GROUNDWATER OF THE SOUTHEASTERN MARGIN OF THE WEST SIBERIAN ARTESIAN BASIN AND ITS PALEOZOIC FRAMING

Yu. K. SMOLENTSEV

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

The territory characterized here occupies the southeastern part of Tomsk Oblast. The western boundary of the district runs along the Ikse River, the eastern boundary along the Chulym River, the northern boundary along latitude 57°20′, and the southern boundary along the boundary of the oblast. Economically, the district is the most densely populated, industrially developed, and settled part of the oblast.

The hydrogeological regionalization of the area described is based on the geostructural principle proposed by G. N. Kamensky for the hydrogeological regionalization of the territory of the USSR [6, 7]. The complex hydrogeological conditions of the territory are determined by its position at the junction of two major hydrogeological regions—the West Siberian artesian basin of the platform type and the Sayan–Altai hydrogeological folded region. The boundary between them runs along the boundary of the outcrop at the surface (beneath a thin cover of anthropogenic deposits) of the folded Paleozoic basement; more precisely, along the boundary between the Oligocene deposits occurring in the trough-shaped structure of the West Siberian Lowland and the Paleozoic deposits of the Tomsk uplift [1, 3, 5, 9]. The northeastern part of the area characterized lies in the peripheral part of the Chulym artesian basin of the second order; the western part, in the marginal part of the Ob artesian basin of the second order; and the southeastern part, in the peripheral part of the Sayan–Altai hydrogeological folded region. The boundary between the Ob and Chulym artesian basins of the second order is drawn by us along the Tomsk swell, and farther to the northwest approximately along the valley of the Ob River, that is, along the boundary between the system of uplifts of the Vasyugan zone and the Chulym depression.

Proceeding from the analysis and generalization of materials on groundwater, on the basis of the classifications of O. K. Lange, F. P. Savarensky, N. I. Tolstikhin, and I. K. Zaitsev, we have proposed a consolidated classification of the groundwater of the district under consideration [1, 3, 5, 9]. The groundwater of the district belongs to the infiltration genetic cycle according to G. N. Kamensky [7]; the role and participation of condensation processes occurring in the aeration zone are not excluded. The principal types of groundwater include waters of the aeration zone, phreatic waters, interstratal waters, fissure waters, and vein waters. Artesian waters—that is, interstratal waters of trough-shaped artesian structures—are distinguished as a subtype of interstratal waters. According to

according to the character of the reservoirs, three principal categories of groundwater are distinguished: pore, fissure, and vein waters. The term “groundwater” is used here in the sense of S. N. Nikitin, O. K. Lange, I. K. Zaitsev, and M. E. Altovsky.

West Siberian Artesian Basin

Geologically, the peripheral part of the West Siberian artesian basin is characterized by a gentle occurrence within the Zhukovo sculptural terrace, the Tomsk swell, and the Yaya uplift, and farther to the west, north, and east by an intensive subsidence of the surface of the Paleozoic folded basement, complicated by uplifts, and by an intensive increase in the thickness of the Meso-Cenozoic deposits of the upper structural stage, which form the cover of the artesian basin; in its composition sandy, gravelly, and pebble material plays an important role. In the cover of the artesian basin two structural tiers are distinguished: the upper one is formed by Quaternary and Paleogene deposits, the lower one by trough-like Cretaceous deposits.

Hydrogeologically, the margin of the artesian basin is characterized by the presence of high-pressure fissure and vein waters in the basement deposits, thick pressure artesian aquifers in the Cretaceous and Paleogene deposits, and also by the wide distribution of interstratal (confined and unconfined) and groundwater in the Quaternary deposits.

In the section of the upper part of the Meso-Cenozoic considered here (down to 500 m), three large aquifer complexes are clearly distinguished along the margin of the artesian basin, separated from one another almost throughout the entire territory by aquiclude strata.

