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Мaximovich N.G., Blinov S.M. Effect of diamond mining on the sulface and subsurface waters in the Vishera river basin (Northern Urals) and measures to be taken to protect the environment//30th Int. Geological Congress. Beijing, China, 1996. P.318.

Effect of diamond mining on the surface and subsurface waters in the Vishera river basin (Nothern Urals) and measures to be taken to protect the environment

N.G.Maximovitch, S.M.Blinov

The Vishera river basin (Nothern Urals) where the diamond placers are mined is composed of the Riphean, Vendian, Ordovician, Silurian, Devonian, Carboniferous and Permian deposits. This region displays the older and recent karst. There are karsted carbonate seamy rocks of the Silurian, Devonian, Carboniferous and Lower Permian of the total thickness up to 4,000 m. Diamond deposits buried in the erosion-and-karst depressions are associated with the overlapped suballuvial and subfluvioglacial karst.
The depressions consist of the deposits of sandstone, quartzitic sandstone, quartz, flint, limestone and dolomite. 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, chlorite.
In this region there are developed 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-stratal waters of the Devonian, Ordovician-Silurian and Asselian-Artinskian terrigenous aquiferous complexes. The first two aquiferous complexes are hydraucally communicating which is due to the karsted rocks.
The diamond deposits are mined by the dredge and hydromechanical methods through 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 rubbly-pebbly and sandy-argillaceous fractions are dumped into the rivers and form the heaps of several meters high. The rivers are dammed up to maintain the required water level for the floating dredges.
Construction of the dams and settler ponds, organization of the circulating water supply system, operation of the hydraulic units result in the change of the hydrodynamic regime of the surface and subsurface waters. The increased level of subsurface waters leads to underflooding and swamping of the territories.
The presence of dams causes hydrostatic head of the subsurface waters. As a result, the polluted river waters can inflow into the ground waters. Leakages from the SCM settler 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 and, in some karsted areas of river valleys, these activate overflow of the surface and ground waters into the underlying aquifers. This phenomenon is promoted by the higher heads when creating the water reservoirs. The surface and ground waters are contaminated as a result of roiling and extraction of the ground components in the course of technological operations.
Once being 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 infiltrate into the subsurface waters.
Sampling of the valleys of the Grand Koltchim, Nothern Koltchim, Koltchim, Grand Shchugor, Tchurotchnaja,Rassolnaja,Volynka rivers showed high content of iron (up to 9.9), nitrites (up to 1.6) and ammonium (up to 11 mg/kg) in the dredge heaps. The iron-bearing minerals in the heaps serve as a source of iron. Maximum contamination of the bottomset beds with iron (up to 10.4 mg/kg) can be seen at the river lengths beside the operating dredges while downstream the concentration of iron becomes lower.
Increased iron content (up to 3.55 mg/l which is 12 times greater than the maximum admissible concentration) can be seen in the pulp of the dredges and SCM. The rivers in this region are characterized by the increased content of sulphates, chlorides, nitrates, nitrites, calcium, sodium, potassium, ammonium, silicon, iron.
Increased content of iron, nitrites, ammonium and sulphates is found in the subsurface waters that are under the influence of the placer mining. In some samples the concentration of iron is 35 times, that of nitrites is 2.5 times, and that of ammonium is 6 times greater than the maximum admissible concentration while the content of sulphates is 10 times greater than the background values. In the course of mining the diamond placer the surface waters are most severely contaminated with the suspended matter. The analysis data have shown that the suspended matter concentration in the Rassolnaja river downstream of the operating dredge zone, depending upon the amount of precipitations, varies from 0.2 up to 12 g/l which is tens and even hundreds times as high as the background values.
To decrease concentration of the suspended matter in the river water the authors proposed to create filters made of the dredge heap rocks in the river bed. The development works have shown that, depending upon the filtration path length, concentration of the suspended matter could be decreased by tens and hundreds times. Based on these development works and by way of solving the inverse problem the authors have calculated optimum parameters of 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 their high efficiency at low costs of their construction.

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