Hydrosphere transformation in the diamond placers mining area in the Vishera river basin, the Urals // Engineering Geology and the Environment. – Rotterdam, Brookfield, 1997. – Vol. 3. – P. 2467-2469. — Лаборатория геологии техногенных процессов

The influence of diamond placers mining in the Vishera river basin (the Urals) on the surface waters, ground waters and bottomset beds composition is examined. The results of experimental works on decreasing the suspended matter concentration using soil filters are shown.

1 INTRODUCTION

Placer deposits mining exerts considerable negative influence on subsurface and surface waters and in turn leads to undesirable changes of other components of the environment (Kalafatovich, Carreno 1994). The Vishera river basin (Northern Urals) where the diamond placers are mined is composed of Riphean, Vendian, Ordovician, Silurian, Devonian, Carboniferous and Permian deposits. This region also contains both ancient and recent karst deposits. There are karsted carbonate seamy rocks of the Silurian, Devonian, Carboniferous and Lower Permian, having a total thickness of up to 4,000 m. Diamond deposits buried in the erosion-and-karst depressions are associated with the verlapped suballuvial and subfluvioglacial karst (Maximovich, Blinov 1996).

The depressions consist of deposits of sandstone, quartzitic sandstone, quartz, flint, limestone and dolomite. The sand portion in the deposits of the erosion-and-karst depressions is occupied mainly by the light fraction as represented by the following minerals: quartz, calcite, dolomite, phosphates and feldspars. The heavy fraction content varies from 0.1 up to 4.2%. Its composition includes: hydrogoethite, magnetite, hematite, siderite, manganese oxides, phosphates, leucoxene, zircon, tourmaline, rutile, ilmenite, chrome-spinellids, anatase, garnets, epidote, pyroxenes, amphiboles, moissanite, fluorite. Terrigenous material of the depression deposits consists mainly of the clay minerals with the following main components: kaolinite, hydromica, montmorillonite and chlorite.

Developed in this area are: the ground waters of the alluvial and eluvial-deluvial deposits; karst waters of the Visean-Artinskian, Fransian-Tournasian and Ordovician-Silurian carbonate aquiferous complexes; fissure-ground and fissure-srratal waters of the Devonian, Ordovician-Silurian and Asselian-Artinskian terrigenous aquiferous complexes. The first two aquiferous complexes are hydraulically communicating, due to the karsted rocks.

2 THE INFLUENCE OF DEPOSIT MINING ON THE HYDROSPHERE

Diamond deposits are mined by dredge and hydromechanical methods through the use of 4 dredges and one seasonal concentrating mill (SCM). The dredges and SCM extract a portion of the heavy fraction containing the diamonds. The rabble-pebbly and sandy-argillaceous fractions are dumped into the rivers and form heaps several meters high. The rivers are dammed up to maintain the required water level for the floating dredges. Construction of the dams and settling ponds, organization of the circulating water supply system and operation of the hydraulic units result in a change in the hydrodynamic regime of the surface and subsurface waters. The increased level of subsurface waters leads to flooding and swamping of the area.

Fig. The content of iron in the surface waters and the bottomset beds of rivers

The presence of the dams causes a hydrostatic head of the subsurface waters. As a result, the polluted river waters can seep into the ground waters. Leakages from the SCM settling ponds also contaminate the ground waters. As a result of the dredge operation there are changes not only in the river bed profile but also in the granulometric composition of the bottomset beds. In some karsted areas of river valleys these changes cause the surface and ground waters to overflow into the underlying aquifers. This phenomenon is promoted by the increase of heads when creating the water reservoirs. The surface and ground waters are contaminated as a result of roiling and extraction of the ground components during mining.

As they come under other physical and chemical conditions, unstable minerals in the heaps are prone to intensive weathering. The products of interaction between the rock heaps and surface waters and atmospheric precipitations percolate into the subsurface waters.

Sampling of the valleys of the Big Kolchim, Northern Kolchim, Kolchim, Big Schugor, Tchurotchnaja, Rassol’naja and Volynka rivers showed high content of iron (up to 9.9 mg/kg), nitrites (up to 1.6 mg/kg) and ammonium (up to 11 mg/kg) in the dredge heaps (Fig). The iron-bearing minerals in the heaps serve as a source of iron. The greatest contamination of the bottomset beds by iron (up to 10.4 mg/kg) can be seen in the river beds nearest the operating dredges, while downstream the concentration of iron becomes lower.

Increased iron content (up to 3.55 mg/1: 12 times greater than the maximum permissible concentration) can be seen in the residues from the dredges and the SCM. The rivers in this region are characterized by an increased content of sulphates, chlorides, nitrates, nitrites, calcium, sodium, potassium, ammonium, silicon and iron.

An increased content of iron, nitrites, ammonium and sulphates is found in subsurface waters affected by placer mining. In some samples the concentration of iron is 35 times greater than the maximum permissible concentration, the concentration of nitrites is 2.5 times greater, that of ammonium is 6 times greater, while the content of sulphates is 10 times greater than in the background values. In the course of mining the diamond placers the surface waters are severely contaminated with suspended matter. Analytical data shows that the concentration of suspended matter in the Rassol’naja river downstream of the operating dredge zone (depending upon the amount of precipitation) varies from 0.2 up to 12 g/1 which is as much as a hundred times greater than the background values.

Table. The change of the suspended matter concentration in the surface waters when using soil filters

3 SUGGESTIONS FOR DECREASING RIVER CONTAMINATION

To decrease concentration of the suspended matter in the river water, the authors propose to create filters made of the dredge heap rocks in the river bed. The experimental works have shown that, depending upon the filtration path length, concentration of the suspended matter could be decreased by as much as a hundred times (Table).

Based on these experimental works the authors have used inverse problem solution to calculate optimum parameters for the soil filters, taking into account initial concentration of the suspended matter, filtration properties of the dredge heaps, river discharge, and service life of the filter.

The use of filters made of the dredge heaps to clean the river waters of suspended matter have thus proved highly efficient at a low cost of construction. According to some data, iron concentration could decrease in this case (Vuorinen, Carlson, Seppanen, Hatva 1988).

4 CONCLUSION

The investigations have shown that the contamination of surface waters by suspended matter and iron is the basic problem of deposit mining. The experimental works on the use of soil filters for the cleaning of the surface waters from suspended matter have shown their high efficiency.

REFERENCE

Kalafatovich, S.C. & R.C.Carreno 1994. Les effets erosifs et environnementaux de 1’exploitation de l’or dans la foret amazonienne: Le cas du sud-est peruvien. Proc. 7 Int. Cong. JAEG, 2753-2756. Lisboa: Balkema.

Maximovich, N.G. & S.M.BIinov 1996. Effect of diamond mining on the surface and subsurface waters in the Vishera river basin (Northern Urals) and measures to be taken to protect the environment. Abstracts 30 Int. Geol. Cong. V. 3, 318. Beijing, China

Vuorinen, A. Carlson, L., Seppanen, H. & T. Hatva 1988. Chemical, mineralogical and microbiological factors: Affecting the precipitation of Fe and Mn from groundwater. Water Sci. and Technol. — 20, №3, 249.