We came across an interesting article published on the igeological blog written by Amanda, Michele, Carina and Isabella, students of geology, and we decided to republish it on our website. Read the full article below.
Today we are going to talk a little about the lowering of the water table carried out in mining activities. Did you know that the practice of lowering the water table has been happening since colonial times, mainly in the 19th century with underground mines?
In mining activities, the lowering of the water table occurs at the moment when the pit reaches the water level and continues until the mine decommissioning process begins.
The “lowering of the water table” involves the removal of a quantity of water from the rock mass or soil profile, in such a way that the water levels are lowered to protect the mine safety and economy. Physically this process is defined as a drainage of the pores within the mass of the soil or rock, which results in the lowering of the water table.
The efficiency of the implementation of these activities depends on hydrogeological studies, which allow the evaluation of the most adequate control systems. The start of the lowering of the water level in mining occurs even before the mine reaches the water level of the aquifer and is closed at the end of the mining activity, which may last for a few decades.
The water level lowering projects are carried out in the medium and large open pit, and such projects require the construction of specific drainage structures. In smaller mines, both open pit and underground, dewatering is carried out by the excavation itself, where the mine itself acts as a drainage structure. The water is removed by gravity or by pumping from tanks where the water is directed.
The drawdown is necessary in works whose presence of water in its subsoil prevents or compromises services, such as foundation or containment. The geotechnical surveys, previously carried out, indicate the most appropriate method of lowering the water table – the choice is made according to the geometry and depth of the excavation, the permeability and the type of the aquifer.
These techniques can be grouped into two main categories: Active techniques or also called advanced drawdown techniques, and passive techniques also known as real-time drawdown techniques. One of the techniques used is that of filter tips (Figures 01 and 02). This procedure is used for surface excavations and consists of a series of small diameter wells, usually 3 to 4 inches, connected to a collection tube to the storage chamber. vacuum, where water and air are separated, in order to prevent the entry of air and reduce the efficiency of the system.
The water table lowering and water control processes, applied in mining, create an extensive depression cone reducing the explosion costs, when the ore is dry, less explosive emulsion is required, reducing even transportation costs (fuel) and the wear of the machine and also improves the trafficability and quality of the ore. A technical-economic analysis must be extensively evaluated so that only what is necessary is carried out by mining for the proper functioning of processes, minimizing impacts on the environment.
In large and medium-sized mining pits, this activity has a considerable potential to generate environmental impacts, which tend to affect other users, creating management problems, which are not always easy to solve. That is why an environmental and water resources structure and legal procedures are required to specifically regulate the activity, through the Grant and Licensing of the Lowering of the Water Level.
To carry out this activity, it is necessary to follow some work steps, such as:
The definition of the geological model, in which in mining the mineral deposit must have its geology very well detailed, in scales from 1: 5000 to 1: 2000. With vertical and horizontal geological sections. Around the mine, the detailing scale is smaller and may be between the scales of 1: 25,000 and 1: 250,000.
The study of the genesis of the deposit in the mineral research phase provides subsidies of high relevance in the constitution of the hydrogeological model of a mineral deposit, since the occurring mineralizations can influence the hydrogeological behavior of the aquifers, creating important compartments.
The analysis of meteorological data, mainly pluviometry, evaporation, air temperature, as well as regional fluviometric data are of fundamental importance in the definition of the preliminary hydrogeological model.
In addition, it is necessary to carry out a systematic inventory of all groundwater emergencies, summers, natural ponds, existing busbars, tubular wells, excavated wells, monitoring wells, piezometers, contained in the sub-basins surrounding the enterprise, contemplating aspects technical and socio-cultural.
The delimitation of the area to be inventoried must include technical aspects, such as the extension of the aquifers, which can potentially be affected by the lowering of the water level, in addition to the socio-cultural aspects. Finally, a monitoring program must be implemented.
From all the collected data, inventory of water points and monitoring of at least one hydrological cycle, it becomes possible to elaborate the hydrological model of the mine. The dynamics of surface water flow and how they interact with groundwater is necessary for a more realistic representation of the phenomena that happen in a hydrogeological system.
The hydrogeological model should provide the boundary conditions for the numerical modeling of the aquifer, which is fundamental to the elaboration of the drawdown project and the prediction of possible impacts on water resources. This numerical modeling of the groundwater flow makes it possible to forecast the drawdown system, as well as its planning throughout the life of the mine and in the decommissioning phase until the recovery of the water levels in the aquifers.