Application of a continuously operating Model for Forecasting a

APPLICATION OP A COITIBUOUSLY OPERATING MODEL POR PORECASTiliG A REGIONAL GROUNDWATER FLOW SITUATION CAUSED BY OPEN-CAST MINE DBWATERING MEASURES

Peukert, D., Reisner, H. and,Kunze, W.

Research Group for Open-Cast lline Dewatering of the GroBrtieehen Institute for Lignite Mining and the Drea4en Technical Uninrllity, GDR

ABSTRAC'l'

The groundwater flow in the south-eastern part of the region Lusatia (Lausitz) is influenced especially by opencast mine dewatering measures. Por forecasting tha expected groundwater conditions in this region a modelsystem wa9 developed, that is capable of taking into account factors influencing the regional groundwater flow proceae. This model system, celled a continuously operating model ( 0011), is opere ted parallel as to t illle, but discontinuously 1m comparison with the running origiDBl process.

'lhe .-odel -• calibrated for th-e per1.od froa 1975 to 1980 by means of -&8\lreaente of water levels and mine -ter pumpage. According to that, the fUture calculations was carried out for the period from 1980 to 2000. Here, the most important reeul ts are

1. future development of the groundw'!ter table

2. future amount of mine water pumpage

). forecast of the influence on the regional gz-oundwater flow of the operation of the newly formed ramaint:ng pits used for water re~oureee management or d:!.apoeal of industrial WQStes

4, :t'orec.aet o:t' the eff'ecte of mine dre:i.nage and use of pits on th~ operation of watarwori<:&.

IMWA Proceedings 1982 A | © International Mine Water Association 2012 | www.IMWA.info

Reproduced from best available copy

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1. INTRODUCTION

All industrialized countries are facing the problem of a steadily increasing demand for water. Its solution leads to an ever more intensified utilization of the natural water resources. This general situation is particularly true for the GDR where already now the water resources are very intensively used. Groundwater is the major source for supplying water of a high quality to population, industry and agriculture.

To an ever increasing extent the groundwater reeourceB are endangered both by accidental spillages of chemicals, oil, fuel, radioactive materials a .. d by planned measures such as the use of chemical fertilizers in agriculture, the disposal of wastes, the underground storage of oil and ua·tural gas etc. Moreover, the groundwater flow ie strongly affected by several man-made impacts. Among them, the lowering of the groundwater table by open-cast mining is most important , also affecting the whole water balance of large regions.

Due to the long-term lowering of the water table in large areas cones of depression are formed that inf~uence the operation of waterworks aud the utilization of areas of agriculture and forestry as well as cause the drainage of ponds and rivers. Not only the lowering of the groundwater table but also its rise after closing down mines has to be taken into account. Then, often the negative effects of the open-cast mine dewatering for several water users are often compensated for. An effective management of the newly formed remaining pits has a particularly important influence on the controlling of the rise of the water table after having closed down mines.

There is no doubt that the controlling of man-made impacts on the groundwater resources require complex economic and social decisions taking into account the conflicting interests of several branches of tne national economy. An important input into such decisions is sufficient information about the development of regional groundwater flow situations to be obtained by means of modelling.

2. GROUNDWATER RELATED PROBLEL~ IN THE SOUTH-EASTERN PART OF THE LUSATIA RE~ION

Problems generally discussed in section 1 of this paper can be found especially in the south-eastern part of the Lusatia (Lausitz). This region of about 1,300 km2 situated between the rivers Scnwarze Elster and Neil3e is one of the most important lignite centres of the GDR. About 25 % of the whole output of lignite of the GDR are produced here from open-cast mines.



Due to dewatering measures carried out in those open-cast mines, the groundwater flow is especially strongly influenced , because the water table depressions of many mines in a relatively small area superpose each other (see fig. 1). Additionally, this superposition of water table depressions is permanently changing in time and place due to the progress of the mines. Therefore, the difficult forecast of effects of lowering the water table on the operation of municipal and industrial waterworks and on mining dewatering well systems is additionally complicated in this region. On the other hand, necessary decisions about providing systems capable of compensating for the negative effects to be expected depend largely on the accuracy of such forecasts.

