Diagnosis of Operating Mine Dewatering Wells Efficiency through Groundwater Modelling

摘要

Pumping wells are used to dewater operating mines at various sites around the world. The usual objective of the pumping from dewatering wells is to intercept most of the groundwater flow such that only a small amount of residual inflow would reach the mine. Decrease of pumping wells production usually occurs over time [1] either due to the extent of the groundwater level drawdown around the mine and depletion of the aquifer storage, or to decline of the well efficiency because of deterioration of the well installation. Therefore, decrease in the pumping rate in a dewatering well does not necessary means that the latter requires rehabilitation or replacement. Borehole rehabilitation or replacement is often costly and should be carried out only if justified upon diagnostic of the reasons behind the decline in the pumping rate.In ideal world, dewatering wells are designed taking into account the hydraulic and physical properties of the target geological formations, and their efficiency estimated upon installation and commissioning. During the mine operation the efficiency of dewatering wells is (or should be) estimated/reassessed regularly to enable better planning of borehole maintenance and replacement where necessary. However, such procedures are not usually implemented and in some cases even measurements of pumping rate and water level in the wells are not collected.Assessment of dewatering wells efficiency during the mine life is usually carried out using pumping tests and down-hole surveys. In some circumstances such costly field tests, which may also interfere with the dewatering operation, are not necessary for all dewatering wells. This paper presents an alternative desktop-based approach for an initial diagnostic of the dewatering wells efficiency and a case study is given for an operating pit. The method consists of using groundwater models to simulate the pumping potential of dewatering wells over time. The estimated achievable pumping rate from a dewatering well is then compared to the actual/observed rate to verify if the well is operating at the expected level. The results of the model can be used by the mine managers to decide if there is need or not for down-hole surveys and borehole rehabilitation or replacement. The results enable more efficient planning of the field work based on the predicted pumping potential of each well, and the priority areas identified by the model as potential source of the residual inflows of groundwater into the mine.