# Assessment and Review of Heavy Metals Pollution in Sediments of the Mediterranean Sea
## 1 Introduction  
Coastal and marine environments represent a crucial interface between human society and the natural world. Moreover, they also function as the primary sink of heavy metals, making them valuable indicators of marine sediment pollution [1, 2]. These accumulated elements could be a contamination risk, releasing toxics elements through chemical and biological processes [3, 4, 5]. Additionally, due to their potential to accumulate in the food chain, they could affect human health [6, 7, 8]. Several studies have highlighted the importance of evaluating aquatic systems through marine sediments, confirming that a high level of heavy metals can become a great problem for the health of the seas [9, 10, 11, 12, 13, 14, 15, 16]. Such contamination often can result from the discharge of wastewater into the sea (from underwater outfalls), especially in areas with high urban development, and elevate pressure of anthropogenic activities [17, 18]. For instance, tourist destinations such as Mallorca Island, southern Spain, Greece, and southern Italy witness an influx of more than 50 million visitors annually, mainly during the spring and summer seasons. In these periods of high population rates, a substantial volume of poorly treated wastewater is discharged, introducing heavy metals into the sea and later into the marine sediments [2, 19, 20]. In this way, numerous efforts to control toxic elements in marine ecosystems have been made. Some good examples can be found in the monitoring carried out on the coasts of Greece, south of Italy, and east and southern Spain [15; 21; 22; 23; 24]. Since the 1980s, assessing marine sediment contamination has involved the use of sediment quality indices [25; 26]. These indices assume that there is a threshold concentration of chemicals in sediments, below which ecosystems are considered unharmed, thus serving as essential tools for detecting contaminated areas. Furthermore, the concentration of heavy metals in marine sediments is classified from highly to slightly contaminated based on background levels [27; 28; 2]. In this order, the studies carried out in the Mediterranean regions mentioned above are useful to understand the pollution of marine sediments in the Mediterranean Sea and to know what the main source of heavy metals that seem to be common among the analyzed areas is. To compare the distribution and concentration of heavy metals in marine sediments, we carried out a study in the southeast of Spain, correlating the data with other regions of the Mediterranean Sea. These selected areas support strong tourism development, with a high number of visitors during the summer and spring seasons, leading to an exponential increase in water use and discharge into the sea. In this context, the present study aimed to investigate the concentration of heavy metals found in marine sediments of the Mediterranean Sea close to the coastal areas. The objective was to identify the most contaminated areas as well as the source of contamination of the toxic elements providing a complementary benefit and/or improvement of the marine coast, such as discharge points of untreated or poorly treated water or chemical precipitation processes associated with the product of the chemical solution of each element that quickly exceeds the equilibrium situation. The study of this process and the identification of the conditions were carried out by evaluating the chemical reaction between sea water and water discharged from submarines outfalls. In addition, a study was carried out on the spatial distribution of heavy metals and metalloids and the chemical mechanisms that best describe the process. The study emphasizes, despite the abundance of available studies focused on the contamination of marine sediments [1; 2; 5; 9; 14; 15; 16; 21; 24; 25; 26; 27; 28; 24; 29; 1; 2; 25; 27; 28] to date, that no comprehensive investigation has been found for the sources of discharges and processes in marine zones close to highly populated areas, which are considered complex.