ISOTOPIC FINGERPRINTING OF GROUND WATER NITRATE SOURCES AROUND ANGLO PLATINUM'S RPM MOGALAKWENA OPERATION (LIMPOPO PROVINCE, SOUTH AFRICA)

摘要

In response to allegations made by the NGO ActionAid in early 2008 that the highly elevated nitrate levels found in groundwater in communities adjacent to Anglo's RPM Mogalakwena operation originate from activities at the platinum mine, Anglo American conducted isotope studies (both in-house and through independent researchers) with the aim to identify the natural and anthropogenic groundwater nitrate sources in the area. Results of the in-house study are presented in this paper.The results of this study demonstrate that at least three isotopically distinct nitrate end-member sources exist in the area and that mixing between these end-members can readily explain the observed compositional variations in the dataset. One of the identified end-member sources is nitrate from community-related septic waste or manure, and contamination from this source is shown to be the most likely cause of highly elevated groundwater nitrate contents in Ga-Molekana village. In contrast, effluents from the mine can be ruled out as a source of nitrate in groundwaters at Ga-Molekana on the basis of the isotopic/hydrochemical data and the westerly/south-westerly groundwater flow direction in the area.Another important nitrate end-member source identified in the area is isotopically consistent with nitrate from soil or possibly from waste rock material, as suggested by leachate data for rock material from a blast site in the Zwartfontein South Pit. The limited data available to date suggest that the source of this end-member may lie within the mine area. Mixing calculations suggest, however, that prior to the addition of nitrate from this end-member the waters must have already had highly elevated nitrate concentrations, probably due to contamination with nitrate from community-related septic effluents or manure upstream from the operation. The extent to which nitrate fluxes from the soil/waste rock end-member to the groundwater are natural or are enhanced/caused by mining-related activities is the subject of ongoing investigation.