Need for water harvesting at Awang Sekmai, Manipur and Artificial Recharge Techniques
- Part 3 -

L Sherjit Singh *

New Year 2013 revellers at Sekmai, Manipur on 1st January 2013 :: Pix - Deepak Oinam

B) SUB SURFACE TECHNIQUE

In this method the structure lies below the surface and recharges ground water directly. The important structures commonly use are Recharge wells, Recharge shaft, Dug wells etc

1 Recharge wells

Recharge wells can be of two types - (a) Injection well, where water is "pumped in" for recharge and (b) Recharge well, where water flows under gravity

The Injection wells are similar to a tube well. This technique is suitable for augmenting the ground water storage of deeper aquifers by "pumping in" treated surface water. These wells can be used as pumping wells during summers. The method is suitable to recharge single aquifer or multiple aquifers. The recharge through this technique is comparatively costlier and required specialized technique of tube well construction and maintenance to protect well from clogging.

It is better if an abandoned tube well is used as a recharge well which will be a cost-effective structure The recharge well for shallow water table aquifers up to 50 m are cost effective because recharge can take place under gravity flow only. These wells could be of two types, one is dry and another is wet. The dry types of wells have bottom of screen above the water table.

In such wells excessive clogging is reported due to release of dissolved gasses as water leaves the well and on other hand redevelopment methods have not been found effective in dry type of wells. The wet type of wells are the wells in which screen is kept below water table. These wet type wells have been found more successful

2. Pits and shafts

In area where impervious layer is encountered at shallow depth the pits & shafts are suitable structure for artificial recharge. These structures are cost effective to recharge the aquifer directly. The diameter of shaft should normally be more than 2 m to accommodate more water. A silt free source water can be put into recharge shaft / pit directly through pipes, if this pipe is kept above water table there are chances of choking of the aquifer by air bubble with water therefore it is always advisable to lower the injection pipe below the water level.

On other hand in the areas where source water is having silt the shaft / pit should be filled with boulder, coarse sand from bottom to have inverted filter or the source water should be passed through a separate filter chamber before it enters the shaft / pit

The advantage of shafts / pits structure is that they do not require large piece of land like percolation tank & other spreading method and there are practically no losses of water in form of soil moisture and evaporation like other methods of spreading.

3 Gravity Head Recharge Wells

In addition to specially designed injection wells, existing dug wells and tube/bore wells may also be alternatively used as recharge wells, as and when source water becomes available. In areas where considerable de-saturation of aquifers have already taken place due to over-exploitation of ground water resources resulting in the drying up of dug wells and lowering of piezometric heads in bore/tube wells, existing ground water abstraction structures provide a cost-effective mechanism for artificial recharge of the phreatic or deeper aquifer zones as the case may be. Schematics of a typical system for artificial recharge through dug wells

II) INDIRECT METHODS

A) Induced recharge

It is an indirect method of artificial recharge involving pumping from aquifer hydraulically connected with surface water such as perennial streams, unlined canal or lakes. The heavy pumping lowers the ground water level and cone of depression is created. Lowering of water levels induces the surface water to replenish the ground water. This method is effective where steam bed is connected to aquifer by sandy formation.

Aquifer modification by Bore blasting and hydro fracturing

i) Bore blast technique

Bore blast technique is adopted to create more storage space of Groundwater artificially in massive and crystalline hard rock by fracturing the bed rocks. Hydrogeological survey is carried out to locate such area where the rock can be blasted to develop cracks below the zone of weathering. The no of boreholes is calculated depending on the quantity of water to be made available. Suitable type of slurry explosive is lowered in the boreholes and is blasted using detonating cord and electrical detonators. At a time about 5 to 6 boreholes are blasted. This technique is applied in assured rainfall areas and where landforms are mostly hilly. These are most suitable for small habitations of about 100 to 150 population, where drinking water shortage occurs.

ii) Hydro fracturing

This technique is applied for rejuvenation of poor yielding or Unsuccessful bore wells. Some of the bore wells in the villages are successful and fitted with pumping device. Poor yielding and dry bore wells in the same village indicate that the fractures do not exist or if existing, they are not connected to the nearby bodies, and the fractures may be closed or sealed. By this technique very high hydraulic pressure is created between the confined sections of the bore well. This very high pressure is responsible for opening the closed or sealed fractures and further connecting it to nearby bodies. When confined section of bore well is not having existing network of fractures then the new fractures are created and these newly created fractures are further extended to connect the water bodies.

