Distribution of mine waste along the Waihou River flood plains

Abstract

Solid waste production, in the form of mine tailings, is the result of mining of natural mineral resources. The Ohinemuri River tributary of the Waihou River was designated as an official sludge channel during the period of mining operations in the catchment as an official sludge channel during the mining period for mining operations in the catchment in 1895. The 1910 Commission recommended it be revoked and only “slimes” be discharged under a range of restrictions. Essentially processing technology had changed by 1910 and it was no longer necessary to discharge most of the waste. However, the designation remained in place as an official sludge channel during the mining period for mining operations in the catchment. Mine waste included metals such as: copper (Cu), Zinc (Zn), Manganese (Mn), Lead (Pb) and Arsenic (As). Regular flooding of the Ohinemuri and Waihou Rivers resulted in the rivers overtopping their banks and dispersing mine waste into the floodplain areas. The result was dispersion of metal contaminants in the Waihou River catchment area. The effects of this regular flooding had an adverse effect on stocks. The flooding destroyed the crops, the slimes and sludge from flood damaged grass paddocks. The cyanide sludge destroyed the fish stock in the lower Ohinemuri River and upper Waihou River. This continued until stop-banks were installed to reduce the occurrence of overbank flooding, mostly long after any mining disposal had ceased. It was assumed that the heavy metals were migrating downstream. However, recent studies show no strong evidence of downstream migration of contaminated sediments in the Waihou River to the Firth of Thames. This research will investigate the heavy metal concentration along the Waihou River floodplain by mapping metal concentrations from mine waste. Field sampling has found that the mine waste has not travelled far beyond Paeroa and is concentrated on the flood plains. Some sample sites have levels of contaminants that exceed the thresholds for chronic toxicity. These results are consistent with research limited to the immediate vicinity of the river channel. Key words: Waihou River, Ohinemuri River, mine tailings, heavy metals, contaminated sediments.