Mine water (also shaft water ) is all water that is or has been in contact with underground and open-cast mines and is brought to the surface by the dewatering . The water that occurs in lignite mining is called swamp water. Depending on its origin or use, it can correspond to the respective natural groundwater quality or be contaminated with pollutants .
Although all the water that penetrates the mine workings ultimately comes from atmospheric precipitation, the miner differentiates between open-pit water and groundwater . Day water is the term used to describe water that does not accumulate in a water-impermeable layer, but penetrates the mine workings from the surface of the day through day openings . Part of the precipitation seeps through the ground into deeper layers of the earth and accumulates above impermeable layers as groundwater. A distinction is made between near-surface and deep groundwater. Because of the earth strata is in a number of areas close to the surface of the deeper ground water by ground water Slightly conductor effectively separated from each other hydraulically. Mine water is mainly groundwater that is located in the pores and crevices of the rock and from there seeps into the mine workings. The groundwater takes a relatively long time to reach greater depths . In the catchment area of the Burgfeyer tunnel of the Mechernich lead ore mining area , the age of the groundwater (mean retention times) was determined to be at least three years, but sometimes more than 100 years. In the Ruhr mining industry, there are additional groundwater. These waters do not come from the infiltration of the respective mine field , but flow in from northern areas via large cross currents. In addition, thermal deep waters rise and mix with the other pit waters.
Temperature and amount of the pit water
The temperature and amount of day water are subject to seasonal fluctuations. In summer it is warmer, in winter it is colder than the rock . Since the groundwater occurs at greater depths, its amount and temperature are fairly constant throughout the year. The temperature of the pit water rises with increasing depth according to the geothermal depth . Mine water at a depth of 1000 meters has an average temperature of 30 ° C. There are also areas where the water is significantly warmer; the deep water near Aachen has a temperature of up to 72 ° C.
The amount of pit water in a mine depends on various factors. First of all, the surface properties of the site play a major role. In mountainous terrain, the rainwater flows quickly into the valley and cannot penetrate the mine workings as quickly. It is different with mines, the deepest floor of which is below the valley surface. The weather conditions also have an influence on the amount of mine water. In regions with low levels of precipitation, there is usually less mine water than in regions with high levels of precipitation. The permeability of the mountain strata on the surface of the earth affects the amount of water in the mine, as does the structure of the deeper rock strata on the distribution of the groundwater. The amount of mine water usually increases at greater depths. How much mine water is produced differs from mine to mine. Most mines have an inflow of one cubic meter per minute. But there are also mines where the inflow of pit water is up to 10 m 3 per minute. In hard coal mining, the average amount of water to be lifted from a depth of 700 meters is around 2 m 3 per usable ton of hard coal.
The pit water is often a mixture of different waters and therefore consists of a mixture of fresh water and brine . Due to numerous mineral substances in the earth's interior, it usually has its own chemistry . Especially when the oxidation of pyrite , pyrites , copper pyrites and similar sulphides leads to acid mine water ( pH values down to −3.6 are known), the mine water is strongly mineralized. In addition to the dissolved minerals, the pit water can also carry mold with it from rotting pit wood . The mine water is partially aerated through contact with the atmosphere, which can lead to reactions of the substances in the water. The additional components of the water differ depending on the mine:
- In hard coal mines, the pit water contains not only 4–6 percent table salt, but also nickel sulphate , iron oxides and manganese. The salt content in the pit water is up to 20 percent in some mines. Iron precipitates in oxygen-rich water in the form of iron (III) oxide hydrates ("iron ocher"), which form a characteristic red-brown precipitate. Depending on the rock layer, the pit water can either contain sulphate or chloride. If these pit waters are mixed with one another, barium sulfate will precipitate . During this precipitation, the radium contained in the water is also precipitated.
- The pit water from lignite mines often contains proportions of calcium, iron oxide , zinc, magnesium, sodium, ammonium and manganese. Similar compositions are often found in the pit water from silver mines .
- Under certain conditions, the pit water from uranium mines contains traces of uranium and radium, this is particularly the case with flooded mine workings.
- But also pollutants that get into the pit water through the production process occur in it. For example, between 1979 and 1984 RAG used PCB-containing hydraulic oil in mines. Less than ten percent of it was properly disposed of. In Emscher and Ruhr, PCB was found in places where the RAG discharges its mine water.