In other parts of the region in question the process of groundwater table lowering caused by open-cast mine dewatering measures is already superposed by the rise of the water table, resulting from closing down dewatering systems of exhausted mines as well as from tne operation of newly formed remaining pits due to Vlater-management or industrial utilization. Taking into account the development of the rise of the water level in the remaining pits and the planned final water levels, we have to calculate effects on the amount of water to be pumped in the mines operating in the immediate vicinity of the remaining pits. The extent of this influence cannot be known correctly from the beginning,. but is expected to require a considerable financial expenditure.

For making decisions on possibly utilizable water discharges for flushing the remaining pits it is neceesa:r¥ to know the run-off balance of the rivers in this region for a long-term period. This run-off balance is decisively determined by the dewatering measures of the open-cast mines both by changing the catchment area of rivers and aquifere and by discharging the pumped water into rivers. At present, it is not possible to give detailed information about the development of the run-off balance of rivers under conditions of superposed water table depressions of the individual open-cast mines and the rising water table in the vicinity of the remaining pits discussed before. The explained problems that are closely interrelated are shown to necessitate comprehensive investigations about the development of the groundwater flow in this region, taking into consideration all influencing factors in order to derive concrete decisions for an optimum investment policy.

). SELECTION AND ESTABLISHMENT OF A MODEL SYSTEM

Due to the complexity of effects influencing the groundwater flow system, tne implementation of control measures



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derived f:;:om a single simulation of the groundwater flow process doea not lead to satisfactory resulta in moat ~asea. Por the forecast of the groundwater de'Jelopment for longer periods a multiple modification of the influencing factors, caused by new technical and technological or economic characteristics aa well as administrative decisions must be expec·ted from the beginning. Consequently, the knowledge about the behaviour of the system needs to be contL~uously ~~proved.

In order to be able to taKe into account the variability of the influencing factors, the method of the contimwusly operating models (.COM) ia presently used, which allows us to organiZcl the proceas:!.ntJ; accordine; to cybernetics~ aspects. According to /11, /2/ and /)/ a continuously \~orking model iL~ a model sye·tem for the monitoring arJd controlling of long-term. industrial or economic proceasee (see also ! 4/ and /5/). It mainly consists of two parts - a apecif·ic data bank tor .info:rmatio!1 storing and a simulation model tor information processing - that are coupled by an analoque or digital simulation method, which is suitable for practical data processing. The system of da:tr, collection, data processing and simulation includ~e tha f'eed.hack of inform.'ltio.o used already during the siml.<lsti<'r: of the particular groC~ndwater flow process or becoming effective during the implementation phase. In such a way the simulation ia gu:u•ar.rtliH'>d to be baaed on the latest data available at th® time of the O?erating of tne mods syatem. By meana of this model ayatem the influence of ter~sible control measure111 can 1-Ja simulated before their implementati<Hl i;;, pracrticz, 'l'ha model sya-t.em ia operated panllel in time, but diacorrt l1:1uously with the :runnitlg original p:t•oc~l!lfl. 1'he ste~e i.ta cpe:ratir;,'! ctepend on th ll'liztl! important~<~ of tt.<!! mo<.1;,l a<Je /4/ and /b/;.

