The potential of phytostabilization on environmental and human risk reduction in mine waste areas
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Worldwide, mining is a source of huge amounts of heavy metal contaminated waste. The most important environmental health risks associated with unmanaged mine waste are caused by the formation of acid mine drainage and the spread of metals into the environment through aeolian dispersion, surface runoff and leaching. Heavy metal pollution and acid mine drainage lead to a decrease of biodiversity, and an increase in death and disease of vegetation, wildlife and humans for many miles around the polluted area. A possible remediation technique is phytostabilization. A long-term vegetation cover would decrease the spread of dust and heavy metals through wind and water erosion, and decrease the leaching of heavy metals and acid mine drainage into the groundwater. Vegetation, possibly aided by microbiota or soil amendments, promotes soil development, nutrient recycling and the development of microbial communities. It is a non-invasive, cost efficient method to reclaim mine waste areas and reduce risks of heavy metal pollution and acid mine drainage. Revegetation has been successful in nutrient-poor heavy metal polluted mine waste areas both in temperate and arid climates. Mycorrhiza and plant growth-promoting bacteria have been found, in some studies, to aid plants in overcoming metal toxicity and decrease leaching and uptake of heavy metals into the plant. However, not all studies have found this effect. Mycorrhiza and growth-promoting bacteria can also decrease the amount of soil amendments needed, lowering costs. Very few studies on the long-term effects of revegetation on risk reduction have been performed. For instance, the decrease of aeolian dispersion and surface runoff due to revegetation has not been studied, and only few have studied the effects on leaching. These studies, possibly due to large differences in experimental design, show conflicting results. It is unclear what influence revegetation has on heavy metal leaching. Metal mobility is influenced by many conflicting processes driven by the plant, bacteria, mycorrhiza and soil status. Most of these processes only influence the rhizosphere and the top soil layer and are not expected to influence metal mobility on a larger scale. Acidification due to vegetation does not seem to be a problem, especially when the right plants are selected. The few studies done show only a very slight decrease in pH levels over time, or an increase under the influence of soil amendments and vegetation. Many plant species do not accumulate heavy metals into their above-ground biomass; these would not pose a risk to foraging animals. In the selection of plants, and after plant establishment, attention should be paid to metal content in leaves and shoots. These should not exceed animal toxicity levels to prevent entry into the food-chain. In order to obtain the best results, a small field study can be performed to select plants with a low metal accumulation and a high tolerance for the local tailing conditions. This ensures higher plant survival rates after planting. After plant establishment, long-term monitoring of the site is necessary. Phytostabilization has great potential as a cost-efficient and non-invasive method for the reclamation of mine waste areas, but more research is needed on the effectiveness of revegetation on risk reduction and to determine which processes mostly contribute to positive effects.