3. Impact of mining on groundwater (mining and its consequences)

Mining of minerals often leads to various environmental impacts [10] including water [11, 12]. The analysis of impact(s) can be done by comparing presentscenario with past or pre-mining scenario [13] and evaluated as either positive or negative orthe combination of both. These are analyzed with respect to the core zone, 5 km radius area (or alternatively consisting ofthe active mining area alone), and buffer zone, consisting of 10 km radius area. The likely impacts of open-pit mining could be in terms of:

a. Drawdown, that is, lowering of water table

b.Water quality deterioration, that is, water pollution

## 3.1 Groundwater lowering due to water table interception

When water is discharged from the pit mine which has intercepted water table, firstly the "collected water" is discharged, and then water from phreatic surface (water table) is sucked, and a "cone of depression" is formed with its axis at the lowest point at the sump bottom having lowest RL. If discharging is done for more time period, this cone of depression continues to enlarge, and pronounced effect is noticed. In technical terminology this is what is referred as "drawdown." Figure 4 explains this drawdown principle in general for a discharge through pit or dug well as applies in hydraulics. If more than one point of water discharge or drawdown exists in a pit mine and kept overlapped, the lowering of water level takes place rapidly, and quarry bottom can be dried with faster speed (Figure 5).

In respect of drawdown, two different kinds of situations come across in an openpit mine: firstly, when the mine is working above the water table and, secondly, when the mine is working below the water table. Water (or drawdown) does not pose any problem in the former case, whereas in the latter case, lowering of water table may be the impact of mining. As a general principle, drawdown is usually in excess of 65% of

Mining of Minerals and Groundwater in India DOI: http://dx.doi.org/10.5772/intechopen.85309

#### Figure 5.

unconfined aquifer thickness [14]. Such drawdown varies from rock type to rock type. Therefore, this statement cannot be taken as a thumb rule.

While analyzing the impact of mining on nearby villages, that is, adjacent to pit, the water-level records or fluctuations (in open dug well/borewell or piezometer) in pre-monsoon and post-monsoon season are taken into account. Because India has monsoonal climate and maximum rainfall occurs during June to September months, pre and post monsoon philosophy is considered best. On the basis of field observations, that is, rock, formations, and aquifer conditions, the impact is assessed of that study area, for example, GW in hard rocks will be present in the fractures/cracks/ and fissures in small quantity while compact soft sandstone rocks contain significant groundwater quantity in rock pores and interstices. The continuity of cracks in aquifer determines the water availability even though stratum has impervious characteristics. Therefore, in such situations drawdown by pumping will be observed as local impact only. Another impact of mining that could be natural also is defined in terms of "radius of influence."

It is often asked how to estimate or quantify the impact of mining on groundwater regime? This question can be scientifically and effectively answered by estimating the influence radius or radius of influence (Ro/Re). The importance of Ro/Re with respect to a mine is that it demarcates a visually assessable picture of impact in terms of a measurable distance and should be kept constant/or minimum as far as possible.

Radius of influence (Ro) in technical terminology is the impact area, spread around the mine due to groundwater extraction or use. It is calculated using Eqs. (1) and (2) given in the below figure.

Ro is directly proportional to "draft magnitude" and "average rainfall" that occurs in an area. Here, the GW extraction is limited to mines only and as an industrial unit which otherwise could be for irrigational, agricultural, or domestic purpose also. For a "single pit" in an open mine, an equivalent radius of influence (Re) is calculated, whereas "Ro" is determined for multiple/concentric pits.

The operative staff, for all practical purposes, can judge the cone of depression, drawdown conditions, and radius of influence in the mine based on their field experience.

### 3.2 Water quality implications

Besides groundwater lowering, water quality implications (in the form of pollution) are a major impact issue of mining on environment globally. The pollutants (or traces of heavy metals) are released into the groundwater by geogenic sources through weathering of the geologic formations [15] and anthropogenic sources. Contamination in groundwater because of anthropogenic sources, for example, agricultural fields and use of fertilizer/pesticides, sewages and solid wastes, return flow due to irrigation, etc., is most often noticed and is far-far larger than the waterlevel lowering impact mentioned above. The water quality implications and environmental impacts are described/covered in appropriate section of this chapter in a scattered manner. It is so because number of cross-connecting factors of land and water has to be looked into for quality evaluation (Sections 2 and 5.2).

