In response to international criteria, the rehabilitation of mine sites in Australia have become more stringent. There is increasing awareness of the potential for detrimental effects on the environment and human health from exposure to metals and metalloids, such as arsenic and lead, liberated in mine waste. Often, high cost is incurred during the mine rehabilitation process in an effort to return the land to its former state and /or accepted future land use. A key issue in Australia is the utilisation of mined land for future pastoral activity. However, there currently are no criteria whereby the risk to grazing animals, from arsenic and lead liberated from rehabilitated mine waste, can be assessed.
One of the key aspects of risk assessment of grazing animals on rehabilitated mine waste is the bioavailability (BA) of contaminants, including arsenic and lead. In this case, 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%. In an effort to quantify arsenic and lead BA, estimates for mine waste have previously been made using a variety of animal models including rats, rabbits, guinea pigs, dogs and swine and various in vitro models. Experiments utilizing cattle were undertaken in order to quantify the BA of arsenic and lead in mine waste. The mine waste materials that were used included:
Material Material Description
Kidston - neutral Gold tailings
Kidston - acid Gold tailings
Gympie - historic Historic gold tailings
Jibbinbar Arsenic Mine Mixed waste: slag, ash, contaminated soil
Red Dome - waste rock Gold waste rock fines
Red Dome - heap leach Gold heap leach
Zinifex Century Zinc Lead-Zinc tailings
Relative BA (RBA) estimates were generated by quantifying arsenic and lead liver accumulation in cattle dosed continuously with selected mine waste for 11 months. The arsenic RBA estimates ranged from 2 ± 0.3 - 29 ± 19 %, with a single lead RBA value of48± 3.2%.
In addition, single dose experiments were conducted with cattle under controlled experimental conditions to quantify the BA of arsenic and lead. This was achieved via analysis of the area under the blood concentration curve for both treatment and control cattle. Both RBA and absolute BA (ABA) values were obtained. The range of RBA values for arsenic was: 11 ± 0.61 - 69 ± 7.24 %, with a single RBA value of 80 ± 13.19 %. The range of ABA values for arsenic were: 7 ± 0.34 - 58 ± 6.49 %, with a single lead ABA value of 3 ± 0.51 %.
Due to the cost involved in conducting large animal studies, BA data were also generated using a laboratory rat model. The range of RBA values for arsenic were: I ± 0.28 - 20 ± 3.09 %, with a single RBA value of 38 ± 6.78 %. The range of ABA values for arsenic were: 0.5 ± 0.14 - 8 ± 1.2 %, with a single lead ABA value of 1 ± 2.15 %.
Despite the significant difference between the BA values for rats and those for cattle, a significant correlation was observed for arsenic BA values. This supports the potential for the laboratory rat model as an alternative, more cost effective target species.
Field validation involving BA assessment of arsenic and lead from rehabilitated mine waste to cattle was also conducted on two North Queensland mine waste facilities. These two trials demonstrated the limited accumulation of arsenic and lead in situ, validating the relatively low BA of arsenic and lead from mine waste material. The amount of mine waste cattle consume in situ was quantified (dose = 150-300 g), as was the relative contribution of arsenic and lead from plant material.
An in vitro physiologically-based extraction test (PBET) was also undertaken using the same mine waste materials dosed to animals. As demonstrated by previous researchers, the arsenic bioaccessibility (BAc) values generated were independent of stomach pH state, whilst lead BAc was pH dependent. The arsenic BAc values correlated well with both the rat and cattle in vivo BA results.
The outcome of the in vivo B A values, and field validation was the development of a risk assessment tool for assessing the risk of contamination in grazing cattle. The risk assessment tool generated a proposed new arsenic investigation level (IL) for rehabilitated mined land for pastoral grazing activity (480 mg/kg). Where the proposed new IL is exceeded, the risk assessment tool utilises a realistic dose calculation (ADR) to provide a framework (Predictive Model) for assessing the safe grazing duration £uid associated management strategy. Also, the risk assessment tool permits the generation of a new human health investigation level (HIL), once BA values fi-om the rat model are used to amend existing HIL calculations. Hence, allowing more informed decisions to be made in regard to the realistic risk of arsenic and lead toxicity to humans (500 mg/kg).