In most situations in surface streams, concentrations of water constituents inversely correlate with the stream flow rate (Stumm and Morgan 1996).
Such a scenario can often be seen after runoff events, where the electrical conductivity, as a measure of total dissolved solids, decreases while the flow increases. However, at most flooded surface and underground mines, there is a steep increase in the concentration of mine water contaminants after the mine water reaches the outflow level (Fig. 13, Fig. 14). At partially flooded mine sites, this commonly occurs after every major storm event (see section 5.4.2), while at mines that are completely flooded, the contaminant concen-trations usually decrease until they stabilise at a relatively constant value –
5.4 Effects Contributing to Mine Water Constituents 47 46 Insights into Mine Closure
metals, such as Fe, Mn, Al, and Zn, which are usually present as hydroxide or sulphate salts. This phase is characterised by fresh water flowing into the system and reacting with the remaining disulphide and the before-mentioned minerals.
Besides the acid-producing component, there is also an acid-consuming component that occurs during the first flush scenario. As described in section 4.5, a range of minerals react as proton acceptors and neutralize acidity. Such buffering can lead to neutral or circum-neutral conditions in the mine water, after all the acidity is flushed out or consumed by the buffering minerals (Demchak et al. 2004).
Most first flush scenarios show a characteristic time lag after which the pollution load reaches its maximum. This time lag can range from several hours to several days, and has not been thoroughly investigated. Yet, physico-chemical measurements in shafts during the flooding process often show that the electrical conductivity in the upper meters of the flooded shaft is less than it is several meters beneath the water surface. Wolkersdorfer (1996) shows that this low conductivity layer (zone № 1) is commonly between 1 and 18 m thick, though in one case it was 190 m. It is followed by zone № 2, where the electrical conductivity decreases within a thickness of 1 to 30 m; finally comes zone № 3, where the electrical conductivity is relatively constant or slightly decreasing (see also Fig. 113). The reason for this behaviour is that the polluted mine water is commonly overlain by less polluted infiltration water. When the mine water finally discharges into the anthroposphere, first the less polluted mine water drains out of the mine followed by the more pol-luted water, which leads to the characteristic first flush.
The duration of the first flush, tf, mainly depends on the acidity removal, acirem, by buffering or dissolution, by the rate, rw, at which acid-containing minerals weather, the volume, V, of the inter-connected workings, the hy-draulic connection, conductivities, K, of the mine workings, and the ground water recharge, RGW. According to the findings of the HERO research team (University of Newcastle upon Tyne), the duration of the first flush after the overspill of the mine water can be calculated (in minutes) using the follow-ing empirical equation (Younger 2000d):
(26) with tr the time it takes for the voids to be flooded (“rebound time”),
min
Wood et al. (1999) commented that tfis a function of time dependent and time independent factors, whereas the lithological setting and the extend of the workings belong to the first, and the removal of vestigial acidity,
car-tf = f aci( rem,r V K Rw, , , GW)≈( .3 95 1 2± . )×tr
bonate dissolution, iron hydroxide armouring, bacterial reduction of sulphate, and change in water flow rate belong to the latter. For the 32 coal mine dis-charges they analyzed, they were able to show that over the long term, the pH ranged between 6 and 7 and total iron decreases to less than 20 mg L-1. A similar approach was used by Younger (2002d), who related the peak con-centration of Fetotto the total disulphide content of the mined coal seams (Table 10).
Alkaline mine water can revert to acidic conditions when all the buffering phases are consumed and the production of juvenile acidity cannot be buffered any longer. Such a scenario has been observed at the Yorkshire Bull-house mine, which became acidic after 60 years of being alkaline (Laine and Dudeney 2000; Younger 2000a). For every mine flooding scenario, it is there-fore essential to calculate the potential net neutralization capacity of the rocks in contact with the flooded mine.
