Restoration of mined lands—using natural processes

Abstract

Mining causes soil damage and destruction. In the process of removing the desired mineral material, original soils become lost, or buried by wastes. In many countries legislation now requires that surface soils be conserved and replaced, but there is a vast heritage of degraded land left by past mining that requires restoration. Since the industry that created this heritage has often gone and there is no money left, the restoration needs to be achieved as cheaply and yet as effectively as possible. The processes of natural succession demonstrate that nature can achieve restoration unaided, and develop fully functioning soils. Although there are problems set by the processes of dispersal, once they are established plants demonstrate that they can readily provide organic matter, lower soil bulk density, and bring mineral nutrients to the surface and accumulate them in an available form. Most importantly, some species can fix and accumulate nitrogen rapidly in sufficient quantities to provide a nitrogen capital, where none previously existed, more than adequate for normal ecosystem functioning. It will normally (but not always) be necessary to introduce artificially, the plant species most appropriate for the restoration process. But natural ecosystem development can then be left to occur on its own. In mined lands, however, certain extreme soil conditions may occur that prevent plant growth, particularly physical conditions, gross lack of certain nutrients and toxicity. It can be important that these are identified and relieved first, otherwise the whole restoration process may either not begin, or fail after a few years. But even so, ecosystem restoration can be achieved at low cost, and the product be self-sustaining in the long term, ecological engineering of the best kind.

Introduction

Because mineral resources are underground and cannot usually be got at without removing the soils and vegetation that overlay them, and nearly always generate wastes which have to be disposed of on the surface, mining causes major damage to whole ecosystems. In both surface and deep mining the original vegetation is inevitably destroyed, and the soils usually lost or buried by wastes. To achieve a successful restoration the soil has to be remediated and the vegetation re-established. This paper will concentrate on the soil and its remediation, because the primary effects of mining are to cause soil damage and if this is not remedied then vegetation will only grow with difficulty, or not at all. In many countries legislation now requires that the original surface soils are conserved and replaced, but there is a vast heritage of degraded land left by past mining that requires restoration. Since the industry that created this heritage has often gone and there is no money left, the restoration needs to be achieved as cheaply and yet as effectively as possible.

`Restoration' is `the act of restoring to a former state or position... or to an unimpaired or perfect condition' (OED, 1971). There are strong implications of perfection. Some people use the word `rehabilitation', but in its definition there is little or no implication of perfection. Nor is there any such implications in the word `reclamation' which is `the making of land fit for cultivation' (OED, 1971). This implies returning the land to a useful, but not necessarily the original, state. The word `restoration' will therefore be used in this paper with the definite purpose of implying that the aim is to reinstate the original functions of the soil in full measure. But whether this, or only some intermediate level of rehabilitation, can be achieved will depend on technique and circumstance.

Although the soil is normally considered to be only part of a complete ecosystem, it has the properties of structure and function analogous to those of whole ecosystems. It is therefore possible to illustrate the process of soil restoration using a diagrammatic representation similar to that developed to illustrate the processes involved in the restoration of whole ecosystems (Magnuson et al., 1980, Bradshaw, 1992) (Fig. 1). The degraded soil has suffered in its properties of both structure and function. Restoration involves returning it to its original state; rehabilitation only a partial return. Reclamation implies ending up in some new state where either structure or function is different from the original; this is quite likely where the soil mineral material has been totally replaced. In such situations it is customary that at least the biological functions of the soil will have been fully restored.