Kaisa Turunen & Kimmo Hentinen. Geological Survey of Finland, P.O. Box 1237, FI-70211 FINLAND, kaisa.turunen(at)gtk.fi, kimmo.hentinen(at)gtk.fi
Water is a crucial resource for mining industry, but it is also the principal agent, alongside with wind, by which potential pollutants can be carried to adjacent environments. Moreover, mine site is also a part of catchment area in meteoric water circulation system and all mines change the hydrological and topographical circumstances of the mining area (Salonen et al. 2014). In fact, the most common and widespread causes of water pollution by mining are impacts arising by the alteration of regional surface flows and ground water flows. Changing the topography involves grading of the site, diversion of runoff and replacement of land masses that increases or decreases infiltration of surface water to the ground water. In general, direct effects of the mining process on the water environment tend to be localized and of limited magnitude. In order to meet the regulatory and environmental standards as well as to minimize the potential for water contamination, mining companies need to develop water management plans prior to mining activities. The management plans should be also updated during the mines life-cycle (Banks et al. 1997, Hedin 2003).
Water volumes and types
Mining operations often have effects on both quantity and quality of water. Moreover, since mining also changes the hydrological and topographical features in wide area, the effects on water systems do not consider only inside of a mine, but also all water systems outside of the mine. Mining operations involve in two main hydrologic components: natural, and mine water systems of which natural waters are associated with the natural hydrological cycle, such as groundwater and meteoric water. During the mine-life-cycle, sufficient and accurate knowledge and characterization of hydrogeological parameters in the mine site are crucial for establishing baseline hydrological conditions for the prediction of drainage release and transport, and monitoring of the environmental conditions and potential impacts. (Morton & Mekerk 1993, EPA 2003). Moreover, the climatic and hydrogeological characteristics of the site control the behaviour of constituents present in mine drainage which is transported through the environment to the receiving water systems (e.g. Wolkersdorfer 2008, Salonen et al. 2014). The hydrological and hydrogeological research related to mine water management is described in Closedure Hydrological and hydrogeological research page.
The amount of water required by a mine varies depending on its size, the minerals being extracted, and the extraction processes used. In general, metal mines require much more water for chemical processing of the ore to concentrate metals than non-metal mines such as gravel mines and quarries. Typically, all water at mines (process, dewatering etc.) is collected and stored in different types of tailings ponds (Fig. 1) before being treated and released to receiving water courses or recycled back to the processes (Banks et al. 1997, Hedin 2003). Nowadays the rate of water reuse and recycling in mineral processing tend to be high at mine sites, especially in areas with arid climates. The recycled water is often treated before recycling to meet the quality of mineral processing (Rankin 2011). The treatment and recirculation lowers the amount of polluted waters as well as reduces the need of water used at mine. The water treatment technologies are described in more detail in water treatment part.
Figure 1. Schematic figure of water circulation in mining operation. (Picture © GTK, 2014. Modified from Kauppila et al. 2013).
The mine waters associated with a mine can be classified according to their quality and potential for contamination as follows (modified from Banks et al. 1997, Younger et al. 2012):
Dewatering water: The precipitation and ground water seeping into open pits and underground mine that is pumped to surface to dewater the mine. Tends to contain residues of blasting agents (e.g. nitrogen compounds) and dissolved minerals and/or metals.
Drainage water: Surface or ground water which flows or has the potential to flow off the mine site. Tends to have low mineralisation and low temperature.
Drill water: Water used in exploration drilling. Usually collected by dewatering.
Mill water: Water used to crush and grind ore, tend to contain dissolved minerals and/or metals. Usually combined and collected with process water.
Mine drainage and leachate: Seepage water which has trickled through solid mine wastes, waste rock sites and tailings dams. Tends to contain dissolved minerals, process chemicals, and metals. The geochemical characters of the flow path media define the quality of the mine drainage: acid producing/non-acid producing, neutral, alkaline, metallogenous and/or saline. Mine drainage can be classified into three types depending on the pH and the element content of the water: Acid mine drainage (AMD) pH < 6, neutral mine drainage (ND) pH > 6 and saline mine drainage (SD) > 1000 mg/l.
