On the Issue of Element Migration in Natural Waters

P. A. Udodov, V. M. Matusevich

(Presented at the scientific seminar of the Department of Hydrogeology and Engineering Geology)

Experience in hydrogeochemical investigations shows that they produce the greatest effect when various prospecting methods are combined—lithochemical, biogeochemical, hydrogeochemical, silt, and others. The use of these methods makes it possible to take into account all the principal forms of element migration: ionic, complex compounds, mineral and organomineral colloids, and so forth. However, the application of each of the methods is determined by the corresponding geological conditions. Thus, for example, in the area of the Tom–Ya interfluve, in individual sectors we used metallometric sampling of core and outcrops, the bottom-sediment method, heavy-mineral sampling, and the hydrogeochemical prospecting method. This was due to the varying thickness of unconsolidated deposits, the nature of the regime of surface watercourses, and the water saturation of the rocks. All the indicated methods were applied in the course of geological investigations.

The study area is located in Western Siberia, in the taiga zone, and is characterized by a humid climate. Geologically, it is confined to the junction of three geological structures: the Kolyvan–Tom folded zone, the Kuznetsk Basin, and the Kuznetsk Alatau.

In the western part of the area, sandy-shale deposits of the Lower Carboniferous and Upper Devonian are widespread. In the eastern part, effusive–tuffaceous sediments of the Middle Devonian of the Kolyvan–Tom folded zone predominate at its junction with the Kuznetsk Alatau, as well as crystalline schists, gneisses, and marbles of the northern continuation of the Kuznetsk Alatau. Here, too, the maximum development of zones of faulting is observed, as well as hydrothermal alteration of rocks—sericitization, chloritization, silicification, carbonatization, and pyritization. The complex of Paleozoic and Cambro-Proterozoic rocks is overlain in places by unconsolidated deposits with a thickness of 10–15 m, sometimes up to 100 m in areas of erosional-tectonic depressions. Outcrops of bedrock are observed mainly along the valleys of small rivers and deeply incised ravines. The unconsolidated formations are represented by lacustrine-alluvial deposits and by a weathering crust of predominantly clayey composition. Sand horizons with pebble are confined to erosional-tectonic depressions and river terraces.

The noted features of the physiographic and geological conditions lead to the fact that the study area is characterized by weak water saturation of the unconsolidated sequence and significant water abundance

zones of fault disturbances. A regionally expressed aquifer horizon, confined to the destroyed roof of Paleozoic rocks, generally has an insignificant water abundance, except for those areas where it is fed by waters of the zones of fault disturbances.

The most widespread elements in the rocks of the district are divided into three main complexes: 1) titanium, zirconium, chromium, vanadium (nickel and cobalt); 2) iron, manganese (phosphorus, cobalt, and nickel); 3) zinc, lead, copper, barium, sulfur, mercury, arsenic, silver (molybdenum, tin, strontium, cobalt).

The first complex of elements is associated with the processes of formation and redeposition of weathering crusts. It consists chiefly of chemically stable elements. Their concentrations in the district are represented by placers, by enriched portions in the weathering crust, and also along zones of fault disturbances, where they are due to the presence of diabase dikes. Regional contamination by this complex of elements is characteristic of the sedimentary rocks of the district, including unconsolidated formations.

The second complex is confined to the weathering crust developed on effusive-tuffaceous rocks.

The third complex is the product of hydrothermal activity, which was manifested most intensely along zones of fault disturbances in the area where the geostructures of the district join.

Study of microcomponents at various water-sampling points everywhere shows, in areas of dispersion trains, the presence in the waters of a complex assemblage of elements, most often reflecting the three types of mineralization noted above. This is explained by the interconnection of different aquifers characterized by definite associations of elements. Thus, for example, in one spring confined to a zone of fault disturbance, the following elements were encountered (content in µg/l):

Table 1

It is quite obvious that here there is mixing of waters enriched with elements of the three complexes indicated above.

In individual cases, when sampling points are located along the periphery of dispersion trains, there is a combination of different, but incomplete, complexes, which is explained by the loss of certain elements during migration.

The migration of elements in the area of study is strongly influenced by such factors as the wide distribution of alkaline waters, humates, the presence of sorbents (clays), elevated carbonic-acid content in the waters, and others. These factors influence the behavior of individual elements in different ways. Thus, alkaline waters promote the migration of chromium and at the same time precipitate certain elements in the form of complex compounds with vanadium (vanadates). This, evidently, explains the somewhat reduced content of vanadium in the natural waters of the district in comparison with chromium, whereas in the rocks the average contents of the two do not differ so sharply (Table 2).

