The environmental and geomorphological impacts of historical gold mining in the Ohinemuri and Waihou river catchments, Coromandel, New Zealand

Between 1875 and 1955 approximately 250,000Mgyr−1 of mercury-, arsenic-, and cyanide-contaminated mine tailings were discharged directly into the Ohinemuri River and its tributaries, in the Coromandel Region, North Island, New Zealand. A devastating flood on 14 January 1907 deposited large amounts of mine waste across the floodplain of the Ohinemuri and Waihou rivers in the vicinity of the township of Paeroa. The 1907 mine-waste flood deposit was located as a dirty yellow silt in cores and floodplain profiles, with a thickness ranging from 0.15–0.50m. Geochemical analysis of the mine waste shows elevated concentrations of Pb (~200–570mgkg−1) and As (~30–80mgkg−1), compared to early Holocene background concentrations (Pb<30mgkg−1; As<17mgkg−1). Bulk sediment samples recovered from the river channel and overbank deposits also show elevated concentrations of Pb (~110mgkg−1), Zn (~140–320mgkg−1), Ag (~3mgkg−1), and Hg (~0.4mgkg−1). Using the mine-waste deposit as a chronological marker shows that sedimentation rates increased from ~0.2mmyr−1 in the early Holocene, to 5.5–26.8mmyr−1 following the 1907 flood. Downstream trends in the thickness of the flood deposit show that local-scale geomorphic factors are a significant influence on the deposition of mine waste in such events. Storage of mine waste is greatest in the upstream reaches of the floodplain. The volume of mine waste estimated to be stored in the Ohinemuri floodplain is ~1.13Mm3, an order of magnitude larger than recent well-publicised tailings-dam failures, such as the 1996 South America Porco, 2000 Romanian Baia Mare and Baia Borsa accidents, and constituted, and was recognised at the time, a significant geomorphological and environmental event. The mine-waste material remains in the floodplain today, representing a sizable legacy store of contaminant metals and metalloids that pose a long-term risk to the Ohinemuri and Waihou ecosystems.