Impact of acid mine drainage on the hydrogeological system at Sia, Cyprus

Abstract

Volcanogenic massive sulphide deposits were mined extensively in Cyprus since Roman and Phoenician times. In the twentieth century extensive open-pit excavation was carried out resulting in considerable mine wastage and concomitant environmental concerns, particularly the production of Acid Mine Drainage (AMD). At Sia, on the northern flanks of the Troodos ophiolite, a hydrologic system consisting of an open-pit lake, waste rock and tailings dumps, a river running into a downstream dam-lake, and localised groundwater circulation, has been investigated to identify the nature and source of environmental contamination, the mechanism and results of acid water generation, and the effects on the system of seasonal climatic variation.

The major metallic cations in the contaminated waters are Al, Mg, Fe, Cu, Zn, and Mn, but there are also quantities of Ni, Co and Cd. Al and Fe dominate the total cation content when pH is lower than 4. Al is derived from the weathering of clay minerals in the hydrothermally altered basaltic country rock especially during the wet season, and Fe is derived predominantly from the oxidation of pyrite. Once pH exceeds 4, Al is removed from solution by complexing and colloidal precipitation, as is Fe at a slightly higher pH. Other elements are controlled by adsorption onto these colloidal particles of Al- and Fe-silicates and oxyhydroxides, or by inclusion in efflorescent salts that crystallise during periods when the Sia river dries up and the mine-pit lake surface area is greatly reduced. These salts form on the muddy substrate through capillary action and include gypsum, hexahydrite and a variety of hydroxysulphate minerals. The salts are highly soluble and dissolve in the first rains of the wet season, producing short-lived toxic concentration of metals.

Groundwaters around the mine site show limited influence of AMD suggesting that the rocks immediately beneath the mine-pit lake are relatively impervious. Local concentrations of boron in some groundwaters have a geogenic origin, either from weathered glassy or hydrothermally altered volcanic rocks or possibly from serpentinised ultramafic rocks which may be found at depth beneath the mine site.