Suer. ~ con )~':.: lO'i.HllY opeHlti.ng model was estab1iahed for tne S'Juth-Eawt Luaai;:!Jll regl.or;. D!!taHa ab<y.::·t informmt1.cm !iiitoring e.nd th~ aimulat:to:n model are de~:or:ibed in /71 and /8/, For the simulation of the regional ci.evelopment of t':"' groundwater flow a two-dimeneionr,l non-etl!I<Hly model W':t!l chosen ·- the HOREGO pr.og:r'Wll. I.t :L;, lmplan1entaa at th<' electronic com!mter ES 1040 or ES 1055 rerm. 'i'h:!.a is based on & mathem...<ttieal model of ·t.he non-a"teady zontal-plane groundwater flow with non-linear paramt~tex·!l! of permeability /9/. The diBcr,;t:J.zation of the flow f:l."ld :I.e done by a grid of orthogonal finite elelemts, The m<mbc:<r of finite elemente is limited only by the CPU memory cf tbc computer used for aimulat~_on. The single elements have to h® arranged in such a way tn'at a minimum nllillbe!' of eleuumtl'l leado to sn opti.m.al adapt 1.on of the model to the inte:rna,l and external bounde.:cy-condit:l.one. In figure 2 th<a grid us~cc for tne South-East Lusatia region is shown.



The following boundary-conditions were considered in the model system for simulating the period from 1975 to 2000

_ development of all open-cast mine dewatering measures as to time and locations

_ operation of all existing and newly formed remaining pits, used for water resources management or:disposal of industrial wastes

_ operation of all waterworks including their planned development

- operation of irrigation systems for agricultural purposes

- natural recharge of groundwater depending on the operation of mines, including their reclamation (this developaant was separately investigated before by means of a special program)

- infiltration/exfiltratiou of rivers and ponds

The model was tested and calibrated for the period from 1975 to 1980 after the installing of all boundary-conditions. Measurements of water levels in gauge wells and measurements of water pumpage in the individual open-cast mines were used for this purpose.

The calibration of the model system was carried out by means of the trial-and-error method. In this process these were changed mainly the transmissibility in single elements and the infiltration or exfiltration of rivers and ponds which is unknown in most cases from the beginning. The degree of the obtained correspondence between measured and calculated results of groundwater levels is shown in figure 2 as an example for 1976. The absolute size of deviation must be seen in connection with tne amount of groundwater lowering at the single elemenis and with tne relatively rough discretization of' the field. The obtained correspondence for the mine water pumpage is shown in figure 4 also as an exawple for one of the mines.

4. RESULTS OP ThE MODEL FORECASTS

In the following, the calibrated model was used for foreoasiing the regional groundwater flow procese with regard to all influencing factors for the period from 1980 to 2000. Of course, the reliability of the model resultll is decreasing with increasing time horizons for predictions. Therefore one muat aim at extending the period of calibration when the model system is operating for later prediction phases.



214

4. 1. DEVELOPME:NT OF GROUiWWATER TABLE

The results of the forecasts of the development of the groundwater table in the region of interest were obtained in the form of water table mapa for 1980, 1985, 1990, 1995 and 2000. Figure 3 shows an example for the year 2000.

When we know development of the regional groundwater table, it is possible to determine the, influences on the dewaterine plants of the single open-cast mines by each other and by remaining pits. Therefore the new information gained about the boundary-conditione as to time and location is precisely known for the calculation of these plants. Based on this, a more economical arrangement and equipment of the dewatering systems as well as an increased protection of the mines against water hazzarda can be achieved. Furthermore the knowledge of tne development of the regional groundwater table permits a more precise establishment of mining protection areas. The water table maps also provide information on tne dates and extent of the limitations to be expected for the operation of the wells of the waterworks which are situated in the influenced area. Dependent on this, decisions can be made in time on the type and extent as well as the implementation time of possibly requir-ed replacements.

4.2. DEVELOPMENT OF THE OPEN-CAST MINE WATER PUMPAGE

As a further result of the investigations on the development of the groundwater flow is the South-East Lusatia region by means of the discribed model system the amount of mine water pumpage of the open-cast mines situated in the interesting area could be determined. In fieure 4 an example is shown for one of these mines. We see clearly the difference between the development forecasted by the regional model system taking into consideration all factors influencing the groundwater flow (line 2) and the development, determined by a former calculation without such considerations (line 3). Line 1 shows the development of the amount of pumpage, as it was actually given out by the model. The difference to the development of line 2 appears to be the share of bottom water that is pumped out of the wells of the dewatering systems and that is impossible to be simulated by the sinrulation-pru(;ram used at present (see /8/ and /9/.

