Water management on mines

Zambia is one of the wettest mining regions with an average ore to water ratio of 11:1. In mining, water is considered both a friend and an enemy. On the positive side, natural water sources can be used as service water for dust control during re-entry examination, drilling operations, hydrometallurgical plant processes, and in some mines, the watering down of haul roads and travelling ways. However, natural water sources also negatively affect ground conditions in underground mines, slope stability in surface mines, and discharged untreated mine contact water can pollute natural water sources used by host communities. From an operational perspective, unstable slopes and poor ground conditions could result in falls of ground, which could cause serious damage to equipment and mine personnel, and even premature mine closure. The slope stability and ground conditions are influenced by local geological and geo-technical characteristics of the slope-forming rock mass or hanging wall strata, and the prevailing ground water conditions. Furthermore, water flowing into underground working can result in slip and fall accidents, create poor visibility, and increase humidity resulting in poor working conditions from an ergonomic perspective. Additionally, water in underground mines creates a humid environment and this negative affects the cooling power of air and reduces equipment and machinery lifecycles due to the corrosive effects of water. The reduction of the negative aspects of water is therefore of paramount importance in ensuring sustainable and cost-effective mining operations. Thus, an effective water management strategy is of pivotal importance. There is no one-size-fits-all solution for mining operations due to the dynamic nature of geology, hydrology, and hydrogeology, but an exploration of the various options is important.

The natural water sources for ingress into mine environments are rain water (climate), surface water (Hydrology), and groundwater (Hydrogeology). The climate of the Copper Belt Province is predominantly determined by the north-south migration of the Inter Tropical Convergence Zone (ITCZ) with seasons. The ITCZ oscillates between the equator and the Tropic of Capricorn (23º S) between November and February. During the winter months, it is located over the northern tropics. The summer rains are a consequence of the southward migration of the ITCZ. Thunderstorms are common and severe at times and bring excessive lightning and sometimes hailstorms. Total annual average rainfall in Zambia is approximately 1,309.1mm, with the majority falling from November to April. A local assessment of the climate is important as it enables the design of an effective water management strategy.

From a hydrological perspective, the Copper Belt Province is drained by several streams that discharge water into the Kafue River. The source of the Kafue River is at the eastern end of the Zambezi – Congo Watershed in the Copper Belt Province, close to the DRC. The direction of flow is generally southward and it meanders close to the Copper Belt mining towns of Chililabombwe, Chingola, Mufulira, and through the outskirts of Kitwe.

The hydrogeology of Zambia is predominantly comprised of low-yielding aquifers with limited water retention potential and aquifers, where the flow is mainly in fissures, channels or geological discontinuities (faults, dykes, joints). These aquifers are then classified as either locally productive aquifers or highly productive aquifers. Within the Copper Belt, low-yielding aquifers consist of the Karroo basalts and older basement rocks, while locally productive aquifers consist of the Lower Roan quartzite, the Muva sediments and the Kundelungu formations. The highly productive aquifers consist of the Upper Roan dolomite and the Kundelungu limestone, which are present only in small portions of the Copper belt. A hydrogeological assessment of the mine specific conditions is important for determining the type of aquifers present (confined or unconfined) and to identify hydrogeological hazard zones that may be affected by rain water and surface water. This will allow for the design and implementation of an effective water management strategy.

The first step in water management is to assess and draw conclusions about the local hydrogeological, hydrological and climate of the proposed mine site. This is usually carried out during the exploration phase of mineral development. The identification of surface water sources and their drainage paths relative to the mineral deposit is investigated. This process will allow potential mine operators to factor this information into the overall mine design. Furthermore, the position and depth below surface as well as the geochemical, geotechnical, and geological characteristics of aquifers are investigated using drill hole data as well as other more complex methods. This data then enables potential mine operators to develop a comprehensive water management strategy for Shaft Sinking, Mine Development and Exploitation based on the type of mining method selected (surface or underground).

The principle strategies used for surface water control are pumping and diversion, for groundwater exclusion and pumping, and for storm water, the primary control mechanism is pumping. If necessary, the pumped water is then treated before it is discharged into tributaries or recycled and re-used on site. The diversion aspect of surface water control from mine sites is done by the construction of canals and pipelines to carry water over hydrological hazard zones, along with herringbone ditches to speed up run-off. To prevent groundwater from entering surface mining sites, exclusion is done by constructing physical cut-off walls and/or pumping from an array of wells or sumps. The type of wells that are used to pump are drainage ditches at the surface of the mine, drainage ditches at the bottom of the mine, horizontal drains, vertical wells drilled from the surface, vertical wells drilled from benches or pit bottom, and dewatering shafts and galleries. These methods can be used in combination based on the hydrological and hydrogeological assessment of the mine site. The type of cut-off wells used to exclude water from open pits are displacement barriers (steel sheet-piles), excavated barriers (concrete diaphragm walls, bored pile walls, bentonite slurry walls and trenches), injected barriers (permeation grouting, rock grouting, jet grouting and mix-in-place methods) and artificial ground freezing.

In underground mines, the same methods used to control surface water on surface mines are used to prevent shaft or decline flooding. Access excavations (tunnels) are developed at a small gradient (typically 1:100) above the horizontal. This allows the mine itself to act as a drain and control groundwater. The water is passed along roadways by gravity or pumping (caused by other factors) to drain holes. The water is then transferred via underground dams to the shaft bottom (mine sump) or main pumping level, where it is pumped to the surface via the shaft or the decline. Where fractured or water-bearing zones are encountered (determined during exploration), exclusion methods are used to reduce water inflow. The most common method of exclusion is grouting with cementitious based grouts predominantly used. Where there are problems with setting times for cement based grouts, specialist chemical polyurethane grouts are available.

Pumping methods are used as part of the water management strategy in all mines. Mine contact water pumped from mines is normally polluted, and in order to protect the environment, it is necessary to treat it before it is discharged into natural surface water drainage systems. Water pollution is caused by Acid Mine Drainage (Sulphuric Acid is produced when sulphide bearing rocks are exposed to air and water), Heavy Metal Contamination and Leaching (when metals like arsenic, cobalt, copper, silver etc. contained in exposed rock faces come into contact with water), Processing Chemicals Pollution (leakages of chemicals from plant processes) and erosion and sedimentation (clogging of riverbeds and smothering of watershed vegetation, wildlife habitats and aquatic organisms caused by erosion). The water treatment technology and techniques used are dependent on the mining method, water management strategy, plant processes and other mine specific factors.

Negating the negative effects of water on mining operations is an important part of mineral development. There are various strategies employed and this is determined by the local climate, hydrology and hydrogeological conditions. Water management is an essential part of determining asset viability as these strategies can be expensive, but at the same time a comprehensive water management strategy can save mines a lot of money, if employed correctly. It can minimise the amount of water treatment needed, enable to steeper mining slopes in surface mines (decrease waste to ore ratios) and allow for the protection of the environment.

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