Coupled hydrodynamic and geochemical models for open pit lakesapplication to the As Pontes Pit Lake

Abstract

The most common solution for an open pit mine after mine closure is the creation of a pit lake. The lake can be formed naturally by raising the groundwater level after stopping dewatering or by diverting water from a nearby river or the sea. Sometimes rivers are diverted for the flooding process to increase the percentage of natural water in the lake and / or stabilize the side walls of the mining void. Acid mine waters containing high concentrations of dissolved species are generated due to the piritous materials that are exposed on the mine walls and dumps. These acid waters may affect the composition of the water in the open pit lake, and, therefore it is necessary to develop tools for predicting the final water quality of the lake and evaluating preventive and remedial actions to achieve water quality that fits the limits established by the regulatory authorities. It is important to quantify the effects and costs of preventive and remedial measures due to the large water volumes to treat and the fact that the negative effects of acidic water can remain for long periods of time in the lake. The final water quality of open pit lakes is determined by hydrologic, climatologic, geochemical and limnological factors. The complexity of the processes and interactions that affect the composition of the water column in an open pit lake make necessary the development of complex models that take into account the stratification and mixing processes and the geochemical reactions such as mineral precipitation. Codes have been developed in the recent years to simulate hydrodynamic processes in lakes such as DYRESM (Imberger y Patterson, 1981), AQUASIM (Reichert, 1994) and CE-QUAL-W2 (Cole and Buchak, 1995). At the same time, there are codes that simulate the chemical and biochemical processes such as PHREEQC (Parkust and Appelo, 1999), EQ3/6 (Wolery and Daveler, 1992) and CORE2D V4 (Samper et al., 2003). Only a few codes can take into account both hydrodynamic and geochemical processes. These codes include MODGLUE (Müller, 2003), the latest version of DYRESM- CAEDYM (Salmon et al., 2008) and the code developed in this dissertation, DYCD-CORE (Samper et al., 2008), by coupling DYRESM and CORE2D V4. DYRESM was developed by Imberger and Patterson (1981) (CWR, 2006). It is a one-dimensional code to simulate the vertical distribution of temperature, salinity and density in lakes and reservoirs. The code is based on the one-dimensional assumption which states that the horizontal transport processes are faster than the vertical ones and in this way the lake can be merged in a set of horizontally homogeneous layers in which energy inputs, mixing processes due to density differences and calculations related to the inflows and outfows are performed. CORE 2 D V4 is a code for modelling non-isotherm water flow, heat transport and multicomponent reactive solute transport under local geochemical equilibrium and kinetic conditions in porous media which has been developed by the Group lead by Professor Javier Samper in the University of La Coruña. DYCD-CORE has been developed to model the hydrodynamic and geochemical evolution of open pit lakes (Samper et al., 2008a; 2008b). DYCD-CORE takes into account the hydrodynamic and mixing processes implemented in DYRESM and the geochemical subroutines of CORE2D V4 (Samper et al., 2003). The water quality model considers transport phenomena in the water column, the density profile that causes stratification and the chemical reactions including aqueous complexation, O 2( g ) and CO 2( g ) in the water column and mineral precipitation. DYCD-CORE solves first the hydrodynamic processes and the transport of the chemical species and minerals. Then, the geochemical calculations are performed using the subroutines extracted from CORE2D V4. The main factors controlling the water quality of an open pit lake are: (1) The composition and the volume of the acid water and (2) The hydrodynamics and stratification that take place in the lake. Most of the lakes situated in temperate climates undergo a thermal stratification cycle due to the seasonality of the meteorological conditions. Since the end of the spring until the autumn, lakes usually present two different zones, the epilimnion and the hypolimnion, which are separated by a maximum thermal gradient zone, called thermocline. A seasonal turnover usually takes place at the end of the autumn due to the cooling of the surface waters and their sink in the water column of the lake. After the turnover, the waters in the lake remain completely mixed during the winter and part of the spring. This favors the homogenization of the dissolved species, nutrientes and oxygen. In some lakes the turnover is not complete and a layer with different chemical properties is formed in the bottom of the lake. This layer, called monimolimnion, can remain undisturbed during one or more stratification cycles. In this case, the lake is called meromictic. Meromixis is common in mine lakes due to the accumulation of dissolved substances in the monimolimnion. The cycles of precipitation/redissolution of the minerals can affect significantly the hydrodynamics of the mine lakes, keeping them stratified through out the year. Inverse temperature profiles are established due to the redissolution of the minerals in the monimolimnion promoted by the anoxic-reductive conditions. Modelling meromictic lakes requires appropiate equations for density calculation considering the chemical composition. In this dissertation, a new code has been developed considering the coupling of hydrodynamics and geochemistry, DYCD-CORE. The coupled code reproduces the temperature profiles observed in the Waldsee meromictic mine lake (Germany). The density equation developed by Chen and Millero (1986) has been modified by a correction term which depends on the concentrations of the dissolved species. DYRESM, CORE 2 D V4 and DYCD-CORE have been used to model the As Pontes pit lake and predict the time evolution of the water quality of the lake. Initially, three independent lakes have been formed inside the mine void of As Pontes (East, West and Intermediate lakes) during the first two years of the flooding process. Hydrodynamic models of the East and the West lakes have been performed. Hydrodynamic model predictions for As Pontes pit lake have also been performed. Full-mixing models have been developed using CORE2D V4. Full-mixing models are not sufficient to simulate the composition of the water column of a mine lake. This is the reason why more sophisticated simulations have been performed using the new coupled code DYCD-CORE. These hydrogeochemical simulations reproduce the measured profiles in the East and West and As Pontes lakes. Finally, DYCD-CORE has been used to reproduce the temperature profiles of the Waldsee meromictic lake (Döbern, Germany). Coupling the hydrodynamics and geochemistry is important to reproduce the permanent stratification observed in this lake. This has been performed by accounting for the chemical composition in the calculation of the water density.