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Water Pollution and Management Strategies from Seepage and Run-off from Overburden Waste Dumps and Mine Benches

A major water pollution problem in mining areas occurs due to the erosion of mine benches, overburden and reject dumps and tailings dams. Due to deforestation and baring of ground, the soil particles are released during the heavy rains. The blasted material in over-burden and mine benches contain fine particles which are also carried away by rain water. Many chemicals also enter the water streams in dissolved state. These sediment carrying effluents are responsible for siltation of agricultural fields and choking of streams and rivers on the downstream side of the mine.

Mining areas in general fall under monsoonal influence with relatively heavy seasonal rainfall with a natural propensity for high erosion and run-off rates. In the overburden dumps, a low angle of slope should be adopted with terracing and bamboo barricades in the slopes, stone barriers at the toe of dumps, check-dams in the channels and gully plugging. These measures apart from considerably reducing run-off also prevent mechanical damage to plants already existing and raised, save agricultural fields and adjoining river water courses from pollution. Some of the measures are briefly described below.

1. Catch-Drains and Diversion Channels: As the very name implies, the function of such a channel is to collect all the run-off water from higher locations and prevent it from running down a vulnerable slope. These are to be laid across the slope on the uphill side before the commencement of a vulnerable slope. Suggested (Changmai, 1999) dimensions of such a channel are :
Width at the bottom of channel - 20 cm to 30 cm
Width at the top of channel - 50 cm to 60 cm
Depth of channel - 30 cm to 45 cm

While carrying such water through diversion channels laid across the slope, it is very much necessary to minimise velocity of water in these while it comes from highland to lowland. Thus diversion channels should be connected by grassed/vegetated waterways along the prevailing slope. The greeneries hinder the run-off for a while, also arrest the solid particles which were being carried by it and hence work as "catch-drains".

Such diversion ditches are useful even in guiding the water to main sumps from the mine benches. In underground mines, garland drains are used to reduce water inflow through subsidence cracks.

2. Gully Plugging: Formation of rills or micro-channels is inevitable in the slopes in the existing conditions, and convergence of a number of rills leads to the formation of a gully. Such gully should be treated by providing suitable plugs filling up the width of the gully from the highest location downwards. A plug generally consists of two rows of driven-in stout branch-cuttings (or species that sprout readily) "Agave" is specially good for gully plugging. The suggested (Changmani, op.cit.) spacing between two row being about 60 cm to 1m. The gap between the rows is to be filled up by prostrate rows of brush-wood. (Fig.5.7a).

In case it is required to form a masonary/stone plug, on the down-stream side of the plug, a boulder-lined apron need to be provided (length of apron being about twice the height of plug) (Fig. 5.7b). The interval between successive plugs should be such that the top of the down-stream plug is at the same level as the bottom of the adjacent plug on the upstream side. To be really effective, the entire length of a gully may be covered by such barriers or plugs.

3. Check Dams or Spoil Dams: These barriers are to be set across channels or water-courses which carry the run-off materials from the overburden dumps and are intended for holding up such run-off materials and preventing transportation/deposition of those in downstream areas. It is suggested (Changmai, 1999) in order to be effective, in each channel there should be series of such check-dams, erected along the width of the channel (to extend and protrude into the firm banks on either side), starting from the commencement of the channel. Each successive dam in a channel should be located in such a way, that a line, joining the bottom (ground level) of the upstream dam and the top of the next down-stream one, should not have a gradient in excess of 1 in 500. On the downstream side of each dam, there should be an apron of 50 cm deep layer of boulders engaged in wire-netting, covering the entire width of the channel. The length of apron (along the length of the channel) should be twice the height of the related check dam. The check-dams are to be built up of boulders (at least 30 cm diam), laid over a wirenetting at the bottom and the same wire netting raised along the sides also, so as to enclose the entire mass of boulders making up the dam. There should be no earth or sand input to fill up the voids in the boulders. The height of the first dam at the highest upstream location will vary according to the height of the banks and those of the subsequent dams on downstream locations will be governed by the thumb-rule indicated earlier in this para. The wires of the wire-netting used in these constructions should be of at least 8-gauge category.

4. Toe Barriers: These barriers, made up of dry stone masonry, are erected parallel to the toe of the dumps, located 6 to 10m away from the base of the toe. These barriers are not designed to serve the purpose of retaining walls for which a much more massive and costly structure will be required. The barriers are intended for containing the finer particles of run-off from spreading onto adjacent areas. For a stone-wall having a height of 2.5m, it is stipulated that the width of the foundation should at least be 4m, the width at the base should be at least 3m, and the slope of the wall on the outer or down-hill side should be flatter (1 in 1) to provide a proper batter (Ghosh et al., 2002).

5. Sedimentation Ponds: All surface run-off water containing sediments should be guided to sedimentation ponds for settlement of suspended solids before discharge of water into natural streams. Provision of such sedimentation ponds have recently been made in a number of mining areas. However, the cleaning and maintenance of these ponds is required to be done on periodic basis. Technique for calculating the size of such ponds and the required detention time of run-off in it can be worked out by following the calculations of Brune (1953), Segarra & Loganathan (1992) and Loganathan et al. (1994) respectively. These have been further used by Raghunath (1985) and Mays (1996).

Installation of oil and grease traps to prevent the discharge of effluents from vehicle wash and workshop is required though quantity of effluents generated is comparatively small in volume. Then the treated effluents can be combined with sewerage system for final treatment.

6. Artificial Rim For Controlling Run-off From Stockpile: Mined out materials should be stacked on prepared floor with an artificial rim for collecting run off to prevent uncontrolled discharge of polluted water from the stockpile. The artificial rim can be made of waste rock in which case it works as a filter and also permits guiding of the run-off water to the central pool from where it can be sent to the sedimentation pond.

7. Collecting and Recycling of Seepage Water From Tailings Dams: This topic has been detailed in Ghosh et al., (2002). Inspite of all efforts it is sometimes not possible to make tailings dams completely seepage free. The common way to control water pollution from such seepage is to prepare a small toe dam on the downstream side and collect the seepage water which can be pumped back to the main reservoir. In this manner zero discharge can be achieved from the slurry pond. While all efforts should be made to reduce and control water pollution in mining areas it must be remembered that hydrologic disturbance is created in another way, namely, reducing availability of water in the aquifer and surface streams on the downstream side of the mining project. The quantity of water used for spraying purposes in crusher houses, and for spraying on haul road etc. is almost wholly lost to the atmosphere by evaporation and only a small proportion goes as run-off to the mine sump. Mining can also destroy the recharge area in many cases, or through deforestation reduce the infiltration ratio and increase the run-off. This is another cause of reduction in the water availability for the community because of the mining project. All efforts should, therefore, be made to reduce the industrial water consumption in mining. The use of wetting agents and road dust control chemicals instead of spraying water should be practised and the process water from mills and washeries should be reused. At the final stage of an opencast mine, it should be the national policy to create a water body of about 30m depth which would be a community asset in providing irrigation water and recharge of ground water in the region.