On the Causes of Increased Water Abundance of Rocks in Topographic Depressions in Kuzbass
Abstract Generated abstract
This paper examines why rocks in topographic depressions of the Kuzbass coal basin show consistently higher groundwater yields than comparable rocks on watersheds and elevated areas. Drawing on borehole yield data, mine inflow observations, drilling fluid samples, and lithological comparisons, the authors argue that the main cause is not simply recharge from rivers or alluvium, but a geomorphologically controlled difference in filtration properties. They propose that erosional incision creates zones of washed, fractured rock in valleys and ravines, where upward or discharge-directed groundwater movement removes clayey weathering products and enhances permeability, while fractures on watersheds tend to become clogged. The study highlights the practical importance of these natural underground drains, especially where sandstones and tectonic fractures intersect valleys, for mine drainage planning and groundwater supply in Kuzbass.
Full Text
On the Causes of Increased Water Abundance of Rocks in Relief Depressions in Kuzbass
L. A. Solomko, G. M. Rogov
(Presented by the scientific seminar of the Department of Hydrogeology and Engineering Geology)
In the exploration of coal deposits, as well as in special hydrogeological investigations in mine fields and for solving the problem of water supply in Kuzbass, a regularity has been established: the more water-abundant rocks are always associated with depressions in the relief.
This regularity is clearly traceable in all districts of Kuzbass. If on watersheds and elevated areas in the Kemerovo district the specific yields from boreholes fluctuate within the range of 0.01–0.2 l/sec, then in relief depressions they reach values of 1.0–2.0 l/sec. In the Leninsk and Belovo districts the specific discharge of boreholes in the valleys of ravines and rivers often reaches 4–5 l/sec; on elevated areas, however, this value does not exceed 0.1 l/sec. The same picture is observed in other districts as well—Tomusinsk, Erunakovo, etc.
The character of the change in specific yields with the altitude position of boreholes is clearly illustrated by the graph (Fig. 1), constructed for
Fig. 1. Graph of the dependence of specific yield on the elevation of the borehole mouth: solid line—data from testing boreholes in the Tomusinsk district; dashed line—data from testing boreholes in the Kemerovo district.
the Kemerovo and the central part of the Tomusinsk districts. The graph includes all hydrogeologically tested boreholes over both areas. For the Tomusinsk district the boreholes are plotted as points
(the upper curve, drawn as a solid line); for the Kemerovo district the wells are marked with crosses (the curve is drawn with a dashed line).
Despite a certain scatter of the points, caused by a whole number of factors, for both districts a regular increase in the specific discharges of wells with a lowering of the terrain relief is traced quite clearly.
The cause of the different water abundance of rocks in depressions and on watersheds is interpreted in different ways.
In some cases this phenomenon is explained by the greater inflow of groundwater from the watersheds; in others, by the low position of the groundwater level on the watersheds, where the most weathered part of the rocks is in the aeration zone, owing to which the aquifer here is confined to a weakly fractured complex.
There is also an opinion that the high discharges of wells in valleys are connected exclusively with infiltration of water from a river or from alluvium.
Until now, the study of this question has not been given great importance, and consequently it has not received due attention.
Nevertheless, the correct interpretation of this phenomenon is of very substantial significance in solving a whole series of questions and, in particular, in the drainage of deposits and in searches for groundwater for water supply.
The explanations cited above are, first, insufficiently substantiated and, second, they touch only on the possibility of replenishing groundwater reserves. The main question here, however, is not the conditions of recharge, but the difference in the filtration properties of rocks on uplands and in depressions of the relief.
Despite the fact that the coal-bearing sequence of the Osinniki deposit of Kuzbass is overlain by a thick horizon of highly water-bearing Jurassic conglomerates, owing to the insignificant filtration properties of the rocks enclosing the coals, the mine workings are located practically under dry conditions; moreover, artificial sprinkling of the faces is often even used.
At the same time, there are cases of increased water abundance of rocks in the complete absence of surface runoff and of overlying water-bearing horizons.
