Water balance modeling
Kimmo Hentinen, Geological Survey of Finland, P.O. Box 1237, FI-70211 KUOPIO, FINLAND, kimmo.hentinen[at]gtk.fi
Water balance models describe the volumes and quality of water entering, circulating and leaving throughout the mine site (IM Mining 2013). Mine operators and regulators can use models to plan water management and assess potential environmental impacts (Janowicz 2011). Model can be created during the initial feasibility and planning stages, but for the purposes of post-closure water management, there are certain characteristics that are needed to determine once again prior mine closure. For instance, the flow patterns that are determined by the geological, geomorphologic and hydrological characteristics of the site have changed during the mine operations and will alter further with time (Wolkersdorfer 2008). After planning, water balance model can be used and developed over the whole life cycle of the mine. At the closure stage of a mine water balance model can be used for example to predict the development of water balance and water quantities at the mine site.
Description of the technology
Modelling starts by defining modelling objectives and developing a conceptual model (Janowicz 2011). Conceptual model is a description of the essential features of the system to be modelled. The mine site water management should include descriptions of the hydrology and chemistry and the effects they cause to nature and mined materials (Maest et al. 2005). Conceptual model should be accurate enough to be able to describe changes to water volumes and quality caused by water management modifications (Water engineering Australia 2011). Haanpää (2013) states that water balance model development (including conceptual model development) consists of the following phases:
- Definition of model objectives
- Development of conceptual model
- Selection of modelling program
- Determination of key limiting factors (e.g. maximum amount of effluent waters)
- Determination of general limitations
- Collection of input data
- Quality assurance
- Calibration and validation
- Sensitivity analysis
- Comparison of results to limiting aspects
Modelling itself can be made using spreadsheets or general purpose simulators (dynamic approach). For spreadsheet modelling (deterministic modelling) Microsoft Excel is probably the most well known spreadsheet environment. For more complex systems, general purpose dynamic simulators are a better option (Janowicz 2011). Their interface for model development is better and they are more flexible for various scenario examinations (Haanpää 2013). Programs such as STELLA, Vensim, MATLAB Simulink and GoldSim can be used for dynamic modelling. Water balance model outputs include water flow rates, volumes and water qualities. For numerical modelling of surface and subsurface waters HydroGeoSphere(HGS) program can be used. Watershed Simulation and Forecasting System (WSFS) is a hydrological model developed by the Finnish Environment Institute (Vehviläinen 1994, Finnish Environment Institute 2013).
One part of the water balance is groundwater quantities/levels and quality. In order to model groundwater flow a hydrogeological model of the area of interest needs to be created. Mathematical groundwater flow modelling is very important tool for estimating groundwater flow directions and possible contaminant transport. Groundwater flow model can be coupled with chemical reaction modelling.
Depending on the accuracy of the model a lot of valuable information about the area of interest can be obtained. With mathematical modelling system behaviour can be studied and predicted. Effects of different modifications to the current system can be simulated (Wang & Anderson 1982). Modelling the future state of the system can be used to support decision making and possible problems can be identified and avoided beforehand. For example effects of extreme climate conditions (heavy rainfall, drought) to water management system can be simulated, problems identified and take actions to avoid these. When planning a whole new mine site or making modifications to already existing one, operators can simulate different options and make the decision hopefully based on the aspect of environmental impact assessment.
Mathematical model is always only a description of a system. This description isn’t necessarily right. There are multiple sources of errors to the simulation results and the model itself can be biased. There are some methods which can be used for validating the model but it is never sure that the model represents reality as it is. This is probably the biggest drawback of mathematical modelling.
Water balance modelling is usually divided into operational units which are modelled separately. When these units are combined, water balance model for the whole mine site can be generated. If different major units affecting water balance are missing, the model is useless. Therefore, water balance model has to always cover the whole mine site. In the case of mine closure, water balance model is used for simulating water storages and flow volumes in the future. Different scenarios can be modelled and with these results the best decisions can be made.
When environmental costs are considered, modelling is the technology which doesn’t have environmental impacts itself at all. For modelling, measured data from the site is needed. This data acquisition has some environmental impacts but it can be neglected when environmental impacts of the mine are considered as a whole. Even though modelling doesn’t have any environmental impacts itself, the decisions made according to the modelling results can have major environmental impacts if the model results are very defective. If the results are wrong, model can be updated to match the current situation and thus model can be used to predict future state of the system. Model never describes the system perfectly, so the model could be updated even after the mine closure.
Input data required for water balance modelling include:
- Physical data, e.g. topography, catchment area, land uses
- Climate data, e.g. precipitation, temperature, snow situation, evaporation
- Hydrological and hydrogeological data, e.g. regional runoff and flow regime, runoff coefficients
- Water quality data, e.g. time series of concentrations, loadings for water quality constituents (all water sources)
- Process waters and dewatering data (mine plan is the main source of information). (Janowicz 2011)
If a closed mine site is to be modelled continuously after the closure, an automated online monitoring system can be used for data input. Mass balance modelling can be made with a desktop PC. A modelling and mining professional is needed to create, adjust and validate the model.
Finnish Environment Institute 2013. Watershed Simulation and Forecasting System (WSFS). Finnish Environment Institute, Freshwater Centre, Modelling and Assessment. http://www.syke.fi/download/noname/%7B4D2F88B9-21F6-4ED5-AEB7-C1AD30A94D70%7D/32817
IM Mining. 2013. Water Management for Mining. International Mining, June 2013. http://www.im-mining.com/2013/06/03/water-management-for-mining/, page visited: 16.5.2014
Janowicz, J. R. 2011. Guidance Document on Water and Mass Balance Models for the Mining Industry. Yukon Government, Environment. December 19, 2011.
Maest, A.S., Kuipers, J.R., Travers, C.L., and Atkins, D.A., 2005. Predicting Water Quality at Hardrock Mines: Methods and Models, Uncertainties, and State-of-the-Art. Kuipers & Associates and Buka Environmental.
Water engineering Australia 2011. Model helps protect environment. March 2011, 18-21. http://www.waterengineeringaustralia.com.au/pdf/wea_0311.pdf, page visited: 3.6.2014.
Haanpää, K-M. 2013. Vesitaseista. Kaivokset vastuunkantajina – Koulutusseminaari 10 – 11.10.2013, Oulu. (in Finnish).
Vehviläinen, B. 1994. The watershed simulation and forecasting system in the national board of water and environment. Publications of the Water and Environment Research Institute. National Board of Waters and the Environment, Finland, No. 17, 3-16.
Wang, H.F. & Anderson M.P. 1982. Introduction to groundwater modeling, Academic Press. 237 pages.
Wolkersdorfer, C. 2008. Water Management at Abandoned Flooded Underground Mines. Fundamentals, Tracer Tests, Modelling, Water Treatment. Springer. 465 p.