Natural recovery rates of a river system impacted by mine tailing discharge: Shag River, East Otago, New Zealand

Gold mining in Deepdell Creek, a tributary of the Shag River, occurred between 1890 and 1946. At least 85,000 tonnes of mine tailings were discharged into the creek during this time. Arsenic is the best geochemical tracer of these tailings, and an estimated 850 tonnes of arsenic in the form of arsenopyrite was discharged. Chromium is also anomalous in source rocks, but As and Cr are decoupled in the sedimentary system. Copper, zinc, and nickel show no mining-related variations down the catchment. Tailings produced 100 times more sediment and 1000 times more arsenic per year than natural erosion of mineralised rocks. Minor dissolved arsenic also discharged from the mining area (ca. 0.024 mg/l at 10 l/s), but this was negligible compared to the tailings. The Shag catchment has little sediment storage capacity, allowing the tailings to pass rapidly through the catchment. Floods are sufficiently frequent and voluminous to remove all tailings within ca. 60 years. Nevertheless, small amounts of arsenic-bearing sediments remain, with concentrations up to 250 ppm 4 km downstream of the mining area in Deepdell Creek. Sediments in the main Shag valley below the Deepdell Creek confluence have low but anomalous As contents (15–25 ppm). Rare relict arsenopyrite grains are trapped in bedrock crevices 25 km from source. Significant tailing deposition is not detectable in the Shag estuary, and the tailings have presumably been discharged into the sea. Natural rehabilitation of the Shag catchment has been rapid and effective.