Concentrations of metals are lower in coal shales collected from recently exposed surface mine wastes than in shales collected from spoils exposed to surface weathering for 45 a. In this period, the character of the spoils changes dramatically as a result of the accelerated breakdown of the freshly exposed minestones. Since the metals are held as unstable sulfides, it is suggested that enrichment may be caused by absorption of metals by colloid of iron or formation of organic complex compound with organic carbon existing in the remnant structure of coal mine spoil. This process may contribute to the triggering of soil contamination thresholds on coal-lands already heavily loaded with metals.
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