Mine effluent: Mining, mill, or process water which is being discharged into surface water, often after being treated.
Mine water: Any surface or ground water present inside the mine site. Tend to have high mineralisation and warm temperature. Mineralisation usually increases with depth.
Mining water: Water which has come into contact with any mine workings.
Process water: Water used in the chemical extraction of metals, commonly contains process chemicals (e.g. SO4, Ca, K and C) as well as dissolved minerals and/or metals.
Service water: Water used for dust depression, cooling or workers needs.
Flow paths and accumulation
In mines, water accumulates and flows e.g. in voids, adits, fissures, faults, rock matrix and microfractures of bedrock and in sediments. Since the permeability of these flow paths differ drastically, also the flow speed and the way of inflow differs in different parts of a mine. For instance, the bigger fractures and bedding planes act as constant feeders, whereas in rock matrix and microfractures water moves more easily by dropping than by flowing. Water does not accumulate evenly in mine workings and in fact, water is depth dependent, which means it tends to accumulate in shallower parts of mine workings. In addition, different shafts at same level of a mine can have different water table level (Wolkersdorfer 2008).
Underground and open pit mines tend to have different effect on mine water. In general, the open pit mining has greater effect on surface waters than underground mines, but often the mine site includes both types of mining. The excavation of open pits inevitably removes large volumes of rock and results in alteration of surface flow paths. As in underground mining, open pits needs to be dewatered to gain the access to the minerals (Younger et al. 2002, Wolkersdorfer 2008).
In general, underground mining has relatively minor impacts on surface water environment. However, since underground mines operate below the ground water table, the water needs to be pumped to surface to dewater the mine. Dewatering of underground mine often affects also the surface water flows through a depression of ground water table which affects the connected surface waters systems. Underground workings may also increase the fracturing of the bedrock and soil resulting in enhanced infiltration of surface water, not to mention a huge open space to water to rush in unless properly managed. Once operations ceases, pumpingis gradually stopped, which results in the flooding of open pits and underground workings as the water table rises (Younger et al. 2002, Wolkersdorfer 2008). Depending on the open space, the availability of infiltration water, and the type of flooding (uncontrolled, controlled, monitored), inundation can last from several months to more than a decade (Wolkersdorfer 2008). More about the controlled flooding of a mine is described in Closedure Mine flooding page.
Post-Closure water monitoring and modelling
Modelling and monitoring of flows in closed mines helps one to understand the hydrodynamics of the mine waters and the possible release of pollutants from the mine site. Monitoring of the water management structures during- and after the closure of a mine is essential, to tell if the measures of closure are effective or not. On the basis of gathered monitoring data, different decisions related to closed/rehabilitated mine site can be made. Objectives of monitoring head to proper mine closure and therefore the process of routinely, systematically, and purposefully gathering information for use in decision making is needed. The monitoring procedures are described more detailed in Closedure Monitoring page.
The data obtained from monitoring is also needed as input data in modelling (e.g. flow velocity, climate data). In water management, modelling is a tool for studying and predicting water resources and their behaviour. In mine closure process, modelling is often used for predicting the changes caused by the end of mining activity. Modelling is usually done also during the mining operations and thus it can be also used at closing stage quite effortlessly, especially if required data for modelling is available. Different scenarios during the mining and their effect to closure can also be tested through modelling. Depending on the data and the model used, modelling only provides a simplified representation of the reality. It’s modeller’s responsibility also to evaluate the quality and realibility of the model for ones purposes. The different types of modelling are described in Closedure Post-closure modelling page.
