitative assessment of soil and groundwater is not cost-effective without using non-invasive techniques for covering large areas with limited costs.
The key objective of this research proposal is the development of vulnerability maps related to aquifer salinization. The research project is intended to build a hydro-geophysical model derived from the acquisition of experimental non-invasive geophysical data of electrical resistivity tomography (ERT), complemented by geochemical, isotopic and geomatic data, in order to retrieve a 3D reconstruction of the studied area, where salinization-prone zones will be detected.
We aim to combine the resistivity model with the assessment of the capacitive behaviour of soil and groundwater (Induced Polarization - IP), by extracting the spectral information contained within the IP decay curve, directly correlated to the physical properties of the sediments. This new theoretical approach will be implemented in a numerical algorithm, which will constitute a further development of the software produced by the PI during the last five years. The subsurface model will be complemented by the assessment of the water table level, provided by a joint inversion of ERT and seismic tomography data. Additionally, remote sensing (LiDAR) and high-resolution multibeam bathymetry will provide surface and submerged (coastal lakes and rivers) information regarding salt accumulation. Finally, surface and subsurface data will be coupled in order to return a salinization map in the GIS environment.
The experimental method will be applied to two case studies in Central Italy (Tiber River mouth and Circeo National Park), where only low-resolution preliminary investigations are available and there is still a lack of vulnerability maps related to salinization.
This project will provide a new interdisciplinary and high-resolution tool for mapping salinity in coastal aquifers, which will include geophysical measurements, geochemical and isotopic analysis and geomatic data processing, with the following improvements with respect to the state of the art:
i) Geophysical mapping of coastal aquifers has been often conducted employing low-resolution methods, such as electromagnetics (Viezzoli et al. 2011) or, where resolution is increased i.e. using the tomographic technique, only investigating small zones (De Franco et al. 2009) down to limited depths. We aim to retain high-resolution, improving the cost-effectiveness by speeding the acquisition process. In this sense, we will take advantage on the instrumental update of the IRIS SyscalPro resistivimeter currently held by DICEA from 48 to 96 electrodes (see Costs section). This new functionality will allow us to improve the benefit-cost ratio by covering large areas in a shorter period, reach greater depths (100 m) and improve the surface resolution by using denser electrode spacing, being unchanged the line length.
ii) Using ERT standalone may not be the best choice, given the high uncertainty arising in complex geological scenarios, as stated before. Therefore, we will explore the additional contribution given by the IP technique compared to the classical ER method. Nowadays, the IP data processing is almost only conducted by a rapid inversion using an integral chargeability, discarding the spectral information contained within the IP decay curves (Fiandaca et al. 2012). Conversely, the spectral analysis can provide directly piecewise parameters (relaxation time and c-exponent) which can be correlated to the physical properties of the sediments for predicting permeability and porosity (Kemna et al. 2012), which are key parameters for coastal aquifers. In light of this, we aim to update the VEMI algorithm (De Donno and Cardarelli 2017b), developed by the PI during the last five years, by inserting the spectral inversion of IP data.
iii) The joint inversion of ER and P-wave seismic tomographic data can improve the accuracy in the WTL determination (Cardarelli et al. 2014). Preliminary results achieved by some participants to this research group using the cross gradient (Palladini and Cercato 2018) are promising, even though they needs to be tested in a complex geological scenario. Furthermore, we aim to explore thoroughly the possibility to produce geospectral images, which can provide a natural visualization of the multiple subsurface parameters found (Gallardo 2007);
iv) Apart from using classical geochemical indicators (i.e. EC, WTL, anions and cations), we will mostly investigate potential and limits of the strontium isotopes for assessing the origin of salinization in the coastal Italian scenarios. The assessment of salinization signatures with strontium isotopes have been already applied, both only to some local case studies in Europe and North America (Vengosh 2003);
v) High resolution DEMs (from remote sensing and multibeam data) and subsurface information will be merged into a salinity vulnerability 3D map in a GIS environment with application to the two zones. Currently in Italy, few aquifer maps are available for areas subjected to salinization, since a quantitative study of the groundwater flow is often not cost-effective, particularly without using geophysical and geomatic non-invasive techniques, which are essential for covering large areas with limited costs (Ogilvy et al. 2009).
References:
De Donno G., Cardarelli E. 2017b. VEMI: a flexible interface for 3D tomographic inversion of time-and frequency-domain electrical data in EIDORS. Near Surface Geophysics 15(1), 43-58.
De Franco R., Biella G., Tosi L., Teatini P., Lozej A., Chiozzotto B., Bassan V. 2009. Monitoring the saltwater intrusion by time-lapse electrical resistivity tomography: The Chioggia test site (Venice Lagoon, Italy). Journal of Applied Geophysics 69(3-4), 117-130.
Fiandaca G., Ramm J., Binley A., Gazoty A., Christiansen A.V., Auken E. 2012. Resolving spectral information from time domain induced polarization data through 2-D inversion. Geophysical Journal International, 192(2), 631-646.
Gallardo L.A. 2007. Multiple cross¿gradient joint inversion for geospectral imaging. Geophysical Research Letters 34(19).
Kemna A., Binley A., Cassiani G., Niederleithinger E., Revil A., Slater L., Kruschwitz S. 2012. An overview of the spectral induced polarization method for near-surface applications. Near Surface Geophysics 10(6), 453-468.
Palladini L., Cercato M. 2018. Structural joint inversion of electrical and seismic tomography data. Atti del XXXVII convegno nazionale di Geofisica della Terra Solida. Bologna, 19-21 novembre.
Viezzoli A., Tosi L., Teatini P., Silvestri S. 2010. Surface water¿groundwater exchange in transitional coastal environments by airborne electromagnetics: the Venice Lagoon example. Geophysical Research Letters, 37(1).