5.4.2 The Effects of Storm Events
Closely related to the first flush is the increase or dilution of contaminant concentrations at a mine water discharge. At partially flooded mines, a storm event can raise the water level in the mine enough to dissolve the secondary minerals that have formed since the last storm event, significantly (in some cases) affecting the discharge quality. Mines that have relatively constant water levels are not subject to this effect. Interestingly, there are many pub-lications touching the matter of increased pollution and sediment loads in mine water affected streams, but no general work about that interesting sub-ject, though Bayless and Olyphant (1993) describe many of the effects that cause stream pollution after surface runoff at mine sites. Most pollution 5.4 Effects Contributing to Mine Water Constituents 49 48 Insights into Mine Closure
Total disulphide content, wt. %
Observed ranges of peak Fetot -concentrations, mg L-1
< 1 0.01–0.5
1–2 0.5–100
2–3 100–350
3–4 350–1,200
4–5 1,200–1,500
Table 10. Disulphide contents of mined coal seams and their relation to observed peak iron concentrations in the United Kingdom. The numbers can be used to pre-dict peak iron concentrations for the first flush period. Modified after Younger (2002d).
the remnants of copper mining, and a high daily load of copper (Parliament of the Commonwealth of Australia 1970; Taylor et al. 1996; Twining and Cameron 1997). The sediments transported by the Kings and Queen Rivers filled up the river mouth with nearly 1.5 m of sediments, so that boats were hindered from entering the river’s mouth (Parliament of the Commonwealth of Australia 1970).
Proper regrading and revegetation is an effective means to minimise acid mine drainage at surface operations (The Pennsylvanian Department of En-vironmental Protection 1998). To prevent runoff, the slopes of tailings or waste rock piles should be regraded to reduce the slope of the piles and to minimise the erosion during storm events (Hawkins 1995). This provides more stable surfaces, supports revegetation, and thus decreases the pollution of surface streams and ground water. Several of the measures described under water diversion (section 5.7) can be used to prevent surface water from flow-ing over uncapped waste rock piles (Adams et al. 2000). Sandén et al. (1997) measured the amount of colloids during a storm event 500 m downstream of a remediated mine waste deposit. At this sampling location, the flow in the stream increased 2 hours after the rain started. They clearly proved that the amount of solids started to increase 1 hour later, increased by about 100%
after another 30 minutes, rose to a maximum of 150% 4 hours after the runoff water reached the measuring location, and stayed that high for at least 24 hours.
In the Picher Mining District (Kansas/Oklahoma/Missouri, USA), meas-ures were taken to prevent surface runoff from flowing into abandoned mine shafts and boreholes (Tar Creek Superfund Task Force Water Quality Sub-committee 2000a; DeHay et al. 2004). In addition, several areas were cov-ered with impermeable soil to prevent storm water from flowing into the abandoned mine workings, but that measure was not very effective and is not recommended to prevent water flowing into abandoned workings. If there is a need to predict surface runoff, Appendices A and B of U.S. Environ-mental Protection Agency (2003) describe the necessary measures to predict the amount of runoff by using several graphical and numerical techniques.
Burbey et al. (2000) observed two different effects of rainfall on the dis-charge water quality. While the main ions in the water, such as Mg, SO4, and Ca, are relatively uninfluenced by the rainfall, the pH is negatively corre-lated with increased flow due to rainfall, and the iron concentrations were positively correlated with the rainfall events. They conclude that the rain-water, which contains no iron, flushes the mine water and also lowers the pH.
Sandén et al. (1997) took 47 samples over a period of 24 hours after a 15.5 mm rainfall event during 7 hours. The flow within the stream increased from 0.2 L s-1to 23 L s-1and decreased to 6 L s-123 hours after the rain comes from the erosion of tailings, waste rock piles, or from adit mines and
the subsequent transport of the sediments and metals into the receiving streams, the flooded mine workings, or pit lakes. Several measures to prevent such runoff by covering waste rock piles were described in Integrated Geot-echnical Engineering Services Specialists in Unsaturated Zone Hydrology (2003) and U.S. Environmental Protection Agency (2000). Field studies, monitoring of surface runoff at abandoned coal mine sites, and the modelling of the runoff were described by Hodgkinson and Armstrong (1996).
In general, the impacts of storm events at abandoned mine sites can be minimised by using a thorough remediation strategy. Many regulations in-clude precautions against surface runoff from active mines, and the recom-mendations should be kept in mind for mine closure activities as well (U.S.
