3D Characterization of potentially toxic elements dispersion in Portman Bay using mineralogical and geochemical indicators

The exploitation of mineral resources is a fundamental activity for the development of societies, but the importance of the alterations caused to the environment has not been considered when carrying out these exploitations. In the case of metallic mining, sulphide oxidation and leaching of potentially toxic elements (PTEs) lead to soil and water pollution.

Sierra Minera of Cartagena-La Unión (SE, Spain) has historically been a major mining district. Technological development caused production to rise during the second half of the 20th century, leading to an increase in tailings dumps, producing one of the biggest ecological disaster on the Mediterranean coast.

Sediments from Portman Bay were collected during the Pilot Project for its regeneration (2008). Samples have been analyzed in 12 cores of 10 meters deep, with one sample every meter. Two cores were selected and analyzed to a depth of 24 meters in order to reach the original bedding material and thus be able to make a 3D model of the extent of the contamination at depth.

The total PTEs content was determined after an acid digestion. In addition, a characterization of the mineralogical composition has been carried out by XRD, both in powder samples and oriented aggregates for the determination of bulk and clay mineralogy respectively.

The materials studied showed sandy texture, except collected close to the discharge point, as in the core located in the internal area of the Bay, where the texture is finer. The total PTEs content is high and follows the following order of concentration: Zn > Pb > As > Cu > Cd.

The mineralogical results showed the presence of quartz, feldespars and phyllosilicates in most of the samples. The rest of the minerals present in the samples correspond to different phases of Fe as sulphides (pyrite), oxides (hematite and magnetite), oxyhydroxides (goethite), carbonates (siderite) and sulphates with different states of hydration, such as jarosite. Gypsum appears in most of the samples, regardless of their depth. The presence of jarosite is relevant in the surface samples of cores in the inner part of the bay, as well as in the first line of cores located near the dumping point. In the case of cores located further away from the coastline, the presence of this phase has been detected both at surface and at depth.

Clay mineralogical data revealed that samples with mining residues contain greenalite, chlorite and mica-illite as main minerals, with minor amounts of minnesotaite, kaolinite and smectite. On the other hand, in the samples from the deeper zones corresponding to the bedding materials, the phyllosilicates found are mica-illite, kaolinite and chlorite in order of abundance.

The statistical integration of the results has allowed the selection of quartz, pyrite, siderite and arsenic as representative variables for the 3D model. Taking into account that the current reclamation project in the bay includes mobilization of part of the waste to a mining cut, the use of these 3D models is very useful as a complement to the geochemical and mineralogical characterization.