SURFACE AND GROUND WATER FLOW MODELLING IN THE RESTORATION OF THE MEIRAMA OPEN PIT MINE

Transcription

1 IMWA Symposium 2007: Water in Mining Environments, R. Ciu & F. Frau (Es), 27th - 31st May 2007, Cagliari, Italy SURFACE AND GROUND WATER FLOW MODELLING IN THE RESTORATION OF THE MEIRAMA OPEN PIT MINE F. Pailla, P. Vellano, J.L. Delgao, R. Juncosa, J. Delgao an J. Fernánez Grupo e Ingeniería el Agua y el Meio Ambiente GIAMA E.T.S. e Ingenieros e Caminos, Canales y Puertos (Civil Engineering Department) Universia e La Coruña. Campus e Elviña La Coruña. Spain Abstract The interaction between the free surface an the groun water flows in a basin is a funamental issue in the stuy of several environmental problems that require a comprehensive knowlege of the balance an quality of the water flows. Nowaays there is a goo number of numerical moels in the market, that allow for an evaluation of the free surface hyroynamics, in one sie, an for the grounwater flow in the other sie. Still, there is a lack of moel skills to evaluate the free surface an groun hyroynamics in a joint way, an moreover incluing water quality criteria. The Ingeniería el Agua y el Meio Ambiente (Water an Environmental Engineering Group) group has been working in these fiels for some years an has evelope a software that allows for the numerical evaluation of this kin of problems. In this work it is shown the application of the evelope coes to the working out of a moel that evaluates the joine surface an groun water flows in an opencast mine to be restore as a lake. The so-preicte results are presente as a conclusion to shortly elaborate other alternatives in orer to restore the opencast mine as a lake. Meirama opencast mine The mining activities that have been taking place in the Meirama opencast lignite mine are to be abanone at the en of The firm LIMEISA (LIgnitos e MEIrama Sociea Anónima) has been exploiting Meirama mine since The lignite eposit is situate in the province of La Coruña, north-western Spain, 25 km from the city of La Coruña. A 550 Mw power station, fuelle by the lignites of Meirama, was built nearby to prouce energy from Meirama coals. At the moment of its closure in 2007 the mine will have prouce some 87 million tons of lignite. Mine tailings of some 160 million m 3 were move to a nearby storage tip. The mine has been proucing from 4 to 10 million m 3 per year uring the first years of the exploitation an some 3 million m 3 uring the last years. Meirama Mine Figure 1. Location an view of the Meirama opencast mine an Power station Meirama pit lake. During the exploitation of the opencast mine many environmental activities have taken place in orer to reuce the environmental impact of the mining an power generating activities. With respect to the earthwork, 350 ha have been restore an 300 ha have been reforeste. At the en of the mining activities the firm LIMEISA has planne to recover the mine pit as a lake with a maximum epth of some 180 m an a surface of nearly 2 km 2. To o so, a am is to be built at the ownwin tip with a free spillway at a height of some 180 m. The prospective pit lake is to lie on the Barcés river basin, an affects some 33 km 2. Base upon the annual mean rainfall an the consiere filling alternative, a perio of about 7 years has been estimate as a reasonable time to complete the filling of the lake.

