Formowanie się zasobów wód podziemnych w utworach węglanowych triasu śląsko-krakowskiego w warunkach antropopresji

Abstract

There are three dominant factors that determine conditions in which groundwater resources are formed: intensive groundwater drainage by wells and mining, urbanization and industrialization. As a result of these impacts groundwater flow systems and groundwater balances have been modified. Long-lasting and intensive groundwater abstraction leads to increase in groundwater recharge. It is due to intensification of existing natural recharge sources and introduction of new sources such as infiltration of surface waters, leakage from shallow aquifers and water inflow from other aquifer systems. At the same time groundwater discharge to rivers and neighboring aquifers decreased. Additional recharge sources, such as water losses from water mains and sewerage systems, and diffuse seepage of sewage occur in urban areas. The region of Upper Silesia is characterized by the highest in the country degree of human impact, including also groundwater resources. Triassic carbonate formation covering an area of about 4000 km2 is the most important and valuable sources of potable water for the region. It is drained by numerous wells, which amount more then 500, and by still working or abandoned Zn-Pb ore mines. Total abstraction of groundwater from this formation is about 10,6 m3/s (ca. 328 million m3 per year). As a result water table in Triassic aquifer declined by 40-70 m maximum in well fields and by 100-260 m in Zn-Pb ore mining areas. The aim of the research presented in this paper is to characterize the conditions and factors controlling sources and quantity of groundwater resources in Silesian- Cracow Triassic carbonate formation, in conditions of human impact. The following detailed tasks were necessary to reach this purpose: 1. Identifying groundwater flow systems, characterizing their structure and hydrodynamic functioning in conditions created by groundwater abstraction. 2. Identifying recharge sources and groundwater discharge, their quantitative estimation aimed at comparing water balance and groundwater resources for predevelopment and actual conditions. 3. Estimating impact of groundwater abstraction on changes in groundwater flow systems and formation of groundwater resources in carbonate Triassic aquifer. 4. Estimating the significance of low-permeable strata in formation of ground-water resources in carbonate Triassic aquifer. The basic method used to solve the problem was groundwater modeling, based on long-term field works. Simulations were performed for steady state hydrodynamic conditions related to the abstraction of groundwater at the end of the 1990s. Besides, there were performed simulations of groundwater flows for the pre-development conditions. The comparison of obtained water balances allowed for assessing the changes in groundwater resources induced by intensive anthropogenic drainage of groundwater by mining and wells. The research comprised Silesian-Cracow Triassic carbonate formation, the area of 4000 km2. The main aquifer of the region is composed of limestone, dolomite and marl of Triassic age (Roethian and Muschelkalk) with the total thickness varying from 20 m to more than 200 m. It is porous-fissured-karst aquifer, disrupted by numerous faults. Hydraulic conductivity of water-bearing formation, determined by pumping tests ranges from 10'6 m/s to 10’4 m/s. Nearly impervious substratum of the aquifer comprises marl, clay, mudstone, siltstone of Early Triassic, Carboniferous and Permian age. Triassic carbonate aquifer is overlain, within the area of about 2700 km2, by Upper Triassic and locally Tertiary clays of very low permeability. Its thickness ranges from 10 m to more than 200 m. Local shallow Quaternary and Jurassic aquifers mainly in the northern and eastern part of the region are present. Groundwater of Triassic aquifer is exploited by wells and zinc and lead ore mines. Mining activity is developed in four regions of: Tarnowskie Góry, Olkusz, Jaworzno-Chrzanów and Bytom. Maximal depth of mining exploitation varies from 60 m to 280 m. In recent years mining has drained ca. 199 million m3 of water per year from Triassic formation. There are ca. 500 wells, which are mostly single well, with capacity varying from 10 m/h3 to more than 600 m3/h. There are also 10 well fields with discharge above 10 000 m3 per day. Maximum water abstraction by wells observed in the middle 1980s was ca. 184 million m3 per year. At the end of the 1990s this abstraction decreased by 30%, to the level of 127 million m3 per year. Total water abstraction by mining and wells in 2000 was 326 million m3 (10,4 m3/s). Mining activity shares 52—54% of this value. Five major Triassic aquifers have been determined in the considered region, basing on geological structure and groundwater flow