## 1 Introduction
Furthermore, places with high contact or superficial resistivity, such as dry conditions, sand, rock outcrops, and paved areas, will cause additional field problems. Alternatively, electromagnetic methods using AC currents, or induction methods, do not require physical contact with the ground and measurements can be done as the operator walks. These methods can also be adapted for airborne use [10]. They are fast and easy to operate and can provide 2D and 3D images of the area under investigation with less field effort. However, if there are sources of electrical noise in the area, such as power lines, they can provide unstable, if not completely improper, data. Groundwater contamination is a dynamic phenomenon. Thus, if reliable information is to be obtained, a 4D approach, namely, time-lapse studies, are required. The use of time-lapse resistivity in groundwater contamination, as well as algorithms for inversion and model resistivity data, have long been discussed [11; 12]. These problems have been recently addressed by a number of authors [13; 14; 15; 16; 17]. However, if data from different epochs are to be used, they must be compared and normalized to a known background, so that resistivity variations can be attributed solely to the variation of groundwater contamination. The apparent electrical conductivity of sediments and soil layers is complex due to the interaction of several factors, such as pore-water salinity, water content, and particle size distribution [18]. Hydrochemical parameters are a useful and complementary tool for assessing environmental aquifer conditions by providing information about chemical signatures of groundwater, distinguishing geological formations with similar hydrogeochemical characteristics, and understanding element mobility and dispersion patterns [19]. Therefore, the use of both electrical exploration methods, to delimit contaminated areas as well as to estimate their temporal and spatial evolution, and hydrochemical parameters, to access the level and type of groundwater contamination, have been applied with success in several studies [20; 21; 22; 23]. The industrial chemical complex of Estarreja (ICCE) in northern Portugal has been operating for almost 70 years. During this period, environmental laws have evolved from non-existent to strict regulations in accordance with the EU regulations. Thus, in the past, effluents were discharged in local streams, man-made ditches, and buried metal pipes, which were easily corroded, playing a major role in the contamination of local aquifers and soils. Approximately 70% of the population living near the ICCE has water wells that are widely used for agriculture (-81%) and domestic (about 24% for drinking) supplies [24]. For this reason, the understanding of the spatiotemporal groundwater contamination is of paramount interest for the population, as well as for authorities and technicians in charge of remediation operations. Since there is a large volume of geophysical and hydrochemical data acquired over several decades in the region, this study integrated such information to investigate temporal (a time span of about 30 years) groundwater contamination in the ICCE region in order to: (a) Map contamination plume(s) and evaluate its/their progression; (b) Estimate the propagation velocity of plume(s) and its/their variation during the study period; (c) Investigate the sources of contamination and their evolution (type and intensity); (d) Carry out an assessment of the remediation works implemented in the early 2000s; (e) Evaluate the benefits of this joint approach in future monitoring and remediation operations, so that the findings of this work can have a wider use and applicability to other case studies.