Evaporation from coal mine pit lakes: Measurements and modelling

Abstract

Open cut coal mines can have large surface water catchments and often extend to depths below the natural water table level with constant dewatering required to maintain coal extraction. When mining ceases, surface water runoff and groundwater start to fill the void, and pit lakes form. Understanding the hydrological and chemical evolution of these lakes is important for appropriate closure planning. One of the key processes controlling pit lake evolution is evaporation loss, and current estimates of this process are highly uncertain. Recognising the importance of evaporation and the uncertainty in current estimation methods, the Commonwealth Scientific and Industrial Research Organisation (CSIRO) has been working in conjunction with BHP to measure and model evaporation from pit lake environments at Norwich Park Mine, a large open cut coal mine in Central Queensland. Compared with typical lakes and reservoirs, pit lakes are a unique environment because of the depth to the water surface below the original land surface. The water surface in a pit is likely to be sheltered from winds experienced at the land surface, and the steep pit walls can shade the water body within. Based on a number of evaporation studies in pit lake environments, CSIRO has developed a pit lake evaporation model that can be run using readily available data sources, but this model is yet to be tested in a coal mine environment. Open cut coal mines in Central Queensland are typically strip mined, resulting in long, rectangular, box-shaped voids. The aim of this study was to measure evaporation in a pit lake at Norwich Park Mine and to use the results to test the performance of the pit lake evaporation model. In addition, the measurement dataset was used to test the performance of the pan coefficient evaporation-modelling approach, which is commonly used in the mining industry for specifying evaporation from pit lakes. Successful measurements were collected over a six-month period using a custom-built floating evaporation monitoring system, and this data was used as the baseline dataset against which to test the performance of two evaporation-modelling approaches. Despite its widespread use, the pan coefficient model was shown to be a very poor predictor of daily evaporation. The CSIRO pit lake evaporation model showed the same trends as daily evaporation measurements, but the model consistently tended to underestimate evaporation totals. Further investigations showed that the wind speed scaling procedure utilised in the model for reducing wind speed at land surface level to that at water level was not suitable for coal mine pits. Observations showed that wind speeds at pit level at the study location were actually greater than those at land level. It is suspected that the long, narrow shape of the coal pits acts to funnel wind flow into the pit, thereby, resulting in an acceleration of airflow. When appropriate wind speed scaling was applied, the CSIRO pit lake model performance was excellent. Total predicted evaporation was within 5% of that measured, and daily trends were very well predicted. These results give confidence in achieving reliable evaporation estimates once modified wind speed scaling approaches can be defined for coal mine environments. This task is now the focus of field investigations at Norwich Park Mine.