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4. Mining and landscape management
It has thus become clear that the opencast hilltop mining of nickel and iron ores in New Caledonia has had a devastating effect on landscapes within and around the ultrabasic massifs and has modified river channels, estuaries, deltas, and some nearshore sea areas (fig. 21). Such effects are not unknown elsewhere in the world.
On the high island of Bougainville, in Papua New Guinea, for example, opencast copper mining has resulted in the discharge of about 150,000 tonnes of rock waste and tailings daily. The tailings have spilled down into a headwater valley of the Kawerong River and thence into the Jaba valley, where they have spread out across the valley floor, destroying large areas of rain forest and killing fish in the river. At its mouth the Jaba has built a new delta out into Empress Augusta Bay, and sand and gravel have been spread northwards along the shore as a prograded beach (Brown 1974). (It is likely that the mining of copper and gold from the mountainous headwater regions of the Ok Tedi, a tributary of the Fly River in western Papua, will generate waste material to augment the already large load of the Ok Tedi, and increase deposition on the Fly delta. But natural rates of sediment yield are here so high that the effects are not expected to be as drastic as on Bougainville or in New Caledonia [Jackson 1982].)
In the Philippines, iron ore mining has led to siltation and pollution of the Larap River on the island of Luzon, adversely affecting the fishery resources of that river and of Larap Bay off its mouth. The Lawis River, in western Luzon, has been similarly affected by chromite mining; the Taft River on Samar Island by iron ore extraction in its headwater regions; and the Abra River, also on Luzon, by gold, silver, and copper mining in its upper tributaries. In each case the river load has been augmented by the inflow of silt and sand (Rabanal and Datingaling 1973).
River channels have been greatly modified and their sediment loads increased by tin mining in various parts of Malaysia and Burma, and there are reports of similar impacts from the copper mines of Chile, Peru, Zaire, and Zambia. In Kelantan and Trengganu, provinces of Peninsular Malaysia, the mining of tin ore (cassiterite), gold, iron ore, and bauxite from hillside and valley-floor deposits has been in progress for over a century, and has resulted in extensive siltation and discoloration of such rivers as the Kinta, the Klang, the Semenyih, and the Pahang. Channel aggradation, levee accretion, increased flooding, and river diversion have been widespread; productive agricultural land has been buried by sand deposition; and villages have had to be relocated (Douglas 1967, 1970; AUSTEC 1974). Successive government acts have put restrictions on the more severely polluting mining activities, for example, "lampanning" - the hydraulic sluicing of hillsides for tin ore, which led to landslides and outwash fans and to accretion along valleys, much as in New Caledonia. Shoaling has occurred rapidly in the estuaries of Malaysian rivers that drain mining areas, damaging fisheries but increasing the tidal mudflat habitat for some shellfish such as cockles.
In south-west England the opencast extraction of kaolin (china clay) from the granitic uplands of Hensbarrow Down near St. Austell and from southwest Dartmoor has generated massive heaps of granular waste (mainly quartz and feld-spar gravels) and has polluted the adjacent rivers, which are white with suspended kaolin. In St. Austell Bay this white sediment discolours the blue waters of the English Channel to various shades of green.
The reddening of sea water by polluted rivers draining opencast mining areas in New Caledonia constitutes "aesthetic pollution" and may disturb and impoverish the flora and fauna of coastal waters, notably coral growth and fisheries. Locally, as in the Baie de Prony, it appears that some marine organisms have adapted to the modified environment. In most cases the deposition of red mud at river mouths has not impeded the growth and spread of mangroves, although these have suffered recurrent damage from discharging river floods. Red-staining of marine waters occurs in a number of places around the world, including streams flowing from the Redruth tin mines into St. Ives Bay, southwest England, and in localities on the coasts of Liberia.
In western Tasmania, sand and gravel washed from the Mount Lyell copper mines at Queens-town and from surrounding hillsides denuded of their forest cover by bushfires and toxic fumes from the smelter have passed into the headwaters of the King River.