We assign to the first aquifer complex the water-bearing deposits of Quaternary age, the continental Oligocene (the Turgas, Azharm, Novomikhailovka, Atlym, and Yurkov formations), and the Sym Formation of the Upper Cretaceous of the Chulym basin. Owing to differences in the lithological composition of the upper part of the section of the Quaternary deposits, the groundwater of the complex in some areas, where an impermeable roof is present, is interstratal; in others, where it is absent, it is typical unconfined groundwater. The waters of the Novomikhailovka, Atlym, and Yurkov formations of the Oligocene and of the Sym Formation of the Upper Cretaceous, occurring below the local base level of erosion, are classed as interstratal artesian waters. The waters of the complex acquire a groundwater character chiefly in river valleys, within alluvial deposits.

Waters of the aeration zone are widely distributed within the complex: bog waters, soil waters, perched water, and waters of the capillary zone. Bog waters occupy considerable areas in river valleys and on interfluve spaces. Their reaction is weakly acidic; they are enriched in iron oxides. Perched water occurs on lenses of impermeable rocks in the upper part of the Quaternary deposits, in the mass of cover loess-like loams, and in interbeds of sands and sandy loams. It is formed by infiltration of atmospheric precipitation. Perched water is of low yield, often polluted, and has an unstable regime. It occurs mainly at depths down to 5 m and is often used by the population for water supply. Perched water has a variegated chemical composition; its mineralization is 0.2–1.4 g/l. It is not recommended for water supply.

Waters of the saturation zone of the aquifer complex are confined to sands, sandy loams, gravel-pebble, and pebble deposits. According to the scheme of zoning of groundwater in the USSR by G. N. Kamensky, the ground-interstratal waters of the Quaternary and Upper Oligocene deposits belong to waters of leaching, formed under conditions of excess-

precise moistening and the predominance of groundwater runoff over evaporation. The groundwater of the complex occurs at depths down to 100 m. The depth to the roof of the aquifer complex increases as the relief rises and with distance from the main water arteries of the region draining the aquifer complex, and reaches a maximum on the highest parts of the interfluves of the Ob—Tom and Tom—Chulym rivers [3, 5, 9]. Thus, in the floodplain parts of river valleys the groundwater occurs at depths down to 15 m, within the limits of above-floodplain terraces at depths down to 27 m, more rarely down to 42 m, etc. [11].

Almost everywhere, the water-bearing Quaternary deposits without an aquiclude are underlain by water-bearing deposits of the continental Oligocene, and in the Chulym basin also by water-bearing Upper Cretaceous deposits, which has made it possible to combine them into a single aquifer complex. Clay beds and lenses within the complex are, in general, of subordinate importance, in individual cases dividing the aquifer complex into a number of closely hydraulically connected aquifer horizons and subcomplexes. Thus, in the southwestern part of the region, in the peripheral part of the Ob artesian basin, in the middle part of the complex there is traced a more or less laterally persistent, substantially clayey unit of the Novomikhailovka Formation of the Middle Oligocene, which divides the aquifer complex into two subcomplexes hydraulically connected with one another. At the base of the first aquifer complex in the Ob basin lies a regionally persistent confining unit of the Lyulinvor Formation of the Eocene, and in the Chulym basin—the clayey deposits of the Upper Simonovka Subformation of the Upper Cretaceous.

The thickness of the water-bearing deposits of the complex is 4–210 m and more; moreover, the complex has the smallest thickness in the southeastern part, at the boundary with the Tomsk hydrogeological massif, and the greatest—in the Chulym basin, in the northeastern part of the region.

As a rule, wells and boreholes penetrate only the very upper part of the aquifer complex, exploiting waters of the Quaternary, more rarely Oligocene deposits. The water table of groundwaters and the piezometric surface of confined waters are located at depths of 0–40 m. In river valleys, in lowered parts of the relief, groundwater levels lie closer to the land surface; on interfluves—at greater depth. In some cases, in the river valleys of the southwestern part of the region, boreholes that penetrated the lower aquifer subcomplex, occurring at depths of 32–200 m, flowed artesian; the piezometric level is established at up to +0.5—+18 m above the wellhead [3, 9, 10, 11]. The head of the waters of the complex is 0–170 m. As a rule, artesian waters of the lower aquifer subcomplex of the deposits of the Yurkovka and Novomikhailovka formations of the Ob artesian basin possess heads exceeding 40, more rarely 60 m.