2.3 SELECTION OF RECHARGING METHODS

The type of artificial recharge system that can be developed at any specific site is controlled, to a large degree, by the geologic and hydrologic conditions that exist at that site. Site selection criteria, in addition to economic considerations should include at least the following (modification from O'Hare et al., 1986)-

1. Movement of ground water
2. Quantity and Quality of source water available
3. Resulting water quality (reactions with native water and aquifer materials)
4 Clogging potential
5. Underground storage space available
6. Depth to underground storage space
7 Transmission characteristics
8. Topography/applicable methods (injection or infiltration)
9. Legal/institutional constraints
10. Cultural/social considerations
11 Thickness and permeability of the material overlying the aquifer, if any.
12 Thickness and permeability of the aquifer.
13 Proximity of the potential recharge site to an appropriate well field cone of depression.
14 Water- level difference between the aquifer and the recharge site
15 Availability of property (land)
16 Environment & Land use pattern
17 Cost and available technology (

2.4 Artificial Recharge Structures Suitable Under Combination of Different Topographic Slopes, Hydrogeologic Groups and Rainfall Distribution (Source-CGWB)

Following methods are suggested for recharging ground water

2.5 Advantages and precaution of artificial ground water recharge

Artificial recharge has several potential advantages like
* Renovation of waste water
* Reduction of flood flows
* Increase well yield
* Decrease the size of areas needed for water supply systems
* Reduction of salt water intrusion or leakage of mineralized water
* Increase stream flow
* Store fresh water derived from rain and snow melt.
* Very few special tools are needed to dig drainage wells.
* In rock formations with high, structural integrity few additional materials may be required (concrete, softstone or coral rock blocks, metal rods) to construct the wells.
* Groundwater recharge stores water during the wet season for use in the dry season
* Aquifer water can be improved by recharging with high quality injected water.
* Recharge can significantly increase the sustainable yield of an aquifer.
* Recharge methods are environmentally attractive, particularly in arid regions.
* Most aquifer recharge systems are easy to operate.
* In many river basins, control of surface water runoff to provide aquifer recharge reduces sedimentation problems. The technology is appropriate and generally well understood by both the technicians and the general population. Reduction of land subsidence

Precaution to be taken for groundwater recharging

* Drainage wells should be maintained adequately so that the wells do not fall into disrepair and to prevent from groundwater contamination.
* There is a potential for contamination of the groundwater from injected surface water runoff, especially from agricultural fields and roads surfaces. So the surface water runoff should be pre-treated before injection.
* Quality of the injected water should be controlled. .
* The hydrogeology of an aquifer should be investigated and understood before any future full-scale recharge project is implemented.
* During construction of water traps, environmental effects should be minimized as far as possible.

3. CONCLUSION

The process of urbanization often causes changes in ground water level as a result of decreased recharge and increased withdrawal. As population increases many individual wells are abandoned in favour of deeper wells. During field investigations, it is found that well located at Sekmai bazar has lowest ground water elevation as a result of reduced ground water recharge due to paved surface area and increased ground water discharge (excessive consumption)

Since there is no any law and legislation about digging wells in our state, the number of well are also increasing day by day. It implies increasing of ground water withdrawal without recharging. This may lead to the damage of aquifer. Once aquifer lost its property, it will not return back. Hence proper measures should be taken up to recharge groundwater for future.

Concluded...