At least 85
000 tonnes of mine tailings were discharged into Deepdell Creek, east Otago
between 1890s and 1940s. The tailings contained about 1 wt % arsenic, so about
850 tonnes of arsenic in the form of arsenopyrite was discharged. Annual floods
have removed nearly all tailings within c. 60 years. Small amounts of
arsenic-bearing sediments (<250 ppm) remain in deepdell creek. about 7>
kg/year dissolved arsenic was discharged in groundwater from mineralised
Hyde-Macraes Shear Zone rocks in the vicinity of the historic mine sites.
Adsorption of this arsenic on to iron oxyhydroxides in the stream sediments may
have contributed to the elevated As content of those sediments. Even less
arsenic remains in the main Shag valley below the Deepdell Creek confluence (15
- 25 ppm As). Rare relict arsenopyrite grains are trapped in bedrock crevices 25
km from source. No tailings deposition is detectable in theShag
River estuary, and the tailings have
presumably been discharged into the sea. Natural rehabilitation of the Shag
catchment has been rapid and effective.