ARTIFICIAL RECHARGE TECHNIQUES AND DESIGNS

A wide spectrum of techniques are in vogue to recharge the ground water acquifers. The artificial recharge techniques can be boradly categorized as follows :

Through water spreading
Recharge through pit and wells
Induced recharge from surface water bodies
Percolation Tanks / Ponds
Recharge tube wells
Sub - surface dykes
Roof top harvesting

These techniques are briefly discussed below :


1. Percolation Tanks :
Percolation Tanks or Ponds are shallow depth tanks formed at appropriate places in natural or diverted water courses and provided with a weir to allow the excess water to continue its course. The percolation tanks are to be located in the midst of cultivable land. The surrounding area is preferably quite flat or gently slopping and the soil in and around the tank should be permeable for quick percolation with minimum evaporation losses.

The existing village tanks which are normally silted and damaged can be modified to serve as recharge structures. By desalting coupled with providing proper waste weir and cut off trench on the upstream side, the village tanks can be converted into recharge structure, for enhancing ground water recharge.

A hydrological frame work of the percolation tanks / ponds both in alluvial soil and hard rock are illustrated in the figure.

2. Recharge tube wells

Recharge tube wells have to be provided in percolation pond / tank bed to hasten the percolation effect where the water table is very deep, but it is possible to get more replenishments by successive floods. The purpose of tube well is to directly feed the deep aquifer with fresh water with less evaporation and transit losses, besides protecting the water quality.

· A general design of the recharging tube well in sedimentary formation consists of drilled bore hole with a diameter of 50 cm. Down to the depth of existing bore wells in the area but usually 30m. below the water table. A PVC / MS tube of 20 cm. Is placed and the annular space between the pipe and the bore well is packed with pebbles or gravel. The section of the tube against the aquifer is slotted and the top of the well is covered by an artificial filter to prevent the suspended material entering into the aquifers with recharged water. The filter consists of a pit of 6 m3 filled with small pebbles, layered with stone chips and sand at the top one metre. The slotted section in the upper part of the tube is wrapped with coir surrounded by gravel pack and sand.
· During flood, water infiltrates through the upper sand layer where most of the suspended material in the water is retained. The sand and gravel pack around the slotted section, filters out the remaining materials and the coir wrapping acts as protective filter before the water enters the tube. The artificial recharge filter is designed to give a high infiltration rate.
· In hard rock area, since the bore is only 150 mm diameter, a slotted pipe is lowered to the top of the hard rock and the annular space between the casing and the weathered rock is provided with coir rope pack to prevent sand and silt entering the bore. However to have a proper functioning of recharging tube well, maintenance work is needed, after every monsoon period.

3. Sub surface Dykes / Sub surface Dams :

Ground water availability in small valleys can be effectively increased by improving the storage potential, by construction of sub surface Dykes with impervious material like clay, bitumen, tarfelt or polythene sheets besides bricks and concrete. The structure need not be thick or project above the surface. Ideal sites are narrow valleys of 100 to 200 m. width underlain by materials having good specific yield. In order to prevent water logging or silt accumulation on the upstream side of the dam, the crest should be slightly longer than the lowest ground elevation along the dam so that the accumulated silts are flushed away.

A dyke is a sub surface wall in the river bed which is made impervious by putting a polythene sheet on the downstream side. Sub surface dykes are constructed in wide streams with high flow levels, for the entire width of the river and for a depth upto the lowest ground water level or at least up to 15 metre below the bed level. When constructed along the sea coast, the sub surface barrier will prevent sea water intrusion into the land, thereby safeguarding the environment and no evgaporation losses occur in this type of structure. During post monsoon period even when the river is dry, sub surface flow occurs which is temporarily stored behind the barrier. This results in maintaining higher water levels in wells in the proximity of the streams for a longer period and less rapid recession in levels during post monsoon period.