Samrit Luoma1 & Kaisa Turunen2, 1Geological Survey of Finland, P.O. 96, FI-02151 Espoo, FINLAND, 2Geological Survey of Finland, P.O. Box 1237, FI-70211 Kuopio, FINLAND, e-mail: samrit.luoma(at)gtk.fi, kaisa.turunen(at)gtk.fi
During mining operation, the precipitation and infiltrating groundwater will be pumped out from the mine pits and underground workings. After the dewatering pumps are switched off, the open pits and underground workings will gradually fill with water and the mined voids will be flooded with surface water or groundwater in areas where water balance is positive. Eventually, as the water table will reach decant points, it overflows and discharges into receiving water courses. Afterwards, the water table stabilizes and stays within a narrow range, responding mainly to seasonal fluctuations. The flooding can last from several months to over a decade, depending on the open space, the availability of infiltration water and whether or not the flooding is carried out in a controlled way (Younger 2002, Wolkersdorfer 2008).
Description of the technology
Mine flooding can be divided into two types: uncontrolled and controlled flooding. Uncontrolled flooding happens usually accidentally due to mine water inrush after a construction damages or major pump failure. In some publications (e.g. Wolkersdorfer 2008) “monitored” flooding is also mentioned, since the publisher quotes controlled does not always mean monitored flooding. However, in Closedure we emphasize that controlled flooding should always include monitoring and thus Closedure focuses only on the term “controlled flooding”.
The controlled flooding may be managed by active or passive methods. In active flooding, water is pumped into a mine, whereas in passive flooding the dewatering is gradually stopped. The mine flooding should always be controlled and interference with the raise of mine water table should be possible at any point. Thus, the pumping system usually remains installed to interfere, as necessary with the raising mine water (NWT 2007).
Mine water rebound
As the pumping stops, the water table will gradually rebound and the mine fills up with water. Although the water level rebound and filling of open space occurs differently in open pits and underground mines, the rate of the water level rebound is always a function of two factors (Younger & Robins 2002):
1) The distribution of available water storage volume within a mine
2) The rate of water inflow to the mine
The overall water inflow rate comprises of head-independent and head-dependent inflows. The head-independent inflow rate is not dependent on the water level in the mine voids which it enters, but only on the water availability in the remote sources and the availability of open pathways the water to enter (Younger & Robins 2002). For instance, precipitation water enters usually the mine as a head-independent flow direct through mine voids which are open from the surface or through infiltration (Banks 2001). In contrast, the head-dependent inflows represent inflows from adjacent aquifers and the rate is dependent on the hydraulic head difference between these two systems, given and adjacent aquifers (Younger & Robins 2002). For instance, when the pumping ceases, the cone of the depression rebounds and the groundwater enters the mine as a head-independent flow from adjacent groundwater reservoirs or via permeable horizons, fractures, and exploration boreholes (Banks 2001). Hence, during the rebound the head difference progressively decreases, being highest in the beginning and resulting in the gradual reduction in the rate of inflow as well as the stabilization of the water table, whereas the head-independent rebound is usually linear. Since, most of the mines in Finland, open pits and underground mines, operate below the groundwater table, the mine rebound is usually mainly controlled by the head-dependent inflows (Younger & Robins 2002).
Since the groundwater inflow is the major component of the pit lake water balance, the long-term hydrological behaviour and filling up of the open pit after flooding is dependent on the elevation of its floor in relation to the local water table. If the mine used to operate above the water table, on permeable strata and it fills up mainly from direct precipitation or surface runoff, the lake would not recharge the aquifer system. This kind of terminal sinks or lakes does not usually have chemical impact on the regional groundwater systems. However, if the strata around the pit is permeable enough, the pit functions as the main groundwater recharge for adjacent aquifers, even if the recharge consists mainly of the direct precipitation and surface runoff. This type of an open pit dries up entirely during dry periods and can also have adverse impact on regional groundwater quality. At the mines that used to operate below the water table, the rebound starts immediately after the cessation of the dewatering and the pit gradually fills by lateral groundwater inflow until the water table reaches its equilibrium with the surrounding aquifers. In these flow-through lake conditions, the water inflows also through direct precipitation and runoff. The water outflows mainly through lateral groundwater outflow or by surface decant over pit wall. The rate of groundwater in- and outflow are dependent by the regional hydraulic gradient and the transmissivity of the country rock around the pit. Since, the flow-through lakes are connected with the adjacent groundwater and/or surface water systems, they may cause a potential chemical impact to down-gradient water systems (Younger & Robins 2002).