The formation of alkaline waters appears to us as the result of the action, on carbonate rocks widely developed in the district, of free CO₂, chiefly along zones of fault disturbances. When bicarbonate waters emerge at the land surface, remo—

CO₂ is removed from them, as a result of which carbonate travertines are deposited, often enriched with hydroxides of iron and manganese. The latter capture and coprecipitate cobalt, as indicated by its elevated content in waterfall-like travertines. Chemical analyses of the travertines have shown the presence in them of elevated contents of certain heavy metals: zinc (0.01%), antimony (0.01%), copper (0.01%), lead (0.003%), traces of silver, etc. Thus, a considerable part of the elements is removed from migration in the waters immediately when they emerge onto the daytime surface.

Humates, on the one hand, protect some colloids from precipitation (Saukov, 1950) and, on the other, form various organic compounds with copper and other elements, which precipitate. The deposition of organo-mineral colloids into bottom sediments is promoted by the elevated content of elements in the waters and by the presence of additional negatively charged coagulation centers (clay particles, silicates, and sesquioxides). With an insignificant content of elements in the waters and an elevated content of organomineral colloids, the phenomenon of opalescence is observed, which is widely developed in regions with a humid climate. In this case, the influx into the waters of elevated contents of elements (for example, in sections where watercourses are fed by fissure waters of zones of rupture disturbances, etc.) leads to rapid deposition of colloids together with the elements into silts. In the region described, sections of zones of rupture disturbances are, as a rule, characterized by an elevated content in the silts of manganese, vanadium, titanium, copper, zinc, lead, etc.; moreover, zinc is present in silt samples only in sections of disturbance zones. The manifestation of these factors is not the same for different sections, which further complicates the conditions of migration of elements in the waters of the region and, at the same time, the interpretation of the data obtained.

All forms of migration are at present determined by B. B. Polynov’s migration-order index (1934), by A. I. Perelman’s coefficient of water migration (1961), and by the water-migration index proposed by P. A. Udodov and Yu. S. Parilov (1961). The coefficients cited reflect the most general regularities in the migration of elements and do not take into account many natural factors on which the conditions of element migration depend. Therefore, as is known, the expediency of applying these coefficients is limited to the clarification of the principal features in the migration of chemical elements.

Meanwhile, for a correct assessment of the migration properties of elements it is necessary to take into account a large number of external migration factors. At present, N. M. Strakhov (1963) evaluates migration capacity according to a complex of geological, paleogeographic, climatic, and other conditions. He has shown that series of element migration may change substantially depending on specific geological conditions.

For weakly dissected territories of the humid type with a semi-closed geological structure, using the Tom–Yaya interfluve as an example, the authors propose two migration indices: 1) the transport index (A), 2) the deposition index (B).

The transport index is the ratio of the average content of elements in the waters to its average content in the rocks of the given region. Thus, the calculation is based on the partial use of the principles of the migration coefficients of A. I. Perelman (average content in the rocks of the given region) and P. A. Udodov and Yu. S. Parilov (expression of content in water in weight percent),

Table 2

Content of heavy metals in rock ores, clarkes, and natural waters in the area of the Tom–Yaya interfluve

Because, in lowland areas, the deposition of microcomponents under the influence of various factors occurs into bottom sediments, the authors propose an index of element deposition. The latter is the ratio of the average content of an element in silty sediments to its average content in the rocks of the district. Table 2 gives occurrence percentages and some initial data for calculating migration indices for the 13 most widespread elements in the district. For the other elements, such as cadmium, mercury, tin, and molybdenum, it was not possible to calculate migration indices because of insufficient initial data for them.

From Table 2 it can be seen that, with a decrease in migration capacity, there is a tendency toward an increase in the deposition index. The latter makes it possible to specify the position of a given element in the migration series. Thus, for example, lead and nickel have identical transport indices (0.4), whereas the deposition index of nickel is significantly higher (1.15) than that of lead (0.14), which permits the conclusion that lead has a greater migration capacity than nickel. The transport index of barium is somewhat lower than that of titanium, but the deposition index of the latter is significantly lower; therefore, in the migration series given, titanium stands after barium. The same picture is observed for manganese and chromium.