Penetration into the mine
When driving through water-bearing layers, the water penetrates through fissures into the mine building. A water ingress is when a mine is unexpectedly flooded with water. If the operation is given up, one speaks of a drained pit. So that these pits can be used again for the extraction of raw materials, they must first be sumped . During the underground mining of mineral resources, the overburden settles. The surface layers above the mining field are loosened and gaps and fissures form through which groundwater can seep into the mine. When sinking shafts, water-bearing layers are pierced. If the manhole lining is not made carefully watertight at these points, water will penetrate the manhole. In older manholes, leaks can occur due to damage to the manhole lining as a result of dismantling close to the manhole . Another source of mine water occurrence are water bubbles that are hit during mining. Large amounts of mine water collect in old, disused mine structures , which the miner calls standing water. Standing water with a clear level to the atmosphere forms a column with hydrostatic pressure rising downwards. Water enclosed all around takes on the pressure of the overlying mountains over time. The amounts of standing water in old closed mines are very large and amount to well over 100 million cubic meters in the Ruhr area alone.
Pit water problems
The ingress of water when sinking shafts was particularly problematic. Sinking work had to be deferred again and again because the pump capacities were insufficient to discharge the considerable amounts of water. A striking example is the sinking of the Rheinpreußen 1 shaft , which took place over a period of 20 years from 1857 to 1877. The sinking work had to be stopped again and again because the pumps that were technically available at the time could not drain the water. It was not until the freezing shaft method developed in the first decade of the 20th century that the problem of water penetration during shaft sinking was solved. Another problem with pit water is standing water, which collects in large cavities and can be approached during mining. By this sudden appearance of large amounts of water can all soles are flooded and there is a significant risk to the miners. The Lassing mine disaster in 1998 was caused by such a flood.
The use of mine water generally depends on the additives that the water contains. Contaminated mine water, especially that with a high salt content, is unusable for many operational processes. While mine water used to be used in a variety of ways (salt extraction, raw material extraction through precipitation, industrial water), this is hardly common today.
The pit water from open and flooded mines is well suited for the use of thermal energy. The pit water is well warmed up due to the loosened mountains and, depending on the depth , reaches temperatures between 20 and 30 ° C. This heat can be used well for heating purposes. However, due to the only moderate temperatures, heat pump heating systems are required to use the mine water . The use of the thermal energy of the mine water has already been implemented in several projects. B. successfully tested in Ehrenfriedersdorf (Saxony) and in Heerlen / NL. In Heerlen, the pit water is used to feed a cold local heating network . For use, certain aspects of licensing law must be taken into account. There are essentially two methods for obtaining geothermal energy: the double and the single probe system. With the doublet system, a larger amount of energy can be obtained continuously, the single probe system is more cost-effective.
Use of domestic water
The swamp water that occurs in lignite opencast mines is often used as service water, depending on the degree of pollution. To do this, it is clarified, if necessary, and then pumped to the industrial plants. As lignite-fired power plants are usually located in the immediate vicinity of the opencast mines, a large part of the water is used as cooling water in power plant operations. However, the sulphate content of the water must not exceed 300 milligrams per liter. In addition, the swamp water is used either directly or, after appropriate filtering, as drinking or service water for the mining operations. A significant part of the water is used for water management compensatory measures. But acid mine water can also be turned into good service water using special water treatment processes. This is particularly useful for mining in arid areas.
Use of drinking water
If the pit water is not particularly contaminated, it can be treated to make drinking water. This was already done in the middle of the 20th century in several villages in the Siegerland . There, the pit water from the disused iron stone mine Pützhorn was filtered and used for drinking water. Under certain conditions, if it complies with the requirements of the Drinking Water Ordinance or if it has been cleaned accordingly, the sump water pumped out during lignite mining can be used as raw water for drinking water treatment.