Figure 1. Schematic diagrams comparing: a) a terminal lake, and b) a flow-through lake. Dashed lines on the right are groundwater table contours. Arrows are groundwater flow lines (modified after Gammons et al. 2009).
Estimating the rebound time and open space volumes for an open pit is more simple compared to the underground mines. A calculation of the rebound time for open pit mines, unless backfilled, can be carried out by assuming the mine pit area as the cone of depression of a big water intake well. Moreover, a simple rebound estimation for an open pit can usually be carried out through a simple water balance or an analytical model. However, if the hydrochemical situation has to be predicted as well, more complex numerical pit filling codes or coupled ground water flow models are needed (Wolkersdorfer 2008). The groundwater adds more complexity to the equation. During the flooding the groundwater inflow and outflow changes due to changes in the water table of the pit lake. For instance, Niccoli et al. (1998 )used a pit lake analytical modelling to predict groundwater flow directions in relationship with the pit lake level. The rebound time of water to fill up an open pit lake is a function of various parameters such as the mined and dewatered volume of rock, the volume of the open pit, the climatic, hydrologic, and hydrogeological conditions (Wolkersdorfer 2008). In both, open pits and underground mines, the flooding process will stop when the surface of the mine water equals the groundwater table or reaches a point of discharge e.g. a drainage adit. The prediction of the rebound scenario is needed to allow the planning of certain remediation measures of mine water treatment options and it should be conducted in advance of mine closure. The well calibrated numerically-based conceptual models can be used for the prediction (ERMITE Consortium 2004, Wolkersdorfer 2008). However, similarly with the estimating of the rebound rate for mines, also the modelling of the flooding is simpler for open pits than for the underground workings. In underground working in the beginning of the flooding, when the hydraulic head gradient is still high, the flow in larger voids and channels is rather turbulent than laminar. This should be taken into account when modelling, since turbulent flow cannot be modelled realistically. Turbulent flow favours erosion and carries high suspended solid loads, which affects also the contaminant loads. A predictive modelling of the water rebound is useful tool for assessing the possible environmental impacts of the flooding due to overspills of deteriorated water (Younger & Robins 2002). The modelling tools are described in Closedure Post-closure modelling page.
Besides modelling and possibility to interfere the flooding by pumps, the flooding should always include a monitoring system for e.g. geotechnical stability of mine workings, water level and pressure. With monitoring the possible deterioration of surface and groundwaters due to overspills and infiltration can be detected and prevented. In addition, any geotechnical errors, collapses or subsidence can be detected by proper monitoring system. The monitoring of mine flooding should consist at least of the water level in the mine and the drainage water quantity. It may include also the installation of physico-chemical probes, regular measurements of the drainage water quality within the flooded part of the mine and at the discharge points, as well as a geotechnical monitoring system. (Early 1999, Younger & Robins 2002, Wolkersdorfer 2008).
Underground water diversion
The unused open voids in underground mines are usually backfilled after the ore has been extracted (Potvin et al. 2005). However, unless the voids are backfilled with impermeable materials the porosity of the backfill usually exceeds the porosity of the adjacent strata and the hydraulic connections between voids still exist. Thus the water flows without any restriction through the open voids, the backfilled parts and even in the pore spaces and fractures in the adjacent strata (Wolkersdorfer 2008). The physical barriers and grouting are often used as a water flow control method in mine inundation/flooding.
