To assess contamination in marine sediments have been used the international indices of contamination of marine sediments, the grain size, the concentration range of chemical elements, the mineralogy, and the mapping of harmful substances. Results were compared with rates from the United States Environmental Protection Agency (EPA) (EPA 1987) and the United States National Oceanic and Atmospheric Administration (NOAA) (Buchman 1999). These authors calculate the degree of contamination in marine sediments with global mean background values (Wedephol 1995; Pekey et al. 2004; Savvides et al. 1995; Wedephol 1995). The background is defined as the concentration of heavy metals in sediments undisturbed by human activity. And the degree to which current concentrations of heavy metals in sediments exceed background levels is the extent of human contamination (Birch 2016; Pekey et al. 2004; Savvides et al. 1995). Nevertheless, by particularities of the marine sediments of coastline in Mallorca, we needed to develop a regional index. This method was developed with the regional background values of the marine sediments of the Mallorca coastline. The result obtained was evaluated according to the geoaccumulation index (Igeo index) (Shin and Lam 2008; Lin et al. 2011; Naifar et al. 2018).
In addition, the type of marine sediments on the Majorcan coastline is mainly sand; while gravel and mud sediments are commonly transported by rivers (Servera et al. 2011; Font-Muñoz et al. 2017). The marine substratum of southern Mallorca is composed of bioclastic sands (foraminifera, bivalves, and bryozoans) of variable size (Font-Muñoz et al. 2017), which, in some areas, may have a greater fine fraction or be composed of gravel. Therefore, to determine the mineralogical characteristics, Folk's ternary diagrams were applied (Folk 1954), which classify the sediments by their granular content of gravel, sand, and mud (silt plus clay). Finally, to have other contamination indicators, chemical parameters such as Organic and Inorganic Carbon (TIC and TOC), phosphorus, and silica were analyzed.
2.1. Sampling
A total of 95 samples of marine sediments were taken in 8 field works with the oceanographic vessel using dredger blades and autonomous diving equipment. Some of the samples taken in the area farthest from the wastewater discharge points have been discriminated against because other factors influence the marine environment, such as greater depth, the direction of the currents, or the geological substrate. The rest of the samples show high concentrations of heavy metals above the natural values of the sediments. In these places the samples were separated by stations, surrounding the outfalls. At the west coast station, Baluard, 17 samples (B01-B17) were taken, in the central part of the study area, Portixol, 11 samples (B01-B11), and at the eastern station, Torrent Gros, 6 samples (T01-T06). In addition, 6 control samples (C01-C06) were between the outfalls and the limits of the study area. Finally, 3 samples (PY1-PY3) were taken from the beaches to obtain background levels.
2.2. Chemical analyses, grain size, and mineralogy
The chemical parameters have been analyzed using the following techniques:
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The determination of trace elements: As, Ba, Cd, Cr, Cu, Ni, Pb, Se, V, and Zn, by Inductively Coupled Plasma-Mass Spectrometry with an Agilent 7500ce equipment based on the EPA 6020 method. Furthermore, the Hg determination is based on EPA 7471 “Mercury in solid or semisolid waste”.
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The granulometric analysis followed the UNE 103 100 Standard and is screened in a decreasing direction with the sieves series of UNE, ASTM, and TYLER (IGME 2008).
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The use of atomic absorption spectrophotometry (Beck and Sneddon 2000) to determine the main elements of minerals by X-ray fluorescence.
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The TIC was calculated by the difference between the total carbon and the carbon determined after the addition of HCl / FeCl2.
The granulometry obtained was grouped according to the particle size classes proposed by Wentworth (1922): gravel (> 2 mm), sand (2 mm − 63 µm), silt (63 µm-4 µm), and clay (< 4 µm). In addition, the textural classification of the sediment is analyzed using the ternary diagrams proposed by Folk (1954), which allows the sediment to be classified according to its grain size of gravel, sand, and mud (silt plus clay).
