## 2 Site Characterisation
### Geology and Hydrogeology  
The study area is part of the so-called Aveiro sedimentary basin, located in the northern sector of the western Meso-Cenozoic Margin. The stratigraphic sequence of this basin is discontinuous, consisting mainly of Holocene-Pleistocene units and, with less representation, Cretaceous and Triassic formations. These sedimentary units are deposited on a substrate of Neoproterozoic age, the Aradas shales formation, mainly composed of fine-grained metapelites of low metamorphic grade, which constitutes the Hercynian basement (Figure 1) [38].  
In detail, the sedimentary sequences (from bottom to top) begin with a red-coloured polygenic conglomerate superimposed by Triassic fine and red sandstones. The Cretaceous units show a greater development towards the Atlantic coast and are deposited over Triassic materials or directly on the Hercynian bedrock. The predominant lithology of this sequence are sandstones with depositional characteristics that vary in depth, allowing for the individualization of different formations. Thus, the base of the sequence is composed of sandstones of variable grain size interbedded with clays and superimposed by a thick clayey layer. Covering this there is a white sandstones sequence, interspersed with clays and conglomerates, and an uppermost layer containing sandstones and clay-silt lagoon facies. Finally, the Quaternary deposits include Pleistocene beach and fluvial deposits related to two sedimentary cycles: (a) the lower unit shows a coarsening downward sequence, with coarse sands and pebbles at the base; and (b) the upper sequence is composed mainly of organic muds, clays, and clayey sands. The most recent Holocene sedimentary units include alluvium and sand-dunes.  
Figure 1: (**a**) Location of the Aveiro aquifer boundary and the industrial chemical complex of Estarreja, northern Portugal; (**b**) Simplified geological map of the study area (adapted from [PERSON] and [PERSON] [38]) and main groundwater flow paths (compiled data from measurements made in 2015 and from Ordens [33]); map coordinates: UTM; WGS84—29N zone.  
Based on the texture and structure of different sedimentary sequences, two main aquifer systems are defined in this region: the Aveiro Cretaceous confined multilayer aquifer system, and the Aveiro Quaternary unconfined (locally semi-confined) multilayer aquifer system [39]. In the studied area, only the Aveiro Quaternary aquifer system is of interest, since the Cretaceous sequences are only expressed to the west of this location (Figure 2). The Aveiro Quaternary aquifer system comprises of three hydrogeological units [39]. The uppermost unit consists of a Holocene shallow unconfined aquifer with thickness varying between 8 and 10 m. This aquifer is highly permeable and shows a main flow path from east to west. Below this exists a semi-confined aquifer, composed of coarse and permeable sediments (sand and gravel), with groundwater flowing from east to west. The top of this aquifer consists of a low permeability mud and fine silt layer that constitutes the top of the Plo-Pleistocene sequence. Lastly is a semi-confined aquifer, formed by old beach and alluvial coarse, permeable sediments (sands, gravel, and pebbles), with groundwater flowing from west to east in opposition to the previous aquifers.  
It should be noted that recharge of this aquifer system is done mainly by rainfall infiltration and irrigation return. In Estarreja, the annual average precipitation is about 1050 mm, the annual average evapotranspiration is 650 mm, and the annual average bulk runoff (including surface runoff and aquifer recharge) is 400 mm [40]. The high permeability and porosity of the aquifer layers favours recharge and the groundwater flow gradient, but this also represents a potential contamination risk since the ICCE is in direct contact with the unconfined aquifer, which facilitates contaminants migration.