Different types of physical barriers such as bulkheads and dams are constructed to delay or fully prevent the water flow in underground working. The materials used in the construction of barriers depend on the purpose of the barrier as well as the hydraulic pressure they have to withstand. The common materials used are steel, wood, sand, concrete and clay (Skousen et al. 1998, Wolkersdorfer 2008). Chekan (1985) divided bulkheads into five types: control, emergency, precautionary, and consolidation bulkheads and open dam walls. Control bulkheads are used in active mining to prevent the flow from closed parts into still actively operating parts. The control bulkheads are usually built to be permanent and they can stand high water pressures. For emergency purposes pipes and/or valves are constructed into the bulkhead to prevent blow outs during maximum water levels. Emergency and precautionary bulkheads are built for accidental water inrushes. The emergency bulkheads are usually permanent and the excess beyond the bulkheads is impossible, whereas the precautionary bulkheads can be shut in case of inrush. Both of these bulkheads have to withstand very high water pressure. Consolidation bulkheads are usually temporary and constructed to prevent the water flow during e.g. maintenance work or grouting. Open dam walls are constructed to seal the adit only partly for water treatment or storage purposes (Chekan 1985, Wolkersdorfer 2008). Moreover, to overcome the accidental collapsing of the complete bulkheads, the partial barriers enable the water flow through the mine workings, but delay the flow. Consequently the oxygen access to the mine is eliminated and oxidation of disulphides hindered. The problem with physical barriers is chemical and physical weathering and blowouts (Wolkersdorfer 2008). The bulkheads are used in mine entrance sealing, which is presented in Closedure Mine entrance sealing part.
Figure 2. Partially sealed mine adit (modified after Foreman 1971 and Skousen et al. 1998)
Through grouting the water and acid producing materials can be separated and the water flow into and within the mine prevented. In grouting different types and mixtures of cement are used depending on the water chemistry as well as hydrostatic pressure. Injection of grouting reduces the groundwater flow through the spoil and infiltration and thereby prevents the oxidation (Skousen et al. 1998). Moreover, the grouting is an effective method to ensure the water tightness in mine entrance sealing, which is presented in Closedure Mine entrance sealing part.
Advantages and disadvantages
Especially with open pits flooding and pit lake forming is often included in the after-care planning, since the pit-lakes work as a wildlife habitats or fisheries as long as the water quality criteria are met. Moreover, the flooding of the open pits and underground workings may decrease the contamination of the water due to decreased rate of weathering and oxidising of minerals. However, especially in the beginning of the flooding, the sudden water rush may dissolve the weathered acid generating minerals (e.g. pyrite) in the exposed ores and waste rock, resulting in deteriorating of water quality (Early 1999, Younger & Robins 2002, Wolkersdorfer 2008). For instance, according to studies of Younger (2000) and Geller et al. (1998), the flooding of an underground mine or an open pit often results in ten-fold increase in the concentrations of especially sulphur and iron within the flooded mine waters. However, Younger (2000) emphasised that this type of deterioration is significant mainly when the mined ore contains more than 1% by weight total sulphur, like in backfilled strip mines or in pyritic ore body open pits. Thus, since the flooding also limits the access of the oxygen and thus hinders further weathering processes, flooding is often recommended in prevention of ARD formation (Early 1999, Wolkerdorfer 2008). In addition, the highest peak in contamination takes its place directly after the flooding and the quality of the discharge water improves with time. This is because the first flushes are strong and rapid enough to flush most of the weathered, contaminant bearing solids from the voids and thereafter the water is replaced with fresh, less contaminated recharge. After the first flush, the contamination rate is dependent on the availability of the weathering minerals and water table fluctuations, mainly on top-most layers of the water body. The completion of the initial water quality after flooding may take years or even decades, and thus, also the water treatment may be needed for years after mine closure (Younger & Robins 2002). According to Glover (1983) and Younger (2000), the first flush is generally exponential in form and its duration can be summarised as follows:
tf = 4tr
Where, tf is the time for first flush, tr is the time for rebound. However, if the mine is only partially flooded the weathering processes and thus also the dissolution of acid producing minerals such as disulphides are often reactivated especially during major storm events, whereas in completely flooded mines the water quality tends to recover faster to background values. Thus the final water table should be as high as possible. In addition, to prevent oxidation of disulphides, the sulphide bearing material should be placed above or below the lowest possible water table (Younger 1997, Demchack et al. 2004, Wolkersdorfer 2008). To accelerate the oxygen consumption in flooded underground workings, a Gas Redox and Displacement (GaRD) method can be applied. In GaRD the oxygen is replaced by reducing gases generated by anaerobic bacteria activity, thus halting the sulphide oxidation and acid generation (Taylor and Waring 2001). More information on proper waste management can be found in Closedure Wastes and waste facilities page.