The knowledge of the development of mine water pumpage enables the lignite factories to economically design treatment plants and discharge systems for mine water. It forme also a major tool for the water authorities which are responEJible for balancing the discaarge into the river net-· work.



Based on this process, information on the available water discharge to be used for filling remaining pits can be provided. Provided that the results obtained are in good agreement with those achieved by the long-therm forecast of the regional groundwater flow, final decisions on the filling rules for remaining pita can be made. Otherwise a simulation run has to b1 repeated using the neweat data available.

4.3. OPERATION OF REMAINING PITS

The simulation of the regional groundwater flow conA:I.dering all influencing factors provides important information on the operation rules of remaining pits and their impacts on the surrounding territory or on dewatering systems of neighbouring open-cast mines.

Figure 5 shows two stages of the course of the filling process to be expected in a big remaining pit situated near the centre of the area in question (see fig. 1). We see that it ie impossible to reach the planned upper or lower level of the water table necessary for a utilization for water management or recreation when only the natural rise of the groundwater table in the surrounding areas after closing down the mines contributes to the filling process (line 1). On the other hand, the lower level of the planned water table can be reached in 1998 and the upper level in 2001, proyided that an additional amount of water of about 100 Mio mJ/year will be available (line 2).

Figure 6 shows the influence of the rise of the water table in the same remeinine pit on the dewatering works of a neighbouring open-cast mine. The western well fields of the mine situated east of the remaining pit are planned to continue operation, in order to avoid water inflow into the dumps.

We can see that nearly 30 million cubic metres of water per year will have to be pumped additionally by these wells when the water table in the remaining pit will reach the level at 118m NN. This cannot be accepted because of the large amounts of material aud energy to be spent. Thus, a different solution must be found. Possible are variations in the mining technology, the changing the dewatering method, the establishing of other water tabel levels to be reached in this remaining pit etc. The newly created conditions form new boundary-conditions to be eonsidered in future eL~ulation runs.

5. CONCLUSIONS

The described problems in the South-East Lusatia region cannot be solved by a single simulation of the regional

215



groundwater flow process. Multiple modifications of the various influencing factors caueed by new technical and technOlogical ae well as economic chances must be expected from -!;he beginniag. '?he problems to be investigated require a contim.:ous access to a model system that repr~eents the most topical state of the original aystem at aay point of time. This requires both, the collection of new information on the system of the groundwater flow and the feedback of infonuation obtained in tne phase of implementing control measures. Consequently a continuously operating model has to be establi.shed.

AcQordir~ to this, it is obvious tnat the work on the continuoufily operating 1110del South-Ea!!t Luflatill cannot be considered a;& being finished. At prese.nt tMs mod!;:~ system in already operating in its thi::d •~>~o x·ldng ph:~.se-, but still a lot of new conditions caused by changi11g of conceptions of lignite exploitation have to be expected..

Besides this, the simulation results obtained eo far have contribu:~ed ve17 much to an improved decision-making pl:'oceae in the lignitt! mining industx"Y, in water rnanagemeJJt and environme-ntal planning as well as in municipal auttw-rities. Because of these facts it is one of the main tasks of the research gNllp for open-cast mine d~watering to hold thie model system continuously in operation and to improve its capabi.li ties.