The classic case of mining impacts on valley and coastal features was that generated by the sluicing of hilltops for gold extraction in the Sierra Nevada, in central California, in the nineteenth century (Gilbert 1917). After the discovery of gold-bearing deposits in 1848, jets of water were used to excavate quarries in gravelly material in the headwater regions of the Yubas, the Bear, and the American rivers, tributaries of the Sacramento River, which flows into San Francisco Bay. This procedure yielded vast quantities of sand, silt, and clay as well as pebbles, cobbles, and boulders to valley-side fans, and thence into the rivers. Valleys that had been deeply incised were aggraded: in the upper Yuba the valley floor was raised by up to 100 metres. With successive floods this detritus was carried downstream, and the major flood of 1862 swept large masses of sediment down to Sacramento and beyond, burying farmland, filling previously navigable river channels, and increasing the risk and scale of future flooding; sand and mud were deposited, shallowing the northern arms of San Francisco Bay. The resulting outcry led to the prohibition of sluicing by 1884, when miners were no longer allowed to cast their tailings into the streams. But the downstream dispersal of mining waste has continued, and the channels of the Yuba, bordered by dissected terraces dating from the mining phase, are still overloaded, with a gradation from coarse to fine channel sediment down the river. In all, some 1,816 million cubic metres of material (about eight times the volume removed to cut the Panama Canal) were excavated between 1849 and 1914 (non-hydraulic mining having continued after 1884), of which 675 million were still in the valleys, 224 million in the Sacramento delta area, and 914 million in San Francisco Bay and the adjacent sea.
Opencast mining in New Caledonia has been on a similar scale. Compared with other mining areas elsewhere in the world, the impact of opencast hilltop mining in New Caledonia has been exceptionally severe and extensive. Large areas have been decertified by the removal of weathered overburden from plateaux and hill crests, and by the spilling and slumping of waste material down hillsides and into neighbouring valleys. It is of course almost impossible to extract the nickel ores, which occur at the base of weathering mantles up to 30 metres deep on these summit plains, without conspicuous devastation; the only way would be to excavate a crater, leaving the rim intact, and working rotationally downwards through the overburden until the richest ores were encountered, although this would be very costly. Mining of superficial iron ores requires the removal of surface layers, but it would be fairly easy to shape the remaining material into a relatively stable landscape on which a soil and vegetation cover could be reestablished.
Unfortunately, natural regeneration of vegetation has been very poor in areas that have been mined and on the waste deposits, even where the latter have become relatively stable at low angles of rest. As has been noted in chapter 2, the rapid expansion of nickel mining in the boom years of the 1970s, when numerous mining companies obtained leases and proceeded to quarry almost every ultrabasic massif in New Caledonia, was followed by a steady fall in nickel prices, which led to the cessation of mining activities in many areas. Abandoned, the hilltop mines stand as stark, derelict, unvegetated landscapes, and the waste products spilling down their bordering slopes are easily mobilized, especially by the heavy downpours that accompany the passage of tropical cyclones. Although commendable attempts have been made in some areas (since the mid-1970s) to arrest downslope movement of mining waste by building rock dams, terraces, and settling basins to trap tailings and by impounding streams polluted by red clay in suspension, the movement of mining waste into rivers, and eventually to the sea, is bound to continue for many decades after mining comes to an end. The next phase of higher prices for nickel ores will certainly revive opencast mining in many areas that have recently been abandoned, and probably extend it into areas not yet mined. It has been suggested that the nickel reserves of New Caledonia (the richest known reserves in the world) will last for at least 200 years, so that the geomorphological and ecological effects of opencast hilltop mining are likely to persist for several centuries.