During the water table rebound, as the table balances to its final position, the water body usually arranges in stratification by the quality. As a rule, a better quality water is found from the top layer of the water body, whereas the highest concentrations of contaminants will be found in the bottom layers, like in the studies of Wheal Jane tin mine in Cornwall UK (Younger 2002). This implicates that since there are no lateral inflows or outflows, the mechanical mixing of the water column is minimal and the stratification stays undisturbed. However, any discharges from the mine might trigger turbulent flow in the water column, resulting in mixing of the whole water body. This, in turn, affects the discharging water quality, that will definitely be lower quality than expected if the topmost layers of the water table were only sampled. Hence, it is essential to sample the mine water rebound over the full depth of the water body (Younger 2000, Younger & Robins 2002). Since the rising of the water table can cause unforeseeable non-point discharges and overspills and the drainage water of a mine flooding has often potential to pollute the adjacent waterways, flooding should be managed to avoid any adverse impact on water reservoirs. Moreover, since the water table resumes close to pre-mining conditions, it is essential to restore the pre-mining hydrogeological conditions as initial as possible (Younger & Robins 2002, Wolkersdorfer 2008). When studying possible pollution derived from the mine water rebound, the study should be based on the baseline and background information of hydrogeology conducted before and during mining operations. This background data is then completed with sampling of mine water rebound points after flooding and combined with storm-event monitoring and flow monitoring to achieve the information on groundwater and surface water interaction and the processes leading to metal inputs. In addition, it is advisable to analyse river sediment samples for total and bio-available metals.
Stability of mine workings
After a mine has been abandoned, the maintenance of the wooden and steel support stops or pillars start to weather and, depending on the physical properties of the support, its stability will gradually decrease. If the rock pressure, caused by the static or dynamic loading, exceeds the supportive forces, the support fails and the workings collapse, eventually causing surface subsidence. In unsupported parts of the mine, especially in the stopes and adits, overburden pressure decreases rock stability and, depending on the extraction ratios, causes roof failures or pillar crushings as well. Alternatively, the buoyancy forces of the rising mine water during mine flooding will cause the pore pressures to decrease and the overburden pressure and the potential for subsidence might be minimised; even uplift can occur. Thus, when engineering the designs for mine flooding, hydrostatic heads and rock mass conditions should be taken into account (Wolkersdorfer 2008). To maximize the stability of underground workings and to prevent collapse, stress transfer, underground failure, and flooding of adjacent mines, the closure plan should include also the estimation about the longevity of these structures (NWT 2007). The failure of underground working may be prevented e.g. by backfilling, which is presented in Closedure Wastes and waste facilities page.
The fracturing of the overburden or the overlying strata may change also the intrinsic permeability of the strata, alter groundwater flow paths and cause fluctuations in the water table. Any changes to the groundwater can take years to establish a new equilibrium. Where the overlying rock strata is thin (less than about 200 metres) between the mined ore and the land surface, rock fracturing associated with mine subsidence can also directly affect the surface water. With respect to groundwater, shallow aquifers could drain into subsidence fractures, or surface waters and recharge could be diverted into fractures. The effects of local geology, occurrence of groundwater and surface water, and mining scenario must be carefully evaluated to ensure an adequate understanding of a particular site (Wolkersdorfer 2008).
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