Environmental Protection Agency 2003). The necessary measures to be taken before, during, and after a storm event should be compiled in a stormwater management plan, similar to all the other management plans for an active or abandoned mine site (Jacobs and Denham 2001). Such procedures include covering of tailings or waste rock piles, covering of potential infiltration zones above the mine workings, water diversion, regrading of slopes, or revegetation. Some of those measures are described in section 11.4.7. Re-tention ponds hold back suspended solids. If diversion ditches are constructed to guide the runoff around waste rock piles, they must be large enough to collect all the water, even during high storm or snowmelt events. At the Aitik/Sweden mine, diversion ditches were too small and erosion caused damage to the cover (Salmon and Destouni 2001). Especially in tropical re-gions, stormwater management is critical, as shown at the Batu Hijau mine, located on Sumbawa Island, Nusa Tenggara Barat, Indonesia. Up to 50 mm in a 15 min interval and 200 mm in a 24 hour interval were measured at the mine site and if no measures were taken to drain the water – even during the post-mining period – severe damage to the surrounding environment could occur. Sediment traps were installed to combat the problem (Jacobs and Den-ham 2001; Jacobs and White 2001).
Besides the erosion of solids and its transport in the runoff water, rainfall also raises the water table in tailings or waste rock piles and can release the easily soluble secondary minerals in tailings (Al and Blowes 1996). Loadings during and after rainfall events can be up to 20% higher than during normal flow situations. Especially after long rainfalls events, when the tailings be-come flooded, significantly high discharges might be expected.
Possibly one of the most interesting sites where such phenomena occur is the Tasmanian Mount Lyell mining area, which affects the King River and, at its mouth, trout farming in Macquarie Harbour (Lake et al. 1977; Locher and Keller 1995). Several hundreds of kilometres downstream, the aquatic life was severely affected by the effects of surface runoff, sediments from
5.4 Effects Contributing to Mine Water Constituents 51 50 Insights into Mine Closure
disulphides are excluded from weathering as soon as possible and the pollu-tant loads are consequently quickly reduced, according to equation 26. In such a scenario, the water treatment plant can be switched off after a rela-tively short time. On the other hand, if the water treatment plant will have to run for a very long time, the flooding process should be deaccelerated. Sev-eral options for an adapted mine water treatment are available, and will be discussed in chapter 11.
The main reasons for mine flooding are:
● the mine is no longer economical
● all the raw material has been exploited
● accident, war, or political reasons
● geotechnical stability of the abandoned mine workings
● prevention of disulphide oxidation
● safety reasons (prevention of unintentional visitors)
Controlled mine flooding is conducted in areas where people or the envi-ronment might be affected by polluted mine water or by raising the water table above a certain level. There, maintenance of pumps or treatment facil-ities will take as long as deemed necessary by the mine operator or the au-thorities. In some mines in the former German uranium mining areas of Sax-ony and Thuringia, controlled flooding was prepared for extensively; several expert opinions about the mine water rebound and the chemical evolution of the mine water quality were sought, taking nearly a decade to draw final con-clusions (Jakubick et al. 2002). Controlled mine flooding does not necessar-started. Their results were similar to those of Burbey et al. (2000), with an
in-crease in pH, turbidity, and TIC and a dein-crease of the other trace element concentrations. The main ions and total iron showed two scenarios: they first increased as salt dissolved from the mine waste disposal site, and after 4–5 hours, they decreased again as a result of dilution.
However, there is no general rule of behaviour of mine sites or flooded mines during storm events. At many sites contaminants decrease due to di-lution. While the contaminant loads stay relatively constant, the contaminant concentrations decrease with increasing flow (Hawkins 1998).
In conclusion, the negative impacts of surface runoff after storm events or snowmelt can be positively influenced by covering the tailings and waste rock piles, regrading the slopes to reduce the slope angles, revegetationg the surface of the piles, and diversion of waters. Compared to many other re-mediation measures, especially mine water treatment, these measures should be an integral part of a mine site remediation to limit AMD/ML.