The water yield of the deposits of the complex, depending on their lithological composition and stratigraphic assignment, is varied; well discharges are 0.002–18.2 l/sec at drawdowns of 0.3–40 m. Specific well discharges are 0.002–4.0 l/sec and more. As a rule, well discharges are greater than 1.0 l/sec. Spring discharges are 0.01–4.0 l/sec; in the valley of the Shegarka River near the village of Kolichkino a spring has been recorded with a discharge of 22.5 l/sec, the height of whose fountain, 10–15 cm above the ground surface, is apparently due to the artesian waters of the continental Oligocene [10]. Good filtration properties, and consequently high water yield, are possessed by sandy, gravel-pebble, and pebble Upper Quaternary deposits of above-floodplain terraces; well discharges are 0.8–3.3 l/sec at drawdowns of 0.3–8.4 m. The sandy-gravelly deposits of the Upper Oligocene also have good water yield; well discharges are 0.45–2.5 l/sec at drawdowns of

drawdowns of 0.3–22.5 m; as a rule, the specific yields of wells are more than 1 l/sec. The sandy-gravelly-pebbly Lower–Middle Quaternary deposits have uneven, and often low, water-yielding capacity; the yields of wells in them are 0.01–3.5 l/sec at drawdowns of 0.4–37 m. The yields of wells exploiting artesian waters of the Novomikhailovka Formation are 0.01–4.3 l/sec at drawdowns of 0.21–45 m; the specific yields of wells rarely exceed 1 l/sec, with their greater water-yielding capacity noted in the Ob–Tom interfluve. The water-yielding capacity of the deposits of the lower aquifer subcomplex of the Novomikhailovka, Atlym, and Yurkovo formations of the Ob artesian basin decreases considerably westward; these deposits are characterized by increased water-yielding capacity within the Ob–Tom and the western part of the Ob–Chulym interfluves, where well yields are 0.2–18.2 l/sec at drawdowns of 3.5–26 m.

The waters of the saturation zone of the aquifer complex are bicarbonate calcium, calcium-magnesium, more rarely calcium-sodium and sodium-calcium, with mineralization of 0.1–0.6 g/l; apparently owing to surface contamination, in a number of cases the waters of Quaternary deposits are variegated in composition, have increased total mineralization (up to 1 g/l) and hardness, and nitrates and nitrite ions are noted in them. Ferrous iron is often present in the waters of the complex, its content reaching 10 mg/l. The waters of the complex are widely used and are recommended for water supply.

Interstratal artesian waters of the Upper–Lower Cretaceous deposits of the Sym, Simonovo, Kiya, and Kiyalino formations have been combined by us into the second major aquifer complex and have been studied much less thoroughly, which makes it necessary to give their joint characterization. They are confined to sands and loose sandstones, occurring at depths of 50–300 and more meters, increasing with distance from the Paleozoic framing. Owing to the presence of fairly thick clay beds, persistent along the strike, the aquifer complex is divided into a number of hydraulically connected aquifer horizons, up to 60–80 m thick. In the Chulym basin the clay horizons are of subordinate importance; here the entire aquifer complex may be regarded as a single hydrodynamic system, reaching a thickness of 400 and more meters.

