The study of the occurrence, distribution and characterisation of metals and metalloids associated with mining activities are identified as key details to assist mining companies with their goals to rehabilitate impacted lands and waters. Also it needs to provide credible estimations of the likely bioavailability of the metals/metalloids present in the mine wastes in order to undertake a risk based assessment of their potential harm to biota. The work described in this thesis has studied the characterisation of metals and metalloids in soil, sediment and aqueous samples collected in the area of the Wainivesi Gold Mine in Fiji, and the exposure pathways of these metals and metalloids within the mining environment. The project assesses the bioavailability of prioritised species of these metals and metalloids that occur within soil, sediment, water, food and PM10 particulate matter in the context of a risk assessment framework using accepted Australian
and USEPA methodology. It has been identified that lead, cadmium and zinc are the metals of concern in this area due to their elevated levels found in the soil, sediment, water and food samples. In the historical tailings, lead concentration was above 25000 mg/kg, cadmium 153 mg/kg, zinc 17700 mg/kg and arsenic 250 mg/kg. The Acid Producing Potential of historical tailings showed that metal solubility and lability will occur and could facilitate the metal/metalloid ions to be transported via mine runoffs into river sediments and waters. X-Ray Diffraction (XRD) analysis indicated the presence of Cerussite, a lead carbonate mineral that is soluble and considered to be highly bioavailable. The far superior technology, using synchrotron induced X-Ray adsorption Spectroscopy (XANES), and linear combination fitting (LCF) confirmed the XRD results of Pb species in the historical tailings, being comprised mainly of PbCO3 (31%) and PbSO4 (68%). Particulate matter (PM10) was analysed to
assess if a potential pathway for dust inhalation exists. With particle size having a significant relationship to potential bioavailability, the identification of elevated concentrations of metals/metalloids in PM10 particles represents an area of concern. However, the geological location of the gold mine with easterly winds and the high rainfall could provide a protective barrier for the local community upwind in this pathway.
The adsorption model of arsenic on dispersed creek/river sediments was also studied for Pb, Cd and Zn and shows that Zn and Cd compete for binding sites on the sediments studied. Maximum adsorption capacities (Bmax) for selected sediment samples from within the mine site and downstream illustrated a maximum Bmax of 588±32 mg/kg-Pb; 2076±1840 mg/kg-Cd and 1088±647 mg/kg-Zn. However, in this project, it is shown that some Bmax values have beenexceeded indicating possible leaching of these metal ions into the water
making them available for aquatic biota. The importance of bioavailability (BA) is important to any risk assessment, where BA refers to the ratio of the element absorbed compared to the amount ingested in the material in question. Traditionally, in the absence of specific BA data, this is assumed to be 100%. No experiments utilising the in-vivo model were undertaken in order to quantify the absolute bioavailability (ABA) of the metals/metalloids in mine waste samples (tailings and sediments). However, recent work has developed the concept of predicting BA by the use of an in-vitro physiologically-based extraction test (PBET) to yield measurements of bioaccessibility (BAc). The PBET aims to simulate human uptake of the target element, and expresses BAc as uptake in terms of a percentage of available element. The BAc for the historical mine tailings samples was 55±7.2% for chromium; 46±4.9% for copper; 15±1.8% for Zn; 10±1.9% for arsenic;
2±0.38% for cadmium; 0.06±0.01% for mercury and 55±4.6% for lead. River sediment samples varied with a maximum BAc of 17±2.5% chromium; 44±6.7% copper; 29±2.4% zinc; 39±3.9% arsenic; 45±5.6% cadmium; 83±8.3 mercury and 86±4.3% lead. A deterministic approach was used to estimate the risk associated with exposure to metals in foods. A ratio of the exposure over the Acceptable Daily Intake (ADI) gives a Hazard Index (HI) which gives an indication of the level of risk. The maximum concentrations (fresh weight) of 154 mg/kg Pb, and 251 mg/kg Zn were found in taro root grown at the mine site. The maximum level of 0.74 mg/kg Cd was found in water cress from the village. These results exceeded Maximum Permissible Level (MPL) recommended by WHO. Results could be attributed to the high bioaccessibility (BAc) of the metals and their preference to grow in water or swampy soils. Further
validation work has to be carried out to qualify the bioaccessibility results obtained in this thesis. Total exposure dose rates and exposure dose rates for each pathway identified were calculated using default enHealth (Australian) estimates. Food ingestion and dust inhalation pathways prove to be the routes that present the most risk.