Geochemistry of Cd in the secondary environment near abandoned metalliferous mines, Wales

Wales, with its long history of metalliferous mining, has many abandonded mines and ore-rich spoil tips, several of which are very extensive. Most of the ore minerals are sulphides, Zn occurring as sphalerite in which Cd is ubiquitous. Soils in the vicinity of the tips contain up to 1000 ppm Cd. Analyses of the ZnS reveal Cd values up to 0.52% with Zn/Cd ratios ranging from 120 to 665, mean 306. Fine spoil from the area contains up to 1050 ppm Cd, the mean Zn/Cd ratio of the most metal-rich samples being 75. Surface samples of tip contain high levels of water-soluble Cd but Zn/Cd ratios are much higher than those for the whole tip material, suggesting that Zn is preferentially leached out. Adit and spoil drainage waters have Cd levels up to 2.5 mg/1 while ground waters from the tips contain up to 2.7 mg/1. The Zn/Cd ratios of these waters are very variable ranging from ~20 to >1000. The Zn/Cd ratio in the ground waters is generally <100, while surface and adit drainage waters are mostly in the range 100–250. The differing ratios in surface and subsurface waters probably reflect variable Eh and pH conditions. In addition, it has been found that adit and tip waters which are rich in Fe and tend to precipitate ochre have relatively high Zn/Cd ratios, typically between 300 and 800, while the ochreous precipitates have ratios between 140 and 250. It is suggested that the Cd content of these waters is controlled by oxidation with consequent precipitation of Fe which scavenges Cd. The Cd content of tip drainage waters shows marked seasonal variation with highest values recorded in winter and after summer droughts. However, the Zn/Cd ratio of individual streams shows remarkable temporal constancy.