Some improvement can be expected with the wider application of the landscaping techniques devised by the Société Le Nickel and the smaller companies to reduce dispersal of mining wastes. The stabilization of mining waste and of eroding surfaces in and around mined areas could also be improved by the planting of vegetation. Natural regeneration of the indigenous maquis, scrub, and forest is very slow, but might be hastened by the planting of native species. Alternatively, afforestation with pine plantations (especially Pinus caribaea and P.eliottli) may prove to be an effective way of stabilizing and revegetating landscapes modified by opencast mining and mining waste (plate 28). Jaffré et al. (1977) have ascribed the poor regeneration of vegetation to the low mineral content and high porosity of surface materials left by opencast mining and waste deposition, as well as to the occasional presence of toxic chemicals in the soils. They advocated the planting of species found to be tolerant of these difficult edaphic conditions, for example, Acacia spirorbis on soils rich in magnesium compounds, and the application of soil improvement techniques (e.g., loosening of rocky surfaces, mulching, and addition of fertilizers such as phosphates) to facilitate plant growth. While it is possible that chemical compounds derived from mining waste are passing into solution in ground water and seeping into the rivers, no specific instances of adverse ecological effects of such chemical pollution have yet been identified.
New Caledonia is remote from centres of conservation activism, and the impacts of opencast hilltop mining have attracted much less criticism than they would have done in Europe, North America, or Africa, where it is likely that severe restrictions would have been placed on the nature and extent of such mining, with an insistence on more effective conservation works to minimize the effects of waste dispersal from mined areas.
In the 1970s a territorial commission for the prevention of damage by mining was established in New Caledonia, composed of representatives from public authorities, local agencies (municipal and Melanesian), mining companies, and scientific organizations. Thereafter, new mining necessitated an authorization from the Service des Mines following consultation with the commission, which is required to make a field inspection of the area to be mined, assessing the risks of erosion on and around the site and compiling a botanical inventory to check if there are rare species or communities requiring site preservation. If mining is authorized, the mining company is obliged to put in conservation works to prevent damage and pollution downslope and downstream from the mining area. These include slope terracing behind boulder walls to achieve soil and waste mantle stabilization (plate 29), barrages to intercept down-washed material (plate 30), and management to maintain percolation and prevent erosion along prospecting trackways and access roads. These procedures are helpful, but have been introduced far too late to prevent extensive damage. In particular, the many mines developed during the period of rapid expansion (1968-1973) and then abandoned are responsible for widespread and still uncontrolled erosion, waste dispersal down slopes, and river pollution and aggradation, the effects of which are still spreading and will persist for many decades, possibly centuries.
In recent years the New Caledonian Association pour la Sauvegarde de la Nature has endeavoured to promote an interest in nature conservation and landscape protection, especially in areas where there are still good stands of indigenous vegetation rich in endemic species, including Araucaria forests and varied maquis. Hilltop mining has been very widespread, and damage by roads and excavations built in the course of mineral prospecting still more so. But some areas with native vegetation remain, either on undisturbed rocky outcrops or on lateritic soils on the hill crests and slopes of ultrabasic massifs, and a few areas where the unusual geomorphological features of peridotitic karst still persist: for example, on the Me Aiu plateau, north of Canala. The Association would like to see some of these preserved as territorial parks or nature reserves to stand as memorials of the native vegetation and original landscape, and as standards against which the changes wrought in and around mined areas can be assessed.
The problem of rehabilitating mined areas and associated deposits of mining waste may prove more difficult and enduring than the recovery of river channels and coastal environments once mining comes to an end. In areas where mining ceased three or four decades ago (e.g., in the vicinity of Voh, on the west coast), river channels still contain coarse gravelly loads, but vegetation has revived on their banks and the conspicuous red clay has been washed away. At the coast, the fringes of deltas that received additional sediment during and after the phase of hilltop mining activity are becoming more stable as the outflow of sediment slackens and the red staining of sea water gradually fades as the ferruginous clay in suspension is either dispersed or coagulated and precipitated into sea floor sediments. In due course, the modern phase of deposition resulting from mining activity will come to an end and quieter sedimentation will ensue. It may eventually be difficult to distinguish the sediments formed by this anthropogenic deposition from those of earlier phases, when relatively widespread deposition of coarse piedmont deposits resulted from tectonic uplift in the hinterland or from climatic changes. At present, however, the impacts of opencast mining on the landscape of New Caledonia are conspicuous and extensive, and they are likely to remain so for a prolonged period. A long-term programme of stabilization and revegetation of the areas subjected to mining and deposition of sedimentary waste materials is now needed to rehabilitate the landscape ecosystems of this ravaged South Pacific island.