Diversion and interception of mine waters
Different combinations of control techniques and water management practices can be used to reduce the potential for water contamination and minimize the volume of water requiring treatment. The selection of strategies is always site-specific and should be made based on hydrological, hydrogeological and hydrogeochemical data. For instance, the excess water is relevant concern in environments with high rates of precipitation, as in Finland, and to prevent unnecessary contamination of clean waters, the off-site surface waters (rain and snowmelt runoff, streams and lakes) should be directed and intercepted to avoid entering the mine site (Lottermoser 2012, Younger et al. 2012). Through diverting the water the volumes are controlled and the effects of water contamination on receiving water courses minimized. If the bedrock is highly fractured there is a significant risk of surface water inflow into the mine voids and diversion can be an extremely useful tool to minimise post closure impacts. This method is therefore highly recommended if a high acid-producing potential in the mine voids is expected during and after mine closure (Younger et al. 2002, Wolkersdorfer 2008).
The diversion of surface waters can be applied by different kind of physical barriers such as dams, diverting ditches and wells. Surface water diversion systems can be used to divert surface water from mines, to prevent the percolation of surface water into the mine and that the water is diverted around the mine as fast as possible before it might enter the mine. Those systems must be securely sealed and well designed for the extreme events e.g. storm events, because the destruction of those system could cause slope failures or sludge movement. The systems should have a regular inspection and periodic maintenance (Younger et al. 2002, Wolkersdorfer 2008). For groundwater, the diversion system consists of e.g. grout curtains, grout injection, large subsurface concrete walls and pumping from wells. These systems are used to protect nearby natural water course e.g. wetland from groundwater dewatering and prevent groundwater flow into the opencast mines or the mine voids and other mine workings (Wolkersdorfer 2008).
Nonetheless, in case of a huge open pit mine diverting is neither practical nor environmentally advisable, since the open pit should function as an aquifer after mine closure. Moreover, diversion of water is not a walk-away option for mine closure and technical installations might need longer maintenance or control than expected. Thus, the post-closure water management should be planned gravity driven to reduce the maintenance costs and prevent problems caused by possible breakage of the equipment e.g. pumps. The water management structures should be designed resilient to physical and chemical erosion to ensure long life span and low maintenance costs. (Younger et al. 2002, Wolkersdorfer 2008).
Closure technologies, methods and aspects related to mine closure water management evaluated in these web pages of the Closedure project include:
Banks, D., Younger, P.L., Arnesen, R.-R., Iversen, E.R. & Banks, S.B. 1997. Mine-water chemistry: the good, the bad and the ugly. Environmental Geology 32(3):157-174.
EPA 2003. Epa and Hardrock Mining: A Source Book for Industry in the northwest and Alaska. A Technical report by the United States Environmental Protection Agency.
Hedin, R.S. 2003. Recovery of Marketable Iron Oxide From Mine Drainage in the USA. Land Contamination and Reclamation, 11(2): p. 93-97.
Lottermoser, B. 2012. Mine Wastes: Characterization, Treatment and Environmental Impacts, Springer, New York. p. 400.
Morton, K.L. & Mekerk, F.A. 1993. A Phased Approach to Mine Dewatering. Mine Water and The Environment, Vol 12. pp 27-34. http://www.imwa.info/bibliographie/12_14_027-033.pdf
Rankin, W.J. 2011. Minerals, metals and sustainability : meeting future material needs, Collingwood, Vic.: CSIRO Pub.
Salonen, V-P., Korkka-Niemi, K., Moreau, J. & Rautio, A. 2014. Kaivokset ja vesi – esimerkkinä Hannukaisen hanke. Geologi 66, 8-19. (in Finnish)
Wolkersdorfer, C. 2008. Water Management at Abandoned Flooded Underground Mines. Fundamentals, Tracer Tests, Modelling, Water Treatment. Springer. 465 p.
Younger, P.L., S.A. Banwart, & R.S. Hedin 2002. Mine Water: Hydrology, Pollution, Remediation, Dordrecht, The Netherlands, Kluwer Academic Publishers.