Characterization of geological heterogeneity through time-domain induced polarization at contaminated sites: the case of a former gravel pit filled with waste

Web page: NA Preferred presentation oral/poster: Oral Abstract. In the recent years, time-domain induced-polarization (TCIP) surveys have been successfully employed in a wide range of environmental applications. Soil and groundwater contaminated sites represent challenges that require the contamination extent definition and a detailed description of geological heterogeneity, to predict the contaminant flows accurately. In this study 9.8 kilometers of 2D DCIP profiles, 400 to 1000 meters long, have been performed in the surroundings of a former gravel pit filled with municipal and industrial wastes after its depletion. Data have been inverted via traditional resistivity inversion and with full-decay spectral IP inversion, directly in terms of hydraulic conductivity (K), using petrophysical relation to re-parameterize the Cole-Cole model in terms in hydraulic properties. Reasonable K models, in accordance with log-wells information are retrieved. Furthermore, the resistivity models improve significantly when inverted along with IP data, in terms of coherence among profiles acquired with different acquisition length. Introduction In recent years TCIP surveys have been implemented in a wide range of environmental applications showing accurate results in depicting geological settings (e.g. Rossi et al., 2017; Maurya et al., 2018). With specific regard to soil and groundwater contamination studies, accurate knowledge about the stratigraphical setting and the distribution of hydraulic properties is crucial to forecast the contamination spreading via groundwater flow and transport modeling. This study aims at depicting geological heterogeneity through TDIP surveying at a contaminated site, with the acquisition of 9.8 km of TDIP data. Survey layout and data processing In this study, 9.8 km of 2D TCIP data have been acquired in the surrounding area of the former gravel pit “La Vallosa”, in the province of Brescia (Italy). After its depletion the gravel pit has been filled with municipal and industrial wastes, contaminating the surrounding aquifers. Profiles with length ranging from 400 to 1000 meters have been acquired, with electrode spacing of 5 or 10 meters (Figure 1a). All data have been collected recording full-waveform data with a 100% duty cycle protocol and on-time of 12.3 seconds. The IP decays have been processed following Olsson et al. (2016), allowing increases in the usable range of IP decays to almost four decades in time. Results and conclusions The gathered data have been inverted i) via traditional resistivity inversion process, ii) implementing hydraulic conductivity (K) as model space inversion parameter (Fiandaca et al., 2022). Figure 1b shows the hydraulic conductivity K estimated from TDIP inversion on a selection of the acquired profiles, with depth range up to 60 m, where unconsolidated formations are present (i.e. where the petrophysical relations for K estimation are valid). Good coherence is evidenced among the profiles, and the K estimates agree with the borehole information at the site. Furthermore, the resistivity models improve significantly when inverted along with IP data, in terms of coherence among profiles acquired with different acquisition length. These findings are being incorporated in the flow and transport model at the site, in order to study the vulnerability of the groundwater resources of the towns downstream of the contaminated site.