A comprehensive assessment of the migration capacity of elements according to these two indices makes it possible to construct the following series for the study district:

$$ \mathrm{Sb} > \mathrm{Ag} > \mathrm{Zn} > \mathrm{Cu} > \mathrm{As} > \mathrm{Pb} > \mathrm{Ni} > \mathrm{Cr} > \mathrm{Mn} > \mathrm{Co} > \mathrm{V} > \mathrm{Ba} > \mathrm{Ti}. $$

The influence of external migration factors is reflected in a certain change in the migration series compiled in an analogous way for different parts of the Tom–Yaya interfluve, depending on the composition of the rocks, tectonics, and regime of the natural waters. For the western part of the district, with sandy-shaly deposits present, the migration capacity of microcomponents in the waters is expressed by the following series:

$$ \mathrm{Sb} > \mathrm{Ag} > \mathrm{Zn} > \mathrm{Cu} > \mathrm{Pb} > \mathrm{Cr} > \mathrm{V} > \mathrm{Mn} > \mathrm{Co} > \mathrm{Ti} > \mathrm{Ni} > \mathrm{Ba}. $$

The eastern part of the district is characterized by the presence of effusive-sedimentary and carbonate rocks. Here the migration capacity is expressed by the series

$$ \mathrm{Zn} > \mathrm{Ag} > \mathrm{Sb} > \mathrm{Cu} > \mathrm{Pb} > \mathrm{Ba} > \mathrm{Mn} > \mathrm{Ti} > \mathrm{Cr} > \mathrm{Ni} > \mathrm{V} > \mathrm{Co}. $$

It is not difficult to note that some elements change their place in the series presented, depending on external factors. For example, the increase in the migration capacity of zinc in the eastern part, compared with silver and antimony, is connected with the maximum manifestation, for the entire district, of disjunctive disturbances enriched with carbon dioxide. This favors the removal of zinc by waters, while the presence of chemically active carbonate rocks hampers the migration of antimony and silver in the waters; zinc, however, is less subject to the influence of the chemical activity of the rocks. In addition, the travertine formation widely developed here (in connection with the abundance of zones of disturbance) leads to partial precipitation of antimony and silver with the travertines. For the same reasons, the migration capacity of barium increases in the waters of the eastern part of the district, which has been confirmed by experimental laboratory work. The formation of alkaline waters is associated with travertine formation, as a result of which the migration capacity of vanadium is considerably reduced.

Of interest is the different behavior of cobalt in the western and eastern parts of the district. In the second case, as we see from the series given, its migration capacity is decreased. Here cobalt ho-

is contained in rocks in an amount of 0.03%, but remains at low concentrations in waters. The iron and manganese, which are widely distributed here, are evidently one of the factors removing cobalt from aqueous migration, along with processes of travertine formation. It is not accidental that cobalt-bearing substances include not only manganiferous compounds, but also iron hydroxides, brown iron ores, and carbonate travertines.

The data presented indicate the great role of geological processes in the migration of elements in natural waters. In this article we have considered the influence of only some of these processes and do not dwell on the question of the influence of many other factors (the nature of water exchange, the type of mineralization, humates, etc.).

Thus, it is evident that, in addition to internal factors, external migration factors exert a substantial influence on the migration of elements in waters; consequently, the behavior of individual elements in migration series, depending on specific geological conditions, may change to a considerable degree. For a more complete study of the migration of elements under various conditions, taking into account their concentration in bottom sediments and other neoformations of the hypergenesis zone, comprehensive geochemical investigations are necessary.

LITERATURE

1. I. I. Ginzburg. An Attempt to Develop the Theoretical Foundations of Geochemical Methods of Prospecting. Gosgeoltekhizdat, 1957.

2. A. I. Perelman. Geochemistry of the Landscape. Geografgiz, 1961.

3. B. B. Polynov. The Weathering Crust. Publishing House of the Academy of Sciences of the USSR, 1934.

4. A. A. Saukov. Geochemistry. Gosgeoltekhizdat, 1950.

5. N. M. Strakhov. Types of Lithogenesis and Their Evolution in the History of the Earth. Gosgeoltekhizdat, 1963.

6. P. A. Udodov and Yu. S. Parilov. On certain regularities in the migration of metals in natural waters. Geochemistry, No. 8, 1961.