Abstract
The mining and roasting of the arsenopyrite ore in the Waiuta area (c. 30 km south of Reefton, Westland) has left a legacy of arsenic-rich material. The scale and extent of elevated arsenic concentrations were established with field portable X-ray fluorescence measurements. The principal source of environmental arsenic at the Prohibition Mill site is the roasting by-product arsenolite, which is very toxic and more environmentally mobile than arsenopyrite in original ore. Resultant arsenic-rich run-off (c. 30 mg/L) is temporarily immobilised by the formation of the secondary arsenic mineral scorodite, which cements the ground around the roaster system. Within the roaster system and the scorodite-rich substrate, arsenic concentrations can be up to 40 and 30 wt%, respectively, and high levels (wt% scale) can persist below 60 cm depth of the substrate. Arsenic in the soil surrounding the site can still exceed 0.1 wt%, and this arsenic is derived from atmospheric fall-out of arsenolite dust, rather than dispersion of arsenolite. The arsenolite dust in the soils as dissolved over the past 60 yr resulting in persistent elevated levels of arsenic adsorbed onto natural iron oxyhydroxide in the soil. An area of c. 32 500 m(2) around the Prohibition Mill site has arsenic levels above the recommended guideline for commercial/industrial sites of 0.05 wt%. On a regional scale, arsenic concentrations do not generally exceed the guideline for commercial/industrial sites, with the exception of a few localised anomalies related to arsenopyrite-rich waste rock.