Risk assessment for the environment, population and infrastructures of some abandoned tailing ponds in the mining district of Cartagena-La Union (SE Spain)

The mining district of Cartagena-La Unión (South-East of Spain) has been among the most important mining centers in Spain in obtaining lead, silver and zinc. For more than two thousand years, different civilizations exploited mineral resources in this area. More than eighty structures of mining wastes (tailing ponds) coming from old mineral washing facilities were built, being currently one of the main sources of pollution. The goal of this chapter is to provide a geophysical and geochemical assessment of eleven tailing ponds selected in the mining district of Cartagena-La Unión, giving a description of the risks for the environment, population and close infrastructures. To achieve this objective, surface soil samples were taken in these tailing ponds where pH, texture, electrical conductivity, and total, bioavailable and water soluble heavy metals were obtained. Geophysics was based on laying out a series of profiles in which physical parameters of the materials in depth were obtained. The equipment used provides 2D and/or 3D sections of electrical resistivity changes (Ohm/m).The geochemical and geophysical studies showed that only "Brunita" tailing pond was structurally instable with a volume of 1000000 m3. The pH values showed that the most acidic tailing ponds were "Descargador" (< 3.5), "Regente" (< 3.5) and "Brunita" (3.5-4.4); oppositely, "Encontrada C. Ponce" was the most alkaline. Regarding electrical conductivity, most ponds showed a low salinity. Brunita was the tailing pond with highest content of salts (> 16 dS m^-1). The predominant textures in the selected tailing ponds were coarse. All tailing ponds had high total concentrations of Pb (maximum values: 21308 mg kg^-1 in " El Lirio" and 21272 mg kg^-1 in "Encontrada- C. Ponce"), Zn (maximum values: 23575 mg kg^-1 in "El Lirio" and 20971 mg kg^-1 in "Cabezo Rajao-La Unión") and Cu (maximum values:1823 mg kg^-1 in "Peña del Águila-La Unión" and 1470 mg kg^-1 in "Cabezo Rajao-La Unión") which are above the limit values for all legislation used in this study. In general trends, the highest fractions of bioavailable and water soluble metals were found in tailing ponds with ultra acidic conditions (pH<3.5), such as "Regente" with 72%, 4%, 17% and 14% of bioavailable Cd, Cu, Pb and Zn, respectively; and with 2.7%, 1.4%; 0.2% and 24% of water soluble Cd, Cu, Pb and Zn, respectively. Therefore, significant risks can arise in these tailing ponds, because the soil particles can reach natural or populated areas by erosion or collapse by structural instability, adversely affecting soil, surface and underground waters, plants, animals and human populations as well as infrastructures close to these tailing ponds. It is important to highlight that in the acidic tailing ponds the problems of mobility of heavy metals are increased and so, higher environmental risks may exist. For the reasons above, rehabilitation plans should be carried out in these tailing ponds.