systems influenced by longterm anthropogenic drainage of groundwaters: Lubliniec-Myszków, Gliwice, Bytom, Olkusz-Zawiercie and Chrzanów. They comprise independent groundwater flow systems, partly open for lateral water flow. In three aquifers: Chrzanów, Bytom and Olkusz-Zawiercie groundwater are drained by mining and wells. In the two remaining - groundwater is exploited by wells only. The exploitation to the largest scale takes place in the Olkusz-Zawiercie aquifer - totally ca. 176 million m3 per year, and to the smallest one in the Chrzanów aquifer - 26 million m3 per year. Total groundwater resources of the Silesian-Cracow Triassic formation in the area of 3808 km2, in present hydrodynamic conditions influenced by anthropogenic discharge, constitute 12,82 m3/s, and drainage of these resources, controlled by wells and mine workings is 10,4 m3/s. These resources, divided into five Triassic major aquifers, amount to (values in brackets show water abstraction): in the Lubliniec-Myszków aquifer - 2,51 m3/s (1,55 m3/s), in the Gliwice aquifer - 1,51 m3/s (0,86 m3/s), in the Chrzanów aquifer - 1,17 m3/s (0,84 m3/s), in the Olkusz-Zawiercie aquifer - 6,5 m3/s (5,6 m3/s), in the Bytom aquifer - 1,41 m3/s (1,41 m3/s). High differentiation of groundwater resources of examined major aqui-fers is justified not only by different hydrogeological conditions, but also by the amount of anthropogenic drainage of groundwater in these aquifers. It is confirmed by proved linear correlation between groundwater resources and groundwater discharge, expressed in dm3/skm2, established for these aquifers. In comparison to average annual precipitation groundwater resources in major Triassic aquifers, mentioned in order given above, are accordingly: 4,8%, 14%, 17,4%, 28,2% and 31%. High values for two aquifers, Olkusz-Zawiercie and Bytorn result from: favorable conditions of recharge from surface, intensified percolation of water from surface due to mine drainage, important losses of water from surface water courses, from water mains and from sewerage system within mining and urban areas. Actual abstraction of groundwater is close to or higher than pre-development recharge rates. It resulted in important increase in groundwater resources in these aquifers by means of increased recharge and reduced natural discharge of groundwater to rivers and other aquifers. Basing on simulation of groundwater flow systems for pre-development conditions, it has been defined that increase in recharge for examined aquifers is: in the Lubliniec-Myszków aquifer - 53%, in the Gliwice aquifer - 41%, and in the Chrzanów aquifer - 21%. This increase in recharge together with reduction in discharge cover actual groundwater abstraction from these aquifers. The main sources of groundwater resources exploited at present are differentiated depending on an aquifer. In the Lubliniec-Myszków aquifer 57% of abstraction is supplied by increased leakage from shallow aquifers, with ca. 125% as compared to the pre-development conditions. It is connected with the fact that ca. 79% of the area of this aquifer is overlain by semi permeable layer. The dominant source of recharge in the Gliwice and in the Chrzanów aquifers is the reduction of natural discharge to rivers and lateral outflows. In relation to pre-development conditions it amounts to 58% and 75% respectively. Dominant factors that determine formation of groundwater resources of Silesian- Cracow Triassic formation are following: the degree to which carbonate aquifer is covered by low-permeable deposits, recharge from precipitation, water infiltration from rivers and leakage from shallow aquifers. In urbanized areas additional anthropogenic water is of some significance: leakage from water supply and sewerage systems as well as diffuse sewage infiltration. Low-permeable deposits which overlain carbonate aquifers comprises 79% of area of the Lubliniec-Myszków aquifer, 21% of area of the Gliwice aquifer and 49% of area of the Chrzanów aquifer. Basing on detailed groundwater modeling results for the Lubliniec-Myszków aquifer, vertical hydraulic conductivity was specified for the low-permeable layer. The thickness of this layer changes from 20 m to more than 300 m and its regional hydraulic conductivity ranges from IO'10 to 10‘8 m/s. These values are higher than those obtained in most laboratory tests for small rock samples. Permeability of aquitard and their thickness, as well as vertical hydraulic gradient determine the intensity with which aquifers are recharged by leakage. This intensity in the mentioned three major aquifers on average amounts to: 26 mm/per year, 31 mm/per year and 61 mm/per year respectively. Variability of leakage in the Lubliniec-Myszków aquifer vary from 4 mm/per year to 102 mm/per year. Recharge from precipitation in outcrop areas of carbonate Triassic aquifer changes from 96 mm/per year