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Boron in plants at a coal mine
Boron (B) is a micronutrient, but can also be toxic to plants and animals at high levels. Boron is a common trace element in coal, commonly up to hundreds of mg/kg. Coal mining can expose coal-bearing waste rock and coal-bearing outcrops to oxidation and leaching by rain and groundwater, allowing enhanced mobilization of B from the coal. Boron exposed in this way can be taken up by plants to high levels, resulting in poor plant performance, and therefore inhibiting revegetation of mine sites. Also, this B mobilization can persist when mines close and rehabilitation has occurred, as rainwater continues to percolate through the waste rock.
The coal mine for this study is at Wangaloa, in the southeastern South Island of New Zealand. The mine was a small (252 ha) open cut operation that wasconfined to a single stream valley, and all waste rock was dumped within the same catchment. The mine was active between 1945 and 1989, and extracted high rank lignite from a sequence of Late Cretaceous marginal marine sediments. Waste rock contains variable amounts of coal, ranging up to 90% locally, on the 1-10 m scale. Abandoned parts of the Wangaloa mine area were initially rehabilitated as plantation forest (Pinus radiata) in the 1980s, but trees on the waste rock piles showed poor growth. At that stage, the site showed evidence of minor acid rock drainage issues. A renewed plan for site rehabilitation was initiated in 2000, and pine trees were removed and replaced by extensive plantings of native New Zealand species in 2003. Poor growth on coal-rich waste rock prompted investigation of the combination of substrate acidity, low nutrient status, and excessive boron uptake. The boron is derived initially from coal, which has up to 450 mg/kg B. The coal also contains pyrite (2-5 wt% S), which oxidizes to yield a low-pH environment (typical pH 2-5). Weathering of coal-bearing waste rock liberates B into rainwater that infiltrates into waste rock or evaporates to leave a gypsum crust enriched in B, possibly as boric acid or colemanite. Surface waters dissolve this evaporative material periodically, yielding total B concentrations up to 6 mg/L, at pH <4.5. Some of the available B is taken up by plants that have been established on the waste rock, resulting in foliage B concentrations of up to 230 mg/kg (dry weight). The highest B contents are in broadleaf (Griselinia littoralis), and this species showed high mortality on coal-rich substrates. Elevated total B levels in native plants established on coal-rich substrates show that similar B accumulation can occur in less than two years despite the provision of fertilizer and peat blocks on initial planting.
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