Relative importance of conceptual and computational errors when delineating saltwater intrusion from resistivity inverse models in heterogeneous coastal aquifers

• Published in: Advances in water resources Vol. 144; p. 103695
• Main Authors: González-Quirós, Andrés, Comte, Jean-Christophe
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.10.2020
• Subjects: Coastal aquifers; Electrical resistivity;Petrophysical model; Heterogeneity; Hydrogeophysics; Heterogeneity; Electrical resistivity;Petrophysical model; Hydrogeophysics; Coastal aquifers
• ISSN: 0309-1708; 1872-9657
• DOI: 10.1016/j.advwatres.2020.103695

•Inverted resistivity data are commonly used to delineate saltwater intrusion (SWI).•Errors associated to inversion and petrophysical transformation are assessed.•A coupled modelling approach was applied to four synthetic heterogeneous aquifers.•Inversion and salinity recovery errors are equivalent when neglecting heterogeneity.•Spatial quantification of the errors is key for geophysics-aided SWI management. In coastal areas, geological heterogeneity influences the geometry and characteristics of the freshwater-saltwater mixing zone. Geophysical electrical and electromagnetic methods are increasingly used to delineate the freshwater-saltwater mixing zone or for groundwater model calibration. However, in practical applications, it is common when assessing pore water salinity from resistivity models to disregard the spatial variability of the aquifer properties assuming pore salinity is the dominant control on bulk electrical conductivity. In this paper we use a coupled hydrogeophysical model to assess the discrepancies in recovered salinity caused by the geophysical inversion and the application of the chosen petrophysical model when heterogeneity is ignored. We find that errors made when neglecting heterogeneity during petrophysical transformation are equivalent or higher in magnitude than inversion errors, more so when an inappropriate petrophysical model is used, but exhibit different spatial distribution. The results show that their combined effect results in significant overestimation of the spreading of the mixing zone. The analysis provides new insights to the groundwater community to better evaluate the reliability of geophysically-derived information when delineating the freshwater-saltwater interface or when geophysics is used quantitatively as part of model scenario simulation, uncertainty analysis or optimisation strategies.