## 4. Groundwater and planning for mining below ground level

When mine becomes deep or excessive watery conditions are encountered in underground mines or when mine is located in the vicinity of a major water body and intensive seepage through strata (more than normal) occurs, then scientific mining and planning for groundwater management becomes essential. At varying locations, different mining and differing groundwater conditions are observed, for example, when mine is located adjacent to sea/in coastal areas, when aquifer encountered is confined and water table is under pressure, etc. In all these situations, mine planning for mineral extraction below the water table has to be carried out differently taking into account the water hydrology. World over, the depth denomination differs from country to country for an open surface mine, operating in pit form (Box 1). But in general and in practical sense, all mines below water table are likely to encounter water or watery condition whether it is an open-pit mine or an underground mine.

Planning for mining below ground level has to consider the effect of deepening of pit. Therefore, an interdisciplinary approach intermingling both planning and engineering aspects is needed. Considering the constraints posed by the dynamics

When open-pit mine is deepened beyond a certain depth, "economic stripping ratio" comes into picture and the underground mining originates in which gaining access to the mineral deposit is by means of vertical shafts, inclined shafts, drift mining, or by other means. The value of mineral exploited, that is, cost of mineral production from mine (ROM cost), govern its excavation depth. For a higher value mineral and lower value mineral, such norms are staggering differently.

Box 1. When mine is deep?

Deep mine or deep mining is simply mining underground, in which the miner and/or machinery work beneath a cover of soil or rock. There is no fixed norm for mine to become "deep" or "shallow." As a rule of thumb, exploitation of fuel minerals (coal/lignite/brown coal) at depths exceeding 300 m depth can be considered as deep mine, whereas for metallic or nonmetallic mineral deposits of modest mineral value, this norm may be taken as 350 m approximately.

### Mining of Minerals and Groundwater in India DOI: http://dx.doi.org/10.5772/intechopen.85309

of groundwater, that is, spatial variability, hydrogeological data and its availability, socio-economic conditions, demographic profile of the area, etc., its quantitative estimation is done. In Indian condition, Groundwater Estimation Committee (GEC-1997) methodology seems practical for calculating water quantity. On this basis, planning of mining below ground level and water management through engineering approach yields desired output. To plan a mine for industrial purpose, obtaining groundwater abstraction permission is necessary. Such statutory compliance, particularly groundwater permission in mining, makes the water management easier [16]. In India and until now, it was mandatory for all new industries to apply for groundwater extraction clearance, but now it is mandatory to obtain these clearance for old as well as new industry (http://times ofindia.indiatimes.com/articleshow/ 49832855.cms?utm\_source=contentofinterest&utm\_medium=text&utm\_campaign=cppst). This has initiated the need and emphasized for estimation of groundwater quantity and its management.

To do the planning as per the approved mining plan, excavation depth (RL/ MRL) and the lowest MRL up to which mining will reach in the future have to be designed scientifically. Depth-wise RL, pit dimensions, and water quantity (Q) are then needed for assessment. It may be noted that the excavated area dimensions keep on changing as per the ultimate pit plan. As per the dug-out area, the water availability in the mine area varies during different periods of a year. Accordingly, water quantity (Q) is first estimated for that particular mining pit. Related to Q or water quantity, three areas are important, namely, "mine lease area," "catchment area," and "pit area" (excavated area/water-filled area).

Geohydrological evaluation of the mine area is extremely helpful for the groundwater assessment and futuristic planning of the mine area. In addition to the GW and SW, seepage water is also accounted for in mine's planning. Seepage water appears through mine walls in open pits, and field observations for seepage flow are generally recorded during post-monsoon season. To get the total water quantity of mine pit, it is simply added to the SW and GW quantity.

By groundwater modeling and simulation methodology, groundwater-level decline (maps, etc.) and the groundwater quantity can be estimated [17]. To understand the groundwater resource position in a mining area, water table depth below ground level and aquifer types are extremely important. If these are known and utilized correctly, the planning for mining will be easier. A general trend indicating rate of groundwater discharge/rate of outflow with time is illustrated below (Figure 6). The help of graph can be taken to know the availability of water during different months in a year, which varies from 200% (100% for surface water and 100% for groundwater) to as low as 55%.

Figure 6. Rate of groundwater discharge (or rate of outflow) with time

As a part of mine planning, operation, and execution, following methodology is helpful for mining below ground level:

	- a. Runoff inside the pit
	- b.Slope erosion and control (including stabilization by natural vegetation, etc.)
	- c. Sediment/silt load accumulation in sump/sedimentation pond, etc.
	- d.Water quality and its deterioration at mine level
	- e. Periodical maintenance/observation