After it has been raised, mine water is usually discharged unfiltered into the next receiving water . Depending on the composition of the mine water, this can disrupt the chemistry of the surface water and result in lasting disturbances for the environment. Strongly alkaline mine water is not as environmentally harmful as acid mine water . If the pH value of the surface water is lowered by acidic pit water, the solubility of metals such as iron or manganese increases. The greater bioavailability of metals and metalloids leads to their accumulation in algae and plants and thus in the entire food chain. If certain toxicity limits are exceeded, this can lead to death from asphyxiation of aquatic life. Water outbreaks from mine buildings can be problematic. These can arise from the formation of standing water or from penetrating surface water in extreme events such as floods. This can lead to massive sediment shifts from the mine workings. These events cause the pollutants to be diluted due to the large amounts of water, but at certain points this can in turn lead to an increase in metal and metalloid concentrations. The discharge of radium-containing mine water leads to an accumulation of radium in the rivers. In the Old Rhine area, radium accumulations could be determined which can be traced back to the discharge of radium-containing mine water. However, the activity concentrations were often below the detection limit.
When mines are abandoned, drainage is usually taken out of service. This leads to the fact that over time the water level rises to its natural level. When the mine water floods the mine workings, the dissolution of easily soluble salts can lead to a rapid increase in the concentration of pollutants in the mine water. The rising water can have a negative effect on the backfill column of the thrown shafts. In extreme cases, the backfill column slips and the manhole breaks . In addition, the rising pit water can lift the concrete plugs of the stored manholes. Parts of the terrain that are deeper than the pit water outlet points can be flooded. In many mines, the pit water is very salty. If this mine water now enters the groundwater-bearing layers, which are used as drinking water, when it rises, the fresh water can be chemically impaired by the chloride content of the mine water . The sharp rise of mine drainage water can cause elevations come of the bottom, characterized elevation may cause damage to buildings. The rise in mine water also causes the groundwater level to rise, which can lead to waterlogging of building foundations and cellars in the affected regions, with corresponding moisture damage.
In the open pit , the water present in the deposit must be pumped out in advance, before the start of mining . This pumped-out water is referred to as swamp water in open-cast mining. Depending on the region, the water can be contaminated with different substances. In contrast to mine water collected underground, marsh water does not come into contact with the ambient air while it is being discharged from the deposit. The groundwater level is lowered by continuously pumping out the water . After pumping out of the reservoir, the marsh water is usually drained into surface water . The amounts of pumped-out swamp water vary in size, depending on the reservoir. In the entire German lignite mining industry, for example, they are between 570 million and 1.4 billion cubic meters. These quantities can temporarily lead to serious changes in the runoff regime of the rivers. On the other hand, the introduction of marsh water into rivers , especially in dry summer months, can ensure that the water flow in the affected rivers remains guaranteed.
There are several methods for cleaning mine water, each of which is tailored to the individual water polluting substances. A basic distinction is made between passive and active mine water purification. With passive mine water purification, only natural energy such as B. Solar energy or bioenergy are used to improve the quality of the mine water. With active mine water purification, the water quality of the mine water is improved through the continuous use of chemical reagents and the permanent use of energy. There are also combined systems in which a passive system is connected after the active mine water cleaning or system combinations in which the active system follows a passive system. Which methods are used depends primarily on the impurities in the mine water. The costs for the respective cleaning process are also a significant factor. For example, the costs for active processes are often very high, so that passive processes are used for the required cleaning whenever possible. If high quality water is required and there is not enough drinking water available, the costs for the water treatment of the mine water only play a subordinate role. Suitable methods for active mine water purification, depending on the contamination of the water, are sulfate reduction , bio-desalination, ion exchange and various membrane processes . Depending on the chemical composition of the mine water, anoxic carbonate channels, open carbonate channels, aerobic wetlands, anaerobic wetlands and reactive barriers are used as passive cleaning methods.
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- ^ Association for the Environment and Nature Conservation Germany (Ed.): Brown coal in the Rhineland. The example of Garzweiler II. Düsseldorf 2017, pp. 6-8.
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- ↑ a b Horst Märten: The latest trends in active water treatment and application examples. In: Technische Universität Bergakademie Freiberg, Institute for Geology, B. Merkel, H. Schaeben, Ch. Wolkersdorfer, A. Hasche-Berger (Eds.), Scientific Communication, No. 31, Treatment Technologies for Waters influenced by Mining GIS - Geoscientific Applications and Developments , Proceedings volume for the workshops at the Geological Institute of the TU Bergakademie Freiberg, 22 + 23 June 2006, ISSN 1433-1284, pp. 13-21.