2 Aims of the present work The management of water uring the filling an post-filling perios shows up like a funamental issue in both their hyroynamic an water-quality aspects, an eserves a close attention in orer to reuce not only the environmental impact but also many unwante effects that can take place ue to a wrong evaluation of the behaviour of such a vast volume of water. The environmental an geotechnical consequences of such a project are still uncertain. Although some effects can be assesse in terms of the previous worl-wie experiences in connection with the own features of Meirama open pit, the final consequences are not certain. Within this frame, the authors are carrying out a numerical evaluation of the behaviour of the water uring the filling an postfilling perios that may cast some light on the real performance of the water in the basin an on the environmental consequences of the filling of the Meirama Lake. This numerical evaluation is intene to assess the hyroynamic behaviour of the joine surface an groun water flows in orer to be able to shortly analyse the water quality evolution. The software to be use in the numerical evaluation inclues some coes evelope within the GIAMA group such as MELEF, HIDRAFEM, FREECORE, an some commercial coes such as SMS or TECPLOT. To carry out such a stuy, many fiel measures are to be mae that provie information about the hyrologic an hyraulic parameters of the basin, such as rainfall, piezometric levels, ischarges an geological characterizations, in terms of which some other parameters such as storage coefficient, hyraulic conuctivity, Manning coefficient an turbulent viscosity can be calibrate. As part of the carrie out work, some measures have been taken in the nearby Eume reservoir, where temperature an conuctivity at ifferent epths an at ifferent seasons have been obtaine. The Eume reservoir is of similar climatological, limnological, an geological characteristics to those of the Meirama pit lake, an in this way these ata provie with relevant ata about the prospective lake. These ata show that there is no winter thermal inversion even for tough winter conitions (see Fig. 2), an therefore, no thermal inversion is expecte in the Meirama lake. Figure 2. Temperature (lower line) an conuctivity (upper line) vs. epth in the Eume Reservoir (December 2006). Hyrogeological issues At present (February 2007) the open pit is still in use an some twenty pumping stations are place in its contours so as to withraw the water table. To prevent the water getting into the pit, two perimeter canals were built. With a mean annual rainfall of some 1500 mm, a rain capacity of m 3 /h is require to rain water from the pit, leaing it to two treatment plants with a total capacity of 10 million m 3. Once the water has been treate, it is spille into the Barcés river. Geologically speaking there are two main regions in the affecte area, one in the NW feature by a granite massif an a secon one in the SE characterize by a schist substratum. Most of the excavation works take place on a seimentary basin with Tertiary materials. The granite an the schist have a scant hyraulic conuctivity. This contributes to promote the free flow to the etriment of that of

3 IMWA Symposium 2007: Water in Mining Environments, R. Ciu & F. Frau (Es), 27th - 31st May 2007, Cagliari, Italy the grounwater. The granite is quite weathere an kaolinize, what contributes to a worse hyraulic conuctivity. These materials constitute not only the bounary of the excavation area but also, in some way, its unerlying substratum, an therefore are responsible for the hyrological behaviour. For these materials hyraulic conuctivities of m/s (schist) an m/s (granite) coul be consiere. Nevertheless, these parameters have been ajuste in a calibration process base on the water level measurements uring a perio of two years. Physical moel The rainage basin of about 33 km 2 has been iscretize in terms of a 7594 noe mesh, with triangular elements, being more refine in the regions where a surface flow is expecte. The mesh has been aapte to fit the situation of the gauging an pumping wells. With these ata an those of the total rainfall an water uses, the moel has been calibrate along the hyrological years 1999/2000 an 2000/2001, ajusting not only the hyrological parameters (namely, hyraulic conuctivity an storage coefficient), but also those relate to the geology, evapotranspiration rates an some others relate to the runoff an the capillary features of the subsoil zone. Figure 3. Finite element mesh. Mathematical moel The GIAMA research group works in the evelopment, valiation an application of a numerical moel in goo accorance with the free surface an groun behaviour of the water, capable to evaluate the physical an chemical variables of the flow, taking into account the interaction of all the water resources in a watershe. Thus, many environmental problems, such as natural or inuce floos, the evolution of water in pons, an continental an coastal basins, coul be evaluate with a special regar to the aspects concerning the water quality of regions subject to natural or acciental spilling ue to inustrial, agricultural, mining or urban activities. The coe is base upon the finite element software MELEF, which solves both surface an grounwater flow by making use of a numerical iffusive wave approach that takes into account the continental, coastal an wetlans environments, together with an evaluation of the evapotranspiration hyrological components. The set of partial ifferential equations to be solve are: xx K ( n ) = K ( h P) + K ( h P) yx ( h P) + K ( h P) xy yy + + Q where the source term Q inclues an evapotranspiration moel evelope by the authors (for etails see Ménez et al., 2003).

