Rainwater harvesting basics: All you need to know about recharge pits

1) Overview:

A recharge pit can be totally invisible when finished. As it is filled of stones, it doesn’t present any danger (contrary to an open well for example). The percolation rate of a recharge pit is much less than of an open well. The water percolates slowly because there is no hydrostatic pressure in the pit.

2) Preparation:

Materials required

1. Graded jelly (different sizes)
2. Sand

Site specification             

• The site should have a sufficiently clean and large catchment
• Location should be such that it permits fast infiltration and percolation
• If the pit aims to recharge a borewell, it should be built as close to it as possible
• Ideally it should be in the valley of the surface   layout

Site selection for infiltration

Many factors affect the suitability of a site as an infiltration facility for the disposal of recharge pit. Among these, the following are most important:

• Depth to groundwater
• Surface
• Underlying soil type

For example the coastal areas - where there is an important layer of sand under the first layer of clay - are fit to the making a recharge pit (sand has a high level of percolation).

3) Construction Technique

Excavation

The diameter of the pit will depend on the catchment area and the rate of percolation of the soil. It can vary between half a meter to 3 meters.

Excavation must be done till a porous soil, weathered rock or fracture is reached. Generally it is found at a depth of 6 to 8 feet.

Backfilling

You need jelly of different sizes, and sand for the top of the pit. The big jelly at the bottom form large gaps for the water to pass through. The smaller ones on the top of it will support the layer of sand.

A mesh between the sand and the jelly will prevent the sand from escaping below.

Instead of sand, you can put a layer of soil, leaves or planted earth. These materials will also filter the water.

4) Cost and caution

• The cost of the pit will roughly depend on the cost of the filling materials - the nature of soil, the cost of sand and jelly – and can be between Rs. 500 to Rs. 5000
• Always ensure that the catchment is free from biological and chemical pollutants
• Never allow polluted water to recharge the ground

Things to remember

• First, construct one recharge pit and measure the amount of water that infiltrates
• Based on the impact, the number of recharge pits in a catchment can be worked out.