The potential for arsenic mobilisation in the Caudal River catchment,north-west Spain

Abstract

Asturias has numerous derelict Hg mine sites as a legacy of a past intensive mining activity, and the most important of these are located in the Caudal River catchment. These mines have been abandoned without remediation, and the legacy of these mine works remains currently in the form of mine wastes stocked in piles on the surface. In the exploited ore deposits, As is quite abundant in the form of sulphides (As-rich pyrite), sulpho-arsenides (arsenopyrite, realgar and orpiment), and supergenic minerals (scorodite). Significant quantities of mine and metallurgical wastes stocked at surface in spoil heaps of different age and size, and old galleries and transversals acting as pseudokarstic aquifers, are potential pollution pathways for the spreading of As to the environment. As a consequence of the weathering of As-rich minerals, As contents are high in mine effluents and spoil heap leachates. In the spoil heaps, waste materials are subjected to the action of a humid environment; in consequence, the instability of sulphides and sulpho-arsenides leads to the production of acid- and metal-rich leachates which pass incorporated into surface water or infiltrate through the land reaching groundwater. Electron microprobe analyses of pyrites confirm its high content of As (up to 10·11% As). Mining wastes stocked in the spoil heap present total As contents reaching up to 6·2%, and mine effluents and spoil heap leachates show As concentrations that can range from 1·4 to 9·2 mg/l. High concentrations of As have been detected too in stream sediments (up to 3·2%). In this context and on the basis of the risk assessment, proposals for remediation strategies are considered with the aim to reduce the leaching and transport of dissolved As from mining wastes.