INCREASED AEOLIAN PARTICULATE HEALTH RISK FROM ULTRAFINE SILICIC GOLD MINE TAILINGS ON THE WITWATERSRAND

Hyperfine milling of ore has dramatically increased efficiency of acid leach mineral extraction, but resulting changes in the waste material are a significant and unrecognized drawback of this technology. Fine milling means higher proportions of inhalable dust. Analysis of 20 bulk tailing samples in South Africa indicate that the proportion of inhalable material has increased from an average of 5 % (2 – 12 %) to 26 % (10 – 50 %) as hyperfine milling is adopted. These more recent tailings storage facilities (TSFs) are rich in fine erodible material which can exacerbate ambient fine particulate loading. The Witwatersrand is prone to a high frequency of aeolian storm episodes in spring, arising from gold mine TSFs. Traditionally these were regarded as a nuisance, but now have become recognized potential health hazards. During August 2009 four weeks of monitoring ambient airborne particles in the range 0.25 – 32 µm was performed with a GRIMM aerosol monitor operating in an urban hotspot (Soweto) in the vicinity (~1 500m) of three TSFs. Concentrations of airborne PM10 dust ranged between 5 and 377 µg/m³ (hourly averages) and 34 and 134 µg/m³ (daily average). During this period there was one sustained heavy dust storm, with wind speeds exceeding 6 m/s. During calmer periods, PM10 at this site was 47 µg/m³ and PM2.5 was 12 µg/m³. During the dust episode the average PM10 in the downwind direction from the TSF increased to 312 µg/m³, while PM2.5 increased to 24 µg/m³. Mineralogical analysis of the TSF materials indicates high proportions of quartz. Occupational health standards for quartz exposure (linked to silicosis deaths in mine workers) were exceeded in this residential environment during this event. Statistical analysis of long term weather data indicates occurrence of wind conditions that will exceed thresholds for dust generation and trigger environmental dust episodes for approximately 1150 hours per year. Considering for example the NE of Soweto (original sampling site) adverse wind speed and direction conditions occur on annual average for 845 hours, which are equivalent to 106 eight-hour shifts exceeding occupational health exposure standards. Thus, strong mitigation measures are imperative in order to reduce exposure and health risk to nearby residents.