in the Gliwice aquifer to 141 mm/per year in the Chrzanów aquifer, and 155 mm/per year in the Lubliniec-Myszków aquifer. In comparison to precipitation it constitutes: 12,2%, 18,1% and 18,7% accordingly. These are global values, referring to the outcrop areas, defined by model research. According to analytical estimation based on water level fluctuations in observation well the recharge rates of carbonate Triassic aquifer by precipitation ranging from 714 mm to 974 mm per year vary from 60 mm/per year to 260 mm/per year, and it is 130 mm/per year on average. It constitutes accordingly from 8,4% to 26,7% of annual precipitation. These values correspond to the values obtained by modeling. Analytical calculations of recharge rates of carbonate Triassic aquifer performed for seven man-made drainage centers, presented in Tab. 7.5, have been based on water inflow to mines or well field. The total recharge rates vary from 158 mm/per year to 440 mm/per year. It constitutes from 19,7% to 55% of annual precipitation. It is however total recharge, coming not only from precipitation but also from surface water courses and anthropogenic water in urban areas, as it was mentioned above. The highest recharge rates refer to the areas characterized by the most intensive human impact on hydrogeological conditions, localized in the Bytom and in the Olkusz-Zawiercie aquifers. Both the results of groundwater modeling and direct measurements of losses of water from rivers point at participation of surface water in recharge of carbonate Triassic aquifer. It is however impossible to compare these results. Basing on performed modeling results it is estimated that share of surface water in total recharge of analyzed Triassic major aquifers amounts to 7-10%. Examinations of environmental tracers in groundwater in the Lubliniec-Myszków aquifer confirm the groundwater flow system assumed in this work and proved by performed research. The occurrence of glacial water on significant area of this aquifer and travel time estimation of vertical seepage through the semi permeable layer, which ranges from 10 000 to more than 80 000 years, indicate that leakage from shallow aquifers is of great importance in groundwater resources. Performed examinations prove that the area of Silesian-Cracow Triassic is divided between two large, regional hydrogeological systems - northern and southern one. They are separated by groundwater divide the localization of which partly differs from localization of surface water divide. They belong to two surface water systems of the Odra and Vistula rivers. Northern hydrogeological system comprises the following major aquifers: Lubliniec-Myszków and Gliwice. It is the system where anthropogenic drainage of groundwater is of diffuse character since it is connected with numerous wells. The southern system includes the Bytom aquifer, the Olkusz-Zawiercie aquifer and the Chrzanów aquifer. Here groundwater drainage is concentrated mainly around Zn-Pb ore mining areas. Locally it is supported by water discharge through hard coal mines. In present conditions hydrodynamic functioning of both regional groundwater flow systems is practically independent. Groundwater discharge from northern system takes place generally westward. In southern system groundwater discharges mainly to inner man-made drainage centers, formed around mining areas. Major groundwater aquifers determined within the examined area of Silesian- Cracow Triassic formation constitute, together with shallow aquifers, independent groundwater flow systems. They are multi-layered systems, with Triassic carbonate aquifer as the major component. One or more overlying shallow aquifers are present and they are separated by semi permeable layers. These are systems partly open for lateral groundwater flow through their boundary but they have, however, internal discharge zones. Complex hydrogeological structure of discussed Triassic aquifer systems, as well as complex relations of these systems with surface intensive and human impact cause that the water balances of these systems are very complex. This is expressed by water balance equation (6), in which the total recharge comprises: 1) effective infiltration from the surface, consisting of recharge from precipitation, losses of water from small water courses, leakage from water mains and sewerage system and diffuse infiltration of sewage, 2) downward leakage from shallow aquifers, 3) lateral inflow from other aquifers, 4) upward leakage from Paleozoic aquifers, mainly Carboniferous and Devonian. The components of discharge are following: 1) abstraction of water by wells, 2) abstraction of water by mines located within the Triassic aquifers, 3) discharge to surface water courses, 4) upward or downward leakage from Triassic aquifer to other, overlying or underlying aquifers, 5) lateral outflow to neighboring aquifers.