Thus, during construction of the Chertinskaya-Yuzhnaya mine (Belovo district), the water abundance of the mine workings of the eastern limb, undercutting a practically dry ravine (the work was carried out in the winter of 1956, with a complete absence of surface runoff and alluvial deposits), proved to be many times greater than in the western limb, although the geological structure of both areas is identical. Into the mine workings of the eastern limb the inflow reached 100–180 m³/hour, whereas from the western limb no more than 8–10 m³/hour of water entered.
It seems to us that the causes of the different water abundance of the rocks of watersheds and lowered areas are closely connected with the process of relief formation.
The main paths of groundwater filtration in the zone of active water exchange are weathering fractures.
In studying the character of fracturing, attention is drawn to the fact that, despite the generally chaotic arrangement of fractures, their dying out is always directed away from the ground surface. Owing to this, on watersheds, where the movement of water coincides with the direction of the dying out of fractures, the latter are subjected to colmatation by clayey material brought from the upper, most destroyed part of the section.
In areas of discharge, however, where water filtration proceeds in the opposite direction, i.e., in the direction of enlargement of the channels,
cracks (according to the principle of a reverse filter), a suffosion process takes place, causing the products of weathering to be carried outward.
Analysis of the drilling fluid when drilling the same horizons (with flushing by clean water) shows that clay particles in the fluid emerging from boreholes in the valleys of small rivers and ravines are either completely absent or are present in insignificant quantities (up to 1%), whereas on watersheds their content reaches 5–8% (Table 1).
Thus, simultaneously with the development of a river valley in the process of erosional incision, a zone of well-washed rocks with enhanced filtration properties is formed.
Because of its great extent and its spread to depth (down to 80–120 m), this zone constitutes a distinctive, powerful natural underground drain, possessing considerable dynamic reserves of groundwater and of practical interest in solving water-supply problems.
Table 1
| Sample No. | Content of clay particles 0.05 mm, % | Note |
|---|---|---|
| 1 | 5 | Watershed |
| 2 | 3 | — " — |
| 3 | 0.05 | River valley |
| 4 | 8 | Watershed |
| 5 | none | River valley |
| 6 | 0.1 | Ravine valley |
| 7 | none | River valley |
It is sufficient to say that practically all production wells in Kuzbass (with a discharge of more than 300 m³/day) tap the waters of this zone. Wells drilled on watersheds (for the water supply of collective farms), as a rule, are characterized by insignificant yields (not more than 2–5 m³/hour).
It would be incorrect to think that all rocks within zones of “washed” rocks have the same water abundance. While recognizing the leading role of the fracture type of filtration, one must not, however, neglect such a factor as the influence of the lithological composition of the rocks.
We say that any rock in depressions of the relief is more water-abundant than on watersheds, but each lithological variety is characterized by a definite water abundance within one and the same geomorphological element.
The sandstone horizons overlying the Polenovskii, Snyatkovskii, Baikaimskii, and other seams, in the valley of the Inya River (the field of the Kirov Mine, Leninsk District), and the sandstones in the roof of seam 5 in the valley of the Bachat River (the field of the “Pionerka” Mine, Belovo District), are characterized by specific discharges from wells of up to 3–5 l/sec, with filtration coefficients of up to 12 m/day.
At the same time, rock complexes of predominantly clayey composition within these same areas produce a specific yield of wells of not more than 0.3–0.6 l/sec.
In depressions of the relief, simultaneously with the suffosion process, leaching processes are activated, as a result of which, in sandstones, along with fracture filtration, a pore type of filtration also takes place.
When drilling sandstones in the core, open cracks are often observed (Raspadsky Beregovoi site, Tomusinsky District), with halos of leaching (up to 1.5–2.0 cm) from rock almost completely deprived of cement and with clearly visible pores. Under slight pressure these areas are easily destroyed, although on the whole the core is sufficiently strong.
The largest open fractures occur in zones of tectonic faults, where the latter are intersected by river valleys. As a rule, the maximum water abundance of rocks is associated with these areas; however, on watersheds, tectonic disturbances practically do not exhibit increased water-bearing capacity.
From all that has been said it follows that, in river valleys and ravines, owing to the distinctive hydrodynamic setting, the processes that promote an increase in the filtration properties of rocks are manifested most strongly. This is chiefly where the causes lie that determine the high yields of wells and large inflows into mine workings, although in individual cases the influence of surface waters is also not excluded.