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Recharging or down the drain? Questions on RWH - Malad, Mumbai

I have a query about rain water harvesting in a housing society located in Malad West, Mumbai.

The society has four buildings each having 7 floors including ground floor. The roof area is 3500 sq ft. While the builder claims that he had installed RWH in the building about 8 years ago, the residents see no change in water table levels. They fear that the water from roof could be going to under ground drains.

What could be the problem? Could it be that due to hard rock in that region, no change in water table is taking place?

Would it be better if they stored the rain water in sumps rather than allowing it to percolate to recharge the groundwater levels? Is there a rule of thumb to be followed?

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Comments

1. How did the you reach this

How did you reach this conclusion that there is no change in the GW levels? Do you actually measure the level periodically? This question is of course just for curiosity, it has no impact on the questions you have asked.

1: Water table is defined as the "underground surface below which the ground is wholly saturated with water"; also as “the level at which the groundwater pressure is equal to atmospheric pressure.” A sandy strata can be saturated with water with a free water surface, and thus have a “water table”. A hard rock is impervious and water is present in hard rock only in cracks and fractures. Strictly speaking, the term “water table” is not applicable in a hard rock area. A better term is - “static water level” in your borehole.

2: RWH simply means collecting rainwater. Artificial Recharge (AR) means putting water in the underground formations. Although RWH and AR go together, these are two different things.  

3: Quantity of water harvested depends only on the roof area. It makes no difference whether the building has 7 floors or 70 floors.

4: Whether water table or static water level, the level will depend not only on success of AR, but also on the quantum of extraction. That is obvious, is it not ? How much money you have in your bank account after 8 years, will depend not only on how much you put in, but also on how much you withdraw. During these 8 years the extraction would have also increased. Therefore, not only water table/ static water level may not increase, it may even go down.

5: It is most unlikely that the water is going in underground drains. Once the water goes below the drain level it can’t get back up and in to the drain on its own. For the water to go into drain, it will have to do so during its downward journey itself, and for that to happen there will have to be a “water express way” connecting the drains to your recharge well. Very unlikely.

6: My guess is, the water is not at all going in to the under ground strata, and is overflowing from the recharge well itself. This is very common in hard rock areas, because in hard rock, the water can not flow fast enough through the cracks and fissures to occupy the empty spaces. It is like trying to pour a lot of water in a bottle that has a very narrow mouth.  

7: It is certainly better to store the rainwater in sumps and use it. But in a place like Malad - high rainfall and high population density - it is unlikely that you will find enough ground space to build a large enough tank. Also, jave you given a thought to us it for what purpose, and how? Will you pump it up into your OH tank and mix it with processed water? Or have you installed dual piping system in your buldings? In absence of clarity and resolute action on these counts, all talk of storing the rain water and using it, in urban areas, is often nothing more than just talk.

But do observe the recharge well during this year monsoons and tell us - does it overflow? If you thus report back, only then the “static knowledge level” about RWH and AR will increase.

Chetan Pandit, National Water Academy

 

2. recharging or down the drain? Questions on RWH, Malad,Mumbai

You mention that the Youbuilder has claimed that he has installed RWH system. Do you have details of what this system is, so that we know where the rainwater is supposed to go? If you have, pl convey these so that remedial measures can be suggested.


Indukanth

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