4 The computer moel MELEF is also able to evaluate the free surface flow making use of a kinematics wave approach through the continuity equation, an provies in that way just a rough approximation. The coe MELEF is being improve by incluing a moule that solves the free surface flow on a Navier-Stokes basis that, therefore, inclues (together with the continuity equation) the resolution of the ynamic equation for the mixe unknowns velocity (u i ) an pressure (p i ): 1 u i, t + u jui, j = p, i + νui, jj + ρ f i The algorithm to be use in the resolution of the free water flow incorporates a stabilization proceure in orer to avoi the instability problems that show up when solving the Navier-Stokes equations even for refine meshes. The coe HYDRAFEM, evelope by the authors of the paper (see Vellano et al., 2002 for etails), is the one chosen to be incorporate to the resolution of the free surface water. A moule that incorporates reactive avection-ispersion flows is also to be inclue. Once the hyroynamics of the surface an groun water flows has been obtaine, the transport equation is solve for the concentration c, in terms of the given velocities q. Possible solute sinks an sources are ae to the left-han-sie of the equation. For a flui source r having a concentration, c *, an a solute sink/source term, R (solute mass ae per unit time an unit flui volume), the transport equation becomes: c C j ( Pj ) ( Wj ) ( Yj ) ( D c) q c + r( c c) + R = ; R = j = 1,2,..., N c where R are the homogeneous an heterogeneous chemical reaction rates, C j are the total issolve, precipitate, exchange an sorbe concentrations of the j-th aqueous chemical species, an N is the number of species Results an Conclusions After the calibration process has been complete, the software has been applie to the resolution of the hyrological regime to be expecte in the Meirama open pit at ifferent conitions, namely, at present, uring filling an post-filling stages. Depth Depth m Figure 4. Water table level expecte in September 2007 (left) an September 2012 (right).

5 IMWA Symposium 2007: Water in Mining Environments, R. Ciu & F. Frau (Es), 27th - 31st May 2007, Cagliari, Italy V= 0.62 m/s Campo e velociaes m/s m/s Figure 5. Velocity fiels in the prospective pit lake at February The results provie ata about the height an velocity of the water flow in the basin which are funamental in orer to epict the environmental effects of the mine closure an the creation of the artificial lake. The results concerning the water quality issues are still in progress. Some results concerning the flow variables uring the filling an post filling situations are shown in Figures 4 an 5. Acknowlegements The work reporte in this paper has been fune by the Ministerio e Eucación y Ciencia, Xunta e Galicia, an LIMEISA. References Essai H.I. (1990). A multilayere sharp interface moel of couple fresh water-salt water flow in coastal systems: moel evelopment an application. Water Resources Research 26(7), Huyakorn P.S., Wu Y.S., Park N.S. (1996). Multiphase approach to the numerical solution of a sharp interface saltwater intrusion problem. Water Resources Research 32(1), Leoux E., Sauvagnac S., Rivera A. (1990). A compatible single-phase/two-phase moel: 1. Moeling the transient salt-water/fresh water interface motion. Groun Water 28(1), LIMEISA (2004). Gestión e la calia el agua. Documentación Técnica para Aguas e Galicia. LIMEISA (2005). Datos e la cuenca hirográfica e la excavación minera e Meirama. (sin publicar). Ménez A., Pailla F., Roríguez J.A., Fernánez R., Juncosa R. (2003). Moelo numérico par la hirología subterránea y superficial el aluvial costero el Bajo Guaalhorce (Málaga, España). Actas el Congreso Tiac 03. Tomo I: SOLUZIONA (2001). Estuio hirológico. Estuios previos al proyecto el lago e la mina e Meirama. Informe para LIMEISA. Vellano P, Puertas J., Colominas I. (2002). SUPG stabilize finite element resolution of the Navier-Stokes equations. Applications to water treatment engineering. Comp. Methos Appl. Mech. Eng.. 191,