## 1 Introduction
Metal pollution in sediments poses an extended environmental issue worldwide because the toxicity of elements affects public health and natural ecosystems [26; 27]. Investigating metal extraction technologies for sediments is crucial to decrease the amount of metal pollution released into the environment. The obtained elements could potentially be recycled in metallurgical industries, contributing to reintroducing the material into production cycles instead of landfilling valuable resources [28]. Other treatments include stabilization and solidification methods, which enable the use of dredged materials in construction (concrete materials, construction products, roadway construction, habitat building, landfill liner/cap, etc.) and are a common strategy internationally [29; 30; 31]. Huelva province, located in the southwest of Spain (Figure 1), is a mining region with ancient mining activity (since Roman times) due to the massive sulphide composition of the territory. This particular composition (pyrite- FeS, arsenopyrite -FeAsS, chalcopyrite-CuFeS\({}_{2}\), galena- PbS, sphalite- ZnS, among others) promotes the natural and forced formation of acid liviaites from mining residues and minerals exposed to oxidation. These acid liviaites, known as acid mine drainage (AMD), are highly acidic fluids with high concentrations of dissolved metals (and metalloids) and sulphates which flow into the water courses of the Rio Tinto and Odiel River basin and eventually reach the HuelvaEstuary, where the Port of Huelva lies. Therefore, mining contributes significantly to the input load of pollution in this region [32; 33; 34]. The Port of Huelva is on the southern Atlantic coast of the Iberian Peninsula (Figure 1). The port underwent its most recent major transformation in the 1960s. At that time, there was a sharp upsurge in the development of the chemical and oil industries, with the creation of its industrial zone and the growth of the port towards the exterior, beyond the Punta del Sebo. At present, bulk solids and liquids are the main goods used by the Port of Huelva. In 2005, port traffic had a turnover of 21 Mt. Since the inception of the Junta Especial de Comercio y Puerto de Huelva (Special Trade Board and Port of Huelva) in 1874, it was considered essential to dredge in the sand bar area and the inner harbour, and these tasks were undertaken and are still carried out today. Dredging activities can be classified into the following two types based on their aims: dredging of the first order, to make the navigable zones deeper or wider, and maintenance dredging, to alleviate the effects of the periodical silting-up processes. In recent years, major dredging work of the first order has been undertaken. The two most outstanding actions were the widening and deepening of a channel in 1995 when approximately 6 Mm\({}^{3}\) were extracted, and the activities carried out in 2000 and 2001, when a volume approaching 4 Mm\({}^{3}\) was removed, this being carried out to improve safety conditions for the larger methane carriers entering the port. Maintenance  
Figure 1: Odiel-Rio Tinto River watershed in the province of Huelva (southwest Spain), the main affected courses according to [35], and the location of the sampling points (Tharsis mine, Puente Gadea/Rio Tinto River and the Port of Huelva-supplier dredged material). dredging has proved necessary because the sediments that build up within the harbour water amount to an average dredging volume of about 0.5 Mm\({}^{3}\) per year. Sediment pollution is mainly associated with fine particles [36]. In the case of the Port of Huelva, the contamination of certain zones by the materials lying on the bed of the Ria can be attributed to the following causes [37]: (i) Natural origins, due to phenomena such as leaching and sediments being washed down and discharged by two mining-impacted rivers such as the well-known Rio Tinto and Odiel rivers. (ii) Ancient anthropic origins, owing to mine workings in the upper basins of these two rivers.