REFERENCES

/1/ LUCK!l'ER, L. Nutzung sHindig arbeitender Madelle zur Steuerung regionaler Grundwasserstromungsfelder. (The Use of Continuously Operating l•iodels for Controlling Regional Groundwater Flow) Beitrag zur Tagung der kybernetischen Kommission des Forschungsrates der DDR, Oberhof 197J

/2/ PEUKERT, D. Beitrag zur Steuerung von groBraumigen Grundwasserabsenkungs- und -wiederanstiegsvorgtingen mit stundig arbeitenden Modellen. (Contribution to Controlling Regional Processes of Lowering and Rising of the Groundwater Table by J,:eans of Continuously Operating Models) Berg- und HUttenmannischer Tag, Freiberg 1975

/J/ PEUKERT, D. Neuere Verfahren zur Steuerung von Tagebauentwasserungsprozessen in der DDR unter besonderer Be~icksichtigung der Anwendung standig arbeitender L:odelle. (Newer Methods of Monitoring Open-Cast Aline Dewatering Processes in the GDR with Special Regard to the Application of Continuously Operating Modele) VII. Grubenwasserschutz-Konferenz, Budapest 1976

/4/ PEUKERT, D. Das standig arbeitende Modell Ostlausitz - ein Grundwasserleitermodell zur komplexen Bewirtschaftung von Wasserressourcen und zur territorialen Planung. (The Continuously Operating !.Iodel East Lusatia -A Model for Complex 1Janagement of Groundwater Resources and for Regional Planning) die Technik, J4. J g. , Heft 9, September 1979 s. 499 - 507

/5/ REIS!l'ER, H. and PEUKERT, D. Stand der Anwendung standig arbeitender Madelle in der Praxis der Tagebauentwasserune. (The State of Application of Continuously Operating Models in Open-Cast Mine Dewatering in the GDR) Neue Bergbautechni.K, 8, Jg., Heft 2, Februar 1978

/6/ ZWIRNMANN, K.-H. 1lodelling Strip .Mining Impacts on Groundwater Resources for Use in Coal Development Planning In: Vorontsov, N. (Ed.), 1980 Szczyrk Conference Papers-Part II, Coal-Issues for the Eighties. IIASA-CP-80-24



/7 I 7.'1/IRNMANN, K. -II. ; BEI/.IS, u. c1n<l PEtJKBH'r. D. Rationelle Speicherung, Aufbereitung und Uereitstellung von k-Wert-De.ten am Beispiel dee SA!.! Ostlauoitz. (Effective Storage, Handling and Use of PermeabilityData for the COM East Lueatia) Zeitechrift fUr A.ngewandte Geologie, Jg. 26, Heft 9, September 1981

/8/ GUTT, B. and PEUKERT, D. Anwendung dee Programmsyeteme IIOREGO zur Simultation gro3raumiger geohydrauliecher Prozesse am Beispiel dee standig arbeitenden Modelle Oetlausitz. (Application of the Computer Program HORiGO for the Simulation of Regional Geohydraulic Processes for the COM East Lusatia) Neue Bergbautechnik, 11. Jg., Heft 6, Juni 1981

/9/ GUTT, B.

1)8

Programmeyetem HOREGO: Berechnung horizontal-cbener Grundwaseeretromungsprobleme auf der Basis von Orthogonalnetzen. (Computer Program HOREGO: Calculation of HorizontalPlane Groundwater Flow Using Grids of Orthogonal Elements) Programmdokumentation der TU Dresden, Sektion Waeserwesen 1978



List of Figures

Figure 11 Map of Objects Affecting Groundwater Resources in South-East Lusatia

Figure 2: Comparison of Measured and Simulated Groundwater Head Values of Selected Single Grid Elements for December 1976

Figure ): Forecast of the Groundwater Table for the Year 2000

Figure 4: Forecast of the Development of Uine Water Pumpage for an Open-Cast !.line in the South-East Lusatia Region

Figure 5: Development of the Filling Process in a Remaining Pit

Figure 6: Effect of the Filling Process in a Remaining Pit on the Development of Water Pumpage in a Neighbouring Open-Cast Mine

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Application of a continuously operating Model for Forecasting a

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