The aquifer complex has been penetrated and tested by a number of wells in the western and northwestern part of the area, in the peripheral part of the Ob artesian basin. The waters of the complex are highly pressurized; in the valley of the Ob River the piezometric levels are established at +3.7–+10.85 m above the well mouths, and on the interfluves at a depth of 1.4–34 m [3, 9, 10, 11]. The head of the waters, amounting to 113–497 m, increases with depth and with distance from the Paleozoic framing. The yields of wells penetrating the aquifer complex are 0.5–12.2 l/sec at drawdowns of 2–57 m; the specific yields of wells are 0.01–0.85 l/sec. The aquifer deposits of the Lower Simonovo Subformation and the Kiya Formation of the Upper–Lower Cretaceous have somewhat greater and more uniform water-yielding capacity; the specific yields of wells in them are 0.11–0.28 l/sec. The aquifer deposits of the Kiyalino Formation have low water-yielding capacity.

In deposits of Cretaceous age there exists a distinctive hydrochemical environment. Here, from top to bottom and from the east and southeast toward the west and northwest, with increasing distance from the peripheral part of the artesian basin, with impeded water exchange, and with replacement of continental facies by marine and coastal-marine ones, a change in the chemical composition of the waters is clearly traceable: from fresh bicarbonate sodium-

calcium-magnesium and hydrocarbonate-chloride-sodium waters, with mineralization of 0.1–0.9 g/l, to strongly saline chloride-sodium waters with mineralization of 4.5–8.1 g/l [10, 11]. The waters of the Cretaceous deposits are not recommended and are not used for water supply.

Fissure and vein waters of the Paleozoic basement of the margin of the artesian basin, which we assign to the third water-bearing complex, have not been studied.

The Sayan-Altai Hydrogeological Folded Region

In geological-hydrogeological terms, the peripheral part of the Paleozoic framing of the artesian basin (the so-called Tomsk hydrogeological massif, as we call it) is characterized by the broad development of dislocated rocks of the Lower Carboniferous and Upper Devonian of the Tom–Kolyvan folded zone, rocks of the Proterozoic and Cambrian of the Kuznetsk Alatau, and sediments of the Kuzbass, occurring at a relatively shallow depth and affected in their upper part by regional fine fissuring in the weathering zone and by more or less large tectonic disturbances and fault zones, with the presence, respectively, of fissure and vein waters, and below the weathering zone, apparently, also of stratal-fissure waters.

Characteristic of this hydrogeological region is the very limited distribution of interstratal waters in Paleogene deposits and the wide distribution of predominantly water-resistant Quaternary deposits with sporadically developed groundwaters.

Within the area under consideration two large water-bearing complexes may be distinguished [1, 9]. The first water-bearing complex includes the water-bearing Cenozoic deposits; the second, the water-bearing Paleozoic deposits.

The waters of the first water-bearing complex are confined to sandy, sand-and-pebble, and pebble deposits and occur at depths of 0.2–60 m, less often 80 m; they are overlain by loams, clays, less often sandy loams and sands, which in general determines the interstratal character of the waters. The thickness of the water-bearing horizons is 4–25 m. Where an impermeable roof is present, the interstratal waters have a head of about 0.1–33.0 m. The piezometric surface of pressure waters and the water table of non-pressure waters occur at depths of 0.11–38 m. The water abundance of the water-bearing deposits, depending on the lithological composition of their bed, is variable; spring discharges are 0.005–0.4 l/sec, sometimes reaching 0.6–0.8–2.0 l/sec; well discharges are 0.00001–1.0–2.5 l/sec with drawdown of 0.05–21.4 m. In most cases the water-bearing beds, lenses, and horizons within the first water-bearing complex are closely hydraulically connected with one another, forming a single hydrodynamic system, a single water-bearing complex.

The waters of the Cenozoic deposits are two-, three-, more rarely four-component, predominantly hydrocarbonate-calcium, calcium-magnesium; ultrafresh, fresh, and with relatively increased mineralization; fresh waters with mineralization up to 0.5 g/l predominate.

The waters of the Cenozoic deposits are used for water supply by the rural population through capped springs, dug wells, and isolated, few boreholes.

Groundwaters confined to the regional fissuring of the weathering zone form a continuous water-bearing horizon throughout the entire area of the second water-bearing complex, which is the main one recommended for water supply.