2.3. NOAA, EPA, regional index, and Igeo index
Heavy metal concentrations were evaluated according to reference indices, EPA (EPA 1987) and NOAA (Buchman 1999). EPA limits determine three levels of contamination in marine sediments and NOAA establishes Screening Quick Reference Tables (SQuiRT) related to effects on biological communities (Buchman 1999, 2008).
However, global references are too broad and a regional index was developed specifically for this study based on local background values of marine sediments. These are the mean values of heavy metal concentrations in beach sand (Geochemical Signal Type (TGS)), whose concentrations are not affected by discharges (Table 1). The upper and lower confidence thresholds between the samples from which the TGS is obtained have a barely appreciable variation, ± 0.77 mg/kg. Concentrations above the TGS belong to the threshold called Probable Pollution Effects (PPE). TGS showed very low heavy metal concentrations, in the range of undisturbed by human activities, making them excellent guide levels.
Table 1
The concentration of heavy metals is calculated in mg/kg (Hg in µg/kg) on the sands of the beaches of Can Pera Antoni and Ciudad Jardin (southwest of Mallorca). These values have been considered background because the concentrations of heavy metals are the natural ranges of the marine sediments in Mallorca
| Ciudad Jardin A | Ciudad Jardin B | Can Pera Antoni | Mean | Standard deviation |
As | 4,65 | 4,79 | 5,51 | 4,98 | 0,46 |
Ba | 17,72 | 14,02 | 17,33 | 16,36 | 2,03 |
Cd | 0,50 | 0,50 | 0,50 | 0,50 | 0,00 |
Cr | 9,25 | 7,73 | 9,34 | 8,77 | 0,91 |
Cu | 1,05 | 0,98 | 1,47 | 1,16 | 0,27 |
Hg | 0,10 | 0,10 | 0,10 | 0,10 | 0,00 |
Ni | 1,35 | 1,36 | 2,36 | 1,69 | 0,58 |
Pb | 12,28 | 9,24 | 8,79 | 10,10 | 1,90 |
V | 7,95 | 8,39 | 8,81 | 8,38 | 0,43 |
Zn | 8,50 | 7,95 | 10,16 | 8,87 | 1,15 |
In addition, the result obtained was evaluated according to the Igeo index (Shin and Lam 2008; Lin et al. 2011; Naifar et al. 2018), as a standard evaluation range. This index considers the spatial asymmetry in the concentrations of some heavy metals, with six categories, from the most to the least polluted áreas (Table 2). Furthermore, the mineralogical characteristics were studied through the Folk ternary diagrams, which classify the sediments by their granular content of gravel, sand, and mud (silt plus clay).
Table 2
Range of contamination determined in the Igeo index proposed by Müller (1969)
0 | Igeo < 0 | Unpolluted |
1 | 0 < Igeo < 1 | Unpolluted to moderately polluted |
2 | 1 < Igeo < 2 | Moderately polluted |
3 | 2 < Igeo < 3 | Moderately to strongly polluted |
4 | 3 < Igeo < 4 | Strongly polluted |
5 | 4 < Igeo < 5 | Strongly polluted |
6 | 5 < Igeo | Extremely polluted |
2.4. Statistics
Statistical analyzes are based on representing box plots, with the interquartile range, where the median is located and therefore its relationship with the first and third quartiles. The dispersion and symmetry of the heavy metals present in the samples are visually represented through Box-Whisker diagrams (box or box and whiskers plots).
The analysis of the variability of the concentrations of each heavy metal, according to the parametric limits of the EPA, the NOAA, and the Regional Index has been carried out with a graphic representation that indicates the concentration of each element per sample. The columns located on the left margin of the graph indicate, in different colors (green, orange, or red), their relationship with the thresholds of each index.
2.5. Mapping
To draw the maps, a database has been developed, georeferencing each sample using the ArcGIS 10.8.1 Software. The maps have been made using the Average Nearest Neighbor (ANN) relationship, which measures the distance between the centroid of each entity and the location of the centroid of its nearest neighbor (Oncina 1996; Mitchell 2005). This is related to the direction of the dominant ocean currents in the study area. The maps represented have been heavy metals, grain size, TIC, CaCO3, and Si.