The aim of this research is, firstly, to adapt a numerical code to analyse the processes of surface - groundwater flows in environmental problems, and secondly, to predict the most likely development of the flood rehabilitation of the Meirama lignite mine considering the entire domain of the Barcés River catchment (~90 Km2). La Coruña, Spain The process begins with the adaptation of MELEF numerical code to the requirements of the Barcés river catchment, which include several hydrogeological characteristics as minor streams; the Meirama open pit mine, with depths up to 200 meters in process of flooding; the Cecebre reservoir at its point of discharge; and the complex geology of a pull-apart type sedimentary tertiary valley of the Meirama basin. The adaptation of the numerical code is crucial for the successful implementation in the analysis of groundwater-surfacewater flows, as well as in the analysis of the flooding evolution of an open pit mine.
The code MELEF evaluates jointly the groundwater-surfacewater flows by the finite element method in two horizontal dimensions. During the implementation of the code we found that the behavior in surfacewater depths up to 200 meters was not appropriate, in addition, the applicability of MELEF code was compromised and limited when it was managed manually. With regard to this, a series of tasks were established in the Project CICYT (CGL2006-01452/HID) to design and develop a numerical code management system implementing Geographical Information Systems (GIS), as well as to improve the code to extend its application to the flood rehabilitation of the Meirama mine.
The GIS also enables the ability to store, represent, manage and take decisions for all the simulated conditions of the numerical code MELEF.
Once the management system was developed, it was possible to generate new and more demanding simulated conditions, and consequently to discover new numerical code deficiencies in the mechanisms of groundwater-surfacewater interaction and the conceptual model of evapotranspiration.
Therefore, during the development of this research, improvements to the numerical code became a major challenge, because: - The surface model needs to be improved to ensure the mass conservation during the flood of the open pit mine. The result was a new formulation based on the linear Muskingum model that keeps the mass conservation, even when dealing with quite deep free surface water.
- Moreover, the behavior of the evapotranspiration model needs an improvement compared to the daily precipitation intensity, because the model was initially designed for a monthly intensity. The solution includes evapotranspiration curves with a new parameter (soil thickness) associated with the depth of roots, as well as a new curve named transpiration under drought stress.
- Meanwhile, the numerical code was provided with an overland runoff model. This model considers the spatial variability of the average capacity of infiltration as a function of slope and the maximum depth of the roots of vegetation. Also, the runoff model is evaluated as an exponential distribution function, which evaluates the apparent infiltration of soil in terms of precipitation and average infiltration capacity.
Once the problem of mass loss has been solved and the main improvements have been made, it was possible to model the present flooding of the Meirama open pit mine considering the whole hydrology domain of the Barcés River catchment. For this, the code was calibrated for a period of three and a half years (2006-2009).
Subsequently, it was able to model the most probable evolution of groundwater-surfacewater flows for a long enough period so as to reach the level of discharge into the Barcés River (~177 m.a.s.l.).
Finally, the analysis of the results during the calibration period reflects how the decisions taken in the management of surface flow during the period in the Meirama basin, defines the flow regime of the Barcés River. Subsequently, the application of the code during a future period predicts the flooding process that will most likely finish in september 2014. However, the evolution of the present flooding is influenced by various factors such as random-stochastic behaviour of precipitation and possible changes in the strategies of flooding of the Meirama open pit mine.