In the roof of the fissure aquifer complex lie clayey products of the weathering crust, which generally have the character of a cover and constitute an excellent aquiclude. In the valleys of rivers that in a number of places expose the aquifer complex under consideration, groundwater of the Paleozoic occurs at depths of up to 20 m, more rarely at depths of 20–40 m. With an increase in relief on the interfluves, the depth to groundwater reaches 140 m.

The fissure-vein waters of the Paleozoic are almost everywhere confined. Their piezometric surface is located at a depth of 0.5–49 m. Sometimes in river valleys the piezometric levels lie above the ground surface, causing the appearance of ascending springs and flowing wells. The magnitude of the head is 0.5–96.6 m.

The aquifer complex has been penetrated and is exploited by a considerable number of wells. The water abundance of the complex varies, which is explained by the varying degree of fracturing of the Paleozoic rocks and by the presence of zones of tectonic disturbances. Spring discharges are 0.01–0.5 l/sec, rarely reaching 1 l/sec, while well yields are 0.11–9.72 l/sec with drawdowns of 0.25–58.4 m; specific well yields are 0.004–2.71 l/sec and more. Such a sharp difference in the specific yields of wells is explained by the fact that, near wells having specific yields of about or more than 1 l/sec, waters of zones of tectonic disturbances have probably been penetrated. Within the marginal part of the Paleozoic framework, a zone of increased water abundance has been established, spatially coinciding with the regional Kolarovo–Semiluzhensk fault zone. The specific yields of wells that have penetrated this zone, as a rule, are 0.5 l/sec or more, whereas in the remaining part of the aquifer complex the specific yields of wells are less than 0.5 l/sec.

The waters of the Paleozoic deposits are hydrocarbonate calcium, calcium-magnesium waters, fresh, with mineralization up to 0.8 g/l. Sporadically there occur radon-radium waters of medium and weak activity, recommended for balneological purposes.

Taking into account all the foregoing, the fissure-vein aquifer complex of the Paleozoic deposits may be regarded as a good source of water supply, the principal aquifer complex recommended for the water supply of rural settlements and individual industrial facilities.

Thus, within the territory under consideration, a clearly expressed hydrodynamic and hydrochemical zonality of groundwater can be traced. The waters of the Cenozoic deposits of the first aquifer complexes of the Paleozoic framework and the margin of the artesian basin circulate in a zone of free water exchange, which lies within the sphere of influence of the erosional incision of the local hydrographic network, intensive drainage, and the effect of modern climatic factors. The fissure-vein waters of the Tomsk hydrogeological massif and the artesian waters of the aquifer complex of Cretaceous deposits of the margin of the artesian basin circulate in a zone of impeded water exchange, where the significance of erosional incision decreases and drainage is impeded. In conditions of very impeded water exchange, apparently, there occurs the circulation of the problematically distinguished fissure-stratal waters and waters of zones of tectonic disturbances below the zone of development of regional fracturing of the Tomsk hydrogeological massif and of the fissure-vein waters of the Paleozoic foundation of the artesian basin.

The Tomsk hydrogeological massif is considerably elevated above the adjoining peripheral part of the West Siberian artesian basin, which determines the runoff of groundwater in western, northern, and eastern directions. In these same directions there also proceeds circu-

[[unclear: beginning of word]] circulation of groundwater in the artesian basin from the marginal, most uplifted areas toward the central parts. A local direction of groundwater movement has been established toward the deeply incised valleys of the Ob, Tom, Chulym, and Yaya rivers and their large tributaries, which drain the basin and its Paleozoic framing.

In the zone of free and impeded water exchange of the Paleozoic framing, and in the zone of free water exchange along the margin of the artesian basin, as well as in the upper part of the zone of impeded water exchange of the Chulym artesian basin (in the area of recharge and significant freshening), a single hydrochemical zone A is traced—fresh oxygen–nitrogen bicarbonate waters. In the zone of impeded water exchange of the marginal part of the Ob artesian basin, zone B is traced—slightly and strongly brackish chloride groundwaters.